Metal Catalyzed Group 14 and 15 Bond Forming Reactions: Heterodehydrocoupling and Hydrophosphination Michael Cibuzar University of Vermont

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Graduate College Dissertations and Theses Dissertations and Theses

2019 Metal Catalyzed Group 14 And 15 Bond Forming Reactions: Heterodehydrocoupling And Hydrophosphination Michael Cibuzar University of Vermont

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Recommended Citation Cibuzar, Michael, "Metal Catalyzed Group 14 And 15 Bond Forming Reactions: Heterodehydrocoupling And Hydrophosphination" (2019). Graduate College Dissertations and Theses. 1023. https://scholarworks.uvm.edu/graddis/1023

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METAL CATALYZED GROUP 14 AND 15 BOND FORMING REACTIONS: HETERODEHYDROCOUPLING AND HYDROPHOSPHINATION

A Dissertation Presented

Michael Patrick Cibuzar

The Faculty of the Graduate College

The University of Vermont

In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Specializing in Chemistry

May, 2019

Defense Date: March 21, 2019 Dissertation Examination Committee:

Rory Waterman, Ph.D., Advisor John M. Hughes, Ph.D., Chairperson Matthew D. Liptak, Ph.D. Adam C. Whalley, Ph.D. Cynthia J. Forehand, Ph.D., Dean of the Graduate College

ABSTRACT

Investigation of catalytic main-group bond forming reactions is the basis of this dissertation. Coupling of group 14 and 15 elements by several different methods has been achieved. The influence of Si–N heterodehydrocoupling on the promotion of α-silylene elimination was realized. Efficient Si–N heterodehydrocoupling by a simple, earth abundant lanthanide catalyst was demonstrated. Significant advances in hydrophosphination by commercially available catalysts was achieved by photo- activation of a precious metal catalyst.

3– Exploration of (N3N)ZrNMe2 (N3N = N(CH2CH2NSiMe3)3 ) as a catalyst for the cross-dehydrocoupling or heterodehydrocoupling of silanes and amines suggested silylene reactivity. Further studies of the catalysis and stoichiometric modeling reactions hint at α-silylene elimination as the pivotal mechanistic step, which expands the 3p elements known to engage in this catalysis and provides a new strategy for the catalytic generation of low-valent fragments. In addition, silane dehydrocoupling by group 1 and 2 metal bis(trimethylsilyl)amide complexes was investigated. Catalytic silane redistribution was observed, which was previously unknown for d0 metal catalysts.

La[N(SiMe3)2]3THF2 is an effective pre-catalyst for the heterodehydrocoupling of silanes and amines. Coupling of primary and secondary amines with aryl silanes was achieved with a loading of 0.8 mol % of La[N(SiMe3)2]3THF2. With primary amines, generation of tertiary and sometimes quaternary silamines was facile, often requiring only n a few hours to reach completion, including new silamines Ph3Si( PrNH) and i Ph3Si( PrNH). Secondary amines were also available for heterodehydrocoupling, though they generally required longer reaction times and, in some instances, higher reaction temperatures. By utilizing a diamine, dehydropolymerization was achieved. The resulting polymer was studied by MS and TGA. This work expands upon the utility of f-block complexes in heterodehydrocoupling catalysis.

Stoichiometric and catalytic P–E bond forming reactions were explored with ruthenium complexes. Hydrophosphination of primary phosphines and activated alkenes was achieved with 0.1 mol % bis(cyclopentadienylruthenium dicarbonyl) dimer, [CpRu(CO)2]2. Photo-activation of [CpRu(CO)2]2 was achieved with a commercially available UV-A 9W lamp. Preliminary results indicate that secondary phosphines as well as internal alkynes may be viable substrates with this catalyst. Attempts to synthesize ruthenium phosphinidene complexes for stoichiometric P–E formation have been met with synthetic challenges. Ongoing efforts to synthesize a ruthenium phosphinidene are discussed.

The work in this dissertation has expanded the utility of metal-catalyzed main- group bond forming reactions. A potential avenue for catalytic generation low-valent silicon fragments has been discovered. Rapid Si–N heterodehydrocoupling by an easily obtained catalyst has been demonstrated. Hydrophosphination with primary phosphines has been achieved with a commercially available photocatalyst catalyst, requiring only low intensity UV light. CITATIONS

Material from this dissertation has been published in the following form:

Cibuzar, M. P., Waterman, R.. (2018). Si–N Heterodehydrocoupling with a Lanthanide

Compound. Organometallics, 23, 4395-4401.

Erickson, K. A., Cibuzar, M. P., Waterman, R.. (2018). Catalytic N–Si coupling as a vehicle for silane dehydrocoupling via alpha-silylene elimination. Dalton Trans. 47,

2138-2142.

ACKNOWLEDGEMENTS

I would like to thank the NSF for funding throughout my graduate studies.

I would like to thank two people at the University of Minnesota who were instrumental in starting this journey. First, I would like to thank Dr. Aaron Massari for his willingness to take a chance on me. Second, a big shout out to Dr. Ian Tonks. His passion for organometallic chemistry is infectious and inspiring. I couldn’t have asked for a better chemistry role model.

I had the pleasure of working with some wonderful people in the Waterman group. A big thanks to Justin for being a great role model. After getting back from some meetings with Rory and not having a clue what he was talking about, without Justin’s help digesting the chemistry and deciphering Rory’s chicken scratches, I’d still be lost.

No one has made me feel like this group is a family like Brandon. Thank you for always being my drinking buddy and my number two. Lastly, I had the pleasure of spending nine months with James. That time was my happiest in all my time at UVM. One cannot ask for a better friend. To my coworkers that don’t see their names here, I love you and thank you for being a part of my journey.

A big shout out to Michelle, Teruki, Scott, Cam, Trevor and all my volleyball friends. Thank you for taking my mind off tough days at work, making me laugh and kicking butt on the court. You guys were so often the highlight of my week.

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Thanks to Manhattan’s Pizza Pub for sustaining me most Friday afternoons. I will greatly miss Matt and Philly Friday.

I’d like to thank Hemi Sawant for always putting life in perspective for me. I couldn’t ask for a more generous friend.

I have had the pleasure of having two great scientists in Matt ‘Matty Ice’ Liptak and (Adam) Whalley on my committee. I looked forward to asking for help from you guys. Thank you for all the help you’ve given and your contributions to my success.

I cannot thank Rory enough for not only helping me to become a better scientist, but also making me a better person. You were the reason I came to UVM, and I’ve never regretted that decision. Thank you for everything that you’ve done.

Dad, you are my inspiration. If I amount to half the man you are, I’ll be doing amazing. Mom, thank you for always reminding me to love. I hope to make you both proud.

Erin, I would like to thank you everything you’ve done for me. I’m not sure how

I would have lived the past few years without you. Besides the insufferable gremlin cat you brought into our home, you’ve made my life better in every way. I love you, and I can’t wait to start the next chapter of my life with you.

TABLE OF CONTENTS

CITATIONS ...... ii ACKNOWLEDGEMENTS ...... iii LIST OF FIGURES ...... viii LIST OF TABLES ...... xii CHAPTER 1: INTRODUCTION ...... 1 1.1. Goals of this thesis ...... 1 1.2. Si–N Coupling ...... 2 1.3. P–C coupling by hydrophosphination ...... 11 1.4. References...... 13 CHAPTER 2: EXPLORING α-SILYLENE ELIMINATION WITH ZIRCONIUM AND ALKALI-BASED CATALYSTS ...... 22 2.1. Introduction...... 22 2.2. Si–N heterodehydrocoupling and α-silylene elimination ...... 31 2.3. Dehydrocoupling by group 1 and 2 metal compounds ...... 45 2.4. Enhancing reactivity using N-heterocyclic carbenes ...... 50 2.5. Future direction ...... 52 2.6. Conclusion ...... 53 2.7. Experimental ...... 54 2.7.1. General Considerations ...... 54 2.7.2.1. Si–N heterodehydrocoupling and α-silylene elimination considerations .... 54 2.7.2.2. Typical procedure for dehydrocoupling experiments ...... 55 2.7.2.3. Reaction of 3 with phenylsilane and diethyl disulfide: ...... 55

2.7.2.4. Reaction of 1 with PhSiH3 and PhSiD3: ...... 56 2.7.3.1. Dehydrocoupling by group 1 and 2 metal compounds ...... 59 2.7.3.2. Typical procedure for dehydrocoupling experiments ...... 59 2.8. References...... 60 CHAPTER 3: Si–N HETERODEHYDROCOUPLING WITH A LANTHANIDE COMPOUND ...... 71 v

3.1. Context ...... 71 3.2. Introduction...... 73

3.3. Coupling of silane and amines by La[N(SiMe3)2]3THF2 ...... 75 3.4. Catalyst comparison...... 83 3.5. Heterodehydropolymerization by 1 ...... 87 3.6. Testing pre-ceramics potential by TGA ...... 89 3.7. Investigation into other E–E bond forming reactions catalyzed by 1 ...... 91 3.8. Conclusion ...... 92 3.9. Experimental ...... 93 3.9.1. General Considerations ...... 93 7 3.9.2. Synthesis of La(N(SiMe3)2)3THF2 (1) : ...... 94 n 3.9.3. Preparation of Ph3Si( PrNH): ...... 94 i 3.9.4. Preparation of Ph3Si( PrNH): ...... 95

3.9.5. Preparation of MePhSi(PhNH)2: ...... 95

3.9.6. Preparation of PhSi(PhNH)3: ...... 96 3.9.7. Catalysis experiments general consideration: ...... 96 3.9.8. General method for heterodehydrocoupling reactions: ...... 97 3.9.9. Direct infusion in pos ESI MS: ...... 98 3.10. References...... 105 CHAPTER 4: RUTHENIUM CATALYZED P–C BOND FORMING REACTIONS . 111 4.1 Introduction...... 111 4.2. Hydrophosphination with 2 ...... 115 4.3. Synthesis of a ruthenium phosphinidene ...... 134 4.4. Future direction ...... 138 4.5. Conclusion ...... 139 4.6. Experimental ...... 140 4.6.1. General Considerations ...... 140 4.6.2. Catalysis experiments general consideration: ...... 140 4.6.3. General method for hydrophosphination reactions ...... 141 4.6.4. Hydrophosphination of styrene with phenylphosphine ...... 142 vi

4.6.5. Hydrophosphination of p-tert-butylstyrene with phenylphosphine ...... 145 4.6.6. Hydrophosphination of p-methylstyrene with phenylphosphine ...... 148 4.6.7. Hydrophosphination of p-bromostyrene with phenylphosphine ...... 151 4.6.7. Hydrophosphination of 4-vinylpyridine with phenylphosphine ...... 154 4.6.7. Hydrophosphination of β-methylstyrene with phenylphosphine ...... 157 4.6.8. Hydrophosphination of ethyl acrylate with phenylphosphine ...... 162 4.6.9. Hydrophosphination of methyl acrylate with phenylphosphine ...... 165 4.6.10. Hydrophosphination of acrylonitrile with phenylphosphine ...... 168 4.6.11. Hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine ...... 171 4.6.12. Hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine ...... 174 4.6.13. Hydrophosphination of styrene with diphenylphosphine ...... 177 4.6.14. Hydrophosphination of ethyl vinyl ether with phenylphosphine ...... 180 4.6.15. Hydrophosphination of phenylacetylene with phenylphosphine ...... 183 4.6.16. Attempted hydrophosphination of benzophenone with diphenylphosphine ...... 186 4.7. References...... 189 CHAPTER 5: CONCLUSIONS ...... 196 5.1. α-Silylene elimination...... 196 5.2. Lanthanum catalyzed Si–N heterodehydrocoupling ...... 196 5.3. Ruthenium catalyzed P–C bond formation ...... 197 CHAPTER 6: COMPREHENSIVE BIBLIOGRAPHY ...... 198

vii

LIST OF FIGURES

Figure 1.1. Periodic table. Bolded elements are central elements in catalysts that have demonstrated Si–N heterodehydrocoupling capabilities...... 11

Figure 2.1: [(Me2N)3Zr(μ-H)(μ-NMe2)2]2Zr ...... 32 Figure 2.2: MS of product of trapping experiment with diethyldisulfide ...... 41 Figure 2.3: Complex 29Si{1H} splitting pattern of product from competition experiment...... 43 Figure 2.4. Complexes 1, 2 and 3 ...... 45 Figure 2.5: Some possible Si–N coupling, redistribution or dehydrocoupling products...... 47 Figure 2.6: 1H-29Si HSQC of the reaction of 3 with phenylsilane and diethyl disulfide...... 56 1 Figure 2.7: Final H NMR of reaction of 1 with PhSiH3 and PhSiD3...... 57 1 29 Figure 2.8: Final H- Si HSQC NMR of reaction of 1 with PhSiH3 and PhSiD3 ...... 58 Figure 2.9: Experimental data and simulation of aryl region of the reaction of 1 with PhSiH3 and PhSiD3 ...... 59 Figure 3.1: Lanthanide based Si–N heterodehydrocoupling catalysts ...... 84 Figure 3.2. Monomer unit of oligomer made from methylphenylsilane and p- phenylenediamine...... 89 Figure 3.3. TGA of polymer made from methylphenylsilane and p- phenylenediamine...... 90 Figure 3.4. Direct infusion ESI MS of a silamine polymer ...... 105 Figure 4.1. Spectral distribution of Rexim G23 UV-A (9W) lamp. Provided by manufacturer...... 116 Figure 4.2. 1,2 and 2,1 addition products...... 119 Figure 4.3. 31P {1H} NMR spectrum of the hydrophosphination of β-methylstyrene ... 120 Figure 4.4. Hydrogens two, three and four bonds from phosphorus ...... 120 Figure 4.5. Overlapping doublet and doublet of doublets in the 1H NMR spectrum of the hydrophosphination of β-methylstyrene ...... 121 Figure 4.6. Hydrogens two, three and four bonds from the methyl protons...... 121 Figure 4.7. Unassigned multiplets in the hydrophosphination of β-methylstyrene ...... 122 viii

Figure 4.8. Six hydrogens (in blue) that comprise the three unassigned multiplets...... 122 Figure 4.9. 1H-1H COSY of the hydrophosphination of β-methylstyrene ...... 124 Figure 4.10. 1H-31P HMBC of the hydrophosphination of β-methylstyrene ...... 125 Figure 4.11. 1H-31P HMBC relevant protons ...... 126 Figure 4.12. Chiral centers on 1,2 addition hydrophosphination product ...... 127 Figure 4.13. Generic metal-alkylidene, imido and phosphinidene complexes ...... 134 Figure 4.14.1. Stacked 1H NMR spectra of the hydrophosphination of styrene with phenylphosphine by 2. Spectra collected every 12 minutes...... 142 Figure 4.14.2. Stacked 31P NMR spectra of the hydrophosphination of styrene with phenylphosphine by 2...... 143 Figure 4.14.3. Final 31P NMR spectrum of the hydrophosphination of styrene with phenylphosphine by 2...... 144 Figure 4.15.1. Stacked 1H NMR spectra of the hydrophosphination of p-tert- butylstyrene with phenylphosphine by 2. Spectra collected approximately every 12 minutes...... 145 Figure 4.15.2. Stacked 31P NMR spectra of the hydrophosphination of p-tert- butylstyrene with phenylphosphine by 2...... 146 Figure 4.15.3. Final 31P spectrum of the hydrophosphination of p-tert-butylstyrene with phenylphosphine by 2...... 147 Figure 4.16.1. Stacked 1H NMR spectra of the hydrophosphination of p- methylstyrene with phenylphosphine by 2. Spectra collected approximately every 20 minutes...... 148 Figure 4.16.2. Stacked 31P NMR spectra of the hydrophosphination of p- methylstyrene with phenylphosphine by 2...... 149 Figure 4.16.3. Final 31P NMR spectrum of the hydrophosphination of p- methylstyrene with phenylphosphine by 2...... 150 Figure 4.17.1. Stacked 1H NMR spectra of the hydrophosphination of p- bromostyrene with phenylphosphine by 2. Spectra collected approximately every 20 minutes...... 151 Figure 4.17.2. Stacked 31P NMR spectra of the hydrophosphination of p- bromostyrene with phenylphosphine by 2...... 152 Figure 4.17.3. Final 31P NMR spectrum of the hydrophosphination of p- bromostyrene with phenylphosphine by 2...... 153 Figure 4.18.1. Initial and final (t = 2 hours) stacked 1H NMR spectra of the hydrophosphination of 4-vinylpyridine with phenylphosphine by 2...... 154 ix

Figure 4.18.2. Initial and final (t = 2 hours) stacked 31P NMR spectra of the hydrophosphination of 4-vinylpyridine with phenylphosphine by 2...... 155 Figure 4.18.3. Final (t = 2 hours) 31P NMR spectrum of the hydrophosphination of 4-vinylpyridine with phenylphosphine by 2...... 156 Figure 4.19.1. Final 1H NMR spectrum of the hydrophosphination of β- methylstyrene with phenylphosphine by 2...... 157 Figure 4.19.2. Final 31P NMR spectrum of the hydrophosphination of β- methylstyrene with phenylphosphine by 2...... 158 Figure 4.19.3. Final 31P{1H} NMR spectrum of the hydrophosphination of β- methylstyrene with phenylphosphine by 2...... 159 Figure 4.19.4. Final 1H-1H COSY NMR spectrum of the hydrophosphination of β- methylstyrene with phenylphosphine by 2...... 160 Figure 4.19.5. Final 1H-31P HSQC NMR spectrum of the hydrophosphination of β- methylstyrene with phenylphosphine by 2...... 161 Figure 4.20.1. Initial and final (t = 12 minutes) stacked 1H NMR spectra of the hydrophosphination of ethyl acrylate with phenylphosphine by 2...... 162 Figure 4.20.2. Initial and final (t = 12 minutes) stacked 31P NMR spectra of the hydrophosphination of ethyl acrylate with phenylphosphine by 2...... 163 Figure 4.20.3. Final (t = 12 minutes) 31P NMR spectrum of the hydrophosphination of ethyl acrylate with phenylphosphine by 2...... 164 Figure 4.21.1. Initial and final (t = 12 minutes) stacked 1H NMR spectra of the hydrophosphination of methyl acrylate with phenylphosphine by 2...... 165 Figure 4.21.2. Initial and final (t = 12 minutes) stacked 31P NMR spectra of the hydrophosphination of methyl acrylate with phenylphosphine by 2...... 166 Figure 4.21.3. Final (t = 12 minutes) 31P NMR spectrum of the hydrophosphination of methyl acrylate with phenylphosphine by 2...... 167 Figure 4.22.1. Initial and final (t = 18 hours) stacked 1H NMR spectra of the hydrophosphination of acrylonitrile with phenylphosphine by 2...... 168 Figure 4.22.2. Initial and final (t = 18 hours) stacked 31P NMR spectra of the hydrophosphination of acrylonitrile with phenylphosphine by 2...... 169 Figure 4.22.3. Final (t = 18 hours) 31P NMR spectrum of the hydrophosphination of acrylonitrile with phenylphosphine by 2...... 170 Figure 4.23.1. Initial and final (t = 2 hours) stacked 1H NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine by 2...... 171

Figure 4.23.2. Initial and final (t = 2 hours) stacked 31P NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine by 2...... 172 Figure 4.23.3. Final (t = 2 hours) 31P NMR spectrum of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine by 2...... 173 Figure 4.24.1. Initial through final (t = 96 hours) stacked 1H NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine by 2. ... 174 Figure 4.24.2. Initial through final (t = 96 hours) stacked 31P NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine by 2. ... 175 Figure 4.24.3. Final (t = 96 hours) 31P NMR spectrum of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine by 2...... 176 Figure 4.25.1. Initial and final (t = 23 hours) stacked 1H NMR spectra of the hydrophosphination of styrene with diphenylphosphine by 2...... 177 Figure 4.25.2. Initial and final (t = 23 hours) stacked 31P NMR spectra of the hydrophosphination of styrene with diphenylphosphine by 2...... 178 Figure 4.25.3. Final (t = 23 hours) 31P NMR spectrum of the hydrophosphination of styrene with diphenylphosphine by 2...... 179 Figure 4.26.1. Initial through final (t = 72 hours) stacked 1H NMR spectra of the hydrophosphination of ethyl vinyl ether with phenylphosphine by 2...... 180 Figure 4.26.2. Initial through final (t = 72 hours) stacked 31P NMR spectra of the hydrophosphination of ethyl vinyl ether with phenylphosphine by 2...... 181 Figure 4.26.3. Final (t = 72 hours) 31P NMR spectrum of the hydrophosphination of ethyl vinyl ether with phenylphosphine by 2...... 182 Figure 4.27.1. Initial and final (t = 48 hours) stacked 1H NMR spectra of the hydrophosphination of diphenylacetylene with phenylphosphine by 2...... 183 Figure 4.27.2. Initial and final (t = 48 hours) stacked 31P NMR spectra of the hydrophosphination of diphenylacetylene with phenylphosphine by 2...... 184 Figure 4.27.3. Final (t = 48 hours) 31P NMR spectrum of the hydrophosphination of diphenylacetylene with phenylphosphine by 2...... 185 Figure 4.28.1. 1H NMR spectrum (t = 96 hours) of the attempted hydrophosphination of benzophenone with diphenylphosphine by 2...... 186 Figure 4.28.2. 31P NMR spectrum (t = 96 hours) of the attempted hydrophosphination of benzophenone with diphenylphosphine by 2...... 187 Figure 4.28.3. Final (t = 96 hours) 31P NMR spectrum of the attempted hydrophosphination of benzophenone with diphenylphosphine by 2...... 188

LIST OF TABLES

Table 2.1: Summary of observations and mechanistic implications...... 44 Table 2.2: Silicon containing product distribution for the reaction between 4a-c or 5a and aryl silanes...... 48 Table 3.1. Coupling of silanes with n-propylamine catalyzed by 1...... 76 Table 3.2. Coupling of silanes with more sterically encumbered amines by 1 ...... 79 Table 3.3. Coupling of silanes and secondary amines by 1 ...... 81 Table 3.4. TON and TOFs of lanthanum-based Si–N heterodehydrocoupling i catalysts with PrNH2 ...... 85

Table 3.5: Coupling of amines with PhSiH3 and Ph2SiH2 ...... 86 Table 4.1. Electronic transitions of 1 and 2...... 114 Table 4.2. Hydrophosphination of styrenes and Michael acceptors ...... 118 Table 4.3. Hydrophosphination with 2 ...... 128

xii

CHAPTER 1: INTRODUCTION

1.1. Goals of this thesis

The primary goal of this thesis is to explore catalytic main group bond forming reactions. With applications ranging from electronics to materials to hydrogen storage, main-group element containing compounds are ubiquitous. Our dependence on main- group containing compounds continues to fuel interest in both improving existing applications and exploring new routes to these compounds. This work is primarily focused on exploring new routes to main-group compounds by catalytic methods. A variety of metals are utilized as catalysts predominantly for coupling group 14 and 15 elements.

A known zirconium-based silicon-phosphorus heterodehydrocoupling catalyst was tested as a silane and amine heterodehydrocoupling catalyst to explore potential advances in Si–N, including reaction rate and selectivity. It was quickly realized that the zirconium compounds of interest were poor Si–N heterodehydrocoupling catalysts, but evidence for low valent silicon chemistry fueled further investigation. If there is evidence for low valent silicon fragments in attempted Si–N heterodehydrocoupling reactions, then

α-silylene elimination may be realized.

Given the utility of heterodehydrocoupling of silanes and amines, increasing reactivity and improving selectivity would of great value. Electropositivity of the metal metal center appears to be a good indicator of utility for this transformation. While the

lanthanides aren’t often utilized, some are earth abundant and very electropositive. If a simple, commercially available lanthanum complex is a highly active Si–N heterodehydrocoupling catalyst, then the utility of commercially available compounds is expanded and efficacy of making new silamines may be achieved.

Hydrophosphination is an atom-economic reaction to make P–C bonds. Recent improvements in hydrophosphination include visible light photo-activation of an inexpensive and commercially available iron complex. Limitations in efficiency, catalyst loading, and substrate scope prevent this method from gaining widespread utility. Though not inexpensive, if the related commercially available ruthenium complex is able to surpass the iron analogue in efficiency and substrate scope, then a highly active, more general, commercially available hydrophosphination catalyst will be increase the ease and potency of hydrophosphination as a P–C bond forming reaction.

1.2. Si–N Coupling

Interest in molecules containing Si–N moieties has existed for over 70 years.

Development of silamine chemistry began to gain traction in the 1950s, with interest in making ceramic materials, specifically silicon nitride (Si3N4, abbreviated as SN). Given its stability at temperatures greater than 2500 °F (1371 °C), it was realized early on that

Si3N4 could be used in high temperature structural engine materials as well as nuclear- powered jets and rockets.1 Some modern applications include refractory materials, molds, thermocouple sheath materials, insulators in electronics, bearings and as an alternative to

titanium in spinal fusion devices.2-6 Over 50 years ago, Chantrell and Popper envisioned than an inorganic polymer could serve as a suitable starting material for ceramic materials.7 It was thought that linear, long chain polymers (20,000–50,000 Da) were necessary for good ceramic fibers to be produced. The high molecular weight was necessary to decrease the brittle nature of the preceramic fibers.8 The synthesis of long chain polysilamines was a challenge however, given that the current method for the synthesis of silamines was by aminolysis of halosilanes. This method produced either low molecular weight rings (n = 3-5) or linear oligomers depending on the halosilane.9-13 In addition to not being an effective reaction in generating high molecular weight polysilamines, stoichiometric salt was produced and inflated the cost of production.14-18

Catalytic synthesis of high molecular weight polysilamines was finally achieved in 1989 when Laine and coworkers utilized triruthenium dodecacarbonyl Ru3(CO)12 to heterodehydrocouple organosilanes with ammonia. Upon pyrolysis, it was clear that this route had significant benefits as well as flaws. Synthetic ease of a variety of polymers allowed for superior structural and chemical variation within the ceramic. Upon thermolysis however, high quantities of undesired ceramic material, mainly silicon carbide (SiC), was present. This decrease in the ceramic yield and the inability to remove the SiC, has stymied the development of polysilamines as precursors for ceramics.19

Heterodehydrocoupling of silanes and amines is still thought to be a promising route to make different polysilamines for ceramic purposes but advances in the pyrolysis of those polysilamines has seen limited success.3, 12, 20-31

Silamines also have a variety of applications in organic chemistry. They have been utilized as part of mild, sterically hindered bases,32 as protecting groups for silanes15, 17 and amines,33, 34 and silylating reagents.35-38 Metal bis-trimethylsilylamides,

M(N(SiMe3)2)x, are sterically encumbered, non-nucleophilic bases widely utilized in organic transformations.39-41 In contrast to metal hydride bases, the hydrocarbon backbone of metal bistrimethylsilylamides improves solubility in non-polar organic solvents. When M = Li, Na, or K, these complexes can act as weaker bases than other common lithium bases, such as LDA, with a pKa of the conjugate acid being ~26 (pKa of the conjugate acid of LDA is ~36).42 Amines have been used as protecting groups for halogens on silanes. Generally, they can easily be removed by acid to restore the halogen functionality to the silane.17 The protection of pyrroles is especially important as there is a growing abundance of pyrrolic components in pharmaceuticals, such as Lipitor.34

Having practical applications in natural product synthesis and new materials with useful optical and electronic properties, pyrrole modification is at the heart of several fields.43-45

N-silylation has been a key component in the development of new pyrroles.34 Similar to protecting groups, silamines can be effective as silylating agents. Silylation of alcohols, aldehydes and ketones can be achieved with an appropriate silamine and catalytic amounts of base.35-38

Before Laine and coworkers successfully dehydropolymerized silanes and amines with Ru3(CO)12 and Rh6(CO)16, the only known catalysts for dehydrocoupling of silanes and amines were noble metals on carbon or alumina supports.19 With goals of making

polysilazanes, Citron and Sommer identified that supported Pd, both on carbon and alumina, dehydrocoupled pyrrolidine and isopropylamine with silanes. Unfortunately for

Citron and Sommer, none of the catalysts attempted was able to form even a disilazane.46

Early development of Si–N heterodehydrocoupling chemistry included coupling of diphenylsilane and aniline with a chromium.47 The available silane substrate scope was

n expanded using the catalyst Bu4NF. This reactivity was plagued by low catalyst turnover due to the affinity of the catalyst to silicon.48 Interest in making polysilamines fueled

Eisenberg to investigate binuclear rhodium complexes as catalysts. Unfortunately, MS data supports oligomers of n = 3 were the largest produced.49 Similar motives drew Liu and Harrod to explore a much cheaper compound, CuCl, as a catalyst. Selectivity proved challenging as a mixture of products was observed in most cases.21

Simultaneously, Liu and Harrod were exploring dimethyltitanocene as a dehydropolymerization catalyst. Reacting phenylsilane with ammonia yielded polymers of molecular weight > 5000 Da. The repeating backbone unit of the polymer was not Si–

N, but Si–Si. Dimethyltitanocene is a known silane homodehydrocoupling catalyst, so competitive Si–Si bond formation produces the polymer backbone.50 It is unclear if Si–N coupling occurs before, after or simultaneously to Si–Si coupling. Harrod proposed a mechanism first requiring Si–H activation to generate a titanium silyl species. This would either react with either another equivalent of silane or ammonia. The reaction with another silane results in chain propagation. Reacting with ammonia generates the silamine. Regardless of whether the titanium silyl species reacts with a silane or

ammonia, a titanium hydride species is produced. This then reacts with a silane, possibly a polysilane, to eliminate H2 and generate a titanium silyl species. Because of silane polymerization, non-polymer-based products were inaccessible.

Scheme 1.1. Dehydropolymerization of silanes by a titanocene catalyst

Takaki and coworkers were the first to explore f-block catalyzed Si–N dehydrocoupling. Tertiary silanes were coupled with a variety of amines by Yb(η2-

Ph2CNAr)(HMPA)n (Ar = Ph, C6H4F-4), HMPA = hexamethylphosphoramide) to produce quaternary silamines.51 Differences in reactivity trends indicated that the mechanism could be different to Harrod’s titanocene species.52 Addition of an equivalent of amine across the Yb–C bond generates a ytterbium bisamido complex. Silanolylsis of the less sterically hindered amine produces a Yb–H species. Activation of an N–H bond of a free amine liberates H2 and regenerates the active catalyst. Shortly after Takaki’s work, the first actinide catalyst was used for Si–N heterodehydrocoupling.51 Eisen and coworkers investigated primary and secondary silanes as substrates. Taking a silane with

~3 eq amine and 1.7 mol % [(Et2N)3U][BPh4] yielded a mixture of mixture of mono- bis- and tris-substituted silamines. Mechanistic studies indicate dehydrocoupling by

51, 53 [(Et2N)3U][BPh4] follows pathway to Takaki’s Yb species.

Several years later, Harder synthesized the calcium analogue of Takaki’s ytterbium catalyst. Si–N heterodehydrocoupling occurred at nearly the same rate. Though still extremely air and moisture sensitive, Harder was able to make a much cheaper, but equally reactive Si–N heterodehydrocoupling catalyst compared to Takaki.51, 54

Selectivity has been challenging whether utilizing aminolysis of halosilanes or dehydrocoupling. The mixture of mono-, bis- and tris-substituted products often requires extensive purification, if even possible, for isolation of a pure product. Until recently, there were no examples of a catalyst selectively producing different silamines. In a seminal report, Sadow and coworkers were able to selectively, and catalytically, produce different silamines in high yields by altering the equivalents of amine in the reaction by utilizing a tris(4,4-dimethyl-2-oxazolinyl)phenylborate magnesium complex (1). Difficult substrates, such as hydrazine, were coupled with silanes, though a mixture of mono- and bis-substituted products were observed.55 Of particular interest is ammonia. The N–H bond of ammonia is strong and nonacidic.56 Each Si–N coupling event of ammonia increases the acidity, and thus reactivity of the other N–H bonds. This has made selectivity with ammonia particularly challenging. Impressively, tertiary silanes and ammonia reacted in the presence of 1 to exclusively produce the mono-silylated ammonia product.55

The most prominent mechanism for Si–N heterodehydrocoupling has involved a metal-amido complex that undergoes silanolysis to produce the silamine and a metal- hydride. It was unclear how that reaction may proceed, as several different mechanisms are possible. Complex 1 is d0, making σ-bond metathesis a possible mechanism. Off- metal mechanisms, such as a nucleophilic attack by the nitrogen of the metal-amido complex to the silane could also be possible. Close monitoring of a series of catalytic reactions produced the rate law in eqn 1, which is independent of amine concentration, and rate constant k’ = 0.060(4) M-1s-1. The dependence on silane indicates that the rate- limiting step involves interaction of the metal-amido complex with the silane. This also suggests that the metal-amido complex is the resting state of the catalyst.

(1)

Activation parameters and kinetic isotope effect calculations support a concerted and highly ordered Si–N coupling transition state similar to σ-bond metathesis. When σ- bond metathesis is the mechanism, electron donating or withdrawing ligands have a minimal effect on reaction rate. This is due to only slight polarization of the four- membered transition state.57, 58 Hammett analysis of 1 provided a positive slope (ρ = 1.4) indicating a decreased activation barrier with electron withdrawing substituents on silicon. This led Sadow and coworkers to conclude that the mechanism of the Si–N coupling step was not σ-bond metathesis, but a nucleophilic attack of the amido onto the silane, producing a five-coordinate silicon species. Rapid β-hydride elimination generates 8

the metal-hydride and formation of the silamine. Sadow’s work has provided a significant advance in mechanistic understanding and selectivity in Si–N heterodehydrocoupling.55

Since Sadow’s report, rapid evolution of Si–N heterodehydrocoupling has been seen.55 Hill and coworkers were quick to realize that other group two metal compounds were able to facilitate this reaction. Hill showed that M[N(SiMe3)2]2 (M = Mg, Ca, Sr) could couple primary, secondary and tertiary silanes to a variety of amines.59 Mechanistic studies indicate that the magnesium and calcium analogues react in a similar fashion to

Sadow’s complex 1.55 The strontium analogue however appears to react by a different mechanism. Given the second order rate dependence on the catalyst, Hill hypothesizes that a dimeric active catalyst center may be involved.59 Interestingly, Bellini and coworkers studied the barium analogue, Ba[N(SiMe3)2]2(THF)x, and experimental and

DFT investigations indicate a nucleophilic amine attack mechanism.60-63 Recent work has expanded the utility of group one, two, and three metal complexes.64-67

The lanthanides have also seen advances in Si–N heterodehydrocoupling, particularly by Cui and Sadow. Cui focused on an NHC-based ytterbium catalyst that able to selectively and efficiently couple several amines with phenylsilane.68 While also studying an NHC-based ytterbium catalyst, Sadow expanded the breadth of lanthanides capable Si–N coupling to samarium.69

Very recently, Conejero and coworkers were able to selectively produce a variety of different amines using part-per million catalyst loadings of a cyclometallated NHC Pt

t t F 70 complex [Pt(I Bu’)(I Bu)][Bar 4]. To date, this represents the most active Si–N heterodehydrocoupling catalyst.

Even with these recent advances, there is still significant room for improvement.

Selectivity remains challenging. Very few catalysts have demonstrated the ability to make mono-, bis- and tris-substituted silamines by simply altering equivalents of starting materials. Significant improvements have been made in adding only one equivalent of amine to a silane, but the ability to exclusively add a second or third equivalent has been somewhat under developed. Like selectivity, catalyst activity has been improved as of late. Group two metal catalysts have been unusually successful for this transformation.

Higher catalyst loadings are still generally used however. The most active catalyst Si–N coupling catalyst to date is a precious metal catalyst. Given the desire for more earth abundant and renewable processes, reliance on precious metals is undesired. Base metal catalyzed Si–N heterodehydrocoupling is highly desired if similar reactivity similar to precious metal catalysts can be achieved. Related to this, few simple, commercially available catalysts have been used for this transformation. An earth abundant, commercially available catalyst which could demonstrate high reactivity for this transformation would continue to drive advances in Si–N coupling.

Based simply on the metals used for this transformation, a relationship between electronegativity and catalyst success can be realized. Less electronegative metal centers appear to facilitate Si–N dehydrocoupling faster than electron-rich metal centers. Given

the motivation to use earth abundant metals, selecting an electron-poor base metal would seem like an appropriate place to start.

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba Ln Hf Ta W Re Os Ir Pt Au Hg Ti Pb Bi Po At Rn

Fr Ra Ac Rf Dd Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Figure 1.1. Periodic table. Bolded elements are central elements in catalysts that have

demonstrated Si–N heterodehydrocoupling capabilities.

1.3. P–C coupling by hydrophosphination

P–C bond-forming reactions have garnered significant attention in recent years.

Given the myriad applications of tertiary phosphines in synthetic and catalytic chemistry, selective P–C bond formation has continued to be a synthetic challenge.71-80 Metal-

catalyzed hydrophosphination is an exciting method for P–C bond formation for a variety of reasons. Hydrophosphination proceeds with perfect atom economy. All components of the phosphine are preserved in the product, eliminating chemical waste. In addition, selectivity can often be imparted, producing specific product and easing purification demands.81-83 Hydrophosphination is not without drawbacks, however.

(1)

Though a variety of catalysts have demonstrated hydrophosphination capabilities, precious metal catalyzed reactions still dominate.81, 82, 84-93 Efforts to promote hydrophosphination with inexpensive or commercially available catalysts has been of increased interest. Several advances have been made using iron catalysts, but most are plagued by high reaction temperatures, which, in some cases decreases selectivity.88, 92, 94-

99 In a seminal report by Pagano and Waterman, visible light photo-activated hydrophosphination by an inexpensive and commercially available iron complex,

88 [CpFe(CO)2]2, was realized.

Photocatalysis has the potential to be a green method of catalysis due to the availability of photons in the visible region from solar irradiation.100-105 High-powered mercury arc lamps, which emit photons in the UV region (λ < 300 nm), are heavily utilized for photochemical transformations. High energy requirements, operation costs as well as cost of the lamp itself, and disposal costs all decrease the renewability of mercury arc lamps. Utilizing low-intensity light decreases the dependence on mercury arc lamps 12

and more renewable light sources, such as natural sunlight, LED, CFL or commercially available UV bulbs, become viable.100, 101, 103

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CHAPTER 2: EXPLORING α-SILYLENE ELIMINATION WITH ZIRCONIUM

AND ALKALI-BASED CATALYSTS

2.1. Introduction

Among p-block elements, C–C, C–O, and C–N bond-forming reactions have been studied most extensively. There has been a significant driving force for new homo- and hetero-bond forming reactions of other main group elements, such as phosphorus, arsenic, silicon, germanium and boron, to name a few.1-6 In addition to being used in ligands in designer catalyst,7-10 compounds containing main group elements can be used for many materials applications, such as for polymers11-14 and hydrogen storage.15-19 They can also possess unique electronic or optical properties.20-22 One particular coupling, Si–

C coupling, has been remarkably important in many fields, including reagents for organic chemistry,23 material science,24 agrochemistry,25 and medicinal chemistry.26

A common method of creating many E–E bonds is through Würtz-type coupling.

Würtz coupling requires halogenated substrates, stoichiometric sodium metal and high elevated temperatures and pressures (eqn 1). Beyond these harsh reaction conditions, large quantities of salt waste are produced. This method is unselective resulting in product separation and purification in all cases. Though these conditions are poorly suited for modern industrial scales, Würtz coupling remains the best option for many of these transformations. Besides Würtz coupling, some industrially catalyzed E–E bond forming reactions, such as olefin metathesis, exist although the catalyst usually contains a

precious metal, such as platinum or ruthenium.27 Separation of the catalyst from the material can be difficult and results in loss of large amounts of catalyst over time. As a result, a cheaper or more efficient method of synthesizing E–E bonds was sought.11

(1)

Interest in finding an alternative to Würtz coupling has fueled research into new methods of E–E bond coupling. Dehydrocoupling emerged as a viable method for the synthesis of new E–E bonds for several reasons.3, 11, 15, 16, 28, 29 Dehydrocoupling is a reaction between two E–H containing compounds in which a new E–E bond is formed and H2 is released as a byproduct (eqn 2).

(2)

Dehydrocoupling is an alternative to Würtz-type coupling because it is atom economical and the byproduct can be harvested as a reagent for other chemistry. As an avenue for rapid hydrogen generation, dehydrocoupling is also enticing for hydrogen storage applications.30 Also, there is no salt or acidic waste that needs to be separated or disposed. Successful dehydrocoupling is facilitated by mild reaction conditions and high turnover. Instead of corrosive halogenated species, main-group hydrides are required.

Many main group element-hydrides are readily available or relatively easily synthesized.

Finally, the thermodynamic advantage of forming hydrogen gas assists in the propensity for reaction.29 23

Dehydrocoupling has been demonstrated with a wide variety of catalysts, from late transition metals12, 31 to early transition metals32-34 to the lanthanides35-37 and actinides38, 39 and recently even main group elements.40, 41 In addition, three distinct mechanisms of dehydrocoupling have been reported. The first mechanism involves both oxidative addition and reductive elimination steps (Scheme 2.1). A redox-active catalyst is necessary for this mechanism. As a result, late transition metals are typically employed.

Though more classically understood, oxidative addition and reductive elimination hasn’t been the most utilized mechanism of dehydrocoupling mechanism.

Scheme 2.1: Oxidative addition and reductive elimination mechanism

σ-Bond metathesis has been the most prominent mechanism for dehydrocoupling catalysts. Compounds containing d0/d0f0 metal centers, such as alkali, early transition group metal complexes, especially ones containing group 4 metals, have demonstrated high dehydrocoupling activity by this mechanism.4, 32, 42 The mechanism involves a characteristic concerted four-membered, kite-like [2σ+2σ] cycloaddition transition state

(Scheme 2.2). This facilitates new E–E bond formation without changing the oxidation state of the metal center.3

Scheme 2.2: σ-Bond metathesis mechanism

Though σ-bond metathesis has been a common mechanism of dehydrocoupling, a newer mechanism emerged: α-elimination. Upon the addition of a stannane to catalytic

5 5 amounts of CpCp*HfHCl (Cp = η -C5H5, Cp* = η -C5Me5), Neale and Tilley observed long chain tin polymers. Utilizing kinetics and isotope labelling experiments, it was concluded that σ-bond metathesis was not the mechanism and instead a deinsertion based mechanism was occurring. Stoichiometric deinsertion from a metal center has been known for decades, but this is the first instance of catalytic deinsertion, and is referred to as α-elimination rather than deinsertion.43-45 The mechanism is simply the microscopic reverse of the insertion of a low-valent main group fragment into a metal hydride bond

(Scheme 2.3). Upon extrusion from hafnium, the stannylene fragment can insert into other bonds, such as another Sn–H bond to form distannane. Consecutive insertions of stannylene into Sn–H bonds enabled the formation of long chain tin polymers, something

that hadn’t been possible by other Sn–Sn bond forming reactions. Trapping experiments with 2,3-dimethylbutadiene provided further evidence in support of α-stannylene elimination.

Scheme 2.3: Mechanism of α-elimination

The scope of α-elimination has expanded dramatically in recent years. Given that heavier main-group elements can exist as stable low valent species and generally contain weaker hydrogen bonds, primed for hydrogen migration, antimony seemed like a reasonable place to start. Shortly after discovering α-stannylene elimination, Waterman and Tilley utilized the same CpCp*HfHCl complex to pursue α-stibinidene elimination.

The reaction of 5 mol % CpCp*HfHCl with MesSbH2 (Mes = 2, 4, 6-trimethylphenyl) yielded a tetrastibine, Sb4Mes4. Deuterium labelling and Eyring analysis yielded similar results to the analogous tin chemistry and it was concluded that an α-hydrogen migration/stibinidene elimination mechanism was occurring.46

Given that α-elimination has enabled the formation of long chain tin polymers, exploring dehydropolymerization with other elements that display unique properties is warranted. A combination of the interest in π-conjugated materials with main-group elements and weak As–H bonds warranted exploration into α-arsinidene elimination.47-51

Utilizing a known phosphine dehydrocoupling catalyst, Waterman and coworkers probed the dehydrocoupling of arsines. It was concluded that the size of the substituents on arsenic was responsible for the mechanism of dehydrocoupling. A smaller arsine,

Ph2AsH, promoted σ-bond metathesis while a larger arsine, such as dmpAsH2 (dmp = dimesitylphenyl), promoted α-arsinidene elimination. Though the formation of long chain arsenic polymers was not observed, several advances in As–C bond forming chemistry were made.2

Stoichiometric phosphinidene transfer has been utilized for the generation of highly valuable phosphorus containing heterocycles, including phospholes.52-65 Layfield and coworkers have provided the first example of catalytic phosphinidene transfer. Using

M[N(SiMe3)2]2 (M = Fe, Co), catalytic phosphinidene transfer to an NHC formed N- heterocyclic phosphinidene.66 While d0/d0f0 complexes have been primarily responsible for advances in α-elimination, Hayfield demonstrated that late transition metals are capable of this transformation. Pagano and Waterman provided significant evidence for catalytic phosphinidene elimination by [CpFe(CO)2Me]. Successful trapping experiments with alkynes, dienes and disulfides, deuterium labelling and reactivity studies all support

α-phosphinidene elimination as the mechanism of dehydrocoupling.67

Though not confirmed, it is plausible that Berry and coworkers were first to observe α-germylene elimination. Using a cis and trans mixture of Ru(PMe3)4(GeMe3)2 as a catalyst, demethanative coupling of trimethylgermane resulted in the formation of highly branched and high molecular weight polygermanes. Berry proposes that after ligand dissociation, α-Me migration occurs to form a ruthenium germylene. This is followed by a germyl migration, which is the new Ge–Ge bond forming step. Oxidative addition of another equivalent of trimethylgermane follows and upon reductive elimination, the new germane terminated from the metal. Addition of another equivalent of trimethylgermane and elimination of an equivalent of methane reestablishes the active catalyst (Scheme 2.4). Before chain termination, reductive elimination of methane enables chain growth and the formation of longer chain polygermanes.22, 68

Scheme 2.4: Don Berry’s proposed demethanative coupling of trimethylgermane

mechanism

An α-germylene elimination mechanism is also a possibility. Instead of a migration, elimination of the germylene from ruthenium is possible. The free germylene would quickly insert into another bond, likely a Ge–H or Ge–Ge bond, to either initiate a new polymer chain or propagate an existing polymer (scheme 2.5). More experiments would be needed to determine whether the germylene remains on ruthenium or is eliminated. In addition to a computational approach, determinization of activation parameters, rate law and kinetic isotope effects could assist in elucidating mechanistic

details. Trapping experiments have also been useful.46, 67 Further exploration into α- germylene elimination chemistry is warranted.

Scheme 2.5: Potential α-germylene elimination mechanism of demethanative coupling

Apart from oxygen, silicon is the most ubiquitous of main-group elements. With an extremely high abundance and wide range of applications, silicon chemistry is of great importance. As many Si–E bonds are still generated by Würtz coupling, investigation into silane dehydrocoupling remains warranted. Being able to catalytically generate a low valent silylene would add a valuable tool to the silicon chemistry toolbox. Unfortunately, advances in transition metal catalyzed silylene elimination have been sparse. The only

example of silylene elimination was reported by Pannell but it was light catalyzed and not specific to a variety of silanes.63, 69 There has been some evidence supporting the notion that certain rhodium catalysts, such as Wilkinson’s catalyst, could be catalytically generating free silylene.70-73 Identifying the right features to promote α-elimination remains challenging. The primary goal of this chapter is to promote and explore α- elimination as a possible mechanism for new silicon bond forming reactions.

2.2. Si–N heterodehydrocoupling and α-silylene elimination

An increased premium has been placed on Si–N heterodehydrocoupling as a reaction that affords products of high synthetic utility and interest.5, 74-89 For example, silazanes have been of use as silylating agents,90, 91 bases,92 ligands for catalysts, and as ceramic or polymer precursors.93 While excellent work has been done in this area discovering and understanding metal and non-metal catalysts, general catalysts for this reaction that tolerate both sterically encumbered substrates or that provide selectivity between a single or multiple Si–N bond forming events on the same substrate (one or two reactions at RNH2 or RR′SiH2, for example) remain a challenge. Despite advances in the development of group 2 metal catalysts, particularly by Hill and Sadow,74, 75 zirconium is an interesting potential candidate being relatively abundant while readily engaging in

3 dehydrocoupling catalysis. Supporting this hypothesis are reports of Zr(NMe2)4 reacting with silanes to give Si–N bonds,94, 95 though this chemistry is limited to stoichiometric

Si–N bond formation and subsequent formation of a poorly soluble polymetallic hydride

product. We report herein our exploration of catalytic Si–N bond formation by zirconium.

Despite meeting limited success in improving upon literature catalysts, this work has led to suggestions of silylene reactivity which is unique within this field. Further studies within provide evidence that zirconium amido derivatives can liberate silylene fragments, insight that may provide more facile routes to the catalytic generation of R2Si: fragments and related synthetic chemistry.

Literature observations of Zr(NMe2)4 reactivity were readily replicated but no catalysis was identified.94, 95 For example, treatment of phenylsilane with 10 mol % of

Zr(NMe2)4 gave Ph(NMe2)SiH2, Ph(NMe2)2SiH, and a yellow precipitate consistent with

95 [(Me2N)3Zr(μ-H)(μ-NMe2)2]2Zr (figure 1.1). It was hypothesized that increased solubility of Zr(NMe2)4 may lead to catalysis, but replication of the above reaction with one equivalent of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) also gave stoichiometric Si–N products and a similar precipitate. Given the success of triamidoamine-supported zirconium in dehydrocoupling catalysis,2, 3, 96, 97 we next considered this ligand to increase catalyst solubility.

Figure 2.1: [(Me2N)3Zr(μ-H)(μ-NMe2)2]2Zr

Treatment of mixtures of silanes and amines with 10 mol % of (N3N)Zr (N3N =

3− N(CH2CH2NSiMe3)3 ) derivatives resulted in limited catalytic activity. For example, heating phenylsilane with excess dimethylamine in the presence of 10 mol % of

98 (N3N)ZrNMe2 (1) to 80 °C in benzene-d6 resulted in 60% formation of Ph(NMe2)SiH2 vs phenylsilane over 24 hours (∼90% conversion after 48 hours) with apparent decomposition of 1 to a complex mixture of unidentified products (eqn 3). The limited efficacy of (N3N)Zr derivatives for Si–N dehydrocoupling was not immediately clear given their past success in P–Si and P–Ge bond forming catalysis.99 The reaction of phenylsilane with isopropylamine in the presence of 10 mol % of 1 at 80 °C in benzene- d6 produced minimal Ph(NMe2)SiH2. However, some diphenylsilane was also observed

1 1 29 by H NMR spectroscopy and confirmed in H– Si HSQC experiments. Silane (SiH4) was not identified in these reactions, and similar phenyl migration has been reported by

Tilley to proceed via a cationic Hf–Ph intermediate.100, 101 The possibility of a rare early transition-metal catalyzed redistribution was nevertheless intriguing because critical metal-mediated silicon catalysis, including the Direct Process and redistribution reactions,102-104 appears to involve unsaturated silicon derivatives such as silylenes

(R2Si:). An aggressive study of such transition-metal silylene (LnM = SiR2) compounds has yielded substantial reactivity and new catalysts.70 Likewise, useful organosilane products can be prepared utilizing silylene fragments generated via photolysis followed by trapping with organic reagents,105 but the direct generation of silylene fragments via metal catalysis, such as α elimination, remains an untapped area.

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Tilley identified that dehydropolymerization of stannanes appears to occur via α-stannylene elimination.43-45 In group 15, α-stibinidene and α-arsinidene elimination reactions have also been reported, and form small rings or distibenes or diarsenes, respectively.2, 46, 106 Layfield first realized catalytic trapping of phosphinidene fragments using bis(NHC) iron and cobalt catalysts,66 while we have expanded on α- phosphinidene elimination as a route to organophosphines via trapping.67 These observations and the value of organosilanes argue for the exploration of α-silylene elimination and the possibility of the resultant organosilane synthesis.

The suggestion of silylene chemistry prompted investigation of zirconium-silyl derivatives supported by the triamidoamine ligand. Reaction of [κ5-N,N,N,N,C-

(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (2) with one equivalent of primary or secondary aryl or alkyl silane failed to produce a silyl complex of the type

(N3N)ZrSiRnH3−n. Extended reactions resulted in the apparent decomposition of 2. It was suspected that any silyl derivative may be unstable with respect to cyclometalation and the restoration of 2. It is already known that (N3N)ZrX derivatives can rapidly exchange the pseudoaxial X ligand, and that if X is of sufficient steric bulk, 2 is favored.2, 107 To 34

test this hypothesis, 2 was treated with one equivalent of phenylsilane-d3, which rapidly

108 afforded a mixture of PhSiHD2, PhSiH2D, and PhSiH3 as well as 2-dn, illustrating equilibrium formation of a disfavored silyl derivative (Scheme 1.6).

Scheme 2.6: Pathway for the incorporation of deuterium on 2 from phenylsilane via a presumed

silyl intermediate, (N3N)ZrSiD2Ph.

With evidence for a transient silyl compound, more aggressive efforts to solicit reactivity were explored. Heating mixtures of 2 with excess (2–10 equiv.) phenylsilane resulted in the ultimate decomposition of 2 with trace quantities of hydrogen, 1,2- diphenyldisilane, and oligosilanes in conversions that were low and unreliable between experiments. Redistribution products were not observed.

Silanes featuring π-basic substituents gave different reactivities. A former

Waterman group member, Dr. Karla Erickson performed the reaction of 2 with

97, Ph(X)SiH2 (X = NMe2 or Cl), which yielded known (N3N)ZrX (X = NMe2, 1; Cl)

98 derivatives as the solely identifiable metal-containing product by 1H NMR spectroscopy (eqn 4). Significant decomposition of 2 was also observed. Phenylsilane was observed as a byproduct for both substrates.

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The mass balance suggests that the byproduct from the reactions of 2 with

Ph(X)SiH2 (X = NMe2 or Cl) is phenylsilylene (PhHSi:). The observation of phenylsilane, even in deuterated solvents, may be related to the low conversion and substantial degradation of zirconium compounds in the reaction. Nevertheless, such a transformation has precedent. Andersen and coworkers have shown that cerocene hydrides react with haloalkanes or ethers to give Ce–X (X = F, Cl, OR) bonds and the corresponding alkane.109, 110 These reactions appear to proceed via a two-step process involving C–H bond activation and H2 formation followed by methylidene elimination and trapping by hydrogen—a stoichiometric α-carbene elimination reaction.109, 110 Based on the same arguments that Andersen has made, it is presumed that the formation of a strong Zr–N bond is a driving force for silylene elimination. Prior studies suggest that

Zr–N bonds are among the strongest for the (N3N)ZrX family of compounds based on structural evidence and computational models.107

Although the relative stability of a Zr–N bond might appear to be detrimental to catalysis, 1 is already known to undergo stoichiometric Si–N bond formation,97 which suggested that catalytic α-silylene elimination may be possible. A testable hypothesis

emerged: does catalytic Si–N bond formation enable α-silylene elimination?

Dehydrocoupling of silanes with 1, a reaction that is unsuccessful with 2 (vide supra), would test this hypothesis.

Treatment of phenylsilane in benzene-d6 at 80 °C with as little as 5 mol % of 1 gave a mixture of silicon-containing products, including predominately 1,2- diphenyldisilane (PhSiH2)2 (2%), Ph(NMe2)SiH2 (3%) and low molecular weight linear and cyclic oligosilanes (5%) as characterized by 1H and 29Si NMR spectroscopy (eqn

5).111 These reactions require heating for several days at 80 °C to achieve limited conversion of phenylsilane. With an increased catalyst loading, typically to 20 mol %,

Ph(NMe2)2SiH, a non-productive byproduct, is generated. Diphenylsilane was far less reactive towards the catalysis. After five days of heating at 80 °C (20 mol % catalyst loading), 10% conversion to Ph2(NMe2)SiH was measured. No reaction was observed between either PhMeSiH2 or Ph3SiH and the catalyst over five days at 80 °C.

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These observations and literature precedents lead to the proposed catalytic cycle

(Scheme 1.7). The first step in the catalysis appears to be a Si–N heterodehydrocoupling reaction between 1 and phenylsilane resulting in Ph(NMe2)SiH2. This transformation necessarily produces a zirconium hydride that is known to rapidly dehydrogenate to the

108 metallacycle 2. The stoichiometric reactions then suggest that Ph(NMe2)SiH2 ring 37

opens 2, resulting in a silyl intermediate (N3N)ZrSiHPhNMe2 that is poised for α-silylene elimination and restoration of 1. The phenylsilylene fragment could then insert into the

Si–H bond of another equivalent of phenylsilane to yield the disilane product, which is consistent with the literature precedents.43, 105, 112, 113

Scheme 2.7: Proposed catalytic cycle for the dehydrocoupling of phenylsilane using

(N3N)ZrNMe2 (1).

In the catalysis, the formation of Ph(NMe2)2SiH presumably results from the reaction of 1with Ph(NMe2)SiH2 generated in situ. This process would be catalyst deactivating, consuming multiple equivalents of NMe2 and leaving 2, which is unstable in the presence of excess phenylsilane (vide supra). Decomposition of 1 limits the consumption of starting phenylsilane. There is insufficient formation of Ph(NMe2)SiH2 to suggest that the decomposition occurs solely from 2, but the complex mixtures of

decomposition products are similar in both the stoichiometric and catalytic reactions. The working hypothesis is that competitive Si–N bond formation occurs between phenylsilane and the amido arms of the triamidoamine ligand to strip the ligand off zirconium.

Although this is consistent with the formation of Si–NMe2 derivatives, there is no direct evidence to support this pathway given the complex mixtures of decomposition products.

The competitive degradation process and the apparent instability of (N3N)Zr-silyl derivatives preclude a detailed mechanistic analysis of the system key to prior instances of α-elimination.45 There is, however, a precedent for silylene elimination. Pannell and co-workers have reported photo-driven iron-catalyzed silylene elimination.63, 69

Trapping was pursued as potential verification of silylene formation.

Phenylsilane was treated with 10 equivalents of a trap in the presence of 10 mol % of 1 at

80 °C in benzene-d6. For traps such as 2-butyne, terminal alkynes, dienes and disulfides, reactions failed to afford organosilane products, and decomposition of 1 resulted. We attribute the lack of trapping to the competitive reactions of 2 with these reagents, many of which have already been described.107 In the case of diphenylacetylene as a trap, some formation of cis-stilbene (∼24%) was observed (eqn 6). It is known that photochemically generated phenylsilylene is trapped with diphenylacetylene to give 1,2,3,4,5- pentaphenylsilole.105 In a limited set of reactions of phenylsilane and diphenylacetylene with catalytic 1, 1,2,3,4,5-pentaphenylsilole was observed by 1H NMR spectroscopy with confirmation by comparison with an authentic sample.105 However, conditions that reliably favor this product could not be identified. Control reactions of H2 and

diphenylacetylene in the presence of 1 give much lower conversions to cis-stilbene than reactions with phenylsilane, which is unsurprising given that 1 and related derivatives are

2 poor hydrogenation catalysts with H2. The reduction of an alkyne by a silane is an unusual transformation, but a report by Marciniec indicates one potential route.114

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The limitations of 1 invited consideration of other catalysts, and Corey's report of phenylsilane dehydropolymerization using Cp2Zr(NMe2)2 (3) was intriguing given their

115 observations of high activity and formation of Ph(NMe2)SiH2 as a byproduct. Because this catalysis is established, only trapping reactions were pursued. Reaction of phenylsilane with three equivalents of the organic trapping reagents diphenylacetylene or

2,3-dimethyl-1,3-butadiene in the presence of 10 mol % of 3 yielded no evidence of a silole and small quantities of PhH2Si–SiH2Ph and Ph(NMe2)SiH2. Competitive silane polymerization was observed in the case of 2,3-dimethyl-1,3-butadiene, whereas diphenylacetylene stunted catalysis entirely. Replicating that reaction with

116 diethyldisulfide as the trap afforded Ph(EtS)2SiH in 21% conversion as identified by

NMR (1H, 1H/29Si HSQC) spectroscopy and GC/MS (eqn 7, figure 1.2). Such a trapping reaction represents the microscopic reverse of the known generation of silylene from dithiolsilanes.63

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Figure 2.2: MS of product of trapping experiment with diethyldisulfide

A competition experiment was performed between phenylsilane and phenylsilane- d3 under the same conditions using 1. It was suspected that the generation of PhHDSi–

SiD2Ph or PhH2Si–SiHDPh must be the result of phenylsilylene or phenylsilylene-

1 2 d1 insertion into a Si–D or Si–H bond, respectively. In the H and H NMR spectra,

respectively, broad multiplets during the chemical shift of 1,2-diphenyldisilane were identified, representing about 6% of the Si–H intensity by 1H NMR integration. Attempts to accurately assign products using simulated spectra were hampered by poor simulation quality. A highly complex resonance was observed in the 29Si NMR spectrum at approximately δ 61.2, indicative of multiple products and non-first order spin systems, but again, accurate simulations could not be made (figure 1.3). Although the observation of apparent crossover products is encouraging, these products may also arise from H/D exchange reactions, such as reaction of 1,2-diphenyldisilane and D2 catalyzed by 2.

Therefore, this experiment represents, at best, supporting evidence of this reactivity rather than direct proof.

Figure 2.3: Complex 29Si{1H} splitting pattern of product from competition experiment

In summary, 1 is a poor catalyst for the heterodehydrocoupling of amines and silanes as well as homodehydrocoupling of silanes. A series of stoichiometric and trapping reactions of 1 and zirconocene 3 (Table 2.1) support the idea that these zirconium compounds engage in α-silylene elimination. This reactivity is unique compared to the prior reports of silane dehydrocoupling by group 4 metals, but the notion that Si–N coupling reaction prompts the α-silylene elimination provides new possibilities for promoting the generation of low-valent fragments.

Table 2.1: Summary of observations and mechanistic implications.

# Observation Implication (s)

1 PhSiH3 + Me2NH + 1(cat) → Ph(NMe2)SiH2 Insufficient data for mechanistic suggestion, though prior work would suggest σ-bond metathesis.

i 2 PhSiH3 + PrNH2 + 1(cat) → Ph(NMe2)SiH2+ Ph2SiH2 Absence of SiH4 is inconsistent with the redistribution, and Ph transfer from a Zr– Ph intermediate (á la Tilley) is possible.

3 2 + PhSiD3 → 2-d1 Deuterium incorporation on the trimethylsilyl substituent must arise from Si–D activation at 2, which demonstrates that a Zr-silyl is unstable with respect to cyclometalation.

4 PhSiH3 + 2(cat) → decomposition This system is not competent for silane dehydrocoupling, despite transient Zr-silyl (#3).

5 2 + Ph(NMe2)SiH2→ 1 + PhSiH3 Reactivity consistent with Andersen's methylidene elimination with cerocenes 2 + PhClSiH2→ (N3N)ZrCl + PhSiH3 suggests a silylene intermediate.

6 PhSiH3 + 1(cat) → (PhSiH2)2 This catalysis may proceed via α-silylene elimination based on observation 5. An alternative is the generation of a hydride that would rapidly form 2 and a σ-bond metathesis pathway, but 2 is not catalytically active (#4).

7 PhSiH3 + R2S2+ 3(cat) → Ph(SR)2SiH This reaction is consistent with silylene trapping by disulfide. An alternative may be disulfide splitting with adventitious hydrogen followed by two sequential Si–S heterodehydrocoupling events, but neither

RSH nor Ph(RS)SiH2 was observed by NMR.

Figure 2.4. Complexes 1, 2 and 3

2.3. Dehydrocoupling by group 1 and 2 metal compounds

Given the importance of silamine formation for promoting α-Elimination, other d0 metal amide complexes were explored.117 After Sadow’s seminal work using ToMMgMe for Si–N coupling, group 2 metal compounds have received more attention.74 Hill and coworkers demonstrated that M[N(SiMe3)2]2 (M = Mg, Ca, Sr) were active Si–N coupling catalysts.75 Continued advances have shown that group 2 metal amide compounds are efficient catalysts for this transformation.81, 86, 118 Based on these successes, it was hypothesized that Si–E dehydrocoupling with group 1 and 2 amide metal compounds could be achieved.

Being either commercially available or previously synthesized, group 1 and 2 metal bistrimethylsilylamides M[N(SiMe3)2]x where x = the group number of the corresponding metal, i.e. when using Li, Na or K, x = 1) were tested for silane homodehydrocoupling properties. An aryl silane (PhSiH3, Ph2SiH2 or Ph3SiH) and 10 mol % M[N(SiMe3)2]x (0.55M solution in benzene-d6) were mixed in a PTFE-valved

NMR tube in a N2 drybox. After removal from the drybox, a freeze-pump-thaw cycle was conducted followed by heating the reaction to 65 °C.

The reaction between 4a (4a = LiN(SiMe3)2 and 4b = NaN(SiMe3)2 and PhSiH3 yielded only one product, the silamine PhH2SiN(SiMe3)2 (SA1) in low yields (10-17% conversion) (Figure 2.4, Table 2.2). There was no visible effervescence or evidence of H2 evolution by 1H NMR. There were several other small resonances that could be other silicon-containing compounds, but due to the low concentration, product identification was inconclusive. In the reaction of KN(SiMe3)2) (4c) with phenylsilane, Ph2SiH2 (RD2), a redistribution product, was the main product (36% conversion) while SA1 was the next most abundant product (27% conversion). In addition, Ph3SiH (RD3), SiH4 (RD0) and

Ph2HSiN(SiMe3)2 (SA2) were observed in even lower concentrations. A dehydrocoupling product was formed, PhH2SiSiH2Ph (DH1) but in very low conversion (1%). There were also two resonances that could not be assigned. If redistribution was occurring, all silanes, from SiPh4 through SiH4, would theoretically have been present. Silane was observed, but due to low concentration the presence of SiPh4 could not be confirmed. The presence of the redistributed silamine Ph2HSiN(SiMe3)2 (SA2) may either be evidence that redistribution is faster than Si–N bond coupling or the steric bulk of the silane is not a large factor going from a primary silane to a secondary silane.

Figure 2.5: Some possible Si–N coupling, redistribution or dehydrocoupling products.

Table 2.2: Silicon containing product distribution for the reaction between 4a-c or 5a and

aryl silanes.

Catalyst Silane Product %

4a PhSiH3 SA1 10

4a Ph2SiH2 SA2 1

4a Ph3SiH - -

4b PhSiH3 SA1 17

4b Ph2SiH2 SA2 3

4b Ph3SiH - -

4c PhSiH3 SA1 27

SA2 2

RD0 1

RD2 36

RD3 2

DH1 5

4c Ph2SiH2 SA1 8

SA2 3

RD1 7

RD3 5

4c Ph3SiH SA2 3

5a PhSiH3 SA1 4

5a Ph2SiH2 - -

5a Ph3SiH - -

Conditions: catalyst loading = 10 mol %. t = 48 hours. T = 65 °C. Values in parentheses equal percent conversion of starting material into the specified product. (-) = no reaction.

When moving to a secondary silane, Ph2SiH2, similar patterns emerge: Si–N coupling is still favored for the lighter earth metal complexes 4a and 4b and redistribution is favored by potassium-based 4c. Reactivity decreased overall, likely due

to increased steric congestion at the active site. While 4a and 4b produced only SA2 in only trace quantities, 4c produced a variety of products, consuming up to 25% diphenylsilane. Redistribution products made up 12%, while Si–N heterodehydrocoupling made up 11% of the products. In addition, effervescence was

1 observed and H2 was observed by H NMR spectroscopy.

Treatment of 4a-c with tertiary silanes gave little conversion. In the case of 4a and triphenylsilane, almost no reactivity was observed over seven days. Besides trace

SA2, no reaction was observed. Similar reactivity was seen with 4b and 4c except SA2 was confirmed as a product by 1H/29Si HSQC spectroscopy for 4c.

The group 2 analogues, M[N(SiMe3)2]2 (M = Mg, Ca), were also investigated. The group 2 analogues are not commercially available but are known literature compounds.119, 120 The magnesium analogue, 5a, has been prepared and preliminary studies with phenylsilane have been conducted. The literature preparation has been modified slightly to aid in collection of pure product.119 Instead of stirring overnight at room temperature, refluxing for four hours significantly increased yield (eqn 8). Reacting

1 phenylsilane with 5a only produced SA1 in 4% conversion by H NMR spectroscopy.

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Synthesis of Ca(N(SiMe3)2)2, 5b, was significantly more challenging. Attempted transmetallation of CaI2 by 4c does not produce 5b. This route is not successful because a calciate species is formed instead, as described by Hanusa (eqn 9).120 49

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Hanusa developed a different synthetic approach to circumvent this issue. CaI2 and benzyl potassium are mixed in THF to form a dibenzylcalcium species. This species can then react with the free amine HN(SiMe3)2 to form 5b without the formation of a calciate. This reaction has proven challenging for many reasons. The product has been synthesized via this route, but impure and only in low yields. Due to the difficulty in synthesizing 5b and low reactivity of 5a, the group 2 analogues were abandoned and efforts to improve the group 1 metal amides, 4a-c, were undertaken instead.

2.4. Enhancing reactivity using N-heterocyclic carbenes

Catalysis using base metal compounds is an enticing approach to reduce dependence on precious metals.121 The decreased reactivity of base metals compared to their precious metal analogues has made this approach challenging. Making base metal compounds more reactive has been achieved in several ways. One such method is adding

N-heterocyclic carbenes. These ligands are good σ-donors and poor π-acceptors. They are

neutral ligands that donate two electrons to the metal center. They are relatively easily synthesized and can be modified rather easily to alter the steric bulk around the reactive site or change the electronics of the complex.

The reactivity of 4a-c in terms of silane dehydrocoupling/redistribution was quite low. In an effort to increase the reactivity a NHC was added. The NHC that was chosen as a starting point was 1,3-bis(2,4,6-trimethylphenyl)imidizol-2-ylidene (IMes), because it was relatively easy to synthesize.122 4c was the most reactive of the group 1 species so it was probed first. In situ generation of complex 4c-IMes was attempted to observe the influence of the NHC. In a PTFE-valved NMR tube, 10 mol % of 4c and 10 mol % of

IMes were added to phenylsilane in benzene-d6. The reaction was freeze-pump-thawed, heated to 65 °C and monitored via 1H and 1H/29Si HSQC NMR for 48 hours. Like the reaction without the NHC, the major products were SA1 (35% conversion) and RD2 (15% conversion). Other products that were observed via HSQC included RD3, SA2 and one other resonance that appears to be from an IMes + PhSiH3 side reaction. Because there was less than 50% conversion of starting material over 48 hours, group 1 NHC complexes were abandoned and lanthanide-based Si–N heterodehydrocoupling catalysts were investigated.

Group 1 and 2 bis(trimethylsilyl)amide complexes were not suitable catalysts for

α-elimination. Commercially available group 1 complexes 4a-c displayed the most reactivity, with 4c catalyzing both dehydrocoupling and redistribution. The introduction of IMes to 4c increased reactivity, but not to a level that merits further study.

2.5. Future direction

The potential utility of catalytically generated silylene fragments will continue to fuel research into α-silylene elimination. At this point however, reliance on silamine formation is a key feature in the generation of free silylene. If this continues to be the only way to promote α-silylene elimination, several limitations persist.

First, the catalyst must also be a Si–N heterodehydrocoupling catalyst to facilitate in situ generation of the silamine. Though there are several catalysts capable of coupling silanes with amines, Si–N heterodehydrocoupling as a field is still developing. Perhaps the simplest direction would be to explore different Si–N heterodehydrocoupling catalysts as potential α-silylene elimination catalysts.

The alternative of requiring a Si–N heterodehydrocoupling catalyst is to eliminate the need for in situ Si–N coupling. Using a silamine as the substrate instead commercially available silanes and amines is the clear choice. The advantages of using commercially available substrates is obvious, but if the catalyst cannot generate a silamine in situ, premade silamines may be the only option. This would have some disadvantages, including increased cost and the need to prepare different silamines. Silamines are generally not that hard to make however and exposing known α-elimination catalyst to silamines may be a promising area to explore.

In addition, trapping continues to be a challenge. Identifying systems that are more amenable to trapping reactions is not trivial. One potential avenue for improved

trapping is to increase the steric bulk around the silylene. This would shield the silylene and likely make it more stable. Part of the issue with trapping is that an extremely reactive fragment is going to react with whatever it finds first. Increasing the lifetime of the reactive fragment may facilitate more well-behaved trapping. The caveat of course, is by increasing the steric bulk at silicon, reactivity would likely slow or even stop. Making slightly more sterically bulky silanes, such as substituted aryl primary silanes like o/m/p- methylstyrene or less bulky secondary silanes, such as methylphenylsilane, could provide longer lived silylene fragments and thus more consistent trapping.

2.6. Conclusion

In summary, 1 is a catalyst for the heterodehydrocoupling of amines and silanes with limited efficacy. It is also a poorly effective silane dehydrocoupling catalyst, but a series of observations from stoichiometric and trapping reactions of 1 and zirconocene 3 (Table 2.1) support the idea that these zirconium compounds engage in α- silylene elimination. This reactivity is unique compared to the prior reports of silane dehydrocoupling by group 4 metals, but the notion that Si–N coupling reaction prompts the α-silylene elimination provides new possibilities for promoting the generation of low- valent fragments.

Group 1 metal complexes 4a-c are inefficient heterodehydrocoupling catalysts. 4c was the most reactive species of those tested, but selectivity and conversion were still low. Steric bulk on silicon played an import role, as decreased reactivity was observed for

secondary silanes and reactivity ceased in most cases with tertiary silanes. The addition of IMes to the reaction had negligible effect on reactivity.

2.7. Experimental

2.7.1. General Considerations

All manipulations were performed under a nitrogen atmosphere with dry, oxygen-free solvents using an M. Braun glovebox or standard Schlenk techniques. Benzene-d6was purchased from Cambridge Isotope Laboratory and then degassed and dried over NaK alloy. Fluorosil was heated under vacuum to temperatures over 180 °C for 12 hours.

NMR spectra were recorded with a Bruker AXR 500 MHz or Varian 500 MHz spectrometer in benzene-d6; reported resonances are with reference to the residual solvent resonance (δ 7.16). GCMS traces were recorded using a Varian 3900 GC with a Varian

Saturn 2100T GCMS.

2.7.2.1. Si–N heterodehydrocoupling and α-silylene elimination

considerations

3- 98 The starting materials (N3N)ZrNMe2 (N3N = N(CH2CH2NSiMe3)3 ) (1),

97 97 [(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (2), (N3N)ZrCl, N,N-dimethyl-1- phenylsilanamine, N,N,N’,N’-tetramethyl-1-phenylsilanediamine123 and 1,3- Bis(2,4,6- trimethylphenyl)imidazolinium chloride (IMes HCl)122 were prepared according to the

i i literature procedure. (N3N)ZrNH Pr was synthesized by the addition of 1 eq of PrNH2 to

2 in benzene, followed by removal of solvent. The dehydrohalogenation of IMes HCl to

yield 1,3- bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) was done by using 3 eq of sodium hydride and catalytic amounts of potassium tert-butoxide in dry THF.

Phenylsilane-d3 was prepared through reduction of trichlorophenylsilane with lithium aluminum deuteride and distillation from diglyme. All other chemicals were purchased from commercial suppliers and dried by conventional means.

2.7.2.2. Typical procedure for dehydrocoupling experiments

All reactions were conducted using a J-Young type polytetrafluoroethylene (PTFE)-

1 valved NMR tube in benzene-d6. Upon addition and obtaining an initial H NMR spectrum, the solution was frozen and the headspace was evacuated. This was repeated at regular intervals during the course of the reaction. After thawing, the NMR tube was heated in an oil bath at 80 °C unless otherwise noted. 1H NMR spectra were collected at

25 °C.

2.7.2.3. Reaction of 3 with phenylsilane and diethyl disulfide:

A mixture of Cp2Zr(NMe2)2 (10 mg, 3.23 x 10-2 mmol), phenylsilane (35 mg, 0.32 mmol), and diethyl disulfide (79 mg, 0.64 mmol) were added in an NMR tube in ca. 0.50 mL benzene-d6. The headspace was removed; after ca 10 minutes, a new Si–H resonance was observed. Reactivity ceased after 5 days, resulting in 15% conversion into one new

Si–H containing species. By 1H and 1H-29Si HSQC and GCMS the product was

1 determined to be the trapped silylene, Ph(EtS)2SiH. H NMR (benzene-d6): δ 5.04 (s, 1H,

29 Si–H), 2.99 (q, 4H, Et), 1.34 (t, 6H, Et). Si NMR (benzene-d6): δ -20.90. GC/MS

(Retention time = 7.56 minutes) is of reaction mixture.

PhSiH3

Figure 2.6: 1H-29Si HSQC of the reaction of 3 with phenylsilane and diethyl disulfide

2.7.2.4. Reaction of 1 with PhSiH3 and PhSiD3:

-2 -1 A mixture of 1 (10 mg, 2 x 10 mmol), PhSiH3 (21.9 mg, 2 x 10 mmol) and PhSiD3

(24.7 mg, 2 x 10-1 mmol, 91% solution in diglyme) were added in an NMR tube in ca.

0.50 mL benzene-d6. The headspace was removed. Over 24 hours, the Si–H resonance at

29 indicating the presence of the dehydrocoupled product, PhH2Si-SiH2Ph, appeared. Si

NMR was run over 15 hours to try to see whether PhH2Si-SiHDPH or PhD2Si-SiHDPh was present. The spectrum was quite complex and elucidating any of these products was not possible. Simulating these compounds was attempted but inconclusive given that the

simulated spectra of simpler compounds were inconsistent with experimental spectra (as seen with PhSiH3).

1 Figure 2.7: Final H NMR of reaction of 1 with PhSiH3 and PhSiD3

1 29 Figure 2.8: Final H- Si HSQC NMR of reaction of 1 with PhSiH3 and PhSiD3

Figure 2.9: Experimental data and simulation of aryl region of the reaction of 1 with

PhSiH3 and PhSiD3

2.7.3.1. Dehydrocoupling by group 1 and 2 metal compounds

3a-c were purchased from Sigma Aldrich. 4a-b were prepared according to literature sources, with deviations discussed above.119, 120

2.7.3.2. Typical procedure for dehydrocoupling experiments

All reactions were conducted using a J-Young type polytetrafluoroethylene (PTFE)- valved NMR tube in benzene-d6. All reactions follow this general procedure: treat an aryl silane (PhSiH3, Ph2SiH2 or Ph3SiH) with 10 mol % 3a-c or 4a (0.55M solution in

1 benzene-d6) in a N2 drybox. After the sample is made, an initial H NMR spectrum is 59

obtained followed by a freeze-pump-thaw cycle. This was repeated at regular intervals during the course of the reaction. After thawing, the NMR tube was heated in an oil bath at 65 °C unless otherwise noted. 1H NMR spectra were collected at 25 °C.

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CHAPTER 3: Si–N HETERODEHYDROCOUPLING WITH A LANTHANIDE

COMPOUND

3.1. Context

The breadth of catalytic reactivity exhibited by [κ5-

N,N,N,N,C(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr, including Si–N heterodehydrocoupling as an avenue for α-silylene elimination, inspires interest in different analogues of this compound.1-5 Beyond modification of the organic backbone, altering the metal can have a dramatic effect on reactivity. Because the non- cyclometallated cerium analogue has been reported but its chemistry remains understudied, it was selected for initial exploration.6

3- t Treatment of Li3(THF)3(NN’3) (NN’3 = [N(CH2CH2NR)3] where R = SiMe2 Bu)

6 with CeCl3(THF)4 in dry THF yields Ce(NN’3) as a yellow solid. This procedure was never performed due to the inability to acquire CeCl3(THF)4 as there is no reported preparation for this complex and all attempts to synthesize it were unsuccessful. Another

7 starting material, Ce[N(SiMe3)2]3, was synthesized according to literature methods. As a common starting place for other cerium complexes, it was thought that through amide exchange, mixing Ce[N(SiMe3)2]3 with H3NN’3 could yield the desired product (eqn 1).

Unfortunately, the only reactivity observed was decomposition of the cerium complex.

Though several other methods were attempted, Ce(NN’3) remained elusive. Given that

the goal was to explore reactivity with cerium complexes, the easily synthesized

Ce[N(SiMe3)2]3 was investigated instead.

(1)

Interest in exploring α-silylene elimination influenced initial reactivity testing.

Reaction of phenylsilane with 5 mol % of Ce[N(SiMe3)2]3 in benzene-d6 yielded

1 29 Ph(H)2SiN(SiMe3)2 in 5% yield over one day, as identified by H and Si NMR (mpc-

03-033). In a related experiment, phenylsilane and 1 equiv of bis(trimethylsilyl)amine with 10 mol % Ce[N(SiMe3)2]3 reacted at 110 °C. After 3 days, nearly complete consumption of phenylsilane was observed, with the Si-N coupled product,

Ph(H)2SiN(SiMe3)2, being the only product (mpc-03-045). The related lanthanum

7 complex, La[N(SiMe3)2]3 was also synthesized and tested. In an identical reaction, complete consumption of phenylsilane was observed in less than 24 hours, with

Ph(H)2SiN(SiMe3)2 being the only product (mpc-03-043). Given the catalytic activity of the two complexes, the lanthanum analogue was selected for further study.

3.2. Introduction

Catalysis is a critical step in sustainable processes, but the relative abundance of the catalyst is a key factor in its overall utility.8 A common approach for reducing the demand for precious metals in catalysis is to replace them with base metals. This methodology has been met with some success, but significant challenges, including achieving comparable turnover metrics with similar catalyst loadings, remain. In fewer cases, rare earth metals have been used instead of base metals. Despite being labelled as rare earth elements, some lanthanides, lanthanum, cerium, and neodymium, for example, are more abundant in the earth’s crust than some base metals such as cobalt.9, 10

Unsurprisingly, exploration of lanthanide-based catalysis has been an exciting area of growth.11

Organolanthanides have been shown to be viable catalysts for a variety of transformation with some critical advances in alkene polymerization,12-17 but main group relevant transformation including dehydrocoupling18 and heterofunctionalization19-23 have been reported, as well. In 1999, Takaki and co-workers first demonstrated

2 heterodehydrocoupling of silanes with amines using Yb(η -Ph2CNAr)(hmpa)n (Ar = 2,6- diisopropylphenyl, hmpa = hexamethylphosphoramide), focusing on quaternary silamines from tertiary silanes.24 In 2012, Cui and coworkers demonstrated that a different ytterbium catalyst, IMesYb[N(SiMe3)2]2 (IMes = 1,3-dimesitylimidazol-2-ylidene), could successfully heterodehydrocouple primary and secondary silanes with amines.25 Sadow has expanded upon Si–N coupling with lanthanides, reporting that

Ln[C(SiHMe2)3]2ImtBu (Ln = Yb, Sm; ImtBu = 1,3-di-tert-butylimidazol-2-ylidene) are also active catalysts for this transformation.26 To date, however, there are no reports of more abundant lanthanides catalyzing the heterodehydrocoupling of silanes and amines.

Silamines are utilized as mild, sterically hindered bases,27 protecting groups for silanes28-30 and amines,31, 32 and silylating reagents.33-36 Polysilazanes are precursors for silicon-nitride-based ceramics.37, 38 Silamines are also important ligands in coordination chemistry.39, 40 Heterodehydrocoupling is an attractive avenue for silamine preparation versus salt metathesis because the H2 byproduct is easily separated and simplifies workup. However, selectivity in dehydrocoupling is also challenging as an excess of amine is required. Recently, improved selectivity has been achieved with dehydrocoupling catalysts, though it remains challenging without designer ligands.41,

42 Overall, dehydrocoupling is a potentially more atom economic process for the generation of silamines, which argues for its continued study.

The primary aim of this work was the investigation of a more earth-abundant, simple lanthanum compound for this transformation. Due in part to the commercial availability, the lanthanide (III) derivatives, Ln[N(SiMe3)2]3, became the focus of this study, some of which have shown excellent activity in different main group bond- forming transformations.21, 43

3.3. Coupling of silane and amines by La[N(SiMe3)2]3THF2

Treatment of lanthanum trichloride with 3 eq of lithium bis(trimethylsilyl)amide in THF for 6 h at ambient temperature affords the five-coordinate compound,

La[N(SiMe3)2]3(THF)2 (1, eqn 1) as a colorless powder upon sublimation. Alternatively,

1 can be prepared by dissolving commercially available La[N(SiMe3)2]3 in THF followed by removal of solvent under reduced pressure. The THF-solvated species has increased solubility in a variety of organic solvents, including benzene-d6 and thus was the preferred complex for this study.

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Compound 1 was tested as a Si−N heterodehydrocoupling catalyst. Treatment of a 3:1 mixture of n-propylamine and phenylsilane with 5 mol % of 1 at 25 °C resulted in

n effervescence of H2. In fewer than 10 min, PhSi( PrNH)3 was produced, but additional ill-defined byproducts were also present. Reducing the catalyst loading to 0.8 mol % and increasing the amine/silane ratio to 4:1 increased the conversion to the desired product to

85% with only a slight increase in reaction time (Table 3.1). Further increasing the amine/silane ratio had no significant impact on yield.

Table 3.1. Coupling of silanes with n-propylamine catalyzed by 1.

Entry Silane Eq.a Product Conversion b n 1 PhSiH3 3 PhSi( PrNH)3 40-50 n 2 PhSiH3 4 PhSi( PrNH)3 85 n 3 MePhSiH2 4 MePhSi( PrNH)2 96 n 4 Ph2SiH2 4 Ph2Si( PrNH)2 100 c n 5 Ph3SiH 2 Ph3Si( PrNH) 100

Conditions: 0.8 mol % of 1 at 25 °C in benzene-d6 solution. Products were characterized by 1H, 29Si and 1H-29Si HSQC NMR spectroscopy. Conversion determined by integration of 1H NMR spectra. aEquivalents of amine per equivalent of silane. b5 mol % of 1. ct = 60

°C.

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Secondary silanes were also effective substrates. Reaction of a 4:1 mixture of n- propylamine and methylphenylsilane with 0.8 mol % of 1 provided the quaternary silane,

n 1 MePhSi( PrNH)2, in 96% conversion as measured by H NMR spectroscopy (eqn 3).

Trace amounts of a byproduct were observed in the 1H and 1H-29Si HSQC NMR spectra at δ 5.19 and δ -12.6, respectively. While we cannot provide definitive data, this product

n is most consistent with PrN(MePhSiH)2 based on Sadow’s reported preparation of i PrN(MePhSiH)2 by magnesium-catalyzed reaction of two equivalents of

methylphenylsilane with isopropylamine.41 Increasing the amine/silane ratio continued to provide the quaternary silane with the same byproduct. However, treatment of a 4:1 solution of n-propylamine and diphenylsilane with 0.8 mol % of 1 produced the expected

n product, Ph2Si( PrNH)2, quantitatively, according to NMR spectroscopy.

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Tertiary silanes were the most challenging substrates for this system. Reaction of 2:1 mixture of n-propylamine and triphenylsilane with 0.8 mol % of 1 at ambient

n temperature affords Ph3Si( PrNH) over a period of 24 hours. Conducting this reaction at

60 °C, however, gave complete conversion in less than three hours. These initial reactions prompted a broader investigation, and a family of silanes and amine substrates were screened based on their appearance in previous studies.25, 26, 41, 44-47

More sterically encumbering amine substrates, isopropylamine, aniline, and tert- butylamine, were tested (Table 3.2). Similar to the n-propylamine reactions, optimal conversions with isopropylamine required an increased amine/silane ratio. Reactions with aniline and tert-butylamine with phenylsilane required a slight excess of amine to achieve quantitative conversions. For secondary silanes, reaction at 60 °C provided optimal product formation in less than 2 h. In reactions with aniline, clean formation of the

tertiary and quaternary silane products was possible from secondary silane substrates, depending only on reaction times.

Due to the steric pressure around silicon, quaternary silamines have been challenging to make by dehydrocoupling. A 2:1 mixture of isopropylamine with

i triphenylsilane in the presence of 0.8 mol % of 1 at 60 °C gives Ph3Si( PrNH) quantitatively in 1.5 h (Table 3.2, entry 4). Under the same conditions, the analogous

n reaction with n-propylamine and triphenylsilane gave Ph3Si( PrNH) quantitatively in 2.75 h (Table 3.1, entry 4; eqn 3). To the best of our knowledge, this is the first time

n i Ph3Si( PrNH) and Ph3Si( PrNH) have been synthesized by any method. Both aniline and tert-butylamine reacted with triphenylsilane but required forcing conditions. Increased catalyst loading (4 mol % of 1), temperature (90 °C), and reaction times (36−48 h) were needed for even minimal conversions. Although NMR spectra demonstrated consumption of silane in all cases, low conversions thwarted product identification. Ultimately, coupling with triphenylsilane was largely dependent on the identity of the amine.

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Table 3.2. Coupling of silanes with more sterically encumbered amines by 1

a b Entry Silane Amine Eq. Temp (°C) Product Conversion Time (hrs) i i c 1 PhSiH3 PrNH2 6 25 PhSi( PrNH)3 90 0.5 i i 2 MePhSiH2 PrNH2 4 25 MePhSi( PrNH)2 100 0.5 i i 3 Ph2SiH2 PrNH2 4 25 Ph2Si( PrNH)2 100 0.5 i i 4 Ph3SiH PrNH2 2 60 Ph3Si( PrNH) 100 1.5

5 PhSiH3 PhNH2 3.1 25 Ph(PhNH)2SiH 100 0.5

6 PhSiH3 PhNH2 4 90 PhSi(PhNH)3 100 24

7 MePhSiH2 PhNH2 2.1 60 MePh(PhNH)SiH 100 3

8 MePhSiH2 PhNH2 2.1 60 MePhSi(PhNH)2 76 36

9 Ph2SiH2 PhNH2 2.1 60 Ph2(PhNH)SiH 100 1.5

10 Ph2SiH2 PhNH2 2.1 60 Ph2Si(PhNH)2 100 72 t t 11 PhSiH3 BuNH2 3.1 25 Ph( BuNH)2SiH 100 1.5 t t 12 MePhSiH2 BuNH2 2.1 60 MePh( BuNH)SiH 100 3 t t 13 Ph2SiH2 BuNH2 2.1 60 Ph2( BuNH)SiH 100 1.5

1 Conditions: 0.8 mol % of 1 in benzene-d6 solution. Products were characterized by H,

29Si and 1H-29Si HSQC NMR spectroscopy. aEquivalents of amine per equivalent of silane. bFor conversions less than 100%, reactions were not complete (i.e., no byproducts observed). cMixture of several silamines.

Generation of a quaternary silamine from a primary or secondary silane is also possible. By slightly increasing the amine:silane ratio to 4:1 and heating the reaction to

90 °C, reaction of aniline and phenylsilane in the presence of 0.8 mol % of 1 provides quantitative conversion to PhSi(PhNH)3 (Table 3.2, entry 6). Due to the increased steric pressure of aniline over isopropylamine, the reactions with secondary silanes take considerably longer. MePhSi(PhNH)2 and PhSi(PhNH)3 have been synthesized before but not by heterodehydrocoupling.48, 49 79

Reactivity with secondary amines was also largely dependent on the identity of the amine. Diethylamine was readily coupled with primary and secondary silanes (Table 3.3).

Use of a bulkier amine, bis(trimethylsilyl)amine, required up to 24 h reaction time for complete conversion under the conditions tested (Table 3.3). Coupling of tertiary silanes with secondary amines was not achieved under conditions screened. In reactions between secondary silanes and bis(trimethylsilyl)amine, even after extended reaction times (24 h or longer), increased catalyst loading (4%), and higher temperatures (90 °C), no consumption of starting materials was observed. Based on these trends, the decrease of reactivity with bis(trimethylsilyl)- amine is likely a result of increased steric crowding at nitrogen or decreased basicity/nucleophilicity compared to diethylamine.

One major limitation of 1 is the inability to successfully heterodehydrocouple silanes with ammonia. Ammonia is a challenging substrate for several reasons, including strong N–H bonds and increased acidity upon silylation.50 In an attempt to prevent the silane with quickly reacting with 1, 1 was added to the NMR tube in benzene-d6 and frozen with liquid nitrogen. While keeping the previous layer frozen a layer of benzene- d6 was added and frozen, creating a barrier to the solution of 1. Finally, a mixture of phenylsilane in benzene-d6 was added and frozen. While keeping the sample frozen solid, the tube was removed from the glovebox and placed on the Schlenk line. The atmosphere in the tube was evacuated and filled with ammonia. Upon melting of the frozen solution of phenylsilane and ammonia should be introduced to 1 at nearly the same time.

Unfortunately, catalyst decomposition occurred and dehydrocoupling was not observed.

Table 3.3. Coupling of silanes and secondary amines by 1

Entry Silane Amine Eq.a Temp (°C) Product Conversion Time (hrs)

1 PhSiH3 Et2NH 3.1 60 Ph(Et2N)SiH2 97 0.5

2 PhSiH3 Et2NH 3.1 60 Ph(Et2N)2SiH 97 24

3 MePhSiH2 Et2NH 2.1 90 MePh(Et2N)SiH 100 1.5

4 Ph2SiH2 Et2NH 2.1 90 Ph2(Et2N)SiH 100 1.5 b 5 Ph3SiH Et2NH 1.1 90 None 0 72

6 PhSiH3 HN(SiMe3)2 3.1 90 PhN(SiMe3)2SiH2 100 24 b 7 MePhSiH2 HN(SiMe3)2 2.1 90 None 0 24 b 8 Ph2SiH2 HN(SiMe3)2 2.1 90 None 0 24 b 9 Ph3SiH HN(SiMe3)2 1.1 90 None 0 24

1 29 Conditions: 0.8 mol % in benzene-d6 solution. Products were characterized by H, Si, and 1H-29Si HSQC NMR spectroscopy. Conversion was determined by integration of 1H

NMR spectra. aEquivalents of amine per equivalent of silane. b4 mol % of 1.

Competitive dehydrocoupling of the bis(trimethylsilyl)- amide ligands on 1 and less sterically hindered amines was not observed except for the reaction of diethylamine and phenylsilane. In this reaction, approximately 2% conversion to PhN(SiMe3)2SiH2 was

25 observed. Neither MePhN(SiMe3)2SiH nor Ph2N(SiMe3)2SiH were observed in the reaction between methylphenylsilane or diphenylsilane with diethylamine, respectively.51

In addition, there are no cases in which bis(trimethylsilyl)amine was coupled with silamine products.

It is likely that the mechanism of dehydrocoupling with 1 is similar to that which

Sadow and co-workers proposed for [ToMMgMe] (ToM = tris(4,4-dimethyl-2-oxazolinyl)- phenylborate).41 The observation of bis(trimethylsilyl)amine in all reactions by NMR spectroscopy suggests in situ amide exchange at 1 that affords the presumed active 81

catalyst, N(SiMe3)3−x(RNH)xLa(THF)z (z = 0−2). It is also possible that one or both THF molecules may dissociate in this step. It is difficult to differentiate bound versus free THF in this system by 1H NMR spectroscopy. Then Si−N bond formation, by either amine attack at silicon or σ-bond metathesis, ensues to generate a hydride intermediate. A Ln−H bond would be primed to react with another equivalent of the amine to regenerate the catalyst. The difference between reactions of methylphenyl and diphenylsilane provides some insight into the mechanism of the Si−N bond-forming step. Despite being more sterically crowded, the reaction time to completion for more electrophilic diphenylsilane with aniline and tert-butylamine was shorter than that with methylphenylsilane (NB: This reaction time is, at best, merely a proxy for relative rate; Table 3.2, entries 7, 9, 13, and

14). As such, these observations give a preliminary indication that nucleophilic amide attack on silicon followed by rapid β-hydride transfer to lanthanum may be more likely than σ-bond metathesis (Scheme 3.1).

Scheme 3.1. Potential mechanism of dehydrocoupling by 1

3.4. Catalyst comparison

To better compare the activity of 1 to that of the other lanthanide catalysts, turnover number (TON) and turnover frequency (TOF) were calculated. Turnover frequency was calculated using total reaction time. We acknowledge that total reaction time, rather than initial rate, significantly underestimates the activity, but these data are available, providing a benchmark to compare catalysts.52 A second feature to consider in the dehydrocoupling of silanes with amines is selectivity. As a result, two different TOFs were calculated. The first is the traditional calculation:

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This calculation describes the turnover frequency of the reaction as it relates to selectivity for a given product. In this calculation, moles of product are defined as only the primary reaction product. A second turnover frequency, TOFSiN, was calculated to approximate the total Si−N coupling events for each catalyst, which approximates overall activity. This is calculated by:

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The TOFSiN value speaks to the Si–N coupling potential of each reaction.

Naturally, this term somewhat underappreciates the ability of these catalysts to

selectively make certain silamines, which is a general goal of this catalysis. It is critical to note that these terms are highly related, essentially rephrasing the same activity values, but these do allow for a comparison between activity and selectivity. With these two terms, however, some general comparative statements can be made about lanthanide- based amine-silane heterodehydrocoupling catalysts (Figure 3.1).

Figure 3.1: Lanthanide based Si–N heterodehydrocoupling catalysts

Compound 1 and the reported lanthanide-based catalysts, Cui’s

25 IMesYb[N(SiMe3)2]2 catalyst (2) and Sadow’s Ln[C(SiHMe2)3]2ImtBu (Ln = Yb (3a),

Sm (3b)) catalysts,26 demonstrate excellent reactivity in the coupling of silanes with

i isopropylamine (Table 3.4). For the generation of PhSi( PrNH)3, it is evident that 1 is the most active catalyst, nearly doubling the TOF and TOFSiN as compared to the next most active catalyst. Compared to 3a and 3b, 2 is more active for coupling secondary silanes with amines. In the reaction between diphenylsilane and isopropylamine, the mono-

i addition product, Ph2( PrNH)SiH, is generated by 2, 3a, and 3b with TOFs of 20, < 1 and

i < 1, respectively. The double-addition product, Ph2Si( PrNH)2, is generated by 1 with a

TOF of 250, an order of magnitude greater than that of 2. Though high TOF is desirable, selectivity is also of high utility. When the ratio of amine to silane approaches unity, 84

selectivity decreases for 1 (vide supra). The TOFs may be lower for 2, 3a, and 3b, but they are more selective and therefore continue to have utility in a variety of applications.

Table 3.4. TON and TOFs of lanthanum-based Si–N heterodehydrocoupling catalysts

i with PrNH2

Entry Silane Cat. Product TON TOFa TOFSiNa b i 1 PhSiH3 1 PhSi( PrNH)3 337.5 225 675 c i 2 PhSiH3 2 PhSi( PrNH)3 59.4 3.3 10 c i 3 PhSiH3 3a PhSi( PrNH)3 37.8 126 378 b i 4 Ph2SiH2 1 Ph2Si( PrNH)2 250 250 500 c i 5 Ph2SiH2 2 Ph2( PrNH)SiH 20 20 20 c i 6 Ph2SiH2 3a Ph2( PrNH)SiH 8.7 < 1 < 1 c i 7 Ph2SiH2 3b Ph2( PrNH)SiH 8.8 < 1 < 1

Conditions: a(hr-1). 2.25 3a-b.26 Catalyst loading = b0.8 mol %, c5 mol %.

Similar trends exist with more sterically encumbered primary and secondary amines (Table 3.5). In bulkier systems, reactions catalyzed by 2 are complete in approximately the same amount of time as that for 1. Again, the reason for the large differences in TOF and TOFSiN is catalyst loading. Cui and coworkers catalyze reactions with 5 mol % of 2, whereas 1 can be utilized with loadings less than 1 mol %. The result is approximately a fivefold increase in TOF and TOFSiN for 1 versus 2. Analyzing TOF and TOFSiN becomes further convoluted when reaction conditions change. Appropriate

TOF and TOFSiN comparisons for reactions of diethylamine with phenylsilane or diphenylsilane are difficult because the reaction temperatures are different for all catalysts. Overall, 1 has generally high TOF and TOFSiN with mild reaction conditions.

Table 3.5: Coupling of amines with PhSiH3 and Ph2SiH2

Entry Cat. Loading (mol %) Amine Product TON TOFa TOFSiNa t t 1 1 0.8 BuNH2 Ph( BuNH)2SiH 250 83 167 t t 2 2 5 BuNH2 Ph( BuNH)2SiH 40 20 40 t t 3 3a 5 BuNH2 Ph( BuNH)2SiH 36.8 1.2 2 b t t 4 1 0.8 BuNH2 Ph2( BuNH)SiH 125 83 83 t t 5 2 5 BuNH2 Ph2( BuNH)SiH 20 20 20 t t 6 3a 10 BuNH2 Ph2( BuNH)SiH 8.7 < 1 < 1 b 7 1 0.8 Et2NH Ph(Et2N)SiH2 121 242 243

8 2 5 Et2NH Ph(Et2N)2SiH 40 20 40

9 3a 5 Et2NH Ph(Et2N)2SiH 32.8 1 2

10 3b 5 Et2NH Ph(Et2N)2SiH 32.4 1 2 c 11 1 0.8 Et2NH Ph2(Et2N)SiH 125 83 83

12 2 5 Et2NH Ph2(Et2N)SiH 20 20 20

13 3a 5 Et2NH Ph2(Et2N)SiH 8.2 < 1 < 1

Conditions: a(/hr). 2.25 3a-b.26 Reaction performed at b60 °C, c90 °C.

There are several possible explanations why 1 is an active catalyst. Many d0 metal complexes have demonstrated the capacity to be Si–N heterodehydrocoupling catalysts.41,

53-60 The decreased electronegativity of these catalysts is likely an important contributor in their reactivity. This trend is similar in the f-block, where La and Yb are among the least electronegative elements. One potential difference among these compounds are the carbene ligands common to compounds 2 and 3, while 1 lacks such strongly electron donating ligands. The relative donation to the metal could be an important factor in either

N–H bond activation or N–Si bond formation. Similarly, the oxidation state of these catalysts likely has a large effect on reactivity. Compounds 2 and 3 are in the +2 oxidation state whereas 1 is in its +3 oxidation state. Similar to having more donating

ligands, decreased oxidation state may slow N–H activation to regenerate the active catalyst. An alternative rationale for increased activity of 1 could be the potential difference in the number of active sites. Up to three bis(trimethylsilyl)amido ligands could be labilized at 1. Presuming the ancillary ligand of 2 and 3 do not dissociate, there would only be up to two actives sites. Further investigation of these differences may yield design features for future catalysts.61

3.5. Heterodehydropolymerization by 1

High TOF and the ability to make tertiary and quaternary silamines make 1 a

potentially useful catalyst for heterodehydropolymerization of silanes and amines. Due

to the inability to activate both N–H bonds of a primary amine, a diamine is necessary.

Given availability and ease of handling and drying, p-phenylenediamine was selected

as the diamine. Reacting p-phenylenediamine and phenylsilane with 1 mol % 1 in dry

THF immediately resulted in the colorless solution becoming yellow. This is

presumably from the amide exchange between bis(trimethylsilyl)amine and p-

phenylenediamine on 1. Within a minute of the color change to yellow, effervescence

began, and the solution quickly returned to nearly colorless. After five minutes,

effervescence ceased, and a small amount of white precipitate had formed. Analysis of

the suspected resulting polymer was challenging due to instability to air and water.

Residual Si–H moieties in the backbone were suspected to be the component

responsible for the decrease in stability.

Moving to a secondary silane would eliminate the possibility of interior Si–H’s.

In an analogous reaction but substituting phenylsilane with diphenylsilane, a white precipitate formed over 24 hours. Upon filtration and washing with THF, analysis of the white precipitate by NMR, MS or GPC was impossible due to insolubility in either organic solvents or water. Methylphenylsilane was then utilized as it may improve solubility in organic solvents. Interestingly, upon addition of the mixture of methylphenylsilane and p-phenylenediamine to 1, the solution turned light pink instead of yellow. Over the course of two days, the solution turned yellow, but no precipitate was observed. Following removal of THF and extraction with Et2O, a yellow solid was isolated. By 1H NMR, several resonances were observed between 0.2-0.5 ppm, likely from different methyl resonances on the silane. Branching and crosslinking could be responsible for slightly different chemical environments that each methyl group observes, leading to slight changes in chemical shift. There is also a distinct lack of Si–

H resonances between 4-5 ppm, indicating that there are likely no primary or secondary silanes present. This data supports the assumption that the reaction produced oligo- or polymeric products. Due to the deteriorated state of the department GPC, only a very rough estimate of molecular weight could be attained. With a maximum retention volume of 9.6 mL, the corresponding molecular weight average of the oligomeric material is between 800-1,200 Da. Defining the monomer as one silamine unit, the potential degree of polymerization range is 3-5.

Figure 3.2. Monomer unit of oligomer made from methylphenylsilane and p-

phenylenediamine.

Decreasing the catalyst loading to 0.05 mol % 1 and increasing the concentration of the overall reaction provided a mixture of species with molecular weight average between 1200-4800 Da. A dilute sample in THF was run by direct infusion in pos ESI MS. Identification of polymers with masses up to 3395 Da were observed. This corresponds to polymers with a degree of polymerization of at least 15. It is possible that higher molecular weight polymers exist, but are in concentrations too low to observe by

MS.

3.6. Testing pre-ceramics potential by TGA

To further probe the identity of the polymers as well as the potential as a preceramic material, thermogravimetric analysis (TGA) was performed. When run a 5

°C/minute, there are three features present in the TGA. The first feature occurs between

100-300 °C and indicates a loss of ~7% of the mass, which corresponds closely to the loss of a -CH3 group (15.04 g/mol out of 226.35 g/mol, 6.6% loss in mass). The second feature, occurring between 330-460 °C, is much larger, indicating a mass loss of ~35% and a total mass loss of ~42%. This corresponds most closely to the loss of a phenyl 89

group (77.11 g/mol out of 226.35 g/mol, 34.1% loss in mass). The final feature, which occurs between 460-700 °C, results in a loss of all volatile organic fragments, a loss of

83.5 mass %. The remaining 16.5 % of the mass equates to ~37 g/mol. The expected product of pyrolysis is SN (42.093 g/mol) but SC (40.097 g/mol) is also potentially present. Given that the sample was not run in triplicate, it is only appropriate to say that there is likely a mixture of SN and SC present.

Figure 3.3. TGA of polymer made from methylphenylsilane and p-phenylenediamine.

It has largely been accepted that silamines made by heterodehydrocoupling do not make good pre-ceramic materials. The high content of carbon in the pre-ceramic material enables the formation of SC in the resulting ceramic. Given that the goal is to make pure

SN, this method doesn’t provide significant improvements over current methods. These experiments were simply a proof of concept study showing that 1 was able to catalyze the synthesis of polysilamines, not to identify and synthesize an optimal pre-ceramic material.

3.7. Investigation into other E–E bond forming reactions catalyzed by 1

A variety of other main-group new bond forming reactions were attempted with 1.

Lanthanide and rare earth catalysts have a well-documented niche application in hydroamination.43 It is somewhat ambiguous if 1 has been utilized for hydroamination in the report by Marks and coworkers. Given the commercial availability of

La[N(SiMe3)2]3, investigating hydroamination potential is warranted. Though perhaps not the best substrates due to their inactivated nature, two α-olefins, 1-hexene and 1-octene were chosen as the unsaturated substrate. N-propylamine and p-tert-butylaniline were selected as the amines. Unfortunately, no reaction was observed with any combination of alkene and amine.

Dehydrocoupling with other substrates was then investigated. Germanium was selected as a heavier analogue to silicon. Using the mildly donating tert-butylgermane and isopropylamine with 10 mol % 1, a slight color change to yellow was observed, but by 1H NMR, no significant consumption of starting material was observed. Similar results were observed when diphenylgermane was used in place of tert-butylgermane.

Perhaps negatively impacting catalysis, a reaction was run with isopropylamine. A small

resonance was observed at 5.82 ppm in 1H NMR, but most of the starting material remained. It doesn’t seem as though 1 is a viable catalyst for Ge–N heterodehydrocoupling or Ge–Ge homodehydrocoupling.

Finally, replacing the amine with a phosphine was explored. Interestingly, upon addition of a solution of phenylsilane and phenylphosphine to 5 mol % 1, the colorless solution immediately became vibrantly yellow. No effervescence was observed and by 1H and 31P NMR, no reactivity was observed beyond catalyst decomposition. In a stoichiometric reaction between 1 and phenylphosphine, the solution again turned vibrant yellow. By 1H NMR, catalyst decomposition is evident, and several new resonances appear. By 31P NMR, only phenylphosphine was present.

3.8. Conclusion

In summary, 1, which is easily obtained in a one-step synthesis or by addition of

THF to commercial La(N(SiMe3)2)3, is perhaps the most active lanthanide-based pre- catalyst for the heterodehydrocoupling of amines and silanes. With catalyst loadings below 1 mol %, dehydrocoupling was efficient in most cases. Coupling silanes with propylamines occurred on the minute time scale, though sometimes yield was lowered due to generation of poorly identified products. Generating quaternary silamines from triphenylsilane with propylamines was also achieved for the first time. Reactivity with bulkier amines was also facile, making tertiary silamines readily. In some cases, even new silamines were generated. Less sterically bulky secondary amines were readily

transformed into silamines in the presence of 1 and primary and secondary silanes. 1 was successfully used to generate a polysilamine of molecular weight near 3400 Da. That polymer was pyrolyzed to generate what appeared to be a mixture of SN and SC.

Additional reactivity for hydroamination, Ge–N, Ge–Ge and Si–P dehydrocoupling was probed but minimal reactivity was observed.

3.9. Experimental

3.9.1. General Considerations

All manipulations were performed under a nitrogen atmosphere with dry, oxygen-free solvents using an M. Braun glovebox or standard Schlenk techniques. Benzene-d6 was purchased from Cambridge Isotope Laboratory and then degassed and dried over NaK alloy. Tetrahydrofuran was dried over sodium and vacuum transferred. NMR spectra were recorded with a Bruker AXR 500 MHz or Varian 500 MHz spectrometer. Reported

1 H NMR resonances are referenced to the residual solvent resonance (benzene-d6 = δ

7.16). ESI-mass spectra were collected on an AB-Sciex 4000 QTrap Hybrid Triple

Quadrupole/Linear Ion trap mass spectrometer. IR data were collected on a Bruker Alpha

FTIR spectrometer as neat samples. Phenylsilane (Oakwood Chemicals), methylphenylsilane (Sigma Aldrich), diphenylsilane (Acros Organics), triphenylsilane

(Gelest), p-tert-butylamiline (Sigma Aldrich), p-phenylenediamine (Sigma Aldrich), phenylphosphine (Sigma Aldrich), tert-butylgermane (Gelest), diphenylgermane (Gelest) and bis(trimethylsilyl)amine (Acros Organics) were used as received. N-propylamine

(Sigma Aldrich), isopropylamine (Alfa Aesar), tert-butylamine (Alfa Aesar), aniline

(Sigma Aldrich), and diethylamine (Acros Organics) were dried by stirring over calcium hydride followed by distillation. was used as received.

7 3.9.2. Synthesis of La(N(SiMe3)2)3THF2 (1) :

Over 20 minutes, lithium bis(trimethylsilyl)amide (4.092 g, 24.45 mmol) in 25 mL THF was slowly added to LaCl3 (2.000 g, 8.15 mmol) in 25 mL THF. The reaction was stirred for six hours. Solvent was removed, and the residue was extracted with hexanes.

Removal of solvent and vacuum sublimation at 80–90 °C provided the product as a

1 colorless powder in 60% yield (3.735g, 4.89 mmol). H NMR (benzene-d6, 500 MHz): δ

3.64 (m, 8H), 1.26 (m, 8H), 0.37 (s, 54H).

n 3.9.3. Preparation of Ph3Si( PrNH): n-Propylamine (38.1 mg, 0.644 mmol) and triphenylsilane (83.9 mg, 0.322 mmol) in 0.5

-3 mL benzene-d6 were added to 1 (2 mg, 2.6 x 10 mmol). The reaction was then heated at

n 60 °C. Over 2.75 hours, Ph3Si( PrNH) was produced in 100% yield. Volatile materials were removed under reduced pressure, and the resultant colorless residue was dissolved in hexanes. The hexanes solution was filtered through a bed of Celite© and concentrated until the first signs of precipitation. The hexanes solution was warmed until all solid was dissolved. The solution was then cooled to –20 °C for 24 hours. The resulting colorless precipitate was isolated by decanting the hexanes solution and washing with minimal,

n 1 cold hexanes. Ph3Si( PrNH) was isolated as a white solid in 96% yield. H NMR

(benzene-d6, 500 MHz): δ 7.72 (m, 6 H), 7.22 (m, 9 H), 2.85 (q, 2 H), 1.31 (sextet, 2 H),

13 0.99 (br, 1 H), 0.71 (t, 3 H). C NMR (benzene-d6, 126 MHz): δ 136.5, 136.0, 129.8,

29 -1 128.1, 44.8, 27.8, 11.4. Si NMR (benzene-d6, 99 MHz): δ 16.48. IR (Neat, cm ): 3402,

3066, 2954, 2922, 2852, 1427, 1396, 1110, 1008, 830, 736, 698, 521, 498. MS (ESI):

318.2.

i 3.9.4. Preparation of Ph3Si( PrNH):

Isopropylamine (38.1 mg, 0.644 mmol) and triphenylsilane (83.9 mg, 0.322 mmol) in 0.5

-3 mL benzene-d6 were added to 1 (2 mg, 2.6 x 10 mmol). The reaction was heated at 60

i °C. Over 1.5 hours, Ph3Si( PrNH) was observed in 100% yield. Volatiles were removed, and the residue was dissolved in hexanes. The hexanes solution was filtered through a bed of Celite© and concentrated until the first signs of precipitation. The hexanes solution was warmed until all solids re-dissolved. The solution was then cooled to –20 ° for 24 hours. The resulting colorless precipitate was isolated by decanting the hexanes solution

i and washing with minimal, cold hexanes. Ph3Si( PrNH) was isolated as a white solid in

1 93% yield. H NMR (benzene-d6, 500 MHz): δ 7.72 (m, 6H), 7.22 (m, 9H), 3.14 (m, 1H),

13 0.96 (d, 6H), 0.92 (br, 1H). C NMR (benzene-d6, 126 MHz): δ 136.8, 136.0, 129.7,

29 -1 128.1, 43.9, 27.8. Si NMR (benzene-d6, 99 MHz): δ -17.86. IR (Neat, cm ): 3387,

3065, 2955, 2924, 2863, 1427, 1379, 1165, 1111, 1017, 871, 739, 699, 523, 497. MS

(ESI): 318.2.

3.9.5. Preparation of MePhSi(PhNH)2:

Dichloromethylphenylsilane (0.511g, 2.67 mmol) and aniline (1.0g, 10.7 mmol) were stirred in 15 mL diethyl ether for 24 hours. The diethyl ether solution was filtered through

a bed of Celite© and concentrated until the first signs of precipitation. The hexanes solution was warmed until solids re-dissolved. The solution was then cooled to –20 °C for 24 hours. The resulting colorless precipitate was isolated by decanting the hexanes solution and washing with minimal, cold hexanes. MePhSi(PhNH)2 was isolated as

1 colorless block crystals in 47% yield. H NMR (benzene-d6, 500 MHz): δ 7.61 (m, 2H),

7.16 (m, 3H), 7.05 (m, 4H), 6.73 (m, 2H), 6.69 (m, 4H), 3.52 (s, 2H), 0.45 (s, 3H). 13C

NMR (benzene-d6, 126 MHz): δ 146.3, 136.2, 134.4, 130.5, 129.6, 128.5, 119.2, 117.4.

29 -1 Si NMR (benzene-d6, 99 MHz): δ -19.80. IR (Neat, cm ): 3375, 3041, 1599,1497,

1376, 1284, 1114, 902, 751, 692, 471. MS (ESI): 305.2.

3.9.6. Preparation of PhSi(PhNH)3:

Phenylsilane (113.3 mg, 1.005 mmol) and aniline (389.9 mg, 4.19 mmol) in 0.5 mL

-2 benzene-d6 were added to 1 (8 mg, 1.05 x 10 mmol). The reaction was heated at 90 °C for 24 hours. Solvent was removed and the colorless solid was washed with pentane.

1 PhSi(PhNH)3 was collected as a colorless solid in 86% yield. H NMR (benzene-d6, 500

MHz): δ 7.78 (m, 2H), 7.13 (m, 1H), 7.08 (m, 2h), 7.02 (m, 6H), 6.74 (m, 9H), 3.84 (s,

13 3H). C NMR (benzene-d6, 126 MHz): δ 145.5, 134.9, 134.0, 130.9, 129.6, 128.6, 119.7,

29 -1 117.6. Si NMR (benzene-d6, 99 MHz): δ -41.47. IR (Neat, cm ): 3371, 3042, 1599,

1496, 1475, 1383, 1280, 1118, 905, 890, 752, 692, 490, 464. MS (ESI): 382.2.

3.9.7. Catalysis experiments general consideration:

All non-polymerization coupling reactions were conducted using a J-Young type polytetrafluoroethylene (PTFE)-valved NMR tube in benzene-d6 solution. Upon

obtaining an initial 1H NMR spectrum, the reaction was placed in an oil bath at the listed temperature. Polymerization reactions were conducted in a scintillation vial and aliquots were periodically taken to determine if the reaction was complete. All NMR spectra were collected at 25 °C.

3.9.8. General method for heterodehydrocoupling reactions:

In a N2 filled dry box, appropriate amounts of an amine and silane measured and mixed in ca. 0.5 mL benzene-d6. This solution was then pipetted into a vial containing 1. The solution was quickly transferred into a J-Young type polytetrafluoroethylene (PTFE)- valved NMR tube. An initial NMR spectrum was collected and then the sample was placed in an oil bath at the listed temperature. The reaction was monitored by NMR spectroscopy until reactivity had ceased.

3.9.9. Direct infusion in pos ESI MS:

100

101

102

103

104

Figure 3.4. Direct infusion ESI MS of a silamine polymer

3.10. References

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110

CHAPTER 4: RUTHENIUM CATALYZED P–C BOND FORMING REACTIONS

4.1 Introduction

Pagano and Waterman demonstrated that [CpFe(CO)2]2 (1) can be photo- activated by commercially available LED light (hν > 500 nm).1 The photo-activation of 1 is quite well studied, and hypothesized upon irradiation by hν > 500 nm, 1 is homolytic

• 2-6 cleaved to generate two radical species, CpFe(CO)2 . This is the same net outcome as

UV irradiation (hν < 300 nm). To test this hypothesis, duplicate reactions were carried out under UV irradiation and visible light irradiation. The observed reactivity was similar in product distribution and reactivity trends, but in some cases, reaction rate was improved by using visible light irradiation.1

Secondary phosphines, specifically diphenylphosphine, was the phosphine of choice for several reasons. First, diphenylphosphine is easily synthesized and purified from an inexpensive starting material.7 Second, the tertiary hydrophosphination products are known and therefore simplify analysis and reactivity benchmarking. Finally, competitive reactivity in the form of homodehydrocoupling was observed with primary phosphines. The unsaturated substrate scope consisted of styrene derivatives and Michael acceptors. Under hν > 500 nm, and 5 mol % of 1, reaction time ranged from 6-72 hours.

Electron rich styrenes were hydrophosphinated slower significantly than electron withdrawing styrenes. Styrene itself was a poor reagent for this transformation, taking upwards of three days to achieve slightly over 10% conversion. Michael acceptors proved

111

to be viable substrates as well. Methyl- and ethyl acrylate ceased reactivity after six hours, consuming 96 and 51% of the diphenylphosphine, respectively. Acrylonitrile also saw 70% starting material consumption in 12 hours, but a 2:5 mixture of Markovnikov and anti-Markovnikov products was observed. This marks the only viable substrate that was not selective for exclusively the anti-Markovnikov product.1 Later, Ackley, Pagano, and Waterman expanded the utility of LED activated 1 to include double hydrophosphination of terminal aryl alkynes with diphenylphosphine.8

Waterman’s exploration of 1 has unearthed a potentially rich area of study for visible light photocatalyzed hydrophosphination. More environmentally friendly and renewable reaction conditions and improved reaction rate highlight advances made thus far. There are several opportunities to expand the scope of visible light photocatalyzed hydrophosphination however. Hydrophosphination with primary phosphines is valuable because a secondary phosphine is produced, leaving a P–H still available for further functionalization. Unfortunately, Waterman observed competitive dehydrocoupling precludes hydrophosphination with primary phosphines using 1.1, 8 Unactivated substrates continue to be challenging for all hydrophosphination catalysts except Waterman’s triamidoamine zirconium catalyst [κ5-

9 N,N,N,N,C(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr. Similarly, reaction times using

1 are still outstripped by d0 metal catalysts. The advances in visible light photo-activation and potential for improvements in reaction rates and substrate variety lend motivation for further exploration.

112

Given ruthenium’s proximity to iron on the periodic table, it is reasonable to expect similar reactivity in related ruthenium complexes.10 In a report by Dixneuf, simple ruthenium complexes, including Cp*(COD)Cl and Cp*Ru(PPh3)2Cl, were used to hydrophosphinate propargyl alcohols with diphenylphosphine. This work represents a facile synthesis of value-added products: vinyl phosphines. Reaction conditions were undesirable however, requiring reflux temperatures over 100 °C, reaction times of 24 hours, catalytic amounts of base with high catalyst loadings.11

The ruthenium analogue, [CpRu(CO)2]2, has similar photochemistry exhibited by

1. In the solid phase, both Ru and Fe analogues exist in the bridged binuclear state. In the solution, 1 exists primarily, over 99% of the time in fact, in the same carbonyl-bridged binuclear state. On the other hand, [CpRu(CO)2]2 (2) exists in equilibrium between the carbonyl-bridged binuclear state and a non-carbonyl bridged form that only contains a

Ru–Ru bond, with the Ru–Ru bridged species predominating. Traditionally, both the ruthenium and iron analogues are photo-activated by UV light. UV-vis absorptions for the carbonyl bridging and non-carbonyl bridging ruthenium species are recorded at λ =

265 nm and 330 nm, respectively. The weaker band at 265 nm corresponds to the

HOMO-1 → LUMO+2 transition (σb → σ*) of the carbonyl-bridged form. Excitation of this band results in CO loss and generation of a triply bridged [Cp2Ru2(CO)3] complex.

The more intense band at 330 nm relates to a HOMO → LUMO+2 transition (σb → σ*) of the non-carbonyl bridged species. Upon excitation, homolytic cleavage of the Ru–Ru

- • 6, 12 bond produces two equivalents of the 17 e radical complex CpRu(CO)2 . 1 has

113

absorption bands in the UV-vis at 346, 410 and 514 nm. The largest band at 346 nm is analogous to the 265 nm band for the ruthenium analogue. Photo-activation at 346 nm results in carbonyl loss to generate a mono-carbonyl bridged species. Quick reintroduction of carbon monoxide results in cleavage of the Fe–Fe bond to generate two

• equivalents of CpFe(CO)2 . Similar to photo-activation of 2 at 330 nm, visible light activation (λ > 500 nm) of 1 generates a non-carbonyl bridged dimer, which then

• 4-6 undergoes homolytic cleavage to generate CpFe(CO)2 . Given the similarities in photo- activation of 1 and 2, it is plausible that hydrophosphination could be observed, possibly even with primary phosphines. The following work is the competition study of photo chemical activation of 2 and it’s hydrophosphination capabilities.

Table 4.1. Electronic transitions of 1 and 2

Compound UV-vis λMax Electronic Transition Assignment

1 514 HOMO→LUMO dnb→σ*

410 HOMO-1→LUMO dnb→σ* σ →σ* 346 HOMO→LUMO+2 b πb→π* σ →σ* 2 330 HOMO→LUMO+2 b Ru–Ru bridged σ →σ* 265 HOMO-1→LUMO+2 b Carbonyl bridged

114

Scheme 4.1. Photo-activation of 1 and 2

4.2. Hydrophosphination with 2

In an attempt to mimic the photo-activation of 1 by visible light, 2 was photo- activated near λ = 330 nm. A broad wavelength 9-W UV/A lamp, which was found to have λmax = 360 nm, was utilized. Importantly, there is negligible generation of photons with wavelengths capable of exciting the 265 nm band of 2. This ensured that the only

• active ruthenium species are derived from CpRu(CO)2 .

115

Figure 4.1. Spectral distribution of Rexim G23 UV-A (9W) lamp. Provided by

manufacturer.

Hydrophosphination of activated alkenes with phenylphosphine was probed.

Irradiation (λmax = 360 nm) of phenylphosphine and olefin, in a three to one ratio, with

0.1 mol % of 2 in benzene-d6 resulted in hydrophosphination of the olefin in yields greater than 90%. Three equivalents of phosphine were used to minimize double P–H activation which results in the formation of undesired tertiary phosphines. Using more than three equivalents did not inhibit the formation of tertiary phosphines. Catalyst loadings down to 0.1 mol % of 2 were achieved without decreased activity. These reactions were irradiated using a commercial UV/A lamp (λ = 360 nm) and shielded from ambient light. Due to the heat given off from the lamp, the temperature of reaction was slightly above ambient temperature, but never achieving temperatures higher than 30 °C.

116

(1)

As shown by Waterman, substituted styrenes were viable substrates for this transformation. A noticeable difference in reactivity was observed for electron rich and poor styrenes with 1, with electron-poor styrenes being more readily hydrophosphinated.1

The opposite trend was observed with 2, with more electron-rich styrenes reacting with a greater relative rate. First, and unlike 1, styrene was a viable substrate, with complete consumption in 60 minutes. Alkyl styrenes, tBu and Me, were both successfully hydrophosphinated in less than 60 minutes. Complete consumption of the bulkier p-tert- butylstyrene occurred faster than p-methylstyrene. Electron-poor styrenes took slightly longer to reach completion. Michael acceptors, though not unexpected, were excellent substrates for this reaction, with both methyl acrylate and ethyl acrylate reaching complete conversion in less than 12 minutes.13-17 In all cases, exclusively the anti-

Markovnikov product was formed.

117

Table 4.2. Hydrophosphination of styrenes and Michael acceptors

Substrate Time Major product Minor product Conversion 60 100 minutes (95 : 5)

40 100 minutes (95 : 5)

60 100 minutes (95 : 5)

70 100 minutes (95 : 5)

120 95 minutes (96 : 4)

23 72 Product unknown Product unknown hours (53 : 47) 12 100 minutes (94 : 6)

12 100 minutes (93 : 7)

18 100a b hours (81 : 19)

Product conversions determined by 31P{1H} NMR spectroscopy. Consumption of unsaturated substrate determined by 1H NMR spectroscopy. acatalyst loading = 1 mol %. b1 unsaturated substrate: 1 phosphine.

An internal olefin, β-methylstyrene, was investigated. Over the course of 23 hours, 72 % consumption of β-methylstyrene was observed. By 31P NMR, there were two new resonances. As there are two potential products in hydrophosphination, the 1,2- 118

addition and 2,1-addition products, providing two potential isomeric products. Generally, the driving force in making the 1,2-addition hydrophosphination product is the absence of steric bulk on one side of the unsaturated substrate, providing easier access for addition chemistry. With β-methylstyrene, a methyl group provides significantly more steric encumbrance than hydrogen, so it is more likely that addition of phosphorus could occur on either side of the olefin. For clarity however, the product of the addition on the less sterically hindered side of β-methylstyrene, resulting in phosphorus adjacent to the methyl group, will be referred to as the 1,2-addition product. The other product, which places phosphorus adjacent to the phenyl ring, will be referred to as the 2,1-addition.

Figure 4.2. 1,2 and 2,1 addition products

There is evidence supporting both products, though some of the assignments are somewhat convoluted. By 31P NMR, there are two resonances within the region that secondary phosphines typically resonate. The coupled 31P-1H spectrum shows two P–H resonances with different splitting patterns. The more complex resonance has more hydrogens two or three bonds away. Typically, only two and three bond coupling from phosphorus is going to be observed. As shown in Figure 4.4., the 1,2-addition has six protons with three bonds while the 2,1-addition product only has three. This supports the identification of the more complex resonance at 32.1 ppm to be the 1,2-addition product. 119

Figure 4.3. 31P {1H} NMR spectrum of the hydrophosphination of β-methylstyrene

Figure 4.4. Hydrogens two, three and four bonds from phosphorus

1H NMR corroborates the formation of both products. First, two separate P–H resonances are apparent. The resonance of the methyl group for both products are around

0.95 ppm. Curiously, there is one doublet integrating for three hydrogens and one double of doublets integrating for three hydrogens. This is due to the inequivalence of the -CH2

120

hydrogens. This difference only manifests in the 2,1-addition product however, as the -

CH2 hydrogens in the 1,2-addition product are too far away to split.

Figure 4.5. Overlapping doublet and doublet of doublets in the 1H NMR spectrum of the

hydrophosphination of β-methylstyrene

Figure 4.6. Hydrogens two, three and four bonds from the methyl protons.

The other six protons, the -CH and -CH2 on each product, were more challenging to assign. By 1H NMR, three multiplets, each integrating to two hydrogens, resonate at

121

2.15, 2.37 and 2.82 ppm. It is possible that each -CH2 resonates at a different location while the -CH resonances of both products overlap. It is also possible that, due to the -

CH2 hydrogens being inequivalent, they resonate at a different location. This second hypothesis is supported by the analogous diphenylphosphine 1,2-addition product which

18 1 1 displays three resonances for -CH2 and -CH hydrogens. To probe this, H- H COSY and 1H-31P HMBC was utilized.

Figure 4.7. Unassigned multiplets in the hydrophosphination of β-methylstyrene

Figure 4.8. Six hydrogens (in blue) that comprise the three unassigned multiplets.

Several features stand out in the 1H-1H COSY. First, cross peaks exist between

2.15 and 2.37 ppm, 2.15 and 2.82 ppm, and also 2.37 and 2.82 ppm. This supports the 122

hypothesis that each multiplet is actually two coincidentally overlapping resonances of one proton from each molecule. This is somewhat convoluted because there is only one cross peak for the methyl groups (0.92 ppm), which is at 2.15 ppm. The absence of a methyl cross peak 2.37 and 2.82 ppm is unexpected. It is possible that in the 1,2-addition product, the -CH2 is simply too far to be observed. In the 2,1-addition product however, the -CH2 is well within the expected range of observance with respect to the methyl group. Given that the COSY supports the notion that each of the three protons in question resonates at a different location, it seems impossible that the methyl group is only coupled to one resonance in the 2,1-addition product.

123

Figure 4.9. 1H-1H COSY of the hydrophosphination of β-methylstyrene

Unfortunately, 1H-31P HMBC doesn’t provide any more clarity. What is most baffling about the 1H-31P HMBC is that there is no coupling at 2.15 ppm. 1H-31P HMBC couples protons two, three and sometimes four bonds away from phosphorus. In both the

2,1-addition and 1,2-addition products, all three hydrogens in question are within three bonds of phosphorus and should therefore have cross peaks. Cross peaks exist for both compounds at 2.37 and 2.82 ppm, which further supports the hypothesis that each proton resonates at a different location. Ultimately, the exact resonances for the six hydrogens

124

on the backbone were not able to be assigned. Most of the evidence supports the identity of both products, however refuting evidence persists.

Figure 4.10. 1H-31P HMBC of the hydrophosphination of β-methylstyrene

125

Figure 4.11. 1H-31P HMBC relevant protons

It is possible that only one of the addition products is being produced but because there are two chiral centers, up to four stereoisomers could be present (RR, RS, SR, SS).

Given the exclusive formation of anti-Markovnikov products by hydrophosphination with

2, it is reasonable to suggest that if only one addition product were being produced, it would be the analogous 1,2 addition product. This product has two chiral centers, one at the tertiary carbon and one at phosphorus. It is possible that each of these stereoisomers has unique resonances, but given that there are only two phosphorus resonances, it is likely that enantiomers are equivalent. This means that when the chiral centers are either

RR and SS, only one resonance is observed. Similarly, RS and SR are equivalent.

Essentially what could be observed is the difference in diastereomers because RR and SS are diastereomers with RS and SR. It is not surprising that diastereomers are different enough to have different chemical shifts. With a simpler complex, diphenyldiphosphine, two chiral centers exist, and a mixture of RR, RS, SR, and SS is possible. By 31P NMR

126

spectroscopy, two separate phosphorus resonances are observed, derived from the two sets of diastereomers. At present, further research needs to be conducted to conclude whether the complexity of the NMR spectra is derived from diastereomers of a single addition product.

Figure 4.12. Chiral centers on 1,2 addition hydrophosphination product

Limited success was observed with cyclohexylphosphine. Reacting cyclohexylphosphine and 2,3-dimethyl,1-3-butadiene (3 eq) with 1 mol % of 2 yielded the 1,4-addition product in 13% conversion over four days. In an analogous reaction but with phenylphosphine, 91% conversion to the 1,4-addition product is achieved in less than two hours. Equal amounts of diphenylphosphine and styrene were combined with 1 mol % of 2 and produced the tertiary hydrophosphination product in 97% yield in 23 hours. Although only demonstrated in one example, diphenylphosphine is likely a viable substrate for this transformation. Unactivated alkenes and terminal alkynes aren’t viable substrates. Unactivated alkenes were unreactive while mixed reactivity was observed with terminal alkynes. When using phenylacetylene as the unsaturated substrate, 1 mol % of 2 yielded 46% consumption of phenylphosphine over two days. Four resonances in the

31P NMR were apparent. Only the smallest resonance, amounting to 1% of the consumed 127

phenylphosphine, was a known product, the single addition hydrophosphination product.

Internal alkynes appear to be somewhat viable substrates. Although less than 30% phosphine consumption before catalyst decomposition, a 9:1 ratio of Z : E products was observed.

Table 4.3. Hydrophosphination with 2

Substrate Time Major products Minor product Conversion 100 a 2 hours Products unknown (91 : 9) 17 a 96 hours Products unknown (69 : 31) 100 a b f 23 hours Products unknown (97 : 3) 13 72 hours Product unknown (97 : 3) 72 hours None None 0

a 26 hours None 4 46 a c e 48 hours Products unknown (45 : 1) 24 hours None None 0 a d

30 a b e 2 hours Products unknown (29 : 1) (9 Z:1 E) 24 hours 20 a b e None

96 hours 93 a b e f Products unknown (87 : 6)

28 hours None None 0 a b

Product conversions determined by 31P{1H} NMR spectroscopy. Consumption of unsaturated substrate determined by 1H NMR spectroscopy. acatalyst loading = 1 mol

128

%. b1 unsaturated substrate: 1 phosphine. c4 unsaturated substrate: 1 phosphine. d1 unsaturated substrate:2 phosphine. eConsumption of phosphine, determined by 31P{1H}

NMR spectroscopy. fDiphenylphosphine used instead of phenylphosphine

The mechanism of hydrophosphination is still under consideration. Photo-

• activation with 360 nm light produces two equivalents of CpRu(CO)2 . It is plausible that

• two CpRu(CO)2 radicals then homolytically cleave a P–H of the phosphine, generating

CpRu(CO)2P(H)R and CpRu(CO)2H. At this point, several variants of a similar mechanism could be possible. Taking CpRu(CO)2P(H)R as the active catalyst, a nucleophilic attack by phosphorus results in the insertion of phosphine into the olefin and generates a Zwitterionic intermediate.19, 20 During this step, the phosphine becomes increasingly acidic and has the potential completely remove an electron from ruthenium, generating a cationic ruthenium species with a neutral, L-type phosphine donor. This intermediate could facilitate what would effectively be a 1,4 addition of a second equivalent of phosphine resulting in protonation of the carbanion, and formation of a new

Ru–P bond and elimination of the hydrophosphination product.

129

Scheme 4.2. Possible olefin insertion and phosphine protonation mechanism

There is also a literature precedent for the formation of a 4-membered metallacycle followed by phosphine dissociation.21-26 Dissociation of the phosphine may happen spontaneously or prompted by either the steric constraint of the ring or the donation of a free phosphine to the ruthenium center. If ring opening is prompted by sterics, a coordination site becomes available for another equivalent of phosphine to coordinate. Protonolysis from the new phosphine to the alkylphosphine eliminates the hydrophosphinated product with regeneration of the active ruthenium phosphido species.27 Instead of a Michael addition, it is possible that a 1,2-insertion of the olefin occurs, followed by protonolysis.28, 29

130

Scheme 4.3. Possible olefin insertion and cyclometallation based mechanism

This mechanism appears to cap the active catalyst loading to 50% of the expected loading since CpRu(CO)2H doesn’t appear in any step. It is possible that a similar mechanism to the one above occurs, but with the ruthenium hydride undergoing a 1,2- insertion into the olefin first, producing a ruthenium alkyl species. By oxidative addition,

P–H of a free phosphine is added to ruthenium and reductive elimination of the phosphido and alkyl moieties results in the hydrophosphinated product and regeneration of the ruthenium hydride.30, 31 It is likely that dissociation of a carbonyl ligand is occurring somewhere in this process to avoid making a 20 e- ruthenium complex. If carbonyl loss is not reversible, regeneration of the active catalyst is not possible and only one turnover is possible. It must be noted that effervescence is not observed, but concentration of 2 is low enough where only trace gas evolution occurs. 131

Scheme 4.4. Possible mechanism of hydrophosphination through a ruthenium alkyl

species

Given that this study was performed as a competition study to 1, a comparison of reactivity is warranted. Generally speaking, 2 is a superior catalyst to 1 when comparing reaction times and conversion of analogous reactions, using phenylphosphine with 2 and diphenylphosphine with 1.

Hydrophosphination with photo-activated 2 has been realized, but further advances can still be made. It is clear that styrenes and Michael acceptors are viable substrates for this reaction. Some success has been observed with internal alkynes as well. Because 1 only hydrophosphinates terminal alkynes, it would be a valuable advancement if 2 is capable of efficiently hydrophosphinating internal alkynes, thus 132

broadening the substrate scope of commercially available catalysts. In addition, initial studies indicate that secondary phosphines may also be promising substrates. Though viable substrates for 1, the increased reactivity of 2 warrants a more thorough exploration of secondary phosphine substrates. Beyond substrate scope, some mechanistic details can be elucidated, possibly facilitating a more directed approach to advancing hydrophosphination. Stoichiometric reactions to attempt to isolate intermediates would be the likely starting point. Deuteration studies could also be useful, such as for determining the origin or fate of CpRu(CO)2H. Between substrate expansion and mechanistic elucidation, there are unexplored opportunities in hydrophosphination by photo- activation of 2.

UV irradiation of the commercially available compound 2 promotes efficient hydrophosphination with low catalyst loading, low intensity light, and no need for thermal activation. Complimentary reactivity was observed compared to the iron derivative, 1. Similar to 1, styrenes and Michael acceptors were hydrophosphinated with ease. While terminal alkynes weren’t hydrophosphinated, internal alkynes appear to be viable substrates. Most importantly, hydrophosphination is not limited to secondary phosphines. Selectivity for the secondary phosphine product, using phenylphosphine as the substrate, is also generally high. This competition study illustrates the ability of commercially available 2 to be photo-activated by inexpensive UV light for efficient hydrophosphination with primary phosphines.

133

4.3. Synthesis of a ruthenium phosphinidene

Another avenue for P–E bond formation is through phosphinidene activation.

After the emergence of terminal alkylidene chemistry, the diagonal relationship between carbon and phosphorus naturally prompted the investigation of terminal phosphinidenes.

These low valent phosphorus species are isolobal with well-studied imido and carbene complexes.32, 33 Similar to Fisher and Shrock-type carbenes, terminal phosphinidene complexes have the ability to be either electrophilic or nucleophilic. Over the past decade, interest in terminal metal phosphinidenes has grown due to their high reactivity and ability to either stoichiometrically or catalytically promote new the formation of new

P–E bonds.33-39 Group 4 and 5 metals have been studied extensively.40-58 Late transition metal phosphinidenes have been studied, but not to the level of the early transition metals.59-73 Though now a variety of these complexes has been isolated, the reactivity and potential utility of these complexes is still an active area of study.

Figure 4.13. Generic metal-alkylidene, imido and phosphinidene complexes

Ruthenium phosphinidene complexes have been known since the early 21st century.61, 66, 68, 71-73 To date however, interest in synthesizing new ruthenium phosphinidenes has generally been prioritized over reactivity studies. One exception is the reactivity study done by Rivière and coworkers. Electron-rich ligands of (η6-

L)(PCy3)Ru(PMes*) (L = benzene, p-cymene) provide sufficient electron donation to the

134

metal-phosphinidene moiety and produce a more nucleophilic species. As such, reactivity with electrophilic reagents was pursued. Reactivity with tetrafluoroboric acid resulted in protonation of phosphorus and a borate anion. The reaction with HCl results in

6 protonation and elimination of free H2PMes* and production of (η -L)(PCy3)RuCl2.

6 Reactivity with Lewis acids, specifically BH3•SiMe2, generated the expected adduct (η -

L)(PCy3)Ru[P(BH3)Mes*]. Introducing alkylating reagents resulted in the alkylation of the phosphinidene. This work was the first to explore the reactivity of the ruthenium phosphinidene moiety but was limited in scope and failed to eliminate any new phosphorus containing species.71

To tweak the electronics of the ruthenium phosphinidene, Lammertsma and coworkers replaced the tertiary phosphine with an NHC. Reacting two equivalents of i i I Pr2Me2 (I Pr2Me2 = 1,3-diisopropyl-4,5-dimethyl-4,5-dihydroimidazol-2-ylidene) with

6 6 i (η -L)(PMes*)RuCl2 produced the analogous complex, (η -L)(I Pr2Me2)Ru(PMes*). The

i decreased back-bonding ability of I Pr2Me2 compared to PCy3 results in a more electron rich and polarized Ru=P bond. This is evident by reactivity towards diiodomethane. The second generation NHC phosphinidene quantitatively produced the phosphaalkene

H2C=PMes* within one minute. The first generation PCy3 ligated complex required an hour to reach 50% consumption of starting material, producing only 45% of the desired product after complete consumption of starting materials.73 This is the first and only example of a ruthenium phosphinidene complex reacting to produce a separate

135

organophosphorus species. There is ample opportunity to advance this field with continued study of ruthenium phosphinidene complexes and their reactivity.

Instead of studying a known ruthenium phosphinidene complex, motivation was derived from Hayes and Tilley’s ruthenium silylene and germylene complexes. Taking

i Cp*Ru( Pr2PMe)Bn with a bulky primary silane or germane resulted in the elimination of toluene and the formation of a ruthenium silylene or germylene, respectively. It was hypothesized that a ruthenium silyl species first forms, followed by toluene elimination and finally an α-H migration to produce the ruthenium silylene. Sterically encumbered silanes and germanes were required for this reaction. Reactions with the phenylsilane or mesitylsilane resulted in a mixture of products, likely generated from undesired intermolecular C–H activation. It was hypothesized that a similar reaction with a bulky phosphine may produce a ruthenium phosphinidene.74 These species would then be reacted with a variety of small molecules to explore new P–E bond forming reactions.

The synthesis of the starting material was altered to avoid the isolation of the finicky reagent [Cp*RuCl]4. Hayes and Tilley showed that [Cp*RuCl]4 can be reacted

i i with one equivalent of Pr2PMe to quantitatively generate Cp*Ru( Pr2PMe)Cl. Treatment

i of Cp*Ru( Pr2PMe)Cl with Mg(CH2Ph)2(THF)2 affords the starting ruthenium material

i 74 Cp*Ru( Pr2PMe)Bn.

136

(2)

To circumvent using isolated [Cp*RuCl]4, it was made in situ. [Cp*RuCl2]2 was

i reduced by excess zinc powder in the presence of Pr2PMe to produced

i Cp*Ru( Pr2PMe)Cl. A more easily synthesized benzylating reagent, benzylpotassium,

i 31 was attempted. Cp*Ru( Pr2PMe)Bn was acquired in yields greater than 90% by P NMR spectroscopy, but significant and unidentified impurities existed by 1H NMR spectroscopy. Unfortunately, purification by literature methods was unsuccessful.74

(2)

If a ruthenium phosphinidene can be synthesized, reactivity of the phosphinidene moiety can be explored. At this point in time, efforts are being made to isolate and purify

i i Cp*Ru( Pr2PMe)Cl. This will be used to in situ make Cp*Ru( Pr2PMe)Bn in the presence

137

i of a bulky phosphine. Cp*Ru( Pr2PMe)Bn may be too reactive to store for long periods of

i time. In addition, transmetallation with Cp*Ru( Pr2PMe)Cl and lithiated phosphines may provide either the phosphinidene or a ruthenium phosphido complex. DMPPH2 and

Mes*PH2 are the phosphines under study. With the bold assumption that a ruthenium phosphinidene is isolated and characterized, small molecule activation studies will be performed. Small molecules of interest include ketones, aldehydes, amines, imines, thiols, disulfides, selenols, diselinides, silanes, germanes and beyond.

Interest in the synthesis of ruthenium phosphinidene complexes continues as little is known about the related reactivity of such molecules. Attempts to make a ruthenium phosphinidene haven’t been successful yet, but alternative synthetic methods provide reason for optimism. Adjusting the size of the tertiary phosphine or primary phosphine may be all that is required. Once synthesized, a comprehensive reactivity study with small molecules will provide a clearer understanding of the reactivity of the phosphinidene moiety.

4.4. Future direction

Hydrophosphination with 2 has proven to be an effective route for P–C bond formation. Short reaction times, low catalyst loading, low intensity irradiation and no thermal impetus all make 2 and exciting hydrophosphination catalyst. Preliminary studies indicate that secondary phosphines are potential candidates for alternative phosphorus sources. Exploring hydrophosphination potential with other primary and secondary phosphines could further broaden the potential for commercially available

138

hydrophosphination catalysts. In addition to varying the phosphorus source, internal alkynes pose valuable targets for hydrophosphination. Because 1 is not capable of hydrophosphinating internal alkynes, if 2 is a competent catalyst, a valuable avenue for the production vinyl- or even diphosphines, via double hydrophosphination, may be realized.

Synthesis of the ruthenium phosphinidene complex may be achieved in a variety of ways. Varying phosphine size, ligand encumbrance and phosphine introduction can all be explored to make the desired moiety. Once synthesized, small molecules, such as CO,

CO2, alkenes, etc. can be introduced to potentially make valuable P–E containing products.

4.5. Conclusion

Low catalyst loadings of 2 is photo-activated by UV-A light to generate a radical species, which then likely activates a P–H and readily undergoes hydrophosphination.

Similar to 1, styrenes and Michael acceptors are excellent substrates, but unlike 1, 2 is capable of hydrophosphination with primary phosphines. Preliminary results indicate that

2 also hydrophosphinates secondary phosphines. Though 2 doesn’t hydrophosphinated terminal alkynes, internal alkynes appear to have potential as unsaturated substrates.

Overall, a commercial catalyst, 2, is photo-activated by a low-intensity commercial UV-

A lamp for efficient hydrophosphination of a variety of substrates.

139

4.6. Experimental

4.6.1. General Considerations

NaK alloy. Tetrahydrofuran was dried over sodium and vacuum transferred. NMR spectra were recorded with a Bruker AXR 500 MHz or Varian 500 MHz spectrometer.

Reported 1H NMR resonances are referenced to the residual solvent resonance (benzene- d6 = δ 7.16). All reagents were acquired from commercial sources and used as received, unless otherwise noted.

4.6.2. Catalysis experiments general consideration:

Reactions were performed in an NMR tube using benzene-d6 as the solvent. The reaction was kept in a foil wrap during transportation between the lamp and the NMR spectrometer. The reaction was placed in a Rexim G23 UV-A (9W) lamp at room temperature and shielded from ambient light. All NMR spectra were collected at 25 °C.

In the spectra below, the single addition product is denoted with a ‘*’. The double addition product is denoted with a ‘**’. Products are not labelled if overlapping with other resonances (i.e. aryl resonances). Phenylphosphine exists at 123.0 ppm in 31P NMR spectroscopy, and is not labelled in the spectra.

140

4.6.3. General method for hydrophosphination reactions

In a N2 filled dry box, appropriate amounts of a phosphine and unsaturated substrate were measured and mixed in ca. 0.5 mL benzene-d6. This solution was then pipetted into a vial containing 2. The solution was transferred into an NMR tube and covered with aluminum foil. The reaction was kept in a foil wrap until an initial 1H NMR spectrum was acquired. The reaction was then placed in a Rexim G23 UV-A (9W) lamp at room temperature and shielded from ambient light. Periodic NMR spectra were collected until reactivity ceased. Phenylphosphine

141

4.6.4. Hydrophosphination of styrene with phenylphosphine

Figure 4.14.1. Stacked 1H NMR spectra of the hydrophosphination of styrene with

phenylphosphine by 2. Spectra collected every 12 minutes.

142

Figure 4.14.2. Stacked 31P NMR spectra of the hydrophosphination of styrene with

phenylphosphine by 2.

143

Figure 4.14.3. Final 31P NMR spectrum of the hydrophosphination of styrene with

phenylphosphine by 2.

144

4.6.5. Hydrophosphination of p-tert-butylstyrene with phenylphosphine

Figure 4.15.1. Stacked 1H NMR spectra of the hydrophosphination of p-tert-butylstyrene

with phenylphosphine by 2. Spectra collected approximately every 12 minutes.

145

Figure 4.15.2. Stacked 31P NMR spectra of the hydrophosphination of p-tert-butylstyrene

with phenylphosphine by 2.

146

Figure 4.15.3. Final 31P spectrum of the hydrophosphination of p-tert-butylstyrene with

phenylphosphine by 2.

147

4.6.6. Hydrophosphination of p-methylstyrene with phenylphosphine

Figure 4.16.1. Stacked 1H NMR spectra of the hydrophosphination of p-methylstyrene

with phenylphosphine by 2. Spectra collected approximately every 20 minutes.

148

Figure 4.16.2. Stacked 31P NMR spectra of the hydrophosphination of p-methylstyrene

with phenylphosphine by 2.

149

Figure 4.16.3. Final 31P NMR spectrum of the hydrophosphination of p-methylstyrene

with phenylphosphine by 2.

150

4.6.7. Hydrophosphination of p-bromostyrene with phenylphosphine

Figure 4.17.1. Stacked 1H NMR spectra of the hydrophosphination of p-bromostyrene

with phenylphosphine by 2. Spectra collected approximately every 20 minutes.

151

Figure 4.17.2. Stacked 31P NMR spectra of the hydrophosphination of p-bromostyrene

with phenylphosphine by 2.

152

Figure 4.17.3. Final 31P NMR spectrum of the hydrophosphination of p-bromostyrene

with phenylphosphine by 2.

153

4.6.7. Hydrophosphination of 4-vinylpyridine with phenylphosphine

Figure 4.18.1. Initial and final (t = 2 hours) stacked 1H NMR spectra of the

hydrophosphination of 4-vinylpyridine with phenylphosphine by 2.

154

Figure 4.18.2. Initial and final (t = 2 hours) stacked 31P NMR spectra of the

hydrophosphination of 4-vinylpyridine with phenylphosphine by 2.

155

Figure 4.18.3. Final (t = 2 hours) 31P NMR spectrum of the hydrophosphination of 4-

vinylpyridine with phenylphosphine by 2.

156

4.6.7. Hydrophosphination of β-methylstyrene with phenylphosphine

Figure 4.19.1. Final 1H NMR spectrum of the hydrophosphination of β-methylstyrene

with phenylphosphine by 2.

157

Figure 4.19.2. Final 31P NMR spectrum of the hydrophosphination of β-methylstyrene

with phenylphosphine by 2.

158

Figure 4.19.3. Final 31P{1H} NMR spectrum of the hydrophosphination of β-

methylstyrene with phenylphosphine by 2.

159

Figure 4.19.4. Final 1H-1H COSY NMR spectrum of the hydrophosphination of β-

methylstyrene with phenylphosphine by 2.

160

Figure 4.19.5. Final 1H-31P HSQC NMR spectrum of the hydrophosphination of β-

methylstyrene with phenylphosphine by 2.

161

4.6.8. Hydrophosphination of ethyl acrylate with phenylphosphine

Figure 4.20.1. Initial and final (t = 12 minutes) stacked 1H NMR spectra of the

hydrophosphination of ethyl acrylate with phenylphosphine by 2.

162

Figure 4.20.2. Initial and final (t = 12 minutes) stacked 31P NMR spectra of the

hydrophosphination of ethyl acrylate with phenylphosphine by 2.

163

Figure 4.20.3. Final (t = 12 minutes) 31P NMR spectrum of the hydrophosphination of

ethyl acrylate with phenylphosphine by 2.

164

4.6.9. Hydrophosphination of methyl acrylate with phenylphosphine

Figure 4.21.1. Initial and final (t = 12 minutes) stacked 1H NMR spectra of the

hydrophosphination of methyl acrylate with phenylphosphine by 2.

165

Figure 4.21.2. Initial and final (t = 12 minutes) stacked 31P NMR spectra of the

hydrophosphination of methyl acrylate with phenylphosphine by 2.

166

Figure 4.21.3. Final (t = 12 minutes) 31P NMR spectrum of the hydrophosphination of

methyl acrylate with phenylphosphine by 2.

167

4.6.10. Hydrophosphination of acrylonitrile with phenylphosphine

Figure 4.22.1. Initial and final (t = 18 hours) stacked 1H NMR spectra of the

hydrophosphination of acrylonitrile with phenylphosphine by 2.

168

Figure 4.22.2. Initial and final (t = 18 hours) stacked 31P NMR spectra of the

hydrophosphination of acrylonitrile with phenylphosphine by 2.

169

Figure 4.22.3. Final (t = 18 hours) 31P NMR spectrum of the hydrophosphination of

acrylonitrile with phenylphosphine by 2.

170

4.6.11. Hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine

Figure 4.23.1. Initial and final (t = 2 hours) stacked 1H NMR spectra of the

hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine by 2.

171

Figure 4.23.2. Initial and final (t = 2 hours) stacked 31P NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with phenylphosphine by 2.

172

Figure 4.23.3. Final (t = 2 hours) 31P NMR spectrum of the hydrophosphination of 2,3-

dimethyl,1-3-butadiene with phenylphosphine by 2.

173

4.6.12. Hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine

Figure 4.24.1. Initial through final (t = 96 hours) stacked 1H NMR spectra of the

hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine by 2.

174

Figure 4.24.2. Initial through final (t = 96 hours) stacked 31P NMR spectra of the hydrophosphination of 2,3-dimethyl,1-3-butadiene with cyclohexylphosphine by 2.

175

Figure 4.24.3. Final (t = 96 hours) 31P NMR spectrum of the hydrophosphination of 2,3-

dimethyl,1-3-butadiene with cyclohexylphosphine by 2.

176

4.6.13. Hydrophosphination of styrene with diphenylphosphine

Figure 4.25.1. Initial and final (t = 23 hours) stacked 1H NMR spectra of the

hydrophosphination of styrene with diphenylphosphine by 2.

177

Figure 4.25.2. Initial and final (t = 23 hours) stacked 31P NMR spectra of the

hydrophosphination of styrene with diphenylphosphine by 2.

178

Figure 4.25.3. Final (t = 23 hours) 31P NMR spectrum of the hydrophosphination of

styrene with diphenylphosphine by 2.

179

4.6.14. Hydrophosphination of ethyl vinyl ether with phenylphosphine

Figure 4.26.1. Initial through final (t = 72 hours) stacked 1H NMR spectra of the

hydrophosphination of ethyl vinyl ether with phenylphosphine by 2.

180

Figure 4.26.2. Initial through final (t = 72 hours) stacked 31P NMR spectra of the

hydrophosphination of ethyl vinyl ether with phenylphosphine by 2.

181

Figure 4.26.3. Final (t = 72 hours) 31P NMR spectrum of the hydrophosphination of ethyl

vinyl ether with phenylphosphine by 2.

182

4.6.15. Hydrophosphination of phenylacetylene with phenylphosphine

Figure 4.27.1. Initial and final (t = 48 hours) stacked 1H NMR spectra of the

hydrophosphination of diphenylacetylene with phenylphosphine by 2.

183

Figure 4.27.2. Initial and final (t = 48 hours) stacked 31P NMR spectra of the

hydrophosphination of diphenylacetylene with phenylphosphine by 2.

184

Figure 4.27.3. Final (t = 48 hours) 31P NMR spectrum of the hydrophosphination of

diphenylacetylene with phenylphosphine by 2.

185

4.6.16. Attempted hydrophosphination of benzophenone with

diphenylphosphine

Figure 4.28.1. 1H NMR spectrum (t = 96 hours) of the attempted hydrophosphination of

benzophenone with diphenylphosphine by 2.

The main product was Ph2PPPh2.

186

Figure 4.28.2. 31P NMR spectrum (t = 96 hours) of the attempted hydrophosphination of

benzophenone with diphenylphosphine by 2.

The main product was Ph2PPPh2.

187

Figure 4.28.3. Final (t = 96 hours) 31P NMR spectrum of the attempted

hydrophosphination of benzophenone with diphenylphosphine by 2.

The main product was Ph2PPPh2.

188

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CHAPTER 5: CONCLUSIONS

5.1. α-Silylene elimination

In summary, tri-amidoamine ligated zirconium compounds are catalysts for the heterodehydrocoupling of amines and silanes with limited efficacy. They are also a poorly effective silane dehydrocoupling catalyst, but a series of observations from stoichiometric and trapping reactions with dimethylamino ligated zirconium complexes support the idea that these zirconium compounds engage in α-silylene elimination. This reactivity is unique compared to the prior reports of silane dehydrocoupling by group 4 metals, but the notion that Si–N coupling reaction prompts the α-silylene elimination provides new possibilities for promoting the generation of low-valent fragments.

5.2. Lanthanum catalyzed Si–N heterodehydrocoupling

La[N(SiMe3)2]3(THF)2, which is easily obtained in a one-step synthesis or by addition of THF to commercial La(N(SiMe3)2)3, is perhaps the most active lanthanide- based pre-catalyst for the heterodehydrocoupling of amines and silanes. With catalyst loadings below 1 mol %, dehydrocoupling was efficient in most cases. Coupling silanes with propylamines occurred on the minute time scale, though sometimes yield was lowered due to generation of poorly identified products. Generating quaternary silamines from triphenylsilane with propylamines was also achieved for the first time. Reactivity with bulkier amines was also facile, making tertiary silamines readily. In some cases, even new silamines were generated. Less sterically bulky secondary amines were readily

196

5.3. Ruthenium catalyzed P–C bond formation

Rp2 is an efficient hydrophosphination catalyst. Photo-activation by low intensity

UV-A light, low catalyst loading, fast reaction times, and lack of thermal dependence all make Rp2 a valuable catalyst. Similar to Fp2, styrenes and Michael acceptors are excellent substrates, but unlike Fp2, Rp2 is capable of hydrophosphination with primary phosphines. Preliminary results indicate that Rp2 also hydrophosphinates secondary phosphines. Though Rp2 doesn’t hydrophosphinated terminal alkynes, internal alkynes appear to have potential as unsaturated substrates. Overall, a commercial catalyst, Rp2, is photo-activated by a low-intensity commercial UV-A lamp for efficient hydrophosphination of a variety of substrates.

197

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