Zwitterionic Nickel Catalyst for Carbonylative Polymerizations

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements of the Degree

Master of Science

Bradley M. Schmidt

December, 2011

Zwitterionic Nickel Catalyst for Carbonylative Polymerizations

Bradley M. Schmidt

Thesis

Approved: Accepted:

______Advisor Dean of the College Dr. Li Jia Dr. Stephen Z. D. Cheng

______Faculty Reader Dean of the Graduate School Dr. Colleen Pugh Dr. George R. Newkome

______Department Chair Date Dr. Ali Dhinojwala

ABSTRACT

The goal of this research project was to develop a catalyst for the copolymerization of carbon monoxide (CO) and epoxides and/or aldehydes. Zwitterionic palladium and nickel complexes were synthesized that contained bidentate phosphine-borate ligands. Under the assumption that a polymerization mechanism similar to the established cobalt-catalyzed copolymerization of CO and aziridines is applicable, the zwitterionic nature of the complexes were expected to posses the high activity of cationic metal-acyl bonds, while maintaining the anionic nature required for ion pairing during the polymerization. Characterization of the nickel complex was completed through NMR spectroscopy, FTIR spectroscopy, and X-ray crystallography. Upon completion of the metal complex syntheses a variety of polymerization conditions were screened, and the products were characterized by NMR and IR spectroscopy.

Although the spectroscopic methods showed the system had activity, a pure polymer product was not obtained.

iii

ACKNOWLEDGMENTS

I would like to thank Dr. Li Jia for his guidance and financial support to make this project possible. I would also like to thank my group members Nishant Kumar, Joseph Scavuzzo, Chao

Wang, Sarang Bhawalkar, and Ilknur Babahan for their assistance with my research, and Jim

Engle for his help with X-ray crystallography.

Finally, I would like to thank my friends and family, especially my parents, who always support me in my endeavors.

TABLE OF CONTENTS

Page

List of Figures ………………………………………………………………………………………………………………………….. ix

List of Tables …………………………………………………………………………………………………………………………… xi

List of Schemes ………………………………………………………………………………………………………………………. xii

Chapter

I. Introduction ……………………………………………………………………………………………………………… 1

II. Historical Background

2.1 Origins of Carbonylation Chemistry ………………………………………………………….… 3

2.2 Polyamides via Carbonylative Polymerizations …………………………………………... 4

2.3 Polyesters via Carbonylative Polymerizations ………………………………………...… 15

2.4 Bidentate Phosphine Ligands in CO/Ethylene Polymerization …………………... 22

III. Experimental

3.1 Handling of Air Sensitive Materials …………………………………………………………... 28

3.2 Polymerization Procedure ……………………………………………………………………...… 29

3.3 Characterization ………………………………………………………………………………....…… 29

3.3.1 Nuclear Magnetic Resonance Spectroscopy (NMR) ………………….... 29

3.3.2 Fourier Transform Infrared Spectroscopy (FTIR) ……………………...... 30

3.3.3 X-ray Crystallography …………………………………………………………...…... 30

3.4 Anhydrous Solvents, Deuterated Solvents, Reagents, and Monomers …...... 31

3.5 Monodentate Phosphine-Borane Ligand Synthesis ……………………………...... 32

3.5.1 Synthesis of BrC6H4-2-(CH2PPh2) …………………………………………...... 32

3.5.2 Synthesis of bromodiphenylborane ……………………………………...... 33

3.5.3 Synthesis of Ph2PCH2C6H4-2-(BPh2) ……………………………………...... 33

3.5.4 Attempted Cobalt Catalyst Synthesis …………………………………...... 34

3.6 Bidentate Pyridine-Borane Ligand Synthesis ………………………………………...... 34

3.6.1 Synthesis of Hydrogen diphenyldi(2-pyridyl)borate ………………...... 34

3.6.2 Synthesis of Trityl diphenyldi(2-pyridyl)borate ……………………...... 35

3.7 Bidentate Phosphine-Borane Ligand Synthesis …………………………………...... 36

3.7.1 Synthesis of Ph2PCH2Li(TMEDA) …………………………………………...... 36

3.7.2 Synthesis of [BPh2(CH2PPh2)2][Li(TMEDA)2] ………………………...... 36

3.7.3 Synthesis of 5-azonia-spiro[4.4]nonane bromide (ASNBr) ……...... 37

3.7.4 Synthesis of [Ph2B(CH2PPh2)2][ASN] …………………………………...... 37

3.8 Palladium Complex Synthesis ……………………………………………………………...... 38

3.8.1 Synthesis of [Ph2B(CH2PPh2)2PdMe2][ASN] …………………………...... 38

3.8.2 Synthesis of [(iPr)2EtNH][BPh4] ……………………………………………...... 38

3.8.3 Synthesis of Ph2B(CH2PPh2)2PdMe(THF) ……………………………...... 38

3.9 Attempted Polymerizations with Palladium Complex …………………………..... 39

3.9.1 Propylene Oxide Attempted Polymerization ………………………...... 39

3.9.2 Benzaldehyde Attempted Polymerization ……………………………...... 39

3.10 Synthesis of Nickel Complex ……………………………………………………………...... 39

3.10.1 Synthesis of (TMEDA)Ni(acac)2 .…………………………………………...... 39

3.10.2 Synthesis of Dimethylaluminumethoxide (Me2AlOEt) …………...... 40

3.10.3 Synthesis of (TMEDA)NiMe2 …………………………………………………..... 40

3.10.4 Synthesis of [Ph2B(CH2PPh2)2NiMe2][ASN] …………………………...... 41

3.10.5 Synthesis of Ph2B(CH2PPh2)2NiMe(CH3CN) ……………………………..... 41

3.10.6 Synthesis of Ph2B(CH2PPh2)2NiCOMe(CO) ……………………………...... 42

vii

3.11 Attempted Polymerizations with Nickel Complex ...... 42

3.11.1 Propylene oxide (run 1) ...... 42

3.11.2 Propylene oxide (run 2) ...... 42

3.11.3 THF (run 3) ...... 43

3.11.4 Butyl Aziridine (run 4) ...... 43

3.11.5 Butyl Aziridine (run 5) ...... 43

3.11.6 N-benzylidenemethylamine (run 8) ...... 43

3.11.7 Butyl Aziridine (run 6) ...... 44

3.11.8 Butyl Aziridine (run 7) ...... 44

3.11.9 Aziridine (run 9) ...... 44

3.11.10 Ethyl Aziridine (run 10) ...... 44

3.11.11 Ethyl Aziridine (run 11) ...... 45

3.11.12 Ethyl Aziridine (run 12) ...... 45

IV. Results and Discussion

4.1 Investigation of monodentate phosphine-borane ligand ………………………….. 47

4.2 Investigation of the Bidentate Pyridine-Borate Ligand ……………………………... 51

viii

4.3 Synthesis and Characterization of Bidentate Phosphine-Borate Ligand ….... 52

4.4 Synthesis and Attempted Polymerizations with Palladium Complex ………... 55

4.5 Synthesis and Characterization of Nickel Complex ………………………………...... 58

4.6 Attempted Polymerizations with Nickel Complex …………………………………..... 68

V. Conclusion ………………………………………………………………………………………………………...... 75

VI. References ...... 76

LIST OF FIGURES

Figure Page

2.1 GPC trace of polymer from cobalt-catalyzed copolymerization of CO and aziridine …..... 9

2.2 (○) Mn by GPC; (+) Mn by end-group analysis; (∆) PDI by GPC for the copolymerization

of n-ehtylaziridine and CO ….…...... 10

2.3 FTIR spectrum of CO/N-butylaziridine copolymerization employing

CH3COCo(CO)3PPh3 (-) or CH3COCo(CO)3P(o-tolyl)3 (-) ……………………………………..…………. 11

2.4 Microstructures of CO / propylene oxide copolymerization …………………………………...... 16

2.5 Examples of the variety of bidentate phosphine ligands synthesized ……………………...... 22

2.6 Variety of bis(phosphino)borates synthesized by the Peters group ………………………...... 26

4.1 (a) 1H NMR spectrum of monodentate phosphine-borane ligand ………………………………...... 48

4.1 (b) 31P NMR spectrum of monodentate phosphine-borane ………………………………………...... 49

4.1 (c) 11B NMR spectrum of monodentate phosphine-borane ligand ………………………………….. 49

4.2 (a) 1H NMR spectrum of trityl-coordinated pyridine-borate ligand ……………………………….... 52

4.2 (b) 13C NMR spectrum of bidentate pyridine-borate ligand …………………………………………..... 53

4.3 1H NMR spectrum of bidentate phosphine borate ligand ………………………………………...... 54

4.4 1H NMR of palladium complex ………………………………………………………………………………...... 58

4.5 1H NMR spectrum of palladium catalyzed CO/propylene oxide copolymerization …...... 59

4.6 (a) 1H NMR spectrum of ligand-coordinated dimethyl nickel complex …………………………..... 61

4.6 (b) 31P NMR spectrum of ligand-coordinated dimethyl nickel complex ………………………….... 62

4.7 1H NMR spectrum of acetonitrile-coordinated nickel complex ………………………………..... 63

4.8 1H NMR spectrum of 5-coordinate nickel complex …………………………………………………..... 65

4.9 (a) 1H NMR spectrum of final zwitterionic nickel catalyst ……………………………………………..... 65

4.9 (b) 31P NMR of final zwitterionic nickel catalyst …………………………………………………………….... 66

4.10 Crystal structure of zwitterionic nickel complex ………………………………………………………... 67

4.11 1H NMR spectrum of product from CO/THF copolymerization trial ………………………….... 70

4.12 1H NMR spectrum of product from CO/butyl aziridine copolymerization in THF ……..... 71

4.13 1H NMR spectrum of product from CO/aziridine copolymerization trail …………………..... 72

4.14 1H NMR spectrum of product from CO/ethyl aziridine copolymerization in THF ……...... 73

4.15 1H NMR spectrum of CO / 1-hexene copolymerization in THF ...... 74

LIST OF TABLES

Table Page

2.1 Effect of bidentate ligand backbone length on CO/ethylene copolymerization

using a Pd catalyst ……………………………………………………………………………………..…………...... 24

4.1 Attempted polymerization conditions with zwitterionic nickel complex ………………….... 69

xii

LIST OF SCHEMES

Scheme Page

2.1 Hydroformylation reaction ………………………………………………………………………………………..... 3

2.2 Reactions of 1 with CO and insertion of the imine into the metal-acyl bond …………...….. 5

2.3 Copolymerization of CO and aziridine ………………………………………………………………………..... 6

2.4 Proposed mechanism of alternating CO/aziridine copolymerization ………………………….... 6

2.5 Formation of amine end group in CO/aziridine copolymerization ….………………………….... 7

2.6 Proposed model for amine formation in CO/aziridine copolymerization …….……………..... 7

2.7 Proposed mechanism of branching in the copolymerization of CO and aziridine ….…...... 8

2.8 Two possible routes of termination resulting in bimodal GPC distribution ……….……….... 9

2.9 Competition of free phosphine and monomer for acyl site on propagating chain ……... 13

2.10 Chain branching mechanism in the copolymerization of CO and N-butylazetidine ..…... 14

2.11 Equilibrium leading to ester formation in the copolymerization of CO and

substituted azetidine …………………………………………………………………………………………………. 14

2.12 Carbonylative polymerization of propylene oxide with CO ………………………………………... 16

2.13 Proposed mechanism of initiation in CO/PO copolymerization ..………………………………… 17

2.14 Proposed mechanism of propagation in CO/PO copolymerization ..…………………………… 17

2.15 Proposed mechanism for the copolymerization of propylene oxide and CO ……………... 19

xiii

2.16 Newly proposed mechanism for the copolymerization of propylene oxide ……………….. 20

2.17 Proposed mechanism for the carbonylative polymerization of epoxides ……………………. 21

4.1 Synthesis of monodentate phosphine-borane ligand ……………………………………………….... 48

4.2 Synthesis of monodentate phosphine-borane cobalt complex ………………………………….. 50

4.3 Synthesis of bidentate pyridine-borate ligand ……………………………………………………………. 52

4.4 Synthesis of bidentate phosphine-borate ligand ………………………………………………………... 54

4.5 Synthesis of zwitterionic nickel complex ……………………………………………………………………. 60

xiv

CHAPTER I

INTRODUCTION

Interest in the synthesis of poly(β-peptide)s and poly(β-hydroxyalkanoate)s stems from their potential use in a broad range of applications.1-3 Carbonylative polymerizations offer the advantage of inexpensive starting materials including CO, aziridines, imines, epoxides, and aldehydes. CO also possesses the advantage of being a renewable resource. Previous work by

Jia and coworkers established a successful system for the copolymerization of CO and aziridines to produce poly(β-peptide)s.4-10 The hopes of improving upon this system and developing an approach to realize a commercially viable carbonylative polymerization of polyesters was the motivation for this research.

Polyester synthesis via CO-epoxide copolymerization has been accomplished by a number of groups. However, the polymerization is always gives undesirable side products and low molecular weight.10-19 Previous reports mainly use a cobalt catalyst with a cocatalyst capable of activating the Co-acyl bond. The approach taken here is different from previous attempts in that zwitterionic nickel and palladium complexes have been designed to effect the

CO-epoxide copolymerization. The palladium analogue of this system has previously been shown to be successful for the copolymerization of CO and ethylene.20

Assuming that the mechanism for the Co-catalyzed copolymerization of CO and aziridines or epoxides is operative here, a zwitterionic complex satisfies the essential properties

1 required by the mechanism. First, the cationic metal center improves the reactivity of the metal-acyl bond. Also, the anionic ligand of the complex should allow the necessary ion-pair between the metal complex and propagating chain to exist in solution. Synthesis and characterization of the metal complexes and screening of monomers and polymerization conditions were carried out.

CHAPTER II

HISTORICAL BACKGROUND

2.1. Origins of Carbonylation Chemistry

The introduction of a carbonyl moiety into organic and inorganic substrates is known as carbonylation. Specifically, transition metal-catalyzed carbonylations have found the most success, and are the standard for production of many organic carbonyl compounds.21 One of the most widely used carbonylation reactions is hydroformylation. Hydroformylation is the reaction of a C-C double bond with hydrogen and carbon monoxide to produce aldehydes as the major product (Scheme 2.1). The reaction was accidently discovered in 1938 by the German

Scientist Otto Roelen while he was attempting to increase the chain-length of Fischer-Tropsch hydrocarbons. It was not until the mid-1950s, however, that the process was commercialized by

BASF. The first generation hydroformylation systems were based on cobalt catalysts, but

Scheme 2.1. Hydroformylation reaction.21

under harsh reaction conditions (200 – 350 bar and 150 – 180°C).21 Perhaps the most important development was the process that allowed for the recycling of the catalyst, making carbonylation reactions such a cost-effective process.21,23 In the 1960s researchers at Shell discovered the importance of phosphine ligands being able to replace CO. This resulted in more stable metal-carbonyls and allowed lower CO pressures to be used in the reaction. Second generation processes that used rhodium as the metal were then commercialized in the 1970s by the Celanese Corporation. The main advantage of the rhodium system was its thermal stability that allowed easier catalyst recycling.24

A variety of transition metals have since been used in carbonylation reactions, including cobalt, nickel, ruthenium, rhodium, palladium, iridium, and platinum.25 Research has now progressed into ligand modification for tailoring catalyst activity and selectivity for specific applications. Carbonylations now find widespread use on the industrial scale and have begun to be employed in polymerizations.

2.2. Polyamides via Carbonylative Polymerization

The palladium-catalyzed alternating copolymerization of carbon monoxide and alkenes has been well studied.26 The facile nature of this reaction is due to the ability of the alkene to form the necessary π-complex with palladium before insertion into the metal-acyl bond.

However, the analogous reaction of a C=N double bond is much more difficult because of the preference for σ-coordination of the lone pair on nitrogen to the metal.27 Nearly simultaneously

4 and independently, Sen28 and Arndtsen29 reported the first observation of imine insertion into a metal-acyl bond. Sen et al. used various bidentate phosphine and nitrogen-donor ligands on Pd and tested the catalytic activity of imine insertion (Scheme 2.2).28 Arndtsen, however, employed bipyridiine (bipy) as the ligand and observed imine insertion. Both groups reported that the reason CO/imine copolymerization was not observed was due to the stability of the cyclic resting state in which the carbonyl group of the insertion product is coordinated to the metal center (structure 3 in Scheme 2.2), which prohibits further monomer coordination.

Scheme 2.2. Reactions of 1 with CO and insertion of the imine into the metal-acyl bond.28

The Jia group realized the copolymerization of carbon monoxide and aziridine employing Heck’s cobalt complex (CH3COCo(CO)3PPh3) as the catalyst the synthesis of poly-β- peptides (Scheme 2.3).4 A propagation mechanism was proposed.

Scheme 2.3. Copolymerization of CO and aziridine.4

First, nucleophilic attack of aziridine on the Co-coordinated acyl group occurs, breaking the Co-acyl bond. This results in formation of a cobaltate anion and aziridinium cation, which is analogous to reactions of acyl chlorides.30 The cobaltate anion then induces ring opening of the coordinated aziridine, followed by CO migratory insertion into the cobalt-carbon bond (Scheme

2.4).4

4 Scheme 2.4. Proposed mechanism of alternating CO/aziridine copolymerization.

It was quickly realized that a more convenient route to an active catalyst was the in situ

5 generation of HCo(CO)3PPh3. This complex behaved in much the same way as Heck’s complex.

However, instead of the polymer containing an acetyl end group, an amine end group is present, caused by the initial insertion of aziridine (Scheme 2.5).5,31 Although the success of the copolymerization showed much promise, a number of side reactions needed to be understood.

Scheme 2.5. Formation of amine end group in CO/aziridine copolymerization.5

Through the synthesis of model compounds and 1H NMR spectroscopy, it was discovered that in addition to amide repeat units there were amine repeat units in the backbone. It was proposed that the formation of the amine units was caused by the repetitive insertion of aziridine. This occurs as a result of monomer competing with the cobaltate anion in the ring-opening process (Scheme 2.6).5 Nucleophilic attack of cobaltate results in amide formation, whereas attack of aziridine results in amine formation. Consistent with the proposed mechanism, it was found that lowering the concentration of aziridine via sequential monomer additions resulted in a decrease in amine microstructure.

Scheme 2.6. Proposed model for amine formation in CO/aziridine copolymerization.5

Another undesirable side reaction in the copolymerization of CO and aziridine was branching and eventually crosslinking. Branching can occur through nucleophilic attack of an amine unit on one chain at the carbonyl group of a propagating chain (Scheme 2.7).5 A decrease in solubility is observed with an increase in amine concentration and crosslink density.

Scheme 2.7. Proposed mechanism of branching in the copolymerization of CO and aziridine.5

Finally, the mechanism of chain termination was studied. Termination was found to occur via one of two routes, as evidenced by the bimodal distribution in the gel permeation chromatography (GPC) traces of the products (Figure 2.1).5 In the first route, chain-transfer occurs via proton abstraction by the cobaltate anion on the acylaziridinium intermediate. The resulting active catalyst (HCo(CO)3L) may initiate propagation of another chain, or may reinitiate the same chain and reform the cobaltate anion (Scheme 2.8). The other possibility when the protic catalyst is used is combination. Combination occurs by nucleophilic attack of an amine end group on the carbonyl group of a propagating chain. The result is a high molecular weight product, seen as the sharp peak in Scheme 2.8.5

Figure 2.1. GPC trace of polymer from cobalt-catalyzed copolymerization of CO and aziridine.5

Scheme 2.8. Two possible routes of termination resulting in bimodal GPC distribution.5

The most straightforward approach to solving the problems associated with amine formation is by using a monomer that prohibits such reactions. The alternative monomers employed were N-substituted aziridines, mainly alkyl substituted aziridines. The advantage of such monomers is that they are less nucleophilic than the unsubstituted aziridine. This favors the cobaltate in the competition between the monomer and cobaltate anion for ring-opening.

With no amine microstructure being present, the problem of branching and crosslinking no longer exists.6 When N-methylaziridine and N-ethylaziridine were employed, there was no

9 detectable amine in the NMR spectra. However, the copolymerization of CO and N- methylaziridine terminated after only 20 catalyst turnovers, tentatively attributed to crystallization- and precipitation-caused blockage of the access of the monomer to the catalyst at the chain end.6 In addition, in the absence of amine formation, the molecular weight distribution dramatically narrowed, and a linear relationship between the number-average molecular weight and the monomer to catalyst ratio was observed (Figure 2.2).6 All of this indicates a living polymerization.

Figure 2.2. (○) Mn by GPC; (+) Mn by end-group analysis; (∆) PDI by GPC for the copolymerization of n-ehtylaziridine and CO.6

To better understand the mechanism of polymerization, in situ FTIR spectroscopy was used to monitor polymer formation and the various intermediates involved in the reaction.

Figure 2.3 shows the IR spectra of the carbonyl region of a typical polymerization in which an

7 acylcobalt catalyst is employed (CH3C(O)Co(CO)3PPh3 or CH3C(O)Co(CO)4P(o-tolyl)3). From this study it was determined that the catalyst resting state is the ammonium-cobaltate ion pair,

10 seen as the peak near 1890 cm-1.7 This discovery was used to synthesize block copolymers by sequential monomer addition. Also notable in Figure 2.3 is the lability of the phosphine ligand which is easily replaced by CO in solution as evidenced by the peak at 1993 cm-1.7 This type of ligand exchange is important when comparing different phosphines due to the competition between aziridine and phosphine for coordination to the propagating acyl site.

CH3COCo(CO)3PPh3

Figure 2.3. FTIR spectrum of CO/N-butylaziridine copolymerization employing CH3COCo(CO)3PPh3 (-) or 7 CH3COCo(CO)3P(o-tolyl)3 (-).

In addition to identifying the intermediates of the polymerization, kinetic data was also studied through in situ FTIR studies. The rate-determining step is the ring-opening process by the cobaltate anion. The reaction is first-order in catalyst as evidenced by the linear relationship of initial rate vs. catalyst concentration. The reaction was also found to be first-order in the aziridine monomer and zero order in CO pressure.7 The final variable to understand was that of the effect of different phosphine ligands on the polymerization.

A number of different phosphine ligands were tested with varying effects on catalyst activity. The two parameters that most greatly affect catalysis are the cone angle of the phosphine ligand and its electron donating ability. The ligands tested were PCy3, PMe2Ph, PPh3,

8 P(para-F-Ph)3, P(meta-F-Ph)3, and P(ortho-tolyl)3. The consequence of the electron donating ability of each ligand is reflected in the equilibrium position of the conjugated or free phosphine during the polymerization (Equation 1).7 The cone angle determines the degree to which the

(1) free phosphine will compete with monomer for the acyl site on the propagating chain (Scheme

8 2.9). Of the ligands tested, P(ortho-tolyl)3 is the most electron donating, therefore complete conversion to the free phosphine is observed via in situ FTIR.8 Aside from the effect in which the donating ability of the phosphine has on the equilibrium, electron donation also plays a role in the activity of the initial catalyst species. The electron donating ability of the ligand strengthens the acetyl-Co bond by increasing the back donation from the metal’s dxz and dyz orbitals to the

π* orbital of the acetyl C=O bond, making it less susceptible to nucleophilic attack by aziridine.8

Steric factors must also be considered when examining the equilibrium depicted in Scheme 2.9.

The P(ortho-tolyl)3 ligand, although considered to be free during the course of polymerization, does not compete well with monomer for the acyl site on the propagating chain due to its large cone angle. Therefore, for multiple reasons it is the most suitable ligand for catalysis and was used in all further polymerizations.

Scheme 2.9. Competition of free phosphine and monomer for acyl site on propagating chain.8

In addition to aziridine monomers, the Jia group also explored copolymerization reactions of CO and azetidine. Three monomers were examined in the study, which included N- n-butylazetidine, N-iso-butylazetidine, and N-benzylazetidine.9 A similar mechanism to that proposed for the aziridine copolymerization is assumed for these reactions. Nucleophilic attack by azetidine on the propagating acyl group is followed by ring opening by the cobaltate anion.

Again, amine repeat units were observed when the monomer concentration was increased, due to competitive ring opening between monomer and cobaltate. This behavior was used to synthesize poly(amide-co-amine) copolymers in which repetitive monomer additions result in a gradient distribution of amine content. Amine microstructure is high at the head of the polymer and falls to approximately zero in the second half of the chain until more monomer is added.9

Slightly different to the aziridine copolymerizations, termination can occur in a polymer with high amine concentration via nucleophilic attack of an amine unit on a propagating azetidinium chain end. The result is a quaternary ammonium group and chain branching (Scheme 2.10).9

The other possibility is intramolecular attack of an amine unit, causing cyclization and the formation of a γ-lactam side product.9

Scheme 2.10. Chain branching mechanism in the copolymerization of CO and N-butylazetidine.9

When CH3COCo(CO)3P(o-tolyl)3 is used as the catalyst and THF as the solvent, the carbonylative polymerization of all three azetidine monomers results in the formation of poly(amide-co-ester)s. However, under the same reaction conditions in the absence of azetidine, polyester formation does not occur. The reason for this observation is due to an equilibrium between the acylazetidinium intermediate and an acyloxonium intermediate

(Scheme 2.11).9 The bulkier N-iso-butylazetidine and N-benzylazetidine shift the equilibrium in favor of the acyloxonium intermediate, resulting in higher ester incorporation. Similarly, lower ester incorporation was observed with higher azetidine monomer loading, and a gradient distribution of the ester microstructure is always seen.10 The polymerization had living characteristics, indicated by the linear increase of molecular weight with increasing monomer concentration. Although not a requirement of a living polymerization, the observed PDI in all polymerizations was relatively narrow (1.18 – 1.55).10 In accordance with the observed THF incorporation, Jia and coworkers, as well as many other research groups,10-19,33 have explored the possibilities of polyester formation using carbonylative polymerizations.

Scheme 2.11. Equilibrium leading to ester formation in copolymerization of CO and substituted azetidine.9

2.3. Polyester via Carbonylative Polymerization

As early as the 1960s carbonylative polymerizations of trioxane and epoxides were reported but with limited molecular weight and undesirable side products.11,12 The products of such polymerizations are poly(hydroxyalkanoate)s, a biodegradable class of polymers with mechanical properties similar to isotactic polypropylene.32 Despite the advantages in development of such a polymerization, little research in the area was done until a patent by

Drent et al. was published in 1994. They reported the carbonylative polymerization of epoxides

13 with CO using Co2(CO)8 and 3-hydroxypyridine as the catalyst and cocatalyst, respectively.

However, Drent claimed β-lactones as the major product and polyester as a side product. Since, reports from the groups of Osakada,14 Rieger,15-18 Alper,19 Coates33, and Jia10 have appeared in the literature.

The use of a cocatalyst to activate the metal-acyl bond is necessary for the carbonylative polymerization of epoxides due to their lower nucleophilicity. The Osakada group took multiple approaches to improve upon the polymerization. Similar to the report by Drent, they employed

3-hydroxypyridine in combination with Co2(CO)8 in THF solvent and 50 atm CO pressure

(Scheme 2.12).14 They report the formation of polyester containing three different microstructures (Figure 2.4).14 Microstructure A is formed by the ring-opening of propylene oxide at the less substituted carbon atom on the ring, whereas microstructure B arises from ring-opening at the more substituted carbon atom. The third, C, is formed by the ring-opening copolymerization of THF. The polymers were characterized by a combination of 1H and 13C NMR spectroscopy and GPC analysis. The system produced polymers of number-average molecular weight of only 1300 Da with a PDI of 1.99.14

Scheme 2.12. Carbonylative polymerization of propylene oxide with CO.14

14 Figure 2.4. Microstructures of CO/propylene oxide copolymerization.

In addition to 3-hydroxypyridine, which showed the highest activity, the group also used pyridine, 2-hydroxypyridine, and 4-hydroxypyridine as cocatalysts to produce polyester, but with lower molecular weight and higher THF incorporation. A number of non-active cocatalysts were also attempted.

Although a mechanism of initiation had previously been reported by Rieger et al.,18 the

Osakada group suggested the following mechanism. Initiation occurs via activation of the Co-Co bond by the amine, forming an ion pair. This allows for insertion of propylene oxide between the cobalt centers and facilitates ring opening of the monomer, followed by CO insertion in the

Co-C bond (Scheme 2.13).14 A bimolecular propagation mechanism is proposed in which the OH end group of an acylcobalt intermediate undergoes nucleophilic addition to another acyl-cobalt bond to form a new ester bond and release a hydrido-cobalt complex (Scheme 2.14).14

Monomer can then insert into the Co-H bond and regenerate the acylcobalt intermediate. The support for this mechanism is based on the ease of insertion of the OH group into a Co-C bond

16 compared to the less nuleophilic propylene oxide. This propagation mechanism was later disputed.

Scheme 2.13. Proposed mechanism of initiation in CO/PO copolymerization.14

Scheme 2.14. Proposed mechanism of propagation in CO/PO copolymerization.14

In addition to pyridine derivatives as a cocatalyst, Osakada and coworkers attempted to use Ru3(CO)12 with success. Again, the resulting polyester contained the same microsctures A,

B, and C from Figure 2.4. The most successful molar ratio of Ru:Co was about 1:3. Addition of bipyridine or N,N,N’,N’-tetramethylethylenediamine (TMEDA) gives the highest molecular

14 weight polymer (Mn = 3000 Da) with the highest amount of microstructure A. Despite the

17 success of the Osakada group, the molecular weight restrictions inhibit the product from finding any commercial value.

Much of the research in this area has been conducted by Bernhard Rieger and coworkers, who originally reported polyester formation as the major product using the same system that Drent had previously employed.18 Through in situ FTIR monitoring of the reaction, they detected formation of lactone but never more than 15% of the total epoxide consumed at one time. They also noted that the lactone peak grows independently of the polyester peak.

This led them to determine that the polymerization mechanism does not involve a lactone intermediate, and polyhydroxybutyrate (PHB) is formed directly from propylene oxide and CO.18

Further evidence is supported by the fact that polymerization with enantiomerically pure propylene oxide results in a purely isotactic polymer with retention of configuration, i.e. R-PO gives R-PHB.17

Rieger and coworkers focused much of their research in the subject on determining the mechanism of polymerization. For the original system employed, the group proposed the following mechanism (Scheme 2.15).18 Initiation occurs by cleavage of the Co-Co bond by

- pyridine, resulting in a Co(CO)4 anion, which is protonated by the hydroxyl group on pyridine.

HCo(CO)4 then ring-opens epoxide to give compound 2, a reaction previously reported by

Heck.34 CO insertion in the Co-C bond readily occurs resulting in the acyl complex. From here, intramolecular attack of the OH end group on the acyl-Co bond produces the lactone side product. The exact mechanism of polymer formation was still in question, but Rieger proposed that pyridine-assisted electrophilic attack of cobaltate on the less hindered carbon of the epoxide results in complex 5, and propagation continues in this manner to generate PHB.18

Scheme 2.15. Proposed mechanism for the copolymerization of propylene oxide and CO.18

Further research led to the discovery that Ph3Si[Co(CO)4] with pyridine is also an effective catalyst for the polymerization, and yields a product with a triphenylsilyl end group.15

In a 2005 publication Rieger and coworkers used in situ FTIR along with density functional theory (DFT) calculations to better understand the roll of the pyridine cocatalyst and the mechanism. A previous study by the group confirmed the need of a Lewis acid for epoxide activation to allow for cobaltate ring-opening and subsequent CO insertion.16 With the system

+ employed, Ph3Si acts as the Lewis acid and serves to form a stable end group, reducing the possibility of β-lactone formation. In the absence of pyridine, the product of this reaction is stable; however, upon the addition of pyridine, immediate PHB formation occurs, as can be seen in the IR spectrum. This prompted the authors to suggest a new mechanism of

+ 15 polymerization (Scheme 2.16), where LA is the Lewis acid Ph3Si . They believe that cleavage of the cobalt-acyl bond by pyridine occurs first, resulting in formation of an acylium cation and cobaltate anion. Interaction of the acylium cation with propylene oxide activates the monomer,

19 making it susceptible to ring opening by the cobaltate anion. This results in formation of an ester group and a new Co-C bond to which CO insertion readily occurs.

Scheme 2.16. Newly proposed mechanism for the copolymerization of propylene oxide.15

Further support for this proposed mechanism was suggested by DFT calculations. The ring-opening reaction was found to be as predicted. A model system employing ethylene oxide in the presence of an acyl-pyridinium species showed a backside attack of the cobaltate anion on the epoxide ring. In addition, energies of activation were calculated for a number of steps in the mechanism. It was determined that the attack of pyridine on the acyl-cobalt bond has an activation energy of 44 kJ mol-1 and a reaction energy of +23 kJ mol-1, but the ring-opening process has an energy yield of -180 kJ mol-1, driving the reaction.15 However, the proposal of intermediate 7 in Scheme 2.16 has been highly disputed, as it suggests multiple activation processes. A more reasonable mechanism was elucidated by the Jia group.16

Jia and coworkers employed their established CO/aziridine copolymerization catalyst

(CH3COCo(CO)3P(o-tolyl)3) with a pyridine derivative as the cocatalyst in the CO/epoxide copolymerization, and found it to be successful in producing polyester. By employing tert-butyl pyridine, molecular weights as high as 4300 Da were achieved, and with m-methoxypyridine

20 molecular weights up to 9600 Da were reported. Also, it was found that by running the polymerization with aziridine and epoxide it was possible to synthesize poly(β-alanoid-block-β- hydroxypropionate)s. Due to the higher nucleophilicity of aziridne compared to ethylene oxide, no polyester formation occurs until complete consumption of the azridine monomer. The synthesis of such a polymer led the group to propose a mechanism similar to the carbonylative polymerization of azridine (Scheme 2.17).10 Pyridine insertion into the Co-acyl bond forms a pyridine-acyl intermediate, to which nucleophilic attack of epoxide can occur. From there the mechanism is parallel to that of the aziridine carbonylative polymerization. The cobaltate anion induces ring-opening, followed by migratory insertion of CO into the Co-carbon bond. Although the details of the reaction have been clarified, the low molecular weight of the polyester product remains the biggest challenge, and research continues in search of a more active catalyst system.

Scheme 2.17. Proposed mechanism for the carbonylative polymerization of epoxides.10

2.4. Bidentate Phosphine Ligands in CO/Ethylene Copolymerization

The design of bidentate phosphine ligands for specific catalytic functions has been an area of research for decades. Their attractiveness lies in the ability to tailor a catalyst’s activity by easily manipulation of the ligand. The main characteristic of diphosphine ligands that determine their properties is their bite angle. The bite angle can affects the steric and electronic properties of the ligand. The bite angle, as defined by Casey and Whiteker in 1990, is the preferred chelation angle, determined only by ligand backbone constraints and not by metal valence angles, as determined by molecular mechanics calculations.35 The interest in altering these ligands for a variety of functions has led to many new developments in the area and many new ligand designs (Figure 2.5).36 To better characterize the effects of ligand modification, a distinction is made between the steric bite angle effect and the electronic bite angle effect.36

Figure 2.5. Examples of the variety of bidentate phosphine ligands synthesized.36

The steric bite angle effect is related to the steric interactions between ligands or ligand and substrate that are generated by changing the ligand backbone but keeping the substituents at the phosphorous donor atoms the same. Steric interactions of this type can affect transition state energies and the catalyst resting state, both of which impact the overall catalyst activity and selectivity.36

The electronic bite angle effect is related to the electronic changes in the metal center when changing the ligand bite angle.37 It can be described as an orbital effect because the ligand’s bite angle determines the metal hybridization and therefore orbital energies and reactivity.36 As a consequence, this effect can stabilize or destabilize the reaction’s initial, final, or transition state energies. However, this is difficult to study because by changing the substituents at the phosphorous donor atoms, the steric properties also change.

One of the first bidentate phosphine ligands used in CO/ethylene copolymerization was

1,2-bis(diphenylphosphino)ethane (dppe) on a palladium metal center, introduced by Drent.38

He found that compared to the typical PPh3 ligands employed at the time, the reaction rates with catalysts containing bidentate ligands were many orders of magnitude faster. The reason is due to the cis coordinating nature of the bidentate ligand as opposed to the trans coordination when PPh3 ligands are employed because of steric restraints. Cis coordination provides a vacant site on the metal cis to the propagating chain, resulting in a lower energy transition state for insertion. As evidence of the large impact bite angle has on catalyst activity,

Drent also employed 1,2-bis(diphenylphosphino)propane (dppp) and 1,2- bis(diphenylphosphino)butane (dppb) (Table 2.1).39 The Pd catalyst containing dppp shows the highest activity because the bite angle is closest to 90°. Based on electron counting and the 18

23 electron rule, group 10 transition metals such as Pd most prefer a square planar geometry.

Therefore, 90° is an optimal bite angle for a bidentate ligand, resulting in a lower energy transition state for insertion reactions. As seen in Table 1, dppe- and dppb-containing catalysts both produce polymer but not nearly as efficiently.

Table 2.1. Effect of bidentate ligand backbone length on CO/ethylene copolymerization using a Pd 39 catalyst.

The active species in the propagation mechanism is a Pd complex in which the ligand occupies two sites and the growing chain occupies a third. The fourth may be occupied by an anion, a solvent molecule, the carbonyl group of the growing chain, or the next monomer. An in-depth study was conducted on CO insertion into Pd-alkyl bonds40,41 and alkene insertion into

Pd-acyl bonds.41,42 The alkene insertion process is irreversible and the thermodynamic driving force of the polymerization.

Many studies have been conducted on the migratory insertion of CO into methyl- palladium bonds, better described as nucleophilic attack of the alkyl group on the carbon of the carbonyl ligand. Insertion into cationic complexes is one order of magnitude faster than neutral complexes.39 In addition, the activities of the complexes containing a dppp or dppb ligand were much faster than those with dppe. Hoffmann and co-workers supported this observation with theoretical studies on the reaction mechanism.43 They found that during the migration of the

24 methyl group toward the CO ligand, the ligand residing next to the methyl group followed its movement, thereby enlarging the bite angle of the diphosphine. This movement is easier for the dppp or dppb ligands than the more rigid dppe ligand, in which the energy of the transition state becomes much higher. In this manner, the stabilization of the transition state by the larger diphosphines can be considered an electronic bite angle effect.

The effect of the diphosphine ligand bite angle on alkene insertion into Pd-acyl bonds is not as well understood. Differing opinions have been reported as to the observed trends when different ligands are employed. It is agreed upon, however, that after alkene insertion the newly formed ketone oxygen occupies the fourth coordination site in the catalyst resting state.

Dekker reports that β-hydride elimination at this stage, resulting in unsaturated ketones, is fastest when using small bite angle diphosphines such as dppe.42 However, van Leeuwen and co-workers report the opposite trend. In either case, the preference for alternating

CO/ethylene insertion depends on a number of factors.

Kinetic studies by Brookhart determined that the barrier for ethylene insertion into a

Pd-acyl bond is lower than that of CO insertion into a Pd-alkyl bond.41 Faster ethylene insertion

(102) is offset by preferred coordination of CO over ethylene (104), and further stabilized by the fact that ethylene has better solubility than CO in most reaction media (~10).36 The steric and electronic bite angle effects of diphosphines on the copolymerization have been well studied, but the Peters’ group further expanded the investigation by incorporating anionic phosphine- borate ligands to form zwitterionic complexes.42

Over the past decade, Jonas Peters and co-workers synthesized and tested the activity of a variety of zwitterionic bis(phosphino)borate platinum and palladium complexes in

CO/ethylene copolymerization.20,43-48 Some examples of the diversity of ligands are shown in

Figure 2.6.46 In-depth analysis of the activities of a formally zwitterionic complex

[Ph2B(CH2PPh2)]PdMe(THF) compared to a traditionally employed cationic complex (dppp) showed the zwitterionic catalyst to be slightly more active, with a turnover number of 39±1 kg polymer per mol catalyst per hour compared to 35±2 kg polymer per mol catalyst per hour for the cationic species.20 However, catalyst activity is just one of several advantages to employing this type of system.

Figure 2.6. Variety of bis(phosphino)borates synthesized by the Peters group.46

Aside from the clear advantage of eliminating the need for a cocatalyst, zwitterionic systems also possess higher solubility in less polar, noncoordinating solvents, and have an increased tolerance to polar or coordinating functional groups.47 Possibly the most attractive property of zwitterionic systems is the difference in electrophilicity that the ligand imparts on the metal center. In CO/ethylene copolymerization as well as other potential applications,

26 controlling the electronic properties of the metal center is crucial for the activity and selectivity of the catalyst. For this reason, a zwitterionic nickel catalyst with a bidentate phosphine-borate ligand has been synthesized and attempted in the copolymerization of CO and propylene oxide, benzaldehyde, and a number of aziridines.

CHAPTER III

EXPERIMENTAL

3.1 Handling of Air-Sensitive Materials

Unless otherwise noted, all manipulations were carried out using standard Schlenk line techniques under a nitrogen atmosphere, or in a nitrogen-filled dry box.

The Schlenk line used in all synthesis and purification was made of Pyrex® glass and joined to a two stage rotary vane pump (Edwards Model E2M0.7) with silicone oil (VWR vacuum pump oil 19). To protect the pump from volatiles, a liquid nitrogen trap with a

Synthware Dewar Flask is placed between the vacuum pump and Schlenk line. Four hoses extend from the manifold, controlled by four glass stopcocks to allow either vacuum or gas to be used. Teflon stopcocks are used to seal all inlets, and are used as a secondary control for gas pressure in each of the two gas manifolds. Aside from the regulator on the nitrogen cylinder

(Union Carbide Corp.), Swagelok needle valves are the primary pressure regulators in the hood.

Swagelok check valves are also placed between the gas outlets and the Synthware oil bubblers to ensure no oxygen enters the lines. Finally, a Thermocouple vacuum gauge (Duniway

Stockroom Corp. Model DST-531) is placed at the end of the vacuum line, sealed by another

Teflon stopcock.

Preparation for synthesis and some manipulations were carried out in a dry box

(MBraun UNILab 07-174). The oxygen level in the dry box was monitored by a probe (MBraun

Model MB-OX-SE1), and vacuum was supplied by a two stage rotary vane pump (Edwards

Model RV12) with silica oil (VWR vacuum pump oil 19). All sealed glassware was taken into the dry box under vacuum, and all empty glassware was taken in warm to avoid water condensation.

3.2 Polymerization Procedure

All polymerization trials were conducted in a stainless steel 125 ml bomb reactor (Parr

Instrument Company) equipped with a pressure gauge (Ashcroft Duralife) under CO atmosphere. The desired amount of catalyst was loaded into the autoclave inside the dry box.

The autoclave was then sealed, taken out of the dry box, and placed on the Schlenk line. The nitrogen atmosphere was exchanged for CO, and under a steady flow of CO at 5 psi, monomer and solvent were injected. The CO pressure was then increased to fit the desired reaction conditions, the autoclave was sealed and placed in an oil bath at the desired temperature. After the allotted polymerization time, the pressure was slowly released and the reaction vessel opened to air. The product was extracted, solvent removed under reduced pressure, and what remained was analyzed by NMR and IR spectroscopy.

3.3 Characterization

3.3.1 Nuclear Magnetic Resonance Spectroscopy (NMR)

1H, 13C, 31P, and 11B NMR spectroscopy were used in the characterization of all

starting materials and products. The deuterated solvents employed were chloroform-d

(δ = 7.27 ppm), benzene-d6 (δ = 7.16 ppm), acetone-d6 (δ = 2.05 ppm), acetonitrile-d3 (δ

= 1.95 ppm), THF-d8 (δ = 3.58 ppm), methylene chloride-d2 (δ = 5.32 ppm), and

methanol-d4 (δ = 3.30 ppm); all spectra were referenced to their respective solvent

peaks. A Varian Mercury-300 MHz spectrometer was used for all spectra obtained at

room temperature, and a Varian Mercury-400 MHz spectrometer was used for variable

temperature studies and all 11B NMR acquisitions.

3.3.2 Fourier Transform Infrared Spectroscopy (FTIR)

Infrared spectrometry was used for characterization of polymerization products

and the final nickel complex. The spectrometer employed was a DIGILAB FTS 3000

Excalibur Series spectrophotometer, and all runs were performed with potassium

bromide salt plates. The polymer products were dissolved in chloroform, transferred to

the salt plates, and then chloroform was allowed to evaporate. Due to the air-sensitive

nature of the nickel complex, the sample was prepared in the dry box as a fluorolube oil

(LG160) suspension, kept in a desiccator, and then quickly transferred to the instrument.

3.3.3 X-ray Crystallography

X-ray crystallography was used in the characterization of the final nickel

complex. Crystal data was acquired on a Bruker SMART APEX CCD-based X-ray

diffractometer, and the structure was solved using the Bruker SHELXTL software

package. Crystals suitable for X-ray diffraction analysis were grown from the slow

evaporation of ether, and protected by Paratone-N oil for transport to the

crystallographer.

3.4 Anhydrous Solvents, Deuterated Solvents, Reagents, and Monomers.

All anhydrous solvents were purchased from Acros Organics unless otherwise noted.

Anhydrous ether, hexane, THF, toluene, chloroform, dichloromethane, and ethanol (Sigma

Aldrich) were kept in bulbs under nitrogen and used on the Schlenk line under a flow of nitrogen. All other anhydrous solvents were used as received.

Deuterated solvents were purchased from Cambridge Isotopes Laboratory, dried according to literature methods50, and stored in a storage flask in the dry box. Deuterated chloroform and deuterated dichloromethane were refluxed over calcium hydride overnight and then distilled under a nitrogen atmosphere into respective storage flasks. Deuterated benzene was stirred over sodium/potassium alloy overnight and then vacuum transferred into a storage flask. Deuterated acetone was stirred over phosphorous pentoxide overnight and then vacuum transferred into a storage flask. Deuterated acetonitrile was also stirred over phosphorous pentoxide overnight, but then distilled under nitrogen into a storage flask. Finally, deuterated

31 tetrahydrofuran was stirred over sodium/potassium alloy overnight and vacuum transferred to a storage flask.

The purification of some reagents was necessary before use. 2-bromopyridine was purified by stirring over KOH for 3 days and then filtered to a new flask containing CaH and stirred overnight. Finally, it was distilled under vacuum into a 250 ml storage flask containing molecular sieves. Trityl chloride was purified via sublimation at 90°C under nitrogen.

Tetramethylethylenediamine (TMEDA) was degassed via repetitive freeze-pump-thaw cycles and stored over molecular sieves. All monomers were previously dried, vacuum transferred or distilled, and stored in the dry box.

The following reagents were used as received:

 Boron tribromide

 Phenyltrimethylsilane

 2-bromobenzyl bromide

 Potassium diphenylphosphide [0.5 M in THF]

 Tert-butyl lithium [1.6 M in pentane]

 N-butyl lithium [2.5 M in hexane]

 Methyldiphenylphosphine

 Tetramethylene bromide

 Pyrrolidine

 (TMEDA)PdMe2

 Sodium tetraphenylborate

 N-ethyldiisopropylamine

 Nickel(II) acetylacetonate

 Trimethylaluminum

3.5 Monodentate Phosphine-Borane Ligand Synthesis

51 3.5.1 Synthesis of BrC6H4-2-(CH2PPh2)

To a 200 ml Schlenk flask on the Schlenk line was added potassium

diphenylphosphide (3.000 g, 13.375 mmol) under nitrogen protection, and then cooled

to -78°C with a dry ice/acetone bath. In a 50 ml Schlenk flask, 2-bromobenzyl bromide

(3.510 g, 14.044 mmol) was dissolved in 23 ml of anhydrous THF and then added drop-

wise to the phosphide. The reaction was stirred for 14 hours and allowed to slowly

warm to room temperature during the process. THF was then removed under reduced

pressure, and to the yellow/brown residual solid was added 100 ml degassed water.

After stirring for about an hour the water was filtered off via cannula and the product

was recrystallized from anhydrous ether and hexane (2.308 g, 46% yield).

3.5.2 Synthesis of bromodiphenylborane52

To a 100 ml Teflon stopper-sealable Schlenk tube cooled to 0°C was added

phenyltrimethylsilane (2.40 g, 15.96 mmol) under nitrogen, followed by boron

tribromide (2.00 g, 7.98 mmol). The tube was sealed and the reaction was stirred at 0°C

for about 10 minutes, then placed in a sand bath and stirred at 180°C for 20 hours. Once

33 cooled to room temperature the brown liquid was transferred to a 50 ml Schlenk flask via cannula and pumped through two liquid nitrogen traps overnight. The resulting liquid was pure bromodiphenylborane (1.523 g, 78% yield).

53 3.5.3 Synthesis of Ph2PCH2C6H4-2-(BPh2)

A solution of BrC6H4-2-(CH2PPh2) (1.530 g, 4.307 mmol) in 25 ml of anhydrous ether was cooled to -78°C on the Schlenk line. To it was added n-butyl lithium [1.6 M in pentane] (5.69 ml, 9.104 mmol) over a five minute period. The reaction was stirred for

16 hours and allowed to slowly warm to room temperature. The solvent was then removed under reduced pressure and the yellow solid product was washed with anhydrous hexane (2 x 10 ml). The product was dissolved in a mixture of 30 ml anhydrous toluene and 20 ml of anhydrous hexane. The solution was cooled to 0°C and to it was added bromodiphenylborane (1.055 g, 4.307 mmol) in 15 ml anhydrous toluene drop-wise. The reaction was stirred for 14 hours and allowed to slowly warm to room temperature. The solvent was then removed under reduced pressure, and the product washed with 30 ml anhydrous hexane. The product was purified via recrystallization from anhydrous ether. (568.4 mg, 44% yield).

3.5.4 Attempted Cobalt Catalyst Synthesis

In the dry box, Ph3SiCo(CO)4 (97.7 mg, 0.227 mmol) and Ph2PCH2C6H4-2-(BPh2)

(100 mg, 0.227 mmol) were loaded into a 50 ml Schlenk flask that was sealed and placed on the Schlenk line. The flask was cooled to 0°C and under nitrogen protection, 20 ml of

anhydrous ether was added. The reaction was stirred for 30 minutes and then placed in

the freezer at -46°C. After three days a white powder was isolated via filtration.

3.6 Bidentate Pyridine-Borane Ligand Synthesis

3.6.1 Synthesis of Hydrogen diphenyldi(2-pyridyl)borate54

To a 100 ml Schlenk flask was added 2-bromopyridine (2.108 g, 13.34 mmol)

under nitrogen protection. The flask was cooled to -78°C and 20 ml of anhydrous THF

was added, followed by n-butyl lithium [2.5 M in hexane] (5.34 ml, 13.34 mmol).

Immediate color change to dark red was observed and the solution was stirred for 15

minutes, after which bromodiphenylborane (1.50 g, 6.67 mmol) was added. The

reaction was stirred at -78°C for 1 hour, then room temperature for 23 hours. The

reaction was quenched by the addition of 50 ml of degassed water, forming aqueous

and organic phases. The organic phase was transferred to a new 100 ml Schlenk flask via

cannula.

1 M acetic acid was prepared from dilution of a 17.4 M stock solution, and then

35 ml was added to the organic phase. The organic phase was again isolated via

filtration with a cannula. Brown crystals which formed upon sitting were isolated via

filtration and then dissolved in 10 ml of acetone and stirred with activated carbon

overnight. The solution was filtered into a new 50 ml Schlenk flask, about one-third of

the solvent was removed under vacuum, and the solution was placed in the freezer at -

46°C. A dusting of white powder was retrieved upon filtration at low temperature.

3.6.2 Synthesis of Trityl diphenyldi(2-pyridyl)borate

To a 100 ml Schlenk flask was added 2-bromopyridine (2.113 g, 13.367 mmol)

under nitrogen protection. The flask was cooled to -85°C in a methanol/dry ice bath and

to it was added n-butyl lithium. The reaction was stirred near -80°C for 40 minutes and

then bromodiphenylborane (1.5969 g, 6.521 mmol) in 4 ml of anhydrous THF was added

drop-wise. The reaction was kept at -80°C for another 30 minutes and the slowly

allowed to rise to room temperature and stir for 19 hours, after which the solvent was

removed under reduced pressure. The porous black product was partially dissolved in a

combination of anhydrous hexane (30 ml) and anhydrous dichloromethane (15 ml). It

was cooled to 0°C and to it was added trityl chloride under a steady flow of nitrogen.

The solution was allowed to rise to room temperature and stir for 23 hours, after which

the solvent was removed under vacuum. Attempted purification of the product via

recrystallization from anhydrous dichloromethane and anhydrous ether or hexane was

conducted with little success.

3.7 Bidentate Phosphine-Borane Ligand Synthesis

55 3.7.1 Synthesis of Ph2PCH2Li(TMEDA)

To a 100 ml Schlenk flask was added n-butyl lithium [2.5 M in hexane] (6.82 ml,

17.05 mmol) followed by TMEDA (1.918 g, 17.05 mmol) and 20 ml anhydrous ether.

Methyldiphenylphosphine (3.192 g, 15.94 mmol) was then added drop-wise at room

temperature, and the reaction was stirred for 3 days. The solid canary-yellow product

36 was isolated by filtration through celite with a cannula, and purified by washing with anhydrous ether (4 x 15 ml). (2.713 g, 53% yield).

56 3.7.2 Synthesis of [BPh2(CH2PPh2)2][Li(TMEDA)2]

Ph2PCH2Li(TMEDA) (4.00 g, 12.41 mmol) was loaded into a 100 ml Schlenk flask, suspended in 80 ml anhydrous ether, and cooled to -78°C. To it was added bromodiphenylborane (1.518 g, 6.20 mmol) in 7 ml anhydrous toluene. The reaction was allowed to slowly warm to room temperature and stir for 14 hours. The white solid product was isolated by filtration and washed with ether (2 x 10 ml). (3.05 g, 76% yield).

3.7.3 Synthesis of 5-azonia-spiro[4.4]nonane bromide (ASNBr)57

The preparation of ASNBr was conducted in the presence of oxygen and moisture. In a 200 ml three-neck round bottom flask NaOH (3.7052 g, 92.64 mmol) was dissolved in 80 ml water. To it was added tetramethylene bromide (20.0 g, 92.64 mmol), and the solution was heated to reflux. While refluxing, pyrrolidine (6.588 g, 92.64 mmol) was added and the reaction was allowed to continue for 45 minutes and then cooled to

0°C. Meanwhile, 45 ml of a 40% w/w NaOH solution was prepared and cooled to 0°C, and then added to the reaction once both were cold. The product was then extracted with chloroform (3 x 40 ml) and the solution concentrated. Finally, at 0°C a large amount of ether was added to precipitate a pure white solid product. (3.24 g, 17% yield).

56 3.7.4 Synthesis of [Ph2B(CH2PPh2)2][ASN]

[BPh2(CH2PPh2)2][Li(TMEDA)2] (2.52 g, 3.14 mmol) was dissolved in 25 ml of

anhydrous ethanol in a 100 ml Schlenk flask. In a 50 ml Schlenk flask was dissolve ASNBr

(0.680 g, 3.30 mmol) in 5 ml anhydrous ethanol. The ASNBr solution was then added

drop-wise to the ligand solution and immediate formation of a white precipitate was

observed. Stirring was continued for 10 minutes after which the product was isolated by

filtration and then washed with anhydrous ethanol (2 x 10 ml) and anhydrous ether (2 x

10 ml). Residual solvent was removed under vacuum overnight. (2.027 g, 94% yield).

3.8 Palladium Complex Synthesis

58 3.8.1 Synthesis of [Ph2B(CH2PPh2)2PdMe2][ASN]

[Ph2B(CH2PPh2)2][ASN] (310 mg, 0.450 mmol) was loaded into a 50 ml Schlenk

flask and suspended in 5 ml anhydrous THF. To it was added (TMEDA)PdMe2 (113.6 mg,

0.450 mmol) previously dissolved in 3 ml anhydrous THF in the dry box. The reaction

was stirred for 20 hours. The solvent was then removed under reduced pressure, and

the orange solid product was washed with anhydrous ether (3 x 10 ml) and dried under

vacuum. (226.9 mg, 61% yield).

56 3.8.2 Synthesis of [(iPr)2EtNH][BPh4]

This procedure was done in the presence of oxygen and moisture. Sodium

tetraphenylborate (4.00 g, 11.69 mmol) was loaded into a 300 ml round bottom flask

and suspended in 50 ml of water. Diisopropylethylamine (1.51 g, 11.69 mmol) was then

added, followed by 3 ml of aqueous HCl. The cloudy white solution was stirred for 15

minutes and then the white product was isolated via suction filtration and washed with

water.

58 3.8.3 Synthesis of Ph2B(CH2PPh2)2PdMe(THF)

In a 25 ml Schlenk flask [Ph2B(CH2PPh2)2PdMe2][ASN] (100 mg, 0.121 mmol) was

dissolved in 1 ml anhydrous THF. In a separate 10 ml Schlenk tube [(iPr)2EtNH][BPh4]

(54.41 mg, 0.121 mmol) was dissolved in 2 ml anhydrous THF and then added to the

palladium solution. The reaction was stirred for 30 minutes after which 1.8 ml of

anhydrous hexane was added to precipitate the ammonium salt side product. The

product was separated by filtration, and then to the filtrate was added 15 ml anhydrous

hexane and allowed to stir for 40 minutes to fully precipitate the product. The red solid

was isolated by filtration and flushed with nitrogen to dry.

3.9 Attempted Polymerizations with Palladium Complex

3.9.1 Propylene Oxide Attempted Polymerization

Following the polymerization procedure described in section 3.2, the following

reaction conditions were employed: palladium complex (9.6 mg, 12.7 µmol), propylene

oxide (414.5 mg, 7.137 mmol), anhydrous THF (2 ml), 200 psi, 70°C, 16 h.

3.9.2 Benzaldehyde Attempted Polymerization

Same reaction conditions as described for the propylene oxide attempted

polymerization in 3.9.1.

3.10 Synthesis of Nickel Complex

58 3.10.1 Synthesis of (TMEDA)Ni(acac)2

Nickel(II) acetylacetonate (Ni(acac)2) (5.204 g, 20.10 mmol) was loaded into a

200 ml Schlenk flask and the green solid was suspended in 40 ml anhydrous hexane. To

it was added TMEDA (4.672 g, 40.20 mmol) at room temperature. The solution

immediately turned blue and homogeneous. The reaction was stirred for 45 minutes at

room temperature and then placed in a dry ice/isopropanol bath (-78°C) for 14 hours.

Blue solid crystals formed and were isolated by low temperature filtration via cannula

and dried under vacuum (6.340 g, 84% yield).

59 3.10.2 Synthesis of Dimethylaluminumethoxide (Me2AlOEt)

In the dry box trimethylaluminum [25% w/w in hexane] (9.458 g, 32.8 mmol) was

loaded into a 100 ml Schlenk flask. The flask was then attached to the Schlenk line and

cooled to 0°C. To it was added 30 ml anhydrous ether, followed by drop-wise addition of

anhydrous ethanol (1.511 g, 32.8 mmol). The solution was stirred for one hour and used

in the next step of the synthesis without purification.

58 3.10.3 Synthesis of (TMEDA)NiMe2

In a 100 ml Schlenk flask (TMEDA)Ni(acac)2 (6.00 g, 16.0 mmol) was dissolved in

25 ml anhydrous ether and cooled to -20°C. The previously synthesized Me2AlOEt was transferred into the flask via cannula in its entirety. The reaction was stirred for 5 hours at -10°C and then placed in a dry ice/isopropanol bath overnight. A yellow/brown solid precipitated and was isolated via filtration at -78°C. The crude product was then redissolved in 20 ml anhydrous ether at 0°C, and recrystallized at -78°C. The supernatant was removed via filtration to give a yellow product (1.44 g, 44% yield). Due to the instability of the product at room temperature, a THF solution of the product was prepared and kept in the freezer at -46°C.

3.10.4 Synthesis of [Ph2B(CH2PPh2)2NiMe2][ASN]

[Ph2B(CH2PPh2)2][ASN] (2.00 g, 2.900 mmol) was suspended in 40 ml of anhydrous acetonitrile in a 100 ml Schlenk flask on the Schlenk line. The flask was cooled to 0°C and to it was added (TMEDA)NiMe2 (0.468 M solution in THF, 15 ml, 7.025 mmol). The reaction was stirred at 0°C for 1 hour and then room temperature for 16 hours. The solvent was then removed under reduced pressure, and the resulting brown oily solid was stirred with 30 ml of anhydrous ether for 8 hours, yielding a yellow solid product. The product was isolated by filtration with a cannula and purified via recrystallization from THF and ether. (1.102 g, 49% yield).

3.10.5 Synthesis of Ph2B(CH2PPh2)2NiMe(CH3CN)

[Ph2B(CH2PPh2)2NiMe2][ASN] (500 mg, 0.642 mmol) was partially dissolved in 13 ml of anhydrous acetonitrile in a 100 ml Schlenk flask. In a 25 ml Schlenk flask

[HN(iPr)2Et][BPh4] was dissolved in 2 ml anhydrous acetonitrile, and then added to the flask with the nickel complex. Immediate methane formation was visible, and the reaction was allowed to stir at room temperature for 30 minutes. Anhydrous ether (15 ml) was added to precipitate the ammonium-borate salt side product, and the product was filtered to a new 100 ml Schlenk flask. Solvent was removed under reduced pressure, and to afford the yellow-orange powder product. (401 mg, 92% yield).

3.10.6 Synthesis of Ph2B(CH2PPh2)2NiCOMe(CO)

[Ph2B(CH2PPh2)2NiMe2][ASN] (500 mg, 0.642 mmol) was partially dissolved in 8 ml of anhydrous acetonitrile in a 100 ml Schlenk flask on the Schlenk line. In a separate

25 ml Schlenk tube [(iPr)2EtNH][BPh4] (289 mg, 0.642 mmol) was dissolved in 3 ml anhydrous acetonitrile and then added slowly to the stirred nickel solution at room temperature. The reaction was allowed to stir for 30 minutes. After this, approximately one-half of the acetonitrile was removed under reduced pressure. The nitrogen atmosphere in the flask was then exchanged for CO. Yellow precipitate was immediately seen upon the atmosphere change. The mixture was stirred for one hour to allow the yellow product to precipitate fully. The yellow solid was isolated via filtration and the

product was dried under vacuum. Purification of the product was conducted via

recrystallization from anhydrous ether and hexane. (189 mg, 43% yield).

3.11 Attempted Polymerizations with Nickel Complex

3.11.1 Propylene oxide (run 1)

Following the polymerization procedure described in section 3.2, the following

reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), propylene

oxide (830 mg, 14.29 mmol), 200 psi, room temperature, 16 h.

3.11.2 Propylene oxide (run 2)

Following the polymerization procedure described in section 3.2, the following

reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), propylene

oxide (83.78 mg, 1.44 mmol), toluene (1 ml), 200 psi, 50°C, 16 h.

3.11.3 THF (run 3)

Following the polymerization procedure described in section 3.2, the following

reaction conditions were employed: nickel complex (15 mg, 0.0216 mmol), THF (889 mg,

12.31 mmol), 200 psi, 50°C, 16 h.

3.11.4 Butyl Aziridine (run 4)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (15 mg, 0.0216 mmol), butyl aziridine (214 mg, 2.16 mmol), toluene (1 ml), 200 psi, 50°C, 16 h.

3.11.5 Butyl Aziridine (run 5)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (15 mg, 0.0216 mmol), butyl aziridine (42.8 mg, 0.432 mmol), THF (1 ml), 200 psi, 50°C, 16 h.

3.11.6 N-benzylidenemethylamine (run 8)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), N- benzylidenemethylamine (34.4 mg, 0.2886 mmol), THF (1 ml), 200 psi, 50°C, 8 h.

3.11.7 Butyl Aziridine (run 6)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (15 mg, 0.0216 mmol), butyl aziridine (64.2 mg, 0.648 mmol), THF (10 ml), 200 psi, 80°C, 16 h.

3.11.8 Butyl Aziridine (run 7)

3.11.9 Aziridine (run 9)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), aziridine

(12.4 mg, 0.2886 mmol), THF (5 ml), 200 psi, 50°C, 16 h.

3.11.10 Ethyl Aziridine (run 10)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), ethyl aziridine (30.8 mg, 0.4329 mmol), ethanol (5 ml), 500 psi, 80°C, 16 h.

3.11.11 Ethyl Aziridine (run 11)

3.11.12 Ethyl Aziridine (run 12)

3.11.13 1-Hexene (run 13)

Following the polymerization procedure described in section 3.2, the following reaction conditions were employed: nickel complex (10 mg, 0.01443 mmol), 1-hexene

(36.4 mg, 0.4329 mmol), THF (5 ml), 500 psi, 50°C, 20 h.

CHAPTER IV

RESULTS AND DISCUSSION

4.1 Investigation of monodentate phosphine-borane ligand

Originally the approach to carbonylative polyester synthesis involved the investigation of a more active cobalt system. Motivation for the synthesis of a catalyst containing the monodentate phosphine-borane ligand stems from the previous success of the cobalt catalyst employed in the CO/aziridine copolymerizations. Based on the pyridine-assisted carbonylative polymerization of ethylene oxide, slight tuning of the catalysts’ electronic properties may be enough to enhance the activity of the carbonylative polymerization of epoxides and realize that of aldehydes. Instead of the using a cocatalyst, the acidity group of a Lewis acid-containing ligand can potentially make the cobalt-acyl bond more susceptible to nucleophilic attack.

The synthesis of the ligand was based initially based on a literature procedure involving

CsOH-promoted P-alkylation.51a However, the preparation was given up after multiple attempts because of low yield and impurities observed in the 1H NMR spectrum. A new synthetic route, similar to that used by Morales-Morales, was used.51b,52 By employing the potassium salt of the phosphine, a much more direct synthesis was achieved with nearly 50% yield. The complete synthetic mechanism is provide below in Scheme 4.1 and the 1H, 31P, and 11B NMR spectra of the final ligand can be seen in Figure 4.1 a-c, respectively.

+ - K P THF, -78 oC - r.t., 14 h t-butyl lithium, -78 oC - r.t., 16 h P P Br +

Br Li

CH3

BBr o 3 2 H3C Si 180 C, 18 h + Br B CH3

P P + Br B Toluene:Hexane (60:40) 0 oC - r.t., 16 h B Li

Scheme 4.1. Synthesis of monodentate phosphine-borane ligand.

214_3-2-11-P-B-ligand

0.9

0.8

a 0.7 P

0.6 B

0.5

0.4 a

Intensity Normalized 0.3

0.2 ether 0.1 ether hexane hexane 0 0.89 1.94 9.35 3.74 2.00 0.94

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.1 (a). 1H NMR spectrum of monodentate phosphine-borane ligand.

1.0 267_5-16-11-P-B-lig_31P

0.9

0.8

0.7 0.6

0.5

0.4

0.3

0.2

Intensity Normalized

0.1

-0.1 -0.2

-0.3

96 88 80 72 64 56 48 40 32 24 16 8 0 -8 -16 -24 -32 Chemical Shift (ppm) Figure 4.1 (b). 31P NMR spectrum of monodentate phosphine-borane ligand.

1.0 231_3-21-11-11B-PBligand

0.9

0.8

0.7

0.6

0.5

0.4

0.3

Intensity Absolute 0.2

0.1

-0.1

-0.2

100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 -140 Chemical Shift (ppm) Figure 4.1 (c). 11B NMR spectrum of monodentate phosphine-borane ligand.

In addition to a 1H NMR spectrum, a 11B NMR spectrum was obtained and showed a single peak near -2.0 ppm, indicative of a 4-coordinate boron species. In addition, a doublet is seen in the 1H NMR spectrum, corresponding to alkyl protons adjacent to a 4-coordinate phosphine. Both of these indicate that there is coordination between the boron and phosphine centers. However, this should not have an effect on the binding to a cobalt center as the phosphine-cobalt bond is much stronger than the phosphine-boron interaction.

Upon completion of the ligand, catalyst synthesis was attempted, but with little success.

The synthesis was attempted with a trimethylsilylcobalt complex (Scheme 4.2), but with very low yield and an unconvincing 1H NMR spectrum. However, the focus of the project quickly turned to the synthesis of bidentate ligands for use on Pd and Ni metal centers.

B P CO CO

o + Ether, 0 C Si Co P Si Co CO + B OC OC CO CO

Scheme 4.2. Synthesis of monodentate phosphine-borane cobalt complex.

4.2 Investigation of the Bidentate Pyridine-Borate Ligand

This ligand, containing an anionic borate center, was synthesized with the intent of placing it on a group 10 transition metal center for catalysis. The borate moiety in the ligand is known to interact with a platinum center and promote hydroxylic substitution for a methyl group residing on boron.54 This suggests that it may also be successful in activation of monomers such as epoxides or aldehydes for carbonylative polymerizations.

The originally attempted synthesis was followed directly from the literature.54 The counter-cation was a proton, assumed to be shared in an equilibrium between the nitrogen atoms on the pyridine rings. However, attempted synthesis provided only a very small amount,

< 10% yield, after all purifications. Upon examining the synthesis more closely, it seemed that a more stable cation should improve the yield and provide a more stable ligand.

To achieve this, trityl chloride was introduced to the reaction mixture after the formation of the lithium salt of the ligand. This should generate LiCl and the trityl-coordinated ligand (Scheme 4.3). This procedure proved successful, as a small amount of pink solid was isolated and characterized by 1H and 13C NMR (Figure 4.2 a-b). The unlabeled peak near 3.6 ppm is attributed to a stabilizer in CD2Cl2. During the synthesis of this ligand, a number of purification steps are necessary in order to recover the highest product yield from different phases of the workup. Although a pure product was achieved, investigation into an electronically similar, but synthetically simpler ligand was undertaken.

Scheme 4.3. Synthesis of bidentate pyridine-borate ligand.

264_5-11-11-BPy2CPh3_CD2Cl2

0.9

0.8

0.7

0.6

0.5

0.4

Intensity Normalized 0.3

0.2

0.1

2.06 3.03 5.44 2.941.69 15.24 2.18 3.54 1.00 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.2 (a). 1H NMR spectrum of trityl-coordinated pyridine-borate ligand.

291_5-24-11-BPy2_13C 0.12

0.11

0.10

0.09

0.08

0.07 0.06

0.05 Normalized Intensity Normalized

0.04 0.03

0.02

0.01

220 200 180 160 140 120 100 80 60 40 20 0 -20 Chemical Shift (ppm)

Figure 4.2 (b). 13C NMR spectrum of bidentate pyridine-borate ligand.

4.3 Synthesis and Characterization of Bidentate Phosphine-Borate Ligand

Following the work of the Peter’s group, *Ph2B(CH2PPh2)2][ASN] was synthesized via a multi-step approach (Scheme 4.4).56 The yield of each step varied from approximately 60% to nearly 95%, and the highest purity in each step of the process was crucial for improving the yield and purity of the final product. The 1H NMR spectrum of the ligand is provided in Figure

4.3, along with the peak assignments. As can be seen, there are a two small, unassignable peaks that cannot be attributed to residual solvent, near 3.00 and 3.45 ppm. Although their origin is unknown, upon placement of the ligand on a metal center these peaks disappear and a pure metal complex is obtained.

N N n-butyl lithium, ether Li Ether P 1/2 Br B r.t., 72 h + + -78 oC - r.t., 14 h N N

- Br N N P P + - + Ethanol - N B Li + N B + r.t., 10 min P P N N

Scheme 4.4. Synthesis of bidentate phosphine-borate ligand.

a c P b + - N B

P d e h f f g

Figure 4.3. 1H NMR spectrum of bidentate phosphine borate ligand.

In addition to ASN, two other options were investigated as possible counter-cations.

First, trityl chloride was reacted with the lithium salt of the ligand in an attempt to generate the trityl cation of the ligand and LiCl. The product was an orange solid that NMR analysis revealed to be ligand decomposition, and upon recrystallization an oil was recovered.

The other cation employed was tetrabutylammonium bromide, following a similar procedure to that used by the Peters group.56 NMR characterization proved that the correct compound was synthesized, but upon reaction with a metal center, purification of the metal complex became a problem. The solubilities of the metal complex and the ammonium borate side product are very similar, making for a difficult separation. For these reasons, ASN was deemed the most suitable cation and used in all further reactions.

The motivation for the synthesis of this ligand is due to a number of advantages. First, the bidentate nature of the ligand is important in that it will coordinate to the d-10 metal center in a cis fashion. This ensures that the propagating chain and incoming monomer will be cis to one another. Also, as previously described, the flexibility of the ligand is of concern. The more rigid bidentate phosphine ligands encounter the problem of extremely high energy transition states upon CO insertion due to migration of the chain adjacent to one of the phosphines. The ligand employed here should not encounter this problem.

In addition, the electronic properties of the ligand are important. In order for epoxides and aldehydes to react, the acyl group must be susceptible to nucleophilic attack and the metal complex must be a good leaving group. The more electron-donating is the ligand, the less susceptible the metal-acyl species is to nucleophilic attack, but the more nucleophilic the resulting Ni(0) or Pd(0) species will be. Therefore, it is necessary to find a balance to promote the carbonylative polymerization, and it is believed that this ligand will suffice.

In addition to the electronic properties of the metal center, which can be tuned by ligand choice, the ligand used here also possesses the advantage of having an anionic borate moiety in the backbone. When examining cationic Ni(II) and Pd(II) species, after the initial

55 nucleophilic insertion of the monomer, the resulting neutral intermediates do not form the required ion pair necessary for propagation. By using this bidentate ligand with an anionic borate center, it is hoped that this ion pair will be realized, and help to induce nucleophilic attack by the metal complex. All of these parameters in combination will hopefully lead to the realization of a viable route to the copolymerization of CO and epoxides or aldehydes.

4.4 Synthesis and Attempted Polymerizations with Palladium Complex

Upon purification of the ligand, the first metal complex synthesized was

58 Ph2B(CH2PPh2)2PdMe(THF), following work by Peters et al. The first step of the procedure was to obtain the dimethyl analogue of the compound. This was achieved by reaction of the ligand with (TMEDA)PdMe2 in anhydrous THF. In order to obtain the final complex, removal of one of the methyl groups was necessary, followed by replacement by a labile ligand such as THF. To do this, an ammonium borate salt ([(iPr)2EtNH][BPh4]) was synthesized that donates a proton and results in the formation of methane. The reaction was run in anhydrous THF; therefore, upon removal of methane, THF fills the vacant coordination site. However, after completion of the reaction, the separation of the [ASN][BPh4] salt from the desired product is not straightforward.

Anhydrous hexane is used as the poor solvent to first precipitate the less soluble salt.

There is very little difference between the amount of hexane necessary to precipitate all of the salt, and the amount required to begin precipitation of the palladium complex. Therefore, unless extremely accurate it is possible that the palladium complex begins to precipitate with

56 the salt and is lost, or some residual salt remains with the complex. Care was taken to minimize either of these possibilities.

The final product is a peach-colored solid, the 1H NMR spectrum of which can be seen in

Figure 4.4. The purity of the product was of concern because there are a number of unidentifiable peaks. However, the characteristic peaks of the desired product are present. The stability of this product is also of concern, as it is reported to decompose upon extended exposure to vacuum. Therefore, it was dried under a stream of nitrogen, hence the large solvent peaks present in the NMR spectrum. Also, as was discovered through polymerization trials, the thermal stability of the complex is an issue, and decomposition was observed under the attempted polymerization conditions.

The active catalyst was generated in situ, because the labile THF ligand is easily replaced by CO when in solution and under a CO atmosphere. The active catalyst is then the Pd-acyl species formed by insertion of coordinated CO into the Pd-carbon bond; another CO molecule fills the fourth coordination site.

342_6-30-11-Ph2BP2PdMeTHF

0.9 a b 0.8 P - + B Pd 0.7 O P c 0.6

0.5

0.4 Normalized Intensity Normalized c hexane 0.3 b hexane 0.2

0.1 a

3.72 0.44 0.53 4.52 2.25 1.74 0.90 0.82 8 7 6 5 4 3 2 1 0 -1 -2 Chemical Shift (ppm)

Figure 4.4. 1H NMR spectrum of final palladium complex.

Polymerizations were attempted with propylene oxide and benzaldehyde, but only propylene oxide showed any sign of ester formation, seen as a small peak in the 1H NMR spectrum near 5.38 ppm (Figure 4.5). This, however, is very promising as it shows that at the very least the Pd-acyl bond is susceptible to nucleophilic attack by PO, and gives hope that a similar complex may show higher activity. Due to the lack of thermal stability of the palladium complex, an analogous nickel complex was synthesized and its effectiveness as a catalyst tested.

a O n

Figure 4.5. 1H NMR spectrum of palladium catalyzed CO/propylene oxide copolymerization.

4.5 Synthesis and Characterization of Nickel Complex

The synthesis of the nickel complex has never before been reported. To begin, an analogous procedure to that used for the palladium compound was employed, and modification was made when necessary. (TMEDA)NiMe2 is not commercially available and was synthesized from a multi-step procedure outlined in Scheme 4.5. The synthesis of EtOAlMe2 was slightly modified from the literature procedure in that the reaction was kept at 0°C throughout, rather that -78°C and allowed to warm to room temperature. The product was then used directly after synthesis without purification. Due to the rapid decomposition of

(TMEDA)NiMe2 at room temperature, the product was kept cold and dissolved in anhydrous

THF, then stored as a stock solution in the freezer at -46°C.

O N O O O O 2+ Hexane, r.t., 1 h 2+ Ni 2 Ni + N O O O N

CH 3 CH H3C o H3C 3 H3C Al Ether, 0 C, 45 min. O Al + OH + CH4 CH3 CH3

O N CH O H3C 3 CH3 O Ether, -10 oC, 4 h 2+ O Al Ni Ni 2 + CH N CH3 3 O N

+ N

+ N N P CH - 3 P CH B o 3 2 Ni CH CN, 0 C - r.t., 14 h - 2+ + 3 B Ni P CH CH3 3 P N

+ N

- O B H3C CH3 P CH P 2+ 3 CH3CN, r.t., 30 min + - - - Ni B Ni + B B + H N + CH3 CO, Ether/Hexane CO P P CH3 H3C CH3 + N CH4

Scheme 4.5. Synthesis of zwitterionic nickel complex.

The first attempted syntheses of Ph2B(CH2PPh2)2NiMe2 were done in anhydrous THF, but with little success due to the lack of solubility of the ligand in THF. The reaction was then attempted in anhydrous acetonitrile with greater success. Purification via recrystallization from

THF and ether provided pure product, as indicated by the 1H and 31P NMR spectra (Figure 4.6(a) and 4.6(b), respectively). Following the procedure for the platinum and palladium analogues, synthesis of Ph2B(CH2PPh2)2NiMe(THF) and Ph2B(CH2PPh2)2NiMe(CH3CN) was attempted.

a b d P c + - 2+ N B Ni h c d P i e j f

Figure 4.6 (a). 1H NMR spectrum of ligand-coordinated dimethyl nickel complex.

1.0 448_9-21-11-BP2NiMe2-ASN_31P

0.9

0.8 P + - 2+ N 0.7 B Ni P 0.6

0.5

0.4 Normalized Intensity Normalized

0.3

0.2

0.1

96 88 80 72 64 56 48 40 32 24 16 8 0 -8 -16 -24 -32 Chemical Shift (ppm)

Figure 4.6 (b). 31P NMR spectrum of ligand-coordinated dimethyl nickel complex.

The reaction of the dimethyl complex in anhydrous THF with [(iPr)2EtNH][BPh4] was performed and worked up similarly to that of the palladium complex. An orange solid was obtained, which was believed to be the desired product as indicated by 1H NMR in THF-d8, but there were a number of unidentifiable peaks. In an attempt to obtain an NMR spectrum in

CDCl3 or C6D6 it was clear that the complex was not stable in either of these solvents. The instability was possibly due to the lability of the THF ligand. As a solution, synthesis in anhydrous acetonitrile was performed.

For steric and electronic reasons, acetonitrile has stronger coordination to nickel than does THF, and was therefore thought to possibly be a superior ligand and result in a more stable nickel complex. The reaction was run in the same manner, but anhydrous ether was used

62 as the poor solvent instead of hexane due to the lack of miscibility of hexane and acetonitrile.

Precipitation of the ammonium salt proceeded as expected, but in attempting to precipitate the nickel complex with ether no product was obtained. Removal of solvent gave a yellow solid that the 1H NMR spectrum showed to be clean with the exception of a few unidentifiable peaks

(Figure 4.7). The inability to isolate a pure solid product led to the attempted generation of the

Ni-acyl complex in situ.

435_9-15-11-NiCat_CD3CN

0.9

c 0.8 P a CD CN - + 3 B Ni b P CH CN 0.7 c 3

ether 0.6 b 0.5

0.4 ether NormalizedIntensity a 0.3

0.2 c

0.1

0 2.24 10.25 16.74 1.49 5.37 8.50 3.44 8.87 5.852.81 5.16 3.00

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.7. 1H NMR spectrum of acetonitrile-coordinated nickel complex.

A number of different reaction conditions were screened in order to obtain the most pure product and the highest yield. Initially, the procedure used for the generation of

Ph2B(CH2PPh2)2NiMe(CH3CN) was used. After precipitation of the ammonium borate salt, the product was filtered into a new flask and placed under CO. Upon doing so the solution darkened, but no precipitate was visible. The solvent was removed under reduced pressure and a yellow solid remained. However, in the attempts to obtain an NMR spectrum in C6D6 the solution became orange and cloudy over time. It was determined that the product is unstable in benzene under a nitrogen atmosphere. A similar approach was then taken and the NMR spectrum in C6D6 under CO atmosphere showed a nearly pure product, with one large unidentified peak near 0.60 ppm (Figure 4.8). This peak is assumed to be coordinated CH3CN, indicative that a 5-coordinate nickel species was obtained. Purification via recrystallization was then attempted.

Initially a toluene/ether mixture was employed for the recrystallization, but after multiple attempts it was discovered that the product has solubility in ether. This knowledge led to improved reaction conditions for the generation of the catalyst. After reaction, instead of precipitating the ammonium borate side product, approximately one-third of the anhydrous acetonitrile was removed under reduced pressure, and then the atmosphere was exchanged for

CO. The bright yellow catalyst has poor solubility in acetonitrile and precipitates from solution, while the ammonium borate side product remains soluble in acetonitrile. The catalyst is isolated via filtration and then recrystallized from anhydrous ether and hexane. The 1H and 31P

NMR spectra are clean, and the coordinated CH3CN peak has disappeared (Figure 4.9 a and 4.9 b, respectively). The complete synthesis is described in Scheme 4.5.

479_10-4-11-BP2NiCOMe

0.9 C6D6

0.8 O b P a - + CH3 0.7 Ni c B CO P i i 0.6 d b CH 3CN e f 0.5 g h 0.4

f g Normalized Intensity Normalized 0.3 d a c h 0.2 e 0.1 b 0 4.25 4.64 17.92 4.25 7.74 3.72 3.00 3.91

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm)

Figure 4.8. 1H NMR spectrum of 5-coordinate nickel complex.

489_10-11-11-BP2NiCOMe

0.9 a 0.8 O b f g P a - + CH3 0.7 Ni e c B CO P 0.6 d b e 0.5 d f

c h 0.4 g h Normalized Intensity Normalized 0.3

0.2 b 0.1

0 4.19 4.53 12.90 4.41 8.36 3.64 3.00

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.9 (a). 1H NMR spectrum of final zwitterionic nickel catalyst.

1.0 484_10-5-11-BP2NiCOMe_31P

0.9

0.8

0.7

0.6

0.5

0.4

0.3

Intensity Normalized 0.2

0.1

-0.1

-0.2 96 88 80 72 64 56 48 40 32 24 16 8 0 -8 -16 -24 -32 Chemical Shift (ppm) Figure 4.9 (b). 31P NMR of final zwitterionic nickel catalyst.

Before polymerizations were attempted with the newly synthesized catalyst, in depth characterization was performed. In addition to 1H and 31P NMR spectroscopy, variable temperature NMR studies were conducted in order to see if the shape of the peak in the 31P

NMR spectrum would change. It is believed that the room temperature broad peak is caused by the fast exchange of the CO ligand when in solution and under a CO atmosphere, and that this peak should become narrower with increasing temperature. Spectra were obtained at 10°C intervals from room temperature up to 70°C. The narrowing of the 31P peak was initially observed, but between the 60°C and 70°C experiments decomposition of the catalyst occurred.

It is believed that decomposition is initiated via loss of the CO ligand followed by deinsertion of the acyl-CO group. However, under high CO pressure this should not occur; therefore, it is possible to run polymerizations at temperatures higher than 70°C.

Perhaps the most convincing characterization method employed was X-ray crystallography. Single crystals suitable for X-ray diffraction analysis were grown from the slow evaporation of ether. They were transferred to a glass slide in the dry box and protected by paratone-N oil for transport to the crystallographer. The crystal structure obtained is provided in Figure 4.10, where the nickel atom is the central green sphere, phosphorous is designated orange, boron is designated pink, and oxygen is designated red. As expected, it is a 4- coordinate, square planar complex in which the bidentate phosphine ligand occupies two sites cis to one another, and the acyl group and carbonyl ligand occupy the third and fourth site. One noteworthy characteristic identified via X-ray crystallography is that the acyl group is perpendicular to the ligand plane. This may improve its susceptibility to attack by monomer.

Also, the anionic borate looks to have little interaction with the cationic nickel center.

Figure 4.10. Crystal structure of zwitterionic nickel complex.

In addition to NMR spectroscopy and crystallography, Fourier Transform Infrared

Spectroscopy (FTIR) was employed. Due to the air-sensitive nature of the nickel complex, the IR sample was prepared in the dry box in fluorolube oil on a KBr plate. The IR spectrum showed two peaks corresponding to CO vibrations; a peak at 2045 cm-1 from the carbonyl ligand and a peak at 1678 cm-1 attributed to the acyl-carbonyl group. The relative reactivity of the Ni-acyl bond can be inferred from the vibrational frequency of the acyl-carbonyl. Unfortunately, 1678 cm-1 indicates that the complex is most likely not susceptible to nucleophilic attack by epoxides and aldehydes.

However, based on a previous study by Brookhart and coworkers, the complex under high CO pressure may become 5-coordinate with the attachment of a second CO ligand.60 When this occurs, the vibrational frequency of the acyl carbonyl was seen to increase, and two peaks appeared in place of the CO ligand single peak. This phenomenon was studied via in situ FTIR.

Under CO at one atmosphere the characteristic peaks were observed at 1684 cm-1 and 2056 cm-

1, the frequency difference is attributed to the use of different instruments. Upon increasing the CO pressure to 200 psi the large peak at 2056 cm-1 began to diminish, and two peaks at

1980 cm-1 and 1911 cm-1 appeared. Also, the peak at 1684 cm-1 quickly disappeared, and although the poor signal to noise ratio made it difficult to detect, a peak at 1714 cm-1 was detected. The pressure was further increased to 500 psi and then 1000 psi, but with little change in the spectrum. Upon releasing the pressure the original peaks should return, indicating a reversible process. This, however, was not the case, which raises some skepticism.

The increase in vibrational frequency of the acyl carbonyl is promising, but still may not be active enough for epoxides and aldehydes. However, it may still be likely that this nickel

68 complex is an active catalyst for the copolymerization of CO and aziridines or imines, which became the main focus of the polymerization trials.

4.6 Attempted Polymerizations with Nickel Complex

A variety of polymerization conditions were screened in search of those which facilitate polymer formation. The monomers attempted include propylene oxide (PO), benzaldehyde

(BA), THF, aziridine (Az), butyl aziridine (BuAz), ethyl azridine (EtAz), N-benzylidenemethylamine

(BI), and 1-hexene (Hx). A table of all conditions attempted is provided below (Table 4.1).

Of the conditions tested, none provided a large amount of polymer. Upon removal of solvent from the product a yellow/brown oil was recovered in most cases. Although this was not desirable, many of the product’s NMR spectra did contain peaks corresponding to some activity in the system.

Table 4.1. Attempted polymerization conditions with zwitterionic nickel complex. CO Solvent Catalyst Monomer Monomer:C Temperature Run Monomer Pressure Solvent Volume (mg) (mg) atalyst ratio (°C) (psi) (ml) 1 PO 10 830 990 200 r.t. Neat - 2 PO 10 83.8 100 200 50 Toluene 1 3 THF 15 889 570 200 50 Neat - 4 BuAz 15 214 100 200 50 Toluene 1 5 BuAz 15 42.8 20 200 50 THF 1 6 BuAz 15 64.2 30 200 80 THF 10 7 BuAz 15 64.2 30 200 50 THF 10 8 BI 10 34.4 20 200 50 THF 1 9 Az 10 12.4 20 200 50 THF 5 10 EtAz 10 30.8 30 500 80 EtOH 5 11 EtAz 10 30.8 30 500 100 THF 5 12 EtAz 10 30.8 30 500 50 DME 4 13 Hx 10 36.4 30 500 50 THF 5

As expected, the PO polymerization trial did not yield any ester as indicated by the 1H

NMR. Similarly, the reactivity of benzaldehyde was tested in a J-Young tube experiment but with little success. Interestingly, the neat polymerization of THF (run 3) showed some activity. A peak in the 1H NMR centered around 4.0 ppm is characteristic of THF insertion (Figure 4.11).

The spectrum, however, is not clean and therefore it is difficult to say with complete confidence. The BI attempted polymerization showed no activity, nor did the BuAz trials under dilute conditions (runs 6 and 7). However, more concentrated BuAz polymerizations were more successful. Polymerization trials 4 and 5 possessed peaks at the positions that correspond to amide formation. In fact, run 5 in THF possessed the peak corresponding to ester incorporation as well (Figure 4.12).

510_10-21-11-THFpzn_200psi_55C

0.9

0.8

0.7

0.6

0.5

0.4

Normalized Intensity Normalized 0.3

0.2

0.1

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.11. 1H NMR spectrum of product from CO/THF copolymerization trial.

515_10-23-11_BuAzirPzn_inTHF

0.9

0.8

0.7

0.6

0.5

0.4 Normalized Intensity Normalized

0.3

0.2

0.1

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 -0.5 Chemical Shift (ppm) Figure 4.12. 1H NMR spectrum of product from CO/butyl aziridine copolymerization in THF.

The attempted polymerization of aziridine (run 9) was very interesting in that the 1H

NMR showed β-lactam formation (Figure 4.13). This most likely occurs through a nickel-hydride intermediate and aziridine insertion, resulting in an amine end group. After CO insertion the amine can then attack the newly formed carbonyl group causing the formation of β-lactam.

Although this is not the desired product, it is promising in that it shows that aziridine has some activity in the system. Slight tuning of the reaction conditions may lead to polymer formation.

519_10-27-11_AzirPzn_50C-80C

0.9

0.8 O NH 0.7

0.6 b a 0.5 a b

0.4 Normalized Intensity Normalized

0.3

0.2

0.1

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.13. 1H NMR spectrum of product from CO/aziridine copolymerization trail.

The final aziridine polymerizations attempted employed ethyl aziridine. First, ethanol was used as the solvent due to its higher polarity. Unfortunately no desirable product was obtained, as it is likely that ethanol reacted rather than aziridine. Next, increased temperature and pressure in THF was attempted. This again resulted in the recovery of an oily product, but the 1H NMR spectrum showed promise (Figure 4.14). In addition, an FTIR spectrum was recorded that contained the signature amide peak at 1635 cm-1. Amide formation is clear, but the activity is still not high enough for polymer formation. A similar result occurred from the attempted ethyl aziridine copolymerization in dimethoxyethane (DME). The 1H NMR spectrum contained broad peaks at the correct chemical shifts in addition to a number of unidentifiable peaks, but again an oily product was obtained.

523_10-29-11_EtAzir_THF_100C

0.9 O 0.8 c 0.7 N d 0.6 n

b 0.5 a

0.4 Normalized Intensity Normalized

0.3

0.2 b a

0.1 c d

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm)

Figure 4.14. 1H NMR spectrum of product from CO/ethyl aziridine copolymerization in THF.

The final polymerization attempted was that of 1-hexene in THF. After removal of solvent, a brown oil was recovered, although much more viscous than usual. 1H NMR analysis showed no signs of polycarbonate, but there was the corresponding peak near 4.0 ppm possibly attributable to THF incorporation (Figure 4.15). As confirmation, an IR spectrum was obtained, and a peak was present at 1737 cm-1, further evidence of ester formation. Further studies must be conducted to improve the yield and purity of the product.

527_11-4-11_1-hexenePzn

0.9

0.8

0.7

0.6

0.5

0.4

Intensity Normalized 0.3

0.2

0.1

8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) Figure 4.15. 1H NMR spectrum of CO / 1-hexene copolymerization in THF.

CHAPTER V

CONCLUSION

The synthesis of a novel zwitterionic nickel complex was completed and well characterized via NMR and IR spectroscopy and X-ray crystallography. The final complex proved to be a four-coordinate square planar species containing a bidentate phosphine-borate ligand, acetyl ligand, and CO ligand. The reactivity was found to be lower than anticipated, and therefore the focus of the project was redirected from attempted polyester synthesis to polyamide synthesis. After screening a number of polymerization conditions, 1H NMR and IR spectra showed the correct amide peaks, but a solid, pure product was never obtained. It is believed that initial insertion of aziridine occurs, and even oligomers may have formed, but the activity of the catalyst is not high enough to promote polymer formation. Also, THF ester incorporation in the copolymerizations may have been achieved. Altering of the ligand backbone by placing electron withdrawing groups on the phosphine donors may suffice to decrease the electron donating ability of the ligand and improve the reactivity of the complex.

Further studies in this area would be necessary.

CHAPTER VI

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