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1,2-Selective Hydrosilylation of Conjugated Dienes 1,2-Selective Hydrosilylation of Conjugated Dienes The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Parker, Sarah Elizabeth. 2014. 1,2-Selective Hydrosilylation of Conjugated Dienes. Doctoral dissertation, Harvard University. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:12274560 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA 1,2-SELECTIVE HYDROSILYLATION OF CONJUGATED DIENES A dissertation presented by Sarah Elizabeth Parker to The Department of Chemistry and Chemical Biology in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the subject of Chemistry Harvard University Cambridge, Massachusetts April 2014 © 2014 Sarah Elizabeth Parker All rights reserved Dissertation Advisor: Professor Tobias Ritter Sarah Elizabeth Parker 1,2-Selective Hydrosilylation of Conjugated Dienes Abstract Selective 1,2-hydrosilylation of 1,3-dienes is a challenging problem to solve for transition metal catalysis. Butadiene, specifically, would be a useful substrate because 3-butenylsilane products have promise as superior coupling reagents for hybrid organic/inorganic materials synthesis. In this thesis, we describe the first selective 1,2-hydrosilylation of conjugated dienes, including butadiene. Chapter 1 describes the relevance of organosilicon compounds to modern applications such as the synthesis of advanced materials and fine chemicals. The history and development of transition metal- catalyzed hydrosilylation methods are discussed, with emphasis on the current state of understanding of the mechanisms of late-transition metal-catalyzed hydrosilylation. Known methodologies for the hydrosilylation of dienes and known 1,2-additions to dienes are discussed, highlighting the mechanistic features of these processes that informed the presented work. In Chapter 2, novel research on the discovery and development of platinum catalysts for the 1,2- selective hydrosilylation of dienes is presented. Discovery of a cyclometallated platinum complex of tri- tert-butylphosphine as a precatalyst enabled the 1,2-selective hydrosilylation of conjugated dienes, including butadiene, isoprene, myrcene, and 2,3-dimethylbutadiene. Catalysis proceeds through a Pt(II)/Pt(IV) catalytic cycle and selectivity for 1,2-addition arises from coordinative saturation at catalyst intermediates that prevents formation of π-allyl complexes. Data are presented that support the proposed Pt(II)/Pt(IV) mechanism and exclude more conventional Pt(0)/Pt(II) mechansims. iii Table of Contents 1. INTRODUCTION ................................................................................................................................ 1 1.1 Importance of Hydrosilylation ...................................................................................................... 2 1.1.1. Organosilanes in Sol-Gel Materials Synthesis ...................................................................... 4 1.1.2. Coupling Agents for Hybrid Materials Synthesis ............................................................... 11 1.1.3. Applications of Organosilanes in Organic Synthesis .......................................................... 17 1.2. Transition Metal-Catalyzed Hydrosilylation............................................................................... 21 1.2.1. Homogeneous Platinum-Catalyzed Hydrosilylation ........................................................... 21 1.2.2. Catalysis by Other Transition Metals .................................................................................. 30 1.2.3. Mechanism .......................................................................................................................... 34 1.3. Transition Metal-Catalyzed Hydrosilylation of Conjugated Dienes ........................................... 45 1.3.1. Challenges of Selective Addition to Conjugated Dienes .................................................... 45 1.3.2. Hydrosilylation of Butadiene .............................................................................................. 48 1.3.3. Hydrosilylation of Substituted Dienes ................................................................................ 53 1.3.4. Development of Selective Additions to Conjugated Dienes in the Ritter Research Group 58 1.3.5. 1,2-Selective Additions to Conjugated Dienes ................................................................... 60 2. RESULTS AND DISCUSSION ......................................................................................................... 64 2.1. Strategy for 1,2-Selective Hydrosilylation Catalyzed by Transition Metals .............................. 66 2.2. Development of Platinum Catalysts for 1,2-Selective Hydrosilylation of Butadiene ................. 68 2.2.1. Catalyst Discovery .............................................................................................................. 68 2.2.2. Hydrosilylation of 1,3-Dienes by Cyclometallated Platinum Complexes .......................... 73 2.3. Mechanism of 1,2-Selective Hydrosilylation .............................................................................. 78 2.3.1. Activation of Platinum Catalyst 16 ..................................................................................... 78 2.3.2. Proposed Mechanism for 1,2-Selective Hydrosilylation..................................................... 80 2.3.3. Counterion Effect ................................................................................................................ 83 2.3.4. Platinacycle Size ................................................................................................................. 93 2.3.5. Evidence Against a Chalk-Harrod-Like Pt(0)/Pt(II) Catalytic Cycle ................................. 96 2.3.6. Alternate Mechanism Proposals ........................................................................................ 102 3. CONCLUSIONS AND OUTLOOK ................................................................................................. 106 4. EXPERIMENTAL ............................................................................................................................ 108 4.1. Materials and Methods .............................................................................................................. 108 iv Table of Contents (Continued) 4.2. Synthesis of Platinum Complexes ............................................................................................. 110 4.3. Synthesis of Butenylsilanes by Hydrosilylation ....................................................................... 135 4.4. Synthesis of Reagents, Ligands, and Substrates ....................................................................... 143 4.5. Optimization of Hydrosilylation Using Precatalysts 16 and 19 ................................................ 149 4.5.1. Comparison of Reductants ................................................................................................ 149 4.5.2. Comparison of Solvents .................................................................................................... 152 4.5.3. Comparison of Silanes ...................................................................................................... 154 4.5.4. Room Temperature Reaction Initiation ............................................................................. 155 4.6. Mechanism Elucidation............................................................................................................. 157 4.6.1. Reaction of 32 with Oxidants ............................................................................................ 157 4.6.2. Observation of Catalyst/Ligand Products after Hydrosilylation ....................................... 157 4.6.3. Detection of Methane During Catalyst Activation ............................................................ 159 4.6.4. Test of Butenylsilane Reductive Elimination from Precatalyst 36 ................................... 161 4.6.5. Comparison of Platinum Pre-Catalysts ............................................................................. 163 4.6.6. Isomerization of 3-Butenylsilane to 2-Butenylsilane ........................................................ 174 4.6.7. Rate Comparison of Precatalysts 32 and 37 ...................................................................... 176 4.6.8. Reaction Timecourse for Hydrosilylation Catalyzed by 16, 19 and 36 ............................ 179 v List of Figures and Schemes Scheme 1. Synthesis of Hydrophobic Silicate Materials .............................................................................. 9 Scheme 2. Mechanism of Organosilane Condensation on a Silicate Surface ............................................. 13 Scheme 3. Mechanism of Adhesion by Covalent Coupling Reagents ........................................................ 13 Scheme 4. Sakurai Allylation ..................................................................................................................... 19 Scheme 5. Peterson Olefination .................................................................................................................. 20 Scheme 6. Tamao-Fleming Oxidation .......................................................................................................
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