|Ph.D Student||Kostenko Arseni|
|Subject||Cooperative M1/M2 (M1=Pt,Pd; M2=Hg,Zn) Homogeneous|
Catalysis of Hydrosilylation
|Department||Department of Chemistry||Supervisors||? 18? Yitzhak Apeloig|
|Dr. Dmitry Bravo-Zhivotovs|
Hydrosilylation is the reaction of addition of a Si-H bond across carbon-carbon and carbon-heteroatom unsaturated bonds (e.g. C=C, C≡C, C=O, C=N). It is used as a critical step in the synthesis of organic molecules. In industry hydrosilylation is a multimillion ton scale process for production of organosilicon compounds that are used as building blocks for polysiloxane polymers known as silicones. Hydrosilylation of unsaturated bonds is most efficiently achieved using transition metal catalysts. Because of the great synthetic significance of hydrosilylation current research in the field is directed to finding new approaches that will improve the efficiency and selectivity of the process by choosing appropriate catalytically active metal complexes, co-factors, ligand design, etc.
In this thesis a new mechanistic approach for hydrosilylation of alkynes (RC≡CR) is presented. We describe, for the first time, that hydrosilylation of alkynes can proceed catalytically via vinylmetals. This is achieved by combination of tris(trialkylphosphine)Pt(0) complexes with a bis(trialkylsilyl)M (M = Zn, Hg) in catalytic loads of less than 1 mol%. This combination forms a catalytic complex that acts as a carrier of the bis(trialkylsilyl)M function to alkynes forming the vinylmetal intermediates R(R3Si)C=C(MSiR3)R. The vinylmetal intermediates, in the presence of a silane and the catalytic complex, are transformed by a radical substitution to the alkenylsilane product R(R3Si)C=CHR with regeneration of the bis(trialkylsilyl)M reagent. The mechanism of this transformation was studied both computationally (using ab initio DFT quantum mechanical calculations) and experimentally and the catalytic abilities of the system were demonstrated on several silanes and acetylenes. We found that the combination of tris(trialkylphosphine)Pt(0) with bis(trialkylsilyl)M allows hydrosilylation using very bulky silanes such as i-Pr3SiH and t-Bu2MeSiH for which conventional catalysts show low efficiency.
The Pt-M heterometallic complexes responsible for the catalytic activity were fully characterized using NMR spectroscopy and X-ray crystallography. These complexes comprise a novel type of hexacoordinated 18-electron Pt(IV) complexes having the general formula (PR3)2Pt(MSiR3)3(SiR3). The processes which lead to their formations were investigated by reactions of different Pt(0) complexes with bis(silyl)M (M = Hg, Zn). These reactions yield a variety of novel Pt-M heterometallic complexes depending on the phosphine ligands, the substituents of the bis(silyl)M and the reaction conditions. Thus, complexes with the general formulas: (dape)Pt(MSiR3)(SiR3) (dape = 1,2−bis(dialkylphospino)ethane), (dape)Pt(MSiR3)2, (dape)Pt(MSiR3)2(SiR3)2, M[(dape)Pt(MSiR3)(SiR3)]2, M[(dape)Pt(SiR3)]2, M[(PR3)3Pt]2 were synthesized and characterized by X-Ray crystallography.
We find that the main processes which result from the reactions of the Pt(0) complexes with bis(silyl)M are: a) oxidative addition of Pt into the M-Si bond; b) reductive elimination of disilane; c) dissociation of the -M-SiR3 substituent. We demonstrate that oxidative addition of Pt to the Si-M bond of (R3Si)2M yields complexes with Si-Pt-M-Si chains in which the Si-M bond is weaker than in (R3Si)2M and therefore undergoes homolytic cleavage under irradiation or mild heating (60-100°C). Thus, these complexes act as radical initiators giving rise to radical chemistry such as radical substitution and radical reduction reactions. We describe experimentally and computationally how the Pt(0) complexes activate M−Si bonds producing previously unknown Pt(III) and Pt(I) radicals which were characterized by EPR spectroscopy.