An Alternative Mechanistic Paradigm for the β- Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle.

The β- Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)₂{κ-C,C,O,O-(bis-NHC)}]BF₄ (M=Ir ( 3 a), Rh ( 3 b); bis-NHC=methylenebis( N-2-methoxyethyl)imidazole-2-ylidene) support a new mechanism, alternative to classical postulations, based on an outer-sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R₃SiO(CH₃)₂]⁺, which reacts with the corresponding alkyne in solution to give the silylation product [R₃SiCHCR]⁺. Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R₃SiCHCR]⁺ affords selectively the β-( Z)-vinylsilane. The β- Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β-( E)-vinylsilanes. [ABSTRACT FROM AUTHOR]