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)}]BF 4 (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 [R 3 SiO(CH 3 ) 2 ] + , which reacts with the corresponding alkyne in solution to give the silylation product [R 3 SiCHCR] + . 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 [R 3 SiCHCR] + 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.