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

• Published in: Chemistry : a European journal Vol. 19; no. 51; pp. 17559 - 17566
• Main Authors: Iglesias, Manuel, Sanz Miguel, Pablo J., Polo, Victor, Fernández-Alvarez, Francisco J., Pérez-Torrente, Jesús J., Oro, Luis A.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16-12-2013; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Acetone; Alkynes; carbenes; Chemistry; homogeneous catalysis; Hydrosilylation; Imidazole; Isomerization; Ligands; Metal hydrides; Selectivity; silanes; carbenes; hydrosilylation; silanes; homogeneous catalysis; alkynes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201303063

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.
Silanes catch the shuttle: An outer‐sphere mechanism that explains the β‐Z hydrosilylation of terminal alkynes based on the role of acetone as a silane shuttle is disclosed. Heterolytic splitting of the silane molecule by the metal centre and acetone affords a metal hydride and the oxocarbenium ion [R3SiO(CH3)2]+, which reacts with the alkyne in solution to give the silylation product [R3SiCHCR]+ (see figure).