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

2013; Wiley; Volume: 19; Issue: 51 Linguagem: Inglês

10.1002/chem.201303063

1521-3765

Manuel Iglesias, Pablo J. Sanz Miguel, Víctor Polo, Francisco J. Fernández‐Álvarez, Jesús J. Pérez‐Torrente, Luis A. Oro,

Catalytic C–H Functionalization Methods

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 (3a), Rh (3b); 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.