Metal-Free Catalytic C–Si Bond Formation in an Aqueous Medium. Enantioselective NHC-Catalyzed Silyl Conjugate Additions to Cyclic and Acyclic α,β-Unsaturated Carbonyls

A metal-free method for enantioselective conjugate addition of a dimethylphenylsilyl group to α,β-unsaturated carbonyls is reported. Transformations are catalyzed by a chiral N-heterocyclic carbene (NHC), performed in an aqueous solution (3:1 mixture of water and tetrahydrofuran) and are operationally simpler to perform than the NHC-Cu-catalyzed variant. The chiral catalyst is generated from an enantiomerically pure imidazolinium salt (prepared in three steps) and a common organic amine base (dbu). NHC-catalyzed processes proceed with 5.0-12.5 mol % catalyst loading at 22 °C within 1-12 h, affording the desired β-silyl carbonyls in 85:15 to >98:2 enantiomeric ratio and in 50% to >98% yield. Cyclic enones or lactones and acyclic α,β-unsaturated ketones, esters, and aldehydes can be used as substrates.