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

摘要

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 hours, affording the desired β-silyl carbonyls in 87:13 to>98:2 enantiomeric ratio and in 50% to >98% yield. Cyclic enones or lactones and acyclic α,β-unsaturated ketones, esters as well as aldehydes can be used as substrates.