Enantioselective Addition-Alkylation of α,β-Unsaturated Carbonyls via Bisguanidinium Silicate Ion Pair Catalysis

Silicon
hydrides, alkynylsilanes, and alkoxylsilanes were activated
by fluoride in the presence of bisguanidinium catalyst to form hypervalent
silicate ion pairs. These activated silicates undergo 1,4-additions
with chromones, coumarins, and α-cyanocinnamic esters generating
enolsilicate intermediates, for a consequent stereoselective alkylation
reaction. The reduction-alkylation reaction proceeded under mild conditions
using polymethylhydrosiloxane, a cheap and environmentally friendly
hydride source. The addition-alkylation reactions with alkynylsilanes
and alkoxylsilanes resulted in the construction of two vicinal chiral
carbon centers with excellent enantioselectivities and diastereoselectivities
(up to 99% ee, >99:1 dr). Density functional theory calculations
and
experimental NMR studies revealed that penta-coordinated silicates
are crucial intermediates.