Enantioselective Diarylcarbene Insertion into Si–H Bonds Induced by Electronic Properties of the Carbenes

Catalytic
enantioselection usually depends on differences in steric
interactions between prochiral substrates and a chiral catalyst. We
have discovered a carbene Si–H insertion in which the enantioselectivity
depends primarily on the electronic characteristics of the carbene
substrate, and the log­(er) values are linearly related to Hammett
parameters. A new class of chiral tetraphosphate dirhodium catalysts
was developed; it shows excellent activity and enantioselectivity
for the insertion of diarylcarbenes into the Si–H bond of silanes.
Computational and mechanistic studies show how the electronic differences
between the two aryls of the carbene lead to differences in energies
of the diastereomeric transition states. This study provides a new
strategy for asymmetric catalysis exploiting the electronic properties
of the substrates.