A new cyclopropanation reaction involving Calpha-Si bond insertion of alkylidene carbenes derived from alpha-silyl ketones has been developed. This unprecedented alkylidene carbene reactivity features excellent selectivity for insertion into Calpha-Si bonds rather than insertion into Cgamma-H bonds or addition to gamma,delta-double or -triple bonds. The selectivity trend clearly indicates that the alpha-oxygen in the tether significantly promotes Cgamma-H insertion, although the Calpha-Si bond insertion still competes effectively.
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