Cyclopropanes
are common building blocks in pharmaceuticals, agrochemicals,
and organic materials. The most general methods for the synthesis
of chiral cyclopropanes are catalytic additions of diazoalkanes to
alkenes. However, a limitation of this approach is that diazoalkanes
can only be safely handled on preparative scales if they possess stabilizing
substituents. Here we show that gem-dichloroalkanes
can serve as precursors to nonstabilized carbenes for asymmetric cyclopropanation
reactions of alkenes. The process uses a cobalt catalyst and is proposed
to involve the formation of a cationic carbenoid species bearing structural
resemblance to the Simmons–Smith reagent. High levels of enantioselectivity
are observed for monosubstituted, 1,1-disubstituted, and internal
alkenes. The reaction is compatible with alkyl-substituted carbenes,
which are susceptible to undergoing competing 1,2-hydride shifts.
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