Vitamin B12 derivatives as natural asymmetric catalysts:: Enantioselective cyclopropanation of alkenes

Vitamin B-12 derivatives were found for the first time to be general and efficient catalysts for asymmetric cyclopropanation of alkenes with ethyl diazoacetate (EDA). Among several common derivatives, aquocobalamin (B-12a) was shown to be the most effective catalyst for a variety of alkenes, providing cis-dominant cyclopropanes in excellent Yields and moderate enantioselectivity. Reactivity studies under different conditions suggest that the active species in the proposed catalytic cycle is the base-on cob(II)alamin (B-12r) that is generated possibly via in situ reduction of B-12a by EDA.