Enantioselective Cyclopropanation of Allylic Alcohols. The Effect of Zinc Iodide.

The effect of zinc iodide on the catalytic, enantioselective cyclopropanation of allylic alcohols is examined with bis(iodomethyl)zinc as the reagent and bis-methanesulfonamide 7 as the catalyst. Significant rate enhancement was observed when 1 equiv of zinc iodide was present, but more importantly, the enantiomeric excess of the product cyclopropane increased from 80% to 89% for the substrate cinnamyl alcohol. Reaction studies and spectroscopic investigations show that this remarkable influence is the result of reagent modification via a Schlenk equilibrium that produces the more reactive and selective species (iodomethyl)zinc iodide.