Mechanism-Based Design and Optimization of a Catalytic Electrophilic Cyclopropanation without Diazomethane

Iodomethylboron compounds, either
the trifluoroborate or a boronic
ester, cyclopropanate electron-rich olefins and unprotected allylic
alcohols with Pd catalysts according to a novel, designed catalytic
cycle. Proposed intermediates in a “diverted Heck” mechanism
are observed by means of spectroscopic studies and by isolation and
X-ray crystallographic characterization, which together with reaction
kinetics point to a separation of rate-determining and product-determining
steps, and a mechanism-based optimization of the yield, selectivity,
and scope of the catalytic electrophilic cyclopropanation. The reaction
with crystalline, air-stable, nonhygroscopic, and nontoxic reagents
provides an alternative to Simmons–Smith-type reactions, as
well as cyclopropanation procedures that require the use of diazomethane.