Reaction Pathways of the Simmons−Smith Reaction

The cyclopropanation reaction of an alkene with a metal carbenoid has been studied by means
of the B3LYP hybrid density functional method. The cyclopropanation of ethylene with a lithium carbenoid
or a zinc carbenoid [Simmons−Smith (SS) reagent] goes through two competing pathways, methylene
transfer and carbometalation. Both processes are fast for the lithium carbenoid, while, for the zinc carbenoid,
only the former is fast enough to be experimentally feasible. The reaction of an SS reagent (ClZnCH2Cl)
with ethylene and an allyl alcohol in the presence of ZnCl2 was also studied. The allyl alcohol reaction was
modeled with an SS reagent/alkoxide complex (ClCH2ZnOCH2CHCH2) formed from the SS reagent and
allyl alcohol. Two modes of acceleration were found. The first involves the well-accepted mechanism of
1,2-chlorine migration, and the second involves a five-centered bond alternation. The latter was found to
be more facile than the former and to operate equally well both with ethylene and with aggregates of SS
reagent/alkoxide complexes. Calculations on the SS reaction with 2-cyclohexen-1-ol offer a reasonable
model for the hydroxy-directed diastereoselective SS reaction, which has been used for a long time in
organic synthesis.