Rh(II)-Catalyzed Reactions of Diazoesters with Organozinc
Reagents
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Abstract
Rh(II)-catalyzed reactions of diazoesters
with organozinc reagents
are described. Diorganozinc reagents participate in reactions with
diazo compounds by two distinct, catalyst-dependent mechanisms. With
bulky diisopropylethyl acetate ligands, the reaction mechanism is
proposed to involve initial formation of a Rh-carbene and subsequent
carbozincation to give a zinc enolate. With Rh<sub>2</sub>(OAc)<sub>4</sub>, it is proposed that initial formation of an azine precedes
1,2-addition by an organozinc reagent. This straightforward route
to the hydrazone products provides a useful method for preparing chiral
quaternary α-aminoesters or pyrazoles via the Paul–Knorr
condensation with 1,3-diketones. Crossover and deuterium labeling
experiments provide evidence for the mechanisms proposed