Cyclopropanation catalyzed by osmium porphyrin complexes

Abstract

The authors report herein the use of osmium porphyrins as stereoselective cyclopropanation catalysts using ethyl diazoacetate with a variety of alkenes. In addition, their studies show that an isolable carbene complex ((TTP)Os[double bond]CHCO[sub 2]Et) is capable of catalytically and stoichiometrically cyclopropanating styrene. Several significant aspects have evolved from the use of osmium meso-tetra-p-tolyporphyrin complexes as catalysts for the cyclopropanation of a variety of alkenes by ethyl diazoacetate. This system provides the highest anti/syn isomer ratio reported to date (a/s = 10) for the catalytic cyclopropanation of styrene by ethyl diazoacetate. Unlike typical cyclopropanation catalysts which produce cyclopropenes from alkyne substrates, the osmium porphyrin catalysts generate bicyclobutanes from phenylacetylene. Moreover, the authors have isolated, on preparative scale, the first carbene complex, (TTP)Os[double bond]CHCO[sub 2]Et, that is catalytically active toward cyclopropanation. The fact that this carbene complex can stoichiometrically cyclopropanate styrene with the same stereoselectivity as in the catalytic process is further evidence for it as an important species in the catalytic cycle. 12 refs., 1 tab.