Cyclopropanation of alkenes can be accomplished catalytically2 or stoichiometrically.3 Catalytic systems typically use a diazo reagent as the carbene source and a metal-containing mediator which forms a postulated metal carbene intermediate. Transfer of the carbene fragment from the metal to an alkene produces the cyclopropane product. Despite the wide variety of catalytic cyclopropanation systems, the putative carbene complex has never been isolated or observed in a catalytic system. This is somewhat surprising since the second category of cyclopropanation reactions involves the stoichiometric reaction of isolated car bene complexes with an alkene to form a cyclopropane. None of the isolated carbene complexes show catalytic cyclopropanation activity. Several years ago Callot demonstrated that rhodium porphyrins catalytically cyclopropanated a variety of alkenes in the presence of ethyl diazoacetate.4 Kodadek and co-workers have expanded this work and have attempted to prepare synthetically useful enantioselective catalysts for the formation of cyclopropanes.5 Their approach has been to use rhodium complexes with optically active porphyrins to induce chirality into the product. A similar approach was used for a variety of non-porphyrin copper catalysts.6 Kodadek has shown that the carbon-bound diazonium complex [(TTP)RhC(H)(C02Et)(N2W is an intermediate in the catalytic cyclopropanation of styrene with ethyl diazoacetate.7•8 In addition, kinetic studies suggest that the formation of a rhodium carbene complex is at least partially rate limiting.8 However, this carbene complex has not been isolated or directly observed. We report herein the use of osmium porphyrins as stereoselective cyclopropanation catalysts using ethyl diazoacetate with a variety of alkenes. In addition, our studies show that an isolable carbene complex ((TTP)Os=CHC02Et) is capable of catalytically and stoichiometrically cyclopropanating styrene.
Available at: http://works.bepress.com/l-woo/16/