Cis-selective asymmetric cyclopropanation of olefins catalyzed by five-coordinate [RuCl(PNNP)](+) complexes

The five-coordinate complex [RuCl(la)]PF6 (3a; la = (S)-N,N ' -bis[2-(diphe nylphosphino)benzylidene]-2,2 ' -diamino-6,6 ' -dimethylbiphenylene) cataly zes the cyclopropanation of styrene by decomposition of diazoesters. The ci s cyclopropane derivative is formed with moderate selectivity (cis:trans ra tio is 48:52), but high enantioselectivity (90-96% eel. The related species [RuCl(lb)]PF6 (3b; Ib = N,N ' -bis [2-(diphenylphosphino)benzylidene]-(1S, 2S)-diaminocyclohexane) gives the cis product with selectivity up to 95% an d enantioselectivity up to > 99% ee. High cis selectivity is obtained also with 2,5-dimethyl-2,4-hexadiene as substrate. Complex 3a yields ethyl chrys anthemate with 94% cis selectivity and enantioselectivity up to 80% ee. The putative carbene intermediate trans-[RuCl(C(H)COOEt)(Ib)](+) (11b) was pre pared and characterized spectroscopically in solution. Its reaction with st yrene gives the cyclopropane derivative with 98:2 cis:trans selectivity. Th e steric constraints in the transition states involving 11b and styrene wer e estimated by means of molecular modeling calculations. The relative total energies of the four diastereomeric aggregates follow the experimental ena ntio- and diastereoselectivity trends. The model proposed also predicts the correct absolute configuration of both cis and trans cyclopropane products .