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-(diphenylphosphino)benzylidene]-2,2 ' -diamino-6,6 ' -dimethylbiphenylene) catalyzes the cyclopropanation of styrene by decomposition of diazoesters. The cis cyclopropane derivative is formed with moderate selectivity (cis:trans ratio 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% and enantioselectivity up to > 99% ee. High cis selectivity is obtained also with 2,5-dimethyl-2,4-hexadiene as substrate. Complex 3a yields ethyl chrysanthemate with 94% cis selectivity and enantioselectivity up to 80% ee. The putative carbene intermediate trans-[RuCl(C(H)COOEt)(Ib)](+) (11b) was prepared and characterized spectroscopically in solution. Its reaction with styrene gives the cyclopropane derivative with 98:2 cis:trans selectivity. The steric constraints in the transition states involving 11b and styrene were estimated by means of molecular modeling calculations. The relative total energies of the four diastereomeric aggregates follow the experimental enantio- and diastereoselectivity trends. The model proposed also predicts the correct absolute configuration of both cis and trans cyclopropane products.