A highly regio-, diastereo- and enantioselective intramolecular cyclopropanation reaction of a racemic α-diazo ketone catalyzed by chiral ortho-metalated dirhodium(II) compounds

A series of racemic dirhodium(II) compounds with two ortho-metalated aryl phosphine ligands in a head-to-tail arrangement Rh-2(O2CR)(2)(Pc)(2) (pc=ortho-metalated aryl phosphine) (1a-k) were tested in the regio- and stereoselective cyclopropanation of racemic 1-diazo-6-methyl-3-(2-propenyl)-5-hepten-2-one 2, which possesses two different reactive C-C double bonds for a five-membered-ring formation. The complexes Rh-2(O2CCH3)(2)(pc)(2) {pc=[(C6H4)P(C6H5)(2)], [(p-CH3C6H3)P(p- CH3C6H4)(2)], and [(C6H4)P(C6H5)(C6F5)]} (1a-d) successfully enhanced the cyclopropanation of trisubstituted versus monosubstituted C=C bonds to give an 80:20 selectivity ratio. The reaction occurred with excellent diastereoselectivity; the syn-products were the only stereoisomers observed in the whole series of the catalysts. Enantioenriched products were obtained when enantiomerically pure dirhodium(II) complexes were used. (C) 2003 Elsevier Science Ltd. All rights reserved.