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

Abstract

A series of racemic dirhodium(II) compounds with two ortho-metalated aryl phosphine ligands in a head-to-tail arrangement Rh 2(O 2CR) 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(O 2CCH 3) 2(pc) 2 {pc=[(C 6H 4)P(C 6H 5) 2], [( p-CH 3C 6H 3)P( p-CH 3C 6H 4) 2], and [(C 6H 4)P(C 6H 5)(C 6F 5)]} ( 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.