A Highly Regio‐, Diastereo‐ and Enantioselective Intramolecular Cyclopropanation Reaction of a Racemic α‐Diazo Ketone Catalyzed by Chiral ortho‐Metalated Dirhodium(II) Compounds.

Abstract

Abstract A series of racemic dirhodium(II) compounds with two ortho-metalated aryl phosphine ligands in a head-to-tail arrangement Rh2(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 Rh2(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.