Part II. Chiral dirhodium (II) catalysts with ortho-metalated arylphosphane ligands in the enantioselective intramolecular cyclopropanation of a racemic α-diazo ketone

In this report, chiral dirhodium (II) with ortho-metalated phosphane ligands, namely (M)-Rh2(O2CR) 2(PC)2 [PC = ortho-metalated aryl phosphane, O2CR = carboxylate bridging ligands) (1a–g), have been used for the intramolecular cyclopropanation of racemic1-diazo-6-methyl-3-(2-propenyl)-5-hepten-2-one (2), containing both a tri- and monosubstituted carbon–carbon double bond, in pentane. The highest level of regiocontrol has been obtained with chiral catalyst Rh2(O2CCH3)2[(p-MeC6H3)P(p-MeC6H4)2]2 (M)-1c, affording favorably trisubstituted cyclopropane 3 versus monosubstituted cyclopropane 4 in 74:26 ratio. An exceptional diastereoselectivity was obtained with the entire catalyst series, leading to the unique formation of the syn products. Excellent enantiocontrol values (80–90% ee) have been achieved with catalysts 1a [(PC = (C6H3)P(C6H4)2, R = C(CH3)3)], 1c [PC = p-MeC6H3)P(p-MeC6H4)2, R = CH3], 1f [PC = m-CH3C6H3)P(m-CH3C6H4)2, R = CF3] and 1g [PC = 3,5-(CH3)2C6H3)P(3,5-(CH3)2C6H4)2, R = CF3] at room temperature. Pentane is found to be a convenient solvent for high enantiocontrol in the cyclopropanation of α-diazo ketone 2.