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RESEARCH PRODUCT

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

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Abstract In this report, chiral dirhodium (II) with ortho -metalated phosphane ligands, namely ( M )-Rh 2 (O 2 CR) 2 (PC) 2 [PC =  ortho -metalated aryl phosphane, O 2 CR = 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 Rh 2 (O 2 CCH 3 ) 2 [( p -MeC 6 H 3 )P( p -MeC 6 H 4 ) 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 = (C 6 H 3 )P(C 6 H 4 ) 2 , R = C(CH 3 ) 3 )], 1c [PC =  p -MeC 6 H 3 )P( p -MeC 6 H 4 ) 2 , R = CH 3 ], 1f [PC =  m -CH 3 C 6 H 3 )P( m -CH 3 C 6 H 4 ) 2 , R = CF 3 ] and 1g [PC = 3,5-(CH 3 ) 2 C 6 H 3 )P(3,5-(CH 3 ) 2 C 6 H 4 ) 2 , R = CF 3 ] at room temperature. Pentane is found to be a convenient solvent for high enantiocontrol in the cyclopropanation of α-diazo ketone 2 .