Rhodium-mediated intramolecular C-H insertion: Probing the geometry of the transition state

The unsymmetrical alpha-diazo ester 1b, derived from the chiral naphthylborneol 5, was cyclized with a set of rhodium carboxylates (L = CF3CO2, CH3CO2, CH3(CH2)(6)CO2, (CH3)(3)CCO2). The ratio of the three major products ((R,R)-3b, (S,S)-3b, 4b) was determined. It was found that the rhodium catalyst derived from pivalic acid gave the highest ratio of 3b:4b. Lowering the temperature of the reaction (L = (CH3)(3)CCO2) increased both the yield and the diastereoselectivity of the cyclization. From these results and from our computational analysis, it is concluded that the ester carbonyl and the rhodium carbenoid are probably syn in the transition state leading to cyclization.