Asymmetric bromination of 4-methylcyclohexene in the presence of dihydrocinchonine

The asymmetric bromination of 4-methylcyclohexene (1) in the presence of dihydrocinchonine gave a levorotatory mixture of trans-3-cis-4- (2) and cis-3-trans-4-dibromo-1-methylcyclohexane (3), containing an excess of (−)-2 and (−)-3. The steric course and optical yields of the reaction were completely analyzed, on the basis of polarimetric, chromatographic and NMR methods, and the absolute configurations of 2 and 3 were deduced from the results of the bromination of (+)-1. The hypothesis that the bromination of alkenes with bromine-dihydrocinchonine gives an excess of the (R:R)-enantiomer was thus confirmed. Partial asymmetric bromination of (±-1 gave (−)-2 of higher optical purity, and the recovered olefin was dextrorotatory. The thermal isomerization of 2 to 3 was also investigated.