Polar addition to olefins. Part II. Stereochemistry of addition of deuterium bromide to cis- and trans-t-butylstyrene. Rotamer populations of sterically crowded trisubstituted ethanes

Abstract

Addition of deuterium bromide to trans- and cis-β-t-butylstyrene in methylene chloride, at 0 °C, gives erythro- and threo-1-bromo-3,3-dimethyl-1-phenylbutane, the polar addition products. With the trans-olefin 81% of cis-addition was found, whereas with the cis-olefin 67% of cis-adduct is formed. The mechanistic implications of these results are discussed. The vicinal proton couplings for the addition product and for the related 2-hydroxy-(and acetoxy-)3,3-dimethyl-1-phenylbutanes are reported. Although the rotamers with gauche phenyl and hydroxy-groups are overwhelmingly populated, the bromo-product exists preferentially as the rotamer with gauche phenyl and t-butyl groups. No evidence for the rotamer with three gauche substituents is observed, in agreement with predictions from previous work.