Electrophilic amination of catecholboronate esters formed in the asymmetric hydroboration of vinylarenes

Article abstract of DOI:10.1016/S0040-4020(97)00722-9

(S)-(4-Methoxyphenyl)-ethyl-1,3,2-benzodioxaborole, (S)-1-(4-chlorophenyl)ethyl-1,3,2-benzodioxaborole and (S)-1-indanyl-1,3,2-benzodioxaborole, intermediates in the catalytic asymmetric hydroboration of 4-chloro- and 4-methoxystyrene, were isolated as pure oils in 75%, 84% and 49% yield respectively. For the first example, amination with N-chloromagnesio-N-methyl-O-trimethylsilylhydroxylamine gave a mixture of (S)-1-(4-methoxyphenyl)-N- methylethylamine in 33% yield, 88% e.e. and (S)-1-(4-methoxyphenyl) ethanol in 31% yield, 86% e.e.. Related results were obtained in the other cases, and the steps of catalytic hydroboration and amination could be combined in a single sequence without isolation of the intermediate Numerous variants were carried out in the amination procedure with only marginal improvements in chemoselectivity. An investigation of the mechanism was carried out using low temperature heteronuclear NMR on ¹³C-1-(S)-1-(4-chlorophenyl)ethyl-1,3,2-benzodioxaborole. The dual pathway is a result of an irreversible and unselective initial step.