ELECTROPHILIC AMINATION OF CATECHOLBORANE ESTERS FORMED IN THE ASYMMETRIC HYDROBORATION OF VINYLARENES

S)-(4-Methoxyphenyl)-ethyl-1,3,2-benzodioxaborole, (S)-1-(4-chlorophen yl)ethyl-1,3,2-benzodioxaborole and (S)-1-indanyl-1,3,2-benzodioxaboro le. intermediates in the catalytic asymmetric hydroboration of 4-chlor o-and 4-methoxystyrene, were isolated as pure oils in 75%, 84% and 49% yield respectively. For the first example, amination with hloromagnes io-N-methyl-O-trimethylsilhydroxylamine gave a mixture of (S)-1-(4-met hoxyphenyl)-N-methylethylamine in 33% yield, 88% e.e. and (S)-1-(4-met hoxyphenyl) ethanol in 31% yield, 86% e.e.. Related results were obtai ned in the other cases, and the steps of catalytic hydroboration and a mination could be combined in a single sequence without isolation of t he intermediate. Numerous variants were carried out in the amination p rocedure with only marginal improvements in chemoselectivity. An inves tigation of the mechanism was carried out using low temperature hetero nuclear NMR on S)-1-(4-chlorophenyl)ethyl-1,3,2-benzodioxaborole. The dual pathway is a result of an irreversible and unselective initial st ep. (C) 1997 Elsevier Science Ltd.