The hydroboration of propargyl bromide. Simple one-pot three-component routes to (Z)-1-bromoalk-1-en-4-ols and to anti-homoallylic alcohols

The hydroboration of propargyl bromide with dialkylboranes takes place regi oselectively to give 3-bromoprop-1-en-1-yl dialkylboranes 13 which, upon qu aternization with bromide ion, undergo a series of transformations into a n umber of allylic boron species. By a suitable choice of the experimental co nditions it is possible to trap the reaction intermediates with aldehydes a nd to steer the process toward either the synthesis of (Z)-1-bromoalk-1-en- 4-ols 6 or anti-homoallylic alcohols 8. Two one-pot three-component process es.were developed based on a sequence of four reactions; preparation of dia lkylborane and hydroboration of propargyl bromide are the first steps. Then , quaternization with TEBABr may be carried out either in the presence of t he aldehyde when (Z)-1-bromoalk-1-en-4-ols 6 are requested, or in the absen ce of the aldehyde in order to allow the formation of gamma -substituted al lyl borane 18 which, successively, adds to the aldehyde affording antihomoa llylic alcohols 8.