Sodium borohydride in dimethyl sulfoxide or sulfolane. Convenient systems for selective reductions of primary, secondary and certain tertiary halides and tosylates

Abstract

SODIUM BOROHYORIDE IN OIMETHYL SULFOXIDE OR SULFOLANE. CONVENIENT SYSTEMS FOR SELECTIVE REDUCTIONS OF PRIMARY, SECONDARY AND CERTAIN TERTIARY HALIDES AND TOSYLATES Robert 0. Hutchins, David Hake', Joseph Keogh' and David Koharski' Department of Chemistry, Drexel Institute of Technology Philadelphia, Pennsylvania 19104 (Received in USA 9 June 1969; received in UK for publication 31 July 1969) The use of various metal hydrides as selective reducing agents has attracted a considerable amount of interest in recent years (2). It occurred to us that one unexplored possibility in this area might be to employ the relatively mild reducing agent sodium borohydride in SNP rate enhancing, polar, aprotic solvents to selectively displace halogens and other good leaving groups with hydrogen without affecting other reducible functional groups. ’ We wish to report that, indeed, sodium borohydride in dimethyl sulfoxide (DMSO) or sulfolane provides a very versatile and convenient method for removing primary, secondary and, in certain cases, tertiary alkyl and benzylic halides and tosylates in the presence of other groups including carboxylic acids, esters and nitro groups. The conditions and yields for a variety of such reductions are shown in Table I. Several features of these reactions make the procedure particularily attractive for synthetic applications. First, yields of hydrogenolysis products are good to excellent (53-98%) for the examples studied. Furthermore, unlike corresponding reductions with lithium aluminum hydride (3), essentially no olefin or other organic soluble side products were isolated or detected with the exceptions of benzhydryl chloride and branide as discussed below. Also, 1,2-dibromides (i.e. entries 24 and 25) are reduced to the hydrocarbons in moderately good yields in contrast to lithium aluminum hydride reductions which afford only olefins (3). Thus, the method can be utilized to hydrogenate double bonds in sensitive compounds by way of bromination and reduction. Heterogeneous reactions appear to offer no problem as illustrated by the smooth reductions of several compounds of limited solubility (i.e. dodecyl derivatives, Table I).