Selective reductions. 35. Reaction of representative organic functional groups with lithium borohydride in the presence of B-methoxy-9-borabicyclo[3.3.1]nonane. A simple, convenient procedure for the catalyzed selective reduction of esters

Abstract

Abstract : The rate of reduction of esters by lithium borohydride is considerably enhanced by a number of Lewis acids of boron. The remarkable catalytic effect of B-MeO-9-BBN and (MeO)3B in enhancing the reactivity of lithium borohydride toward the reduction of other representative functional groups has been explored. Alkyl halides are not reduced. Epoxides are readily reduced in 0.25 h. Carboxylic acids are reduced rapidly up to 50%, with further reaction being very slow. Acid salts are not reduced. Tertiary amides are slowly reduced in the presence of 100 mol % of B-zMeO-9-BBn, 40% in 24 h. Nitriles, under the same conditions, are reduced completely in 5H. Pyridine and nitrobenzene are not significantly affcted by this system. Sulfides, sulfoxides, and sulfones are also inert. However, tosylates are reduced rapidly. These results indicate the utility of this catalytic effect for the ready reduction of esters by lithium borohydride, as well as the ability of this reducing system to tolerate many substituents in such reductions.