Selective Reduction of Carbonyl Compounds with B-Trifluoromethanesulfonyldiisopinocampheylborane in Ethyl Ether

Abstract

In the previous reports, 1-4 we showed that Lewis acidity of trisubstituted borane derivatives, a series of new MeerweinPonndorf-Verley (MPV) type borane reagents, plays a role in part on their reactivity toward the reduction of carbonyl compounds: for example, the strong electron-withdrawing effect of fluorine substituent of B-trifluoroacetoxydiisopinocampheylborane (Ipc2BO2CF3) makes it much more reactive than Ipc2BOAc itself. This phenomenon strongly indicates the reaction proceeds by the activation of carbonyl group through coordination of Lewis acidic boron atom followed by hydride transfer from β-hydrogen source to the carbonyl acceptor via the six-membered transition state, quite similar to the mechanism which is generally accepted in the original MPV reaction using aluminum isopropoxide. 5 From such mechanistic point of view, we extended to design other derivative B-trifluoromethanesulfonyldiisopinocampheylborane (Ipc2BOSO2CF3). Evidently, trifluoromethanesulfonyl group is one of the strongest electron-withdrawing ones. Accordingly, we examined the general reducing characteristics of the reagent in order to understand its nature and explore its role in organic synthesis.