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Since its discovery over forty years ago (1), sodium borohydride, NaBH^sub 4^, has been exhaustively studied. Standard organic chemistry texts discuss the lower reactivity of NaBH^sub 4^ compared with lithium aluminum hydride, LiAlH^sub 4^: whereas LiAlH^sub 4^ reduces carboxylic acids to primary alcohols, NaBH^sub 4^ does not reduce carboxylic acids. This differentiation is the basis of a recent experiment described in this Journal (2).
Three recent articles (3-5) led us to investigate the reaction of sodium borohydride with carboxylic acids. These reports show that after initial addition of the carboxylic acid to NaBH^sub 4^, subsequent addition of an electrophile, either I^sub 2^ (3, 5) or H^sub 2^SO^sub 4^ (4), reduces the carboxylic acid to the primary alcohol. The probable mechanism is shown in Figure 1. The intermediate 2 is not susceptible to hydride attack at carbonyl carbon, but 2 does react with the added electrophile "E^sup +^", producing intermediate 3. The left resonance form of 3 shows that trivalent boron serves to withdraw electrons from the adjacent oxygen, leaving the carbonyl carbon susceptible to nucleophilic attack by hydride. This proposed mechanism also explains the observation (6) of aldehydes 4 produced in some borohydride reducing media.
Our goal was to develop the new reduction conditions into a procedure applicable to the first-year organic chemistry laboratory, where reduction of the carboxylic acid group has remained an obstacle, notwithstanding the use of borane or LiAlH^sub 4^ (2) on the microscale. The NaBH^sub 4^ method with either electrophile can be modified to any scale; in our hands, the use of I^sub 2^ as the electrophile performed...