Reducing Characteristics of Borohydride Exchange Resin–CuSO4 in Methanol

Reducing characteristics of borohydride exchange resin (BER)–CuSO4 (cat.) were studied in methanol at room temperature. Carbon–carbon double bonds conjugated with benzene or carbonyl group were more rapidly reduced than was the case with isolated double bonds. Carbonyl groups were readily reduced, whereas esters and amides were inert, and nitriles were slowly reduced. High chemoselectivity was also observed in halide reductions: p-bromochlorobenzene and p-bromoiodobenzene were reduced quantitatively to chlorobenzene and bromobenzene, respectively. Aliphatic epoxides were inert to this reagent; however, styrene oxide derivatives were readily reduced to the corresponding deoxygenated products. Aliphatic azides were reduced slowly during 6 h, whereas phenyl azide was transformed to aniline in 1 h. Nitrocyclohexane was reduced at room temperature, but nitrobenzene, nitrosobenzene, azobenzene, and azoxybenzene required an elevated temperature (65 °C) for rapid reductions (1 h). Similarly, N,N-dimethylaniline N-oxide was reduced at room temperature, whereas pyridine N-oxide required refluxing. Finally, among the sulfur compounds tested, only diphenyl disulfide was reduced readily, and sulfide, aliphatic disulfide, sulfoxide, sulfone, and tosylate were inert to this reducing system.