Unlike borohydride reduction of carbonyl compounds which proceeds readily at the natural pH of the solution, reduction by sodium cyanoborohydride is very slow unless the pH is reduced to 3-4. Selective reduction of iminium ions occurs, however, at pH 6-8, leading to the widespread application of the reagent in reductive aminations of carbonyl compounds: J. Am. Chem. Soc., 93, 2897 (1971); Org. Synth. Coll., 6, 499 (1988). For improved methods using combinations with Ti(IV) chloride or isopropoxide, see: Tetrahedron Lett., 31, 5547 (1990); J. Org. Chem., 55, 2552 (1990). Tosylhydrazones are reduced to methylenes. The relatively slow reduction of carbonyl compounds enables the tosylhydrazone to be generated in situ: J. Am. Chem. Soc., 93, 1793 (1971); 95, 3662 (1973). For a detailed study of the mechanism of tosylhydrazone reduction, see: J. Org. Chem., 54, 4175 (1989).
