Selective reductions with complex hydride reducing agents

This paper describes the selective reductions with thioborane derivatives, zinc—modified cyanoborohydride, and the ate complex from diisobutylaluminum hydride and n-butyllithium. Reaction of carboxylic acids with 1,3,2-dithiaborinane in the presence of stannous chloride or boron trifluoride etherate in tetrahydrofuran gives 1,3-dithianes in high yields. The reaction of acids with thexylphenylthioborane in dichloromethane gives S,S'-diphenyl acetais, whereas esters are converted into phenyl sulfides in the presence of zinc iodide. Zinc-modified cyanoborohydride in ether selectively reduces allylic, benzylic, tertiary alkyl halides without reducing primary and secondary alkyl, vinyl, and aryl halides. Furthermore, the reagent in ether reduces aldehydes, ketones, and acid chlorides to alcohols and the reagent in methanol is very effective in the reduction of enamines, reductive amination of aldehydes and ketones, and deoxygenation of aldehdyes and ketones. The ate complex from diisobutylaluminum hydride and n-butyllithium, capable of reducing most reducible functional groups, is found to be a highly selective reducing agent. The reagent rapidly reduces simple primary alkyl bromides, benzylic, and allylic halides but slowly primary alkyl chlorides and secondary alkyl bromides. Tertiary amides are cleanly reduced to aldehydes and various selective reductions can be achieved by the use of the reagent.