chapter  20
47 Pages

Reduction

BySir Richard Norman, J.M. Coxon

This chapter describes the reductive processes, which fall into three categories: the removal of oxygen, the addition of hydrogen, and the gain of electrons. Alkenes and alkynes are reduced at room temperature and atmospheric pressure, usually in benzene solution. Aryl halides can be reduced by the formation of the Grignard reagent or the lithium compound followed by treatment with water, or with hydrazine over a palladium catalyst. Alkyl systems are generally much less easy to hydrogenolyze than the corresponding benzyl systems. Allylic systems behave in most reactions analogously to benzylic systems. Electrons are transferred to alkynes more readily than to alkenes. The Mozingo reaction is useful for reducing carbonyl compounds which are sensitive to mineral acid and bases, for the Clemmensen and Wolff-Kishner methods are then unsuitable. Triphenylmethyl systems readily form the trityl cation and this is particularly susceptible to reduction by hydride-transfer agents. Aromatic ketones are reduced by lithium aluminium hydride in the presence of aluminium trichloride.