This chapter presents a critical review of synthetically useful variations of ionic methods for hydrogenation of organic compounds. In practice ionic hydrogenation involves the formal introduction of hydride from a donor source to an electron-deficient carbon center.
In this chapter, organosilicon hydrides are used as the source of ionic hydride with the goal of completing and updating earlier review work. Organosilicon compounds with at least one Si-H bond have the ability to serve as mild-air and water-stable sources of hydrides and, thus, have reducing properties. In general, organosilicon hydrides do not undergo spontaneous reactions with organic compounds unless the organic substrate is a reasonably strong electrophile or the silane is first activated by the interaction of a nucleophilic species with a nucleophilic center. The organosilicon hydrides are covalent compounds that have little or no nucleophilic properties on their own. The use of these hydrides often provides a means of effecting reductions of organic substrates under very mild conditions and with excellent functional group selectivity. Consideration of the nature of the Si-H bonds provides insight into the chemical behavior of these hydrides. Enhanced hydridic nature manifests itself in the chemical behavior of hydrosilanes.