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8.3.2 Product Subclass 2: Potassium Hydride

DOI: 10.1055/sos-SD-008-01119

Venturello, P.; Barbero, M.Science of Synthesis, (200681315.

General Introduction

Potassium hydride acts as a base and as hydride donor. It is used for deprotonation, cyclizationcondensation, elimination, and rearrangement reactions, and also as a reducing agent. Potassium hydride undergoes reaction quickly and quantitatively with acids, and of particular note is its capability to rapidly deprotonate tertiary alcohols where sodium hydride or potassium metal do so slowly or not at all. The reactions of metal hydrides take place at the crystal surface. The crystal lattice energies decrease from lithium to cesium hydride, and potassium hydride appears to have the optimal lattice energy and hydride radius for surface reactions. The presence of 18-crown-6 enhances the reactivity of potassium hydride. The crown ether can operate as a phase-transfer agent or as a simple pickling agent of the potassium hydride surface, dissolving the formed inorganic salts. Potassium hydride is usually superior to lithium and sodium hydride in the reactions described in this section. Unusually active potassium hydride can be prepared easily from hydrogen and superbasic reagents (t-BuOKTMEDA) in hexane. Superactive potassium hydride is very active in deprotonation as well as in reduction. The reactivity of commercially available potassium hydride, which is prepared by the reaction of hydrogen gas with elemental potassium, depends upon the impurities in different lots (mainly potassium or its reaction products), thus leading to side reactions and variable yields. The superactive metal hydride contains no alkali metal. The methods described in this section deal with the literature published from the year 1980 onwards. The previous literature concerning potassium hydride in organic synthesis has been reviewed.[‌1‌]