Potassium Hydride, Highly Active New Hydride Reagent. Reactivity, Applications, and Techniques in Organic and Organometallic Reactions

Abstract

Potassium hydride is extremely reactive both as a base and as a hydriding agent. In both of these reactions it is far more reactive than either sodium hydride or lithium hydride. Potassium hydride surpasses finely divided potassium metal as a base yet possesses none of the latter's electron-transfer properties (reduction, coupling, etc.). KH reacts rapidly at 20-25° with excess triethylmethanol to yield the alkoxide quantitatively in <1 min, in contrast to K (80% complete in 20 min with very slow further reaction), NaH (5% in 20 min), and LiH (0% in 20 min). KH also rapidly metalates unhindered amines (e.g., pyrrolidine) and dimethyl sulfoxide. In tetrahydrofuran suspension, metalation similarly proceeds rapidly with highly hindered weak acids such as bis(trimethylsilyl)amine (pKA = 28-29), N-isopropylaniline (pKA = 26), triethylmethanol (pKA = 22), and 2,6-di-tert-butylphenol (pKA = 17-18). Under comparable conditions, only the latter reacted significantly with NaH; none react with LiH. Ketones are metalated to potassium enolates in high to quantitative yield; no reduction of carbonyl groups by hydride is observed. KH reacts rapidly with weak and hindered Lewis acids under conditions where NaH is very sluggish and LiH is essentially inert. Hindered trialkylboranes are readily converted to the corresponding borohydrides (K+-HBR3) at room temperature; the very weakly acidic triisopropyl borate reacts similarly. Reactions with KH appear to be entirely heterogeneous, occurring at the crystal surface. Potassium hydride is thermally stable and readily handled as a dispersion in mineral oil. In the absence of the protective oil, it must be protected from air and moisture. Detailed handling procedures are discussed. © 1974, American Chemical Society. All rights reserved.