Vanadium(I) Chloride and Lithium Vanadium(I) Dihydride as Selective Epimetallating Reagents for π‐ and σ‐Bonded Organic Substrates

Subvalent vanadium(I) salts, of empirical formulas, VCl, vanadium(I) chloride and LiVH 2 , lithium vanadium(I) dihydride, whose efficient preparation, structural constitution and mode of reaction toward certain organic substrates have been described in a preceding article, are here evaluated in their reactions toward a wide variety of π‐ and σ‐bonded organic substrates, namely carbonyl, imine, azo, alkene, 1,3‐diene, nitrile π‐bonds and C–X, C–O, C–N and N–N σ‐bonds. Compared with the high reactivity of CrCl and LiCrH 2 reagents in attacking both types of bonds, the VCl and LiVH 2 reagents were much milder and selective in epimetallating π‐bonds, often forming the 1:1 adduct of LiVH 2 and π‐bonded substrate as the major product. Finally, the vanadium reagents showed little tendency to cleave C–O, C–S and C–N bonds and a smaller scope in cleaving C–X bonds than their chromium counterparts. Because of their selectivity these vanadium reagents offer the following preparative promise: 1) smooth McMurry carbonyl coupling to their reductive dimers; 2) deoxygenation of epoxides; 3) selective aromatic C–X reduction; 4) high yields of epimetallated carbonyls or imines as intermediates to ‐hydroxy and ‐amino acids; 5) 1,4‐reductions of 1,3‐alkadienes; 6) reductive dimerization of nitriles to ketones; 7) 1,4 or 1, n ‐epimetallations leading to acyloins or indoles; and 8) reductive dimerizations of azines to produce unusual imidazole derivatives. In explaining the greater kinetic stability of the 1:1 LiVH 2 adduct with carbonyl or imine substrates it is pointed out that such epimetallated adducts from LiVH 2 would likely be diamagnetic, whereas such adducts from LiCrH 2 have an unpaired electron on the Cr center and hence would rupture, so that the electron would be on the C center.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)