Vanadium(I) Chloride and Lithium Vanadium(I) Dihydride as Epimetallating Reagents for Unsaturated Organic Substrates: Constitution and Mode of Reaction

Subvalent vanadium(I) salts, of empirical formulas, VCl, vanadium(I) chloride and LiVH 2 , lithium vanadium(I) dihydride, can be conveniently prepared in THF solution, starting at –78 °C, by treating either VCl 3 or VCl 4 with an appropriate number of equivalents of n BuLi. As judged by the stability of solutions or solid samples of LiVH 2 , the preparation of LiVH 2 from VCl 4 is the preferred method. Individual physical characterization of solid samples of VCl or of LiVH 2 , admixed with their LiCl by‐product, was carried out after removal of all volatiles in vacuo and by the following measurements: 1) gasometric protolysis with glacial acetic acid and measurement of the H 2 evolved in the oxidation of V I to V II ; 2) infrared spectroscopic search for V–H bands; and 3) examination for unpaired electrons by EPR activity. Such measurements applied to VCl lend strong support for a V I oxidation state but only probable evidence for paramagnetism and for the association of VCl units. Similar measurements applied to LiVH 2 give unambiguous gasometric and IR evidence favoring the LiVH 2 stoichiometry and the biradical nature of the VH 2 anion with a linear array of H–V–H atoms. Chemical characterization of both VCl and LiVH 2 toward individual organic substrates, such as olefins, ketones, epoxides and organic halides, yielded convincing evidence that organic radical mechanisms are involved, both for the proven biradical, LiVH 2 , as well as for the diamagnetic VCl. Finally, the question of why LiVH 2 prepared from VCl 4 is more stable than the LiVH 2 obtained from VCl 3 is addressed in terms of the actual coordination sphere of the VH 2 anion in THF solution and in the solid state. Preliminary studies comparing the reactivities of LiVH 2 and LiCrH 2 toward organic substrates indicate that LiVH 2 is the distinctly more moderate and usefully selective reductant. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)