Stabilizing Effect of HMPA (Hexamethylphosphoric Triamide) on Vanadium−Carbon Bonds and Their Application to Carbon−Carbon Bond Formation Reactions

Allylation of carbonyl compounds with allyl halides mediated by a vanadium(II) complex in a mixed solvent system of THF and HMPA has been accomplished. The addition of HMPA stabilized a vanadium−carbon bond of an allyl vanadium species and prevented dimerization of allyl halides or pinacol coupling of carbonyl compounds. Other alkylations such as a benzylation of carbonyl compounds or a Reformatsky-type reaction could also be mediated by a low-valent vanadium(II) species and benzyl bromide or bromo acetate in a THF−HMPA solvent system. By isolation of the vanadium−HMPA complex, [V 2 Cl 3 (hmpa) 6 ][ZnCl 3 (hmpa)] (HMPA = hexamethylphosphoric triamide), and clarification of its structure, it became clear that the coordination of bulky HMPA to vanadium centers played an important role in the carbon−carbon bond-forming reactions.