Grignard reagent formation

It is likely that reactions of magnesium metal with organic halides RX in ether solvents are typical metallic corrosions in which the stabilization of Mg2+, substantially through its coordination by the solvent, drives its loss from the metal and consequently the reductions of RX and reaction intermediates such as R-. at the metal surface. Although alkyl halides form Grignard reagents through non-chain mechanisms in which intermediate radicals diffuse in solution, very small amounts of radical isomerization occur in Grignard reactions of certain vinyl and aryl halides, even when intermediate radicals R-. would isomerize very rapidly. This suggests a dominant non-radical mechanism for these vinyl and aryl halides or a mechanism in which intermediate radicals R-. have extremely short lifetimes. Since the former seems more likely, a dianion mechanism, through a transition state [RX2-](double dagger), is proposed. Surface studies of polycrystalline Mg show that the "oxide" layer is mostly Mg(OH)(2) and that it is mechanically passivating. In the absence of promoters, Grignard reactions occur very slowly until enough RX has seeped to the magnesium surface and reacted there to undercut and cause the Mg(OH)2 layer to flake off. (C) 2004 Elsevier B.V. All rights reserved.