Mechanism of diol formation in the reaction of benzylic Grignard reagents with aldehydes
2-Hydroxymethyl-1-methylene-1,2-dihydronaphthalene (8) was obtained from the reaction of 1-naphthyl-methylmagnesium chloride with monomeric formaldehyde. This compound slowly produces approximately equimolar amounts of 1-methylnaphthalene and 1-(2-hydroxyethyl)-2-hydroxymethylnaphthalene (9) in the presence of MgCl2. In the light of these results and studies on the reactivity of 1-naphthylmethylmagnesium chloride towards acetone and halide-free dibenzylmagnesium towards monomeric formaldehyde, a new mechanism of diol formation in the reaction of benzylmagnesium chloride with aldehydes is described. This mechanism involves an ortho addition on the Grignard reagent, leading to a trienic magnesium alkoxide intermediate (4), which decomposes by a reversible process into the Grignard reagent and aldehyde. The latter undergoes a Prins-type reaction with the magnesium alkoxide intermediate in the presence of MgCl2, to give the corresponding diol and toluene.