Selective reactions using metal phenoxides. Part 1. Reactions with formaldehyde

Abstract

The reactions between formaldehyde and a series of aryloxymagnesium bromides (1) and their complexes with hexamethylphosphoramide (HMPA) in benzene have been investigated. In the absence of ligand 2,2′-dihydroxydiphenylmethanes (2) are obtained, while in the presence of stoicheiometric amounts of HMPA 2-hydroxybenz-aldehydes (4) are produced in high yield. Both products are the result of an exceptional ortho-regioselective attack on the aromatic nucleus of the phenol. The formation of (4) has been shown to occur via an oxidation–reduction process, promoted by HMPA, between the 2-hydroxybenzyl alcohol intermediate (5) and formaldehyde. The difference in acidity between the free and HMPA-complexed magnesium counterion is invoked to explain the two reaction pathways.