Phosphides and arsenides as metal–halogen exchange reagents. Part 2. Reactions with aromatic dihalides

Abstract

1,2-Dihalogenobenzenes react with two molar equivalents of lithium diphenylphosphide in the presence of furan to give, after oxidation, triphenylphosphine oxide (major) and the corresponding 2-halogenophenyldiphenylphosphine oxide. No trace of furan–dehydrobenzene adduct was observed in these reactions; however, 1- and 2-naphthyldiphenylphosphine oxides (9:4) were detected. These compounds were the major products from the reaction of the furan–dehydrobenzene adduct with diphenylphosphide, although the ratio of 1- to 2-isomers was quite different (1:9) in this case.

Reaction of lithium diphenylphosphide with 3,4-dichloronitrobenzene gave tetrachloroazobenzene (40%) as the only isolated product and attempts to replace 1,2-dihalogenobenzenes with 2-halogenobenzenediazonium salts gave much lower yields of products. Although dehydrobenzene is thought to be involved in many of these reactions, alternative mechanisms also operate, depending on the halides used.

The reaction of lithium diphenylarsenide with 1,2-dihalogenobenzenes gave triphenylarsine as the major product and it seems likely that similar mechanisms to those involved in reactions of diphenylphosphide are operating. Surprisingly, dehydrobenzene–furan adduct reacted with diphenylarsenide to give naphthalene in high yield.