Organoboron compounds. Part VII. Photochemical reactions of boron halides with aromatic hydrocarbons and aromatic ethers

Abstract

The u.v. irradiation and subsequent hydrolysis of a mixture of boron tri-iodide and benzene, and of mixtures of boron tribromide and benzene, or toluene, or biphenyl, or naphthalene results in the formation of the corresponding arylboronic acids. The initial reaction products, the arylboron dihalides, appear to be formed by photolysis of the boron trihalides followed by reaction of the resulting dihalogenoboryl radicals with the aromatic compounds. The reaction induced by visible light between benzene and boron tribromide gives low yields (after hydrolysis) of phenylboronic acid and p-phenylenediboronic acid and may occur via a loosely bound complex of the reactants. Naphthalene reacts with boron tribromide on irradiation with u.v. light to give, after hydrolysis, 2-naphthylboronic acid. A similar, but much less efficient, reaction occurs when these reactants are heated together in the dark. Irradiation and subsequent hydrolysis of a mixture of diphenyl ether and boron tribromide gives 10-hydroxy-10-bora-9-oxa-aro-anthracene; the corresponding reaction with 4-bromodiphenyl ether gives only a trace of 3-bromo-10-hydroxy-10-bora-9-oxa-aro-anthracene, the major product being 4-phenoxyphenylboronic acid. Irradiation of a mixture of phosphorus tribromide and benzene gives phenylphosphonous dibromide in low yield.