The object of this note is to draw attention to the possible importance of concerted molecular reactions, of the type termed 'pericyclic' by Woodward and Hoffmann', in the mechanism of coal liquefaction.
This type of pathway has not hitherto been suggested and will be illustrated with reference to model compounds. Among these, experiments concerning the decomposition of 1,2_diphenylethane (abbr. DPE) and of the related benzylphenyl ether (abbr. BPE) under coal liquefaction conditions have been reported by several author?' in attempts to model the reactivity of aryl-substituted carbon-carbon and carbonoxygen sigma bonds known 'e-8 to exist in coal. The data from these model compound experiments, and for coal liquefaction in general 9~10 have most commonly been interpreted in terms of a free-ridical mechanism, wherein the relatively weak central C-C (or C-O) bond in the substrate ruptures to form delocalized benzyl (and phenoxy) radicals which abstract hydrogen from a donor to yield toluene (and phenol), the observed product(s), along with dehydrogenated donor. In what follows we attempt to show how the experimental observations might be accommodated by an alternate mechanism that involves only concerted reactions.
A pericyclic pathway for the decomposition of DPE substrate with Al-dialin as hydrogen-donor is sketched in Fk-ure 1. The overall reaction: