Mechanisms of Condensation of Biaryl Hydrocarbons

Article abstract of DOI:10.1021/jo00248a018

Results of kinetic studies of the condensed phase thermal reactions of the following biaryl hydrocarbons are reported; 1,1'-binaphthyl, 1,2'-binaphthyl, 1-phenylnaphthalene, and 9-phenylanthracene.Condensation generally occurred in parallel with both isomerization and dissociation and rates depended on concentrations of hydrogen donors.In order to simplify mechanisms, detailed studies used donors capable of providing just one H atom.These were xanthene, fluorene, and diphenylmethane.Condensation of 1,1'-binaphthyl in the presence of xanthene, the most effective donor, followed a second-order rate low (first order in both xanthene and binaphthyl concentration), k/M^-1s^-1=10^5.9+/-0.6exp(-36.0+/-2.0 kcal/RT)(360-560 deg C).Fluorene was one-fifth as effective as xanthene while diphenylmethane was nearly inert.A mechanism is proposed in which the key intermediates in all reactions are radicals created by H-atom transfer to the biaryls.In the condensation of naphthyl-containing biaryls, it is suggested that reactions are initiated by H transfer to a position next to the condensation site.Details of the unimolecular steps leading to condensation and isomerization, however remain unclear.Over the conditions studied, the reaction order with respect to donor varied between one and zero.It is proposed that this variability is a result of competition between two pathways for H transfer, one involving a simple, selective H transfer from the donor to a biaryl molecule, the other involving a free H-atom intermediate.