Pyrolysis of silica-immobilized benzyl phenyl ether: Competing radical rearrangement pathways under restricted diffusion

Pyrolysis studies of silica-immobilized benzyl phenyl ether (approximate to PhOCH2Ph or approximate to BPE), a model for related ether structures in fuel resources, have been conducted at 275-325 degrees C to examine the impact of restricted mass transport on the pyrolysis mechanism compared with previous studies in fluid phases. Significant rearrangement chemistry is observed for approximate to BPE occurring through two competitive free-radical pathways that are both promoted by the diffusional constraints. One path involves recombination of incipient benzyl and surface-bound phenoxy radicals to form benzylphenol isomers, 10. The second, previously unreported rearrangement path for approximate to BPE involves a 1,2-phenyl shift in an intermediate radical, approximate to PhOCH . Ph, leading to formation of benzhydrol (8) and benzophenone (9) as principal products. The rearrangement products 8-10 typically account for ca. 50% of the pyrolysis products. However, the path selectivity is a sensitive function of approximate to BPE surface coverage and the presence of spacer molecules that either facilitate or hinder hydrogen atom transfer steps on the surface.