Kinetics of Intramolecular Phenyl Migration and Fused Ring Formation in Hexylbenzene Radicals

A combined ab initio/TST study was conducted to study the phenyl migration and fused ring formation of a series of 1-phenyl-hex- x-yl radicals. Phenyl shift proceeds through a two-step mechanism with a ring closure by ipso-cycloaddition, followed by a ring-opening by β-scission leading to x-phenyl-hex-1-yl radical isomers. Both steps involve a spirocyclic transition state connected by a spirocyclic intermediate. Barrier heights at the CBS-QB3 level range from 9.9 to 17.7 kcal mol^-1 depending on the size of the newly formed ring. Formation of fused rings by peri-cycloaddition reactions occur through barrier heights that range from 8.8 to 28.8 kcal mol^-1. The addition sites are the substituted and ortho carbons of the phenyl ring for the ipso- and peri-cycloadditions, respectively. For both types of reaction, the Arrhenius A factor decreases as the ring size increases. This is related to the number of internal rotors lost across the reaction. In addition, length of the side chain also impacts on the kinetics as longer side chains decrease the barrier height required to form a second cycle. Electrons in the cyclohexadienyl group of the spiro and fused rings are much more localized than expected, with the presence of two distinct π bonds. Rate coefficients for each reaction are provided. The unimolecular reactions are fast at most conditions, implying that ring formation and/or ring-opening processes will equilibrate 1-phenyl-hex- x-yl and x-phenyl-hex-1-yl radicals and the spiro and fused isomers.