Synthetic Applications of Conjugated Azocarbinols. Radical Chain Hydrophenylation and Hydrocyclohexenylation of Haloethenes

Article abstract of DOI:10.1021/ja00414a025

2-(Phenylazo)-2-propanol (5) and (phenylazo)diphenylmethanol (6) decompose in solution by process involving phenyl radicals.Similarly, 1-(1-azocyclohexenyl)cyclohexanol (7) decomposes to generate the 1-cyclohexenyl radical.Evidence for radical intermediates includes the formation of chlorobenzene and 1-chlorocyclohexene, respectively, from decomposition of 5 (or 6) and 7 in CCl4.Evidence for induced, chain decomposition by radical abstraction of hydroxyl hydrogen, in concert with breaking of at least one C-N bond of the azo function, includes faster decomposition in CCl4 than in benzene, acceleration of decomposition in CCl4 by thiophenol, and acceleration of decomposition in benzene by trityl radicals.That decomposition mechanism is supported also by the finding that methyl ethers and acetate esters of the azoalcohols decompose much more slowly than the alcohols themselves.Phenyl radicals from either 5 or 6, and 1-cyclohexenyl radicals from 7, can be trapped with some alkene by addition.Such radical adducts subsequently pick up a hydrogen atom, presumably by abstracting from the hydroxyl group of the azocarbinol in concert with C-N bond breaking.The overall processes, then, are hydrophenylation of alkenes with 5 and 6 and hydro-1-cyclohexenylation of alkenes with 7 by a radical chain mechanism.The processes are of preparative value only in cases of alkene substrates which are neither highly polymerizable nor prone to radical attack on allylic substituents.Several highly halogenated compounds prepared by treatment of haloethenes with 6 or 7 are reported.The reaction between 5 and benzaldehyde, to form acetone and 1-benzoyl-2-phenylhydrazine, was found to be second order overall, first order in 5 and first order in benzaldehyde between 0.4 M and neat benzaldehyde.This result does not appear to be compatible with a mechanism involving decomposition of 5 to acetone and phenyldiazene, with subsequent reaction of the latter with benzaldehyde.