A kinetic EPR study of the dissociation of 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals: release of aminoacyl radicals and their cyclisation

Abstract

Hydrogen atom abstraction from 1-carbamoyl-1-methylcyclohexa-2,5-dienes generated the corresponding delocalised 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals at temperatures below ca. 300 K. At higher temperatures suitably substituted examples dissociated to produce toluene and aminoacyl (carbamoyl) radicals. Both types of intermediate were detected and characterised in solution by EPR spectroscopy. From measurements of the concentrations of the initial and released radicals, rate constants and Arrhenius parameters for dissociation of 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals were determined. It was found that dissociation to give a secondary aminoacyl radical [˙C(O)N(R)CH₂Ph] took place with a rate constant in the range 50 to 90 s⁻¹ at 300 K. The alternative dissociation of the 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals to release methyl radicals and an aromatic amide was much slower and did not compete. Analogous dissociations giving primary aminoacyl radicals [˙C(O)NHR] were significantly slower. Aminoacyl radicals with allyl, butenyl or similar side chains underwent cyclisation and, in the case of the 1,2,2-trimethylbut-3-enyl derivative, cyclisation was faster than dissociation of the parent cyclohexadienyl radical throughout the accessible temperature range. Semi-empirical AM1 and ab initio DFT computations indicated the decarbonylations of the aminoacyl radicals did not compete with cyclisations.