Intramolecular cyclisations of biphenyl-2-carboxyl radicals: evidence for a II-state aroyloxyl radical

Abstract

Biphenyl-2-carboxyl radicals generated by homolysis of acyl hypoiodites cyclise intramolecularly giving mainly δ-lactones through Ar₂-6 cyclisation. 2′-Alkoxybiphenyl-2-carboxyl radicals do not give the expected Ar₁-5 cyclisation product but undergo a homolytic ipso-substitution of the 2′-substituent. The phenanthrene-4-carboxyl radical gives 5H-phenanthro[4,5-bcd]pyran-5-one. Consideration of the molecular orbitals involved suggests that the biphenyl-2-carboxyl radicals are in the π-ground state and have a higher energy, and, therefore, a less thermally accessible Σ-state than the corresponding amido-radicals. It is suggested that acyloxyl radicals which readily decarboxylate have either a Σ-ground state or a thermally accessible excited Σ-state.