Azepinones. Part 1. Formation of simple 1H-azepin-3(2H)-ones by gas-phase pyrolysis: crystal and molecular structure of 1-phenyl-1H-azepin-3(2H)-one

Abstract

Flash vacuum pyrolysis of the Meldrum's acid derivatives (4)–(6) at 500 °C (10^–3 Torr) gives good yields of the 1H-azepin-3(2H)-ones (7)–(9) respectively. X-Ray crystallography of the 1-phenylazepinone (8) shows that the dienaminone conjugated system is approximately planar though the seven-membered ring as a whole is markedly non-planar. The coalescence temperature for ring-inversion of the 2,2-dimethyl derivative (9) is –96 °C, corresponding to a ΔG^‡ of 36.4 kJ mol^–1.