Generation of eight-pi non-Kekulé polyenes from the 2π+ 4π, diethyl azodicarboxylate + tropone cycloadduct

Abstract

In this paper it is shown that the 2π+ 4π, diethyl azodicarboxylate + tropone cycloadduct can be used to make precursors to eight-pi non-Kekule polyenes. These are the first eight-pi non-Kekule polyenes that have been observed spectroscopically (triplet ESR spectra) and the first non-Kekule polyenes for which distinct spectra can be associated with particular cis/trans isomers. The enone moiety of diethyl 4-oxo-6,7-diazabicyclo[3.2.2]non-2-ene-6,7-dicarboxylate was elaborated into a triene by reaction with hexynyllithium, a Meyer–Schuster rearrangement using water–acetic acid–sulfuric acid, and a Peterson methylenation [(i) trimethylsilylmethylmagnesium chloride, (ii) thionyl dichloride, (iii) aq. ammonium chloride]. The carbamate groups were hydrolysed with potassium hydroxide–methanol and the resultant –NHNH– system was oxidised with mercury(II) oxide. The resultant Z and E isomers of 4-(2′-butylallylidene)-6,7-diazabicyclo[3.2.2]nona-2,6-diene were separated by column chromatography. UV irradiation of the Z isomer in an EPA glass at 77 K gave a triplet ESR spectrum |D/hc| 0.0223, |E/hc| 0.0022 cm–1. Similar irradiation of the E isomer gave |D/hc| 0.0133, |E/hc| 0.0044 cm–1. A study of the temperature dependence of the intensities of the Δm= 2 transitions confirmed that both spectra are associated with biradicals with a triplet ground state.