Use of the diazoalkane–thione reaction in the synthesis of a vinyltrimethylenemethane precursor

Abstract

Previous studies have shown that the reaction of diazoalkanes with 3,5-dihydro-3,3,5,5-tetramethylpyrazole-4-thione (6) provide a flexible route for the synthesis of precursors to trimethylenemethane (TMM)(1) and tetramethylene-ethane (TME)(2) biradicals. We now show that this route can be adapted for the synthesis of a precursor to another type of non-Kekulé polyene, a vinyl-TMM biradical (3). The thermal isomerisation of 1-(2,4-dimethylpent-1-en-3-ylidene)-2,2,3,3-tetramethylcyclopropane (27) to 4-isopropyl-3-isopropylidene-2,5-dimethylhexa 1,4-diene (11) which involves a vinyl-TMM intermediate (3-isopropyl-4-isopropylidene-2,5-dimethylhex-2-ene-1,5-diyl) was shown by deuterium labelling to involve a 1,6-hydrogen shift. It is suggested that such sigmatropic rearrangements of non-Kekulé polyenes may involve orbital symmetry control. Related examples are discussed. Two reactions discovered incidentally in the course of this work are of some interest, one involving the synthesis of an allylic amine through the thermal rearrangement of an aziridine (2-isopropyl-3,3-aziridine)(16) and the other the formation of an oxathiole (20) through the reaction of a thione (6) and an α-diazo ketone (3-diazo-4-methylpentan-2-one)(18).