The synthesis of 3H-1,2-diazepines by the base-induced elimination of toluene-p-sulphinic acid from 3,4-dihydro-2-tosyl-1,2-diazepines, and some observations on sigmatropic hydrogen shifts in the 3H-1,2-diazepine system

Abstract

The reaction of 3,4-dihydro-2-tosyl-1,2-diazepines (13) with sodium ethoxide provides the first general synthesis of 3H-1,2-diazepines (9). The ¹H and ¹³C n.m.r. spectra of the products show that they exist as diazepines (3) rather than as the diazanorcaradiene valence tautomers (4). In some cases the synthesis produced mixtures of isomeric 3H-1,2-diazepines via a rapid [1,5]sigmatropic hydrogen shift in the primary product. A preliminary kinetic study of the isomerisation of 3,5-dimethyl-3H-1,2-diazepine (14c) to 5,7-dimethyl-3H-1,2-diazepine (9c) at 0 °C has shown that the hydrogen shift is ca. 10¹⁰ faster than in an analogous cycloheptatriene.