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
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.