Diazepines. Part XIX. Kinetics of addition of bromine to position 6 in 2,3-dihydro-1,4-diazepinium salts

Abstract

Measurements on the aqueous bromination of 2,3-dihydro-1,4,6-trimethyl- and of 2,3-dihydro-6-methyl-1,4-diazepinium perchlorate show that addition of bromine at position 6 is a fast reaction between bromine molecules and dihydrodiazepinium cations, for which the rate constant at 298 K is ca. 5 × 10⁶ I mol^–1 s^–1. The activation parameters are consistent with such a reaction. In contrast, bromination at position 6 of 6-bromo-2,3-dihydro-5,7-dimethyl-1,4-diazepinium perchlorate is an equilibrium bromination, proceeding only to a small extent, followed by an equilibrium proton dissociation step, and concluded by rate-determining hydrolyses of the two intermediates. Again, the activation parameters are consistent with the mechanism. Both types of bromination kinetics can be accommodated within a single reaction scheme, the rate-determining steps being different, and possible reasons for the difference are discussed. The influence of mesomerism and other stability effects on the behaviour of 2,3-dihydro-1,4-diazepinium cations towards bromine is discussed.