Mechanism of oxidative cleavage by bromine of diazabutadienes (azines) to aldehydes or ketones

Abstract

2.3-Diazabuta-1,3-dienes (1; R = Me or H) are smoothly cleaved by bromine in 7 : 3 acetic acid–water at 25° to give quantitative amounts of the corresponding substituted acetophenone(s) or benzaldehyde(s), with the evolution of nitrogen. Kinetic studies under these conditions show that two moles of bromine are consumed per mole of (1) oxidized; one mole of bromine is involved at or before the rate-determining step while the second mole reacts rapidly in a subsequent step. Surprisingly, electron-donating substituents slow and electron-withdrawing substituents enhance the reactivity of (1)(Hammett ρ+0.8 when R = H and +0.32 when R = Me) which is unexpected for a reaction involving electrophilic attack by bromine. A mechanism is proposed in which a small equilibrium concentration of the hydrate (6) is formed; the reaction of (6) with bromine [to give the diazoalkane (9)] is rate determining. The second mole of bromine reacts with the diazoalkane (9) which is shown independently to react rapidly, yielding the ketone (11).