Reactivity of 1-Chloro-3-phenyldiazirines

p-Substituted 1-chloro-3-phenyldiazirines (5), the putative
intermediates of the reaction of N,N,N'-trichlorobenzamidines
(10) with excess of bromide ions, react further to afford
mixtures of 3-bromo- (4) and 3-chloro-3-phenyldiazirines (6).
The 6:4 ratios inversely correlate with the Hammett σp and
σp+ constants of the p-substituents. The formation of 4,
proposed to proceed by anti-SN2‘ mechanism, is predominant
with electron-withdrawing p-substituents. Compounds 6, the
major products with electron-donating p-substituents, may
arise from 5 by a [1,3]-sigmatropic shift of chlorine proceeding via polar transition structures 12. The results of a gas-phase DFT (B3LYP/6-31+G*) study on the two mechanisms
are consistent with experiment.