The first known 3H-diazirines bearing a carbonyl group and a halogen atom on C-3 have been prepared by a novel synthetic method. Carboalkoxychloro- and bromodiazirines 1a-d are formed in up to 45% yields by reductive dechlorination of carboalkoxy-N,N,N'-trichloroformamidines 9a,b using chloride or bromide ion. This method constitutes the first example of the use of N,N,N'-trichloroamidines as starting materials in organic synthesis.

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 sigma(p) and sigma(p)+ constants of the p-substituents. The formation of 4, proposed to proceed by anti-S(N)2' 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.

Experimental and computational evidence suggests that the cleavage of n-butyl and tert-butyl 3-bromodiazirine-3-carboxylates by sodium n-butoxide in DMF, affording high yields of dibutyl carbonates, may proceed by nucleophilic acyl displacement of the bromodiazirinyl anion, a weakly bound complex of a cyclic carbene c-CN 2 (diazirinylidene) and a bromide ion. We explain the formation of substantial

Diazirinylidene (c-CN2) is formally the simplest of the N-heterocyclic carbenes. The intermediacy of this elusive species in the fragmentation of butyl 3-bromodiazirine-3-carboxylate (1a) with pent-4-en-1-ols and their sodium alkoxides in DMF is supported by the formation of 2-oxabicyclo[4.1.0]heptanes and dipentenoxymethanes. These products result from an intramolecular [2 + 1] cycloaddition and O-H insertion, respectively, of pentenoxymethylenes suggested to originate from the reaction of the electrophilic c-CN2 with an alkoxide ion. The reaction of 1a with primary or secondary amines in methanol affords the corresponding 3-bromodiazirine-3-carboxamides.

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