The Mechanism of Interaction of Vinyl Azides with Electrophilic Bromine

The pathway of the reaction of vinyl azides with bromine under various conditions has been examined for the first time. In contrast to protonation, bromination of several vinyl azides did not give products resulting from alkyl or phenyl migration to N, but instead involved trapping of electron-deficient nitrogen species (iminodiazonium ions) by nucleophiles. 1-Azido-2-butylethylene (17), as an example of a terminal azidoolefin, produced mainly nitrile 18 and dibromoaldehyde 20 on bromination. Bromination of 17 or 22 in methanol proceeded with solvent incorporation giving rise largely to methoxyazides 21 and 23, respectively. The regiochemistry of the latter products, proven by spectral data and hydrolysis to bromoaldehydes, indicates that the azide function exerts a stronger directive effect in these additions than an alkyl or even a phenyl group. Bromination of nonterminal vinyl azides 25 and 31 led to isolation of imminium salts which were very easily hydrolyzed to α-bromo or α,α-dibromo ketones. Fragmentation to nitriles was also observed. Bromination in methanol led, as for 17 and 22, to regiospecinc solvent incorporated adducts.