Reactions of N-(ethoxycarbonyl)-3-halo-3-(halomethyl)azetidines with DBU. The halogen dance

Reaction of N-(ethoxycarbonyl)-3-(bromomethyl)-3-chloroazetidine (3) with DBU results in the formation of two haloalkenes, i.e., N-(ethoxycarbonyl)-3-(bromomethylene)azetidine and N-(ethoxycarbonyl)-3 (chloromethylene)azetidine (4a and 4b, respectively). The results of control experiments suggest that Cl- is capable of promoting nucleophilic displacement of bromine in the CH2Br groups of both 3 and N-(ethoxycarbonyl)-3-bromo-3-(bromomethyl)azetidine (8), but Br- is incapable of displacing chlorine in the CH2Cl groups of both N-(ethoxycarbonyl)-3-chloro-3-(chloromethyl)-azetidine (7) and N-carbethoxy-3-bromo-3-(chloromethyl)azetidine (10). Thus, it is suggested that the formation of 4b via reaction of 3 with DBU is a result of DBU-promoted elimination of HCl from 7, a key reaction intermediate. The observations reported herein can be explained without invoking the intermediacy of any bridged halonium ion.