Attempted condensation of o-phenylenediamine with phenylpropiolic acid with
a view to preparing 2-beta-phenylacetylenylbenzimidazole under a variety o
f conditions such as in refluxing methanol, in hot PPA, in refluxing benzen
e with Dean-stark removal of water etc., leads to the recovery of starting
materials. However, condensation of o-phenylenediamine dihydrochloride and
dihydrobromides with phenylpropiolic acid in refluxing ethylene glycol yiel
d the respective 2-haloarylvinylblenzimidazoles: The latter are dehydrohalo
genated under basic conditions to obtain the desired 2-beta-phenylacetyleny
lbenzimidazole. The mechanism of formation of 2-haloarylvinylbenzimidazoles
has been discussed.