The insertion of dimethylvinylidene carbene into azo moiety was investigated in order to synthesize 4,6-dimethyl-5-[2-(2-methylpropl-enyl)-1H-benzimidazol-1-yl] pyrimidine-2(5H)-thiones [7a-j] under kinetically controlled phase transfer catalysis conditions. In situ generation of dimethylvinylidene carbene was facilitated by the interaction between 3-chloro-3-methyl-1-butyne and alkali at the interface. Interestingly, insertion of this carbene into the -N=N- linkage of 2,4-dimethyl-3-arylazo-6-thiopyrimidine afforded newly synthesized desired benzimidazolo pyrimidines. The reaction follows the pseudo-first order rate law. Rational mechanism of the reaction is proposed according to the experimental evidence. The compounds were synthesized in excellent yields (70-80%) and their structures were established on the basis of their IR and H-1 NMR spectral data. (c) 2005 Elsevier B.V. All rights reserved.