2,2’-Diaryl-1,1’-bibenzimidazoles (10) were prepared in 32-66% yields from N-benzylidene-o-phenylenediamines (1) through oxidation with dioxygen catalyzed by copper(I) chloride in pyridine. Substitution on the benzene ring in the benzylidene group seems to determine the reactions. Ortho substitution causes cyclization to 2-arylbenzimidazoles (5). With the p-methoxy derivative 1d only oxidative coupling of the amino groups to the diazo compound 9d was found. In unsubstituted and para- and/or meta-substituted cases of 1 cyclodehydrogenation with coupling to 1,1’-bibenzimidazoles (10) was established. A mechanism is proposed involving intermediate radicals. The stabilities of these radicals-depending upon the substitution-govern partitions in parallel reactions leading to the diverse products 5, 10, and 9d. The X-ray structure determination of 10i reveals an N-N bond distance of 1.380 (1) A and a 71.9° angle of twist between the two benzimidazole rings.
ASJC Scopus subject areas
- Organic Chemistry