5-Chloro-1-[(E)-3-(dimethyl­amino)­acrylo­yl]-3-methyl-1H-benzimidazol-2(3H)-one–6-chloro-1-[(E)-3-(dimethyl­amino)­acrylo­yl]-3-methyl-1H-benzimid­azol-2(3H)-one (4/1)

In the reaction of 7-chloro-1,5-benzodiazepine-2,4-dione with N,N-dimethyl­formamide/dimethyl­acetal, the diazepine seven-membered ring undergoes a contraction to form the five-membered ring. The reaction yields two isomers the title compound, C(13)H(14)ClN(3)O(2); the major component has the chlorine-atom substituent in the 5-position of the benzimidazolone ring and the minor component has the chlorine atom in the 6-position. The two isomers form a disordered co-crystal, the chloro­methyl­benzimidazolone portion of both components are disordered with respect to each other in a 4:1 ratio [the refined ratio is 0.816 (5):0.184 (5)]; the dimethyl­amino­cryloyl substitutent is ordered. The double bond of the dimethyl­amino­acryloyl substituent has an E configuration.