New sulfur derivatives of 2,3-dichloropyrrolo[1,2-a] benzimidazol-1-one. Demonstration of regioselective thiolate addition and X-ray diffraction structures of 2-chloro-3-methylthiopyrrolo[1,2-a]benzimidazol-1-one and 2,3-di(benzylthio)pyrrolo[1,2-a]benzimidazol-1-one

The reaction of thiols with the heterocyclic compound 2,3-dichloropyrrolo[1,2-a] benzimidazol-1-one (1) has been investigated as a route to new redox-active, bidentate sulfur ligands. Treatment of 1 with either methylthiol or benzylthiol in the presence of pyridine affords monosulfide compounds 2-chloro-3-methylthiopyrrolo[1,2-a] benzimidazol-1-one (2) and 2-chloro-3-benzylthiopyrrolo[1,2-a]benzimidazol-1-one (3) and the disulfide derivatives 2,3-di(methylthio)pyrrolo[1,2-a]benzimidazol-1-one (4) and 2,3-di(benzylthio)pyrrolo[1,2-a]benzimidazol-1-one (5). The substitution of the first chlorine group in 2,3-dichloropyrrolo[1,2-a]benzimidazol-1-one (1) occurs regioselectively at C-3 to produce 2-chloro-3-methylthiopyrrolo[1,2-a]benzimidazol-1-one (2) and 2-chloro-3-benzylthiopyrrolo[1,2-a]benzimidazol-1-one (3), followed by replacement of the remaining chlorine group to furnish the disulfide compounds 4 and 5. The new thiols have been isolated by column chromatography and characterized in solution by spectroscopic methods. The molecular structures of 2-chloro-3-methylthiopyrrolo[1,2-a]benzimidazol-1-one and 2,3-di(benzylthio)pyrrolo[1,2-a]benzimidazol-1-one have been determined by X-ray crystallography. Compound 2 crystallizes as two independent molecules in the monoclinic space group P2₁/c, a = 13.221(2) Å, b = 18.478(2) Å, c = 8.948(1) Å, = 100.088(3)°, V = 2152.3(5) ų, Z = 8, and d_calc = 1.547 Mg/m³; R = 0.0354, R_w = 0.0739 for 2820 reflections with I > 2(I). Compound 5 crystallizes in the triclinic space group P-1, a = 5.180(1) Å, b = 11.494(2) Å, c = 17.243(3) Å, = 86.024(3)°, = 88.606(4)°, = 81.235(3)°, V = 1012.1(4) ų, Z = 2, and d_calc = 1.360 Mg/m³; R = 0.0354, R_w = 0.0692 for 2655 reflections with I > 2(I). The redox properties of the disulfide compounds 4 and 5 have been explored by cyclic voltammetry, where a one-electron reduction at ca. –1.10 V has been observed for each compound. The site of electron accession in has been established by carrying out molecular orbital calculations at the extended Hüuckel level on the model compound 2,3-di(thio)pyrrolo[1,2-a]benzimidazol-1-one.