Abstract
The formation of carbon–sulfur bond is one of the fundamental and important transformations in organic synthesis because C–S bonds are abundant and important skeleton in the field of pharmaceuticals and material science. For example, the antidepressant vortioxetine, and an antifungal medication sertaconazole are useful medicines. π-Expanded benzo[b]thiophene derivatives, such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and dinaphtho[2,3-b:2',3'-d]thiophene (DNT), have received considerable attention as key building blocks for use in organic field effect transistors.
The electrochemical carbon–heteroatom bond formation is known to be as an environmentally benign strategy that proceeds under mild conditions. Recently, several novel electrochemical carbon–heteroatom coupling reactions, such as those for C–N and C–O, have been reported. For instance, Lin and co-workers reported the 1,2-diazidation of alkenes, and Baran reported electrochemical allylic oxidations. Lei reported electro-oxidative intermolecular C–S bond formation via C–H/S–H bond cleavage. Intramolecular dehydrogenative coupling between an arene and a thiocarbonyl group was also effective for constructing C–S bonds. While electrochemical transformations have been studied intensively, to the best of our knowledge, there has been no report on electrochemical carbon–heteroatom bond formation for the construction of π-expanded thienoacene derivatives, which are potent candidates for useful organic materials.
From this perspective, we are focusing on an electrochemical C–S bonds formation for the synthesis of thienoacene derivatives, such as [1]benzothieno[3,2-b]benzo[2,3-d]furan (BTBF) and [1]benzothieno[3,2-b]benzo[2,3-d]thiophene (BTBT). We first assumed that intramolecular dehydrogenative C–H/S–H coupling of 2-arylbenzene-1-thiol would form π-expanded thiophenes, but the desired product was not obtained. During the course of further study, we found that the desired dehydrogenative C–S bond formation proceeded smoothly in the presence of electrogenerated [Br+] as a promoter. This strategy was effective for the synthesis of π-expanded thienoacenes such as BTBF and BTBT. We report here the first electro-oxidative dehydrogenative C–S bond formation promoted by electrogenerated [Br+] for the synthesis of BTBFs and BTBTs.
[1] Koichi Mitsudo, Ren Matsuo, Toki Yonezawa, Haruka Inoue, Hiroki Mandai, Seiji Suga, Angew. Chem., Int. Ed. 2020, 59, 7803–7807.
Figure 1