Synthesis of Benzo[c]thiophene

Benzo[c]thiophene, also known as isobenzothiophene, is a bicyclic heteroaromatic sulfur heterocycle, constituted by fusion of benzene with the“c” site (3,4-positions) of the thiophene ring. It is less stable than benzo[b]thiophene because the resonance energy of benzo[c]thiophene (184 kJ/mol) is comparatively higher and is considered to be contributed by following resonating structures. 

The stability of the ring system is increased by the presence of substituents at the 1,3-positions of the benzo[c]thiophene ring.The first derivative of the benzo[c]thiophene ring system, 1,3-diphenylbenzo[c]thiophene, was synthesized and reported in 1922 but its structure was not correctly elucidated until 1937. The parent compound was isolated and reported in 1962.

Structure

Based on X-ray diffraction, benzo[c]thiophene is best represented as an ortho-quinonoid structure in which the CS bond length is similar to those in thiophene. 

Synthesis 

A reaction of 1,2-bis(halomethyl)benzene with Na2 S gave 1,2-dihydrobenzo[c]thiophene, which on dehydrogenation over Pd/C or Pt in the vapor phase at 300°C afforded parent benzo[c]thiophene. 

1,3-Diarybenzo[c]thiophenes have been synthesized in high yields by the reaction of 1,2-(diaroyl)benzene with Lawesson’s reagent in DCM at room temperature.

Conveniently, benzo[c]thiophene has been prepared in three steps. The first step is the formation of benzo[c] thiophene-1,3-dione from a reaction of phthaloyl chloride with Na2 S, which was chlorinated with a mixture of POCl3 and PCl5  to 1,3-tetrachlorobenzo[c]thiophene in the second step. Finally, the intermediate 1,3-tetrachlorobenzo[c] thiophene on treatment with NaI in DMF afforded 1,3-dichlorobenzo[c]thiophene.

1,3-Diarylbenzo[c]thiophene has been prepared by replacing oxygen by sulfur in 1,3-diarylbenzo[c]furan by  Lawesson’s reagent.