Grignard reactions on ortho dicarboxylic arene derivatives. Synthesis of 1,3-dithienylisothianaphthene compounds

Abstract

1,3-Dithienylisothianaphthene (10a) is obtained through ring closure of 1,2-dithienoylbenzene (7a). The synthesis of 7a has been accomplished based on a Grignard reaction by adding 2-thiophene-magnesium bromide to 1,2-di(S-(2-pyridinyl)) benzenedithioate (6b) to obtain 7a in a yield of 95%. The use of 6b avoids the formation of the corresponding 3,3-dithienyl-3H-isobenzofuran-1(3H)-one (dithienylphthalide, 8). The same procedure is applied to obtain 1,3-dithienyl-4,5,6,7-tetradeuterioisothianaphthene (10b) and 1,3-dithienyl-4,5,6,7-tetrafluoroisothianaphthene (10c). The synthesis of the 2,3-dithienoylpyridine (12a), 3,4-dithienoylpyridine (12b), and 2,3-dithienoylpyrazine (12c) however fails. The presence of nitrogen in the central ring system influences the result of the Grignard reaction. Possibly the free electron pair of the nitrogen interferes with the formation of a stable six-membered ring intermediate which is essential for the diketone formation.