Cyclization of 1,4‐Phenylenediacrylic Acid with Thionyl Chloride and Subsequent Suzuki–Miyaura Reactions Revisited. The Products are Benzo[1,2‐b;5,6‐b′]dithiophenes and not Benzo[1,2‐b;4,5‐b′]dithiophenes

The reaction of 1,4‐phenylenediacrylic acid with thionyl chloride was reinvestigated. In earlier reports [Liebigs Ann. Chem. 1980 , 1172; Heterocycles 1995 , 41, 2691; Adv. Synth. Catal. 2009 , 351, 2683] it was claimed that 3,7‐dichlorobenzo[1,2‐b;4,5‐b′]dithiophenes were formed in these reactions. Herein, we provide unambiguous evidence that the assignment of these structures is wrong and that, in contrast, 3,6‐dichlorobenzo[1,2‐b;5,6‐b′]dithiophenes are formed. As a consequence, the structures of these parent molecules and of numerous aryl‐substituted derivatives prepared by Pd‐catalyzed cross‐coupling reactions have to be revised. As many of these dithiophenes were reported to show interesting optical, thermal and electronic properties, the theoretical explanations for these properties have to be reconsidered in the light of the corrected structures reported herein. Our structural assignments are based on X‐ray crystal structure analyses of the parent molecules and on NMR spectroscopic studies of the first unsymmetrical derivatives. Besides, mechanistic investigations based on quantum chemical calculations have been carried out which support the formation of the 3,6‐dichlorobenzo[1,2‐b;5,6‐b′]dithiophene isomers.