Nonsymmetrical 3,4-Dithienylmaleimides by Cross-Coupling Reactions with Indium Organometallics: Synthesis and Photochemical Studies

The synthesis and photochemical study of novel nonsymmetrical 1,2‐dithienylethenes (DTEs) with a maleimide bridge have been carried out. The synthetic approach to the DTEs was based on successive selective palladium‐catalyzed cross‐coupling reactions of 5‐susbtituted‐2‐methyl‐3‐thiophenyl indium reagents with 3,4‐dichloromaleimides. The required organoindium reagents were prepared from 2‐methyl‐3,5‐dibromothiophene by a selective (C‐5) coupling reaction with triorganoindium compounds (R3In) and subsequent metal–halogen exchange. The coupling reactions usually gave good yields and have a high atom economy with substoichiometric amounts of R3In. The results of photochemical studies show that these novel dithienylmaleimides undergo a photocyclization reaction upon irradiation in the UV region and a photocycloreversion after excitation in the visible region, thus they can be used as photochemical switches. ON–OFF operations can be repeated in successive cycles without appreciable loss of effectiveness in the process. Photochromic compounds: Triorganoindium reagents are useful reagents for the synthesis of nonsymmetrical 3,4‐dithienylmaleimides by palladium‐catalyzed cross‐coupling reactions (see scheme). Photochemical studies show that these novel compounds undergo photocyclization–photocycloreversion upon irradiation, which allows their use as photochemical switches.