Palladium-Catalyzed Direct C-H Arylation of Thieno[3,4-b]pyrazines: Synthesis of Advanced Oligomeric and Polymeric Materials

The first examples are reported of an efficient regioselective direct CH arylation of thieno[3,4-b]pyrazine (TP) and its 2,3-dimethyl derivative with bromoalkylthiophenes (BATs), under Heck experimental conditions using Pd(OAc)2/Bu4NBr as the catalytic system, giving rise to a variety of valuable aryl-substituted thienopyrazines. The obtained results suggested that the 2-position of the TP moiety is less reactive towards CH arylation than the 5- and 7-positions. Moreover, the 3-position of the TP moiety showed almost no significant reactivity when all other positions were arylated. The CH arylation of 2,3-dimethyl-TP with an excess amount of BATs proceeded smoothly, affording the corresponding diarylated thienopyrazine derivatives in excellent yields, without any additional products. Compared to usual cross-coupling reactions, the present synthetic methodology has been used to prepare interesting donoracceptor p-conjugated polymeric materials in a facial manner in a simple way. Microwave-assisted polymerization proved to be efficient for obtaining reasonable molecular weight copolymers ranging from 18.8 to 24.3 kg?mol1. Incorporating the thienopyrazine unit into polyhexylthiophene chains affected the photophysical and electrochemical properties. The optical band gaps were estimated to be in the range of 1.631.06 eV. All copolymers exhibited a diffraction peak at around 2? = 5.72 degrees corresponding to a d spacing of 15.43 angstrom, which was assigned to an interchain spacing between polymer main chains similar to that found in P3HT. Moreover, a peak around 2? = 23.09 (3.84 angstrom) was also observed and is believed to be related to p-p stacking of the polymer backbones.