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

Abstract

The first examples are reported of an efficient regioselective direct C–H 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 C–H 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 C–H 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 donor–acceptor π-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 mol^–1. 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 1.63–1.06 eV. All copolymers exhibited a diffraction peak at around 2θ = 5.72° corresponding to a d spacing of 15.43 Å, which was assigned to an interchain spacing between polymer main chains similar to that found in P3HT. Moreover, a peak around 2θ = 23.09 (3.84 Å) was also observed and is believed to be related to π-π stacking of the polymer backbones.