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

• Published in: European journal of organic chemistry 2012-10, Vol.2012 (28), p.5540-5551
• Main Authors: Abdo, Nabiha I., El-Shehawy, Ashraf A., El-Barbary, Ahmed A., Lee, Jae-Suk
• Format: Article
• Language: English
• Subjects: Arylation; Chemical reactivity; Chemistry; Cross-cou­pling; Donor-acceptor systems; Exact sciences and technology; Heck reaction; Heterocycles; Heterocyclic compounds; Microwave chemistry; Organic chemistry; Polymers; Preparations and properties; Reactivity and mechanisms; Synthetic methods
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201200769

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. Regioselective C–H arylation of thieno[3,4‐b]pyrazine derivatives with heteroaryl bromides under “Heck‐type” conditions gives donor–acceptor π‐conjugated oligomeric and copolymeric materials in a facial manner. The thienopyrazine unit altered the photophysical and electrochemical properties. Diffraction peaks indicate interchain spacing similar to P3HT and suggest π‐π stacking of the backbones.