Mechanistic Origin of β‑Defect Formation in Thiophene-Based Polymers Prepared by Direct (Hetero)arylation

A computational study of the direct arylation reaction was undertaken to understand the origin of nonselective β-couplings observed in thiophene-based polymers prepared by direct (hetero)­arylation polymerization (DHAP). The reactivities and selectivities of C–H bond activations were studied for the concerted metalation–deprotonation mechanism on model thiophene substrates using density functional theory (DFT) and domain-based local pair natural orbital coupled cluster with singles, doubles, and perturbative triples (DLPNO-CCSD­(T)). Substituent effects are shown to control experimentally observed regioselectivities. Electron-rich or -deficient thiophene substrates with multiple C–H bonds possess an intrinsic selectivity for bond activation at the α-position, while halogens in the α-position substantially reduce coupling selectivities by activating the adjacent Cβ–H bonds. Therefore, the halogenated monomer is responsible for the reported poor selectivities in DHAP. The effect of the substrate undergoing oxidative addition is also reported for the first time along with design principles for monomeric units amenable to high-selectivity DHAP polymerizations