Mechanistic Origin of Ligand-Controlled Regioselectivity in Pd-Catalyzed C-H Activation/Arylation of Thiophenes

The use of ligands to control regioselectivity in transition-metal-catalyzed C-H activation/functionalization is a highly desirable but challenging task. Recently, Itami et al. reported an important finding relating to Pd-catalyzed ligand-controlled a/beta-selective C-H arylation of thiophenes. Specifically, the use of the 2,2'-bipyridyl ligand resulted in a-arylation, whereas the use of the bulky fluorinated phosphine ligand P[OCH(CF3)2]3 resulted in beta-arylation. Understanding of this surprising ligand-controlled a/beta-selectivity could provide important insights into the development of more efficient catalyst systems for selective C-H arylation, and so we carried out a detailed computational study on the problem with use of density functional theory methods. Three mechanistic possibilitiesSEAr and migration, metalation/deprotonation, and Heck-type arylation mechanismswere examined. The results showed that the SEAr and migration mechanism might not be plausible, because the key Wheland intermediates could not be obtained. On the other hand, our study indicated that the metalation/deprotonation and Heck-type arylation mechanisms were both involved in Itamis reactions. In the metalation/deprotonation pathway the a-selective product (C5-product) was preferred, whereas in the Heck-type arylation mechanism the beta-selective product (C4-product) was favored. The ligands played crucial roles in tuning the relative barriers of the two different pathways. In the 2,2'-bipyridyl-assisted system, the metalation/deprotonation pathway was energetically advantageous, leading to a-selectivity. In the P[OCH(CF3)2]3-assisted system, on the other hand, the Heck-type arylation mechanism was kinetically favored, leading to beta-selectivity. An interesting finding was that P[OCH(CF3)2]3 could produce a C-H....O hydrogen bond in the catalyst system, which was crucial for stabilization of the Heck-type transition state. In comparison, this C-H...O hydrogen bond was absent with the other phosphine ligands [i.e., P(OMe)3, PPh3, PCy3] and these phosphine ligands therefore favored the metalation/deprotonation pathway leading to a-selectivity. Furthermore, in this study we have provided theoretical evidence showing that the Heck-type arylation reaction could proceed through an anti-beta-hydride elimination process.