Role of Silver Salts in Palladium-Catalyzed Arene and Heteroarene C–H Functionalization Reactions

Silver carboxylate
salts are widely used as additives in palladium-catalyzed
C–H functionalization reactions. However, the role of these
silver additives is often not fully understood. This paper describes
an investigation of the role of AgOPiv in the stoichiometric activation
of C6F5H at a well-defined PdII complex
as well as in the PdII-catalyzed oxidative dimerization
of 2-alkylthiophenes. Both in situ NMR spectroscopy and H/D exchange
studies of the reactions of C6F5H implicate
a role for AgOPiv in the C–H cleavage event, generating Ag–C6F5 as an intermediate. The catalytic studies show
similar trends despite the different conditions and substrates, suggesting
that AgOPiv promotes a similar metalation of the thiophene in the
catalytic transformations. This proposal is supported by DFT calculations,
which show energetically feasible pathways for concerted metalation–deprotonation
of both 2-methylthiophene and pentafluorobenzene at [Ag­(OPiv)]2. These studies suggest that initial metalation of C–H
substrates at AgI carboxylates should be considered as
a plausible pathway in C–H functionalization reactions involving
mixtures of Ag and Pd salts.