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

Abstract

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 C 6 F 5 H at a well-defined Pd II complex as well as in the Pd II -catalyzed oxidative dimerization of 2-alkylthiophenes. Both in situ NMR spectroscopy and H/D exchange studies of the reactions of C 6 F 5 H implicate a role for AgOPiv in the C−H cleavage event, generating Ag−C 6 F 5 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 2methylthiophene and pentafluorobenzene at [Ag(OPiv)] 2 . These studies suggest that initial metalation of C−H substrates at Ag I carboxylates should be considered as a plausible pathway in C−H functionalization reactions involving mixtures of Ag and Pd salts.

Is silver a mere terminal oxidant in palladium catalyzed C–H bond activation reactions?

“…It is instructive to note that homobimetallic catalytic systems were recently suggested to participate in similar catalytic reactions. [38][39][40] However, such homobimetallic systems are found to be energetically less preferred in the case of the present examples. 38 The above-mentioned analysis is based on the relative energies of the transition states computed with respect to a global reference point, i.e., the separated reactants.…”

Deuterium Exchange between Arenes and Deuterated Solvents in the Absence of a Transition Metal: Synthesis of D‐Labeled Fluoroarenes

“…Transition metal catalysis has also been tested and Larrosa et al used a silver salt in the presence of triphenylphosphine and D 2 O to deuterate a number of fluoroaryls in moderate yields (Scheme b) . The role of silver in this exchange was also observed later by Sanford et al for C 6 F 5 H . Photochemical conversion of C‐X (X = halogen) to C‐D bonds with D 2 O catalyzed by porous CdSe nanosheets has been applied to several fluorosubstituted aryl halides .…”

C–H Bond Functionalization of 1,4‐Benzoquinone by Silver‐Mediated Regioselective Phosphination and Amination Reactions

“…Oxidants such as AgF, AgBF 4 , and Cu(OAc) 2 were ineffective in this reaction (entries 6–8). Recently, Larrosa and Sanford and their co‐workers independently came to the same conclusion that Ag I is capable of activating the electron‐deficient arene C–H bond through a concerted metalation/deprotonation (CMD) mechanism when an appropriate base such as CO 3 2– , OAc – , or OPiv – is the counter ion to the silver salt. Although the capacity of Ag I to oxidize HQ intermediates to the corresponding BQ derivatives is known, its other roles have not yet been identified in this reaction.…”