Synthetic and Mechanistic Studies of Pd-Catalyzed C−H Arylation with Diaryliodonium Salts: Evidence for a Bimetallic High Oxidation State Pd Intermediate

Abstract

This paper describes the substrate scope and mechanism of Pd-catalyzed ligand-directed C-H arylation with diaryliodonium salts. This transformation was applied to the synthesis of a variety of different biaryl products, using directing groups including pyridines, quinolines, pyrrolidinones, and oxazolidinones. Electronically and sterically diverse aryl groups (Ar) were transferred in high yield using iodine(III) reagents of general structure [Mes-I-Ar]BF 4 . Mechanistic investigations have been conducted that establish the kinetic order of the catalytic reaction in each component, determine the resting state of the catalyst and the iodine(III) reagent, quantify the electronic influence of the arylating reagent on the reaction rate, and establish the intra-and intermolecular 1º H/D kinetic isotope effect. On the basis of these studies, this transformation is proposed to proceed via turnover limiting oxidation of the Pd dimer [Pd(N∼C)(OAc)] 2 (N∼C = 3-methyl-2-phenylpyridine) by [Mes-I-Ph]BF 4 . This mechanism implicates a bimetallic high oxidation state Pd species as a key catalytic intermediate. The significance of this and other aspects of the proposed mechanism are discussed in detail.

Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

“…The first reduction peak potential of PhI(OAc) 2 was found to occur equally both in the CH 3 CN solution and in the carbon paste electrode (1.70 V vs. Fc + /Fc) as a two-electron process. These redox properties are in good agreement with the experimental data on their high oxidative ability, as hypervalent iodine oxidants have been used for the two-electron oxidations of numerous Pd(II) to Pd(IV) cyclometallated complexes [3,7,8,[10][11][12][26][27][28][29]31].…”

“…Pd(II)/Pd(III) or Pd(II)/Pd(0) redox shuttles are proposed [11,12,31]. In both cases, the reactions start from ligand-directed C-H activation at Pd(II) centers affording cyclopalladated intermediates, which then undergo functionalization by two distinct pathways.…”

“…The chemistry of palladium (Pd) compounds in higher oxidation states is extremely useful in oxidative functionalizations, in which a C-H bond can be catalytically converted into another functional group [1,[6][7][8][9][10][11][12]. One of the major problems in Pd(II)/Pd(III)/Pd(IV) chemistry, however, is that the co-oxidants used are either expensive to purchase (silver and other metal salts are often used), or to separate from the product mixture (particularly in the case of high molecular weight organic oxidants).…”

Direct ortho‐Trifluoroethylation of Aromatic Ureas by Palladium Catalyzed C‐H activation: A Missing Piece of Aromatic Substitutions

“…Comparison of the barriers for the C-H activation and the trifluoroethyl migration steps along the dimer route shows that the rate-determining step is the CF3CH2 group transfer from the iodonium salt to the palladium center with an energetic span of 22.1 kcal/mol. This is in nice accord with earlier experimental 18 and theoretical predictions for analogous reactions.…”

Diaryliodonium Salts in Organic Syntheses: A Useful Compound Class for Novel Arylation Strategies

“…96 The mechanism of this palladium catalyzed oxidative C-H activation was also investigated. 97 Initially, the monophenylation of diverse heterocycles including pyridines (125) …”