Palladium(III) in Synthesis and Catalysis

Abstract

While the organometallic chemistry of Pd in its (0), (+II), and (+IV) oxidation states is wellestablished, organometallic Pd(III) chemistry remains widely unexplored. Few characterized Pd(III) complexes are known, which has inhibited detailed study of the organometallic chemistry of Pd(III). In this review, the potential roles of both mono-and dinuclear Pd(III) complexes in organometallic chemistry will be discussed. While not widely recognized, Pd in the (+III) oxidation state may play a significant role in a variety of known Pd-catalyzed reactions.

Cationic Pd(II)-catalyzed C–H activation/cross-coupling reactions at room temperature: synthetic and mechanistic studies

“…Liu [ 73 ] and Sanford reported the stoichiometric reaction of palladacycles and aryliodonium salts at high temperature to give the corresponding products through Pd(II)/Pd(IV) or Pd(III)/Pd(III) dimeric species bridged by acetates ( Scheme 26 ) [ 68 , 175 , 249 – 250 ]. Sequences involving Pd(III) intermediates have been suggested as alternatives to Pd(II)/Pd(IV) cycles in some cases, but most well-studied examples involve Pd(III)/Pd(III) dimers formed with the aid of bridging anionic ligands such as acetate or nitrate [ 251 – 252 ] that do not match as well with the cationic palladium conditions employed here. Silver-mediated one electron oxidations to form monomeric Pd(III) complexes have also been studied [ 251 ], but the successful implementation of silver-free conditions with stoichiometric palladium herein would appear to eliminate this as a key step.…”

Silver-Mediated Trifluoromethoxylation of Aryl Stannanes and Arylboronic Acids

“…The ability to effect challenging carbon–heteroatom bond transformations from silver complexes may be a consequence of synergistic metal–metal interactions that can lower activation barriers, as identified for bimetallic catalysis. 12 We hypothesize that trifluoromethoxylation proceeds from discrete high-valent silver complexes, formed via oxidation of aryl silver complexes with F-TEDA-PF 6 ( 4 ), followed by fluoride to trifluoromethoxide ligand exchange. The use of two equivalents of AgPF 6 gave the highest observed yields of trifluoromethoxylation, which could suggest a dinuclear silver complex as a key intermediate in C–OCF 3 bond formation.…”

Transition-metal-catalyzed acetoxylation of heterocycles: all that glitters is not palladium

“…Thus, electron-rich indoles readily underwent acetoxylation, whereas electron-deficient indoles required increased amounts of PhI(OAc) 2 (up to 2.5 equiv) and prolonged reaction times to achieve synthetically useful yields; (iii) the acetoxylation of C-2 or C-3 unsubstituted indoles occurred regioselectively at the C-3 position*. These observations together with the commonly accepted mechanism for Pd-catalyzed C-H activation/acetoxylation of benzene rings [5][6][7] led authors to propose that the first step of the catalytic cycle is electrophilic palladation of indole [2,3]. In fact, it has been demonstrated earlier that electrophilic metallation of indoles with Pd(II) species occurs regioselectively at the more electron-rich C-3 position rather than at C-2 [8].…”

“…In fact, it has been demonstrated earlier that electrophilic metallation of indoles with Pd(II) species occurs regioselectively at the more electron-rich C-3 position rather than at C-2 [8]. Subsequent oxidation of indolyl-Pd(II) species 1 with PhI(OAc) 2 delivers Pd(IV) or bimetallic Pd(III) complexes 2 [6], which undergo reductive elimination to form acetoxyindoles and Pd(II) salts (Scheme 3) [7]. Scheme 3 _______ * Formation of 2,3-bisacetoxyindole side product was observed in AcOH (see [2]).…”