Dehydrogenative (Hetero)arene Alkoxylations Triggered by Pd^II‐Catalyzed C(sp²)–H Activation and Coordinating Substituent: Pd^II,III or Pd^IV Complex as Key Intermediate?

Abstract

This review summarizes the intermolecular PdII‐catalyzed dehydrogenation procedures leading to regioselective coupling of alcohols with (hetero)arenes bearing tethered coordinating groups. Unusual oxidative conditions of catalyst regeneration are required, and the optimum oxidant varies with the substrate. Emphasis is placed on the mechanism of these intriguing reactions. Either PdII–Pd0 or PdII–PdIV redox catalytic cycles, possibly with PdIII intermediates, could be involved, depending on the directing group.

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 ability of Pd(II) to activate C-H bonds through chelation-assisted processes has been known since the 1960s. Nitrogen-containing directing groups were found to promote the insertion of palladium into an ortho-C-H bond, which led to the formation of stable and isolable cyclopalladated complexes from Pd(II) starting material (Scheme 6) [25,[31][32][33][34][35][36][37][38][39][40]. The ability of Pd(II) to activate C-H bonds through chelation-assisted processes has been known since the 1960s.…”

“…The ability of Pd(II) to activate C-H bonds through chelation-assisted processes has been known since the 1960s. Nitrogen-containing directing groups were found to promote the insertion of palladium into an ortho-C-H bond, which led to the formation of stable and isolable cyclopalladated complexes from Pd(II) starting material (Scheme 6) [25,[31][32][33][34][35][36][37][38][39][40]. Despite the large literature devoted to the synthesis of various palladacycles and studies of their reactivity, their redox properties, which are necessary to describe and understand the mechanisms of catalytic reactions involving them, are elucidated only in few works [11,15,24,25,[41][42][43][44].…”

“…The recently reported work [31] analyzes the oxidant dependence and radical participation in Pd-catalyzed dehydrogenative alkoxylation of arenes (Scheme 7), as well as the efficiency of the procedures and key intermediates proposed in the literature. The latter are mainly Pd(IV) complexes for reactions with PhI(OAc) 2 , K 2 S 2 O 8, or oxone, which seems to be fair and corresponds to the strength of these oxidants (the strongest).…”