Observation and Mechanistic Study of Facile CO Bond Formation between a Well‐Defined Aryl–Copper(III) Complex and Oxygen Nucleophiles

Abstract

A well-defined macrocyclic aryl–Cu(III) complex (2) reacts readily with a variety of oxygen nucleophiles, including carboxylic acids, phenols and alcohols, under mild conditions to form the corresponding aryl esters, biaryl ethers and alkyl aryl ethers. The relationship between these reactions and catalytic C-O coupling methods is demonstrated by the reaction of the macrocyclic aryl–Br species with acetic acid and p-fluorophenol in the presence of 10 mol% Cu(I). An aryl-Cu(III)-Br species 2(Br) was observed as an intermediate in the catalytic reaction. Investigation of the stoichiometric C-O bond-forming reactions revealed nucleophile-dependent changes in the mechanism. The reaction of 2 with carboxylic acids revealed a positive correlation between the log(k(obs)) and the pK(a) of the nucleophile (less-acidic nucleophiles react more rapidly), whereas a negative correlation was observed with most phenols (more-acidic phenols react more rapidly). The latter trend resembles previous observations with nitrogen nucleophiles. With carboxylic acids and acidic phenols, UV-visible spectroscopic data support the formation of a ground-state adduct between 2 and the oxygen nucleophile. Collectively, kinetic and spectroscopic data support a unified mechanism for aryl-O coupling from the Cu(III) complex, consisting of nucleophile coordination to the Cu(III) center, deprotonation of the coordinated nucleophile, and C-O (or C-N) reductive elimination from Cu(III).

Direct observation of two-electron Ag(I)/Ag(III) redox cycles in coupling catalysis

“…Once 1 ClO4 was fully characterized, we turned our attention towards its reactivity. We speculated that this complex could show a similar reactivity to its analogous copper complex, that is, to undergo reductive eliminations to functionalize the C sp2 -M(III) (M ¼ Cu or Ag) bond in the presence of nucleophiles of different nature 19,35 . The reaction of 1 ClO4 with sulfonamides, aromatic and aliphatic carboxylic acids, phenols and thiophenols resulted in the formation of C arylnucleophile coupling products in excellent yields, characteristic of a reductive elimination reaction (products 2a-2g, Table 1).…”

Pincerlike Cyclic Systems for Unraveling Fundamental Coinage Metal Redox Processes

“…Later in 2010, Ribas and Stahl's groups demonstrated that the final C-N coupling products could be achieved in a catalytic fashion by employing [Cu I (CH 3 CN) 4 ] PF 6 as catalyst (<5 mol%) and the macrocyclic aryl bromide 19e-Br as substrate via aryl-Cu III -Br intermediates [39]. The same operative Cu I /Cu III catalytic cycle has been now validated for O-, S-, Se-, and P-nucleophiles (Scheme 16) [52,55].…”

“…Our research group and others have demonstrated the capability of aryl-Cu III complexes to generate C aryl -heteroatom bonds upon reaction with nitrogen-based nucleophiles [41] (lactams, imides, sulfonamides, oxazolidone, and benzamides), oxygen-based nucleophiles [45,[52][53][54] (carboxylic acids, alcohols), sulfur-based nucleophiles [45,55] (thiols, thiocyanate), selenium-based nucleophiles [55] (benzeneselenol), and phosphorus-based nucleophiles [55] (H-phosphonate diesters). This step can be envisioned to occur through two possible mechanisms:…”

“…While for halogenation reactions it is clear that the first pathway is functional, being aryl-Cu III -X complexes isolated and fully characterized, for C-nucleophile reactions, the authors have only been able to detect by low-temperature UV-Vis and NMR techniques aryl-Cu III -nucleophile adducts prior to the nucleophile deprotonation step [52,55]. Further, both oxidative addition and coupling steps could be merged to realize a catalytic process with the whole range of nucleophiles (Scheme 16).…”

Understanding the Heteroatom Effect on the Ullmann Copper-Catalyzed Cross-Coupling of X-Arylation (X = NH, O, S) Mechanism

“…While most of the evidence collected so far has shown that the active catalyst is a ligand-copper(I)-nucleophile [(L)Cu I (Nu)] species, there is no such agreement on the way this catalyst activates the aryl halide (ArY, Y = Cl, Br and I). The most invoked routes are based on either one-electron redox processes through the radical intermediate Cu I /Cu II catalytic cycle namely Single Electron Transfer (SET) and Halogen Atom Transfer (HAT) [4,[39][40][41][42], or on two-electron redox processes via a Cu I /Cu III catalytic cycle like Oxidative Addition-Reductive Elimination (OA-RE) or σ-bond Metathesis (MET) (Scheme 1) [37,38,[43][44][45][46][47][48][49][50]. Scheme 1.…”