Chan–Evans–Lam Couplings with Copper Iminoarylsulfonate Complexes: Scope and Mechanism

Abstract

Copper­(II) pyridyliminoarylsulfonate complexes with chloride or triflate counteranions were employed in Chan–Evans–Lam (CEL) couplings of N-nucleophiles and arylboronic acids. The complexes avoided typical side reactions in CEL couplings, and an excess of boronic acid was not required. Water was tolerated, and addition of neither base nor other additives was necessary. Primary amines, acyclic and cyclic secondary amines, anilines, aminophenol, imidazole, pyrazole, and phenyltetrazole can be quantitatively arylated at either 25 or 50 °C with 2.5 mol % of the catalyst. Reaction kinetics were investigated in detail. Kinetic and spectroscopic studies provide evidence for the formation of unproductive copper–substrate complexes. Formation of an aniline–phenylboronic acid adduct was responsible for the zero-order dependence of reaction rates on phenylboronic acid concentration. Kinetic evidence indicates that the order of reaction steps is transmetalation, nucleophile coordination, and oxidation. Couplings performed poorly with electron-deficient arylboronic acids, due to a slower Cu­(II)/Cu­(III) oxidation in the catalytic cycle. Photoredox catalysis partially resolved this problem, but addition of copper acetate as an external oxidant proved to be more efficient.

Tuning the Copper(II)/Copper(I) Redox Potential for More Robust Copper‐Catalyzed C–N Bond Forming Reactions

“…This CEL coupling is the only reported type of copper‐dependent coupling reaction that is proposes to be initiated through a copper(II) species and uses O 2 as an oxidant within the cycle to regenerate the copper(II) state during turnover . These coupling reactions occur with a boronic acid as the coupling agent to form C–C or C–heteroatom bonds and have begun to be extensively studied with most of the research focus on using copper salts or copper salts with a supporting ligand as the catalyst, where very few discrete complexes have been reported . CEL coupling reactions have also been reported to be more active under protic conditions when using copper(I) sources, where the current consensus mechanism for this process involves copper(I), copper(II), and copper(III) states .…”

Chan‐Evans‐Lam C−N Coupling Promoted by a Dinuclear Positively Charged Cu(II) Complex. Catalytic Performance and Some Evidence for the Mechanism of CEL Reaction Obviating Cu(III)/Cu(I) Catalytic Cycle

“…The generally held view of the catalytic cycle includes the redox Cu(III)/Cu(I) couple as the main driving force of the oxidative conversions. Scheme illustrates this point, as was recently discussed by Stahl, Schaper and Watson . According to the mechanism, one Cu(II) ion oxidizes another ion into the Cu(III) version, at which both components of the coupling are coordinated.…”

Cu(II)-silsesquioxanes as efficient precatalysts for Chan-Evans-Lam coupling

“…No less important process, namely, the oxidative coupling of nucleophiles with arylboronic acids (Chan-Evans-Lam (CEL) coupling), is also catalyzed by copper compounds. This reaction was firstly reported in 1998 [26e28] and since then many examples of simple copper salts' application for CEL coupling were reported [29,30]. In turn, examples of MOF-[31e36], bi- [37,38] or polynuclear copper-based [39,40] catalysts remain scarce.…”