Sulfonato-diketimine Copper(II) Complexes: Synthesis and Application as Catalysts in Chan–Evans–Lam Couplings

Abstract

Copper complexes bearing a diketimino-sulfonate ligand, LCu(NO 3 )(NCMe), were prepared and proved to be stable to water for several hours in solution. Prolonged standing in the presence of water or strong bases led to desulfonation of the ligand. LCu(NO 3 ) was inactive in the polymerization of lactide (in the presence of benzyl alcohol), but showed high activity for the Chan−Evans−Lam coupling of a variety of amines and anilines. Couplings do not require addition of base, ligand, or molecular sieves. Mechanistic investigations indicate a catalytic cycle involving dioxygen as the required oxidant, precoordination of boronic acid to the sulfonate group, and transmetalation occurring prior to reaction with the nucleophile.

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 inhibition of the CEL reaction by the nucleophilic anilines (compare the yields of 7 a , 7 d′ and 7 q′ , Scheme ) could also be linked to the same cause. The analogous mechanism was earlier suggested by Shaper and supported by competition experiments in the paper on CEL reaction promoted by his sulfonato−diketimine copper(II) complexes …”

“…Air is routinely used for the reaction as an oxidant. The CEL coupling later was elaborated to allow C−N, C−O, C−S, C−Se, C−P bond formation via copper(II)‐catalyzed pathways of arylboronic acids with a wide range of nucleophiles, including sensitive substrates, which is critically important in numerous commercial chemical processes . There are several reports for three‐component, tandem CEL‐type reactions and heterocycles synthesis .…”

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

“…These catalytic data are on par with reactivity reported for other N‐donating copper complexes that have been reported . To date, Schaper's catalysts for CEL coupling still show the best catalytic turnover, but have significantly different donor abilities . However within the context of this work, it appears that the more electron deficient phen derivatives support the CEL coupling reaction better than those that are more electron rich, where complex 4 generated the highest yields for C–N bond forming reactions.…”

Synthesis and Characterization of a Tetradentate, N-Heterocyclic Carbene Copper(II) Complex and Its Use as a Chan–Evans–Lam Coupling Catalyst

“…The CEL reaction, however, is much less understood, where the reaction has been proposed to proceed through several different copper oxidation states including Cu + , Cu 2+ , and Cu 3+ . The mechanistic role of each of these species is still actively debated. − Another issue with adopting the CEL coupling is that many using this reaction still employ a large excess of aryl boronic acids and 1–2 equiv of a copper “catalyst,” even though, under these conditions, copper should be considered more of a reagent than a catalyst. Several groups have made dedicated progress in exploring the ligands that support CEL reactivity, but overall, the scope of these reports is limited. ,− Previously, we reported a series of bidentate ligands including diazafluorenone, which stabilized the Cu + state more efficiently than phenanthroline, and this shift in potential was linked to increased catalytic yields for the CEL process .…”