Gas‐Phase Study on the CC Coupling of Naphthol Catalyzed by Cu^II⋅TMEDA: Evidence for the Key Role of Binuclear Clusters

Abstract

The mechanism of oxidative coupling of two naphthol molecules to form binaphthol catalyzed by Cu(OH)ClTMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) was approached by means of a gas-phase model system. Concise evidence is provided that the coupling reaction proceeds in clusters with two Cu(II) centers, whereby the intermediacy of free naphthoxy radicals in the coupling step is avoided. In the absence of TMEDA, the cluster is bound via a bridging counterion and the coupling reaction is followed by cluster cleavage. The coordination of one or two TMEDA molecules to the reactive complex results in more efficient coupling of naphthol molecules, and moreover, the binuclear cluster is also conserved after the reaction is completed. The effect of TMEDA is twofold: First, it supports clustering of copper and, second, as a ligand bound to a copper center in the reactive complex, it weakens the bond between copper and the naphtholato ligand such that the naphtholato unit is more prone to undergo C--C coupling. Furthermore, a pronounced counterion effect is found that correlates well with condensed-phase data: weakly bridging counterions (e.g., NO3(-)) yield less stable dicopper clusters and the coupling reaction hardly occurs, whereas better bridging counterions (e.g., Cl(-) or Br(-)) provide more stable clusters that make the coupling reaction more efficient.

Studies on gas-phase cyclometalations of [ArNi(PPh3)n]+ (n = 1 or 2) by electrospray ionization tandem mass spectrometry

“…A similar effect of the ortho-substitution of carboxymethyl was observed in the coupling of naphthol [66], which indicates the participation of ortho-ester in binding to the metal center. For ion 9, we postulate that the coordination of ortho-carboxymethyl to nickel weakens the Ni-PPh 3 bonding, which facilitates the leaving of one PPh 3 ligand.…”

Theory meets experiment: Gas-phase chemistry of coinage metals

“…A gas-phase study of the reaction mechanism of copper(II)-mediated coupling of methyl ester of 2-hydroxy-3-naphthoic acid (NaphOH) in presence of the chelating ligand N,N,N ,N -tetramethylethylenediamine (TMEDA) [195,196] has shown that rather than mononuclear copper complexes [Cu(NaphO)(TMEDA)] + , which do not exhibit any significant radical reactivity, binuclear clusters can serve as essential reaction intermediates [197,198]. In such complexes each naphthol molecule is bound to one copper atom and the coupling reaction occurs between two of these "controlled" radicals (Scheme 4).…”

Reactions Catalysed by a Binuclear Copper Complex: Aerobic Cross Dehydrogenative Coupling of N‐Aryl Tetrahydroisoquinolines

“…This study shows, for the first time, that Cu II salicylate dimer 1 is a powerful catalyst for the aerobic oxidative coupling of amines with carbon‐, nitrogen‐ and phosphorus‐based nucleophiles. Mechanistically, the binuclear Cu II salicylate is distinct from the widely used simple copper salts such as CuBr and CuCl 2 in the CDC . Only in the presence of an oxidant can it oxidize the amine to the iminium intermediate, and this process is notably accelerated by the chloride ion.…”