Naphthol Coupling Monitored by Infrared Spectroscopy in the Gas Phase

The reaction mechanism of copper(II)-mediated naphthol coupling in the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine) is studied using infrared multiphoton dissociation (IRMPD) spectroscopy and DFT calculations. It is shown that the coupling reaction proceeds in ad hoc formed binuclear clusters [(1-H)(2)Cu2Cl(TMEDA)(2)](+), where (1-H) is a deprotonated naphthol molecule (methyl ester of 3-hydroxy-2-naphthoic acid). The IRMPD spectra of the isolated cluster in the gas phase reveal that it contains two uncoupled naphtholate subunits and only the irradiation promotes the coupling reaction, which is thus observed as a genuine gas-phase reaction. The driving force for the C-C coupling is a keto-enol tautomerization of the initial coupling product, and the formation of the corresponding binol in the cluster is exothermic by 0.61 eV. In contrast, analogous C-O and O-O couplings are endothermic reactions.