Multitechnique Approach to Reveal the Mechanism of Copper(II)-Catalyzed Arylation Reactions

Abstract

Multiple in situ and time-resolved spectroscopic techniques (EDXAFS, UV-vis, EPR, and NMR), with a focus on simultaneously acquired EDXAFS and time-resolved UV-vis, are described to reveal detailed structural and electronic information on reaction intermediates of an important Cu(II)-catalyzed N-arylation of imidazole. The N-arylation of imidazole was performed in a NMP/ H 2 O solvent mixture, at ambient temperature and atmosphere, using the commercially available Cu catalyst [Cu(OH)(TMEDA)] 2 Cl 2 (I). The spectroscopic study resulted in the characterization of most reaction intermediates, and a novel mechanism for the Cu(II)-catalyzed arylation reaction is proposed. The first and selectivity-determining step is the reaction of the dimeric Cu(II) starting complex with imidazole, forming a mononuclear Cu(II)(imidazole) intermediate, II. After subsequent addition of phenylboronic acid, we propose the formation of a Cu(III)(imidazolate)(phenyl) intermediate, III, which after reductive elimination forms the phenylimidazole product, and a known Cu(I) monomeric species, IV, is identified. Finally, this Cu species is reoxidized, forming back an equilibrium mixture of Cu(II) mononuclear and dinuclear complexes. Inhibition of the reaction by imidazole and phenylimidazole is observed. The phenylboronic acid is, in combination with H 2 O, involved in the oxidation and reoxidation steps in the described catalytic cycle.

Insights in the mechanism of selective olefin oligomerisation catalysis using stopped-flow freeze-quench techniques: A Mo K-edge QEXAFS study

“…13 More recently, stopped flow systems have been developed for homogeneous catalysis studies, to probe homogeneous catalytic intermediates in situ and time-resolved (milliseconds). 14,15,16 In the stopped flow system the evolution of the reaction mixture can be monitored in time with EDE or Quick-XAS (QEXAFS), allowing full EXAFS spectra to be obtained in the millisecond to second time-scale. 17 The problem with this set-up is that the energy dispersive XAFS data acquisition (but also the QEXAFS if the high time-resolution wants to be achieved) is performed in transmission geometry and therefore a relatively high concentration of the species under investigation is required.…”

“…The stopped-flow EDE method has been proven for homogeneous Pd 14 and Re 18 systems with a high energy Pd K-and Re L-edge, enabling the in situ transmission experiments. However, although the method has been proven for homogeneous Ni (8.333 keV) 14 and Cu (8.979 keV) 16 systems, the Cr K-edge is at an even lower energy of 5.989 keV. Not only the solvent and reactant molecules, but even a few cm of air, are highly absorbing at these energies, severely hampering the transmission experiment.…”

“…Therefore, the set-up as developed here is based on the stopped-flow EDE approach, using a commercially available stopped-flow system with a proven performance for combined EDE/UV-Vis experimentation. 14,16 The stopped-flow (SFM400/QS, Biologic, France) consists of four syringes which can be filled with reaction solution or solvent. 16,20 The system is computer controlled and allows injection of very precise volumes with controlled injection rates (and thus injection times).…”

“…14,16 The stopped-flow (SFM400/QS, Biologic, France) consists of four syringes which can be filled with reaction solution or solvent. 16,20 The system is computer controlled and allows injection of very precise volumes with controlled injection rates (and thus injection times). The solutions are injected using stepper motors via the delay lines and the mixers into a cuvette or capillary, depending on the requirement.…”

“…Using cuvettes with two transmission pathways perpendicular to each other will allow the addition of a complementary technique, like UV-vis, to be performed simultaneously. 16 An optical fibre UV-vis spectrometer (MCS-UV-VIS-NIR 1/500-3 Fast Diode Array Detector, BioLogic, France), specially customised for the stopped flow, is also used in this study. The UV-Vis technique is used to perform time-resolved UV-vis studies (in the laboratory) as well as combined EDE/UV-Vis experiments (at the synchrotron).…”