Mechanism of the Aerobic Homocoupling of Phenylboronic Acid on Au₂₀⁻: A DFT Study

Abstract

The mechanism of the gold nanocluster-catalyzed aerobic homocoupling of arylboronic acids has been elucidated by means of DFT calculations with Au20(-) as a model cluster for the Au:[poly(N-vinylpyrrolidin-2-one)] catalyst. We found that oxygen affects the adsorption of phenylboronic acid and, by lowering the energy barrier, a water molecule enhances dissociation of the C-B bond, which is probably the rate-determining step. The key role of oxygen is in activating the surface of the gold cluster by generating Lewis acidic sites for adsorption and activation of the phenylboronic acid, leading to the formation of biphenyl through a superoxo-like species. Moreover, the oxygen adsorbed on the Au nanocluster can act as an oxidant for phenylboronic acid, giving phenol as a byproduct. As shown by NBO analysis, the basic aqueous reaction medium facilitates the reductive elimination process by weakening the Au-C bond, thereby enhancing the formation of biphenyl. The coupling of phenyl and reductive elimination of biphenyl occur at the top or facet site with low-energy-barrier through spillover of phenyl group on Au NC. The present findings are useful for the interpretation or design of other coupling reactions with Au NC.

Encapsulated Gold Nanoparticles as a Reactive Quasi‐Homogeneous Catalyst in Base‐Free Aerobic Homocoupling Reactions

“…As the temperature‐dependent reactivity is often explained by the collision theory, the activation energies of the reaction were calculated from the Arrhenius plot (Figure S7). Apparently, the base‐free homocoupling reaction with the composite (Oxa) (∼31.8 kJ/mol) is easier than that with the composite (Cit) (∼41.4 kJ/mol), which were within the range of reported values …”

“…The coupling of hydrophilic 4‐hydroxyphenylboronic acid led to a relatively low yield, which was possibly caused by the deprotonation of the slightly acidic alcohol in the phenol group. This process might compete against the formation of 4‐hydroxyphenyl‐B(OH) 3 − species in the base‐free coupling reactions . Extending the reaction time (e. g., 8 h) appreciably increased the yields of all products to moderately overcome the steric and miscibility barriers under the same reaction conditions.…”

Intramolecular Hydroamination by a Primary Amine of an Unactivated Alkene on Gold Nanoclusters: A DFT Study

“…A twenty‐atom gold cluster (Au 20 ) was selected based on our previous works. The Au 20 − model cluster has been used successfully to simulate methanol oxidation on a Au:PVP system, as well as for modeling homocoupling reactions of chloroarenes and phenylboronic acid on the same colloidal Au NC catalyst. The selection of this cluster was based on the observation that it has a significantly lower electron affinity than other gold clusters up to 58 atoms .…”

“…The M06 functional has been demonstrated to be a useful functional for applications in transition‐metal‐containing systems . In particular, we have recently shown that DFT calculations with the M06 functional is suitable for the study of chemical reactions on Au NCs . Benchmark calculations using the left‐eigenstate complete renormalized‐coupled cluster method with singles, doubles, and noniterative triples (CR‐CC(2,3)) for the reaction on Au NCs showed that M06 and B3LYP work well for the activation energy barrier, whereas B3LYP overestimates the reaction energy .…”

“…Gold nanoclusters (NCs) stabilized by water‐soluble polymers, for instance, poly( N ‐vinylpyrrolidone) (PVP), represented as Au:PVP, also serve as promising catalysts for the intramolecular hydroamination of unactivated alkenes under aerobic conditions . In contrast to Au I , the Lewis acid character of zero‐valent Au NCs is generated by the adsorption of O 2 , which allows the nucleophilic substrates to be adsorbed effectively on the Au surface and might play a key role as a catalytic center . The use of Au:PVP enables intramolecular hydroamination of unactivated alkenes to be performed with toluenesulfonamide in aqueous or polar solvents under relatively basic and aerobic conditions, and can lead to highly selective formation of the desired products (Scheme b) .…”