Suzuki Miyaura cross-coupling reactions are essential synthetic tools for constructing C-C bonds. The ligand-free versions of Suzuki cross-coupling reactions are gaining attention recently due to the sustainability and cost-effectiveness of the former over the ligand-supported counterparts. Unlike the palladium-catalyzed versions, the deeper mechanistic insights for the copper-catalyzed Suzuki cross-coupling reactions are semioccasional. In the present work, a detailed computational investigation into the mechanism of the ligand-free copper-catalyzed Suzuki cross-coupling reaction between aryl boronic acid and alkynyl halides is carried out using the Density Functional Theory method, employing B3LYP-D3 functional augmented with CPCM solvation model, using CuI as the catalyst, ethanol as the solvent and K3PO4 as the base. Our investigation revealed that the unsaturated halides like alkynyl halides could act as good ancillary ligands for the copper by forming a Cu- π intermediate and promoting a facile transmetalation process. The study further showed that the reaction proceeds via an initial transmetalation step, followed by a concerted oxidative nucleophilic substitution-reductive elimination step. This is owing to the ease of reductive elimination involving the cross-coupling of Csp2 -Csp carbon atoms and the instability of a Cu (III) intermediate. This shows that the mechanism of Cu catalyzed Suzuki coupling is different from the Pd catalyzed counterparts. A two-coordinated Cu(I) species is identified as the active catalyst in the reaction. The oxidative nucleophilic substitution step is found to be the rate-determining step. A thermodynamically stable π-coordinated intermediate species where the substrates and base molecule are coordinated to the metal center is identified as the rate-determining species for the ligand-free Suzuki-cross-coupling reaction. The overall reaction is found to be exergonic. Understanding these reactions at the molecular level with the aid of computational techniques will help the researchers and industrialists to impart effective changes to improve the catalytic efficiency of copper metal.

Deciphering the mechanism of ligand-free copper-catalyzed Suzuki cross-coupling reaction: A DFT study

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