A detailed theoretical investigation to unravel the molecular mechanism of the ligand-free copper-catalyzed Suzuki cross-coupling reaction. Issue 22 (7th April 2022)

• Record Type: Journal Article
• Authors: Rajalakshmi, C.; Krishnan, Anandhu; Saranya, Salim; Anilkumar, Gopinathan; Thomas, Vibin Ipe
• Is Part Of: Organic & biomolecular chemistry. Volume 20:Issue 22(2022)
• Page Start: 4539
• Page End: 4552
• Publication Date: 2022-04-07
• Subjects: Chemistry, Organic -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1039/d2ob00371f
• Languages: English
• ISSNs: 1477-0520

Abstract

Abstract : A DFT study into the mechanism of ligand-free Cu catalyzed Suzuki cross-coupling reactions of unsaturated halides with aryl boronic acid that portrays the effect of substrate molecules on reaction feasibility by acting as pseudo-ancillary ligands. Abstract : The Suzuki–Miyaura coupling (SMC) represents a very efficacious method for constructing C–C bonds in organic synthesis. The ligand-free variants of SMC have been grabbing attention these days. Despite this momentousness, the mechanistic details of the ligand-free variants are scant in the literature. Herein, we have carried out a detailed mechanistic investigation into the ligand-free Cu-catalyzed SMC of unsaturated organic halides with aryl boronic acid with the aid of density functional theory (DFT) calculations employing the conductor-like polarizable continuum model (CPCM) method. The present study elucidates that in the absence of ancillary ligands on the metal, the substrates, base, and solvent molecules could act as pseudo-ancillary ligands to facilitate the cross-coupling reaction. The investigation further revealed that unsaturated halides like alkynyl halides/vinyl halides could act as good ancillary ligands for copper by forming a Cu–π intermediate and promoting a facile transmetalation process. However, regarding the oxidative addition and reductive elimination steps, a concerted pathway is observed contrary to Pd catalyzed Suzuki coupling, owing to the instability of Cu(iii ) species and the favourability of Csp 2 –Csp bond formation. In the whole set of mechanisms explored, oxidative addition/oxidative nucleophilic substitution was the rate-determining step in all the cases. A thermodynamically stable π-coordinated intermediate species where the substrate 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 presence of the aforesaid intermediate increases the energy span and consequently the activation barrier for the rate-determining step. This study unveiled a theoretical rationale for the high-temperature requirement in the ligand-free Cu-catalyzed SMC reaction.