Unveiling the Novel Mechanistic Insights and Role of Base in Zn-Catalyzed Csp–Csp2 Cross-Coupling Reaction

Abstract

A detailed mechanistic investigation of the Zn (II)-catalyzed Csp–Csp² (Sonogashira-type) cross-coupling reaction is reported herein, using the Density Functional Theory method. The present study unveiled an unconventional non-redox mechanism for Zn-catalyzed cross-coupling reaction, where the oxidation state of Zn remains intact throughout the catalytic cycle. Our study further revealed the significant role of the base in controlling the feasibility of cross-coupling reactions that are catalyzed by electron-deficient metal centers. Our study indicates that K₃PO₄ acts as an ancillary ligand (Lewis base) for the electron-deficient Zn (II) catalytic center rather than as a proton abstractor for the nucleophilic coupling partner (phenylacetylene) in this reaction. The active catalyst was identified to be a four-coordinate bis-DMEDA Zn (II) complex. The mechanism proceeds via the initial activation of the nucleophilic coupling partner (phenylacetylene), followed by the electrophilic coupling partner (organic halide) activation liberating the cross-coupled product by a concerted nucleophilic substitution pathway. The turn-over limiting step was identified to be the activation of the electrophilic coupling partner. The activation barrier obtained for the reaction, 31.0 kcal/mol concords well with experimental temperature requirements (125°C). The coordination by base is found to stabilize the rate-determining intermediates and transition states involved in the reaction. The mechanistic insights gained from this study could aid in the rational design and development of sustainable cross-coupling reactions using zinc as the catalyst.