MECHANISTIC INSIGHTS INTO CATALYTIC SYSTEMS IN ORGANIC SYNTHESIS INVOLVING TRANSITION METAL COMPLEXES
Keywords:
transition metal catalysis, organic synthesis, reaction mechanism, oxidative addition, ligand effects, computational chemistry, cross-coupling, radical pathways.Abstract
Background: The transition metal complexes have been proven as key catalysts in the new generation of organic synthesis industry in an efficient and highly selective manner in many cases. Mechanism cycles (oxidative addition, migratory insertion, ligand exchange, and reductive elimination) are involved in these catalytic systems and are complex, involving multiple steps, which are strongly influenced by the identity and environment of the metal. This study's background is based on the rationale for the need to gain a better mechanistic understanding of these processes in order to design more efficient synthetic routes and catalysts in complex organic transformations.
Aim: This study focuses on understanding the mechanisms of metal-catalyzed organic reactions by a combination of experimental and computational studies.
Method: A catalytic screening of Pd, Ni, Cu, Rh, and Ru complexes, along with spectroscopic characterization (NMR, IR, UV-vis, EPR and XAS) and isotopic labeling experiments, combined with density functional theory (DFT) calculations, has been used for examining reaction intermediates and energy profiles.
Results: The results indicated that the Cu(I) systems showed the best selectivity (100%) in the cyclo-addition reactions, the Pd catalysts in the cross-coupling reactions, and the Ni catalysts in the cross-electrophile transformations. Kinetic isotope effect studies proved that the rate-determining step for palladium and rhodium systems is the C–H activation step, but for nickel catalysis, the low activation barriers indicate a radical pathway. Calculations confirmed the experiments by demonstrating different energy landscapes for the different metal systems and that the oxidative addition barriers were lowest for nickel.
Conclusion: The nature of the hybrid mechanistic pathways for transition metal-catalyzed reactions, which involve both organometallic and radical processes, and the strong influence of ligand and electronic effects on the efficiency of these reactions are discussed. These are the results of a deeper understanding of the mechanism, which allows rational optimization of the catalysts and further improvement of sustainable synthesis.
