Mechanistically Driven Development of Kumada Catalyst-Transfer Polymerizations: A Rapid Injection NMR Study

Transmetalation is a pivotal step in the Kumada-Tamao-Corriu cross-coupling reaction, yet its mechanistic details have remained relatively unexplored compared to other palladium-catalyzed processes. Herein, we systematically investigate how diverse phosphine ligands influence the transmetalation rate, using rapid injection NMR (RI-NMR) to directly monitor the formation of the cross-coupling product. The study reveals that both ligand electronic and steric effects significantly affect k _obs, with electron-rich ligands tending to slow down the reaction and heteroatom-substituted ligands (CPhos) dramatically accelerating transmetalation. Computational studies suggest that the relatively lower transmetalation transition state barriers for CPhos over SPhos oxidative addition complexes are due to both increased electrophilicity of the Pd-atom as well as the ability to access a more favorable square-pyramidal transition state geometry. Finally, we leverage these kinetic findings to guide the synthesis of poly-(3-hexylthiophene) (P3HT) by catalyst-transfer polymerizations. Fast transmetalation ligands, such as CPhos, give higher-molar-mass polymers with controlled dispersities, whereas slower ligands afford inferior control. These results collectively emphasize the centrality of transmetalation in dictating overall cross-coupling performance and pave the way for the rational design of palladium catalysts for both small-molecule synthesis and advanced polymer applications.