Electronically Asynchronous Aryl Transfer in Carbonate-Assisted Suzuki-Miyaura Transmetalation Revealed by Electron Localization Function Topology

The transmetalation step in the ligand-free Suzuki-Miyaura reaction remains a topic of debate, especially regarding the roles of the base and the order of bond-breaking and bond-forming processes. In this study, we investigate the carbonate-assisted boronate pathway and demonstrate that aryl transfer proceeds through an electronically asynchronous mechanism, in which B–C bond cleavage precedes Pd–C bond formation despite a smooth and featureless free-energy profile. Free-energy calculations combined with intrinsic reaction coordinate analysis reveal a kinetically accessible and highly exergonic pathway. However, analysis of the electron localization function (ELF) shows that the reaction involves a series of discrete electronic events that are not directly visible in the energy profile. Pretransmetalation involves stepwise η 2 -aryl coordination, where Pd–C ortho interaction occurs before Pd–C ipso bonding. Transmetalation happens through B–C ipso bond cleavage followed by Pd–C ipso bond formation in separate steps, revealing that aryl transfer occurs through electronically asynchronous steps. These findings reveal that key bond-reorganization events in palladium-catalyzed cross-coupling can proceed through electronically distinct stages even in the absence of detectable energetic signatures, highlighting a fundamental limitation of energy-based mechanistic analysis and establishing ELF topology as a powerful tool to uncover hidden electronic processes in organometallic reactivity.