Dissecting Transmetalation Reactions at the Molecular Level: Role of the Coordinated Anion in Gas-Phase Models for the Transmetalation Step of the Hiyama Cross-Coupling Reaction

Palladium-catalyzed cross-coupling protocols have become a cornerstone in organic synthesis. Here, a gas-phase model of the Hiyama cross-coupling reaction was designed to shed light on the roles of coordinated anions (fluoride versus chloride) in transmetalation from Si to Pd. A combination of mass spectrometry experiments and DFT calculations was used. The ligated palladium fluoride and chloride cationic complexes, [(phen)Pd(X)](+) (X = F and CI), readily react with vinyltrimethylsilane, Me3Si(CH=CH2), via transmetalation to give [(phen)Pd(CH=CH2)](+) as the major product. DFT calculations reveal that this transmetalation reaction is concerted and proceeds via a four-centered transition state, illustrating the role of coordinated halide in this gasphase system. Two minor side products are observed corresponding to transmetalation to give [(phen)Pd(CH3)](+) and [(phen)Pd(SiMe2X)](+). DFT calculations suggest that these arise from the same initial Si to Pd methyl transmetalation pathway to give the [(phen)Pd(CH3) + Me-2(CH=CH2)SiX](+) intermediate, which either then loses Me-2(CH=CH2)SiX or reacts via C-C bond coupling to ultimately yield propene and [(phen)Pd(SiMe2X)](+). [(phen)Pd(CH=CH2)](+) undergoes a reaction with a second molecule of vinyltrimethylsilane to form an adduct, which upon collision-induced dissociation liberates 1,3-butadiene to form [(phen)Pd(SiMe3)](+). DFT calculations suggest a mechanism in which C-C bond formation is followed by migration of SiMe3 from C to Pd. Links between the observed gas-phase chemistry and solution-phase Pd-mediated homocoupling reactions of vinyltrimethylsilanes are explored.

Automated summary

Gas-phase model of the Hiyama cross-coupling reaction was established to investigate the roles of coordinated anions in transmetalation, using a combination of mass spectrometry experiments and DFT calculations. The results revealed the concerted nature of transmetalation reactions and provided insights into the mechanism of Pd-catalyzed cross-coupling reactions. The study sheds light on the correlation between gas-phase chemistry and solution-phase Pd-mediated homocoupling reactions.