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 Cl), 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 gas-phase 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) + Me₂(CH═CH₂)SiX]⁺ intermediate, which either then loses Me₂(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.