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, Me 3 Si(CHCH 2 ), via transmetalation to give [(phen)Pd(CHCH 2 )] + 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(CH 3 )] + and [(phen)Pd(SiMe 2 X)] + . DFT calculations suggest that these arise from the same initial Si to Pd methyl transmetalation pathway to give the [(phen)Pd(CH 3 ) + Me 2 (CH CH 2 )SiX] + intermediate, which either then loses Me 2 (CHCH 2 )SiX or reacts via C−C bond coupling to ultimately yield propene and [(phen)Pd(SiMe 2 X)] + . [(phen)Pd(CHCH 2 )] + 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(SiMe 3 )] + . DFT calculations suggest a mechanism in which C−C bond formation is followed by migration of SiMe 3 from C to Pd. Links between the observed gas-phase chemistry and solution-phase Pd-mediated homocoupling reactions of vinyltrimethylsilanes are explored.

Gas-phase experiments and computation provide fundamental model reactions for aryl and fluoride transfer between silver and boron centres.