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.