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

Abstract

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.

Cracking and Dehydrogenation of Cyclohexane by [(phen)M(X)]+ (M = Ni, Pd, Pt; X = H, CH3) in the Gas Phase

“…This is consistent with recent findings of Wang et al who achieved efficient dehydrogenation of cyclohexane at low temperatures using Pt nanoparticle catalyst supported on nitrogen-doped carbon . Our results highlight the value of using gas-phase studies in conjunction with DFT calculations to probe the role of coordinated anions in C–X bond activation reactions that form organometallic intermediates …”

“…10 Our results highlight the value of using gas-phase studies in conjunction with DFT calculations to probe the role of coordinated anions in C−X bond activation reactions that form organometallic intermediates. 36 ■ MATERIALS AND METHODS Materials. Phenanthroline, palladium acetate, nickel acetate, and acetic acid were all purchased from Sigma-Aldrich (St. Louis, MO) and used as received.…”

Ion-pairs as a gateway to transmetalation: aryl transfer from boron to nickel and magnesium

“…4 Although in some instances the resultant organometallic has been isolated (Scheme 1B), 5 rarely has detailed mechanistic evidence been provided for the specific involvement of ion-pairs in transmetalation. 6 Here we report the first use of ion–ion reactions in the gas-phase 7 to directly probe transmetalation reactions 8 via formation of ion-pairs. DFT calculations are used to explore the mechanisms and energetics of these reactions.…”