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...
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Published in | Organometallics Vol. 40; no. 12; pp. 1822 - 1829 |
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Main Authors | , , , , , |
Format | Journal Article |
Language | English |
Published |
American Chemical Society
28.06.2021
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Online Access | Get full text |
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Summary: | 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(CHCH2), via transmetalation to give [(phen)Pd(CHCH2)]+ 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(CHCH2)SiX]+ intermediate, which either then loses Me2(CHCH2)SiX or reacts via C–C bond coupling to ultimately yield propene and [(phen)Pd(SiMe2X)]+. [(phen)Pd(CHCH2)]+ 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. |
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ISSN: | 0276-7333 1520-6041 |
DOI: | 10.1021/acs.organomet.0c00795 |