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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

Stewart, AWE, Ma, HZ, Weragoda, GK, Khairallah, GN, Canty, AJ and O'Hair, RAJ 2021 , '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' , Organometallics, vol. 40, no. 12 , pp. 1822-1829 , doi: 10.1021/acs.organomet.0c00795.

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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, 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.

Item Type: Article
Authors/Creators:Stewart, AWE and Ma, HZ and Weragoda, GK and Khairallah, GN and Canty, AJ and O'Hair, RAJ
Journal or Publication Title: Organometallics
Publisher: Amer Chemical Soc
ISSN: 0276-7333
DOI / ID Number: 10.1021/acs.organomet.0c00795
Copyright Information:

Copyright 2021 American Chemical Society

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