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A modified cationic mechanism for PdCl2-catalyzed transformation of a homoallylic alcohol to an allyl ether

Farshadfar, K, Chipman, A, Hosseini, M, Yates, BF ORCID: 0000-0001-9663-3301 and Ariafard, A ORCID: 0000-0003-2383-6380 2019 , 'A modified cationic mechanism for PdCl2-catalyzed transformation of a homoallylic alcohol to an allyl ether' , Organometallics, vol. 38, no. 15 , pp. 2953-2962 , doi: 10.1021/acs.organomet.9b00276.

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Abstract

Density functional theory calculations were utilized to investigate a PdCl2-catalyzed transformation involving double-bond migration (alkene isomerization), followed by condensation with methanol starting from a homoallylic alcohol. Against the proposed mechanism in the literature [Tan, J.; Org. Biomol. Chem. 2008, 6, 1344−1348], which assumes involvement of PdIV intermediates for both double-bond migration and condensation, our calculations preclude this supposition. The double-bond migration process is found to proceed through the cationic mechanism, accessed by the increased acidity of the allylic hydrogen in the Pd-activated alkene. The cationic mechanism commences with allylic C–H bond deprotonation by MeOH (solvent), giving an η1-allyl complex which then rearranges through an η3- to another η1-allyl complex, followed by protodemetalation. The allyl rearrangement was identified as an essential step in order for the double-bond migration to proceed via a lower activation energy. This double-bond migration mechanism which does not involve a PdIV intermediate is similar to the one reported earlier [Senan, A. M.; ACS Catal. 2016, 6, 4144−4148]. Once double-bond migration is completed, nucleophilic attack of MeOH to the PdII-activated new double bond initiates the condensation reaction. The nucleophilic attack transition structure gains some stability from a hydrogen bond between the entering alcohol and the available hydroxyl group at the allylic position of the isomerized substrate. In the final step of condensation, the hydroxyl abstracts the proton from the carbon-bonded MeOH to give an allyl ether product and a free water. The findings of this paper are anticipated to add value in the areas of the alkene isomerization and condensation processes involving transition metal complexes as catalysts.

Item Type: Article
Authors/Creators:Farshadfar, K and Chipman, A and Hosseini, M and Yates, BF and Ariafard, A
Keywords: palladium catalysis, mechanistic study, gold catalysis, alkene Isomerisation, Density Functional Theory
Journal or Publication Title: Organometallics
Publisher: Amer Chemical Soc
ISSN: 0276-7333
DOI / ID Number: 10.1021/acs.organomet.9b00276
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Copyright © 2019 American Chemical Societ

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