A case study of proton shuttling in palladium catalysis

The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule...

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Bibliographic Details
Published in:Chemical science (Cambridge) 2016-01, Vol.7 (3), p.2179-2187
Main Authors: Monot, Julien, Brunel, Paul, Kefalidis, Christos E, Espinosa-Jalapa, Noel Ángel, Maron, Laurent, Martin-Vaca, Blanca, Bourissou, Didier
Format: Article
Language:English
Subjects:
Alkynes
Carboxylic acids
Catalysis
Chemical Sciences
Derivatives
Isomerization
Ligands
Palladium
Polyols
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Summary:The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule of substrate. It acts as a proton shuttle in the activation of the acid, it directs the nucleophilic attack of the carboxylic acid on the π-coordinated alkyne and it relays the protonolysis of the resulting vinyl Pd species. A variety of H-bond donors have been evaluated as external additives, and polyols featuring proximal hydroxyl groups, in particular catechol derivatives, led to significant catalytic enhancement. The impact of 4-nitrocatechol and 1,2,3-benzenetriol is particularly striking on challenging substrates such as internal 4- and 5-alkynoic acids. Endo/exo selectivities up to 7.3/1 and 60-fold increase in reactivity were achieved. Thanks to mechanistic studies, the catalytic performance of SCS indenediide Pd pincer complexes has been spectacularly enhanced using catechol additives as proton shuttles.
ISSN:2041-6520
2041-6539
DOI:10.1039/c5sc04232a