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Preventing Pd-NHC bond cleavage and switching from nano-scale to molecular catalytic systems: amines and temperature as catalyst activators

Abstract

Many reactions catalyzed by Pd complexes with N-heterocyclic carbene (NHC) ligands are performed in the presence of amines which usually act as coupling reagents or mild bases. However, amines can react with Pd/NHC complexes in a number of ways: enhancing molecular catalysis, causing the catalyst deactivation or triggering the ligandless modes of catalysis by producing NHC-free active palladium species. This study gains insight into conditions required for the efficient use of amines as activators of molecular Pd/NHC catalysis and preventing the undesirable reductive cleavage of the Pd-NHC bond in catalytic systems. Reactions of Pd/NHC complexes with various amines within a temperature range of 25–140 °C and thermal stability of the resulting amino-complexes are examined. The results indicate the major influence of the amine structure and reaction temperature on the catalyst transformations. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to the reductive Pd-NHC bond cleavage, while deprotonation of the complexes with primary and secondary aliphatic amine ligands in the presence of strong bases at 25–60 °C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd-PEPPSI complex – amine systems suitable for the strong-base-promoted C-S cross-coupling reactions between aryl halides and thiols are suggested on the basis of these findings.

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Publication details

The article was received on 10 Oct 2019, accepted on 03 Dec 2019 and first published on 04 Dec 2019


Article type: Paper
DOI: 10.1039/C9CY02041A
Catal. Sci. Technol., 2019, Accepted Manuscript

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    Preventing Pd-NHC bond cleavage and switching from nano-scale to molecular catalytic systems: amines and temperature as catalyst activators

    O. Khazipov, M. Shevchenko, D. Pasyukov, A. Chernenko, A. Astakhov, V. Tafeenko, V. Chernyshev and V. Ananikov, Catal. Sci. Technol., 2019, Accepted Manuscript , DOI: 10.1039/C9CY02041A

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