Journal
CATALYSIS SCIENCE & TECHNOLOGY
Volume 10, Issue 5, Pages 1228-1247Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c9cy02041a
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Funding
- Russian Foundation for Basic Research, RFBR Grants [19-03-01053, 16-29-10786]
- M. V. Lomonosov Moscow State University Program of Development
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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 25140 degrees 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 transformation. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to 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 degrees C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd-PEPPSI complex-amine systems suitable for 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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