The first synthesis of Pd-PEPPSI N-heterocyclic carbene complexes derived from the abundant and renewable natural product caffeine is reported. The catalysts bearing 3-chloro-pyridine, pyridine and N-methylimidazole ancillary ligands were readily prepared from the corresponding N9-Me caffeine imidazolium salt by direct deprotonation and coordination to PdX in the presence of N-heterocycles or by ligand displacement of PdX(Het). The model Pd-PEPPSI-caffeine complex has been characterized by x-ray crystallography. The complexes were successfully employed in the Suzuki cross-coupling of aryl bromides, Suzuki cross-coupling of amides, Heck cross-coupling and Sonogashira cross-coupling. Computational studies were employed to determine frontier molecular orbitals and bond order analysis of caffeine derived Pd-PEPPSI complexes. This class of catalysts offers an entry to utilize benign and sustainable biomass-derived Xanthine NHC ligands in the popular Pd-PEPPSI systems in organic synthesis and catalysis.
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| http://dx.doi.org/10.1021/acs.organomet.2c00262 | DOI Listing |
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