Synthesis, Characterization, and Catalytic Study of Caffeine-Derived N-heterocyclic Carbene Palladium Complexes

Abstract

Homo- and heterodicarbene palladium complexes bearing caffeine-derived N-heterocyclic carbene ligands were synthesized and fully characterized by NMR spectroscopy, mass spectrometry, and X-ray diffraction analysis. The superior acidity of the alkylated caffeine-based salts also allowed the isolation of zwitterionic caffeine monocarbene palladium complexes, which showed outperforming catalytic activities in the cyanation reactions of aryl halides.

Methanol-Mediated Formation of an Iridium(III) NHC/Azolato Chelate Complex: An Experimental and Theoretical Study

“…Such high oxidation state iridium species have previously been observed as intermediates in various transformations but we do not know any example where it happened during the oxidative addition of a C–halogen bond to Ir III . While C NHC ^C NHC chelate complexes bearing two different NHC ligands have been prepared by deprotonation of suitable di-azolium salts, − the resulting complexes do not feature an azolato donor such as [ 2 ]. In further investigations, we will try to functionalize the unsubstituted, negatively charged theophyllinato N9 nitrogen atom in [ 2 ] in a post synthetic manner by protonation , or by alkylation to transform the azolato ligand into an NH,NR-NHC or a classical NR,NR′-NHC ligand, respectively.…”

Greening up organic reactions with caffeine: applications, recent developments, and future directions

“…Several homo- and heterodicarbene palladium complexes bearing caffeine-derived N-heterocyclic carbene ligands were synthesized by Teng et al 65 Amongst all the synthesized complexes, caffeine-pyrazole-derived complexes i.e. cis -[PdI 2 (Caf-Py)] (Cat I) and [PdI 3 (Caf-Py)] (Cat II) ( Scheme 39 ) demonstrated highest catalytic potential for the cyanation of aryl halides 103 with K 4 [Fe(CN) 6 ]·3H 2 O 104 to afford 105 ( Scheme 40 ).…”

Advances in the Synthesis and Applications of N-Heterocyclic Carbene Metal Complexes with a Focus on the Weak Base Route

“…Finally, inspired by the recent reports concerning the functionalization of xanthines (e.g., caffeine, theobromine, and theophylline) and their corresponding NHC complexes with different transition metals, 131 Huynh and Meng described the preparation of Pd(II)-NHC complexes from bridged xanthinium-azolium salts. 132 Interestingly, the bis(carbene) complex can be obtained only when cesium carbonate is used as the base. Conversely, the use of Pd(OAc) 2 as a basic source of palladium afforded mixtures of the desired biscarbene complexes and unusual zwitterionic complexes (see Scheme 71).…”