Pd-NHC complex catalyzed C-H bond activation reactions of caffeine and 2-isobuthylthiazole

A series of new Pd-NHC complexes were synthesized, characterized, and utilized as catalysts on 8-(hetero)ary-lation of xanthines and C-5 (hetero)arylation of 2-isobuthylthiazole. All the synthesized derivatives were characterized by NMR, Q-TOF-LC/MS, FTIR, and X-ray (for 4a, 5a, and 5b) analysis. In addition, DFT calculations and computational NBO studies for Pd-NHC complexes were examined, and HOMO and LUMO energy levels and electron density of each Pd-NHC complex were defined. 4a-c and 5a-b complexes showed good catalytic activity in C-H bond activation reactions. DFT studies have also been conducted to examine the reaction mechanism following the CMD pathway. Complex 5a was chosen as a representative catalyst for the reaction of unsubstituted phenyl with caffeine and 2-isobuthylthiazole. Although the delta E values of the complexes are so close, slight difference in the catalytic activity were observed for 4a-c and 5a-b. Since this low delta E value facilitates the oxidative addition reaction of (hetero)aryl bromides, thanks to this catalytic system, new 8-(hetero)aryl xan-thines and 5-(hetero)aryl thiazole derivatives could be synthesized with high yields and low catalyst loading.