Ligand controlled orthogonal base-assisted direct C-H bond arylation in oxa(thia)zole-4-carboxylate series. New insights in nCMD mechanism

Alkyl- and arylphosphines have been screened in competitive C2-H/C5-H direct phenylation of oxa(thia)zole-4-carboxylates using Cs2CO3 and Rb2CO3 carbonate bases. nCMD-based C2-H selective direct phenylation was highly kinetically reduced (or enhanced) in favor (or to the detriment) of CMD-based direct C5-H phenylation with bromo- and chlorobenzene, respectively, using highly electron-rich ligands. These results gave novel experimental proof in favor of the electrophilic substitution-type mechanism for nCMD process based upon a prior nitrogen-arylpalladium complex interaction that preludes the deprotonation step.