meta-Selective C-H Functionalization of Pyridines.

The pyridine moiety is an important core structure for a variety of drugs, agrochemicals, catalysts, and functional materials. Direct functionalization of C-H bonds in pyridines is a straightforward approach to access valuable substituted pyridines. Compared to the direct ortho- and para-functionalization, meta-selective pyridine C-H functionalization is far more challenging due to the inherent electronic properties of the pyridine entity. This review summarizes currently available methods for pyridine meta-C-H functionalization using a directing group, non-directed metalation, and temporary dearomatization strategies. Recent advances in ligand control and temporary dearomatization are highlighted. We analyze the advantages as well as limitations of current techniques and hope to inspire further developments in this important area.