In conclusion, we have demonstrated the electrochemical ring expansion reaction using α-H diazo esters as a carbynyl anion equivalent. This approach facilitated a straightforward single-carbon insertion into various polysubstituted pyrroles to produce pyridine derivatives. Importantly, the insertion position was controlled through electronic perturbation by the N-protecting groups, and unprecedented para-selective insertion was achieved by introducing an electron-withdrawing PG to the pyrrole derivative. The desired transformation also occurred with various substituted cyclic compounds such as indole, imidazole, indene, and cyclopentadiene, yielding the desired carbon-inserted product. In-situ spectroscopy and theoretical calculations supported the reaction mechanism involving a distonic radical cation intermediate. This study introduces a novel concept for single-carbon insertion chemistry, expanding the chemical toolbox for synthesizing polysubstituted (hetero)aromatic compounds.