Electrochemical reactor dictates site selectivity in N-heteroarene carboxylations

Pyridines and related N-heteroarenes are commonly found in pharmaceuticals, agrochemicals and other biologically active compounds1,2. Site-selective C–H functionalization would provide a direct way of making these medicinally active products3–5. For example, nicotinic acid derivatives could be made by C–H carboxylation, but this remains an elusive transformation6–8. Here we describe the development of an electrochemical strategy for the direct carboxylation of pyridines using CO2. The choice of the electrolysis setup gives rise to divergent site selectivity: a divided electrochemical cell leads to C5 carboxylation, whereas an undivided cell promotes C4 carboxylation. The undivided-cell reaction is proposed to operate through a paired-electrolysis mechanism9,10, in which both cathodic and anodic events play critical roles in altering the site selectivity. Specifically, anodically generated iodine preferentially reacts with a key radical anion intermediate in the C4-carboxylation pathway through hydrogen-atom transfer, thus diverting the reaction selectivity by means of the Curtin–Hammett principle11. The scope of the transformation was expanded to a wide range of N-heteroarenes, including bipyridines and terpyridines, pyrimidines, pyrazines and quinolines. An electrochemical strategy is described in which the direct carboxylation of pyridines and related N-heteroarenes with CO2 shows divergent site selectivity depending on the type of reactor used.