Iridium Catalysed C‐H Borylation of Heteroarenes: Balancing steric and electronic regiocontrol

Over the past twenty years, the iridium catalysed borylation of aromatic C-H bonds has become the method for the synthesis of aromatic organoboron compounds required for many purposes including applications in natural product synthesis, material science, and medicinal chemistry. The reaction is highly efficient, tolerant of a broad range of substituents and can be applied to both carbocyclic and heterocyclic substrates. The regioselectivity of C-H activation is dominated by steric considerations and there have been considerable efforts in recent years to develop more selective processes for less constrained substrates. However, most of these have focused on benzenoid type substrates and in contrast, heteroarenes remain much desired but more challenging substrates with the position and / or nature of the heteroatom(s) significantly affecting reactivity and regioselectivity. This review will survey the borylation of heteroarenes focusing on the influence of steric and electronic effects on regiochemical outcome and, by linking to current mechanistic understandings, aims to provide insights to what is currently possible and where further developments are required.