Catalytic Electrophilic C-H Borylation Using NHC•Boranes and Iodine Forms C2-, not C3-, Borylated Indoles

Activation of N-heterocyclic carbene boranes (NHC center dot BH3) by I-2 enables the metal-free catalytic C-H borylation of heteroarenes with formation of H-2 as the by-product in a process that uses only bench stable precursors. The borylation of indoles using NHC center dot BH3/I-2 produces C2-borylated indoles exclusively in contrast to other catalytic electrophilic C-H borylation methods. Mechanistic studies indicate that this is due to C3 to C2 boron migration facilitated by the absence of exogenous Bronsted bases. Thus this C-H borylation methodology proceeds under sufficiently Bronsted acidic conditions to enable the thermodynamic C2-borylated indole isomer to be formed instead of the C3 borylated-isomer. This demonstrates that electrophilic C-H borylation can be used to access a wider range of borylated regioisomers than reported to date.