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

Abstract

Activation of N-heterocyclic carbene boranes (NHC⋅BH3) by I2 enables the metal-free catalytic C−H borylation of heteroarenes with formation of H2 as the by-product in a process that uses only bench stable precursors. The borylation of indoles using NHC⋅BH3/I2 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 Brønsted bases. Thus this C−H borylation methodology proceeds under sufficiently Brønsted 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.