Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One‐Pot Route to C7‐Borylated‐Indolines

Abstract

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B•••B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B•••B separation enables double EÀ H (E = N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf 2 generates an electrophile that transforms NÀ H indoles and indolines into N/C7diborylated indolines, with NÀ H borylation directing subsequent C7À H borylation. Indole reduction to indoline occurs before CÀ H borylation and our studies indicate this proceeds via hydroboration-C3-protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7-BPin-N-H-indolines. Overall, this represents a transient directed CÀ H borylation to form useful C7-BPin-indolines.