Inter- and intra-molecular C–H borylation for the formation of PAHs containing triarylborane and indole units

Abstract

Inter-/intra-molecular electrophilic C-H borylation of C4-substituted indoles enables the formation of fused polycyclic aromatic structures containing triarylborane and N-heterocyclic units. These compounds are B-(C)-N isosteres of carbocyclic PAHs that do not contain B-N bonds and comparison of one pair of BN/CC isosteres reveals that different resonance structures dominate. These compounds are highly sensitive to protodeboronation, of both the chloroborane intermediates and the mesityl protected products, which results in low isolated yields of the latter. Protodeboronation can be utilised productively for a C-H directed, C-H electrophilic borylation to make a previously unknown pinacol boronate ester by selective protodeboronation of the chloroborane intermediate. Intermolecular and double intramolecular electrophilic C-H borylation of a C4-substituted indole leads to a more highly fused structure containing two boracycles which represents a B-(C)-N analogue of the unknown carbon isostere indeno[1,7ab]perylene.

A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C–H borylation

“…A related outcome has been recently observed on incorporation of a B–C C–N moiety to generate an indole derived-PAH. 10 Comparison of 2-BMes and 7-BMes with dibenz[ a , kl ]anthracen-7-one (which is isoelectronic to 2-BCl , Fig. 3 , left) reveals a similar distortion from planarity in the pentacyclic structure with a C19–C20–C9–C8 angle of 33.4° in this fused cyclic-ketone.…”

“…Naphthalene functionalised alkynes were selected as: (i) they are available in one step from commercial materials ( e.g. aryl bromides and terminal alkynes), (ii) post borylative cyclisation they are correctly arranged to form six membered boracycles by intramolecular electrophilic borylation, 10 and (iii) non-planar PAHs will be formed (due to the presence of cove regions) enhancing solubility. 11 Herein we report that this single methodology facilely leads to B-, B 2 - and B,N-doped PAHs all from simple alkyne precursors.…”

Intramolecular (directed) electrophilic C–H borylation

“…For example, Ingleson and co-workers developed a sequential inter-/intra-molecular electrophilic C-H borylation of an indole substrate using an amine, BCl 3 and AlCl 3 . 72 The propensity of the indole C3 position to undergo electrophilic substitution enables facile intermolecular C-H borylation with a borenium cation to give 67. A double intramolecular C-H borylation to form 68 then was achieved by addition of 2,6-dichloropyridine (Cl 2 -py) and AlCl 3 to 67 (proposed to form a reactive borenium cation in situ that is effective for electrophilic borylation at room temperature, inset Scheme 33).…”

Acyl‐Directed ortho‐Borylation of Anilines and C7 Borylation of Indoles using just BBr3

“…[12] Directed electrophilic CÀHb orylation is dominated by directing R 2 N-or Nheterocycle groups with borylation generally forming six membered boracycles preferentially over other ring sizes. [13] Thee xtension of CÀHb orylation using BX 3 to the C5/C7 positions of indoles would be highly attractive.However,this requires conditions that disfavour electrophilic C3 À Hb orylation (which is relatively facile) and ad irecting group that: (i)iscompatible with BX 3 ;(ii)enables selective borylation at the desired position;( iii)isr eadily deprotected post CÀH borylation. Tr ansition metal-catalysed C7ÀHindole function-Scheme 1.…”