Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins

Abstract

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2–N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the “aromatic metamorphosis” of indoles into 1,2-benzazaborins via the insertion of boron into the C2–N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

One tool to bring them all: Au-catalyzed synthesis of B,O- and B,N-doped PAHs from boronic and borinic acids

“…Scheme 2c) is based on reductive ring opening of indoles with excess Li metal, trapping of the dilithiated intermediate with RBpin, and aqueous workup. 103 Apart from our current work on the second most stable 1,2-B,Nnaphthalenes, the only other report comes from the Cui group. 104 They used benzylic imines as key intermediates (3 synthesis steps) and converted them into the corresponding enamidyl dibromoboranes, which were cyclized via an Et3Npromoted intramolecular electrophilic aromatic borylation reaction at 80 °C (2 steps; yields: 30-51% with respect to the corresponding benzylic imine).…”

“…3 The most stable 2,1-B,N-naphthalenes are also by far the most thoroughly studied isomers. 18,19,[92][93][94][95][96][97][98][99][100][101][102][103] The dominant synthesis strategies rely on ring-closing reactions between o-vinyl-/oalkynylanilines and R'BCl2 or K[R'BF3]/SiCl4/NEt3 (cf. Scheme 1a; R' = halogen, alkyl, alkenyl, alkynyl, (hetero)aryl).…”

Ring-expanding and Ring-opening Transformations of Benzofurans and Indoles with Introducing Heteroatoms

“…Moreover, addition of phenylboronic acid pinacol ester (PhB(pin)) as an electrophile to the reaction mixture after filtration furnished azaborin 20aa in a good yield (Scheme 15b). 8 Notably, the filtration was an important operation to obtain the product because the residual Li metal caused decomposition of the resulting azaborin. The use of organoboronic acid esters should be a great advantage over the conventional azaborin syntheses because various organoboronic acid esters are commercially available and easily handled.…”

“…1,2 Previously our group achieved transformations of dibenzothiophenes, 3 dibenzofurans, 4 benzofurans, 57 and indoles. 8 To establish aromatic metamorphosis as a truly useful strategy in organic synthesis, we need to understand the reactivity of each heteroaromatic ring.…”

Borazatruxenes

“…4,5 Moreover, the electronic properties of PAHs are intimately related to their structure; therefore small modications in their scaffold can lead to vastly different optical and electronic properties. [6][7][8][9][10][11][12][13][14] Truxene is a C 3 -symmetric PAH that has been intensively studied for the past 15-20 years. 15 In particular, it has been used as a precursor to polyarenes, 16,17 liquid crystals 18,19 and hemifullerenes, 20,21 all of which have potential to be used in organic electronics.…”