Transition‐Metal‐Free Synthesis of Carbazoles and Indoles by an S_NAr‐Based “Aromatic Metamorphosis” of Thiaarenes

Abstract

Dibenzothiophene dioxides, which are readily prepared through oxidation of the parent dibenzothiophenes, undergo nucleophilic aromatic substitution with anilines intermolecularly and then intramolecularly to yield the corresponding carbazoles in a single operation. The "aromatic metamorphosis" of dibenzothiophenes into carbazoles does not require any heavy metals. This strategy is also applicable to the synthesis of indoles. Since electron-deficient thiaarene dioxides exhibit interesting reactivity, which is not observed for that the corresponding electron-rich azaarenes, a combination of a thiaarene-dioxide-specific reaction with the SN Ar-based aromatic metamorphosis allows transition-metal-free construction of difficult-to-prepare carbazoles.

Synthesis of N‐Alkyl and N‐H‐Carbazoles through SNAr‐Based Aminations of Dibenzothiophene Dioxides

“…Treatment of carbazolide anion C with 2‐fluoropyridine in the same pot induced an intermolecular S N Ar reaction, providing N‐( 2‐pyridyl) carbazole 6 which is an attractive scaffold as organic light emitting diodes . Of note is that the synthesis of carbazole 6 has not been achieved by means of our previous synthesis of N ‐aryl carbazoles from the reaction of dibenzothiophene dioxides with anilines using KN(SiMe 3 ) 2 because of the low nucleophilicity of 2‐aminopyridine . Furthermore, a one‐pot addition of Boc 2 O to carbazolide anion C successfully gave N ‐Boc carbazole 7 in 86 % yield.…”

“…Recently, we have been interested in ‘aromatic metamorphosis’ of heteroatom‐containing aromatic compounds, which represents a transformation of a heteroaromatic skeleton to another ring system through the partial disassembly of the starting aromatic core . Particularly, we have developed the transition‐metal‐free transformation of dibenzothiophenes into N ‐aryl carbazoles through twofold S N Ar reactions of dibenzothiophene dioxides with anilines in the presence of KN(SiMe 3 ) 2 , the potassium cation of which played a key role to promote S N Ar aminations (Scheme ) . However, the reaction of dibenzothiophene dioxides with alkyl amines by using KN(SiMe 3 ) 2 was sluggish.…”

“…Considering the fact that KN(SiMe 3 ) 2 is the irreplaceable base in our previous report for the synthesis of N ‐aryl carbazoles, we envisioned that the salt metathesis reaction of dianion A with additional KN(SiMe 3 ) 2 promotes the intramolecular S N Ar cyclization to deliver carbazole 3 aa . We thus performed the reaction of 1 a with amine 2 a in the presence of LiN(SiMe 3 ) 2 to form dianion A , followed by treatment with KN(SiMe 3 ) 2 at 100 °C as a one‐pot procedure.…”

Intramolecular Consecutive Dehydro‐Diels–Alder Reaction for the Catalytic and Enantioselective Construction of Axial Chirality

“…As examples of the synthetic transformations of the bis(dibenzothiophene) derivatives, we oxidized 3 a into the disulfone 4 a , which could be further transformed into the bis(benzocarbazole) derivatives 5 a and 5 a′ , and the retention of enantiomeric excess of 5 a was ascertained [Eq. (1) in Scheme ] . In addition, 3 d could be converted into the ditriflate 7 d in two steps, which could be used for further couplings [Eq.…”

Intramolecular Consecutive Dehydro‐Diels–Alder Reaction for the Catalytic and Enantioselective Construction of Axial Chirality

“…(1) in Scheme 6]. [19] In addition, 3d could be converted into the ditriflate 7d in two steps,w hich could be used for further couplings [Eq. (2) in Scheme 6].…”