Base‐catalyzed Sulfurative Condensation of 2‐Oxoindoles to Isoindigos Using Elemental Sulfur

Abstract

Herein, the base-catalyzed formation of isoindigos is described through the cascade reaction of 2-oxoindoles with elemental sulfur (S 8 ) serving as an environmentally friendly and cheap sulfurating reagent. Readily available organic amines, especially 1,4-diazabicyclo[2.2.2]octane (DAB-CO), exhibit excellent activity for this protocol, thus simply and efficiently providing structurally diverse isoindigos in moderate to high yields with a broad substrate scope. A plausible mechanism involving the generation of 3-thioisatin intermediates and the subsequent self-condensation, was proposed on the basis of mechanistic studies. The practicality of the methodology was further demonstrated by the synthesis of gram-scalable drug molecules and value-added spirothiopyrans.

Iron-catalyzed selective construction of indole derivatives via oxidative C(sp3)–H functionalization of indolin-2-ones

“…[42][43][44] In 2018, Wang's group utilized the oxidation relay strategy to selectively synthesize isatins and isoindigos by NaI-mediated transformation of indoline-2-ones in the presence of pure oxygen as an oxidant. 45 Recently, we reported the organocatalyzed self-condensation of 3-thioisatins, in situ generated from COS-triggered oxygen/sulfur exchange of isatins or the sulfuration of 2-oxoindoles with S 8 , to effectively construct a series of structurally diverse isoindigos, along with the generation of undesirable byproducts, such CO 2 or H 2 S. 46,47 Meanwhile, Punji's group further reported iron-catalyzed dimerization of isatins to isoindigos with oxygen as the sole byproduct. 48 In the case of iron-catalyzed oxidative alkylation and hydroxylation of indolin-2ones reported by Xu's group in 2020, the oxidative product (indoline-2,3-dione) and radical coupled product (3-hydroxy-3,3′-biindoline-2,2′-dione) were detected in the absence of alkyl-substituted N-heteroarenes (Scheme 1(II)).…”