Ruthenium‐Catalyzed Vinylene Carbonate Annulation by C−H/N−H Functionalizations: Step‐Economical Access to Indoles

Abstract

A convenient and effective method of ruthenium‐catalyzed C−H/N−H annulations using vinylene carbonate as oxidizing acetylene surrogate has been disclosed. This method is scalable and compatible with a wide range of functional groups, providing a step‐economical access to indole synthesis Preliminary mechanistic studies provided support for a reversible, acetate‐assisted C−H ruthenation, along with a subsequent olefin insertion.

Advancement of vinylene carbonate as a coupling partner in metal-catalyzed C–H functionalization

“…A tentative reaction mechanism was proposed by Miura and Lan as shown in Scheme 9. [47][48][49] The amide-directed intramolecular C-H activation with the pre-catalyst Cp*RhX 2 leads to the formation of a five-membered rhodacyclic species, which subsequently undergoes VC insertion into the C-Rh bond to generate a seven-membered metallacycle 25-II. At this stage, there are two possible pathways: (A) a formal rhodium migration, which involves sequential reductive elimination and oxidative addition into the adjacent C-O bond of rhodium to form the complex 25-III, followed by β-O elimination to liberate the desired coupling product 26 and the active Rh(III) complex.…”

“…To further explore more sustainable and practical synthetic tactics for indoles, afterwards, the Ma group reported the theme of carboxylate-aided low-cost ruthenium-catalyzed C–H/N–H annulations of functionalized anilines with VC without any external oxidants. 48 This method was scalable and more productive in yields, and the directing group could be easily avoided with LDA (lithium diisopropylamide) as a base. The author also evaluated the effect of N substituents for the reaction, which indicated that the bigger t Bu group attached to the carbonyl group reacted clearly more efficiently than Me, Et or amide substituents.…”

Rh(III)-Catalyzed C–H Annulation of N-Nitrosoanilines with Iodonium Ylides for the Synthesis of N-Alkyl Indoles

“…In recent years, DG-assisted transition-metal-catalyzed C–H bond annulation of N-substituted anilines with alkynes has been a straightforward tool for synthesis of indoles. Afterward, diazo, alkene, sulfoxonium ylides, and vinylene carbonate were further extensively employed as coupling partners in these annulations in the presence of Ru, Rh, or Co catalysis (Scheme ). Therefore, diverse indole derivatives were synthesized in this methodology.…”

Ruthenium‐Catalyzed C7‐Formylmethylation or Sequential Acetalization of Indolines with Vinylene Carbonate in Different Solvents

“…Very recently, the team of Zhu and Hao realized the Rh(III)‐catalyzed C7‐formylmethylation of indolines with vinylene carbonate (Scheme 1b) [21j] . However, summarizing the above studies, it can be found that the catalysts for the C−H functionalization involving indoline or vinylene carbonate are usually rhodium and palladium, and cheaper ruthenium catalysts are less employed [21k] . Obviously, it is worth exploring the C−H activation reactions catalyzed by ruthenium and even construct the catalytic results different from rhodium and other catalysts.…”