Gold-catalyzed heteroannulation and allyl migration: synthesis of highly functionalized indole derivatives

Oxidative Addition to the N-C Bond Vs Formation of the Zwitterionic Intermediate in Platinum(II)-Catalyzed Intramolecular Annulation of Alkynes to Form Indoles: Mechanistic Studies and Reaction Scope

In this study, Pt(II)-catalyzed intramolecular translocation annulation of ortho-alkynylamides to the formation of indoles is presented, where a proposed intermediacy of zwitterionic intermediate has been substantiated over the oxidative addition. We focused our attention on Pt(II)-catalyzed aminoacylation of alkynes both theoretically and experimentally using low boiling solvent where the formation of deacylation product was suppressed simultaneously. One-step intramolecular [1,3]-acyl migration from the zwitterionic intermediate is highly unlikely, which imparts a high energy barrier of +99.0 kcal mol^-1. Another possible approach involving oxidative addition to the N-C bond, migratory insertion to alkyne, and subsequent reductive elimination is also explored through DFT studies to justify the reaction consequence. However, based on the computational studies, it is suggested that initial zwitterion formation is highly favored over oxidative addition. We suggest the formation of an acylium intermediate, which can further react with indol-3-ylplatinum species in an intramolecular manner, albeit within the same solvent cage to form 3-acyl indoles.

1,2-Difunctionalizations of alkynes entailing concomitant C–C and C–N bond-forming carboamination reactions

Vicinal carboamination of alkynes is a highly reliable and efficient practical strategy for the quick preparation of valuable and diverse amine derivatives starting from simple synthons. The last decade has witnessed numerous practical methods employing transition-metal-based/metal-free carboamination approaches using alkynes for the synthesis of these N-bearing entities. Driven by the renaissance of transition metal catalysis, intermolecular and intramolecular carboamination of alkynes comprising concomitant C–N and C–C bond formation has been studied extensively. In contrast to metal catalysis, though analogous metal-free approaches have been relatively less explored in the literature, they serve as alternatives to these expensive approaches. Despite this significant progress, reviews documenting such examples are sporadic; as a result, most reports of this type remained scattered throughout the literature, thereby hampering further developments in this escalating field. In this review, different conceptual approaches will be discussed and examples from the literature will be presented. Further, the reader will get insight into the mechanisms of different transformations.

The 1,2-difunctionalization of alkynes happening through concomitant C–C and C–N bond formation strategies have provide an unified access to diversely functionalized N-bearing heterocycles.

1,2-Aminofunctionalization Reactions of Pyridino-Alkynes via Carbophilic Activation

Transition metal-catalyzed 1,2-difunctionalization reactions of alkynes have emerged as a powerful tool to forge carbon-carbon and carbon-heteroatom bonds for the rapid synthesis of polyfunctionalized molecular scaffolds. In this regard, our group has persistently been developing transition metal-mediated 1,2-aminofunctionalization reactions of alkynes through a rationally designed pyridino-alkyne core by utilizing the carbophilic activation strategy. In this account, we present an array of such 1,2-aminofunctionalization reactions which have been successfully executed on this core to afford important polycyclic frameworks such as functionalized quinalizinones, pyridinium oxazole dyads (PODs), N-doped polycyclic aromatic hydrocarbons (PAHs), N-doped spiro-PAHs. Additionally, the synthesis of phosphine ligated gold complexes bearing pyrido-isoquinoline scaffold from the pyridino-alkynes will be discussed briefly.

Sequential Sonogashira/intramolecular aminopalladation/cross-coupling of ortho-ethynyl-anilines catalyzed by a single palladium source: rapid access to 2,3-diarylindoles

We have developed a practical and efficient one-pot protocol for the synthesis of 2,3-diarylindoles via Pd-catalyzed bis-arylative cyclization of various o-ethynylanilines with aryl iodides. Mechanism studies showed that a Pd-catalyzed Sonogashira reaction took place firstly, giving an internal alkyne intermediate, which subsequently underwent intramolecular aminopalladation/cross-coupling to give access to 2,3-diarylindoles. The present methodology exhibits a broad substrate scope, producing various 2,3-diaryl indoles bearing two different aryl groups.

Lewis Acid Catalyzed Atom-Economic Synthesis of C2-Substituted Indoles from o-Amido Alkynols

Herein we have disclosed a Zn(OTf)₂ catalyzed synthesis of C2-alkyl substituted indole derivatives via unprecedented carbonyl group migration from o-amido alkynols. The key features of this protocol involve N,O-carbonyl group migration, broad substrate scope with varied functionality tolerance, moderate to good yields, and 100% atom economy. The crossover experiments proved that the migration is happening via an intramolecular pathway.

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

This review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates.

Domino reaction of a gold catalyzed 5-endo-dig cyclization and a [3,3]-sigmatropic rearrangement towards polysubstituted pyrazoles

Pyrazoles are important heterocyclic compounds with a broad range of biological activities. A new procedure toward tri- or tetrasubstituted pyrazoles has been developed, via a one-pot gold catalyzed synthesis from hydrazines with alkynyl aldehydes or ketones. The reaction proceeds through consecutive hydrazone formation, 5-endo-dig cyclization and an aza-Claisen rearrangement resulting in the desired polysubstitued pyrazoles.

Dissecting the Gold(I)-Catalyzed Carboaminations of N-Allyl Tetrahydro-β-carbolines to Allenes

N-Allyl tetrahydro-β-carbolines undergo gold-catalyzed cyclizations that lead to tetracyclic compounds, resulting from both ring closure and the transfer of the allylic group from the nitrogen to the carbon backbone. The final skeleton obtained depends on the nature of both the R² group of the allene and the R³ group of the allylic residue. Mechanistic studies and DFT calculations allowed the determination of all the mechanistic pathways involved in these processes, stemming from a common intermediate that evolves differently according to the substituents nature.

Synthesis of 2,3-dihydro-1H-pyrroles by intramolecular cyclization of N-(3-butynyl)-sulfonamides

Hydroamination of 3-butynamine derivatives to give non-aromatic 2,3-dihydropyrroles was achieved by using PdCl₂ or AuCl as the catalyst.

Efficient access to alkynylated quinalizinones via the gold(i)-catalyzed aminoalkynylation of alkynes

The gold-catalyzed aminoalkynylation of alkynes for the synthesis of quinalizinones from pyridinoalkynes using 1-[(triisopropylsilyl)-ethynyl]-1,2-benziodoxol-3(1H)-one (TIPS-EBX) is reported.

Highly selective intramolecular addition of C–N and S–N bonds to alkynes catalyzed by palladium: a practical access to two distinct functional indoles

Palladium-catalyzed highly selective intramolecular addition of C–N and S–N bonds to alkynes has been achieved, and two distinct kinds of functional indoles were synthesized rapidly from the same set of substrates in a catalyst-controlled manner.