Synthesis of functionalized indolizines via gold(i)-catalyzed intramolecular hydroarylation/aromatization of pyrrole-ynes

Regioselective Access to 1,2,4-Triazole-Fused N-Heterocycle, Pyrrolo[1,2-a][1,2,4]triazolo[5,1-c]pyrazine, via Double Dehydrative Cyclizations

A highly efficient and regioselective approach to a novel 1,2,4-triazole-fused N-heterocyclic scaffold, pyrrolo[1,2-a][1,2,4]triazolo[3,4-c]pyrazine, was established by base-promoted reaction of pyrrole-2-carbonitrile-derived substrate with acyl hydrazide where domino double ring closures comprised of enamine formation, attack on nitrile, and cyclodehydration enabled sequential construction of pyrazine and 1,2,4-triazole ring systems with formation of three C-N bonds.

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.

Gold-Catalyzed Hydroarylation Reactions: A Comprehensive Overview

The hydroarylation of alkynes, alkene, and, allene is a cost-effective and efficient way to incorporate unsaturated moieties into aromatic substrates. This review focuses on gold-catalyzed hydroarylation, which produces aromatic alkenes,...

Dual Gold/Silver Catalysis: Indolizines from 2-Substituted Pyridine Derivatives via a Tandem C(sp3)-H Alkynylation/Iminoauration

A dual gold/silver-catalyzed cascade C(sp³)-H alkynylation/iminoauration of 2-substituted pyridines with hypervalent iodine(III) reagents for the synthesis of indolizines is described. This novel reaction involves the formation of an alkynyl Au(III) species, a dual gold/silver-catalyzed C(sp³)-H functionalization, and a subsequent iminoauration process. A number of indolizines bearing diverse functionalities were prepared in good to excellent yield. Furthermore, a gram-scale reaction was efficiently conducted.

Recent Advances in Functionalization of Pyrroles and their Translational Potential

Among the known aromatic nitrogen heterocycles, pyrrole represents a privileged aromatic heterocycle ranging its occurrence in the key component of "pigments of life" to biologically active natural products to active pharmaceuticals. Pyrrole being an electron-rich heteroaromatic compound, its predominant functionalization is legendary to aromatic electrophilic substitution reactions. Although a few excellent reviews on the functionalization of pyrroles including the reports by Baltazzi in 1963, Casiraghi and Rassu in 1995, and Banwell in 2006 are available, they are fragmentary and over fifteen years old, and do not cover the modern aspects of catalysis. A review covering a comprehensive package of direct functionalization on pyrroles via catalytic and non-catalytic methods including their translational potential is described. Subsequent to statutory yet concise introduction, the classical functionalization on pyrroles using Lewis acids largely following an ionic mechanism is discussed. The subsequent discussion follows the various metal-catalyzed C-H functionalization on pyrroles, which are otherwise difficult to implement by Lewis acids. A major emphasize is given on the radical based pyrrole functionalization under metal-free oxidative conditions, which is otherwise poorly highlighted in the literature. Towards the end, the current development of pyrrole functionalization under photocatalyzed and electrochemical conditions is appended. Only a selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. The aromatic nucleophillic substitution on pyrroles (being an electron-rich heterocycle) happened to be the subject of recent investigations, which has also been covered accentuating their underlying conceptual development. Despite great achievements over the past several years in these areas, many challenges and problems are yet to be solved, which are all discussed in summary and outlook.

Palladium catalyzed synthesis of indolizines via the carbonylative coupling of bromopyridines, imines and alkynes

We report herein the development of a palladium-catalyzed, multicomponent synthesis of indolizines. The reaction proceeds via the carbonylative formation of a high energy, mesoionic pyridine-based 1,3-dipole, which can undergo spontaneous cycloaddition with alkynes. Overall, this provides a route to prepare indolizines in a modular fashion from combinations of commercially available or easily generated reagents: 2-bromopyridines, imines and alkynes.

A palladium catalyzed, multicomponent synthesis of indolizines is described via the carbon monoxide driven generation of reactive, pyridine-based 1,3-dipoles.

Copper-Catalyzed Chemodivergent Cyclization of N-(ortho-alkynyl)aryl-Pyrrole and Indoles

Herein, we described an efficient copper-catalyzed chemo-divergent tandem reaction of N-(ortho-alkynyl)aryl-pyrrole and (iso)indoles, delivering ring-fused N-heterocycles in good yields in an atom-economical manner. N-(ortho-alkynyl)aryl-pyrrole and indoles undergo the tandem cyclization/migration reaction, in which the group at 2-position was migrated to 3-position. In contrast, the dearomative cyclization of N-(ortho-alkynyl)aryl-isoindoles would occur to deliver the N-fused tetracyclic products efficiently.

Synthesis of Indolizine Derivatives Triggered by the Oxidative Addition of Aroyl Chloride to Pd(0) Complex

An efficient synthesis of indolizine derivatives from propargylic pyridines and aroyl chlorides was developed. The 5-endo-dig cyclization was initiated by the in situ formed acylpalladium species from the facile oxidative addition of aroyl chloride to Pd(0) complex. This transformation successfully occurred in the presence of an N-nucleophilic moiety and acid chlorides, a good electrophilic partner, affording highly functionalized indolizines in good-to-excellent yields.

Facile Synthesis of Polysubstituted Indolizines via One-Pot Reaction of 1-Acetylaryl 2-Formylpyrroles and Enals

An efficient method for the synthesis of polysubstituted indolizines has been developed based on formal [4+2] annulation of 1-acetylaryl 2-formylpyrroles with enals, followed by oxidative aromatization. Pyridine-type six-membered rings were constructed in this transformation. This transition metal-free reaction features mild reaction conditions, a broad substrate scope, and excellent functional group tolerance. Notably, the formyl group is well tolerated under reaction conditions.

Synthesis of indolizine derivatives containing eight-membered rings via a gold-catalyzed two-fold hydroarylation of diynes

A novel strategy for a gold(i)-catalyzed synthesis of indolizine derivatives containing eight-membered rings has been developed, which may have potential usefulness as blue or green OLEDs.