Counterion Effects in a Gold-Catalyzed Synthesis of Pyrroles from Alkynyl Aziridines

Regiodivergent Gold-Catalyzed Rearrangement-Addition Reactions of Sulfenylated Propargylic Carboxylates with Indoles

Sulfenylated propargylic carboxylates were introduced to investigate the influence of sulfur substitution in gold-catalyzed alkyne activation pathways. Regiodivergent gold-catalyzed rearrangement and indole capture reactions proceed under mild conditions to give functionalized indole products bearing sulfenylated (Z)-enol carboxylate motifs. Pathways involving both 1,2- and 1,3-carboxylate migrations are achieved selectively, with indole being added in a 1,4 relationship to the sulfenyl group in each case. High levels of selectivity are influenced by the catalyst system, counterion, and carboxylate group.

Three-Component Synthesis of Di-Keto Aziridines and Highly Functionalized Alkenes from Sulfoxonium Ylides, Nitrosoarenes, and Alkynes

A catalyst-free one-pot three-component method of sulfoxonium ylides, nitrosoarenes, and alkynes for the synthesis of highly functionalized di-keto aziridines and alkenes is described. This strategy features the catalyst-free and additive-free approach, the employment of safe, more stable, and readily accessible sulfoxonium ylides, which bear a much wider substrate scope as starting materials. In terms of terminal alkynes, a cascade reaction of nitrone formation/1,3-diploar cycloaddition/Baldwin rearrangement is involved to afford a wide variety of di-keto aziridines. However, highly functionalized alkenes could be obtained instead of di-keto aziridines through the same nitrone formation/1,3-diploar cycloaddition and another different rearrangement reaction when internal alkynes are employed as starting materials.

Gold(I)-Catalyzed Synthesis of Heterocycles via Allene Oxide from Propargylic Alcohols

A new mechanistic pathway of propargylic alcohol activation by gold(I) catalysis has been proposed toward the efficient synthesis of N-protected pyrroles, 5,6-dihydropyridin-3(4H)-ones from N-protected 5-aminopent-2-yn-1-ol, and 5-aminopent-2-yn-1-ol. Control experiments support that the reaction proceeded via the neighboring group participation of the oxygen atom of propargylic alcohol to form an allene oxide intermediate where the nucleophilic heteroatom attacks intramolecularly. Further, this methodology is successfully extrapolated toward the atom-economic synthesis of hydroxyalkyl indoles and benzofurans. The short reaction time of 30 s, low catalyst loading of 0.5 mol %, high yield, variation in the substrate scope, and procedurally simple open-flask reaction conditions make this methodology highly applied.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Balancing Bulkiness in Gold(I) Phosphino-triazole Catalysis

The syntheses of a series of 1-phenyl-5-phosphino 1,2,3-triazoles are disclosed, within which, the phosphorus atom (at the 5-position of a triazole) is appended by one, two or three triazole motifs, and the valency of the phosphorus(III) atom is completed by two, one or zero ancillary (phenyl or cyclohexyl) groups respectively. This series of phosphines was compared with tricyclohexylphosphine and triphenylphosphine to study the effect of increasing the number of triazoles appended to the central phosphorus atom from zero to three triazoles. Gold(I) chloride complexes of the synthesised ligands were prepared and analysed by techniques including single-crystal X-ray diffraction structure determination. Gold(I) complexes were also prepared from 1-(2,6-dimethoxy)-phenyl-5-dicyclohexyl-phosphino 1,2,3-triazole and 1-(2,6-dimethoxy)-phenyl-5-diphenyl-phosphino 1,2,3-triazole ligands. The crystal structures thus obtained were examined using the SambVca (2.0) web tool and percentage buried volumes determined. The effectiveness of these gold(I) chloride complexes to serve as precatalysts for alkyne hydration were assessed. Furthermore, the regioselectivity of hydration of but-1-yne-1,4-diyldibenzene was probed.

Au-catalyzed skeletal rearrangement of O-propargylic oximes via N-O bond cleavage with the aid of a Brønsted base cocatalyst

Au-catalyzed skeletal rearrangement reaction of O-propargylic oxime proceeded via N–O bond cleavage with the aid of a Brønsted base cocatalyst.

O-Propargylic oximes that possess an electron-withdrawing aryl group on the oxime moiety undergo Au-catalyzed skeletal rearrangements via N–O bond cleavage to afford the corresponding 2H-1,3-oxazine derivatives. Our studies show that the inclusion of a Brønsted base cocatalyst not only accelerates the reaction but also switches pathways of the skeletal rearrangement reaction, realizing divergent synthesis of heterocyclic compounds. Computational studies indicate that the elimination of propargylic proton in the cyclized vinylgold intermediate is rate-determining and both electron-withdrawing substituents at the oxime moiety and base cocatalyst facilitate the proton elimination. Moreover, the protodeauration process proceeds stepwise involving N–O bond cleavage followed by recyclization to construct the oxazine core.

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions.

Heterogeneous Gold(III)-Catalysed Double Hydroamination of 2-Alkynylanilines with Terminal Alkynes Leading to N-Vinylindoles

Heterogeneous double hydroamination of 2-alkynylanilines with terminal alkynes was achieved by using a magnetic nanoparticles-supported gold(III)-2,2′-bipyridine complex and silver trifluoromethanesulfonate as catalysts to afford the corresponding N-vinylindoles in moderate to good yields under mild and solvent-free conditions. The heterogeneous gold catalyst can easily be separated from the reaction mixture by simply applying an external magnet and can be recycled up to seven times without significant loss of activity.

Regioselective Synthesis of 2,5-Disubstituted Pyrroles via Stepwise Iododesilylation and Coupling Reactions

A new protocol has been developed for the regioselective preparation of 2,5-disubstituted pyrroles. This approach is based on a stepwise iododesilylation and a subsequent coupling reaction, involving a 6-step pathway starting from the simplest pyrrole. A variety of 2,5-disubstituted pyrrole derivatives are accessible in moderate to good yields. In this study, the protection group for the pyrrole nitrogen is carefully chosen and N,N-dimethylaminosulfonyl is the final choice, which facilitates the subsequent double lithiation and makes the pyrrole moiety more stable. However, the attempted removal of this group fails under several different conditions. Instead, unexpected dimethylaminosulfonyl migration to the β-position of the pyrrole ring in the presence of tetrabutylammonium fluoride is observed.

O, Bao X. Computational insights into the mechanisms of Au(i)-catalysed intramolecular addition of the hydroxylamine group onto alkynes

Computational studies were carried out to understand the reaction mechanisms and the origin of the substrate-dependent chemo- and regio-selectivities of the Au(i)-catalysed intramolecular addition of the hydroxylamine group onto alkynes.

Ring Expansion and Skeletal Rearrangement of Propargyl Alcohol Substituted Aziridines Induced by Ruthenium Complexes

The ring expansion and skeletal rearrangement of two types of propargyl alcohol substituted aziridines with or without cycloalkane moieties was induced by a ruthenium cyclopentadienyl phosphine complex. In the simple aziridine system with no cycloalkane, the unique cycloisomerization process altered the absolute connectivity of the two-carbon unit in the three-membered ring to give organometallic products with substituted pyridine or dihydropyridine ligands. For the aziridine on a cyclohexyl ring, the cycloisomerization process was controlled by an interchange process between vinylidene and allenylidene species, thus creating a better relative configuration of the aziridinyl and the alkynyl units. This determines the stereochemistry of the metal carbene products of the octahydroindole derivatives. The structures of five products were determined by X-ray diffraction analysis.