Heterobimetallic Catalysis: Platinum-Gold-Catalyzed Tandem Cyclization/C-X Coupling Reaction of (Hetero)Arylallenes with Nucleophiles

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.

(Z)-γ-Alkylidenebutenolide Synthesis through Au(I)-Catalyzed 1,3-Acyloxy Migration and the Carbonyl-Ene Reaction

A bimetallic protocol has been developed to construct (Z)-γ-alkylidenebutenolide compounds from readily available propargyl α-ketoesters. It involves a gold-catalyzed 1,3-acyloxy migration of propargyl α-ketoesters and a carbonyl-ene cyclization of in situ generated allenyl esters. DFT calculations suggest that the copper salt might play dual roles as both chloride abstractor facilitating the generation of highly active gold catalyst and Lewis acid promoting the stepwise intramolecular carbonyl-ene reaction.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Brønsted Acid Catalyzed (4 + 2) Cyclocondensation of 3-Substituted Indoles with Donor–Acceptor Cyclopropanes

Acylcyclopropanes are employed as useful donor–acceptor cyclopropanes that undergo formal (4 + 2) cyclocondensation with N-unprotected 3-substituted indoles in the presence of a Brønsted acid catalyst. The reaction involves the simultaneous alkylation of both the N and C-2 positions of the indole and provides access to the 8,9-dihydropyrido[1,2-a]indole scaffold that is the central core of several biologically relevant indole alkaloids in excellent yields and good selectivities.

Chemoselective synthesis of heterobimetallic bis-NHC complexes

Bis-NHC precursors composed of an azolium and a 2-halogenoazole group connected by different linkers have been site-selectively metallated to give heterobimetallic complexes from a single-frame bis-NHC ligand. The azolium group reacts with a base and oxidized metal centers to give NHC complexes with no participation of the halogenoazole, while the halogenoazole reacts in an oxidative addition with low-valent transition metals to give azolato complexes with no participation of the azolium group.

Asymmetric construction of pyrido[1,2-a]-1H-indole derivatives via a gold-catalyzed cycloisomerization

Pyrido[1,2-a]-1H-indoles are important scaffolds found in many biologically active compounds. Herein, we first developed an IPrAuCl/AgSbF₆-catalyzed cycloisomerization of N-1,3-disubstituted allenyl indoles affording pyrido[1,2-a]-1H-indoles. Then the axial-to-central chirality transfer starting from enantio-enriched N-1,3-disubstituted allenylindoles affording optically active pyrido[1,2-a]-1H-indoles has been realized in excellent yields and enantioselectivities. A mechanism has been proposed based on mechanistic studies. Synthetic applications have also been demonstrated.

Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes

The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a SEOx process via Pt(IV)–H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed.

Gold catalysed transformation of 2-arylindoles to terphenyl amines via 3-dienyl indoles and Brønsted acid promoted formation of 2-carboxyindoles to 3-indenylindoles via 3-allenylindoles

Gold-catalysed intramolecular reaction of unprotected/protected 3-dienyl indoles leads to the formation of functionalised terphenylamines with high efficiency; this reaction can also be accomplished in one pot by starting with 2-arylindole and propargylic alcohols. This reaction occurs via 3-dienyl indoles and proceeds likely by gold-catalysed cascade cyclisation, aromatisation and fragmentation of the indole heterocyclic moiety. We have also found that PTSA can work well giving comparable yields, but at least one mole equivalent of PTSA is required. On the other hand, 3-indenylindoles have been synthesised from indole-2-carboxylates (2-carboxyindoles) and propargylic alcohols through Brønsted acid mediation. Many of these indenylindoles exhibit isomerism (probably diastereomers) in the solution state (1H NMR). Two examples of indolo-indenyl acetates have been prepared by reacting the above terphenylanilines with PhI(OAc)2. Key products are characterised by X-ray crystallography.