Synthesis of Polysubstituted 2‑Iodoindenes via Iodonium-Induced Cyclization of Arylallenes

A new chemoselective iodocarbocyclization of allenyl arenes was developed leading to the formation of 2-iodoindenes. In acetonitrile or nitromethane, electrophilic sources of iodine cations react selectively with the C₂–C₃ double bond of 1-arylallenes to give, after <i>anti</i> nucleophilic attack of the aromatic ring, 2-iodoindene products in high yields. Variations of the allenic skeletons revealed the high 5-<i>endo</i> selectivity and some competitive pathways of cyclization. Postfunctionalization reactions of the carbon–iodine bond, via Pd- and Cu-cross-couplings, gave rise to substituted indenes in good to excellent yields

One-Pot Gold-Catalyzed Aminofluorination of Unprotected 2‑Alkynylanilines

A tandem gold(I)-catalyzed aminocylization/fluorination and a two-step, one-pot gold(III)-catalyzed cyclization/electrophilic fluorination provide a convenient and general method for the synthesis of 3,3-difluoro-2-substituted-3<i>H</i>-indoles in good yield under mild conditions. Extension of the procedure to the synthesis of 2-aryl-3-fluoro-1<i>H</i>-indoles is described. The reaction proceeds smoothly in green ethanol and does not require any base, acid, or <i>N</i>-protective group

Ruthenium-Catalyzed [2 + 2 + 2] Cycloaddition Reaction Forming 2‑Aminopyridine Derivatives from α,ω-Diynes and Cyanamides

A novel, efficient, and mild synthetic route for the preparation of 2-aminopyridines via ruthenium-mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and cyanamides has been developed. This atom-economical catalytic process demonstrated remarkable regioselectivities to access pyridine derivatives of high synthetic utility

Access toward Fluorenone Derivatives through Solvent-Free Ruthenium Trichloride Mediated [2 + 2 + 2] Cycloadditions

An efficient and practical route for the preparation of highly substituted fluorenones and analogues via solvent-free ruthenium trichloride mediated [2 + 2 + 2] cycloaddition of α,ω-diynes and alkynes has been developed. This green chemistry approach involves a solventless and atom-economical catalytic process to generate densely functionalized fluorenones and related derivatives of high synthetic utility

Synthesis of Functionalized 1<i>H</i>‑Isochromene Derivatives via a Au-Catalyzed Domino Cycloisomerization/Reduction Approach

A Au-catalyzed versatile and efficient access to 1<i>H</i>-isochromenes is reported. The efficiency of the [AuCl₂(Pic)] complex (1–5 mol %) was demonstrated and allowed a domino cycloisomerization/reduction reaction process starting from a wide range of functionalized <i>ortho</i>-alkynyl­benz­aldehydes and one example of <i>ortho</i>-alkynyl­pyridinyl­aldehyde. The smooth reaction conditions were amenable to aryl- and alkyl-substituted alkynyl derivatives, as well as functionalized halogen and ether moieties, leading to a chemo- and regioselective 6-<i>endo</i>-cyclization with good to excellent yields

HNTf₂‑Catalyzed Regioselective Preparation of Polysubstituted Naphthalene Derivatives Through Alkyne–Aldehyde Coupling

We report herein the preparation of polysubstituted naphthalene derivatives by the original Brønsted-acid-catalyzed benzannulation reaction of phenylacetaldehydes with alkynes. This reaction, which was usually performed with Lewis acids under thermal activation, is efficiently promoted by 15 mol % of triflimide (HNTf₂) at room temperature under metal-free and mild reaction conditions and leads with a perfect regioselectivity to a wide variety of diversely functionalized naphthalenes in 41–78% yield. A catalytic cycle is proposed together with some further applications of this catalytic system in the related benzannulation transformations of epoxide and acetal derivatives

Silver-Catalyzed Domino Hydroarylation/Cycloisomerization Reactions of <i>ortho</i>-Alkynylbenzaldehydes: An Entry to Functionalized Isochromene Derivatives

A Ag-catalyzed versatile and efficient access to 1<i>H</i>,1-arylisochromenes is reported. Starting from <i>ortho</i>-alkynylbenzaldehydes bearing various substitution patterns on the benzaldehyde and alkynyl units, the use of silver triflate (10 mol %) allowed a domino hydroarylation/cycloisomerization reaction process, leading to aryl-functionalized 1<i>H</i>-isochromene (>10 compounds, 80–98% yields). Notably, the reaction conditions were also compatible with benzaldehydes bearing an aliphatic-substituted alkynyl moiety with modest to good yields (34–88%, 10 compounds)

Gold-Catalyzed Access to 1<i>H</i>‑Isochromenes: Reaction Development and Mechanistic Insight

The gold-catalyzed domino cyclization/nucleophilic reaction of <i>ortho</i>-carbonylalkynylaryls has been studied. Thus, 2-(pyridin-2-ylethynyl)­benzaldehyde has been chosen to isolate key intermediates that may take part in the reaction mechanism. Employing Hantzsch ester (HEH) as nucleophile, it has been impossible to isolate the corresponding gold–alkenyl specie; however, when methanol was used as solvent (and nucleophile), the expected chelate gold–vinyl complex was isolated and unambiguously characterized by X-ray analysis. When HEH is present in the alcoholic reaction mixture, isotopic studies show that the cleavage of the Au–C bond of gold–vinyl complex proceeds through a protodemetalation pathway, rather than a plausible metal–hydride reductive elimination mechanism. Finally, with the aim of broadening the scope of the cyclization/reduction reaction previously reported, we present that the catalytic system is robust and applicable for a diverse family of challenging substrates presenting ester, aldehyde, ether, alkene, and alkyne functionalities

Water-Soluble Gold(I) and Gold(III) Complexes with Sulfonated <i>N</i>‑Heterocyclic Carbene Ligands: Synthesis, Characterization, and Application in the Catalytic Cycloisomerization of γ‑Alkynoic Acids into Enol-Lactones

Zwitterionic imidazolium salts bearing 3-sulfonatopropyl, and 2-pyridyl, 2-picolyl, and 2-pyridylethyl substituents have been synthesized and employed as precursors for the preparation of novel water-soluble Au­(I)- and Au­(III)-NHC complexes of general composition [AuCl­(NHC)] and [AuCl₃(NHC)] (NHC = <i>N</i>-heterocyclic carbene), respectively. These complexes proved to be active, selective, and recyclable catalysts for the intramolecular cyclization of γ-alkynoic acids under biphasic toluene/water conditions, leading to the desired enol-lactones in high yields under mild conditions (r.t.). Remarkably, despite the well-known ability of gold complexes to promote the hydration of CC bonds, the competitive hydration process was not observed, even during the cycloisomerization reactions of 1,6-diynes