Gold-catalyzed Intermolecular Oxidations of 2-Ketonyl-1-ethynyl Benzenes with N-Hydoxyanilines to Yield 2-Aminoindenones via Gold Carbene Intermediates

Lewis acid-mediated transformations of 5-acyl-N-fluoroalkyl-1,2,3-triazoles to cyclopentenones, indenones, or oxazoles

We present a transition metal-free approach to 2-N-substituted indenones, cyclopentenones, and 4-carbonyl oxazoles, based on the reaction of 5-acylated N-fluoroalkyl substituted 1,2,3-triazoles (prepared by a three-component click reaction of copper acetylides, fluoroalkyl azides, and acyl chlorides) with Lewis acids aluminium trichloride or boron trifluoride etherate, proceeding via the generation and cyclization of vinyl cations.

Gold-Catalyzed Oxidation Reactions of Thioalkynes with Quinoline N-Oxides: A DFT Study

Mechanistic investigation of the gold-catalyzed oxidative reactions of thioalkynes with quinoline N-oxides was performed using density functional theory (DFT) calculations. For the oxidative rearrangement of thioalkynes with quinoline N-oxide to yield the same products, the Cβ-oxidation of thioalkynes was predicted to be competitive with Cα-oxidation, with the Cβ-oxidative process slightly more favorable. However, for the oxidative alkenylation of propargyl aryl thioethers with quinoline N-oxides, the Cβ-oxidation of thioether by quinoline N-oxide generated the product 3-hydroxy-1-alkylidene phenylthiopropan-2-one. Moreover, the ring opening of the four-membered sulfonium intermediate was achieved by the nucleophilic attack of quinoline N-oxide rather than a water molecule.

Synthesis of 2-amidoindenone derivatives through an ynamide carbosilylation/Houben-Hoesch cyclization 2-step sequence

Herein we report the efficient and selective two-step synthesis of various 3-silyl-2-amidoindenones from easily accessible ynamides. This sequence involves a regio- and stereo-selective silylcyanation followed by a Houben-Hoesch type cyclization. Thanks to post-transformations, various 3-substituted 2-amidoindenones could be obtained.

Theoretical Analysis of a Three-Component Reaction between Two Diazo Compounds and a Hydroxylamine Derivative: Mechanism, Enantioselectivity, and Effect of Cooperative Catalysis

The mechanism, enantioselectivity, and effect of chiral phosphoric acid (CPA) cocatalyst were investigated by the density functional theory (DFT) for the three-component asymmetric aminohydroxylation between two diazo compounds and a hydroxylamine derivative. This type of cascade process is cooperatively catalyzed by Rh2(OAc)4 and CPA. The obtained results clearly indicate that the first step of the global reaction involves a nucleophilic attack at the nitrogen center of N-hydroxyaniline by rhodium-carbene intermediates producing imines. Subsequently, an enolate intermediate was recognized as the key species generated from the second diazo compound and the leaving benzyl alcohol (BnOH) fragment of the first step and in the presence of the same dirhodium catalyst. Then, the reaction is terminated by the asymmetric Mannich-type addition, delivering the aminohydroxylation products of an S-R conformation with the assistance of chiral phosphoric acid. The distortion/interaction analysis shows that the relative distortions of CPA and the enol play a vital role in the energy ordering of the stereocontrolling transition states (TSs). Furthermore, the influence of different substituents in CPA was fully rationalized by distortion/interaction analysis. This study opens up novel synthetic possibilities and improves the reaction predictability when exploring the related types of cooperatively catalyzed organic transformations.

Gold-Catalyzed [4 + 1] Heterocyclization of Hydroxamic Acid and Nonactivated Alkyne: A Protocol to Construct 5-Methyl-1,4,2-dioxazole

A novel gold-catalyzed [4 + 1] heterocyclization of nonactivated alkyne and hydroxamic acid is developed for the regiospecific synthesis of 5-methyl-1,4,2-dioxazole, which is an important structural motif in various bioactive molecules. The current methodology is characterized by high efficiency, simple operation, mild reaction conditions, and good functional group compatibility. Moreover, gram-scale synthesis and synthetic application toward bioactive molecular skeletons have been realized.

Cobalt-Catalyzed Thioamide Directed C(arene)-H Annulation with Ynamide: Regioselective Access to 2-Amidoindenones

Demonstrated herein is an unprecedented thioamide-directed cobalt (Co)-catalyzed umpolung annulation of sulfoximines enabled aryl thioamide with ynamide for the synthesis of highly substituted 2-amidoindenones. The cyclization is regioselective, making β-C-C and α-C-CO bonds. The transformation is even successful on a gram scale, exhibiting broad scope with labile functional group tolerance and constructing 43 unusual 2-amidoindenones of structural diversity. Control experiments and mechanistic investigation validate the regioselectivity outcome in this transformation.

Direct Access to 2,3-Disubstituted Amido-Indenones through Annulation of 2-Iodobenzaldehydes with Ynamides

In this paper we report the annulation reaction between 2-iodobenzaldehyde derivatives and various ynamides. This palladium-catalyzed reaction leads to rare polysubstituted amino-indenones in good yields with a regioselectivity up to complete. Remarkably, a regiodivergent selectivity has been identified between aryl and alkyl or silyl ynamides, with the first leading mainly to 2-amido-indenones and the second to 3-amido-indenones.

Palladium(II)-Catalyzed [2+2+1] Annulation of Alkynes and Hydroxylamines: A Rodox-Neutral Approach to Fully Substituted Pyrroles

A palladium-catalyzed [2+2+1] annulation of alkynes and hydroxylamines has been developed for the rapid construction of fully substituted pyrroles. This transformation involves sequential nucleophilic-addition of hydroxylamine to alkyne, alkyne migratory insertion, and synergistic demetallization cyclization, which provides a redox-neutral annulation approach to pyrrole derivatives. Moreover, the strategy enabled alteration of the photophysical properties of pyrrole products by varying the aryl substituents, thus leading to the development of N-functionalized tetraarylpyrroles as new fluorophores.

Ag(I)-Promoted homo-dimerization of 2-(alk-2-yn-1-onyl)-1-alkynylbenzenes via a [4 + 2] cycloaddition of benzopyrylium ions: access to structurally unique naphthalenes

Herein we report a Ag(I)-promoted homo-dimerization of 2-(alk-2-yn-1-onyl)-1-alkynylbenzenes for the synthesis of structurally novel and functionalized naphthalene derivatives. This transformation exhibits a broad scope for the alkyl as well as aryl groups present on alkynes. Observations made from control experiments suggest the possible mechanism as (i) the homo-dimerization of the in situ generated benzopyrylium ion intermediates through a head-tail [4 + 2] cycloaddition, followed by (ii) the competitive ring-opening vs. decarbonylative aromatization of the adduct to give formylated and deformylated naphthalenes, respectively.

Gold Catalysts Can Generate Nitrone Intermediates from a Nitrosoarene/Alkene Mixture, Enabling Two Distinct Catalytic Reactions: A Nitroso-Activated Cycloheptatriene/Benzylidene Rearrangement

Gold-catalyzed reactions of cycloheptatrienes with nitrosoarenes yield nitrone derivatives efficiently. This reaction sequence enables us to develop gold-catalyzed aerobic oxidations of cycloheptatrienes to afford benzaldehyde derivatives using CuCl and nitrosoarenes as co-catalysts (10-30 mol %). Our density functional theory calculations support a novel nitroso-activated rearrangement, tropylium → benzylidene. With the same nitrosoarenes, we developed their gold-catalyzed [2 + 2 + 1]-annulations between nitrosobenzene and two enol ethers to yield 5-alkoxyisoxazolidines using 1,4-cyclohexadienes as hydrogen donors.

Gold(I)-Catalyzed Oxidative 1,4-Additions of 3-En-1-ynamide with Nitrones via Carbon- versus Nitrogen-Addition Chemoselectivity

This work reports gold-catalyzed 1,4-oxofunctionalizations of 3-en-1-ynamides with nitrones, yielding two distinct E-configured products. We obtained 1,4-oxoarylation products from 3-en-1-ynamides bearing C(4)-electron-donating substituents and 1,4-oxoamination products from those analogues bearing C(4)-aryl substituents. We propose that if vinylgold carbenes are stable, imines undergo a para-arylation on these gold carbenes. If vinylgold carbenes are highly electron-deficient, this N-attack is irreversible to enable 1,4-oxoaminations.

Silver-catalyzed desulfurizative annulation of 1,2-benzisothiazoles with ynamides to construct multi-substituted isoquinolines

An unprecedented silver-catalyzed desulfurizative annulation of 1,2-benzisothiazoles with ynamides to access multi-substituted isoquinoline derivatives is disclosed.

Revealing the Mechanism and Origin of Reactivity of Au(I)-Catalyzed Functionalized Indenone Formation of Cyclic and Acyclic Acetals of Alkynylaldehydes

A density functional theory study is presented here to offer mechanistic insights and explications of experimentally intriguing observations in the Au(I)-catalyzed cyclization of cyclic and acyclic acetals of alkynylaldehydes that leads to indenone formation. The reactivity of catalytic cycles with and without methoxy migration is clearly defined when the alkyne terminus is phenylated. The reaction mechanism of indenone formation proceeds first with the coordination of Au(I) to alkyne to initiate the reaction with 1,5-H shift as a rate-determining step (RDS), and the fastest 1,5-H shift is achieved when one phenyl ring carries an electron-donating group and the other one is substituted with an electron-withdrawing group. Following the 1,5-H shift, the reaction undergoes feasible steps that are cyclization and 1,2-H shift before elimination to persist the iterative cycle, but the reactivity of both steps is highly affected by the existence of the phenyl group on the alkyne terminus. The unreactivity of the alkyne terminus not bearing a phenyl ring is because the cyclization is thermodynamically disfavorable, subsequently deactivating the 1,2-H shift kinetically and thermodynamically. The absence of a tether in the acetal unit considerably outpaces any 1,5-H shift and instead activates 1,5-methoxy migration, giving methoxy-migrated indenone, with the 1,2-OMe shift being an RDS.

Hydroxylamines As Bifunctional Single-Nitrogen Sources for the Rapid Assembly of Diverse Tricyclic Indole Scaffolds

Conventional approaches on using hydroxylamine derivatives as single nitrogen sources, for the construction of n-membered (n > 3) N-heterocycles, rely upon two chemical operations by involving sequential nucleophilic and electrophilic C-N bond formations. Here, we report a highly efficient cascade of alkyne insertion/C-H activation/amination for the rapid preparation of a myriad of tricyclic indoles, in a single-step transformation, by using bifunctional secondary hydroxylamines. It is noteworthy that judicious selection of applicable amino agents, for enabling the prior oxidative addition of aryl iodide to initial Pd(0) species and subsequent two C-N bonds formation, was the key to the success of this reaction. Control experiments indicated that a five-membered palladacyclic intermediate played a crucial role in promoting the final aminative ring closure.

Gold-Catalyzed Aromatizations of 3-Ene-5-siloxy-1,6-diynes with Nitrosoarenes To Enable 1,4-N,O-Functionalizations: One-Pot Construction of 4-Hydroxy-3-aminobenzaldehyde Cores

This work describes gold-catalyzed aromatizations of 3-ene-5-siloxy-1,6-diynes with nitrosoarenes to form 4-hydroxy-3-aminobenzaldehyde derivatives, manifesting the use of nitrosoarenes as 1,4-N,O-functionalization sources. Various 3-ene-5-siloxy-1,6-diynes in benzoid and nonbenzoid types are applicable substrates. A series of ¹⁸O- and ²H-labeling experiments have been conducted to exclude gold-π-alkyne intermediates. We postulate a mechanism of dual gold catalysis involving initial formation of gold-π-alkynylgold species that activates a 1,5-hydrogen shift to form reactive 1,6-dipoles, thus furnishing intramolecular Michael-type reactions with nitrosonium electrophiles.

Transition metal-free functionalized hydration of alkynes: one-pot synthesis of fluorinated β-keto-imidates using Selectfluor

A transition metal-free, four-component one-pot synthesis of polyfunctionalized fluorinated β-keto-imidates via the functionalized hydration of alkynes has been described. The intermediate 1,3-ketoamino moiety was obtained from easily accessible arylpropioladehyde and arlyhydroxylamine and was treated with Selectfluor delivering fluorinated β-keto-imidates. A wide variety of functional groups are tolerated under mild reaction conditions and the product applicability is highlighted.

Gold-catalyzed oxidative cycloalkenations of alkynes with quinoline N-oxides

This work reports gold-catalyzed oxidative cycloalkenations of phenyl propargyl ethers and phenoxyalkynes with quinoline N-oxides to afford 4-alkylidenechroman-2-ones and 3-alkylidenebenzofuran-2-ones respectively.

Gold-catalyzed N,O-functionalization of 1,4-diyn-3-ols with N-hydroxyanilines to form highly functionalized pyrrole derivatives

This work describes new N,O-functionalization of 1,4-diyn-3-ols with N-hydroxyanilines to yield highly functionalized pyrrole derivatives.

Gold-Catalyzed Regiodivergent [2 + 2 + 2]-Cycloadditions of Allenes with Triazines

Gold-catalyzed regiodivergent cycloadditions of functionalized allenes with 1,3,5-triazines, providing diverse N-heterocycles in moderate to excellent yields under mild reaction conditions, are reported. Importantly, different types of allenes exhibit distinct selectivity and reactivity for the reactions. Mechanistic investigations reveal that all of the cycloadditions proceed through a stepwise [2 + 2 + 2]-cycloaddition process.

Gold-Catalyzed Vinyl Ether Hydroalkynylation: An Alternative Pathway for the Gold-Catalyzed Intermolecular Reaction of Alkenes and Alkynes

In this report, the gold-catalyzed intermolecular reaction of vinyl ethers and terminal alkynes is investigated. Utilizing a triazole gold catalyst lessens gold decomposition in the presence of the vinyl ether and affords an alkynylation product instead of the [2 + 2] product. This protocol has been expanded to include glycal substrates, which undergo a one-pot alkynylation-Ferrier reaction to produce functionalized sugars in moderate to excellent yields with high diastereoselectivity.

Silver-Catalyzed Domino Reaction of ortho-Carbonylated Alkynyl-Substituted Arylaldehydes with Conjugated Dienes: Stereoselective Access to Indanone-Fused Cyclohexenes

A silver-catalyzed domino reaction of ortho-carbonylated alkynyl-substituted arylaldehydes with conjugated dienes is described here. Through this reaction, the synthesis of a variety of indanone-fused cyclohexene derivatives can be achieved efficiently. The formation of these tricyclic products could involve a key Diels-Alder reaction of in situ generated indanenone dienophiles with conjugated dienes. Particularly, the products can be accomplished in a high endo/exo selective way.