In(III)/PhCO2H Binary Acid Catalyzed Tandem [2 + 2] Cycloaddition and Nazarov Reaction between Alkynes and Acetals

Visible Light-Promoted Three-Component Reaction for the Synthesis of Pyrrolopyrazoles

A visible light-enabled three-component cascade assembles pyrrolopyrazoles from arylalkynes, benzoquinones, and 5-aminopyrazole through a mechanistically validated carbonyl-alkyne metathesis/(3 + 2)-cycloaddition/1,2-aryl migration sequence. This protocol delivers pyrrolopyrazole heterocycles in up to 96% yield with excellent functional group tolerance. Preliminary biological screening identified promising antitumor activity in selected products, highlighting the potential value of this method. The proposed reaction mechanism is supported by control experiments.

Palladium-catalyzed cross-coupling of arylcarboxylic acid 2-pyridyl esters with terminal alkynes

In the presence of Na2CO3, the combination of PdCl2(dppf), dppp and CuI catalyzes the decarbonylative coupling of arylcarboxylic acid 2-pyridyl esters with terminal alkynes to afford 1,2-disubstituted acetylenes. (Hetero)aryl, alkyl, and silylacetylenes and various electron-donating and -withdrawing group-substituted arylcarboxylic acid 2-pyridyl esters can be used in this transformation, with a range of functional groups showing compatibility.

Site-selective synthesis of indanyl-substituted indole derivatives via 1,3-dithiane induced Nazarov cyclization

We report an efficient site-selective synthetic method to C2 and C3 indanyl-substituted indole derivatives via 1,3-dithianyl induced Nazarov-type cyclization. In particular, C2-selective indanyl-substituted indoles were directly obtained by a BF3·Et2O-promoted sequence of intramolecular C3-C2 migration and Nazarov-type cyclization process.

Palladium-Catalyzed Synthesis of Indanone via C-H Annulation Reaction of Aldehydes with Norbornenes

A novel methodology for the synthesis of indanone derivates has been developed. The palladium-catalyzed annulation reaction of o-bromobenzaldehydes with norbornene derivatives is achieved through extremely concise reaction processes. The indanone skeleton was established directly via C-H activation of the aldehyde group under a mild reaction condition. This method is simple and practical, which simplified the traditional synthesis method for the rapid construction of indanone.

An efficient approach to angular tricyclic molecular architecture via Nazarov-like cyclization and double ring-expansion cascade

A modular and efficient method for constructing angular tri-carbocyclic architectures containing quaternary carbon center(s) from 1,3-dicycloalkylidenyl ketones is established, which involves an unconventional synergistic cascade of a Nazarov cyclization and two ring expansions. It features high selectivity, mild conditions and convenient operation, wide scope and easy availability of substrate. Substitution with R¹ and R² at the 4πe-system with electron-donating group favors this reaction, while that with electron-withdrawing group or proton disfavors. The electron-donating group as R¹ directs the initial ring expansion at its own site, while the p-π- or n-π- associated substituent as R² favors selectively the later ring expansion near its location because of the beneficial maintenance of an original conjugated system. The stereoselectivity has proved to be governed by either the steric effect of R³ and R⁴ at the expanded rings, or the migration ability of the migrating atom. Density Functional Theory calculation suggests the initial Nazarov cyclization would be the rate-determining step. A racemic total synthesis of the natural (±)-waihoensene is realized in 18 steps by use of this methodology.

Rh-Catalyzed Annulation of Benzoic Acids, Formaldehyde, and Malonates via ortho-Hydroarylation to Indanones

A three-component reaction from readily available low-cost materials of benzoic acids, formaldehyde, and malonates for the preparation of indanones by rhodium catalysis is reported. The annulation is initiated by an ortho-hydroarylation of benzoic acids, and a Lewis acid is not required. The solvent has a significant influence to the reaction, and 2-substituted or nonsubstituted indanones are obtained by the change of solvent.

Zinc-Catalyzed Alkyne-Carbonyl Metathesis of Ynamides with Isatins: Stereoselective Access to Fully Substituted Alkenes

A zinc-catalyzed intermolecular alkyne-carbonyl metathesis reaction of ynamides with isatins followed by an amide to ester conversion has been developed, which produces the indolone derivatives with a fully substituted alkene species in good to high yields. The salient features of this reaction include the following: mild reaction conditions, an inexpensive zinc catalyst, a broad substrate scope, the excellent regiocontrol and stereoselectivity, and amenable to the gram scale.

Inter- and Intramolecular Alkyne-Carbonyl Metathesis/1,6-Addition/Oxidative Aromatization Tandem Reactions between 1,6-Diynes and Aldehydes

We report an efficient synthetic route to 9-arylfluorenes and biaryl compounds from 1,6-diynes and aldehydes via inter- and intramolecular alkyne-carbonyl metathesis/1,6-addition/oxidative aromatization reactions. These tandem reactions are initiated by a BF₃·Et₂O-promoted tandem inter- and intramolecular alkyne-carbonyl metathesis of 1,6-diynes with carbonyl compounds followed by an In(OTf)₃-catalyzed 1,6-addition/oxidative aromatization with iodobenzene diacetate.

Electrostatic polarization of nonpolar substrates: a study of interactions between simple cations and Mo-bound N2

Although a great deal of catalytic studies have focused on covalent interactions between substrates and catalyst centers, recognition of the importance of noncovalent and ionic interactions is driving new approaches to catalyst design. Electrostatic interactions with simple cations (those with little covalency, such as alkali metals) play crucial roles in many catalytic processes, but these effects are challenging to study due to their complicated solvation and speciation behaviour. These effects are particularly difficult to study during cation-mediated reactions with weakly-polar or non-polar substrates. Dinitrogen is one of the most nonpolar substrates known to be affected by electrostatic interactions in both heterogeneous and homogeneous reactions but understanding the significance of these effects requires further exploration. To examine these effects systematically, a new multidentate ligand framework bearing pendent crown ethers has been developed and incorporated into a series of Mo(0)-based dinitrogen complexes. Prepared via both reduction and ligand substitution routes, the strength and impact of cation-N₂ interactions have been studied experimentally (IR spectroscopy) and computationally. Although the smallest cation (Li⁺) has the largest impact on the ground-state heterobimetallic activation of N₂, solvation interactions are highly competitive and result in low Li⁺-(N₂)Mo binding affinities. Thus, although smaller cations can have the largest electronic impact on substrates, these interactions are also the least persistent.

Iron(III)-Mediated Bicyclization of 1,2-Allenyl Aryl Ketones: Assembly of Indanone-Fused Polycyclic Scaffolds and Dibenzo[a,e]pentalene Derivatives

The rapid construction of three-dimensional fused carbocycles is a key challenge in synthetic chemistry. Herein, an unprecedented and practical tandem Nazarov/oxidative umpolung 4π-ring closure of readily available 1,2-allenyl aryl ketones mediated by iron(III) chloride has been developed, furnishing a new family of indanone-fused molecular architectures in moderate to excellent yields. The indanone-containing blocks can be efficiently converted to unsymmetrical dibenzo[a,e]pentalenes. Significantly, divergent synthetic applications have been achieved to provide densely functionalized polycyclic arrays.

A Method for Synthesis of 3-Hydroxy-1-indanones via Cu-Catalyzed Intramolecular Annulation Reactions

We report a facile and highly efficient method that copper-catalyzed intramolecular annulation to synthesize 3-hydroxy-1-indanones employing simple 2-ethynylbenzaldehyde as starting materials was achieved successfully. This protocol provided a simple synthetic approach to afford 3-hydroxy-1-indanones under mild conditions in good to excellent yields.

Beyond olefins: new metathesis directions for synthesis

The olefin-olefin metathesis reaction has emerged as one of the most important carbon-carbon bond-forming reactions, as illustrated by its wide use in the synthesis of complex molecules, natural products and pharmaceuticals. The corresponding metathesis reaction between carbonyls and olefins or alkynes similarly allows for the formation of carbon-carbon bonds. Although these variants are far less developed and utilized in organic synthesis, they possess attractive qualities that have prompted chemists to incorporate and explore these modes of reactivity in complex molecule synthesis. This review highlights selected examples of carbonyl-olefin and carbonyl-alkyne metathesis reactions in organic synthesis, in particular in the total synthesis of natural products and complex molecules, and provides an overview of current advantages and limitations.

Enantioselective indium(i)-catalyzed [4 + 2] annulation of alkoxyallenes and β,γ-unsaturated α-keto esters

An indium(i)–chiral phosphoric acid complex was found to catalyze the enantioselective [4 + 2] annulation reaction of β,γ-unsaturated α-keto esters with alkoxyallenes, affording cyclic O,O-acetals in good yields and with high regio- and stereo-selectivities.

Friedel-Crafts Acylation Reactions of BN-Substituted Arenes

The acylation reaction of BN-arenes has been studied using BN-arene and acyl chloride in good to excellent yields, which led to the first synthesis of indanone BN-analogue. The BN-aromatic ketone products have been characterized by ¹H NMR spectroscopy with their molecular structures unambiguously confirmed by X-ray crystallography. The annulation reaction of BN-arenes promoted by AgBF₄ occurs in a completely regioselective manner and a mechanism for this transformation is proposed.

Rhodium(iii)-catalyzed synthesis of indanones via C–H activation of phenacyl phosphoniums and coupling with olefins

Phosphonium ylide acts as an efficient directing group in the oxidative carbocyclization between arenes and activated olefins for indanone synthesis.

Recent Progress on Nazarov Cyclizations: The Use of Iron Salts as Catalysts in Ionic Liquid Solvent Systems

Nazarov cyclization is an important and versatile method for the synthesis of five-membered carbocycles, and extensive studies have been conducted to optimize the reaction. Among recent studies, several trends are recognized. One is the combination of different reactions with Nazarov cyclization in a one-pot reaction system which enables the preparation of unique cyclization products. The second is the use of a transition-metal catalyst, though Lewis or Brønsted acids have generally been used for the reaction. The third is the realization of the asymmetric Nazarov cyclization. The fourth is the base-catalyzed Nazarov cyclization. Furthermore, several useful protocols for realizing Nazarov cyclization have also been developed. The recent progress on Nazarov cyclizations is summarized in Section 2. Section 3 is our chronicle in this field. We focused on the use of iron as the catalyst in Nazarov cyclizations and ionic liquids as solvents: Nazarov cyclization of thiophene derivatives using FeCl3 as the catalyst was accomplished and we succeeded in demonstrating the first example of an iron-catalyzed asymmetric Nazarov reaction. We next established Nazarov cyclization of pyrrole or indole derivatives using Fe(ClO4 )3 ·Al2 O3 as the catalyst with high trans selectivities in excellent yields. Since the cyclized product was reacted with a vinyl ketone in the presence of the same iron salt, the system allowed realization of the sequential type of Nazarov/Michael reaction of pyrrole derivatives. Furthermore, we demonstrated the recyclable use of the iron catalyst and obtained the desired Nazarov/Michael reaction products in good yields for five repetitions of the reactions without any addition of the catalyst using an ionic liquid, [bmim][NTf2 ], as the solvent. We expect that the iron-catalyzed Nazarov cyclization, in particular, in an ionic liquid solvent might become a useful method to synthesize functional molecules that include cycloalkene moieties.

Barluenga's reagent with HBF4as an efficient catalyst for alkyne-carbonyl metathesis of unactivated alkynes

Barluenga's reagent (IPy₂BF₄, Py = pyridine) treated with HBF₄efficiently catalyzes the inter- and intramolecular alkyne-carbonyl metathesis of unactivated alkynes. This work represents the first catalytic application of the Barluenga's reagent.

MeOTf-catalyzed annulation of aldehydes and arylalkynes leading to 2,3-disubstituted indanones

MeOTf-catalyzed arylalkyne–aldehyde metathesis and Nazarov cyclization represent a sustainable and atom-economical approach for the construction of indanones with high stereoselectivity.

Efficient catalytic enantioselective Nazarov cyclizations of divinyl ketoesters

An efficient catalytic enantioselective Nazarov cyclization of divinyl ketoesters is developed using a BOX/Cu(ii) catalyst, affording various optically active cyclopent-2-enone esters.

A facile access to substituted benzo[a]fluorenes from o-alkynylbenzaldehydes via in situ formed acetals

Brønsted acid catalysed reaction of o-alkynylbenzaldehydes with arylalkynes in the presence of trimethyl orthoformate takes a different reaction pathway and results in benzo[a]fluorenes.