Copper-catalyzed asymmetric propargylic substitution with salicylaldehyde-derived imine esters

Copper-catalyzed asymmetric propargylic substitution with salicylaldehyde-derived imine esters and propargylic carbonates has been successfully realized, generating a wide range of chiral amino acid derivatives containing propargylic groups with excellent results (up to 95% yield and 94% ee). The ortho-hydroxy group of the salicylaldehyde-derived imine esters is crucial to increase the reactivity and stabilize the azomethine ylide, which may be due to the formation of an intramolecular hydrogen bond between the hydroxyl group and the imine group. A series of synthetic transformations were carried out to access other important chiral compounds, which displayed the synthetic versatility.

Stereoselective synthesis of C3-tetrasubstituted oxindoles via copper catalyzed asymmetric propargylation

Herein, a copper catalyzed asymmetric propargylation of 2-oxindole-3-carboxylate esters with terminal propargylic esters is described. This strategy successfully provides a direct approach to constructing a broad range of chiral C3-tetrasubstituted oxindoles with contiguous tertiary and quaternary carbon stereocenters in high yields and excellent enantioselectivities (16 examples, up to 99% yield and 98% ee). Moreover, the diastereoisomers of the two newly formed stereocenters can be separated by silica gel chromatography, thereby providing a valuable stereoselective access to all four possible stereoisomers of C3-tetrasubstituted oxindoles.

Herein, a copper catalyzed asymmetric propargylation of 2-oxindole-3-carboxylate esters with terminal propargylic esters is described.

An Isoxazole Strategy for the Synthesis of Fully Substituted Nicotinates

A four-step quasi one-pot procedure for the preparation of fully substituted nicotinates from ketone enamines and 4-methylideneisoxazol-5-ones has been developed. The reaction sequence involves (1) reaction of 4-methylideneisoxazol-5-ones with ketone enamines with the formation of isoxazole-5-ols, (2) their O-methylation with diazomethane, (3) hydrogenative cleavage of the O-N bond in 5-methoxyisoxazoles under action of Mo(CO)₆/H₂O and simultaneous isomerization and condensation of the formed enamines, with the formation of dihydropyridines, and (4) aromatization of the latter.

Cp*RuCl-Vinyl Carbenes: Two Faces and the Bifunctional Role in Catalytic Processes

Ruthenium vinyl carbenes derived from Cp/Cp*RuCl-based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5-pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs-type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α-unsubstituted and α-substituted alkynals showcases the peculiarities of these intermediates. Although Z-vinyl dihydrooxazines are preferentially obtained with the former, Z-vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η³ -coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species.

Copper-Catalyzed Asymmetric Propargylation of Indolizines

Methods to diversify indolizines are valuable for the discovery of medications and fluorescent molecules. The utilization of copper-catalyzed asymmetric propargylation to install a terminal alkyne handle on indolizine heterocycle is reported. This method delivers C3-propargylation products from C2-substituted indolizines or C1-propargylation products from C2,C3-disubstituted indolizines through a stereoconvergent pathway.

Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis

With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

[4 + 2]-Cycloaddition and 1,4-Addition of ortho-Quinone Methides by a Chiral Crotyl Silane

Anhydrous FeCl₃ in the presence of 2,6-lutidine promotes the substrate-controlled enantioselective [4 + 2]-cycloaddition and crotylation reaction between an enantioenriched ( S, E)-crotyl silane and in situ generated ortho-quinone methides ( oQMs). The reaction produces both the chiral chroman and crotylation products in a ratio reflective of the electronic nature of the parent oQM with overall combined yields up to 96%. A ring-opening and elimination sequence was subsequently developed to provide direct access to the crotylation products, containing two contiguous tertiary carbon stereocenters, in good yields and enantioselectivities.

Copper-catalyzed propargylic [3+3] cycloaddition with 1H-pyrazol-5(4H)-ones: enantioselective access to optically active dihydropyrano[2,3-c]pyrazoles

A copper-catalyzed propargylic [3+3] cycloaddition with 1H-pyrazol-5(4H)-ones as C,O-bisnucleophiles through the desilylation-activated strategy has been developed. With the support of a chiral tridentate P,N,N-ligand, the reaction gave rise to a variety of optically active dihydropyrano[2,3-c]pyrazoles cyclohexadienone derivatives with up to 96% ee.

Scope and advances in the catalytic propargylic substitution reaction

Nucleophilic displacement of the propargylic alcohol is one of the sought-after methods in the current scenario. The highly nucleophilic alkyne functional moiety along with its considerably acidic terminal hydrogen atom allows the propargylic unit to play a crucial role in organic synthesis by offering a handle for further synthetic transformations. Until 2000, the most fundamental propargylic substitution reaction was the Nicolas reaction, a multi-step transformation, developed in 1972, which involved cobalt as a stoichiometric promoter. Therefore, the direct catalytic substitution of propargylic alcohols was a highly desirable method for development. The pioneering work on the Ru-catalyzed propargylic substitution reaction in 2000 encouraged many researchers to develop several novel catalytic propargylic substitution reactions, which have made rapid progress since then. The purpose of this review is to emphasise the involvement of diverse types of Lewis acid, transition metal and Brønsted acid catalysts in the propargylic substitution reaction and provide an updated summary of the recent developments in this field. The selected examples presented here are the most significant and relevant ones and we believe that this will help the readers to comprehend the scope of the propargylic substitution reaction with diverse types of catalysts and will envisage the scientific community for the future developments in this field.

Direct nucleophilic displacement of the alpha-hydroxy of the propargylic alcohol is one of the sought-after methods in the current scenario. An updated summary of the recent developments in this field is presented here.

Chiral ligands designed in China

Asymmetric catalysis has become an indispensable and productive field within the Chinese organic chemistry society. The design of chiral ligands is one of the most prominent research areas in this field. Since the late 1990s, Chinese organic chemists have developed numerous chiral ligands possessing novel chiral skeletons and design concepts. Some of these ligands have been widely adopted and can be regarded as ‘privileged ligand’, which have shown excellent performance in many asymmetric catalytic reactions. In this review, we provide an overview of the chiral ligands designed by Chinese scientists with the aim of promoting the development of this area in China and with the hope of encouraging more scientists across the world to use these ligands when designing asymmetric reactions.

Recent advances in chiral imino-containing ligands for metal-catalyzed asymmetric transformations

In this review, the recent applications of a variety of chiral imino-containing ligands classified by different types of metal-catalyzed asymmetric reactions are summarized. The progress made in this area would encourage us to design and synthesize more novel chiral imino-containing ligands, and explore their applications in metal-catalyzed asymmetric transformations.

Cu-Catalyzed asymmetric Friedel–Crafts propargylic alkylation of phenol derivatives

A copper-catalyzed asymmetric Friedel–Crafts propargylic alkylation of electron-rich phenol derivatives with a variety of propargylic esters has been described. With Cu(OTf)₂ decorated with a chiral tridentate ketimine P,N,N-ligand as the catalyst, the reaction proceeded smoothly in high yields and with good to excellent enantioselectivities (up to 95% ee).

Ir-catalyzed asymmetric hydrogenation of β-keto esters with chiral ferrocenyl P,N,N-ligands

The Ir-catalyzed asymmetric hydrogenation of β-keto esters with chiral ferrocenyl P,N,N-ligands has been developed, providing the corresponding β-hydroxy esters in good to excellent enantioselectivities.

Highly diastereo-/enantioselective Cu-catalyzed propargylic alkylations of propargyl acetates with cyclic enamines

Highly diastereo- and enantioselective copper-catalyzed propargylic alkylations of morpholine-derived cyclic enamines have been developed by the employment of a less sterically hindered chiral tridentate P,N,N-ligand.

Tridentate P,N,N-ligand promoted copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters: synthesis of highly functionalized pyrroles

By employment of a newly developed tridentate P,N,N-ligand, a copper-catalyzed [3 + 2] cycloaddition of propargylic esters with β-enamino esters for the construction of highly functionalized pyrroles has been developed.