Enantioselective α-heterofunctionalisation of carbonyl compounds: organocatalysis is the simplest approach

Enantioselective α-heterofunctionalization reactions of catalytically generated C1-Lewis base enolates

Chiral Lewis base (LB) organocatalysis has emerged as a powerful covalent catalysis concept which allows for highly selective asymmetric C-C and C-heteroatom bond formations. Considering significant recent progress in the development of strategies to access α-heterofunctionalized carboxylic acid derivatives under chiral LB catalysis, we wish to summarize the most significant concepts and advances in this field within this mini review now.

α-Amination of Carbonyl Compounds by Using Hypervalent Iodine-Based Aminating Reagents Containing a Transferable (Diarylmethylene)amino Group

Hypervalent iodine-based aminating reagents containing a transferable (diarylmethylene)amino group can be used for the α-amination of simple carbonyl compounds such as esters, amides, and ketones in the presence of a lithium base. The (diarylmethylene)amino groups of the products can be readily modified, thus providing access to primary amines and diarylmethylamines. The developed method features transition-metal-free conditions and a simple one-pot procedure without the need to prepare enolate equivalents separately, thus offering a general and practical approach to the synthesis of a wide variety of α-amino carbonyl compounds. Experimental mechanistic investigations indicate that this amination proceeds through a unique radical coupling of an α-carbonyl radical with an iminyl radical; they are generated through a single-electron transfer between a lithium enolate and the hypervalent iodine reagent.

An Improved and Suitable Method for Large Scale Production of α-Monochlorocyclodecanone

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α-Monochlorocyclododecanone was synthesized from cyclododecane and 1,3-Dichloro-5,5-dimethylhydantoin (DCDMH) catalyzed with silica gel in MeOH. The product was obtained by crystallization with nearly 85% yield. The presented method used cheap raw materials, mild reaction conditions, simple separation method and environment-friendly process. Additionally, the current synthesis provided an ideal approach for large-scale production.

Recent Advances in Organoselenium Catalysis

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: Organoselenium chemistry has developed as an important tool in the field of synthetic and medicinal chemistry. Various organoselenium reagents have been developed and used successfully to achieve different organic transformations such as the selenocyclizations, oxyselenenylations and selenoxide eliminations etc. Additionally, the potential of organoselenium reagents is not limited their use as stoichiometric reagents but they have successfully used as organocatalyst in number of synthetic transformations. Various organic and inorganic oxidants have been identified as terminal oxidants to regenerate the active catalytic specie. In this review article, the recent progress of organoselenium reagents in catalysis is being highlighted along with their asymmetric variants.

A Molecular-Wide and Electron Density-Based Approach in Exploring Chemical Reactivity and Explicit Dimethyl Sulfoxide (DMSO) Solvent Molecule Effects in the Proline Catalyzed Aldol Reaction

Modelling of the proline (1) catalyzed aldol reaction (with acetone 2) in the presence of an explicit molecule of dimethyl sulfoxide (DMSO) (3) has showed that 3 is a major player in the aldol reaction as it plays a double role. Through strong interactions with 1 and acetone 2, it leads to a significant increase of energy barriers at transition states (TS) for the lowest energy conformer 1a of proline. Just the opposite holds for the higher energy conformer 1b. Both the ‘inhibitor’ and ‘catalyst’ mode of activity of DMSO eliminates 1a as a catalyst at the very beginning of the process and promotes the chemical reactivity, hence catalytic ability of 1b. Modelling using a Molecular-Wide and Electron Density-based concept of Chemical Bonding (MOWED-CB) and the Reaction Energy Profile–Fragment Attributed Molecular System Energy Change (REP-FAMSEC) protocol has shown that, due to strong intermolecular interactions, the HN-C-COOH (of 1), CO (of 2), and SO (of 3) fragments drive a chemical change throughout the catalytic reaction. We strongly advocate exploring the pre-organization of molecules from initially formed complexes, through local minima to the best structures suited for a catalytic process. In this regard, a unique combination of MOWED-CB with REP-FAMSEC provides an invaluable insight on the potential success of a catalytic process, or reaction mechanism in general. The protocol reported herein is suitable for explaining classical reaction energy profiles computed for many synthetic processes.

A solid-supported organocatalyst for asymmetric Mannich reaction to construct C2-quaternary indolin-3-ones

A simple and novel solid-supported organocatalyst from a 2-chlorotrityl chloride resin-immobilized 4-hydroxyproline was developed, and this organocatalyst has been used for the asymmetric Mannich reaction of 2-aryl-3H-indol-3-ones and aldehydes/ketones. A series of C2-quaternary indolin-3-ones were prepared in good yields (up to 83%) and with excellent diastereoselectivities (up to 20 : 1) and enantioselectivities (up to 99% ee). In addition, the organocatalyst can be recovered by simple filtration and also be reused for the asymmetric Mannich reaction without significant loss of catalytic efficiency.

A simple and novel solid-supported organocatalyst from a 2-chlorotrityl chloride resin-immobilized 4-hydroxyproline was developed, which has been used for the asymmetric Mannich reaction of 2-aryl-3H-indol-3-ones and aldehydes/ketones.

Investigation of the Organocatalytic Chlorination of 2-Phenylpropanal

Results of an examination of the organocatalytic enantioselective α-chlorination of 2-phenylpropanal are described. Synthetic investigation including the screening of primary and secondary aminocatalysts, many different reaction conditions, and other α-branched aldehydes show that especially primary aminocatalysts can catalyze the formation of the α-chloro branched aldehydes in good yields, but only with moderate enantioselectivities. In order to try to understand the challenge in obtaining high enantioselectivity for the aminocatalytic α-chlorination of α-branched aldehydes a series of experimental investigations were performed employing 2-phenylpropanal as a model system. These investigations have been coupled with computational investigations, which provided important insight into the moderate enantioselectivity of this chlorination reaction. Analysis of the reaction showed, that the lack of control over the selectivity of formation of the (E)- and (Z)-enamine intermediate, and the clustering of reaction barriers of possible reaction pathways help to rationalize difficulties in producing high enantioselectivity.

Enantioselective α-Chlorination Reactions of in Situ Generated C1 Ammonium Enolates under Base-Free Conditions

The asymmetric α-chlorination of activated aryl acetic acid esters can be carried out with high levels of enantioselectivities utilizing commercially available isothiourea catalysts under base-free conditions. The reaction, which proceeds via the in situ formation of chiral C1 ammonium enolates, is best carried out under cryogenic conditions combined with a direct trapping of the activated α-chlorinated ester derivative to prevent epimerization, thus allowing for enantioselectivities of up to e.r. 99:1.

Asymmetric α-Chlorination of β-Keto Esters Using Hypervalent Iodine-Based Cl-Transfer Reagents in Combination with Cinchona Alkaloid Catalysts

We herein report an unprecedented strategy for the asymmetric α-chlorination of β-keto esters with hypervalent iodine-based Cl-transfer reagents using simple Cinchona alkaloid catalysts. Our investigations support an α-chlorination mechanism where the Cinchona species serves as a nucleophilic catalyst by reacting with the chlorinating agent to generate a chiral electrophilic Cl-transfer reagent in situ. Using at least 20 mol-% of the alkaloid catalyst allows for good yields and enantioselectivities for a variety of different β-keto esters under operationally simple conditions.

Photochemistry of 2-Nitroarenes: 2-Nitrophenyl-α-trifluoromethyl Carbinols as Synthons for Fluoroorganics

Facile synthesis of a new series of 2,2'-bis(trifluoroacetyl) azoxybenzene derivatives and trifluoromethylated benzo[c]isoxazoline systems, along with trifluoroacetyl nitrosobenzene derivatives was achieved by solvent controlled photolysis of appropriate 2-nitrobenzyl alcohols. Corresponding photoactive 2-nitrobenzyl chromophore plays a distinct role in this photosynthetic process, while, quite unprecedented, pertinent fluoromethyl substitution leads to high value fluoromethylated products, whose direct access is not feasible by common synthetic protocols. The significance of fluorine and fluoroalkyl substitution and its prominent biological effects makes this new photochemical approach an important discovery in synthetic methodology. Plausible mechanistic pathways involved in the formation of the products during steady-state photolysis are further established by picosecond laser flash photolysis experiments.

Selenium reagents as catalysts

Organoselenium chemistry has become an important tool in synthetic and medicinal chemistry.

L-Proline functionalized magnetic nanoparticles: A novel magnetically reusable nanocatalyst for one-pot synthesis of 2,4,6-triarylpyridines

In this work, an efficient method for the immobilization of L-proline on magnetic nanoparticles was offered and evaluated as a recoverable magnetic nanocatalyst for synthesis of 2,4,6-triarylpyridines through one-pot three-component reaction of acetophenone, aryl aldehydes and ammonium acetate. This article is the first report of the catalytic application of L-proline functionalized magnetic nanoparticles in organic reactions as a magnetic nanocatalyst. This novel magnetic nanocatalyst proved to be effective and provided the products in high to excellent yield under solvent-free conditions. The structure of obtained nanoparticles was characterized by Fourier transform infrared spectrophotometry (FT-IR), field-emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA) and energy-dispersive X-ray spectroscopy (EDX). TGA result revealed that it is stable up to 200 °C for using as a catalyst in organic reactions. FE-SEM image of the synthesized nanocatalyst showed that it has nearly core-shell spherical shape and uniform size distribution with an average size about 80 nm. Moreover, the catalyst could be easily recovered by facile separation by magnetic forces and recycled for several times without significant loss of its catalytic activity. The benefits of this study are simplicity, nontoxicity, low cost, simple workup, and an environmentally benign nature.

Multifunctionalization of Unactivated Cyclic Ketones via Synergistic Catalysis of Copper and Diarylamine: Access to Cyclic α-Enaminone

A multifunctionalization of unactivated cyclic ketones via synergistic catalysis of copper and diarylamine for the direct synthesis of cyclic α-enaminone is reported for the first time. This reaction goes through oxidative α-amination, followed by a desaturation, and features mild reaction conditions, a broad substrate scope, and great functional group tolerance.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.

Organocatalytic Asymmetric Decarboxylative Amination of β-Keto Acids: Access to Optically Active α-Amino Ketones and 1,2-Amino Alcohols

An organocatalytic asymmetric decarboxylative amination reaction of β-keto acids is described. Under mild reaction conditions, a series of chiral α-amino ketones were obtained in good to high yields (up to 99%) and enantioselectivities (up to 95% ee). A chiral 1,2-amino alcohol was synthesized from the corresponding decarboxylative amination product in several steps without loss of enantioselectivity.

Enantioselective electrophilic cyanation of β-keto amides catalysed by a cinchona organocatalyst

An operationally simple protocol for the enantioselective electrophilic α-cyanation of β-keto amides catalyzed by cinchona-derived catalysts has been demonstrated.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

Chemo- and Stereoselective Transition-Metal-Free Amination of Amides with Azides

The synthesis of α-amino carbonyl/carboxyl compounds is a contemporary challenge in organic synthesis. Herein, we present a stereoselective α-amination of amides employing simple azides that proceeds under mild conditions with release of nitrogen gas. The amide is used as the limiting reagent, and through simple variation of the azide pattern, various differently substituted aminated products can be obtained. The reaction is fully chemoselective for amides even in the presence of esters or ketones and lends itself to preparation of optically enriched products.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Chemoselective room temperature E1cB N–N cleavage of oxazolidinone hydrazides from enantioselective aldehyde α-hydrazination: synthesis of (+)-1,4-dideoxyallonojirimycin

A room temperature method for hydrazide N–N cleavage is presented that completes enantioselective aldehyde α-hydrazination methodology.

Catalytic asymmetric α-amination of β-keto esters and β-keto amides with a chiral N,N′-dioxide–copper(i) complex

The highly efficient asymmetric α-amination of β-keto esters and β-keto amides was realized by using a chiral N,N′-dioxide/Cu(i) complex. Under mild reaction conditions, a series of chiral α-amino dicarbonyl compounds were obtained in good result.

Phase-transfer catalyzed enantioselective α-alkylation of α-acyloxymalonates: construction of chiral α-hydroxy quaternary stereogenic centers

The enantioselective synthesis of α-acyloxy-α-alkylmalonates was developed as an efficient method for producing chiral α-tertiary alcohols, which are potentially valuable intermediates in the synthesis of natural products and pharmaceuticals.

Iron(III)-salan complexes catalysed highly enantioselective fluorination and hydroxylation of β-keto esters and N-Boc oxindoles

Chiral iron(iii)-salan complexes catalysed highly enantioselective α-fluorination and α-hydroxylation of β-keto esters and N-Boc oxindoles to give the corresponding products in high yields and good-to-excellent ee values under mild reaction conditions.