Asymmetric Friedel–Crafts Alkylation of Indoles with Trifluoromethyl Pyruvate Catalyzed by a Dinuclear Zinc Catalyst

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

Electron-Deficient Fluoroarene-Mediated Synthesis of Trifluoromethyl Ketones from Carboxylic Acids

Fluoroarene-mediated trifluoromethylation of carboxylic acids for the synthesis of trifluoromethyl ketones is disclosed. The fluoroarene activates the acid group and generates the fluoride source in situ for the trifluoromethylation reaction. The present protocol is safe and metal-free, operates under mild reaction conditions, and does not require any external additives to generate trifluoromethyl anion. The current transformation provides good functional group tolerance and also delivers 92% and 88% yields of trifluoromethyl ketones in batch and continuous flow, respectively.

Zinc-Catalyzed Enantioselective [3 + 3] Annulation for Synthesis of Chiral Spiro[indoline-3,4'-thiopyrano[2,3-b]indole] Derivatives

With a dinuclear zinc-ProPhenol complex as a catalyst, an efficient and novel [3 + 3] annulation of indoline-2-thiones and isatylidene malononitriles has been successfully developed via the Brønsted base and Lewis acid cooperative activation model. This practical methodology gives access to a broad range of chiral spiro[indoline-3,4'-thiopyrano[2,3-b]indole] derivatives in good yields with excellent levels of enantioselectivities (up to 88% yield and 99% ee).

Design, Catalyst-Free Synthesis of New Novel α-Trifluoromethylated Tertiary Alcohols Bearing Coumarins as Potential Antifungal Agents

A new method for the synthesis of α-trifluoromethylated tertiary alcohols bearing coumarins is described. The reaction of 3-(trifluoroacetyl)coumarin and pyrrole provided the target compounds with high yields under catalyst-free, mild conditions. The crystal structure of compound 3fa was investigated by X-ray diffraction analysis. The biological activities, such as in vitro antifungal activity of the α-trifluoromethylated tertiary alcohols against Fusarium graminearum, Fusarium oxysporum, Fusarium moniliforme, Rhizoctonia solani Kuhn, and Phytophthora parasitica var nicotianae, were investigated. The bioassay results indicated that compounds 3ad, 3gd, and 3hd showed broad-spectrum antifungal activity in vitro. Compound 3cd exhibited excellent fungicidal activity against Rhizoctonia solani Kuhn, with an EC₅₀ value of 10.9 μg/mL, which was comparable to that of commercial fungicidal triadimefon (EC₅₀ = 6.1 μg/mL). Furthermore, molecular docking study suggested that 3cd had high binding affinities with 1W9U, like argifin.

Recent Advances in the Catalytic Asymmetric Friedel-Crafts Reactions of Indoles

Functionalized chiral indole derivatives are privileged and versatile organic frameworks encountered in numerous pharmaceutically active agents and biologically active natural products. The catalytic asymmetric Friedel-Crafts reaction of indoles, catalyzed by chiral metal complexes or chiral organocatalysts, is one of the most powerful and atom-economical approaches to access optically active indole derivatives. Consequently, a wide range of electrophilic partners including α,β-unsaturated ketones, esters, amides, imines, β,γ-unsaturated α-keto- and α-ketiminoesters, ketimines, nitroalkenes, and many others have been successfully employed to achieve a plethora of functionalized chiral indole moieties. In particular, strategies for C-H functionalization in the phenyl of indoles require incorporation of a directing or blocking group in the phenyl or azole ring of indole. The discovery of chiral catalysts which can control enantiodiscrimination has gained a great deal of attention in recent years. This review will provide an updated account on the application of the asymmetric Friedel-Crafts reaction of indoles in the synthesis of diverse chiral indole derivatives, covering the timeframe from 2011 to today.

Dinuclear Zinc Catalyzed Enantioselective Dearomatization [3+2] Annulation of 2-Nitrobenzofurans and 2-Nitrobenzothiophenes

The application of dinuclear zinc catalysts in a dearomatization reaction has been developed. Catalytic asymmetric dearomatization [3+2] annulations of 2-nitrobenzofurans or 2-nitrobenzothiophenes with CF₃ -containing N-unprotected isatin-derived azomethine ylides catalyzed by dinuclear zinc catalysts are realized with excellent diastereomer ratios (dr) of >20 : 1 and enantiomeric excess (ee) of up to 99 %. This protocol provides a practical, straightforward access to structurally diverse pyrrolidinyl spirooxindoles containing a 2,3-fused-dihydrobenzofuran (or dihydrobenzothiphene) moiety, and four contiguous stereocenters. Reactions can be performed on a gram scale. The absolute configuration of products is confirmed by X-ray single crystal structure analysis, and a possible mechanism is proposed.

Dinuclear zinc-catalyzed enantioselective formal [3 + 2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with low reactivity aurone derivatives

Highly enantioselective formal [3 + 2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with aurone derivatives of low reactivity using chiral dinuclear zinc catalysts has been developed via a Brønsted base and Lewis acid cooperative activation model. These transformations involving a domino Michael/Mannich reaction sequence led to efficient construction of a range of chiral spiro[benzofuran-pyrrolidine] scaffolds bearing three biologically relevant heterocyclic moieties and two adjacent spiro quaternary stereocenters in high yields (up to 95%) and with good enantioselectivities (up to 99% ee).

Dinuclear zinc-catalyzed asymmetric Friedel-Crafts alkylation/cyclization of 3-aminophenols with α,α-dicyanoolefins

An enantioselective Friedel-Crafts alkylation/cyclization tandem reaction of 3-aminophenols with α,α-dicyanoolefins has been performed successfully using a chiral dinuclear zinc catalyst, leading to a range of chiral 2-amino-4H-chromenes (up to 98% yield and >99% ee). To the best of our knowledge, this is the first asymmetric example of the dinuclear zinc-catalysed functionalization of aromatic C(sp²)-H bonds.

Recent Advances in Selected Asymmetric Reactions Promoted by Chiral Catalysts: Cyclopropanations, Friedel–Crafts, Mannich, Michael and Other Zinc-Mediated Processes—An Update

The main purpose of this review article is to present selected asymmetric synthesis reactions in which chemical and stereochemical outcomes are dependent on the use of an appropriate chiral catalyst. Optically pure or enantiomerically enriched products of such transformations may find further applications in various fields. Among an extremely wide variety of asymmetric reactions catalyzed by chiral systems, we are interested in: asymmetric cyclopropanation, Friedel–Crafts reaction, Mannich and Michael reaction, and other stereoselective processes conducted in the presence of zinc ions. This paper describes the achievements of the above-mentioned asymmetric transformations in the last three years. The choice of reactions is related to the research that has been carried out in our laboratory for many years.

Trifluoromethylation of Benzoic Acids: An Access to Aryl Trifluoromethyl Ketones

The trifluoromethylation of benzoic acids with TMSCF₃ was achieved through nucleophilic substitution with the use of anhydrides as an in situ activating reagent. Under the reaction conditions, a wide range of carboxylic acids including the bioactive ones worked well, thus providing a facile and efficient method for preparing aryl trifluoromethyl ketones from the readily available starting materials.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

Enantioselective synthesis of indanone spiro-isochromanone derivatives via a dinuclear zinc-catalyzed Michael/transesterification tandem reaction

An enantioselective Michael/transesterification tandem reaction of α-hydroxy indanones with ortho-ester chalcones was realized using dinuclear zinc catalysts. A series of enantiomerically pure spiro[indanone-2,3'-isochromane-1-one] derivatives were obtained in good yields with excellent stereoselectivities (up to >20 : 1 dr, up to >99% ee). This protocol could be conducted on a gram scale without affecting its stereoselectivities. In addition, the absolute stereochemistry of the products was determined by X-ray crystallographic analysis of 3ac, and a positive nonlinear effect was observed. Finally, a possible catalytic cycle was proposed to explain the origin of the enantioselectivity.

Reaction of indoles with aromatic fluoromethyl ketones: an efficient synthesis of trifluoromethyl(indolyl)phenylmethanols using K2CO3/n-Bu4PBr in water

A new, mild and efficient protocol for the synthesis of trifluoromethyl(indolyl)phenylmethanols by the reaction of indoles with a variety of aromatic fluoromethyl ketones in the presence of K₂CO₃ (15 mol %) and n-Bu₄PBr (15 mol %) in water. The desired products were obtained in good to excellent yields without requiring a column chromatographic purification. The reusability of the catalytic system and large-scale synthesis of indolyl(phenyl)methanols, which would further transform into biological active indole-derived compounds, are further advantages of this protocol.

Brønsted Base and Lewis Acid Cooperatively Catalyzed Asymmetric exo'-Selective [3 + 2] Cycloaddition of Trifluoromethylated Azomethine Ylides and Methyleneindolinones

A Brønsted base and Lewis acid cooperatively catalyzed 1,3-dipolar cycloaddition is reported through chiral dinuclear zinc catalysts. An asymmetric exo'-selective [3 + 2] cycloaddition of CF₃-containing N-unprotected isatin-derived azomethine ylides is realized. In the presence of 10 mol % of catalyst, azomethine ylides react efficiently with methyleneindolinones, giving a series of trifluoromethyl-substituted 2,3-pyrrolidinyl dispirooxindoles with highly enantio- (up to 99% ee) and exo'-selectivity (>20:1 dr). Up to four contiguous stereogenic centers, including two adjacent spiro quaternary stereocenters, are constructed in one step.

Brønsted Acid-Catalyzed Asymmetric Friedel-Crafts Alkylation of Indoles with Benzothiazole-Bearing Trifluoromethyl Ketone Hydrates

An efficient Brønsted acid-catalyzed asymmetric Friedel-Crafts alkylation of indoles with benzothiazole-bearing trifluoromethyl ketone hydrates as electrophiles has been developed. The mild organocatalytic reactions proceeded well with low catalyst loading to afford a range of enantioenriched α-trifluoromethyl tertiary alcohols containing both benzothiazole and indole rings with excellent yields and enantioselectivities.

Azetidine-Derived Dinuclear Zinc-Catalyzed Asymmetric Conjugate Addition of Bioactive Heterocycles to β,γ-Unsaturated α-Keto esters

A general AzePhenol dinuclear zinc catalytic system has been successfully developed and applied into the asymmetric Michael addition of 4-hydroxyl pyrones and 4-hydroxycoumarins to β,γ-unsaturated α-keto esters. Excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) are obtained for a wide range of substrates under mild conditions in the absence of additives. This bimetallic catalytic approach represents a new and effective asymmetric synthetic protocol to access a variety of bioactive compounds with pharmacological interest. The possible mechanism is proposed to explain the origin of the asymmetric induction.

Dinuclear zinc synergistic catalytic asymmetric phospha-Michael/Michael cascade reaction: synthesis of 1,2,3-trisubstituted indanes bearing phosphoryl groups

A new dinuclear zinc synergistic catalytic asymmetric phospha-Michael/Michael cascade reaction of o-dienones and dialkyl phosphates is reported. This method has been proven to be general and efficient for the formation of a range of chiral 1,2,3-trisubstituted indane compounds containing phosphorus groups in good yields (up to 92%) with excellent stereoselectivities (up to >99% ee and up to >99 : 1 dr). The relative configuration of the product was identified as having a trans,trans substitution pattern via two-dimensional (2D) nuclear Overhauser effect spectroscopy 1H-1H NMR experiments. A possible mechanism was proposed.

Dinuclear Zinc-AzePhenol Catalyzed Asymmetric Aza-Henry Reaction of N-Boc Imines and Nitroalkanes under Ambient Conditions

The asymmetric aza-Henry reaction of N-Boc imines and nitroalkanes was realized in the presence of 10 mol % dinuclear zinc-AzePhenol catalysts under ambient conditions. A variety of nitroamines were obtained in good yields (up to 97%) with excellent enantioselectivities (up to 99% ee) and high diasteroselectivity (up to 14:1 dr). Our protocol combined the operational simplicity and mild reaction conditions, thus making the process amenable for technical applications.

Dinuclear zinc cooperative catalytic three-component reactions for highly enantioselective synthesis of 3,3′-dihydrofuran spirooxindoles

A new dinuclear zinc cooperative catalytic enantioselective three-component reaction via a domino Knoevenagel/Michael/cyclization sequence has been developed.