One-Pot Synthesis of CF3-Substituted Pyrazolines/Pyrazoles from Electron-Deficient Alkenes/Alkynes and CF3CHN2Generated in situ: Optimized Synthesis of Tris(trifluoromethyl)pyrazole

Multicomponent syntheses of pyrazoles via (3 + 2)-cyclocondensation and (3 + 2)-cycloaddition key steps

Pyrazoles are rarely found in nature but are traditionally used in the agrochemical and pharmaceutical industries, while other areas of use are also actively developing. However, they have also found numerous other applications. The search for new and efficient syntheses of these heterocycles is therefore highly relevant. The modular concept of multicomponent reactions (MCR) has paved a broad alley to heteroaromatics. The advantages over traditional methods are the broader scope and increased efficiency of these reactions. In particular, traditional multistep syntheses of pyrazoles have considerably been extended by MCR. Progress has been made in the cyclocondensation of 1,3-dielectrophiles that are generated in situ. Limitations in the regioselectivity of cyclocondensation with 1,3-dicarbonyls were overcome by the addition-cyclocondensation of α,β-unsaturated ketones. Embedding 1,3-dipolar cycloadditions into a one-pot process has additionally been developed for concise syntheses of pyrazoles. The MCR strategy also allows for concatenating classical condensation-based methodology with modern cross-coupling and radical chemistry, as well as providing versatile synthetic approaches to pyrazoles. This overview summarizes the most important MCR syntheses of pyrazoles based on ring-forming sequences in a flashlight fashion.

In situ studies of reversible solid-gas reactions of ethylene responsive silver pyrazolates

Solid-gas reactions and in situ powder X-ray diffraction investigations of trinuclear silver complexes {[3,4,5-(CF3)3Pz]Ag}3 and {[4-Br-3,5-(CF3)2Pz]Ag}3 supported by highly fluorinated pyrazolates reveal that they undergo intricate ethylene-triggered structural transformations in the solid-state producing dinuclear silver-ethylene adducts. Despite the complexity, the chemistry is reversible producing precursor trimers with the loss of ethylene. Less reactive {[3,5-(CF3)2Pz]Ag}3 under ethylene pressure and low-temperature conditions stops at an unusual silver-ethylene complex in the trinuclear state, which could serve as a model for intermediates likely present in more common trimer-dimer reorganizations described above. Complete structural data of three novel silver-ethylene complexes are presented together with a thorough computational analysis of the mechanism.

Chemoselective Reactions of Functionalized Sulfonyl Halides

Chemoselective transformations of functionalized sulfonyl fluorides and chlorides are surveyed comprehensively. It is shown that sulfonyl fluorides provide an excellent selectivity control in their reactions. Thus, numerous conditions are tolerated by the SO2 F group - from amide and ester formation to directed ortho-lithiation and transition-metal-catalyzed cross-couplings. Meanwhile, sulfur (VI) fluoride exchange (SuFEx) is also compatible with numerous functional groups, thus confirming its title of "another click reaction". On the contrary, with a few exceptions, most transformations of functionalized sulfonyl chlorides typically occur at the SO2 Cl moiety.

One-Pot (3 + 2) Cycloaddition-Isomerization-Oxidation of 2,2,2-Trifluorodiazoethane and Styryl Derivatives

A facile access to 5-aryl-3-trifluoromethylpyrazoles has been developed by a one-pot (3 + 2) cycloaddition-isomerization-oxidation sequence employing 2,2,2-trifluorodiazoethane and styryl derivatives. A broad variety of functional groups and good yields are achieved under mild conditions. Additionally, the functionalization of 3-trifluoromethylpyrazoles was studied. DFT calculations of the cycloaddition transition state energies are consistent with the experimentally observed reactivity.

Towards Axially Chiral Pyrazole-Based Phosphorus Scaffolds by Dipeptide-Phosphonium Salt Catalysis

Given the comparatively lower rotational barriers, the catalytic asymmetric construction of axially chiral biaryl structures, especially those containing a five-membered heterocycle, still remains a challenge. Herein, we described a general and modular protocol to access atropisomeric arylpyrazole scaffolds containing a phosphorus unit by a dipeptide phosphonium salt catalyzed reaction involving an oxidative central-to-axial chirality conversion. This reaction features excellent yields and enantioselectivities, broad substrate scope, and a low catalyst loading, delivering axially chiral phosphine compounds.

Iodine-Mediated Coupling of 2,2,2-Trifluorodiazoethane and Alkynes To Access Bistrifluoromethylated 1,3,5-Trienes

Multi-component reaction of higher degree ultilization of diazo molecules for polyene formation is highly intriuging but still underexplored. We present herein an unprecedented coupling of 2,2,2-trifluorodiazoethane and aryl alkynes mediated by iodine under visible light. The multi-component reaction involving two diazo units and two alkyne units provides a straightforward and step-economic access to bistrifluoromethylated 1,3,5-trienes in high stereoselectivity by creation of three C═C bonds in a single step under mild conditions.

Base-Promoted (3 + 2) Cycloaddition of Trifluoroacetohydrazonoyl Chlorides with Imidates En Route to Trifluoromethyl-1,2,4-Triazoles

A base-mediated (3 + 2) cycloaddition of trifluoroacetohydrazonoyl chlorides with imidates for the construction of 3-trifluoromethyl-1,2,4-triazoles has been described. This reaction is characterized by readily starting materials, simple reaction conditions, good yields, a broad substrate scope, and excellent regioselectivity. The utility of this protocol has been validated by the synthesis of a drug-like molecule.

Solvent-controlled base-free synthesis of bis(trifluoromethyl)-cyclopropanes and -pyrazolines via cycloaddition of 2-trifluoromethyl-1,3-enynes with 2,2,2-trifluorodiazoethane

A highly efficient solvent-controlled synthesis of bis(trifluoromethyl)cyclopropanes and bis(trifluoromethyl)pyrazolines via a [2 + 1] or [3 + 2] cycloaddition reaction of 2-trifluoromethyl-1,3-conjugated enynes with CF₃CHN₂ was developed. The reactions of 2-trifluoromethyl-1,3-conjugated enynes with CF₃CHN₂ proceeded smoothly under transition-metal and base-free conditions, affording the expected cycloaddition products in good to excellent yields. When DMAc (N,N-dimethylacetamide) was used as the solvent, bis(trifluoromethyl)pyrazolines were obtained; however, in contrast, bis(trifluoromethyl)cyclopropanes were formed by changing the solvent from DMAc to DCE (1,2-dichloroethane).

SuFExable NH-Pyrazoles via 1,3-Dipolar Cycloadditions of Diazo Compounds with Bromoethenylsulfonyl Fluoride

"Click" reactions have transformed the molecular sciences. Augmenting cycloaddition reactions, sulfur(VI) fluoride exchange (SuFEx) chemistry has diversified the landscape of molecular assembly. Herein, we report a facile strategy to access SuFExable NH-pyrazoles via strain and catalyst-free 1,3-dipolar cycloadditions of stabilized diazo compounds under mild conditions. Subsequent SuFEx proceeds efficiently with various N- and O-nucleophiles. Access to SuFExable NH-pyrazoles─a class of compounds containing two common pharmacophores─enables future opportunities within drug discovery, chemical biology, materials chemistry, and related fields.

Metal-free [2 + 1 + 3] Cycloaddition of Trifluoroacetaldehyde N-Sulfonylhydrazones with Hexahydro-1,3,5-triazines Leading to Trifluoromethylated 2,3,4,5-Tetrahydro-1,2,4-triazines

A transition-metal-free [2 + 1 + 3] cycloaddition of trifluoroacetaldehyde N-sulfonylhydrazone and hexahydro-1,3,5-triazine was described. This operationally simple protocol provides a general synthesis of diverse trifluoromethylated 2,3,4,5-tetrahydro-1,2,4-triazines in 81-97% yield with a broad substrate scope, including aryl, benzyl, and alkyl hexahydro-1,3,5-triazine.

Zinc-Enabled Annulation of Trifluorodiazoethane with 2H-Azirines to Construct Trifluoromethyl Pyrazolines, Pyrazoles, and Pyridazines

A diethylzinc-promoted unconventional annulation reaction of 2,2,2-trifluorodiazoethane with 2H-azirines is described. This transformation involves two [3 + 2] cycloaddition steps and one dinitrogen extrusion process in one pot, thus giving a broad array of 3-trifluoromethyl pyrazolines in good yields with excellent diastereoselectivities. Further transformations provide facile access to 3-trifluoromethyl pyrazoles and 3,5-ditrifluoromethyl pyridazines with good efficiency.

Silver-Catalyzed Direct Synthesis of Trifluoromethylated Enaminopyridines and Isoquinolinones Employing Trifluorodiazoethane

This Letter reports a Ag-catalyzed three-component approach for the N-alkenylation of 2-aminopyridines employing aldehydes and trifluorodiazoethane. Unlike the known reactions of trifluorodiazoethane with imines, which generate Mannich adducts, aziridines, or triazolines depending on the substrates and conditions, this reaction, after Mannich addition, proceeds via a carbene formation and 1,2-aryl migration sequence to afford (E)-enaminopyridines. This surprising selectivity, which is effective for a wide range of aldehydes and 2-aminopyridines, has been subsequently explored to access trifluoromethylated isoquinolinones.

Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenes

A general catalytic methodology for the synthesis of pyrazolines from α-diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α-diazo compounds promoted by the tert-butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon-carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron-deficient alkenes in traditional dipolar [3+2]-cycloaddition of α-diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds.

In Situ Generation of Unstable Difluoromethylphosphonate-Containing Diazoalkanes and Their Use in [3 + 2] Cycloaddition Reactions with Vinyl Sulfones

A [3 + 2] cycloaddition reaction of unstable difluoromethylphosphonate-containing diazoalkanes with vinyl sulfones under simple reaction conditions is developed, which provides an efficient route toward functionalized fluorinated pyrazolines derivatives in good chemical yields. The difluoro diazoalkanes are generated in situ using t-BuONO for the diazotization of (β-amino-α,α-difluoroethyl)phosphonates, and their stabilities and reactivities were carefully investigated.

Regioselective Synthesis of 5-Trifluoromethylpyrazoles by [3 + 2] Cycloaddition of Nitrile Imines and 2-Bromo-3,3,3-trifluoropropene

A general and practical method for the synthesis of 5-trifluoromethylpyrazoles is reported that occurs by the coupling of hydrazonyl chlorides with environmentally friendly and large-tonnage industrial feedstock 2-bromo-3,3,3-trifluoropropene (BTP). This exclusively regioselective [3 + 2] cycloaddition of nitrile imines and with BTP is catalyst-free and operationally simple and features mild conditions, high yields, gram-scalable, a broad substrate scope, and valuable functional group tolerance. Significantly, our method has been applied for the synthesis of the key intermediate of an active agonist of sphingosine 1-phosphate receptor.

But-3-ene-1,3-disulfonyl difluoride (BDF): a highly selective SuFEx clickable hub for the quick assembly of sultam-containing aliphatic sulfonyl fluorides

A versatile selectively addressable SuFEx click chemistry hub, but-3-ene-1,3-disulfonyl difluoride (BDF) was designed and synthesized through head-to-tail dimerization of ethenesulfonyl fluoride (ESF). BDF possesses three active sites to selectively participate in the construction of 4-membered, 5-membered and 6-membered cyclic sulfonamide (sultams) bearing aliphatic sulfonyl fluoride moieties for further manipulations in a fast, efficient and practical manner.

Metal-free [3+2] cycloaddition of glycosyl olefinic ester with in situ generated CF3CHN2: Access to CF3-substituted pyrazoline glycoconjugates

An efficient [3 + 2] cycloaddition of glycosyl olefinic ester with in situ generated CF₃CHN₂ for the syntheses of CF₃-substituted pyrazoline glycoconjugate has been developed. This mild, one-pot reaction condition avoiding the use of metallic catalyst and additive will be useful in the pharmaceutical industry. This reaction features are the broad substrate scope, good functional group tolerance with good to high yields.

Alkynes and Nitrogen Compounds: Useful Substrates for the Synthesis of Pyrazoles

The easy preparation and functionalization of pyrazoles associated with their innumerable biological properties have made this class of N-heterocycles very attractive for the development of new synthetic routes and applications. The cyclization reactions of alkynes and nitrogen compounds represent a powerful tool for the preparation of pyrazoles. This Review covers the recent advances in the preparation of pyrazoles by reacting alkynes and nitrogen compounds under transition-metal-catalyzed or metal-free conditions.

Ag-Catalyzed Trifluoromethylative Ring Expansion of Isatins and Isatin Ketimines with Trifluorodiazoethane

A general method for the construction of trifluoromethylated 2-quinolinones has been established herein by using a trifluoromethylative ring expansion of isatin with trifluorodiazoethane. The strategy provides a platform for the rapid synthesis of a wide range of substituted 3-hydroxy-4-trifluoromethyl-2-quinolinones. This operationally simple and robust Ag-catalyzed protocol successfully transforms isatin ketimines to 3-amino-4-trifluoromethylquinolinones in excellent yields. The utility of this novel method is further illustrated by the conversion of the products into various synthetically and medicinally relevant molecules.

A rhodium-catalyzed three-component reaction of arylisocyanides, trifluorodiazoethane, and activated methylene isocyanides or azomethine ylides: an efficient synthesis of trifluoroethyl-substituted imidazoles

A novel method for the synthesis of trifluoroethyl-substituted imidazoles is reported via a rhodium-catalyzed three-component reaction of isocyanides, 2,2,2-trifluorodiazoethane and activated methylene isocyanides or azomethine ylides.

Regiodivergent Synthesis of Pyrazolines with a Quaternary Carbon Center via Cycloaddition of Diazoesters to N-Purine-Substituted Allenes

Diversity-oriented synthesis of pyrazoline derivatives that contain a quaternary carbon center has been achieved via the 1,3-dipolar cycloaddition between N-purine-substituted allenes and α-alkyl/aryl diazoesters. Using Pd₂(dba)₃ as a catalyst, only 1-pyrazoline derivatives are produced in a regioselective manner. When DPPB is used as a catalyst, diverse 1-pyrazolines and 2-pyrazolines are obtained in moderate to good total yields.