Transition Metal‐free C−H Sulfonylation and Pyrazole Annulation Cascade for the Synthesis of 4‐Sulfonyl Pyrazoles

From Conventional to Sustainable Catalytic Approaches for Heterocycles Synthesis

Synthesis of heterocyclic compounds is fundamental for all the research area in chemistry, from drug synthesis to material science. In this framework, catalysed synthetic methods are of great interest to effective reach such important building blocks. In this review, we will report on some selected examples from the last five years, of the major improvement in the field, focusing on the most important conventional catalytic systems, such as transition metals, organocatalysts, to more sustainable ones such as photocatalysts, iodine-catalysed reaction, electrochemical reactions and green innovative methods.

Synthesis of pyrazoles from sulfonyl hydrazone and benzyl acrylate under transition-metal-free conditions

Pyrazoles as an important class of heterocyclic compounds, are widely found in pharmaceuticals and bioactive natural products. Herein we report a [3 + 2] cycloaddition reaction for the synthesis of a series of pyrazoles, with the yield up to 77%. This approach exhibits many notable features, such as convenient operating conditions, excellent functional group compatibility and readily accessible raw materials, providing an alternative route for the construction of pyrazole derivatives.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Electrochemical Synthesis of Polysubstituted Sulfonated Pyrazoles via Cascade Intermolecular Condensation, Radical-Radical Cross Coupling Sulfonylation, and Pyrazole Annulation

Electrochemical synthesis of polysubstituted sulfonated pyrazoles from enaminones and sulfonyl hydrazides was established under metal-free, exogenous-oxidant-free, and mild conditions. By judicious choice of different electrochemical reaction conditions, NH₂-functionalized enaminones or N,N-disubstituted enaminones can react with aryl/alkyl sulfonyl hydrazides to afford tetra- or trisubstituted sulfonated pyrazoles in moderate to good yields, respectively. The gram-scale electrochemical transformation demonstrated the efficiency and practicability of this synthetic strategy. In addition, the sulfonated NH-pyrazole can be obtained via the dissociation of the N-tosyl group. Mechanistic studies reveal that the electrochemical cascade reaction synthesis of polysubstituted sulfonated pyrazoles proceeded via the sequence of intermolecular condensation, radical-radical cross coupling sulfonylation, and pyrazole annulation.

Cyclization/hydrolysis of 1,5-enenitriles initiated by sulfonyl radicals in the aqueous phase in the presence of the I2/TBHP system

A novel cyclization/hydrolysis of 1,5-enenitriles for the synthesis of valuable pyrrolidine-2,4-diones in the aqueous phase using I₂ as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant is reported. In the presence of the I₂/TBHP system, sulfonyl hydrazides produce sulfonyl radicals, which undergo radical addition, intramolecular cyclization, hydrogen abstraction, and hydrolysis to give the final products. The use of the inexpensive and environmentally friendly I₂/TBHP catalytic oxidation system in the aqueous phase makes it a benign and sustainable strategy.

Asymmetric Synthesis of Structurally Sophisticated Spirocyclic Pyrano[2,3-c]pyrazole Derivatives Bearing a Chiral Quaternary Carbon Center

A carbene-catalyzed enantio- and diastereoselective [2 + 4] cycloaddition reaction is developed for quick and efficient access to structurally complex multicyclic pyrano[2,3-c]pyrazole molecules. The reaction tolerates a broad scope of substrates bearing various substitution patterns, with the multicyclic pyrano[2,3-c]pyrazole products afforded in generally good to excellent yields and optical purities. The chiral molecules obtained from this approach has found promising applications in the development of novel bacteriacides for plant protection.

Copper-catalyzed C-H/N-H annulation of enaminones and alkynyl esters for densely substituted pyrrole synthesis

Herein, the copper-catalyzed annulation of enaminones with alkynyl esters for the facile synthesis of different pyrroles with a 2,3,4,5-tetrasubstituted structure has been developed. With Cu(OAc)₂ as the only catalyst, the tunable synthesis of 2-vinyl and 2,3-dicarboxyl-functionalized pyrroles has been achieved by using terminal and internal alkynyl esters, respectively. The synthesis of 2-vinyl pyrroles represents the first example accessing 2-vinyl substituted pyrroles via direct cascade reactions involving vinylation and pyrrole ring formation.

Facile construction of C,N-disulfonated 5-amino pyrazoles through an iodine-catalyzed cascade reaction

A green and facile synthesis of previously unreported C,N-disulfonated 5-amino pyrazoles was established through an iodine-catalyzed cascade reaction.

Recent advances in transition metal-free annulation toward heterocycle diversity based on the C-N bond cleavage of enaminone platform

The enaminones and analogous stable enamines are well known as platform building blocks in organic synthesis for construction of heterocyclic compounds, especially N-heterocycles. To date, especially enaminones have been successfully...

DMSO as a C1 Source for [2 + 2 + 1] Pyrazole Ring Construction via Metal-Free Annulation with Enaminones and Hydrazines

A cascade reaction between enaminones, hydrazines, and dimethyl sulfoxide (DMSO) for the synthesis of 1,4-disubstituted pyrazoles catalyzed by molecular iodine in the presence of Selectfluor has been realized. DMSO plays a dual role as the C₁ source and the reaction medium. In addition, the synthesis of 1,3,4-trisubstituted pyrazoles using aldehydes as alternative C₁ building blocks has also been achieved.

Transition Metal-Free De Novo Synthesis of Sulfonated Pyrazoles from Sulfonyl Hydrazides, 1,3-Diketones, and Sodium Sulfinates at Room Temperature

A transition metal-free method for de novo construction of diverse sulfonated pyrazoles from readily available sulfonyl hydrazides, 1,3-diketones, and sodium sulfinates was established under mild conditions. Pyrazoles bearing two different sulfonyl groups were obtained in one step. The method features a diversity of substituents of the pyrazole products and a remarkably simple work-up.

Recent Advances in Organic Synthesis Based on N,N-Dimethyl Enaminones

Enaminones are gaining increasing interest because of their unique properties and their importance in organic synthesis as versatile building blocks. N,N-Dimethyl enaminones offer a better leaving group (a dimethylamine group) than other enaminones, and allow further elaboration via a range of facile chemical transformations. Over the past five years, there have been an increasing number of reports describing the synthetic applications of N,N-dimethyl enaminones. This review provides a comprehensive overview on the synthetic applications of N,N-dimethyl enaminones that have been reported since 2016.

1 Introduction

2 Direct C(sp2)–H α-Functionalization

2.1 Synthesis of α-Sulfenylated N,N-Dimethyl Enaminones

2.2 Synthesis of α-Thiocyanated N,N-Dimethyl Enaminones

2.3 Synthesis of α-Acyloxylated N,N-Dimethyl Enaminones

3 Functionalization Reactions via C=C Double Bond Cleavage

3.1 Synthesis of Functionalized Methyl Ketones

3.2 Synthesis of α-Ketoamides, α-Ketoesters and 1,2-Diketones

3.3 Synthesis of N-Sulfonyl Amidines

4 Construction of All-Carbon Aromatic Scaffolds

4.1 Synthesis of Benzaldehydes

4.2 Synthesis of the Naphthalenes

5 Construction of Heterocyclic Scaffolds

5.1 Synthesis of Five-Membered Heterocycles

5.2 Synthesis of Six-Membered Heterocycles

5.3 Synthesis of Quinolines

5.4 Synthesis of Functionalized Chromones

5.5 Synthesis of Other Fused Polycyclic Heterocycles

6 Conclusions and Perspectives

Transition-metal-free sulfonylations of methylthiolated alkynones to synthesize 3-sulfonylated thioflavones

A set of transition-metal-free NaI/TBHP-mediated sulfonylation cyclization reactions of methylthiolated alkynones with sulfonyl hydrazides was developed, by which various 3-sulfonylated thioflavones were prepared under mild reaction conditions.