Synthesis of tetrasubstituted pyrazoles containing pyridinyl substituents

Homocoupling Reactions of Azoles and Their Applications in Coordination Chemistry

Pyrazole, a member of the structural class of azoles, exhibits molecular properties of interest in pharmaceuticals and materials chemistry, owing to the two adjacent nitrogen atoms in the five-membered ring system. The weakly basic nitrogen atoms of deprotonated pyrazoles have been applied in coordination chemistry, particularly to access coordination polymers and metal-organic frameworks, and homocoupling reactions can in principle provide facile access to bipyrazole ligands. In this context, we summarize recent advances in homocoupling reactions of pyrazoles and other types of azoles (imidazoles, triazoles and tetrazoles) to highlight the utility of homocoupling reactions in synthesizing symmetric bi-heteroaryl systems compared with traditional synthesis. Metal-free reactions and transition-metal catalyzed homocoupling reactions are discussed with reaction mechanisms in detail.

Introducing chirality in halogenated 3-arylsydnones and their corresponding 1-arylpyrazoles obtained by 1,3-dipolar cycloaddition

New 1-arylpyrazoles substituted with halogen atoms (Br, I) were synthesized from the corresponding sydnones by 1,3-dipolar cycloaddition. By introduction of a prochiral group such as isopropyl, in the ortho position of the benzene ring, in the starting phenylglycine 1 the rotamers caused by the hindered rotation between the phenyl and the heterocyclic ring were detected by NMR spectroscopy for 1-arylpyrazoles and for the first time for 3-arylsydnones. The N-nitrosophenylglycines present E–Z stereoisomerism due to the partial C–N double bond character. All the new compounds were structurally characterized by NMR spectroscopy and confirmed by X-ray crystallography. The crystal structures of N-nitrosophenylglycine 2c and of the sydnone 3c present similar Br⋯Br type II halogen contacts.

New 1-arylpyrazoles substituted with halogen atoms (Br, I) were synthesized from the corresponding sydnones by 1,3-dipolar cycloaddition.

Domino C-H Sulfonylation and Pyrazole Annulation for Fully Substituted Pyrazole Synthesis in Water Using Hydrophilic Enaminones

The cascade reactions between NH₂-functionalized enaminones and sulfonyl hydrazines have been developed for the synthesis of fully substituted pyrazoles. By making use of the hydrophilic primary amino group in the enaminones, the reactions proceed well in the medium of pure water in the presence of molecular iodine, TBHP, and NaHCO₃ via cascade C-H sulfonylation and pyrazole annulation. The cleavage of the C-N bond in enaminones is confirmed by the experiment using ¹⁵ N-labeled enaminone.