Homocoupling Reactions of Azoles and Their Applications in Coordination Chemistry

Abstract

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.

Simultaneous Denitrative C–C Bond Formation and Construction of Pyrazole Ring Leading to 1,1'-Diphenyl-4,4'-bipyrazole

“…Bipyrazoles are often found in biological active compounds and used as a ligand constructing MOF. 7 Accordingly, numerous synthetic methods have been reported. 5,[7][8][9][10] The main methods include transition-metal-catalyzed coupling reaction 5,8 and halogenating reagent-promoted coupling reaction of pyrazoles, 9 or construction of pyrazole rings using polyfunctionalized substrate.…”

“…7 Accordingly, numerous synthetic methods have been reported. 5,[7][8][9][10] The main methods include transition-metal-catalyzed coupling reaction 5,8 and halogenating reagent-promoted coupling reaction of pyrazoles, 9 or construction of pyrazole rings using polyfunctionalized substrate. 10 To the contrary, the present reaction includes a construction of pyrazole ring and a coupling reaction accompanied by denitrative C-C bond formation under neutral conditions, which is a hitherto unknown reaction type.…”

“…In other words, nitronic acid appears to be the actual active species in this homo-coupling (Scheme 4). Nitroalkanes (7) or (8) tautomerize to nitronic acid (8), forming a C-C bond to afford dimeric product (9) (Scheme 4). Nitroamine, nitrosamine, and hydrazine are thought to eliminate more readily than that of nitrous acid or amine to release the strain by steric repulsion of amino and nitro groups, yielding bipyrazole (4a).…”

Synthesis, Crystal Structure, and Luminescence of Cadmium(II) and Silver(I) Coordination Polymers Based on 1,3-Bis(1,2,4-triazol-1-yl)adamantane

“…1,2,4-Triazole is a versatile nitrogen-donor ligand capable of forming strong coordination bonds with various metal ions [8,9]. Derivatives of 1,2,4-triazole are known as anticancer, antiproliferative, anti-inflammatory, anticonvulsant, analgesic, antioxidant agents, and possess activity against wide, but specific spectrum of living organisms [10].…”

Special Issue “Recent Advances in the Synthesis, Functionalization and Applications of Pyrazole-Type Compounds”

“…Lee et al summarized the recent advances in homocoupling reactions of pyrazoles and other azoles (imidazoles, triazoles and tetrazoles) to highlight the utility of homocoupling reactions in the synthesis of symmetric bi-heteroaryl systems compared with traditional synthesis. Metal-free reactions and transition-metal catalyzed homocoupling reactions were discussed with reaction mechanisms in detail [ 14 ]. In the field of energetic materials (EMs), the nitrated pyrazoles have attracted wide attention due to their high heat of formation, high density, tailored thermal stability, and detonation performance, having good applications in explosives, propellants, and pyrotechnics.…”