Copper(I)/Copper(II)-Assisted Tandem Catalysis: The Case Study of Ullmann/Chan-Evans-Lam N1,N3-Diarylation of 3-Aminopyrazole

Recent developments in selective N-arylation of azoles

Transition-metal based carbon-heteroatom (C-X) bond formation has attracted the attention of synthetic chemists over the past few years because the resultant aryl/heteroaryl motifs are important substructures in many natural products, pharmaceuticals, etc. Several efficient protocols such as Buchwald-Hartwig amination, Ullmann coupling, Chan-Lam coupling and metal-free approaches have proved beneficial in C-X bond formation. Selective arylation of one hetero-centre over other centres without protection/deprotection thus allowing minimum synthetic manipulation has been achieved for several substrates using these protocols. Azoles are one such novel five-membered heterocyclic core with huge pharmaceutical applications. Though N-arylation on azole-bearing analogues has been extensively practised, selective N-arylation either on one N-centre or the exocyclic N-site of the azole ring in competition with other hetero-centres in the framework has been recently explored for azole-carrying systems. Thus, this review would focus on recent advances in chemo- and regio-selective N-arylation (either on one N-centre or the exocyclic N-site of the azole ring) on azole-containing frameworks.

Copper-catalysed N-arylation of 5-aminopyrazoles: a simple route to pyrazolo[3,4-b]indoles

A copper-catalysed intramolecular N-arylation of 5-aminopyrazoles is demonstrated for the first time. Highly substituted pyrazolo[3,4-b]indoles are synthesized. In particular, the indole core is decorated with halogens and alkyl and methoxy groups. Furthermore, a selective N-arylation of unsymmetrical diaryl bromide containing pyrazoles is exemplified, resulting in valuable pyrazolo[1,5-a]benzimidazoles.

C4-C5 fused pyrazol-3-amines: when the degree of unsaturation and electronic characteristics of the fused ring controls regioselectivity in Ullmann and acylation reactions

Pyrazol-3-amine is a scaffold present in a large number of compounds with a wide range of biological activities and, in many cases, the heterocycle is C4-C5 fused to a second ring. Among the different reactions used for the decoration of the pyrazole ring, Ullmann and acylation have been widely applied. However, there is some confusion in the literature regarding the regioselectivity of such reactions (substitution at N1 or N2 of the pyrazole ring) and no predictive rule has been so far established. As a part of our work on 3-amino-pyrazolo[3,4-b]pyridones 13, we have studied the regioselectivity of such reactions in different C4-C5 fused pyrazol-3-amines. As a rule of thumb, the Ullmann and acylation reactions take place, predominantly, at the NH and non-protonated nitrogen atom of the pyrazole ring respectively, of the most stable initial tautomer (1H- or 2H-pyrazole), which can be easily predicted by using DFT calculations.

Zirconium complexes supported by a ferrocene-based ligand as redox switches for hydroamination reactions

The synthesis of (thiolfan*)Zr(NEt2)2 (thiolfan* = 1,1'-bis(2,4-di-tert-butyl-6-thiophenoxy)ferrocene) and its catalytic activity for intramolecular hydroamination are reported. In situ oxidation and reduction of the metal complex results in reactivity towards different substrates. The reduced form of (thiolfan*)Zr(NEt2)2 catalyzes hydroamination reactions of primary aminoalkenes, whereas the oxidized form catalyzes hydroamination reactions of secondary aminoalkenes.

Nickel-catalyzed Chan–Lam cross-coupling: chemoselective N-arylation of 2-aminobenzimidazoles

A complementary set of Ni- and Cu-based catalyst systems for the chemoselective N-arylation of 2-aminobenzimidazoles have been developed.