Oxidative conversion of N-substituted 3-aminopyrazoles to azopyrazoles using electrogenerated NaOCl as the mediator

Abstract

The possibility of electrooxidative one-stage (one-pot) and two-stage conversion of N-alkylated aminopyrazoles to azopyrazoles involving electrogenerated NaOCl as the mediator has been studied for the first time. It has been found that the process involving NaOCl generation at the 1 st stage and its reaction with an aminoazole at the 2 nd stage occurs much more efficiently. If the starting aminopyrazole has no substituent at ring position 4 (1-methyl-3-amino-1H-pyrazole), the process results in the generation of azo-and 4,4'-dichloroazopyrazoles. It has been shown with 3-amino-1,4-dimethyl-1H-pyrazole as an example that under similar conditions, 4-substituted aminopyrazoles selectively give the corresponding azo derivatives.

Electrooxidation Is a Promising Approach to Functionalization of Pyrazole-Type Compounds

“…The use of such redox-mediators is a trend in modern electroorganic chemistry [ 18 ], since it allows the processes to be carried out under milder conditions, increasing their efficiency and selectivity. This Section describes the original approaches to the synthesis of azopyrazoles using electrogenerated redox mediators NiO(OH) [ 160 , 161 , 162 ] and Br 2 [ 163 ], or electrogenerated hypohalites as oxidants [ 164 , 165 ].…”

“…It oxidizes aminopyrazoles ( Az–NH 2 ) to azopyrazoles ( Az–N = N–Az ) and forms Ni(OH) 2 , after which the cycle repeats [ 80 , 161 ]. In Approach B, the metal-free oxidant is Br 2 [ 164 ], which is effectively electro(re)generated on the ruthenium–titanium oxide anode (RTOA).…”

“…In addition, special voltammetric tests showed that an increase in the Ni(OH) 2 peak ( Figure 3 , A, peak A 1 , E p ox = 0.46 V) or a decrease in the Br 2 peak ( Figure 3 B, peak B 2 , E p red = 0.69 V) after the adding of aminopyrazole is proportional to the process efficiency [ 162 , 164 ].…”

“…Two-stage approaches ( Scheme 19 ) include preliminary electrogeneration of hypogalites followed by addition of aminopyrazoles [ 164 , 165 ]. Note, that hypohalites exist in equilibrium forms: predominantly HOCl (HOBr) in a neutral medium, and predominantly NaOCl (NaOBr) after adding NaOH (approaches C, D, C’, D’, respectively).…”

“…According to the data [ 80 , 152 , 154 , 155 , 159 , 163 , 164 , 165 ], the possible mechanisms ( Scheme 20 ) involve the oxidation of 1-methyl-1 H -pyrazol-3-amine 1k (step 1k→[Az–NH 2 ] ·+ ) or its N–H halogenation (step 1k→[NH–X] ) followed by N–H amination to hydrazopyrazole [NH–NH] and its oxidation to the target azopyrazole 1k–1k (see also Scheme 3 and Scheme 9 ).…”