A green protocol ball milling synthesis of dihydropyrano[2,3-c]pyrazole using nano-silica/aminoethylpiperazine as a metal-free catalyst

Abstract

Background: Ball mill is an effective, and green method for the synthesis of heterocyclic compounds in very good yields. This method is a simple, economical, and environmentally friendly process. In this work, an efficient approach for the synthesis of pyranopyrazoles (PPzs) using ball milling and metal-free nano-catalyst (Nano-silica/aminoethylpiperazine), under solvent-free conditions was reported.

Results: The new nano-catalyst silica/aminoethylpiperazine was prepared by immobilization of 1-(2-aminoethyl)piperazine on nano-silica chloride. The structure of the prepared nano-catalyst was identified by FT-IR, FESEM, TGA, EDX, EDS-map, XRD, and pH techniques. This novel nano-catalyst was used for the synthesis of dihydropyrano[2,3-c]pyrazole derivatives under ball milling and solvent-free conditions.

Conclusions: Unlike other pyranopyrazoles synthesis reactions, this method has advantages including short reaction time (5-20 min), room temperature, and relatively high efficiency, which makes this protocol very attractive for the synthesis of pyranopyrazoles derivatives.

Keywords: Ball milling; Green chemistry; Nano-silica/aminoethylpiperazine; Pyranopyrazoles.

Fig. 1

Synthesis of PPz using Nano-SiO₂/AEP under solvent-free ball milling conditions

Fig. 2

Preparation of Nano-SiO₂/AEP

Fig. 3

FT-IR of (a) 2-AEP, (b) nano-silica chloride, (c) Nano-SiO₂/AEP

Fig. 4

XRD pattern of Nano-SiO₂/AEP

Fig. 5

TGA curve of Nano-SiO₂/AEP

Fig. 6

FESEM images of Nano-SiO₂/AEP

Fig. 7

a EDX spectrum of Nano-SiO₂/AEP, b Element mapping images for Nano-SiO₂/AEP nano-catalyst

Fig. 8

Influence of frequency and milling time on the yield of product

Fig. 9

Reusability of Nano-SiO₂/AEP nano-catalyst

Fig. 10

Proposed mechanism for synthesis of PPz in the presence of Nano-SiO₂/AEP