Simultaneous different mechanisms for the efficient synthesis of β-enaminones: 12-tungstocobaltic acid-supported on nano silica as an electron transfer and Brønsted acid nano catalyst

Abstract

In the present study, 12-tungestocobaltic acid, H5CoW12O40, was immobilized on nano silica from rice husk (CoW@NSiO2) to develop a novel, efficient, heterogeneous and recyclable nano catalyst for the synthesis of β-enaminones. It is apparent from acidity and cyclic voltammetric measurements that, the catalyst is electroactive and undergoes reversible redox transitions, as well as it is contains strong acid sites and mobile protons. As evidenced from mechanistic investigations, CoW@NSiO2 can catalyze the synthesis of β-enaminones with two simultaneous ways: electron transfer and Brønsted acid mechanisms. In order to confirm the synthesis of enaminones through simultaneous mechanisms of electron transfer and acidity, the model reaction was carried out in the presence of K5Co as an electroactive catalyst and CoW@NSiO2 with electron scavenger as an acid catalyst. The results showed that the reaction proceeded simultaneously through both mechanisms. There is evidence that the electron transfer property of this catalyst is most pronounced in this type of organic reactions. The catalyst demonstrated outstanding performance, and the methodology proved to be versatile, yielding excellent results across a wide range of substrates. It is worth mentioning that aliphatic amines were well-tolerated in the process and produced β-enaminone compounds with excellent yields and short reaction times. Also, reactions with dimedone, a cyclic 1,3-diketone, delivered moderate product yields. Additionally, the catalyst showed remarkable recyclability, maintaining its activity for a minimum of five consecutive cycles without any noticeable decline. Notably, the cyclic voltammetric and acidity measurements revealed that the catalyst’s electron transfer property and Brønsted acidity remained unchanged after five runs.