Graphene acid: A potent carbocatalyst for the friedel-crafts arylation of aldehydes with indoles

Abstract

Carbocatalysis represents a highly attractive and effective field within the realm of metal-free nanocatalysis, significantly advancing sustainability in synthetic chemistry. Graphene acid (GA) emerges as a well-defined graphene derivative, characterized by a high density of homogeneously distributed carboxylic groups over graphene lattice. This unique and uniform structure positions GA as an elegant alternative to other 2D carbocatalysts, namely graphene oxide. GA was successfully employed as the catalyst in a Friedel-Crafts-type reaction between indoles and aldehydes, facilitating the synthesis of bis(indolyl)methanes, organic compounds exhibiting interesting biological properties and significant pharmaceutical potential. The metal-free nature of GA, combined with the performance of the reaction "on water" under mild conditions, highlight the green credentials of the developed protocol. Comprehensive substrate screening, including a plethora of aliphatic or aromatic aldehydes and various 1- or 2-substituted indoles, resulted in moderate to high yields of variously functionalized bis(indolyl)methanes. Mechanistic and recovering studies showed that GA acts catalytically as a Br & oslash;nsted acid, maintaining its catalytic activity at a high rate for six subsequent cycles.