Regenerable Acidity of Graphene Oxide in Promoting Multicomponent Organic Synthesis

Abstract

The Brønsted acidity of graphene oxide (GO) materials has shown promising activity in organic synthesis. However, roles and functionality of Lewis acid sites remain elusive. Herein, we reported a carbocatalytic approach utilizing both Brønsted and Lewis acid sites in GOs as heterogeneous promoters in a series of multicomponent synthesis of triazoloquinazolinone compounds. The GOs possessing the highest degree of oxidation, also having the highest amounts of Lewis acid sites, enable optimal yields (up to 95%) under mild and non-toxic reaction conditions (85 °C in EtOH). The results of FT-IR spectroscopy, temperature-programed decomposition mass spectrometry, and X-ray photoelectron spectroscopy identified that the apparent Lewis acidity via basal plane epoxide ring opening, on top of the saturated Brønsted acidic carboxylic groups, is responsible for the enhanced carbocatalytic activities involving Knoevenagel condensation pathway. Recycled GO can be effectively regenerated to reach 97% activity of fresh GO, supporting the recognition of GO as pseudocatalyst in organic synthesis.

Allylic and Allenylic Dearomatization of Indoles Promoted by Graphene Oxide by Covalent Grafting Activation Mode

“…Delightfully, sonication of the recovered GO under acidic conditions (HCl 1 m , 1 h, Figure a) enabled the simultaneous restoring of the acidity profile and part of the epoxydic content of the pristine GO. As testified by the XPS analysis, the epoxy group increased from 24 % to 30 % upon acid treatment (Figures S7) . Once again, the feedback gained from catalysis was in agreement with the structural characterization.…”

“…The protonation of the epoxide ring on the GO surface leads to an unstable oxonium unit that opens barrierless ( Rx ) . Ring opening of the epoxide groups relieves ring strain and forms a highly stabilized α‐carbocation, as already observed by Chen et al . Then, the resulting α‐carbocation undergoes a facile nucleophilic attack by the allylic alcohol (transition state Ts1 ).…”

Recent advances in the catalytic applications of GO/rGO for green organic synthesis

“…Ebajo et al [178] reported the synthesis of triazoloquinazolinone compounds by utilizing Brønsted acidic edges and Lewis-acid sites in GO as heterogeneous promoters. GOs with the maximum number of Lewis acid sites possess the highest degree of oxidation resulting in the best yields (up to 95%) under moderate reaction conditions (85°C in EtOH).…”

Graphene Oxide as a Mediator in Organic Synthesis: a Mechanistic Focus

“…Notably, the intrinsic Brønsted acidity of GO is known to promote ring‐opening of strained epoxides of the surface, releasing stabilized α‐carbocations by the π‐conjugate motif of GO (Scheme 6). [28, 29] …”

“… Experimental evidence (FT‐IR and XPS, adapted from ref. [28]) of the covalent anchoring of allylic alcohols on the GO surface.…”