Oxidant-free annulation of α-hydroxy ketones with diamines using aluminum(III) chloride: A supramolecular approach for practical quinoxaline synthesis

Abstract

The conventional reaction of α-hydroxy ketones (acyloins) with arene-1,2-diamines typically requires an oxidant to facilitate a tandem oxidative condensation, resulting in quinoxaline derivatives. In this study, an innovative approach has been introduced that eliminates the need for an oxidant, serving as an alternative to tandem oxidative protocols. Inspired by the ability of 3-hydroxyflavones to chelate Al(III) ions, this study focused on exploring the self-activation of acyloins with AlCl₃ in the oxidant-free synthesis of quinoxalines via the formation of the acyloin@AlCl₃ complex. Thus, when aromatic 1,2-diamines were exposed to the activated acyloin@AlCl₃ complex under ethanolic conditions, high to excellent yields of quinoxalines were achieved. Additionally, the effect of other metal chlorides on the reaction was systematically investigated. To elucidate the specific role of aluminum(III) chloride in the annulation process, density functional theory was employed to show that the benzoin is activated in the presence of AlCl₃ for further condensation reaction. Experimental validation of the model reaction, conducted through cyclic voltammetry corroborated the theoretical findings, demonstrating that benzoin exhibits greater electrochemical activity than benzene-1,2-diamine (ortho-PD) in the presence of AlCl₃. Collectively, these data revealed the self-activation of benzoin as the host through the incorporation of AlCl₃ as the guest.