Photosensitizer-free singlet oxygen generation via a charge transfer transition involving molecular O2 toward highly efficient oxidative coupling of arylamines to azoaromatics

Abstract

Photosensitizer (PS)-mediated generation of singlet oxygen, O₂ (a¹Δ_g) is a well-explored phenomenon in chemistry and biology. However, the requirement of appropriate PSs with optimum excited state properties is a prerequisite for this approach which limits its widespread application. Herein, we report the generation of O₂ (a¹Δ_g) via direct charge-transfer (CT) excitation of the solvent–O₂ (X³Σ_g⁻) collision complex without any PS and utilize it for the catalyst-free oxidative coupling of arylamines to azoaromatics under ambient conditions in aqueous medium. Electron paramagnetic resonance (EPR) spectroscopy revealed the formation of O₂ (a¹Δ_g) upon direct excitation with 370 nm light. The present approach shows broad substrate scope, remarkably fast reaction kinetics (90 and 40 min under an open and O₂ atm, respectively), high selectivity (100%), and excellent yields (up to 100%), and works well for both homo- and hetero-coupling of arylamines. The oxidative coupling of arylamines was found to proceed through the generation of amine radicals via electron transfer (ET) from amines to O₂ (a¹Δ_g). Notably, electron-rich amines show higher yields of azo products compared to electron-deficient amines. Detailed mechanistic investigations using various spectroscopic tools revealed the formation of hydrazobenzene as an intermediate along with superoxide radicals which subsequently transform to hydrogen peroxide. The present study is unique in the way that molecular O₂ simultaneously acts as a light-absorbing chromophore (solvent–O₂ complex) as well as an efficient oxidant (O₂ (a¹Δ_g)) in the same reaction. This is the first report on the efficient, selective, and sustainable synthesis of azo compounds in aqueous medium under an ambient atmosphere without any PCs/PSs and paves the way for further in-depth understanding of the chemical reactivity of O₂ (a¹Δ_g) generated directly via CT excitation of the solvent–O₂ complex toward various photochemical and photobiological transformations.