Solvent-free aerobic oxidation of benzylic C–H bonds via nanoconfined cobalt–porphyrin frameworks: a green and safe catalytic strategy

Abstract

The solvent-free aerobic oxidation of inert benzylic C–H bonds using molecular oxygen represents an atom-economical and environmentally benign route to high-value oxygenated chemicals. However, conventional approaches often suffer from safety risks associated with radical chain reactions and peroxide accumulation, alongside poor sustainability profiles due to extensive solvent use and stoichiometric oxidants. Herein, we develop three-dimensional cobalt–porphyrin-based porous aromatic frameworks (CoPor-PAFs) synthesized via Suzuki–Miyaura coupling, which serve as efficient heterogeneous catalysts for the selective oxidation of benzylic secondary C–H bonds. The optimized catalyst, CoPor-PAF-4, features highly dispersed Co²⁺ sites, tailored micropores (0.50–0.80 nm), and passivated residual groups via a dual-capping strategy. Under solvent-free conditions, CoPor-PAF-4 achieves 60.1% conversion of p-ethylnitrobenzene with 94.5% selectivity toward p-nitroacetophenone at 130 °C and 1.0 MPa O₂, delivering a high turnover number (TON) of 13,151. More importantly, this system delivers outstanding green metrics, including an E-factor of 0.81 and a reaction mass efficiency (RME) of 55%, significantly outperforming most solvent-dependent and stoichiometric oxidant-based protocols. Integrated mechanistic investigations, combining spectroscopic analysis, kinetic modeling, and DFT calculations, elucidate that the nanoconfined space exerts control by restricting radical diffusion (as quantified by configurational diffusion models) and promoting a low-barrier oxygen-rebound pathway. This unique environment effectively suppresses hazardous peroxyl radical chain reactions. Together, this work establishes a green, efficient, and intrinsically safe catalytic system for C–H oxidation that aligns with key principles of green chemistry, including waste prevention and catalytic efficiency.