Sustainable One-Pot Metal-Free Oxidative Carboxylation of Styrenes with Molecular Oxygen and Carbon Dioxide

The development of sustainable, metal-free catalytic systems is pivotal for advancing green chemistry and CO₂ utilization. To avoid using transition metal catalysts, this study proposes an optimized one-pot oxidative carboxylation of styrene into styrene carbonate, applying molecular oxygen as a benign oxidant and tetra-<i>n</i>-butylammonium bromide ([Bu₄N]⁺Br^–) as the sole catalyst. This strategy integrates Mukaiyama epoxidation and CO₂ cycloaddition in a single step, with isobutyraldehyde as a coreagent to enable mild epoxidation conditions. Under optimized parameters (10 mol % [Bu₄N]⁺Br^–, 0.8 MPa O₂, 4.5 MPa CO₂, 130 °C), styrene carbonate is obtained in a 61% yield with high selectivity. A comprehensive study of reaction parameters, including O₂ and CO₂ partial pressures, catalyst loading, temperature, and solvent effects, highlights the critical role of phase equilibrium as investigated using high-pressure view cell experiments. Kinetic analysis shows that styrene epoxidation proceeds via a high-energy radical-chain pathway, where hydrogen abstraction and peroxy-radical propagation constitute the rate-limiting steps (<i>E</i>_<i>a</i> = 124.8 kJ·mol^–1), whereas CO₂ cycloaddition to the resulting epoxide follows with a lower barrier (<i>E</i>_<i>a</i> = 91.2 kJ·mol^–1). This metal-free transformation offers a viable and sustainable alternative to conventional cyclic carbonate synthesis, contributing to CO₂ valorization and green chemical manufacturing.