Overview of green oxidation of benzyl and allyl H

Abstract

This review focuses on green oxidation methods for benzylic and allylic C-H bonds. Traditional synthesis methods often require pre-introducing functional groups like halogens or hydroxyl groups to activate substrates. Such approaches are not only cumbersome but also pose environmental risks. In contrast, modern methods take advantage of the σ-π hyperconjugation effect, which facilitates the activation of benzylic and allylic C-H bonds. This allows for direct functionalization without pre-functionalization, aligning with the principles of green chemistry. Photocatalysis activates C-H bonds via hydrogen atom transfer (HAT) by exciting metal complexes through ligand-metal charge transfer (LMCT) and generates products in combination with green oxidants such as oxygen. Photoelectric cooperative catalysis integrates electrochemical and photochemical steps to break through the redox window limitations of single catalytic systems and achieve highly selective C-H functionalization. In terms of green oxidants, methanol as an oxygen source in electrochemical oxidation is compatible with various benzylic substrates containing functional groups, while water enables metal-free and oxidant-free carbonylation through radical-polar crossover mechanisms in photocatalysis. Challenges include nanosecond-scale radical lifetimes and low catalytic efficiency, mitigated by substrate-catalyst pre-coordination. Future directions involve dual-ligand design and "metal-photocatalyst-electrochemistry" ternary systems to enhance selectivity and atom economy for sustainable synthesis.