Metal-Free Selective Oxidations

By incorporating boron atoms into the polymeric structure of carbon nitride, a team of chemists based in Germany and China has created a metal-free material that selectively mediates oxidation of aliphatic C–H moieties under mild conditions (Chem. Sci., DOI: 10.1039/c0sc00475h). The development may lead to new types of low-cost and environmentally benign metal-free catalysts for hydrocarbon conversion reactions. Selectively oxidizing hydrocarbons at sp³-hybridized carbon centers to produce alcohols, aldehydes, and ketones is challenging. The key issue is that the increased reactivity of the products relative to the starting materials often leads to overoxidation and poor product selectivity. Efforts to replace metal-based catalysts currently used for those transformations with enzymes and biomimetic catalysts have met with limited success thus far. Yong Wang of Germany’s Max Planck Institute of Colloids & Interfaces, Haoran Li of China’s Zhejiang University, and coworkers report that the polymeric solid formed by reacting dicyandiamide, H₂NC(NH)NHCN, with ammonia borane, BH₃NH₃, readily catalyzes oxidation of benzylic aromatics in the presence of H₂O₂ or O₂. For example, by using O₂ as an oxidant, the team converted fluorene to fluorenone in 46% yield and greater than 99% selectivity.