Immobilized di(hydroperoxy)propane adducts of phosphine oxides as traceless and recyclable oxidizing agents

Oxidations represent important reactions that are ubiquitous in academia and industry. Hydrogen peroxide (H2O2) is a common source of active oxygen for oxi-dations. H2O2 is usually diluted in water as it is too unstable to be used in pure form. However, the presence of water can complicate reactions because biphasic mixtures with organic solvents form. Furthermore, secondary reactions with water may lead to side products. Therefore, alternative forms of H2O2, such as peroxide adducts, are an active area of research. Di(hydroperoxy)alkane adducts of phosphine oxides are one attractive solution because they are soluble in organic solvents, crystallizable, shelf-stable and active towards a variety of oxidation reactions. The only drawback is that the phosphine oxide carrier has to be removed after the reaction. In this contribution, the bifunctional ligand (EtO)3Si(CH2)2PPh2 is immobilized on a silica (SiO2) support which is subsequently end-capped with EtOSi (CH3)3. The new surface-bound di(hydroperoxy)propane adduct is then generated with the immobilized phosphine oxide as carrier. The adduct and a deuterated analog are characterized with solid-state and solution NMR spectroscopy. It has been demonstrated that substrates in organic solvents easily access the surface-bound peroxide and are oxidized quantitatively. The phosphine oxide carrier remains bound to the surface and can be removed easily by settling of the silica. Using the oxidative esterification of nonyl aldehyde it is proven that the immobilized peroxide adduct does not leach from the silica support and is active and reusable over multiple cycles.

Automated summary

Oxidations are important reactions in both academia and industry, and hydrogen peroxide (H2O2) is a commonly used source of active oxygen for these reactions. However, using H2O2 in its pure form is impractical due to its instability, so it is usually diluted in water. This can complicate reactions as biphasic mixtures with organic solvents are formed, and secondary reactions with water can produce side products. Therefore, researchers are actively exploring alternative forms of H2O2, such as peroxide adducts. One promising solution is the use of di(hydroperoxy)alkane adducts of phosphine oxides, which are soluble in organic solvents, stable, and effective for various oxidation reactions. The only drawback is the need to remove the phosphine oxide carrier after the reaction. In this study, a bifunctional ligand is immobilized on a silica support, creating a surface-bound di(hydroperoxy)propane adduct with the phosphine oxide as the carrier. The adduct is characterized using NMR spectroscopy and it is demonstrated that it remains bound to the silica support, allowing for easy removal of the carrier. The immobilized peroxide adduct is shown to be active and reusable for multiple cycles.