Over the years, we have reviewed several high-yielding PTC oxidations using catalytic amounts of TEMPO or hydroxy-TEMPO with hypochlorite or other oxidants. Several examples are shown at the end of this post. A patent was issued this month [Velankar, A.; Vaze, K. R.; Shinde, P. D. (S.H. Kelkar and Company Limited), US 12,516,012, 06-Jan-2026], in which the inventors describe the oxidation of a mixture of unsaturated C10 alcohols using hydroxy-TEMPO in the presence of tetrabutylammonium chloride and N-chlorosuccinimide (NCS). The desired unsaturated aldehydes were isolated in 63% yield. While this is a respectable result, the yield is significantly lower than other PTC oxidation systems we reported previously (cited below).
In this system, NCS likely serves as the oxidant, regenerating the active hydroxy-TEMPO species. However, NCS is not a strong oxidant and may not regenerate hydroxy-TEMPO as efficiently as hypochlorite-based systems. This could be one reason for the modest yield. Furthermore, potential losses during workup, including extraction, aqueous washes, and fractional distillation, may also contribute to the reduced isolated yield. It’s possible that the actual conversion is higher, but the isolation method results in losses.
Over-oxidation to the carboxylic acid was not reported but that could potentially be a source of yield loss. However, hydroxy-TEMPO is recognized for its ability to selectively oxidize primary alcohols to aldehydes without significant over-oxidation to carboxylic acids. This level of selectivity is obviously a key advantage for chemists seeking clean, controlled oxidations under mild conditions. Avoiding over-oxidation not only improves product purity, but can also simplify downstream processing and reduce the need for purification steps.
It may be worthwhile for the inventors of this new patent to explore whether the PTC systems we reported (shown below) could offer improvements. The chemistry here is clearly sound, but there could be room for optimization.
- PTC oxidation with hypochlorite and hydroxy-TEMPO
- PTC hypochlorite oxidation with catalytic TEMPO
- PTC TEMPO hypochlorite oxidation
- Combining PTC with high shear
For those working in oxidations under phase-transfer conditions, this recent patent is another reminder that even familiar systems like TEMPO oxidations can benefit from revisiting older, well-characterized PTC systems. Depending on the substrate and desired selectivity, combining the right oxidant, the right PTC and the right conditions can make a significant difference.
