Phase-transfer catalysis excels in the selective mild oxidation of primary alcohols to aldehydes without overoxidation to the carboxylic acids as first reported by Lee and Freedman in the 1970’s as we teach in our 2-day course “Industrial Phase-Transfer Catalysis” (soon to be conducted in Philadelphia in October). TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl radical) is a well known catalyst for oxidant that is used in catalytic quantities, due to its high cost, in conjunction with other less expensive oxidizing agents. Hydroxy-TEMPO (HO-TEMPO) can be used in many cases in which TEMPO is used since it is readily made from less expensive starting materials.
This patent shows that careful choice of PTC reaction conditions using hypochlorite and HO-TEMPO results in a very rapid and selective oxidation of alcohols to the aldehydes. Similar to the Lee and Freedman publication, the reaction must be stopped at the right time to minimize over-oxidation. In this case, the reaction time needs to be stopped at 8 minutes to achieve the highest amount of desired product.
Tetrabutylammonium hydrogen sulfate is usually used as the phase-transfer catalyst in PTC hypochlorite oxidations (quat hydrogen sulfates are also used for hydrogen peroxide oxidations). TBAB can make hypobromite in-situ and cause further complications.
A high shear Silverson mixer was used in this patent. PTC reactions that use sensitive reactants and products are sometimes better performed with lower agitation efficiency in order to minimize non-catalyzed interfacial side reactions. It is often better to let the phase-transfer catalyst control the contact between the reactants instead of the less controlled brute force of high shear agitation. It is not known if less agitation would be beneficial in this case but it is worthwhile to consider this concept in the future for compounds that are sensitive to water, oxidants, etc.
When you need to control selectivity to achieve the highest performance for your commercial process in development or in production, contact Marc Halpern of PTC Organics to integrate highly specialized expertise in industrial phase-transfer catalysis with your commercial needs.
