In the November 2016 PTC Catalyst of the Month, we suggested that tetrabutylammonium oxone can be replaced by tetrabutylammonium hydrogen sulfate and Oxone (TM). A patent was issued this month used that combination to convert a thioether to the corresponding sulfone shown in the diagram.
Oxone (registered trademark of DuPont) is a commercial oxidizing agent that is a triple salt mixture of 2KHSO5·KHSO4·K2SO4. This mixture has a higher stability than the potassium peroxymonosulfate by itself. Under PTC conditions, the quat cation transfers the HSO5- anion, a peroxy oxidizing agent, into the organic phase to perform efficient oxidation.
Note that one equivalent of Oxone contains two equivalents of KHSO5. Therefore, 775 mmol of Oxone contains 1,550 mmol of active peroxy oxidizing equivalents. Since 516 mmol of sulfide requires 1,032 mmol active oxygen, there is a 50 mole% excess of oxidizing agent to form the sulfone.
Even though the inventors chose to use PTC only for the oxidation of the sulfide to the sulfone, all three reactions in the sequence shown in the diagram should have been PTC candidates. As we teach in our 2-day course “Practical Phase-Transfer Catalysis”, PTC excels in transferring ethoxide into toluene and PTC excels even more in transferring and activating mercaptide anions in thioetherifications.
In other words, one could choose a single phase-transfer catalyst that would continue with the toluene phase and perform all three reactions using PTC in a streamlined process.
Then again, with 87% overall yield for the three steps, all three reactions must be quite good as reported. We are always looking for ways to minimize unit operations to streamline processes.
If your company can benefit from streamlining processes by performing multiple consecutive PTC reactions using a single solvent and a single phase-transfer catalyst, now contact Marc Halpern of PTC Organics and/or complete the form at www.phasetransfer.com/projectform.pdf to obtain a free estimate of the probability of success for using PTC to meet your performance targets.
