The use of PTC with dithionite is a known to be effective for several reductions that we teach in our 2-day course “Industrial Phase-Transfer Catalysis.”
In the reaction shown in the diagram, tetrabutylammonium transfers dithionite into the organic phase (methyl t-butyl ether as solvent) into which 1,2-dibromo-1,1,2,3,3,3-hexafluoro-propane is added dropwise. A radical is formed at the secondary bromide of the perhalopropane (more stable than at the primary bromide in this reactant). The radical adds to aniline with regioselectivity as shown in the diagram. The HBr that is formed is neutralized by the bicarbonate. The reaction is performed under mild conditions and rather dilute with MTBE and water.
A very similar reaction was reported in the January 2016 PTC Reaction of the Month. See http://phasetransfercatalysis.com/ptc_reaction/ptc-sodium-dithionite-initiated-free-radical-addition/. In that case, heptafluoroisopropyl iodide was reacted with dithionite to form the iodine radical and the perfluoroisopropyl radical. The perfluoroisoproyl radical added to the aniline in the para position in 71% yield. The January 2016 patent also used MTBE as solvent, bicarbonate as the base to neutralize (HI in that case) and TBA HSO4 as the phase-transfer catalyst.
The reaction performed in this April 2022 patent by Syngenta is more challenging than the January 2016 patent by Mitsui since the product in the Syngenta patent needed to contain a primary bromide. The inventors of the Syngenta patent used phase-transfer catalysis well to achieve their challenging goal.
When you need to react a water-soluble anion with an organic-soluble reactant, phase-transfer catalysis should almost always be on your short list of options to screen (after proper safety analysis). When you need help choosing PTC conditions, now contact Marc Halpern of PTC Organics to explore collaboration through consulting.
