PIPO- Polymer immobilised TEMPO
Mechanism + Description
These can be added as the nitroxide radical form – PIPO, or as the secondary amine from which the active catalyst is generated via oxidation in situ – these are the Chimassorb 966-type materials sold as (BASF).
A number of polymeric TEMPO-based nitroxide radical catalysts exist. These often show high activity and can be easier to remove and re-use than free TEMPO. PIPO is a polymer immobilised TEMPO that is produced from a cheap solid support. The reagent was published recently by Roger Sheldon, there is very limited scale-up information with this system but Sheldon reports there are three advantages over TEMPO/Bleach
(i) NaBr/KBr are not required
(ii) There is less tendency for over oxidation to the acid with this system hence less reliance on chlorinated solvents (which tend to prevent over oxidation)
(iii) The catalyst can be filtered off and reused.
Chem. Commun.,2000, 271-272 – Overview of PIPO, free from bromide and halogenated solvent.
Synlett 2001; 2001(1): 0102-0104 – comparison of PIPO and silicon supported TEMPO
Relevant scale up example
Additional Solid phase TEMPO options
Green note – There are a wide range of solid supported catalysts. The options for easier work-up and reuse potentially make these green options, however considerations should be given to their preparation and lifespan (number of reuses before depletion of results) to make an informed decision if its a better option than the standalone/homogeneous reagents.
Org. Process Res. Dev., 2007, 11 (4), pp 766–768 – SiliaCat TEMPO, reusable solid supported TEMPO
J. Org. Chem. 2009, 74, 8510–8515 - Tetraarylphosphonium Salts as a soluble and recoverable supported TEMPO
Chemistry-A European Journal (2011), 17(22), 6056-6060 - Recyclable Magnetic Core-Shell Nanoparticle-Supported TEMPO cat. for Aerobic Oxidation of Alcohols in Water (20 reuses)
Angewandte Chemie, International Edition (2007), 46(38), 7210-7213 – TEMPO on mesoporous material, 14 x reuse
Heterocycles (2010), 81(11), 2539-2553 – Polymer supported TEMPO in combination with Oxone