PhosphonicS™ Heterogeneous Metal Oxidation Catalysts

Chemfiles Volume 10 Article 2

        Dr. Josephine Nakhla

Dr. Josephine Nakhla
Product Manager
Email: josephine.nakhla@sial.com

Allylic and benzylic oxidations, alcohol oxidations, sulfoxidations, and epoxidations represent key synthetic transformations. Many traditional oxidants (chromium, manganesebased reagents, and peracids) are inherently toxic and often present difficulties in reaction work-up and purification, particularly from metal residues.

The development of new heterogeneous oxidation catalysts which would obviate the use of hazardous reagents such as those described above, would significantly advance the field. Under mild conditions in the presence of a re-oxidant, PhosphonicS heterogeneous oxidation catalysts cleanly, efficiently and selectively perform key oxidative transformations (Scheme 1) without the use of stoichiometric quantities of hazardous metalbased oxidants.1–4

Scheme 1:Versatility of PhosphonicS™ Heterogeneous Metal Oxidation Catalysts

Scheme 1:Versatility of PhosphonicS™ Heterogeneous Metal Oxidation Catalysts

Advantages of PhosphonicS Heterogeneous Metal Oxidation Catalysts:

  • Catalytic oxidants which circumvent the use of toxic stoichiometric oxidants
  • Simplified reaction work-up
  • Facile product isolation
  • Chemo and regioselective

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N-Heterocyclic Carbene Organocatalysts

Organocatalysis has been an active field with over 2,000 publications in the past 10 years. Organocatalysis offers multiple advantages, such as simplified procedures, chemical waste reduction, obviating the need for a metal catalyst and subsequent product contamination, and inherent cost savings (since no metal catalyst is required). Scheidt and coworkers have developed a series of N-heterocyclic carbene organocatalysts for various transformations and have also developed chiral variants, which afford the desired products with good levels of enantioselectivity. The unusual but efficient conversion of α,β- unsaturated aldehydes to their saturated ester counterparts using a recently commercialized N-heterocyclic carbene as catalyst is depicted below (Scheme 2).

Scheme 2: NHC-catalyzed preparation of saturated esters from a,ß-unsaturated aldehydes

Scheme 2: NHC-catalyzed preparation of saturated esters from a,ß-unsaturated aldehydes

Subsequently, Scheidt and coworkers reported the use of a chiral triazole for the enantioselective addition of homoenolates to nitrones affording γ-amino esters (Scheme 3). The reaction scope was broad, proving amenable to both the use of electron-rich and electron-poor aldehydes. Using 20 mol % of the chiral triazole at –25 °C affords the desired products in good yields and selectivities.

Scheme 3: NHC-catalyzed enantioselective additions of enals to nitrones to afford disubstituted hydroxyl amines

Scheme 3: NHC-catalyzed enantioselective additions of enals to nitrones to afford disubstituted hydroxyl amines

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Materials

     

References

  1. Al-Haq, N.; Sullivan, A. C.; Wilson, J. R. H. Tetrahedron Lett. 2003, 44, 769.
  2. Jurado-Gonzalez; M.; Sullivan, A. C.; Wilson, J. R. H. Tetrahedron Lett. 2003, 44, 4283.
  3. Jurado-Gonzalez; M.; Sullivan, A. C.; Wilson, J. R. H. Tetrahedron Lett. 2004, 45, 4465
  4. Al-Hashimi, M.; Roy, G.; Sullivan, A. C.; Wilson, J. R. H. Tetrahedron Lett. 2005, 46, 4365.
  5. Phillips, E.M.; Chan, A.; Scheidt, K.A. Aldrichimica Acta 2009, 42, 55.
  6. Chan, A.; Scheidt, K. A. Org. Lett. 2005, 7, 905.
  7. Phillips, E. M.; Reynolds, T. E.; Scheidt, K. A. J. Am. Chem. Soc. 2008, 130, 2416.

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