A schematic of a generic Pd-PEPPSI type precatalyst
A schematic of a generic Pd-PEPPSI type precatalyst. R1, R2, and R3 represent carbon or heteroatom substituents.

PEPPSI is an abbreviation for pyridine-enhanced precatalyst preparation stabilization and initiation. It refers to a family of commercially available[1][2][3] palladium catalysts developed around 2005 by Prof. Michael G. Organ and co-workers at York University,[4][5] which can accelerate various carbon-carbon and carbon-heteroatom[6] bond forming cross-coupling reactions. In comparison to many alternative palladium catalysts, Pd-PEPPSI-type complexes are stable to air and moisture and are relatively easy to synthesize and handle.

Structure and synthesis

In the basic structure of Pd-PEPPSI, R1 can be a methyl (CH3, Me), ethyl (C2H5, Et), isopropyl (C3H7, iPr), isopentyl (C5H11, iPent), or isoheptyl (C7H15, iHept) group, and starting from the second in the row the resulting catalysts are thus labeled as PEPPSI-IEt, PEPPSI-IPr, PEPPSI-IPent, and PEPPSI-IHept respectively, with or without "Pd-" added in front.[7] Commonly used PEPPSI catalysts such as Pd-PEPPSI-IPr[8] contain an unsubstituted imidazole core (R2=H) and a 3-chloro substituted pyridine ligand (R3=3-Cl). However, structural modifications of the imidazole backbone[9][10][11][12][13] and pyridine ligand[7][10][11] can profoundly affect the catalytic activity of these complexes.

The synthesis and structure of Pd-PEPPSI catalysts were presented in 2005[4][1] and published in 2006.[14] PEPPSI catalysts are organopalladium complexes containing N-heterocyclic carbene (NHC) ligands. They can be obtained by reacting an imidazolium salt, palladium(II) chloride, and potassium carbonate in 3-chloropyridine as a solvent, under vigorous stirring at 80 °C for 16 hours in air. The yield of PEPPSI in this reaction is 97–98%.[1][14] Contrary to other common palladium-based catalysts, such as tetrakis(triphenylphosphine)palladium(0), PEPPSI is stable to exposure to air[15] and moisture.[16] Even heating in dimethyl sulfoxide at 120 °C for hours does not result in significant decomposition or deactivation of PEPPSI catalysts.[1]

iPEPPSI

Examples of abnormal NHCs based on the mesoionic 1,2,3-triazol-5-ylidene structure have been used for palladium catalysis. In this manner, pyridine fused tzNHCs were prepared to yield palladium complexes with pyridine attached to the carbene core. With this ligand, air stable and highly active palladium complexes of iPEPPSI (as in internal PEPPSI) were synthesized.[17]

An example of iPEPPSI complex.[17]

Properties and applications

PEPPSI can catalyze various palladium cross-coupling reactions including Negishi coupling,[15] Suzuki coupling, Sonogashira coupling, Kumada coupling,[18] and the Buchwald–Hartwig amination[6] as well as aryl sulfination[19][10][6] and the Heck reaction.[1][20] In Negishi coupling, PEPPSI promotes reaction of alkyl halides, aryl halides or alkyl sulfonates with alkylzinc halides,[21] and the important advantage of PEPPSI over alternative catalysts is that the reaction can be carried out in a general chemical laboratory, without a glove box. PEPPSI contains palladium in the +2 oxidation state and is thus a "precatalyst", that is the metal must be reduced to the active Pd(0) form in order to enter the cross-coupling catalytic cycle. This is usually achieved in situ in the presence of active transmetalating agents such as organo-magnesium, -zinc, -tin, or -boron reagents.[7] Once activated, the NHC-Pd(0) species becomes rather air-sensitive.[15][1][22][23]

Suzuki coupling (a) and Buchwald-Hartwig reaction (b) can be activated by PEPPSI complexes.

An efficient, cationic palladium catalyst of iPEPPSI (internal PEPPSI) type was demonstrated to efficiently catalyse the copper-free Sonogashira reaction in water as the only solvent, under aerobic conditions, in the absence of copper, amines, phosphines and other additives.[17]

Sonogashira coupling under green reaction conditions using iPEPPSI.[17]

Additionally, the cationic palladium iPEPPSI complex shown above was used in the hydroamination of alkynes as well. The authors have demonstrated that the ligands actively participate in the reaction mechanism since the pyridine group acts as an internal base to enable the intramolecular proton transfer between active sites of intermediates.[24][25]

Palladium iPEPPSI complex with designated catalytic region and pyridine wingtip that actively participates in the catalytic reactions as internal base.[24][25]

References

  1. 1 2 3 4 5 6 PEPPSI Catalysts, Sigma-Aldrich
  2. ↑ PEPPSI™-IPent for Demanding Cross-Coupling Reactions, Sigma-Aldrich
  3. ↑ PEPPSI Catalyst, Sigma-Aldrich ChemFiles
  4. 1 2 Hadei, N.; Kantchev, E.A.B.; O'Brien, C.J.; Chass, G.; Hunter, H.H.; Penner, G.; Hopkinson, A.C.; Organ, M.G. (2005). Rational catalyst design and its application in sp3-sp3 couplings. 230th ACS National Meeting. Washington, DC: American Chemical Society. pp. Abstract 308.
  5. ↑ Hadei, N.; Kantchev, E.A.B.; O'Brien, C.J.; Organ, M.G. (2005). "The First Negishi Cross-Coupling Reaction of Two Alkyl Centers Utilizing a Pd−N-Heterocyclic Carbene (NHC) Catalyst". Org. Lett. 7: 3805–3807. doi:10.1021/ol0514909. PMID 16092880.
  6. 1 2 3 Valente, C.; Pompeo, M.; Sayah, M.; Organ, M.G. (2014). "Carbon–Heteroatom Coupling Using Pd-PEPPSI Complexes". Organic Process Research & Development. 18: 180–190. doi:10.1021/op400278d.
  7. 1 2 3 Nasielski, J.; Hadei, N.; Achonduh, G.; Kantchev, E.A.B.; O'Brien, C.J.; Lough, A.; Organ, M.G. (2010). "Structure-Activity Relationship Analysis of Pd-PEPPSI Complexes in Cross-Couplings: A Close Inspection of the Catalytic Cycle and the Precatalyst Activation Model". Chem. Eur. J. 16: 10844–10853. doi:10.1002/chem.201000138. PMID 20665575.
  8. ↑ PEPPSI™-IPr catalyst, Sigma-Aldrich
  9. ↑ Pompeo, M.; Froese, R.D.J; Hadei, N.; Organ, M.G. (2012). "Pd-PEPPSI-IPentCl: A Highly Effective Catalyst for the Selective Cross-Coupling of Secondary Organozinc Reagents". Angew. Chem. Int. Ed. 51: 11354–11357. doi:10.1002/anie.201205747. PMID 23038603.
  10. 1 2 3 Sayah, M.; Lough, A.J.; Organ, M.G. (2013). "Sulfination by Using Pd-PEPPSI Complexes: Studies into Precatalyst Activation, Cationic and Solvent Effects and the Role of Butoxide Base". Chem. Eur. J. 19: 2749–2756. doi:10.1002/chem.201203142. PMID 23296748.
  11. 1 2 Pompeo, M.; Farmer, J.L.; Froese, R.D.J; Organ, M.G. (2014). "Room-Temperature Amination of Deactivated Aniline and Aryl Halide Partners with Carbonate Base Using a Pd-PEPPSI-IPentCl-o-Picoline Catalyst". Angew. Chem. Int. Ed. 53: 3223–3226. doi:10.1002/anie.201310457. PMID 24677620.
  12. ↑ Atwater, B.; Chandrasoma, N.; Mitchell, D.; Rodriguez, M.J.; Pompeo, M.; Froese, R.D.J.; Organ, M.G. (2015). "The Selective Cross-Coupling of Secondary Alkyl Zinc Reagents to Five-Membered-Ring Heterocycles Using Pd-PEPPSI-IHeptCl". Angew. Chem. Int. Ed. 127: 9638–9642. doi:10.1002/anie.201503941. PMID 26110577.
  13. ↑ Lu, D.-D.; Xu, X.-X.; Liu, F.-S. (2017). "Bulky Yet Flexible Pd-PEPPSI-IPentAn for the Synthesis of Sterically Hindered Biaryls in Air". J. Org. Chem. 82: 10898–10911. doi:10.1021/acs.joc.7b01711. PMID 28925697.
  14. 1 2 O'Brien, C.J.; Kantchev, E.A.B.; Valente, C.; Hadei, N.; Chass, G.A.; Lough, A.; Hopkinson, A.C.; Organ, M.G. (2006). "Easily Prepared Air- and Moisture-Stable Pd–NHC (NHC=N-Heterocyclic Carbene) Complexes: A Reliable, User-Friendly, Highly Active Palladium Precatalyst for the Suzuki–Miyaura Reaction". Chem. Eur. J. 12: 4743–8. doi:10.1002/chem.200600251. PMID 16568494.
  15. 1 2 3 Li, Jia Jack; Corey, E.J. (2009). Name reactions for homologations, Part 1. John Wiley and Sons. ISBN 978-0-470-48701-3.
  16. ↑ Valente, C.; Belowich, M.E.; Hadei, N.; Organ, M.G. (2010). "Pd-PEPPSI Complexes and the Negishi Reaction". Eur. J. Org. Chem.: 4343–4354. doi:10.1002/ejoc.201000359.
  17. 1 2 3 4 Gazvoda, M.; Virant, M.; Pevec, A.; Urankar, D.; Bolje, A.; Kočevar, M.; Košmrlj, J. (2016). "A mesoionic bis(Py-tzNHC) palladium(II) complex catalyses green Sonogashira reaction through an unprecedented mechanism". Chem. Commun. 52: 1571–1574. doi:10.1039/c5cc08717a. PMID 26575368.
  18. ↑ Ackerman, Lutz (2009). Modern Arylation Methods. Verlag: Wiley-VCH. doi:10.1002/9783527627325. ISBN 9783527319374.
  19. ↑ Sayah, M.; Organ, M.G. (2011). "Carbon–Sulfur Bond Formation of Challenging Substrates at Low Temperature by Using Pd-PEPPSI-IPent". Chem. Eur. J. 12: 11719–11722. doi:10.1002/chem.201102158. PMID 21898625.
  20. ↑ Luis, Santiago V.; Garcia-Verdugo, Eduardo (2009). Chemical Reactions and Processes Under Flow Conditions. Green Chemistry Series. Royal Society of Chemistry. doi:10.1039/9781847559739. ISBN 978-0-85404-192-3.
  21. ↑ Cazin, Catherine S.J. (2010). N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis. Catalysis by Metal Complexes. Vol. 32. Netherlands: Springer. pp. 169–173. doi:10.1007/978-90-481-2866-2. ISBN 978-90-481-2866-2.
  22. ↑ Organ, M.G.; Avola, S.; Dubovyk, I.; Hadei, N.; Kantchev, E.A.; O'Brien, C.J.; Valente, C. (2006). "A User-Friendly, All-Purpose Pd–NHC (NHC=N-Heterocyclic Carbene) Precatalyst for the Negishi Reaction: A Step Towards a Universal Cross-Coupling Catalyst". Chem. Eur. J. 12: 4749–4755. doi:10.1002/chem.200600206. PMID 16568493.
  23. ↑ PEPPSI: Instructions for Use, Sigma-Aldrich
  24. 1 2 Virant, M.; Mihelač M.; Gazvoda M.; Cotman, A.E.; Pinter, B.; Košmrlj, J. (2020). "Pyridine Wingtip in [Pd(Py-tzNHC)2]2+ Complex Is a Proton Shuttle in the Catalytic Hydroamination of Alkynes". Org. Lett. 22: 2157–2161. doi:10.1021/acs.orglett.0c00203. PMC 7308070. PMID 31999464.
  25. 1 2 Virant, Miha (2019). Development of homogeneous palladium catalytic systems for selected transformations of terminal acetylenes (PhD). University of Ljubljana.
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