PEPPSI

General structure of PEPPSI catalysts. R stands for a small organic fragment.

PEPPSI is an abbreviation for pyridine-enhanced precatalyst preparation stabilization and initiation. It refers to a group of palladium catalysts developed around 2005 by Prof. Michael G. Organ and co-workers at York University,[1][2] which can accelerate various aminations and cross-coupling reactions. In comparison to many alternative palladium catalysts, 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 PEPPSI, R can be a methyl (CH3, Me), ethyl (C2H5, Et), isopropyl (C3H7, iPr), isopentyl (C5H11, iPent), or isoheptyl (C7H15, iHept) group, and the resulting catalysts are thus labeled as PEPPSI-IMes, PEPPSI-IEt, PEPPSI-IPr, PEPPSI-IPent, and PEPPSI-IHept respectively, with or without "Pd-" added in front.[3] Contrary to common palladium-based catalysts, such as tetrakis(triphenylphosphine)palladium(0), PEPPSI is stable to exposure to air[4] and moisture.[5] Even heating in dimethyl sulfoxide at 120 °C for hours does not result in significant decomposition or neutralization of PEPPSI catalysts.[6]

The synthesis and structure of PEPPSI catalysts were presented in 2005[1][6] and published in 2006.[7][8] PEPPSI catalysts are based on an organometallic complex palladium N-heterocyclic carbene (NHC). They can be obtained by reacting imidazolium salt, palladium(II) chloride, and potassium carbonate in a 3-chloropyridine solvent, under vigorous stirring at 80 °C for 16 hours in air. The yield of PEPPSI in this reaction is 97–98%.[6][8]

Properties and applications

PEPPSI can catalyze various cross-coupling reactions including Negishi coupling,[4] Suzuki coupling, Kumada coupling,[9] and the Buchwald–Hartwig amination as well as the Heck reaction.[6][10] In Negishi coupling, it promotes reaction of alkyl halides, aryl halides or alkyl sulfonates with alkylzinc halides,[11] 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.[3] Once activated, the NHC-Pd(0) species becomes rather air-sensitive.[4][12][13]

          
Suzuki coupling (left) and Buchwald-Hartwig reaction (right) can be activated by PEPPSI

References

  1. 1 2 Organ, M. G. Rational catalyst design and its application in sp3-sp3 couplings. Presented at the 230th National Meeting of the American Chemical Society, Washington, DC, 2005; Abstract 308.
  2. ↑ Hadei, Niloufar; Kantchev, Eric Assen B.; O'Brie, Christopher J.; Organ, Michael G. (2005). "The First Negishi Cross-Coupling Reaction of Two Alkyl Centers Utilizing a Pd−N-Heterocyclic Carbene (NHC) Catalyst†". Organic Letters. 7 (17): 3805–7. PMID 16092880. doi:10.1021/ol0514909.
  3. 1 2 Nasielski, Joanna; Hadei, Nilofaur; Achonduh, George; Kantchev, Eric Assen B.; O'Brien, Christopher J.; Lough, Alan; Organ, Michael 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". Chemistry: A European Journal. 16 (35): 10844. doi:10.1002/chem.201000138.
  4. 1 2 3 Jie Jack Li, E. J. Corey Name reactions for homologations, Part 1, John Wiley and Sons, 2009, ISBN 0-470-08507-X p. 74
  5. ↑ Valente, Cory; Belowich, Matthew E.; Hadei, Niloufar; Organ, Michael G. (2010). "Pd-PEPPSI Complexes and the Negishi Reaction". European Journal of Organic Chemistry: n/a. doi:10.1002/ejoc.201000359.
  6. 1 2 3 4 PEPPSI Catalysts, Sigma Aldrich
  7. ↑ Issue cover of Chemistry: A European Journal, June 2006
  8. 1 2 O'Brien, Christopher J.; Kantchev, Eric Assen B.; Valente, Cory; Hadei, Niloufar; Chass, Gregory A.; Lough, Alan; Hopkinson, Alan C.; Organ, Michael 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". Chemistry: A European Journal. 12 (18): 4743. doi:10.1002/chem.200600251.
  9. ↑ Lutz Ackermann Modern arylation methods, Wiley-VCH, 2009, ISBN 3-527-31937-9 p. 52
  10. ↑ S. V. Luis, Eduardo García-Verdugo Chemical Reactions and Processes Under Flow Conditions, Royal Society of Chemistry, 2010, ISBN 0-85404-192-3 p. 153
  11. ↑ Catherine S. J. Cazin Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, Springer, 2010, ISBN 90-481-2865-X pp. 169–173
  12. ↑ Organ, MG; Avola, S; Dubovyk, I; Hadei, N; Kantchev, EA; O'Brien, CJ; 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". Chemistry (Weinheim an der Bergstrasse, Germany). 12 (18): 4749–55. PMID 16568493. doi:10.1002/chem.200600206.
  13. ↑ PEPPSI: Instructions for Use, Sigma-Aldrich
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