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Palladium Catalysts: Pd(dmdba)2 and Pd EnCat™ Systems

ChemFiles Volume 5 Article 10

C–C coupling reactions are arguably the most powerful transformations in synthetic organic chemistry, as simple molecules can be expeditiously converted into architecturally diverse structures. Among C–C bond forming reactions, the great importance of palladium catalysts has encouraged scientists to search for very active and, at the same time, stable palladium complexes that may advance research scale Heck, Suzuki, and Sonogashira processes to the industrial arena.1 The cross-coupling of aryl halides and boronic acids in an efficient and selective manner, known as the Suzuki–Miyaura reaction,2 represents one benchmark that a new Pd catalyst must be applied toward prior to entry into industrial process applications.

Pd2(dba)3 is the most commonly used non-phosphine based Pd(0) precursor because of its air stability and proclivity to undergo dibenzylidene ligand dissociation, thus granting Pd faster access into the catalytic cycle. Fairlamb and co-workers have explored varying the p-electron accepting characteristics of the dba ligand and thus affected the Pd-h2-dba bond strength (Scheme 13).3 They found that electron donating groups on the dba ligand decreased the magnitude of ligand back-bonding, thus increasing the concentration of the most reactive Pd(0) species in solution. This is the greatest advantage over Pd2(dba)3 and is directly translated into improved catalyst activity and product yields in the reactions of various aryl chlorides with arylboronic acids (Scheme 14, Table 3).

Scheme 13

Scheme 14

Table 3

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References

  1. Beller, M. et al. Angew. Chem., Int. Ed. Engl. 2005, 44, 674.
  2. (a) Suzuki, A. In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; VCH: Weinheim, 1998; p 49. (b) Fu, G. C. et al. Angew. Chem., Int. Ed. Engl. 1998, 37, 3387. (c) Buchwald, S. L. et al. J. Am. Chem. Soc. 1998, 120, 9722. (d) Beller, M. et al. Angew. Chem., Int. Ed. Engl. 2000, 39, 4153. (e) Noonan, A. F. et al. J. Org. Chem. 2001, 66, 8677. (f) Nolan, S. P. et al. J. Am. Chem. Soc. 2003, 125, 16194.
  3. Fairlamb, I. J. S. et al. Org. Lett. 2004, 6, 4435.

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