Nájera Catalysts

Aldrich ChemFiles 2007, 7.5, 6.

Aldrich ChemFiles 2007, 7.5, 6.

Oxime-derived palladacycles 3 and 4 (Figure 2) introduced by Professor Carmen Nájera and co-workers at the Universidad de Alicante (Alicante, Spain) are very efficient phosphine-free precatalysts for a variety of C–C bond-forming processes.4 They are thermally robust and insensitive to both oxygen and moisture, slowly forming nanoparticulate palladium under the reaction conditions.5

Figure 2.

These palladacycles are readily available from the corresponding benzophenone and acetophenone oximes via metallation of the aromatic moiety with lithium tetrachloropalladate (Scheme 3).4

Scheme 3.

Catalyst 3 provided the highest activities in these bond-forming processes carried out in organic solvents, while catalyst 4 proved to be more active in water or aqueous solvents.4 Examples of the bond-forming processes mediated by these palladacycles include the Heck-Mizoroki,6 Suzuki-Miyaura,7,8 Sonogashira,9 Sila- Sonogashira,9 Ullmann,4 and Stille10 coupling reactions, examples of which are shown in Scheme 4.

Scheme 4.

Palladacycle 4 is an efficient precatalyst for mono- and b,bdiarylation of electron-deficient olefins (Scheme 5),5 while 3 effectively catalyzes the homocoupling of terminal acetylenes in a direct synthesis of conjugated diynes (Scheme 6).9

Scheme 5.

Scheme 6.

Palladacycle 3 also promotes the copper-free synthesis of ynones11 and the direct synthesis of 2,3-disubstituted indoles6 from terminal and internal acetylenes (Scheme 7).

Scheme 7.

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  1. Alonso, D. A. et al. Synthesis 2004, 1713.
  2. Botella, L.; Nájera, C. J. Org. Chem. 2005, 70, 4360.
  3. Alonso, D. A. et al. Adv. Synth. Catal. 2002, 344, 172.
  4. Alonso, D. A. et al. J. Org. Chem. 2002, 67, 5588.
  5. Botella, L.; Nájera, C. J. Organomet. Chem. 2002, 663, 46.
  6. Alonso, D. A. et al. Adv. Synth. Catal. 2003, 345, 1146.
  7. Alonso, D. A. et al. Org. Lett. 2000, 2, 1823.
  8. Alonso, D. A. et al. J. Org. Chem. 2004, 69, 1615.

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