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Gold catalysis - chemfiles volume 9 article 2

By: Josephine Nakhla, chemfiles volume 9 article 2

The Friedel-Crafts reaction is well-documented in organic chemistry; however, one major limitation is the common use of strong acids. There have been studies on the use of stoichiometric and catalytic metals to accelerate this important class of reactions. Since gold catalysts can be considered as metallic sources of H+, several research groups have successfully demonstrated the utility of Au(I) catalysts for Friedel-Craftstype reactions. Tarselli and Gagne recently reported an efficient and functional-group-tolerant method for the cyclization of 4-allenyl arenes to afford benzocycles in the presence of chloro(triphenylphosphite) gold. The reaction was generally amenable with electron-rich arenes, but heterocyclic aromatic compounds with coordinating abilities such as triazoles, isoxazoles, and oxazoles led to catalyst poisoning. On the other hand, the reaction was tolerant of functional groups and moieties such as ethers, acetals, and pyrroles (Scheme 9).1


Scheme 9.

Echavarren and co-workers used a Au(I) catalyst to effect the transformation of substrate 4 to 5. This reaction proceeds through a 5-exo-dig cyclization followed by trapping with MeOH (Scheme 10).2


Scheme 10.

Echavarren and co-workers have also reported the intermolecular addition reactions of nucleophiles, including electron-rich arenes and heteroarenes, allylsilanes, and 1,3-dicarbonyl compounds to 1,5- and 1,6-enynes. Using the electron-rich Au(I)-phosphite based catalyst below (6), the authors reacted 1,6-enynes with arenes and heteroarenes to afford carbocycles. This reaction occurs via 5-exo-dig cyclization to afford a cyclopropyl metal carbene species, which upon reaction with the nucleophile affords the carbocyclic product (Table 7).3


Table 7.

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References

  1. Tarselli, M. A.; Gagne, M. R. J. Org. Chem. 2008, 73, 2439.
  2. Nieto- Oberhuber, C. et al. Chem.-Eur. J. 2006, 12, 1677.
  3. Amijs, C. H. M. et al. J. Org. Chem. 2008, 73, 7721.

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