Palladium Catalysts for Carbonylation

Chemfiles Volume 6 Article 1

Metal-catalyzed carbonylation functions as one primary and efficient route for introducing carbonyl groups into an organic molecule. The versatility of carbonylation technology has been extended to the formation of a diverse array of organic carbonyl compounds via reactions of aziridines,1 epoxides,2 oxazolines,3 and primary alkyl- or arylmethyl halides.4 This last class of compounds, following their carbonylation to the corresponding esters, represents important chemical intermediates produced on an industrial scale. The traditional means of synthesizing arylacetic esters is tedious, initially proceeding through a stoichiometric reaction of arylmethyl halides with metal cyanides, followed by hydrolysis and esterification.5 Preston and co-workers have spearheaded the development of a mild, catalytic system that focuses on Pd as the active metal component.4 Pd-mediated carbonylation reactions were known prior to the methodology illustrated below; however the original catalysts suffer from the necessity of high pressures and temperatures.6

Pd catalyst 8 efficiently carbonylates benzyl halides in methanol at pressure ranging from 1 to 4 bar (Scheme 11). The carbonylation also proceeds favorably in an aqueous (biphasic) system, but arylmethyl chlorides were shown to be more robust substrates than the corresponding bromides. A side-by-side comparison of catalyst 8 versus PdCl2(PPh3)2 (9) at 3.45 bar CO pressure is shown in Table 3. High product selectivity is furnished by organopalladium complex 8, whereas the latter system produces a substantial amount of byproducts. Most importantly, the industrial usefulness of this carbonylation system is found in the experiments conducted at atmospheric CO pressure (Scheme 12). Simply bubbling CO through the methanolic solution containing the catalyst and benzyl halide formed a series of aryl halide esters in quantitative yields in 2 h. Reduced Pd species were not observed under these conditions. Sigma-Aldrich has commercialized the innovative Pd catalyst 8 and the related 2-benzyl alcohol complex 10 in collaboration with a Heriot-Watt University research team (Scheme 13).7 These catalysts fuel the formal addition of carbon monoxide to benzyl halides affording benzyl esters under low pressure and temperature conditions.


Scheme 11.

Table 3.

Scheme 12.

Scheme 13. (666327) (665932)

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Materials

     

References

  1. (a) Coates, G. W. et al. Angew. Chem., Int. Ed. Engl. 2001, 41, 2781. (b) Alper, H. et al. J. Org. Chem. 2001, 166, 5424 and references therein.
  2. Coates, W. G. et al. J. Am. Chem. Soc. 2005, 127, 11426 and references therein.
  3. Jia, L., Xu, H. Org. Lett. 2003, 5, 1575.
  4. (a) Preston, P. N. et al. Tetrahedron Lett. 2005, 46, 8695. (b) Preston, P. N. et al. Organometallics, 2005, 24, 1119.
  5. Beller, M. et al. J. Mol. Catal. A 1997, 116, 259.
  6. (a) Geissler, H. Clariant Corporation. US Patent 6653502, 2003; Chem. Abstr. 2003, 136, 385941. (b) Ziolkowski, J. et al. J. Mol. Catal. A 2000, 154, 93. (c) Gardano, A. J. Organomet. Chem. 1976, 121, C55.
  7. Manufactured by Aldrich under exclusive license from Heriot-Watt University, PCT/GB2005/002738 patent pending.

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