Lindlar catalyst

A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate or barium sulfate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation of alkynes to alkenes (i.e. without further reduction into alkanes).Thus if a compound contains a double bond as well as a triple bond, only the triple bond reduces to a double bond as opposed to both bonds being reduced down to a single bond.^{[citation needed]}

As described by its inventor Herbert Lindlar,^[1][2] the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline enhances its selectivity, preventing formation of alkanes. An example of alkyne reduction is the reduction of phenylacetylene to styrene. A recent description on how the poisons act on the selectivity of the Lindlar catalyst identifies a complementary role of the poisons while Pb reduces the amount of hydrogen content in the bulk of Pd, quinoline blocks the formation of oligomers.^[3]

Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.^[4] An example of commercial use is the organic synthesis of vitamin A which involves an alkyne reduction with the Lindlar catalyst.

Other heterogeneous catalysts useful for hydrogenation are Adams' catalyst, Palladium black, and Raney nickel.

References

1. ↑ Lindlar, H.; Dubuis, R. (1973), "Palladium Catalyst for Partial Reduction of Acetylenes", Org. Synth. ; Coll. Vol. 5: 880 
2. ↑ Lindlar, H. (1952). "Ein neuer Katalysator für selektive Hydrierungen". Helv. Chim. Acta 35 (2): 446. doi:10.1002/hlca.19520350205. .
3. ↑ Garcia-Mota, M; Gomez-Diaz, J; Novell-Leruth, G.; Vargas-Fuentes, C.; Bellarosa, L.; Bridier, B.; Perez-Ramirez, J.; Lopez, N. (2010). "A density functional theory study of the 'mythic' Lindlar hydrogenation catalyst". Theor. Chem. Acc. 128 (4–6): 663–673. doi:10.1007/s00214-010-0800-0. 
4. ↑ Overman, L. E.; Brown, M. J.; McCann, S. F. (1993), "(Z)-4-(Trimethylsilyl)-3-Buten-1-ol", Org. Synth. ; Coll. Vol. 8: 609