Lindlar hydrogenation

What is Lindlar hydrogenation?

The reduction of alkynes to cis-alkenes by hydrogen in the presence of palladium impregnated onto calcium carbonate and deactivated by lead acetate and quinoline was optimized by Lindlar in 1952. This reaction is commonly known as the Lindlar reduction or Lindlar hydrogenation, and the palladium on calcium carbonate CaCO3 poisoned with lead acetate Pb(OAc)2 and quinoline is referred to as the Lindlar catalyst.

The Lindlar catalyst exists as a slurry mixture, resulting in a heterogeneous hydrogenation. Palladium coated on BaSO4 has been found to be superior to CaCO3 due to the solid support’s stability towards acid.

Lindlar hydrogenation - general reaction scheme - Lindlar reduction
Lindlar hydrogenation
  • R1 = alkyl, aryl
  • R2 = H, alkyl, aryl

The Lindlar catalyst is prepared by reducing PdCl2 with a CaCO3 slurry, followed by the addition of lead acetate and 0.05–1 equivalent quinoline. The addition of quinoline lowers the reduction rate but efficiently enhances the selectivity by inhibiting alkene surface interaction. Although lead acetate has been suggested to block the most active catalyst sites and prevent overreduction of alkenes to alkanes, it modifies the surface structure of the catalyst, such as the formation of the Pd3Pb alloy particle. Such modifications on the catalyst surface also increase the selectivity.

It is believed that the stereoselective reduction of alkynes to cis-alkenes is because one face of the triple bond is shielded by the Lindlar catalyst so that hydrogen is restricted to approach the triple bond from the other side. The solvent also plays an important role in this reduction. The cis/trans ratio was dramatically enhanced from 10:1 to 25:1 by changing the solvent from MeOH to hexane/EtOAc (1:1), and the cis/trans ratio was further increased to 86:1 by the addition of quinoline in hexane/EtOAc (1:1).

Although the Lindlar catalyst‘s palladium is reduced to a zero valence state, it remains electron-deficient, with a bond energy about 2.1 eV higher than Pd(0). This electron deficiency causes stronger absorption of alkynes onto the catalyst, facilitating their reduction to alkenes, which are released without overreduction to alkanes due to the weaker interaction with the catalyst. However, an unprotected amino group near the alkyne group accelerates overreduction of alkenes, reducing selectivity due to its strong electron-donating ability. Adding 1 equivalent of ethylenediamine to the reaction mixture reduces overreduction to a minimum level.

References

Lindlar, H. (1952), Ein neuer Katalysator für selektive Hydrierungen. [A new catalyst for selective hydrogenations] Helvetica Chimica Acta, 35: 446-450. https://doi.org/10.1002/hlca.19520350205

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