Semihydrogenation of alkynes
In practical work, the two most commonly used methods for semihydrogenation of alkynes are the Lindlar and P-2-nickel hydrogenations. For synthesis of mono- or polyunsaturated fatty acids from acetylenes, hydrogenation using Lindlar catalyst is in our opinion always the method of choice. Although traces of trans-isomers are also formed, the Lindlar hydrogenation is simple to perform, relatively rapid and in most cases cleanly stops at the alkene stage. In contrast, P-2 hydrogenation often results in significant amounts of both trans-isomers and over-hydrogenated products.
A few colleagues of us claim that Lindlar hydrogenations are unpredictable, saying that on occasion the process is very slow and at other times does not work at all. They blame the Lindlar catalyst used for this problem, however, in our experience impurities in the sample to be hydrogenated are more often the cause of the problem. The samples have to be free from acetylenic oxidation or polymerization products and this can be achieved by purification on a silica gel column just before the hydrogenation. Another reason for poor hydrogenation is the presence of starting material or byproducts remaining after the acetylenic coupling reactions. Such products can be shorter chain acetylenic acids or esters which are difficult to remove by silica gel chromatography. Especially impurities having a terminal acetylenic bond tend to inhibit the hydrogenation. In such cases the sample should to be purified by crystallization or reversed-phase HPLC before hydrogenation is attempted.