Hydrogenation, catalytic, alkene conjugated compounds

In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp2 carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides of sp2 carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C rr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes, conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /(-hydrogen. At the same time, the PdfO) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg. Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

As mentioned in Section 3.2, hydrogenation is by far the most investigated catalytic reaction and palladium the most commonly employed metal, followed by platinum. The most common substrates for catalytic hydrogenation tests are simple alkenes, cyclic alkenes and unsaturated carbonylic compounds. In the latter case, conjugated substrates (a,P-unsaturated aldehydes, acrylic acid) have received particular attention. 

The reactions of 1,3-butadiene are reasonably typical of conjugated dienes. The compound undergoes the usual reactions of alkenes, such as catalytic hydrogenation or radical and polar additions, but it does so more readily than most alkenes or dienes that have isolated double bonds. Furthermore, the products frequently are those of 1,2 and 1,4 addition ... 

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