Syn addition of hydrogen

The catalytic hydrogenation reaction can be stopped after adding one molar equivalent of hydrogen gas to form an alkene with a modified palladium catalyst. One such palladium catalyst, called the Lindlar catalyst, has palladium coated on calcium carbonate that contains a small amount of lead acetate. Hydrogenation of an alkyne using the Lindlar catalyst is stereospecific. Syn addition occurs, giving the Z (cis) isomer. This syn addition indicates that the reaction occurs on the surface of the palladium catalyst and that both hydrogen atoms bond on the same face, while the compound remains bound to the catalyst. 

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Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

There are two products that can be formed by syn addition of hydrogen to 2 3 dimethylbi cyclo[2 2 1] 2 heptene Write or make molecular models of their structures... 

Draw the other stereoisomer that might have been obtained from syn addition of hydrogen to each alkene. Is the observed product for each addition also the thermodynamic product Compare energies for alkene A+ H2 observed and not observed and alkene B+H2 observed and not observed. What structural factors seem to be responsible for the relative stabilities of the two products of each hydrogenation reaction ... 

Hydrogenation of alkynes in the presence of P-2 catalyst causes syn addition of hydrogen to take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration. 

Catalytic hydrogenation delivers hydrogen to one face of the alkene the consequence is syn addition of hydrogen. This is a departure from our usual observations with ionic mechanisms, where the groups typically add to a double bond with anti stereochemistry (see Section 8.1.2). 

Fig. 11.5 Proposed intermediacy of a cyclic Schiff Base (CSB) producing respectively trans- and c/s-PDC through two-step electron transfer and syn-addition of hydrogen atoms at platinum surface.

Alkynes normally react with two equivalents of hydrogen to produce alkanes. However, it is also possible to react an alkyne with only one equivalent of hydrogen and stop the reaction at the alkene stage if a special catalyst, called Lindlar catalyst, is used. The Lindlar catalyst is a deactivated form of palladium that is less reactive than normal catalysts. This method provides good yields of the cr. v-alkene, resulting from syn addition of hydrogen ... 

It has been generally accepted that the orientation of adsorption of an unsaturated molecule onto the catalyst is controlled by a steric interaction or hindrance between the substrate and the catalyst in other words, the adsorption at a less hindered side of the substrate is more favored.149 The stereochemical outcomes of many hydrogenations have thus been explained by syn addition of hydrogen (from the catalyst) to the substrate at a less hindered side. Unless isomerization or some other opposing factors are concerned, such a theory may be successfully applied to those cases where the adsorption of substrate or the formation of half-hydrogenated state is the key step that... 

A non-catalytic procedure for the syn-addition of hydrogen to alkenes makes use of the unstable compound diimide (N2H2). This strongly exothermic reaction is favoured by the elimination of nitrogen gas. The diimide reagent must be freshly generated in the reaction... 

The reason that catalytic hydrogenation often results in syn addition of hydrogen to aikenes was discussed in Chapter 24. 

The treatment of an alkene with osmium tetroxide results in the syn-addition of hydrogen peroxide. 

Because the alkyne sits on the surface of the metal catalyst and the hydrogens are delivered to the triple bond from the surface of the catalyst, only syn addition of hydrogen occurs (Section 5.19). Syn addition of hydrogen to an internal alkyne forms a cis alkene. 

Syn addition of hydrogen across an alkyne Trans selective reduction of alkyne... 

STRATEGY AND ANSWER We make use of two reactions that we have just studied in this chapter syn addition of hydrogen to an alkyne, and alkylation of alkynide ions. 

Because the insertion step 4 and the reductive elimination step 5 are stereospecific, the net result of the hydrogenation using Wilkinson s catalyst is a syn addition of hydrogen to the alkene. The following example (with D2 in place of H2) illustrates this aspect. 

Alkenes are good nucleophiles by virtue of their JI electron density, but may be readily reduced under a hydrogen atmosphere in the presence of a palladium-charcoal catalyst, to give alkanes arising by stereospecific syn-addition of hydrogen. 

Catalytic reduction is stereoselective, the most common pattern being the syn addition of hydrogens to the carbon-carbon double bond. The catalytic reduction of 1,2-dimethyl-cyclohexene, for example, yields afrl,2-dimethylcyclohexane along with lesser amounts of trans-, 2-dimethylcyclohexane. 

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