Haptophilicity

Some olefinic molecules have a second function that anchors itself on the catalyst surface in such a way so as to enforce addition of hydrogen to its own side of the molecule. This anchoring effect, dubbed haptophilicity 112,113), has been observed by many investigators 0,19,74,78,90,120). An example of Gula and Spencer 47) illustrates how the anchoring tendencies of a function remote from the point of saturation may influence the stereochemistry. 

The anchoring effect, which is also sometimes called the haptophilic effect, results from the fact that certain groupings increase the strength of adsorption of... 

Despite the haptophilic behavior of the hydroxy group towards both heterogeneous and homogeneous noble metal catalyst systems,a C—OH bond unactivated by adjacent alkenic or aromatic groups (see Chapter 4.7) is quite resistant to hydrogenolysis, and forcing conditions are usually required. 

As discussed in Chapter 13, the hydrogenation of mixtures of simple alkenes and olefinic alcohols in non-polar solvents results in the selective saturation of the hydroxy olefin because of the enhanced adsorption of this material caused by the presence of the hydroxy group. In alcoholic solvents, however, this haptophilic assistance is negated by the adsorption of the solvent molecules on the catalyst, and hydrogenation selectivity is controlled by the steric environments of the competing olefins.7i.72... 

The propensity of a functional group to bind to the catalyst surface, thus orienting hydrogenation from its own side is known as haptophilicity. Intramolecular complexation is most effective with an hydroxyl group . Acetates show little selectivity but sulfon-imides afford great diastereofacial selectivity . This effect is related to the polarity of the solvent, e.g., in the hydrogenation of 22 over Pd in various solvents ... 

Haptophilicity of the hydroxyl function may account for a selective hydrogenation. A difference of strain is responsible for the preferred reduction of one of two similarly substituted double bonds in 12 ... 

Hydrogenation of 1 at high P and moderate T stops after reduction of one benzene ring [equation (a)] . The second ring is not hydrogenated probably because the substrate molecule is bent and only one ring lies flush on the catalyst surface. Haptophilicity probably allows the full reduction of 2 [equation (b)] . 

---