Haptophilic effect

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

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 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 ... 

The solvent effect can be physical or chemical. The physical effect is usually manifested in the form of solubility or the dielectric constant. Here, we consider a new feature known as haptophilicity. The chemical effect depends on the acid/ base character or reactivity of the solvent. We shall demonstrate these effects through selected examples (see Gilbert and Mercier, 1993). 

The effect of the reactant is basically a kinetic effect and is hence part of the kinetic analysis considered in Chapter 7. We confine our discussion in this section to the effect of solvent polarity, which has a direct influence on the performance of the catalyst. It is known (Thompson and Naipawer, 1973 Sehgal et al., 1975) that certain functional groups in a substrate can bind to the catalyst surface during a reaction in such a way as to enforce the addition of hydrogen from its own side of the molecule, which is opposite to that expected on the basis of steric hindrance, namely binding occurs from the opposite side. This effect, termed haptophilicity, can be exploited in selecting the right kind of solvent to enhance the selectivity to a desired product. 

Thompson, H. W. McPherson, E. Lences, B. L. Stereochemical Control of Reductions. 5. Effects of Electron Density and Solvent on Group Haptophilicity /. Of. Chem. 1976, 4i, 2903-2906. 

---