Hydrogen bonding effects chemoselectivity

The stereoselective hydrogenation of alkynes to alkenes can be effected by a wide variety of homogeneous catalysts. The appropriate choice of catalyst and reaction conditions allows the selective formation of either the (Z)- or the (l )-a1-kene. Most of the catalysts display a very high chemoselectivity, as they are not reactive towards reducible functional groups such as carbonyl, ester, and double bonds. Many of the details related to catalyst behavior and intricate mechanistic details concerning semihydrogenation of alkynes have often not been unraveled, and will remain a topic of research for the coming years. 

Similar effects can be observed in the hydrogenation of molecules with two competing C=C double bonds (54). As an example, the hydrogenation of geraniol to citronellol or 3,7-dimethyl-2-octen-l-ol has been performed with supported Pt°. With Pt particles in a non-shape selective environment such as MCM-41, the chemoselectivity for citronellol amounts to 85% at 80% conversion. However, in a shape selective host such as zeolite Pt-HBeta, 100% chemoselectivity is obtained at the same conversion level ... 

In all cases the catalyst favored the phenyl derivative chemoselectivity was in the 80-90% range. The (3-CD unit selectively incorporates the phenyl ring and this assists the transport of the substrate from the organic into the aqueous phase. Formation of such a host/guest molecular complex keeps the alkene double bond at a suitable distance to interact with the Rh-phosphine catalyst. Both effects increase the rate of hydrogenation of 4-phenyl 1-butene. 

During our studies on the use of supported copper catalysts in selective hydrogenation reactions, we found that these systems can be effectively applied to the chemoselective hydrogenation of a,P-unsaturated ketones giving quantitative yields in the corresponding saturated ketone also when another olefinic bond is present in the molecule [3]. 

We recently reported that prereduced CU/AI2O3 is an effective catalyst for the selective hydrogenation of the C-C double bond of a,p-unsaturated carbonyl compounds when an isolated olefinic bond is also present in the molecule, showing unique chemoselectivity for a heterogeneous catalyst... 

---