Three-component reaction 336 INDEX

Although optical resonance spectroscopy is suitable for studying chemical reactions involving two reactants and a single product, it cannot be extended to more than two or three components because one cannot interpret the resonance spectrum uniquely to obtain the composition from the refractive index. Raman spectroscopy is an attractive alternative approach. 

Now the possibility that a reaction mixture that contains one equivalent of all three components cyclopropene, butadiene, and furan can be explored. As mentioned befor, it is now possible to form four products exo and endo cyclopropene adduct with butadiene [83], and exo and endo cyclopropene adduct with furan [84]). The reactivity indexes were 0.0740 (endo adduct with butadiene), 0.0649 exo adduct with butadiene), 0.0617 exo adduct with furan), and 0.0592 endo adduct with furan). The selectivity index of formation of the endo cycloadduct with butadiene over the exo cycloadduct with furan was 0.0091 for cyclopropene as the dienophile. If higher selectivity favored the endo addition of cyclopropene to butadiene and if reaction were conducted under kinetic control, only the endo cyclopropene adduct with butadiene should be isolated as a product. This finding was also confirmed experimentally. 

Since the catalytic activity takes place not only on the three-component system Cu/ZnO/AljOj, but already on pure ZnO, the first step in a thorough analysis of the reaction mechanism is to investigate how the reactants, intermediates and products adsorb on ZnO. However, since ZnO catalysts are mostly prepared in wet chemical processes, it is difficult to find out whether the active sites are Zn or 0 atoms on regular low-index surface planes, edges or corners of ZnO micro crystals, defects such as O vacancies, or even more complicated species. Of course, the simplest possibility is to study processes at regular ZnO surface planes, as is always done in the surface science approach to heterogeneous catalysis. 

Table II lists some furan and thiophene components substituted with sulfur at the three position on both heterocyclic rings generated in one reaction system. Also included are Kovats retention index data and references reporting the occurrence in foods or model systems.

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