Abstract
The effect of a range of solids on some simple Diels-Alder reactions has been explored by the development of the methodology for the monitoring of a suspended solid in a liquid phase reaction mix as well as a solid 'impregnated' with reaction mix. The specific solids chosen were silica, quartz, sodium chlorate, aluminium chloride, cyanoguanidines, magnesium perchlorate, aspartic acid, a-D-glucose, carbon, sodium sulphate and guanidinimn carbonate. These were selected due to their structural features, literature evidence of effectiveness and evidence of effect in solution. Solid aluminium chloride, magnesium perchlorate, carbon and silica significantly enhance reactivity of the acrolein/1,3-cyclohexadiene reaction compared to control. Furthermore, aluminium chloride and silica affect diastereoselectivity to give an exo/endo ratio of approximately 0.5:1 and 0.1:1, respectively (for acrolein/cyclopentadiene) compared with 0.25:1 for the control. These solids, (AICI3, Mg(C104)2, Si02 and carbon) were then studied in more detail. Kinetic analysis of heterogeneous catalysis for each solid was carried out with measurement of rate constants and the enhancement due to each solid quantified. From this, an order of catalysis effect for solid catalysts was shown to be Lewis acid " general acid > compression or localisation. The effect of a solid organic material, bis-resorcinol anthracene was examined. Its preparation and a kinetic analysis of its effect as a solid catalyst are described. Differential enhancement for different dienes has been discovered and is interpreted as due to a selective cavity effect. Preparation of a group of acyclic achiral imides (having no stereogenic atom), which were likely to crystallise in a chiral confoixnation, for absolute asymmetric Diels- Alder reactions is also described. The aim was to effect enantioselective cycloaddition of a simple diene to a dienophile whose chirality is due solely to its solid state conformation and where its chirality is reflected in the crystallisation. Crystallisation of one imide to give chiral space group P212121 is seen, with another giving the achiral Pcba crystal structure. Some evidence of reaction with diene was observed. In cases where no reaction was seen, steric hindrance and lack of conjugation due to twisted chiral conformations are proposed. The scope for further work is explored. This work has given a clearer insight into heterogeneous catalysis of the Diels-Alder reaction, allowed quantification of rate enhancement by solids, and given a greater understanding of how and why some solids are more effective than others.