DIELS-ALDER REACTIONS CATALYZED BY CATION-EXCHANGED CLAY-MINERALS

Catalysis of Diels-Alder reactions by various clay minerals exchanged with a variety of transition metal cations was shown to occur readily at ambient temperatures in a variety of solvents. Non-transition and l anthanide cation-exchanged clays were found to be ineffective. alpha,b eta-Unsaturated carbonyl compounds (such as methyl vinyl ketone, methy l acrylate and methyl methacrylate) were used as dienophile. Cyclopent adiene, furan, pyrrole, isoprene and cyclohexa-1,3-diene were successf ully employed as the diene, whilst thiophene was unreactive. Such reac tions (except that with isoprene which produces a mixture of 1,3- and 1,4-isomers) produced a mixture of endo- and exo- isomers, the ratios of which could be altered by manipulation of the inter-layer reaction space of the clay catalyst. Choosing a clay with a higher layer charge , and hence lower basal spacing, DELTAd, gave increased selection for the kinetically less favoured, but less bulky isomer.