Diels–Alder reactions catalysed 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. Nontransition and lanthanide cation-exchanged clays were found to be ineffective. α,β-Unsaturated carbonyl compounds (such as methyl vinyl ketone, methyl acrylate and methyl methacrylate) were used as dienophile. Cyclopentadiene, furan, pyrrole, isoprene and cyclohexa-1,3-diene were successfully employed as the diene, whilst thiophene was unreactive. Such reactions (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, Δd, gave increased selection for the kinetically less favoured, but less bulky isomer.