Base-catalysed prototropic rearrangement. Part I. Comparison of the base-catalysed and the metal carbonyl-catalysed isomerisation of allyl ethers

Abstract

The light-induced isomerisation of allylic ethers in the presence of pentacarbonyliron has been compared with that brought about by a basic heterogeneous catalyst, KNH₂–Al₂O₃. The reaction on the basic catalyst proceeds through the usual cis-mechanism. In contrast, the carbonyliron reaction gives cis- and trans-isomers, in a 1 : 1 ratio in the case of allyl phenyl ether. A π-complex was detected (by n.m.r. spectroscopy) as an intermediate in the reaction. The formation of a catalyst which was particularly active for the thermal isomerisation of olefinic double bonds was detected during the irradiation of pentacarbonyliron in allyl phenyl ether. 3-Propoxycyclohexene was readily isomerised under these conditions, whereas it remained unchanged in the presence of very strong bases. Conformational effects of the ring were observed.