7-Deuterocycloheptatriene - prepared from tropylium bromide and lithium aluminium deuteride in 94% isotopic purity - rearranges on heating at 100-140°. Mass spectrometry and proton magnetic resonance (NMR) spectroscopy showed that by an intramolecular process a mixture of the 7-, α-, β- and γ-monodeuterocycloheptatrienes is formed. The changes in shape and intensity of the NMR signals proved that these isomers arise in successive steps, each of which involves a trans-annular 1-5 shift of hydrogen, combined with a shift of two double bonds. The Arrhenius parameters of the shift are E = 31-32 kcal/mole and log A (s−1) = 10.9-11.5. In transition-state theory this corresponds with an appreciable negative entropy of activation, indicating a rigid transition state.
A number of known isomerizations in 7-membered ring systems, particularly those of the Buchner esters and acids, can be explained by a trans-annular 1-5 shift.