The chemistry of cycloheptatriene: Part VII: Thermal isomerization of 7‐deuterocycloheptatriene by trans‐annular 1‐5 shifts of hydrogen

Abstract

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.