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

Authors


  • Communicated at the meeting of the Section for Organic Chemistry of the Koninklijke Nederlandse Chemische Vereniging on December 22, 1961. Cf. Chem. Weekblad 57, B310 (1961).

  • R. van Helden, A. P. ter Borg and A. F. Bickel, The chemistry of cycloheptatriene. VI: Interaction of 2,6-dimethylphenol and tropylium salts, Rec. trav. chim. 81, 599 (1962).

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.

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