Bond Fixation in Annulenes. 14. Synthesis of and Bond Shifting Equilibrium between 1,4- and 1,6-Di-tert-butylcyclooctatetraenes

Article abstract of DOI:10.1021/jo00156a023

Photolysis of the Diels-Alder adduct of 3,6-di-tert-butyl-o-benzoquinone and cyclobutadiene at ice-bath temperatures afforded the bicyclooctatriene 12.During warming to room temperature, this hydrocarbon underwent kinetically first-order valence isomerization to provide 5 and its bond shift isomer 4.This finding adumbrated the facility with which 4 and 5 are interconverted.Reaction of this cyclooctatetraene mixture with N-methyltriazolinedione gave urazoles 16 and 17 as chromatographically separable entities.Like 13, the related cycloadduct of 12 wherein both bridgehead tert-butyl groups exhibit restricted rotation , the angular tert-butyl substituent in 17 is sterically perturbed.Hydrolysis-oxidation of either 16 or 17 returned only mixtures of 4 and 5 because of their rapid bond shifting rates.When the equilibrium constant between these two isomers was determined by 1H NMR spectroscopy, it was found that 5, the apparently more congested compound, was the more stable in CDCl3 solution.The possible underlying causes of this phenomenon are discussed.