Stability of heteroazulene-substituted tropylium ions: synthesis and properties of the (2-oxo-2H-cyclohepta[b]thiophen-3-yl)tropylium ion, and its oxygen and nitrogen analogues

Abstract

Synthesis and properties of a novel type of heteroazulene-substituted tropylium tetrafluoroborate, 2-oxo-2H-cyclohepta[b]thiophen-3-yltropylium tetrafluoroborate, and its oxygen and nitrogen analogues, 9a–d·BF4− are reported. The synthesis is based on a simple reaction of the tropylium ion with the corresponding heteroazulenes in the presence of NEt3, followed by oxidative hydrogen abstraction with DDQ and subsequent exchange of the counter-anion by using aq. HBF4 solution. The stability of the tropylium ion derivatives 9a–d is found to be in the order 9a < 9b < 9c < 9d from pKR+ values of 3.2–5.7 obtained spectrophotometrically and reduction potentials measured by cyclic voltammetry (CV). The important canonical structures for the cations 9a–d are discussed on the basis of the 1H and 13C NMR spectral data and the energy levels of HOMOs of heteroazulenes as well as the stable conformation obtained by AM1 method (MOPAC97). A good linear correlation between the pKR+ values and the reduction potentials was obtained for cations 9a–d. The stabilizing effect of heteroazulenes toward cations 9a–d is similar to that of heteroazulene-substituted methyl cations based on the similarity of the regression line slopes.