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 pK_R+ 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 pK_R+ 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.