Longer polyenyl cations in relation to soliton theory

Abstract

The carotene-like polyenes decapreno-β-carotene (C₅₀), C₅₄-β-carotene (C₅₄, first synthesis) and dodecapreno-β-carotene (C₆₀) with 15, 17 and 19 conjugated double bonds, respectively, were synthesized by double Wittig reactions. Introduction of a leaving group in allylic position failed, and cations were obtained by hydride elimination effected by i) triphenylcarbenium tetrafluoroborate-d₁₅, prepared by a new method, or ii) treatment with trifluoroacetic acid-d. Deuterated reagents were employed for product analysis by ¹H NMR. Parallel experiments were performed with β,β-carotene (C₄₀). NIR spectra at room temperature and at –15 °C were employed for characterisation and stability studies of the cationic products. In CH₂Cl₂λ_max in the 900–1350 nm region was recorded. NMR data for the cationic product of β,β-carotene obtained by the two new preparation methods were consistent with the two monocations previously characterised. The cationic products of the longer polyenes provided downfield-shifted, broadened signals, compatible with C₅₀-monocation, mixed C₅₄-mono- and dication and C₆₀-dication. Combined NIR and NMR data suggest that the extent of charge delocalisation is limited by the maximum soliton width for cations obtained from linear polyenes with more than ca. 20 sp²-hybridized carbon atoms.