- On the Synthesis and Optical Properties of Sulfur-Bridged Analogues of Angulenium Cations and Derivatives
Cyril Nicolas, Gérald Bernardinelli, Jérôme Lacour
J. Phys. Org. Chem. 2010, 23, 1049-1056
Several sulfur-bridged heterocyclic carbocations of the acridinium and triangulenium family were prepared using 9-(2,6-dimethoxyphenyl)-1,8-dimethoxy-thioxanthenium tetrafluoroborate [BF4] as key synthetic intermediate. Thanks to simple yet not so facile aromatic nucleophilic substitution reactions, thiachromenoacridinium, azaoxa- and dioxathiotriangulenium salts (e.g., [BF4], [BF4], and [BF4], respectively) were obtained in moderate to good yields. Importantly, from the crystal structural analysis of 9-(2,6-dimethoxyphenyl)-1,8-dimethoxy-9H-thioxanthen-9-ol 16, bond lengths between sulfur and carbon atoms were confirmed to be much longer than that of classical CO or CN in analogous frameworks. This clearly triggers a distortion of the sulfur-containing six-membered ring which can be invoked to explain the relative lack of stabilization of the resulting carbenium ions. The increased electrophilicity of the central carbon set off a strongly reduced reactivity towards SNAr reactions. A comparison between electronic absorption spectra of the sulfur-bridged dyes and their aza analogues was also carried out. A ca. 50 nm shift towards the low energy region was found to occur upon the insertion of the sulfur atom.
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