On the reaction of prop-2-enylidenetriphenylphosphorane derivatives. Novel synthesis of 4,9-methanocyclopentacycloundecene derivatives and their spectroscopic and chemical properties
Novel 3-ethoxycarbonyl-2-ethoxy- and 2-ethoxy-4,9-methanocyclopentacycloundecenes 12 and 13 are synthesized by the reaction of 9-chloro-1,6-methanoannulen-8-one 8, a 10π-electron vinylogue of 2-chlorotropone, with 2-ethoxy-3-(ethoxycarbonyl)prop-2-enylidene- and 2-ethoxyprop-2-enylidenetriphenylphosphorane, 7a and 7b, respectively. The reaction pathways involve a Michael-type addition of phosphoranes 7a,b onto ketone 8, subsequent proton migration regenerating the phosphorane moiety, which undergoes intramolecular Wittig reaction, and aromatization, eliminating HCl. The 1H NMR spectra show that compounds 12 and 13 are aromatic molecules having a diatropic 14-electron cyclic π system. The UV–visible absorption spectra of compounds 12 and 13 exhibit extended cyclic conjugation of the aromatic perimeter according to a bathochromic shift of the longest absorption maxima as compared to those of the related azulene derivatives having the same substituents. In addition, the first example of an aromatic substitution reaction of the methanocyclopentacycloundecene ring system has been studied, and shows that the compounds 12 and 13, like azulene, undergo trifluoroacetylation under exceptionally mild conditions and exhibit the same site-selectivity as does azulene. The electrochemical properties of compounds 12 and 13 as well as the related azulene derivatives are also analyzed and discussed on the basis of MNDO calculations.