Synthesis and redox behavior of 1,2-dihydro-1-oxabenz[a]azulen-2-ones

Three new 1,2-dihydro-1-oxabenz[a]azulen-2-one derivatives, 1a (R ¹=H, R²=Me), 1b (R¹=H, R²=Ph), and 1c (R¹=COOEt, R²=Me), have been synthesized by the reaction of 2-hydroxyazulene (2a) and its 1-ethoxycarbonyl derivative 2b with ethyl acetoacetate (3a) or ethyl benzoylacetate (3b) in the presence of aluminum chloride. To our knowledge, these are the first examples of this type of compound, although the yield of the products is low in some cases. Their electronic properties were studied in detail utilizing the analyses of 1,2-dihydro-1-oxabenz[a]azulen-2-one derivative 1a by the spectroscopic and voltammetric analyses. The analyses revealed that the fused α-pyrone system lowers both the HOMO and the LUMO energies, relative to those of parent azulene (10), but has much pronounced effect on the LUMO, consequently, leading to decrease in HOMO-LUMO gap, compared with those of 10. These results should be attracted to the development of amphoteric redox materials. Reactivity toward electrophilic reagents was also examined by bromination and Vilsmeier-Haack formylation reactions of 1a. To evaluate the scope of the reaction products we have examined Sonogashira cross-coupling reaction of the bromination products with trimethylsilylacetylene and conversion of the formylation product to dibromoolefin by the reaction with phosphorous ylide prepared with CBr ₄ and Ph₃P. Effective extension of the π-electron system in the ethynyl products has been revealed by the spectroscopic analysis. These reaction products would be attracted to the application as a terminal group for electronic applications.