Syntheses, Thermal Reactivities, and Computational Studies of Aryl-Fused Quinoxalenediynes: Effect of Extended Benzannelation on Bergman Cyclization Energetics

摘要

A series of [b]-fused 6,7-diethynylquinoxaline derivatives have been synthesized through an imine condensationnstrategy to examine the effect of extended benzannelation on the thermal reactivity of enediynes. Absorption and emissionnspectra of the highly conjugated quinoxalenediynes were red-shifted approximately 100−200 nm relative to those of 1,2-ndiethynylbenzene. Strong exotherms indicative of enediyne cyclization were observed by differential scanning calorimetry, whilensolution cyclizations in the presence of 1,4-cyclohexadiene confirmed C1−C6 Bergman cyclization. To provide further insight intonBergman cyclization energetics, computational studies were performed to compare changes in the cyclization enthalpy barrier,nreaction enthalpy, and barrier of retro-Bergman ring-opening. Extension of benzannelation from 1,2-diethynylbenzene to eithern2,3-diethynylnaphthalene or the 6,7-diethynylquinoxalines had a minimal effect on the cyclization barrier. In comparison, thenenthalpies of cyclization were increased upon linearly extended benzannelation, which resulted in reduced barriers to retro-nBergman ring-opening. In addition, the orientation of extended benzannelation was found to have a significant effect on thencyclization endothermicity. In particular, 5,6-diethynylquinoxaline exhibited a 6.9 kcal/mol decrease in cyclization enthalpyncompared to 6,7-diethynylquinoxaline due to increased aromatic stabilization energy in the respective angularly versus linearlynfused azaacene cyclized products.