Enediynes can undergo multiple cyclization channels to produce reactive diradical intermediates upon exposure to heat or light. The most common reaction pathway, C1-C6 Bergman cyclization, is preferred for simple enediynes such as (Z)-hex-3-ene-1,5-diyne due to a large gain in aromatic stabilization energy. Upon inserting the alkene unit of the enediyne within an aromatic core, the gain of aromatic stabilization energy is attenuated and the resulting Bergman cyclization becomes reversible. Furthermore, introduction of bulky substituents on the alkyne termini hinders C1-C6 cyclization and can lead to an alternative C1-C5 cyclization pathway. To examine how extended benzannelation orientation, combined with alkyne substitution pattern, influence reaction channels of enediynes we have undertaken a combined theoretical and experimental study on the reactivity pathways of isomeric 5,6-bis(phenylethynyl)quinoxaline and 6,7-bis(phenylethynyl)quinoxaline derivatives. With increased linear benzannelation the 5,6-isomer shows limited reactivity thermally at temperatures up to 300 °C. In comparison, the angular 6,7-bis(phenylethynyl)quinoxaline shows evidence of C1-C6 cyclization thermally as well as photochemically while calculations support this pathway as the preferred reaction channel. A full study including thermal and photochemical reactivity profiles for these two isomers, along with calculated barriers and reaction energies for C1-C5 and C1-C6 cyclizations, will further elaborate features that control diradical cyclization channels of these highly conjugated enediynes.

Syntheses and reactivity of phenylethynyl-substituted quinoxalenediyne isomers

251st National Meeting (2016)

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