| Abstract
| - Chemoselective photorearrangements of pyrazino-, quinoxalino-, and benzoquinoxalinobarrelenes wereinvestigated by deuterium-labeling experiment. Photolysis of pyrazinobarrelene 4 and quinoxalinobarrelene5 with 300 nm region light under either direct or sensitized conditions afforded semibullvalenes 7 and 8,respectively; benzoquinoxalinobarrelene 6 was inert to both reaction conditions. Irradiation of deuteratedpyrazinobarrelene 4-d4 and quinoxalinobarrelene 5-d4 afforded deuterated semibullvalenes 7-d4-A−7-d4-F and 8-d4-A−8-d4-F, respectively. Of the two a priori possibilities of bridging, the deuterium-labelingexperiment has shown that deuterated pyrazinobarrelene 4-d4 afforded 98% (C6D6) and 95% (CD3CN)of semibullvalenes generated through aryl−vinyl (A−V) bridging, whereas the quinoxalinobarrelene 5-d4furnished 79% (C6D6) and 71% (CD3CN) of semibullvalenes generated through vinyl−vinyl (V−V)bridging. The contrasting photochemical behavior of heteroarene-fused barrelenes 4−6 was explainedqualitatively in terms of triplet energy minimization and relative stability of diradicaloid intermediates.
|
