Chemoselective photorearrangements of diazinobarrelenes. Deuterium labeling study

Chemoselective photorearrangements of pyrazino-, quinoxalino-, and benzoquinoxalinobarrelenes were investigated by deuterium-labeling experiment. Photolysis of pyrazinobarrelene 4 and quinoxalinobarrelene 5 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 deuterated pyrazinobarrelene 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-labeling experiment 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-d4 furnished 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 explained qualitatively in terms of triplet energy minimization and relative stability of diradicaloid intermediates.