Substituent effects on the bridging modes of photochemical rearrangements of pyrazino-, quinoxalino-, and benzoquinoxalinobarrelenes

Chemoselective photorearrangements of symmetrically substituted pyrazinobarrelene 4, quinoxalinobarrelene 5, and benzoquinoxalinobarrelenes 6 and 7 containing polar and non-polar groups under various conditions are described. In both direct and sensitized irradiation conditions, barrelenes 4–7 afforded similar bridging modes and photoproduct distributions suggesting a resemblance in the multiplicities of photoreactants upon excitation. Irradiation of pyrazinobarrelene 4a furnished almost equal amounts of photoproducts derived from DPM (vinyl–vinyl bridging) and ADPM (aryl–vinyl bridging) pathways. Pyrazinobarrelenes 4b–d underwent chemoselective rearrangements via the ADPM route. In the case of quinoxalinobarrelenes 5a–c and benzoquinoxalinobarrelenes 6b,c, vinyl–vinyl bridging was strongly favored. Benzoquinoxalinobarrelene 6a was insensitive to photochemical reactions. Heteroaryl–vinyl bonding was the preferred primary interaction in benzoquinoxalinobarrelene 7a whereas 7b favored the DPM route via vinyl–vinyl bridging. The photochemical behavior of the title compounds was explained in terms of energy minimization of the perturbed triplet state and diradical stabilization by polar and non-polar substituents. Plausible mechanisms for the photochemical reactions are also described.