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.