Thermal Reaction Behavior of Triphenylimidazolyl Radical with a Bulky Substituent.

The triphenylimidazolyl radical (TPIR) is generated by the irradiation of the photochromic molecule hexaarylbiimidazole (1,2'-HABI). Usually, the unsubstituted TPIRs form 1,2'-HABI thermally at room temperature. In this study, we report the thermal reaction behavior of TPIR with a Bu group (Bu-TPIR) under N atmosphere and the novel reactivities of TPIRs. Under N atmosphere at room temperature, Bu-TPIRs form SpI with a spiro carbon and a novel HABI isomer Bu-1,4'-HABI, whose bonding pattern is different from that of the original unsubstituted HABI (1,2'-HABI). The results of H NMR spectroscopy, EPR measurements, and DFT calculations revealed that SpI is generated via three steps: (1) intramolecular hydrogen transfer from the Bu group to the nitrogen atom of the imidazole ring, (2) intramolecular cyclization of alkyl radicals, and (3) intermolecular hydrogen transfer with another Bu-TPIR. Furthermore, we found that the thermal reaction of Bu-TPIR at a low temperature affords the diastereomers of other isomers (Bu-4,4'-HABI_RS and Bu-4,4'-HABI_RR); in other words, the thermal reaction of Bu-TPIR depends on temperature.