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 t Bu group ( t Bu-TPIR) under N 2 atmosphere and the novel reactivities of TPIRs. Under N 2 atmosphere at room temperature, t Bu-TPIRs form SpI with a spiro carbon and a novel HABI isomer t Bu-1,4'-HABI, whose bonding pattern is different from that of the original unsubstituted HABI (1,2'-HABI). The results of 1 H NMR spectroscopy, EPR measurements, and DFT calculations revealed that SpI is generated via three steps: (1) intramolecular hydrogen transfer from the t Bu group to the nitrogen atom of the imidazole ring, (2) intramolecular cyclization of alkyl radicals, and (3) intermolecular hydrogen transfer with another t Bu-TPIR. Furthermore, we found that the thermal reaction of t Bu-TPIR at a low temperature affords the diastereomers of other isomers ( t Bu-4,4'-HABI_RS and t Bu-4,4'-HABI_RR); in other words, the thermal reaction of t Bu-TPIR depends on temperature.