Non-statistical dynamics and aromaticity as the reaction driving force on the mechanism of the Zimmerman Di-π-methane rearrangement

Abstract

The mechanism of the Zimmerman Di-π-methane rearrangement of dibenzobarrelene (DBB) has been studied using computational methods including high-level calcns., quasi-classical dynamics, and the reaction electronic flux (REF). In the triplet T_1, upon sensitization, the mechanism involves the formation of a 1,4-biradical intermediate (BR-I) from DBB* and then the formation of a 1,3-biradical bicyclic (BR-II) intermediate before the intersystem crossing leading to the dibenzosemibullvalene product in the ground state. According to previous work, the triplet di-π-methane rearrangement undergoes non-statistical dynamics in the triplet surface, thus competing one and two step mechanisms altogether. Although substituent effects can switch the mechanism when destabilizing the transition states, thus detg. The regioselectivity. Recently, even carbon tunneling effects have been reported for this photochem. reaction. We have also studied the driving force for this photorearrangement using the REF approach together with methods to assess aromaticity such as NICS, HOMA, and ACID. The triplet antiaromaticity alleviation and the so-called Baird's rule were also assessed.