Experimental and theoretical exploration of the detailed mechanism of the rearrangement of barrelenes to semibullvalenes: diradical intermediates and transition states

Visible‐Light‐Induced Di‐π‐Methane Rearrangement of Dibenzobarrelene Derivatives

“…Their structural assignment is based on the NMR‐spectroscopic data, especially the NOE effects between the protons 4b‐H and 4‐H, 4b‐H and 5‐H, and 8b‐H and 8‐H as determined by ROESY NMR spectroscopy (Scheme ; see also the Supporting Information). These highly regioselective DPM reactions are in agreement with results from Zimmerman et al . who have shown that the irradiation of dibenzobarrelene 1 j leads to the selective formation of the biradical intermediate BR1 and subsequent reaction to the dibenzosemibullvalene 2 j .…”

The Triplet Surface of the Zimmerman Di‐π‐Methane Rearrangement of Dibenzobarrelene

“…The vinylcyclopropane product is formed after coupling of the excited and the ground state through a conical intersection, which affords a fast and effective decay channel to reach the ground state. [6] However, a mechanism skipping the BR-I intermediate has been suggested invoking a direct 1,2-aryl shift [6][7][8][9][10] (dashed line in Scheme 1). [5] In polycyclic molecules, the DPM rearrangement occurs via a triplet state.…”

“…This is consistent with a mechanism in which the transition states leading to isomeric triplet biradicals of BR-I determine the regioselectivity of the reaction. [6] However, a mechanism skipping the BR-I intermediate has been suggested invoking a direct 1,2-aryl shift [6][7][8][9][10] (dashed line in Scheme 1).…”

The photochemistry of 8-methyl 1-2,3-benzobicyclo[4.2.0]octa-2,4,7-triene

“…Interestingly, Robb and co-workers obtained results from a theoretical approach that suggest some or all the steps in the traditional Zimmerman mechanism may merge into one step for singlet DPMRs, and the reaction may become concerted (17). On the other hand, for the triplet reaction in constrained systems, Zimmerman recently reported further experimental and theoretical work in support of the stepwise mechanism (18).…”