On the question of 1,4-diradical intermediates in the di-.pi.-methane rearrangement of benzobicyclo[3.2.1]octadienes: azoalkanes as mechanistic probes

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

“…Here, it is suggested that the DPM mechanism involves closure to an alternative vinylcyclopropane, shown as octavalene 33 (Scheme 7, route i), which then undergoes thermal reorganization to the COTS. A similar rationale was invoked to account for the formation of the cyano-COTS 27 and 28 from the direct photolysis of triene 2 (6), and independently generated 1,3-cyclobutanediyls like the proposed intermediate 32 are reported to give bicyclobutanes by bonding at the radical centres (20 COT 24) may also be via 2u, + 2~r , electrocyclic opening (21) of the cyclobutene ring of the triene (note Scheme 7, route ii). Unfortunately, the labelling pattern used in this study does not permit us to discern to what extent this pathway is used in direct irradiations.…”

“…The results of deuterium-labelling experiments have shown that at least three routes can be used to generate the SB product, and in some cases their mode of formation has been shown to be multiplicity dependent, e.g., triene 7 gives SB 9 via a l,2-shift mechanism (route i, Scheme 2) from S,, but utilizes a Zimmerman di-'TFmethane rearrangement (DPM rearrangement) (20), which starts with the addition of benzyne to o-methylanisole, is outlined in Scheme 3. The first four steps in the synthetic sequence follow those used in the reported preparation of the 7,s-dimethyl triene 3 (8).…”

“…For example, in both trienes the "donor" portion is common (styrene, IPv = 8.43 eV (25)), and the electron affinities of models for the "acceptor" vinyl groups are nearly identical (cf. EAv = 2.27 eV (26) for the tetramethylethylene moiety in 3, versus an EAv = 2.24 eV (26) for the trimethylethylene moiety in 20).…”

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).…”