Regioselectivity of the Di-π-Methane Rearrangements of 1,4-Dialkoxy-9,10-bis(methoxycarbonyl)dibenzobarrelenes in Solution and in the Solid State

Abstract

Irradiation of solutions of the title compounds led to the regioselective formation of one of the two possible dibenzosemibullvalenes, as a consequence of the stabilization of intermediate biradicals by the alkoxy groups present. Continued irradiation of the reaction mixture converted the main product into a dibenzocyclooctatetraene as secondary photoproduct. Irradiation of the solid neopentoxy-substituted dibenzobarrelene afforded, almost exclusively, the dibenzosemibullvalene that is the minor photoproduct on irradiation in solution. Presumably, due to steric constraints in the crystal, the formation of the other isomer is prevented.

Photochemical Reactions as Key Steps in Organic Synthesis

“…The di-π-methane rearrangement is frequently described with barrelene derivatives . Transformation of the dibenzobarrelene 76 has been carried out in a crystal (Scheme , eq 18) . The reaction also occurs with heterocyclic aromatic compounds or in ionic liquids …”

“…146 Transformation of the dibenzobarrelene 76 has been carried out in a crystal (Scheme 27, eq 18). 147 The reaction also occurs with heterocyclic aromatic compounds 148 or in ionic liquids. 149 A heteroatom variant of this reaction also exists.…”

Photoisomerisation of dibenzobarrelenes—a facile route to polycyclic synthons

“…23 Dibenzocyclooctatetraenes are also formed from dibenzosemibullvalenes. A recent report 24 states that the dibenzocyclooctatetraene 45 is formed as a secondary photoproduct from the dibenzosemibullvalene 42 (Scheme 12). It has been suggested that the reaction proceeds through s-bond cleavage to give the 1,3-diradical 43, which subsequently undergoes an aryl 1,2-shift to give the 1,4-diradical 44.…”