Controlling the regioselectivity of the di-π-methane rearrangements of 1,2-naphtho-annelated barrelene derivatives--solution versus solid-state photochemistry

The synthesis of 1,2-naphtho-annelated barrelene derivatives, namely dimethyl-7,12-dihydro-7, 12-ethenobenzo[a]anthracene-13,14-dicarboxylate (4a) and dimethyl-7,14-dihydro-7,14-ethenodibenzo [a,j]-anthracene-15,16-dicarboxylate (4b), and the investigation of their photoreactivity in solution and in the solid state is reported. The irradiation of 4a and 4b resulted in regioselective di-π-methane rearrangements to give semibullvalene products; however, the product distribution is inverted upon changing the reaction medium from solution to the solid state. In solution, an [alpha-naphtho-vinyl bridging predominates as the initial photochemical step because of the selective stabilization of the corresponding transition state, as discussed on the basis of the structures of the related biradical intermediates, whereas the solid-state photoreaction is significantly influenced by the steric constraints within the crystal lattice that only allow a β-naphtho-vinyl bridging. Key words: di-π-methane rearrangement, barrelene derivatives, solid-state photochemistry. On a reahse la synthese de derives condenses 1,2-naphtobarrelenes, a savoir le 7,12-dihydro-7,12-ethenobenzo[a]anthracene-13,14-dicarboxylate de dimethyle (4a) et le 7,14-dihydro-7,14-ethenobenzo[a,j]anthracene-15,16-dicarboxylate de dimethyle (4b), et on a etudie leur photoreactivity en solution et a l'etat solide. L'irradiation des composes 4a et 4b conduit a des rearrangements di-π-methane regioselectifs et a la formation de produits semibullvalenes; toutefois, la distribution des produits est inversee lorsque le milieu reactionnel passe de la solution a 1'etat solide. En solution, un pont α-naphtovinyle predomine comme etape photochimique initiale en raison de la stabilisation selective de l'etat de transition correspondant, tel que discute sur la base des structures des intermediaires biradicalaires apparentes, alors que la photoreaction a l'etat solide est fortement influencee par des contraintes steriques dans le reseau cristallin qui ne permettent que la formation d'un pont β-naphtovinyle. Mots-cles : rearrangement di-π-methane, derives du barrelene, photochimie a l'etat solide. [Traduit par la Redaction]
Introduction The di-π-methane rearrangement (1) of benzo-annelated derivatives of barrelene (bicyclo[2.2.2]octatriene) (2) and norbornadiene (bicyclo[2.2.1]heptadiene) (3) has been shown to be an appropriate model reaction for the assessment of the [...]