Stereocontrolled photo-reaction pathways of endo/exo-2-benzoyl-substituted bicyclo[2.2.2]oct-5-en-2-ol:: Paterno-Buchi reaction versus α-cleavage

Two stereoisomeric ketones, endo- and exo-(2-hydroxy-[2.2.2] bicyclo-5-en-1-yl)-phenyl methanone (endo-2 and exo-2) were synthesized via a Lewis acid catalyzed Diels-Alder reaction. Both compounds were tested in terms of their efficiency as photoinitiators for radical polymerization. Whereas the exo isomer serves as a good photoinitiator, the curing efficiency is poor in the case of the endo derivative. CIDNP investigations and product analysis by NMR and GC-MS together with density functional calculations reveal the distinctly different reaction pathways of the two isomers. On one hand, exo-2 undergoes alpha-cleavage from the triplet excited state forming a radical pair that is able to induce polymerizations. On the other hand, endo-2 cyclizes in a Paterno-Buchi reaction yielding a tetracyclic product 11 and no formation of radicals is observed.