Copper(I)-Catalyzed [2 + 2] Photocycloadditions with Tethered Linkers:  Synthesis of Syn-Photodimers of Dicyclopentadienes

Cu(I)-catalyzed intramolecular photocycloadditions of diesters made of endo-dicyclopentadiene derivatives linked by the ester bonds with tethers are highly regio- and stereoselective and complete within hours, and the tethers can be easily cleaved afterward upon reduction with LiAlH4. Irradiation of the diesters afforded a 1:1 mixture of the heretofore unknown exo-cis-exo dimer, originating from the (R,S/S,R) diastereomer of the diester and the exo-trans-exo, deriving from the (R,R/S,S) diastereomer. The intermolecular photodimerization yielded, instead, only exo-trans-exo isomers and side products after irradiation for several days. The role of the tether's length and structure on the course of the photocycloadditions was investigated, and it was observed that short tethers introduce considerable strain in the products' framework. Adamantyl-containing tethers provided the shortest reaction times and highest yields. X-ray diffraction analysis of an exo-cis-exo stereoisomer containing adamantane in the tether exhibited an unusually close approach between H atoms on the methylene bridges and a long C−C distance in the cyclobutane ring. A rearrangement induced by X-ray irradiation was observed in this molecule.