Copper(I)-catalyzed [2+2] photocycloadditions with tethered linkers: Synthesis of syn-photodimers of dicyclopentadienes

Cu(I)-catalyzed intramolecular photocycloadditions of diesters. made of end o-dicyclopentadiene derivatives-linked by the eater bonds with tethers are highly regio- and stereoselective and complete within hours; and the tether s can be easily cleaved afterward upon reduction with LiAlH4. Irradiation o f 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 pr oducts after irradiation for several days. The role of the tether's; length and structure on the course of the photocycloadditions was investigated, a nd it was observed that short tethers introduce considerable strain in the products' framework. Adamantyl-containing tethers provided the shortest rea ction times and highest yields. X-ray diffraction analysis of an exo-cis-ex o stereoisomer containing adamantane in the tether exhibited an unusually c lose approach between H atoms on the methylene bridges and a long C-C dista nce in the cyclobutane ring, A rearrangement induced by X-ray irradiation w as observed in this molecule.