Chemo- and stereoselective intermolecular [2 + 2] photocycloaddition of conjugated dienes using colloidal nanocrystal photocatalysts

The use of visible-light photosensitizers to power [2 + 2] photocycloadditions that produce complex tetrasubstituted cyclobutanes is a true success of photochemistry, but the scope of this reaction has been limited to activated α,β-unsaturated carbonyls. This paper describes selective intermolecular homo- and hetero-[2 + 2] photocycloadditions of terminal and internal aryl-conjugated dienes—substrates historically unsuited for this reaction because of their multiple possible reaction pathways and product configurations—through triplet-triplet energy transfer from CdSe nanocrystal photocatalysts to generate valuable and elusive syn-trans aryl vinylcyclobutanes. The negligible singlet-triplet splitting of nanocrystals' excited states allows them to drive the [2 + 2] pathway over the competing [4 + 2] photoredox pathway, a chemoselectivity not achievable with any known molecular photosensitizer. Reversible tethering of the cyclobutane product to the nanocrystal surface results in near-quantitative yield of the syn-trans product. Flat colloidal CdSe nanoplatelets produce cyclobutanes coupled at the terminal alkenes of component dienes with up to 89% regioselectivity. [Display omitted] •A photocatalytic system to selectively generate syn-trans aryl vinylcyclobutanes•CdSe nanocrystal photocatalysts enable selectivity for [2 + 2] over [4 + 2] coupling•Reversible tethering of substrates to nanocrystals enables syn-trans selectivity Cyclobutane derivatives are a valuable class of molecules for the pharmaceutical industry. [2 + 2] photocycloadditions provide access to these challenging scaffolds from simple olefins. Previously, the scope of stereoselective [2 + 2] photocycloadditions did not include terminal or aryl-conjugated dienes because they have several competing coupling and isomerization pathways and their valuable coupling products, vinylcyclobutanes, have many potential stereoisomers. Here, we achieve exceptional chemo- and stereoselectivity for intermolecular [2 + 2] photocouplings of internal and terminal dienes to syn-trans vinylcyclobutanes by using the unique electronic structure and surface templating ability of colloidal CdSe nanocrystals. This work not only expands the scope of [2 + 2] photocycloaddition reactions but also introduces fundamentally new mechanisms of selectivity for these couplings using colloidal photocatalysts. Direct coupling of alkenes into cyclic molecules through photocycloaddition is a shortcut for forming complex molecules wi