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.