Intramolecular 10,10a-[2+2] photocycloaddition reactions of phenanthrenes with linked styrene

•Unprecedented photocycloaddition to the 10,10a-positions of phenanthrene occurs.•High-yield synthesis of novel eight membered ring compounds is achieved.•Donor-acceptor effects govern the structures of singlet intramolecular exciplexes.

Intramolecular photocycloaddition reactions of styrene linked 9-cyanophenanthrenes led to formation of C-9 C-10 [2+2] cycloadducts which underwent cycloreversion under the prolonged irradiation conditions to regenerate the starting substrates. Unusual 8-membered ring products were irreversibly formed in these processes. Product distributions were governed by substituent controlled, donor-acceptor interactions that direct generation of two intramolecular singlet exciplexes, which serve as intermediates in the pathways to formation of 9,10- and 10,10a-[2+2] adducts.

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