Photocyclization can be carried out with ortho-, meta-, and para-substituted stilbene substrates.
ortho-Substituted substrates generally give 1-substituted phenanthrenes, unless the substituent
is a good leaving group, in which case elimination to form unsubstituted phenanthrene occurs. meta-
Substituted substrates give mixtures of 2- and 4-substituted products.
Substitution of the exocyclic double bond is well tolerated. Polycyclic aromatic compounds can be
synthesized using substrates containing multiple aromatic rings.
Stilbene derivatives containing fused aromatic systems may cyclize using either of two nonequivalent
ortho carbons. Which carbon reacts depends on both steric and electronic factors. Electronically, the
dihydrophenanthrene intermediate exhibiting greater aromatic stabilization is preferred. For instance, in
1-naphthyl-2-phenylethylene, electronic factors favor the formation of 1 over 2 in a ratio of 98.5:1.5.
ortho-Terphenyl substrates cyclize to the corresponding triphenylenes in the presence of an
oxidant, such as iodine. Oxygen is unsatisfactory because ring-opening to highly stabilized terphenyl is
faster than oxidation when oxygen is used.
Amides may cyclize to form lactams. Esters, which exist primarily in the trans conformation about the
C-O single bond, do not undergo this process efficiently.
Photocyclization can also form five-membered rings. In the vinyl naphthalene series, both oxidative and
non-oxidative processes are possible; although the latter requires a proton-transfer catalyst.
Cyclization of arylvinyl- or diarylamines provides indolines and carbazoles, respectively. In one
interesting example, the use of circularly polarized light provided 3 in slight enantiomeric excess.
In 2015, Li and Twieg reported a novel derivative of Mallory type photocyclizations and named it as photocyclodehydrofluorination (PCDHF). In the cyclization a stilbene (or ortho-terphenyl) with a pentafluorophenyl group, the fluorine atom can be used as a facile leaving group.