In organic chemistry, the Mallory reaction is a photochemical-cyclization–elimination reaction of diaryl-ethylene structures to form phenanthrenes and other polycyclic form polycyclic aromatic hydrocarbons and heteroaromatics. This name reaction is named for Frank Mallory, who discovered it while a graduate student.
Under UV irradiation, stilbene and its derivatives undergo intramolecular cyclization to form dihydrophenanthrenes. In the presence of an oxidant, the dihydrophenanthrenes aromatize to give polycyclic aromatics. Typically, the dihydrophenanthrenes themselves are relatively unstable, and revert to cis-stilbenes in the absence of a hydrogen-trapping agent. Suitably substituted stilbenes may undergo irreversible, rearomatizing elimination or [1,n]-shift processes in the absence of an oxidant. Aryl enynes, heteroatomic stilbene derivatives (e.g. amides), and substrates containing a single heteroatom in place of the stilbene double bond also undergo the reaction.