Photoaddition of maleimide to anisole

Abstract

Irradiation in the maleimide–anisole charge-transfer band gives three isomeric 2 : 1 adducts which result formally from initial 1,2-, 2,3-, and 3,4-additions to the aromatic ring followed by Diels–Alder addition of a second maleimide molecule. The additions are strongly inhibited by the strong proton donor trifluoroacetic acid, yet are markedly promoted by methanol, though not by acetonitrile. In the case of methanol, unfiltered irradiation greatly promoted formation of the isomeric adduct derived formally by 1,2-cycloaddition to anisole, but this selective effect was not observed on irradiation exclusively in the charge-transfer band. Maleic anhydride does not undergo corresponding reactions under comparable conditions.