Photoaddition of N-ethylmaleimide to cumene

The photoaddition of N-ethylmaleimide (NEMI) to cumene is compared with that of benzene. The equilibrium constants of the CT-complexes were determined by means of ¹H-N.M.R. spectroscopy (K_benzene-NEMI = 0.035 kg mole^?1, K_cumene-NEMI = 0.049 kg mole). By irradiation with light of wavelength 254 nm the 2:1 adduct cumene-NEMI is formed. The structure was determined by ¹ H-N.M.R. and mass spectrometry. These data indicate that only one of the possible isomers is formed and that the free rotation of the isopropyl group is hindered in the adduct. Furthermore two isomeric 1 : 1 adducts could be detected and assigned to the 1 : 1 cycloaddition product and to cumyl-N-ethyl-succinimide.