FUNNELS AND BARRIERS IN THE PHOTOCYCLOADDITION OF ARENES TO ALKENES AND DIENES

In the photocycloaddition of benzene to alkenes, products are formed i n the ortho and meta addition modes; in the photoaddition of benzene t o the terminal atoms of a diene system, products are formed in the met a and para modes. On the basis of orbital symmetry correlations, meta photocycloaddition has been described as an allowed process. The situa tion regarding the ortho and para modes is less clear. It is shown tha t orbital symmetry correlations can also be used to predict the occurr ence of reactive minima or funnels in the potential energy surface of the first excited state of the reactants. The presence of these funnel s can be used to rationalize the abundance of ortho and meta adducts i n the photoaddition of arenes to alkenes, and the occurrence of meta a nd para adducts in the photoaddition of arenes to dienes. The role of charge transfer in the selection of the addition mode and in the regio selectivity in the ortho mode for arenes to alkenes is discussed.