[4+2]-photocycloadditions of 2-morpholinopropenenitrile to three polycyclic aromatic hydrocarbons

Electronic excitation of the fused polycyclic aromatic hydrocarbons anthrac ene (1), benz[a]anthracene (2) and dibenz[a,c]anthracene (3) in the presenc e of the captodative olefin 2-morpholinopropenenitrile (4) gives rise to th ermally inaccessible [4+2]-adducts. Quenching experiments suggest participa tion of an excited triplet state only in case of 2. A stepwise [4+2]-photoc ycloaddition is assumed.