Diels-Alder reactions of anthracenes with dienophiles via photoinduced electron transfer

Photochemical Diels-Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of anthracenes in chloroform at 298 K. The dependence of the quantum yields on the concentrations of dienophiles has revealed that the photochemical Diels-Alder reaction proceeds via electron transfer from the singlet excited states of anthracenes to dienophiles. The rates of photoinduced electron transfer are diffusion limited, agreeing with the largely negative free energy change of electron-transfer judging from the more negative one-electron oxidation potentials of the singlet excited states of anthracenes than the one-electron reduction potentials of dienophiles. The radical ion pair produced in the photoinduced Diels-Alder from the singlet excited states of anthracenes to p-benzoquinone and fumaronitrile has been detected as the transient absorption spectrum at 298 K with use of laser flash photolysis. The diradical intermediates prior to the Diels-Alder adduct formation following the photoinduced electron transfer have successfully been detected by ESR at 77 K under photoirradiation of chloroform solutions of anthracene derivatives and p-benzoquinones.