Metal ion-catalyzed Diels-Alder and hydride transfer reactions. Catalysis of metal ions in the electron-transfer step.

Rates of Diels-Alder cycloaddition of anthracenes with p-benzoquinone and its derivatives as well as rates of hydride-transfer reactions from 10-methyl-9,10-dihydroacridine to the same series of p-benzoquinones are accelerated significantly in the presence of metal ions in acetonitrile. An extensive comparison of the catalytic effects of metal ions in electron transfer from one-electron reductants (cobalt tetraphenylporphyrin and decamethylferrocene) to p-benzoquinones with those in the Diels-Alder reactions of the quinones as well as the hydride-transfer reactions has revealed that the catalysis of metal ions in each case is ascribed to the 1:1 and 1:2 complexes formed between the corresponding semiquinone radical anions and metal ions. The transient absorption and ESR spectra of the semiquinone radical anion-metal ion complexes are detected directly in the electron-transfer reduction of p-benzoquinone derivatives in the presence of metal ions. The catalytic reactivities of a variety of metal ions in each reaction are well correlated with the energy splitting values of pi(g) levels because of the complex formation between O(2)(.-) and M(n+), which are derived from the g(zz) values of the ESR spectra of the O(2)(.-)-M(n+) complex.