キノン類の電子移動還元反応における希土類イオンの触媒作用と対アニオン効果

摘要

Scandium ion-promoted electron transfer reactions of rho-benzoquinone are remarkably accelerated when tetrakis(pentafluorophenyl)borate anion is used instead of trifluoromethanesulfonate anion as the counter anion of scandium ion. Only a catalytic amount of scandium borate salt (Sc[B(C_6F_5)_4]_3) accelerates significantly the Diels-Alder reaction of 9,10-dimethylanthracene with rho-benzoquinone, which proceeds via Sc~(3+)-promoted electron transfer from the anthracene to rho-benzoquinone. The triplet-triplet absorption maximum of the photo-excited triplet state of rho-benzoquinone formed by the laser flash photolysis of an acetonitrile solution of rho-benzoquinone was red-shifted in the presence of scandium ion due to the complex formation between the triplet excited state of rho-benzoquinone and scandium ion. The shift of the triplet-triplet absorption maximum is correlated with the the Lewis acidity and the catalytic reactivity of metal ions.