Scandium Ion-Promoted Reduction of Heterocyclic NN Double Bond. Hydride Transfer vs Electron Transfer

Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH2) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph2Tz), which contains a NN double bond, occurs efficiently in the presence of Sc(OTf)3 (OTf = OSO2CF3) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc3+. The observed second-order rate constant (k obs) increases with increasing Sc3+ concentration to approach a limited value. When AcrH2 is replaced by the dideuterated compound (AcrD2), the rate of Sc3+-promoted hydride transfer exhibits the same primary kinetic isotope effect (k H/k D = 5.2±0.2), irrespective of Sc3+ concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP2- = tetraphenylporphyrin dianion) and 10,10‘-dimethyl-9,9‘-biacridine [(AcrH)2] to Ph2Tz, whereas no electron transfer from CoTPP or (AcrH)2 to Ph2Tz occurs in the absence of Sc3+. In each case, the observed second-order rate constant of electron transfer (k et) shows a first-order dependence on [Sc3+] at low concentrations and a second-order dependence at higher concentrations. Such dependence of k et on [Sc3+] is ascribed to formation of 1:1 and 1:2 complexes between Ph2Tz•- and Sc3+ at the low and high concentrations of Sc3+, respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph2Tz•-. The 1:2 complex formation results in the saturated kinetic dependence of k obs on [Sc3+] for the Sc3+-promoted hydride transfer, which proceeds via Sc3+-promoted electron transfer from AcrH2 to Ph2Tz, followed by proton transfer from AcrH2 •+ to the 1:1 Ph2Tz•-−Sc3+ complex and the subsequent facile electron transfer from AcrH• to Ph2TzH•. The effects of counteranions on the Sc3+-promoted electron transfer and hydride transfer reactions are also reported.