Hydride transfer from 9-substituted 10-methyl-9,10-dihydroacridines to hydride acceptors via charge-transfer complexes and sequential electron- proton-electron transfer. A negative temperature dependence of the rates

The reactivity of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) in the reactions with hydride acceptors (A) such as p-benzoquinone derivatives and tetracyanoethylene (TCNE) in acetonitrile varies significantly spanning a range of 10⁷ starting from R = H to Bu(t) and CMe₂COOMe. Comparison of the large variation in the reactivity of the hydride transfer reaction with that of the deprotonation of the radical cation (AcrHR·⁺) determined independently indicates that the large variation in the reactivity is attributed mainly to that of proton transfer from AcrHR·⁺ to A·^- following the initial electron transfer from AcrHR to A. The overall hydride transfer reaction from AcrHR to A therefore proceeds via sequential electron-proton-electron transfer in which the initial electron transfer to give the radical ion pair (AcrHR·⁺ A·^-) is in equilibrium and the proton transfer from AcrHR·⁺ to A·^- is the rate- determining step. Charge-transfer complexes are shown to be formed in the course of the hydride transfer reactions from AcrHR to p-benzoquinone derivatives. A negative temperature dependence was observed for the rates of hydride transfer reactions from AcrHR (R = H, Me, and CH₂Ph) to 2,3- dichloro-5,6-dicyano-p-benzoquinone (DDQ) in chloroform (the lower the temperature, the faster the rate) to afford the negative activation enthalpy (ΔH((+))(obs) = -32, -4, and -13 kJ mol^-1, respectively). Such a negative ΔH((+))(obs) value indicates clearly that the CT complex lies along the reaction pathway of the hydride transfer reaction via sequential electron- proton-electron transfer and does not enter merely through a side reaction that is indifferent to the hydride transfer reaction. The ΔH((+))(obs) value increases with increasing solvent polarity from a negative value (-13 kJ mol^-1) in chloroform to a positive value (13 kJ mol^-1) in benzonitrile as the proton-transfer rate from AcrHR·⁺ to DDQ·^- may be slower.