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

Abstract

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 But 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.