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 10 7 starting from R ) H to Bu t and CMe 2 COOMe. 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 CH 2 Ph) 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 (∆H q obs ) -32, -4, and -13 kJ mol -1 , respectively). Such a negative ∆H q 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 ∆H q 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.

Hydride-exchange reactions between NADH and NAD+ model compounds under non-steady-state conditions. Apparent and real kinetic isotope effects

“…25 In view of our results obtained for the proton transfer reactions, we regarded a similar 2-step mechanism as a likely possibility for the hydride exchange reaction (2) (3) (4) between MAH and BQCN ϩ . In this paper we report kinetic data consistent with the 2-step mechanism and present evidence that an intermediate electron donor acceptor (EDA) complex lies on the reaction coordinate.…”

Detection of a Radical Cation of an NADH Analogue in Two‐Electron Reduction of a Protonated p‐Quinone Derivative by an NADH Analogue

“…[20] The subsequent proton (or hydrogen) transfer from NADHC + and its analogues to singly reduced species (QC À ) acting as strong bases is generally too fast to detect the radical cation. [11][12][13][15][16][17] [ ) n (n = 1, 2)].…”

“…[31] AcrHC produced by deprotonation of AcrH 2 C + is a much stronger reductant than AcrH 2 , and rapid ET from AcrHC (E ox = À0.46 V vs SCE) [11] À2 m). The resulting ESR spectrum (Figure 1 c) reasonably agrees with the computer simulation spectrum (Figure 1 d) produced using values of the hyperfine coupling constants (hfc) (a H (C-9) = 24.2, a N (NCH 3 ) = 14.0, a H (NCH 3 ) = 10.4, a H -(C-2,7) = 3.4, and a H (C-4,5) = 1.0 G) of AcrH 2 C + that were previously reported.…”

Solar Photochemical Oxidations of Benzylic and Allylic Alcohols Using Catalytic Organo-oxidation with DDQ: Application to Lignin Models

“…32 However, the authors note that the use of alcohols containing electron-withdrawing groups gave unsatisfactory yields (Scheme 1). This is most likely due to the one-electron reduction potential of DDQ (E red = 0.51 V vs SCE), 33 which may be too low to oxidize these electron-poor alcohols. We now report the remarkable effect of visible light on such catalytic DDQ oxidations that allows the oxidation not only of less reactive benzylic and allylic alcohols (Scheme 1) but also of β-O-4 lignin models under mild conditions.…”