Electrochemistry of Electron Transfer Probes. Observation of the Transition from Activation to Counterdiffusion Control in the Fragmentation of α-Aryloxyacetophenone Radical Anions1a
摘要:
The cleavage of radical anions of substituted alpha-phenoxyacetophenones, X-C6H4COCH2OPh, IIa-k, has been studied in DMF by voltammetric and coulometric techniques. The standard potentials (E degrees) for formation of and rate constants, k, for the cleavage of the radical anions were determined using linear sweep voltammetry, LSV, together with digital simulation and previously reported laser flash photolysis data. The rate constants cover a range of almost eight orders of magnitude (0.4 s(-1) for X = p-MeCO- to 1.3.10(7) s(-1) for X = p-MeO-). The relative driving forces, Delta Delta G degrees(het)(RX.-), for the heterolytic cleavage of the radical anions (to give R-. + X-) were estimated from thermochemical cycles. A combined plot of log(k) versus Delta Delta G(het)degrees(RX.-) for the radical anions of IIa-k and of alpha-aryloxyacetophenones gave a curve with alpha = 0.5 at high driving forces and alpha = 1 at low driving forces, where alpha = partial derivative Delta G(0)(double dagger)/partial derivative Delta G degrees. The plot was analyzed using a model in which reversible cleavage of the radical anions takes place inside the solvent cage followed by (counter)diffusion of the fragments out of the solvent cage. The change in the value of a is interpreted as a change in the rate limiting process from chemical activation (i.e.,, fragmentation) to counterdiffusion. The model allowed the determination of the absolute values of Delta G(het)degrees(RX.-) and the intrinsic barrier, Delta G(0)(double dagger), for the fragmentation of the radical anions (8 +/- 1 kcal mol(-1), 0.35 eV). This leads to an estimate of the homolytic bond dissociation free energy of the C-OPh bond in the unsubstituted alpha-phenoxyacetophenone.
Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones
作者:Claude P. Andrieux、Jean-Michel Savéant、André Tallec、Robert Tardivel、Caroline Tardy
DOI:10.1021/ja963674b
日期:1997.3.1
of the dissociative electron transfer reaction, and, in the stepwise cases, the determination of the cleavage rate constants and the standard potentials for the formation of the anionradical. Analysis of the data, using thermodynamical parameters derived from experiment and from literature points to three mechanism governing factors, the oxidability of the leavinggroup, the bond dissociation energy
Jovanovic, S. V.; Renaud, J.; Berinstain, A. B., Canadian Journal of Chemistry, 1995, vol. 73, # 2, p. 223 - 231
作者:Jovanovic, S. V.、Renaud, J.、Berinstain, A. B.、Scaiano, J. C.
DOI:——
日期:——
Solvent Reorganization as a Governing Factor in the Kinetics of Intramolecular Dissociative Electron Transfers. Cleavage of Anion Radicals of α-Substituted Acetophenones
作者:Claude P. Andrieux、Jean-Michel Savéant、André Tallec、Robert Tardivel、Caroline Tardy