1,721,081 research outputs found
Electrochemical and stereochemical investigation on the mechanism of the decay of 2-halo amide anions. The intermediacy of aziridinones
No full text
The Role and Relevance of the Transfer Coefficient Alpha in the Study of Dissociative Electron Transfers. Concepts and Examples from the Electroreduction of Perbenzoates
No full textThe electrochemical transfer coefficient R is shown to be a sensitive probe of the mechanism by
which electron transfer and bond cleavage may be coupled in dissociative electron transfers. R is particularly
useful in detecting the transition between stepwise and concerted dissociative electron transfers. Whereas linear
potential dependencies of R are in agreement with either mechanism, a mechanism transition can be evidenced
upon observation of a nonlinear R pattern. Under favorable circumstances, a wavelike potential dependence of
R can be observed. This is a function of main parameters describing the mechanism competition such as, in
particular, the difference between the two relevant standard potentials, the intrinsic barriers, and the
preexponential factors of the two rate-constant equations. The analysis of R was applied to study the
electroreduction of a series of perbenzoates, XC6H4CO3But, in DMF. The reduction leads to the irreversible
cleavage of the O-O bond. The R data were obtained by cyclic voltammetry followed by convolution analysis.
For all compounds investigated, the experimental trend could be simulated satisfactorily by reasonable selection
of the main parameters. Whereas the analysis showed that the reduction of the unsubstituted peroxide proceeds
by a pure concerted mechanism, a stepwise mechanism holds when X ) 4-NO2. On the other hand, R-wave
patterns were found for X ) 4-COMe and 3-NO2, as previously described for X ) 4-CN. For the latter
compounds, the R analysis is in agreement with a dissociative electron transfer process in which the mechanism
changes from stepwise to concerted by increasing the applied potential. Finally, although the reduction of the
4-OCOMe perbenzoate basically occurs by a concerted mechanism, a transition pattern seems to emerge at
the most negative potentials explored. Further support to the experimental outcome and conclusions was provided
by studying the temperature effect on the reduction of the 4-COMe derivative, which led to the expected shift
toward the stepwise mechanism by lowering the temperature
Evidence for the Transition between Concerted and Stepwise Heterogeneous Electron Transfer-Bond Fragmentation Mechanisms.
No full textThe electrochemical reduction of tert-butyl p-cyanoperbenzoate in DMF leads to the cleavage of the oxygenoxygen
bond. A subsequent reaction yielding tert-butyl p-cyanobenzoate could be avoided by the addition of a mild
acid, with the result that the main reduction peak was affected only by the kinetics of the electron transfer bond
fragmentation process. Unusual features of the heterogeneous electron transfer mechanism were evidenced by the
conventional voltammetric criteria, which pointed to a nonlinear potential dependence of the transfer coefficient R.
The fine details of the electron transfer mechanism were obtained by convolution analysis of the voltammetric curves,
a procedure that led to a bell-shaped potential dependence of alfa. The observed behavior is explained by the assumption
that when the electrode potential is decreased, the process changes from a concerted electron transfer bond cleavage
to a partially stepwise mechanism. The observed potential dependence of alfa is in agreement with the results from
simple calculations aimed to obtain the potential dependences of alfa expected for the two pure mechanisms
Dependence of Intramolecular Dissociative Electron Transfer Rates on Driving Force in Donor-Spacer-Acceptor Systems
No full textThe voltammetric reduction of a series of phenyl-substituted 4-benzoyloxy-1-methylcyclohexyl bromides
has been investigated in DMF. The reduction leads to the cleavage of the C-Br bond. On a thermodynamic
ground, the direct reduction of the tertiary C-Br function is easier than that of the selected benzoates
by at least 0.5 V. However, since the direct reduction of bromides is affected by a large activation overpotential,
the electron is first located in the benzoate moiety. The rate constant for the following exergonic intramolecular
dissociative electron transfer was determined by kinetic analysis of the cyclic voltammetry curves. The
intermolecular rate constants for the reaction between the radical anions of methyl benzoates and 4-tert-butyl-
1-methylcyclohexyl bromide were also determined and found to correlate very well with related literature data
pertaining to tert-butyl bromide. The intramolecular rate constants were found to be more sensitive to variation
of driving force than the corresponding intermolecular data. This result can be attributed to a shift of the
center of the ð* orbital of the radical anion donor away from the acceptor moiety, the shift being larger for
the most easily reduced donors. The resulting distance increase is therefore envisaged as responsible for a
more rapid rate drop, compared to the intermolecular pattern, when smaller driving forces are considered
Correlation between the reduction potential of benzylic type halides and the redox potential of the pertinent radicals
No full text
Intramolecular dissociative electron transfer
No full textDissociative electron transfers (ET) are reactions in which the ET is associated with the cleavage
of a sigma bond. Although a rather satisfactory amount of information is currently available on
the intermolecular and heterogeneous dissociative ET reactions, less is known for the
corresponding intramolecular processes, despite the relevance of these reactions in both chemistry
and biochemistry. This tutorial review focuses on the most recent developments in this area, with
particular emphasis on the reactions occurring in well-defined Donor–Spacer–Acceptor molecular
systems. The goal is to provide the reader with the essential background to understand and
possibly predict the feasibility and rates of these reactions, as well as to stimulate the application
of the intramolecular dissociative ET concepts and related issues to still unexplored molecular
systems
Dependence of nonadiabatic intramolecular dissociative electron transfers on stereochemistry and driving force
No full textAbstract: The intramolecular dissociative electron transfer (ET) across donor-bridge-acceptor (D-B-A) systems consisting in a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,N-dimethylformamide has been studied by cyclic voltammetry. X-Ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated D-B-A molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation and the same D/A orientation. As previously found with the corresponding cis equatorial,axial conformers (S. Antonello, F. Maran, J. Am. Chem. Soc. 120 (1998) 5713), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the C-Br bond (acceptor A) through the 1,4-cyclohexanediyl spacer (bridge B). The intramolecular ETs are exergonic, with driving force in the range from 0.5 to 1.2 eV. The electrode process follows the same mechanism previously established for the cis series of isomers, but the ET rate constants obtained with the trans axial,axial isomers are larger by 1.1 order of magnitude. Molecular models and X-ray crystallography structures show that the rate increase cannot be ascribed to a decrease of the distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the German-Kuznetsov theory of nonadiabatic dissociative ET (E. D. German, A. M. Kuznetsov, J. Phys. Chem. 99 (1995) 9095) shows that the rate increase is caused by a more favorable coupling between the electronic wave functions describing the reagent and product states. The data and trends are discussed in comparison with other nonadiabatic intramolecular dissociative ET processes
- …
