Search

The achiral, nitroxyl-contg. a-amino acid TOAC (TOAC = 2,2,6,6- tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral a-amino acid Ca-Me valine [(aMe)Val], was used to prep. short peptides (from di- to hexa-) that induced the enantioselective oxidn. of racemic 1-phenylethanol to acetophenone. The best catalyst was an Na-acylated dipeptide alkylamide with the -TOAC-(aMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded b-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc- TOAC-L-(aMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.

The intramolecular dissociative electron transfer (ET) across donor–bridge–acceptor (D–B–A) systems consisting of a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,Ndimethylformamide 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. As previously found with the corresponding cis equatorial, axial conformers (Antonello and Maran, 1998 [25]), 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 orders of magnitude. X-ray crystallography structures show that the rate increase cannot be ascribed to a simple decrease of the geometrical 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 (German and Kuznetsov, 1994 [35]) shows that the rate increase is caused by a more favorable coupling etween 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

Monodisperse Au25L18(0) (L = S(CH2)2Ph) and [n-Oct4N+][Au25L18(-)] clusters were synthesized in tetrahydrofuran. An original strategy was then devised to oxidize them: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral or the negatively charged clusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au25L18+][C6F5CO2-]. As opposed to other reported redox methods, this dissociative ET approach is irreversible, easily controllable, and clean, particularly for NMR purposes, as no hydrogen atoms are introduced. By using this approach, the -1, 0, and +1 charge states of Au25L18 could be fully characterized by 1H and 13C NMR spectroscopy, using one- and two-dimensional techniques, in various solvents, and as a function of temperature. For all charge states, the NMR results and analysis nicely match recent structural findings about the presence of two different ligand populations in the capping monolayer, each resonance of the two ligand families displaying distinct NMR patterns. The radical nature of Au25L18(0) is particularly evident in the 1H and 13C NMR patterns of the inner ligands. The NMR behavior of radical Au25L18(0) was also simulated by DFT calculations, and the interplay between theory and experiments revealed a fundamental paramagnetic contribution coming from Fermi contact shifts. Interestingly, the NMR patterns of Au25L18(-) and Au25L18(+) were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.

To study the relationship between rate and driving force of intramolecular dissociative electron transfer, a series of donor-spacer-acceptor systems (D-Sp-A) has been devised and synthesized. Cis-1-aminocyclohexane-4-carboxylic acid and a perester functional group were kept constant as the spacer and the acceptor, respectively, whereas unsubstituted or variously substituted phthalimide groups were selected as the donors. X-ray crystallography and ab inition conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido) - axial(perester) conformation and the same D/A orientation. The electrochemical analysis provided clear evidence of a concerted dissociative electron-trasfer mechanism. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter being obtained by using model molecules.

The concerted dissociative reduction of di-tert-butyl peroxide (DTBP), dicumyl peroxide (DCP), and di-n-butyl peroxide (DNBP) is evaluated by both heterogeneous and homogeneous electron transfer using electrochemical methods. Electrochemical and thermochemical determination of the O−O bond energies and the standard potentials of the alkoxyl radicals allow the standard potentials for dissociative reduction of the three peroxides in N,N-dimethylformamide and acetonitrile to be evaluated. These values allowed the kinetics of homogeneous ET reduction of DTBP and DCP by a variety of radical anion donors to be evaluated as a function of overall driving force. Comparison of the heterogeneous ET kinetics of DTBP and DNBP as a function of driving force for ET allowed the distance dependence on the reduction kinetics of the former to be estimated. Results indicate that the kinetics of ET to DTBP is some 0.8 order of magnitude slower in reactivity than DNBP because of a steric effect imposed by the bulky tert-butyl groups. Experimental activation parameters were measured for the homogeneous reduction of DTBP with five mediators, covering a range of 0.4 eV in driving force over the temperature range −30 to 50 °C in DMF. The temperature dependence of the kinetics leads to unusually low preexponential factors for this series. The low preexponential factor is interpreted in terms of a nonadiabatic effect resulting from weak electronic coupling between the reactant and product surfaces. Finally, the data are discussed in the context of recent advances of dissociative electron transfer reported by Savéant and by German and Kuznestov. In total the results suggest that these peroxides undergo a nonadiabatic dissociative electron transfer and represent the first reported class of compounds where this effect is reported.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.