No abstract

The voltammogram of aryldiazonium tetrafluoroborates in acetonitrile (ACN), at low concentration, shows a first one-electron wave followed at a more negative potential by a small second wave; this last one corresponds to the reduction of the radical formed at the level of the first wave. Simulation of the voltammogram permits one to determine the standard redox potential of the radical/anion couple Eo(Ph./Ph-) = 0.05 V/SCE and the reduction mechanism of the diazonium cation. An electron transfer concerted with the cleavage of the C-N bond furnishes the aryl radical; this radical undergoes two competitive reactions: reduction at the electrode and H-atom transfer.

An approach has been described for structural modelling of relatively small molecules by NMR. The fmt step involves setting up primary restraints to account for the covalent structure of the molecule of interest. For 12,13-diepi-F430, covalent angles and distances were derived from X-ray crystallographic data published for related coenzyme B12 derivatives. In general, standard van der Waals radii were employed, with the exception that atoms of the five-and six-membered rings of the macrocycle were allowed to approach to distances less than the sums of their van der Waals radii in order to maximize conformational sampling. It is essential to utilize proper primary restraint parameters when extending algorithms designed for structural modelling of biopolymers to small molecules, and one must demonstrate that the primary restraints alone allow for sufficient conformational flexibility.Structures generated by our distance geometry approach using only primary restraints exhibited significant conformatiomal diversity, with each of the unsaturated fivemembered pyrrole units and the six-membered ring existing in several low-penalty conformations. Back-calculated N O S Y spectra for these DG/SA structures were shown to deviate significantly compared to the experimental NOESY spectra.Structures generated subsequently with the addition of "loose" NOE-derived distance restraints converged to a unique conformation, and the similarity of the back-calculated and experimental NOESY spectra indicates that the DG(+NOE) structures are reasonable representatives of the solution-state conformation of 12,13-diepi-F430. The DG(+NOE) structures compare favorably with the X-ray structure of 12,13-diepi-F430M, with pairwise RMSDs for the 24 macrocyclic ring atoms of ca. 0.28 A. The DG(+NOE) and X-ray structures exhibit a similar corphin ring conformation that is characterized by an extreme saddle-shaped ring deformation, which was predicted on the basis of X-ray studies of Ni(I1)-containing hydrocorphinoid c~mplexes.l~-'~ The NMR-based approach has several limitations; in particular, regardless of the quality of N M R data, the "resolution" of the N M R structures will certainly never approach the high resolution obtainable by X-ray diffraction techniques. In addition, the N M R approach relies intimately on knowledge of the primary structure, including atomic stereochemistry and electronic conjugation, and without full knowledge of these parameters the implementation of the NMR-based approach should be avoided. Nevertheless, for molecules that have proven difficult or impossible to crystallize, such as native F430, the N M R approach provides an attractive alternative for structural modelling. Applications to other small-molecule systems for which high-resolution X-ray structural results are available will establish the reliability and ultimate potential of this technique.Abstract: A systematic investigation of the kinetics of the electrochemical reduction of a series of 11 arylmethyl halides in acetonitrile and N,N'-dimethylformamide re...