No abstract

Analysis of the ESR spectrum of the radical cation, derived from pp'-bis-{ phenyl(triphenylphosphorylidene)methyl}biphenyl reveals that spin density is found at both 31P nuclei. Zero spin density is indicated for the outer phenyl rings. In the radical cation, derived from the corresponding m,m'-isomer the spin density is delocalized over one half of the molecule and interaction is found with only one phosphorus atom. In this system the spin density in the outside ring exceeds that in the biphenyl ring. The localization of the odd electron on one half of the molecule must be caused by a strong Jahn-Teller effect. The assignment of the hyperfine coupling constants was based on spin density calculations according to the Unrestricted Hartree-Fock (UHF) method. From these calculations we also obtained an indication of the geometry of the radicals.The synthesis of these compounds is reported.

Abstract. Pyridoxal and its iminium derivatives form cyclic compounds via an intramolecular nucleophilic attack. This results in a configurational chirality on C4' which can be related to out-of-plane orientations around the C4-C4' bond in the open forms. 'H-and I3C-NMR structural studies of the cyclic compounds were performed in solution and in the solid state ( I3C NMR only). It was found that the presence of an additional chiral centre in the iminium moiety gives rise to an excess of one of the diastereomers for the cyclic aminal. This indicates a preference for one of the axially chiral C4-C4' conformers prior to ring closure. The present results may be of relevance with respect to the function of the coenzyme pyridoxal phosphate, which forms iminium compounds with L or D amino acids. Axial chirality in these systems has been proposed by us as a mechanism for the stereospecificity of enzymatic amino acid transformations.