The photoreactions of electron transport quinones vitamin K1 (1) and plastoquinone-1 (2) were studied by picosecond pump-probe spectroscopy, nanosecond flash photolysis, step-scan FTIR spectroscopy, and by irradiation in glassy solvents at 77 K with optical and EPR detection. In polar solvents, charge transfer from the beta,gamma-double bond to the quinone moiety initiates intramolecular proton transfer from the side chain, k = 2.7 x 10(11) s-1, yielding 1,3-quinone methide diradicals, which establish a metastable equilibrium between the singlet and the triplet state. Subsequent proton transfer through the solvent forms 1,2-quinone methides. In apolar solvents the predominant primary photoreaction is formation of cyclic 'preoxetane' diradicals (kform > 3 x 10(11) s-1, kdecay approximately 1 x 10(9) s-1), which revert to a photostationary E/Z mixture of the starting materials, unless they are trapped by oxygen. The beta,gamma-double bond in the isoprenoid side chain thus provides two efficient deactivation processes, which prevent the intermolecular photoreactions commonly observed with the parent quinones. A combined mechanistic scheme rationalizes the known photoreactions of 1, 2 and related compounds in solution.

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Chiral ligands coordinated to metal ions exert a selectivity towards the additional coordination of racemic substrates. Experimentally determined equilibria distributions of [CO(L'),]~' and [Co(L3)(L2)(X)y+ are compared with calculated data based on strain-energy minimization (L3: trap = propane-1,2,3-triamine; 1,2,4-trdb = butane-1,2,4-triamine; 1,2,3-trab = butane-I ,2,3-triamine; 1,3,4-trpe = pentane-1,3,4-triamine; 1,3,4-tmeb = 2-methylbutane-1,3,4-triamine; 1,2,4-trpe = pentane-l,2,4-triamine; L2: en = ethane-l,2-diamine; pn = propane-1,2-diamine; X: NH,, OH2, OH-). Equilibration of Co(II1) complexes was achieved by oxygenation of aqueous solutions of Co(1I) salts in presence of the ligands. Quantitative isomer distribution was investigated with HPLC, and quantitative analysis of the enantiomeric excess (ee) of the racemic substrate (present in a two-fold excess) was studied, after chromatographical recovery, by 'H-NMR analysis of its Mosher-acid derivative. There is good agreement between calculated and experimental data. Systems with L = 1,2,4-trab are, as expected, relatively poorly discriminating (ee([C0(1,2,4-trab)~]~') -5 %; ee([Co(l,2,4-trab)(pn)(X)y') -10%). Calculations indicate that Me substitution of the ligand backbone of 1,2,4-trab (and trap) leads to an increased enantioselectivity (with practically constant isomer selectivity), and at the optimum site for substitution -90% ee is predicted.Introduction. -Our studies of steric interactions of coordinated ligands are aiming at producing chiral discrimination upon coordination of racemic ligands to chiral matrices (formation of stable diastereoisomers) [ 1-61. This involves: i ) ligand design by molecularmechanics calculations and ligand synthesis; ii) stereoselective ligand-exchange reactions, uiz. equilibration of the chiral matrix with a racemic substrate; iii) recovery of the (partly) resolved racemic substrate and analysis of the amount of chiral induction.The ligand systems described here are summarized in Fig. 1. They are based on trap and 1,2,4-trab, which are the smallest achiral and chiral tridentate triamines, respectively.