Reaction of R-styreneoxide with glycine-tert-butylester yielded amino alcohols of the general formula NR1R2R3, where R1 = CH2COOtBu and R2 = R3 = 2-phenyl-2-hydroxyethyl (H2LA); R2 = 2-phenyl-2-hydroxyethyl and R3 = 1-phenyl-2-hydroxyethyl (H2LB); R2 = H and R3 = 2-phenyl-2-hydroxyethyl (HLC); and R2 = H and R3 = 1-phenyl-2-hydroxyethyl (HLD). The corresponding reaction with sarcosine-tert-butylester and subsequent hydrolysis provided the zwitterion +NH(CH3){CH2CHPh(OH)}(CH2CO2-), HLE* (asterisk refers to unprotected carboxylate). Reaction of these ligands with VO(OiPr)3 in CH2Cl2 gave the oxovanadium(V) complexes [VOL(OiPr)2] and [VOL2(OiPr)] (for LC and LD) or, when reacted in the presence of MeOH, [VOL'(OMe)], where L' represents the methyl ester of LA, LB, and LE. The crystal and molecular structures of R-HLC, S-HLD, R,S-HLE* x H2O, and lambda-[VO(R,S-LB')OMe] have been determined. The complex [VOLB'(OMe)] contains vanadium in a distorted trigonal-bipyramidal array (tau = 0.72), the oxo group in the equatorial plane, and methoxide and N in the apical positions, and thus, it structurally models the active center of vanadate-dependent haloperoxidases. The structure and the bonding parameters, including a particularly long d(V-N) of 2.562 A, are backed up by DFT calculations. The isolated oxovanadium(V) complexes and the in situ systems L + VO(OiPr)3 catalyze the oxidation, by cumylhydroperoxide HO2R', of prochiral sulfides (MeSPh, MeSp-Tol, PhSBn) to chiral sulfoxides plus some sulfone. The best results with respect to enantioselectivity (enantiomeric excess (ee) = 38%) were obtained with the system VO(OiPr)3/LA, and the best selectivity with respect to sulfoxide (100%) was obtained with [VOLA(OiPr)]. The reaction with the hexacoordinated [VO(OMe)(HOMe)LD*] was very slow. Oxidation of PhSBn is faster than that of MeSPh and MeSpTol. Turn-over numbers are up to 60 mol of sulfoxide mol-1 of catalyst h-1 (-20 degrees C). The unspectacular ee apparently is a consequence of flexibility of the active catalyst in solution, as shown by the 51V NMR of the catalysts [VOL(OR)] and the oxo-peroxo intermediates [VOL(O2R')]. As shown by DFT calculations, the peroxo ligand coordinates in the tilted end-on fashion in the axial or equatorial position (energy difference = 17.6 kJ/mol).

The active center of vanadate-dependent peroxidases (VPOs) is represented by vanadate covalently attached to a histidine, with vanadium in a trigonal-bipyramidal environment. Protein phosphatases and kinases are inhibited by the phosphate analog vanadate [VVO2(OH)2- and VIVO(OH)3-], which can be related to the coordination of vanadium to histidine or a hydroxide function as provided by tyrosinate or serinate. The vanadium centers in these proteins have been modeled by employing chiral ONO ligands. The penta-coordinated chiral complexes [VO(OMe)(L1)] (H2L1 = substituted diethanolamine) are distorted trigonal-bipyramidal with the methoxy group and the amine-N in the axial positions. These structural models of VPO also mimic the sulfide-oxidation activity of the peroxidases. The complexes [VO(H2O)L3] (H2L3 = Schiff-base ligands based on salicylaldehyde derivatives (o-vanillin; 2-hydroxy-naphthylaldehyde) and L- or D,L-tyrosine, or D,L-serine are tetragonal-pyramidal; the OH functions of the amino acid moieties are not directly coordinated to vanadium; they are involved, however, in complex hydrogen-bonding networks. The oxo/peroxo anion [VO(O2)(L2)2]3- (H2L2 = 2,5-dipicolinic acid) contains a slightly asymmetrically bonded O22-, featuring structural characteristics of the peroxo/hydroperoxo intermediates of the peroxidases. XD structure results are reported for the following complexes: R,S- and R,R-[VO(OMe)(L1)], K3[VO(O2)(L2)2].4.5H2O, the Tyr derivatives L-[VO(H2O)L3].MeOH and D,L-[VO(H2O)L3].H2O, and the Ser derivative D,L-[VO(H2O)L3].2H2O.

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The overall protonation constants of N,N′-piperazine-dipropionic acid (PDPA) were calculated with SUPERQUAD and PSE-QUAD computer programs, at 25 and 37°C and an ionic strength of 0.1 mol dm -3 (KCl). There is a good agreement between the values calculated with the two programs. The obtained values were checked by a simulation titration curve and by the protonation (observed and calculated) curves for C L = 2.0716 mmol dm -3 at 25°C. A two-step deprotonation mechanism is proposed. In the first step the deprotonation occurs to the carboxyl group and in the second step it is the nitrogen atom of the piperazine ring which undergoes deprotonation. To cite this article: C.