Stereoselective control by face-to-face versus edge-to-face aromatic interactions: the case of C(3)-Ti(IV) amino trialkolate sulfoxidation catalysts.

The stereoselective oxidation of differently functionalised benzyl phenyl sulfides has been examined by using enantiopure Ti(IV) trialkanolamine complexes. These complexes efficiently catalyse the sulfoxidation with good stereoselectivities. The data highlight the contribution to the stereoselectivity of steric effects and non-covalent pi-pi interactions between the aromatic rings of the Ti(IV) complex and those pertaining to the substrates. Enantiomeric excesses have been correlated with the electrostatic potential surfaces (EPS) of the reacting sulfides. The overall study leads to a mechanistic interpretation that explains the stereoselectivity of the system and dissects the role of aromatic and steric interactions in the stereoselective process.

Modelling the sulfoxygenation activity of vanadate-dependent peroxidases.

Vanadate-dependent peroxidases contain, in their active center, vanadate covalently attached to histidine in an overall trigonal-bipyramidal array. We describe here the synthesis and characterization of optically active amino alcohols and their vanadium(V) complexes, and we show that the structural models of the active center thus obtained are also functional models for the sulfide-peroxidase activity of the enzyme in heterogeneous catalysis. The heterogeneous systems were obtained by immobilizing the complexes on silica gel and mesoporous silicas, and by aggregation. The following ligands, ligand precursors, and V compounds have been structurally characterized: (R)-(2-phenylethanol)-(R)-1-phenylethylamine (HL(A)), (R,R)-bis[2-phenyl(ethylmethylether)]ammonium chloride ([L(D)]+Cl(-)), the carbasilatranes (R,R)-methoxy{N,N',N''-2,2',3-[bis(1-phenylethanolato)propyl]amino}silane ((R,R)-Si(OMe)L(E)), (R,R)-methoxy-{N,N',N''-1,2',3-[(1-phenylethanolato)-(2-phenylethanolato)propyl]amino}silane ((R,R)-Si(OMe)L(E')), and [VO(L(F))(OSiMe2(t)Bu)], where H2L(F)=ethylbis(2-hydroxy-2-phenylethyl)amine.

Substrate binding to vanadate-dependent bromoperoxidase from Ascophyllum nodosum: a vanadium K-edge XAS approach.

The EXAFS region of vanadium K-edge XAS spectra of native vanadate-dependent bromoperoxidase (isoenzyme I) from Ascophyllum nodosum in the presence of the substrate bromide can be fitted to three shells (at 1.62, 1.73-1.78 and 1.99-2.07 A) in the first coordination sphere of vanadium plus two more distant shells at 4.1A, possibly corresponding to bromide, and 4.7 A due to light scatterers stemming from the protein pocket. Bromide does not directly bind to the vanadium centre. The XANES and the EXAFS features for the enzyme are essentially reproduced by model complexes of the general composition [VO(H(2)O)(n)(ONO)] (n= 1 or 2) where ONO is the dianion of a Schiff base from bromosalicylaldehydes (Brsal; with the Br substituent in the position 3, 4, 5 or 6) and amino acids. The 3-Brsal derivatives exhibit an outer sphere shell at 3.8 A, which is traced back to intermolecular contacts. The data obtained from EXAFS are compared to those obtained from single crystal X-ray diffraction of [VO(H(2)O)(2)(4-Brsal-gly)] and [VO(H(2)O)(2)(6-Brsal-gly)] (gly = glycinate). In the complex [VOBr(2)(ONO)']] ((ONO)' is the Schiff base from o-anisole and o-hydroxyaniline), the V-Br distance is 2.44 A.

Structural models for the reduced form of vanadate-dependent peroxidases: vanadyl complexes with bidentate chiral Schiff base ligands.

The penta-coordinated vanadyl complexes [VO(ON)(2)] have been obtained by reaction between [VOX(2)] (X = acetylacetonate or chloride) and the Schiff base ligands HON = (R)-sal-am, (R)-Clsal-am and (S)-naph-am, where sal and naph are the salicylidene and naphthalidene moieties, and am derives from phenylethylamine. The three complexes and the ligand (R)-Clsal-am have been structurally characterized. The geometry of the complexes is in-between trigonal-bipyramidal (with the two imine functions in the axis) and square-pyramidal; tau values range from 0.66 to 0.44. Structural and EPR (electron paramagnetic resonance) features are in accord with the coordination environment proposed for the inactive, reduced (V(IV)) form of the bromoperoxidase from the marine brown alga Ascophyllum nodosum.

Catalysis of oxo transfer to prochiral sulfides by oxovanadium(v) compounds that model the active center of haloperoxidases.

The catalytic properties of a new class of chiral vanadium compounds--[(S,S,S)-VO(OMe)L1] (5), [(S,S)-VO(OMe)L2] (6), [(S,S)-VO(OMe)L3] (7), and [(R,R,R)-VO(OMe)L4] (8), as well as the system VO(OiPr)(3)/(R,R,R)-H(2)L4 [H(2)L1=(S,S)-bis(2-hydroxypropyl)-(S)-1-phenylethylamine, 1; H(2)L2=(S,S)-bis(2-hydroxypropyl)benzylamine, 2; H(2)L3=(S,S)-bis(2-hydroxypropyl)isopropylamine), 3; (H(2)L4)=(R,R)-bis(2-phenylethanol)-(R)-1-phenylethylamine, 4]--in the asymmetric oxidation of prochiral sulfides by organic hydroperoxides have been investigated. Particular attention has been paid to the factors that guide the discrimination between the two prochiral faces of the sulfides (methyl p-tolyl sulfide and benzyl phenyl sulfide), to steric implications stemming from the oxidant (cumyl hydroperoxide and tert-butyl hydroperoxide), and to the specific complex used. As an example, (S)-methyl p-tolyl sulfoxide was obtained in a 31 % enantiomeric excess by use of cumyl hydroperoxide as oxidant and complex 5 as the catalyst, after 150 min at 0 degrees C and with 100 % conversion of the sulfide. The crystal and molecular structures of 5 and 6 reveal the close relationship between these complexes and the active center of vanadate-dependent haloperoxidases: the vanadium is in a slightly distorted trigonal-bipyramidal environment with the nitrogen and the methoxy group in the axial positions, and the oxo and alkoxide functions of L2 and L3 are the plane. The presence and equilibrium situation of isomers of the catalysts in solution has been investigated by (51)V EXSY and variable-temperature multinuclear NMR spectroscopy. An intermediately formed peroxo (ROO(-)) vanadium complex was detected by (51)V NMR spectroscopy.

Double-stranded W/Ag/S chain structure generated from [syn-(WO(NCS)(2))(2)(mu-S)(2)] building blocks.

The dimeric W(V) complex [Et(4)N](4)[syn-(O=W(NCS)(3))(2)(mu-S)(2)], 1, prepared from [Et(4)N](2)[WS(4)], SCN(-), and Cd(2+), shows interesting reactivity patterns in that the thiocyanate trans to the oxo group can in part be replaced, initiated by Mn(2+), by dimethylformamide (DMF) to form [Et(4)N](2.5)[(O=W(NCS)(2.25)(DMF)(1.25))(2)(mu-S)(2)], 2. With Ag(+), 1 undergoes partial replacement of SCN(-) by DMF and coordinates to the silver ions to generate ([Et(4)N](2.5)[(W(2)O(2)(NCS)(2)(mu-S)(2))(mu-NCS)(2)(DMF)(Ag(0.5)(SCN))])(n), 3. Compound 3 constitutes a polymeric double-stranded chain, with normal bonding interactions [via W-(mu-NCS)-Ag] between the two strands, and moderate intrastrand [W-(mu-NCS).Ag] bonding. The crystal and molecular structures of the three compounds are described.