Asymmetric sulfoxidation of prochiral sulfides using aminoalcohol derived chiral C3-symmetric trinuclear vanadium Schiff base complexes

A series of trimeric variants of the efficient and well known Bolm’s chiral vanadium salen catalysts are reported. These C₃-symmetric trinuclear chiral Schiff bases were synthesized by condensing a variety of trialdehydes with optically active aminoalcohols. The catalytic activity of the chiral vanadium complexes of these ligands was investigated for the enantioselective oxidation of prochiral sulfides using hydrogen peroxide as an oxidant. The procedure afforded the corresponding sulfoxide in good yield and the enantioselectivities were comparable to those obtained with the mononuclear complexes.     

Asymmetric synthesis of Sulindac esters by enantioselective sulfoxidation in the presence of chiral titanium complexes

High ee values (up to 94–96% ee) and moderate isolated yields (48–50%) were achieved in the preparation of Sulindac alkyl esters. These molecules are simple and straightforward precursors of Sulindac, an anti-inflammatory drug that has also been recently investigated in anti-cancer therapy. The key step of the overall procedure was the enantioselective hydroperoxide oxidation of the Sulindac sulfide alkyl esters in the presence of chiral complexes of titanium with either (S,S)- or (R,R)-hydrobenzoin. Various reaction conditions were investigated in order to obtain the best balance between yield and enantioselectivity.     

Asymmetric hydrogenation using chiral phosphinite rhodium complexes

The Cu(I) complex of (1R,3R)-bis(diphenylphosphinoxy)-1,3-diphenylpropane (BDPODP) has been prepared and used for the transfer of the ligand to Rh(I). The Rh(I) complexes of this new phosphinite obtained by this method act as efficient asymmetric homogeneous hydrogenation catalysts for (Z)-α-(acylamino)-cinnamic acids.     

Asymmetric sulfoxidation using [(3,3-Dimethoxycamphoryl)sulfonyl]oxaziridine

[(3,3-Dimethoxycamphoryl)sulfonyl]oxaziridine oxidizes sulfides to sulfoxides cleanly and efficiently in up to 98% ee.     

Asymmetric synthesis of 3-substituted unsaturated prolines from chiral sulfoximine substituted allyl titanium(IV) complexes

An asymmetric synthesis of the 3-substituted Δ^3,4-unsaturated prolines 7a-e and 3-substituted 4-methylene prolines 14a-c starting from the corresponding γ,δ-unsaturated α-amino acids 4a-e and 11a-c, respectively, is described. Amino acid derivatives 4a-e and 11a-d were obtained through aminoalkylation of the corresponding sulfoximine substituted allyl titanium(IV) complexes 2a-e and 10a-d, respectively, with the N-tert-butylsulfonyl imino ester 3. Activation of sulfoximines 4a-e and 11a-c through methylation of the sulfoximine group followed by a KF mediated isomerization of the vinyl aminosulfoxonium salts 5a-e and 12a-c, respectively, to the corresponding allyl aminosulfoxonium salt 6a-e and 13a-c, respectively, and a subsequent intramolecular nucleophilic substitution of the allylic aminosulfoxonium group afforded the enantio- and diastereomerically pure proline derivatives in medium to high yields.     

Non-oxo vanadium(IV) complexes with acetylacetone 4-R-benzoylhydrazones

Vanadium(IV) complexes of formula [V(acRbh)₂] with acetylacetone 4-R-benzoylhydrazones (H₂acRbh, R=H, Cl, OMe and NO₂) have been synthesized and characterized by elemental analysis, IR, UV–vis, and electron paramagnetic resonance (EPR) spectroscopic measurements. All the complexes are one-electron paramagnetic and show very similar axial EPR spectra in frozen solution. In dimethylformamide, the complexes display a V(IV)?V(III) redox couple in the E _1/2 range ?0.16–?0.25?V (versus Ag/AgCl). Single crystal X-ray structures of [V(acHbh)₂] and [V(acClbh)₂] have been determined. The metal center in each complex is in a distorted trigonal prismatic N₂O₄ coordination sphere assembled by the enolate-O, the imine-N, and the iminolate-O donor acRbh^2?.     

o,o′-Alkylene dithiophosphato complexes of vanadium(IV) and vanadium(V)

Reactions of VO(acac)₂ with alkylene dithiophosphoric acids, POGOS₂H, and of VOCl₃ with the ammonium salts NH₄(POGOS₂) in 1:2 molar ratio gave the oxovanadium(IV) alkylene dithiophosphates, [VO(POGOS₂)₂], and monochloroxovanadium(V) alkylene dithiophosphates, [VOCl(POGOS₂)₂], respectively, where G = —CH₂CMe₂-CH₂—, —CH₂CEt₂CH₂—, —CHMeCH₂CMe₂— or —CMe₂CMe₂—. These complexes are green solids, soluble in common organic solvents and sensitive to moisture. They were characterized by elemental analysis, molecular weight and spectral studies including i.r. and n.m.r. (¹H, ¹³C and ³¹P), which suggested bidentate bonding of the POGOS₂ ligands to give a square pyramidal for the V^IV complexes and an octahedral geometry for the V^V complexes.     

Vanadium(IV) and vanadium(V) complexes of salicyladimine ligands

The synthesis and characterization of a V(IV) and a V(V) complex of the salicyladimine ligand system are described. The reaction of salicylaldehyde and 1,3-diaminohydroxypropane with vanadyl sulfate produced a monomer (VOL1) which, upon heating in methanol, crystallized as a V(V) complex (VO(2)L1). The reaction of 3-methoxysalicylaldehyde, 1,3-diaminohydroxypropane, and vanadyl sulfate resulted in a binuclear complex held together by hydrogen bonding (VOL2). VOL1 was determined to catalyze the epoxidation of cyclohexene better than VOL2. The synthesis and characterization of VOL1, VOL2, and VO(2)L1 are described. The role of each complex as a catalyst for the epoxidation of cyclohexene is investigated. Results indicate that the V(V) complex performs better than either of the V(IV) complexes.     

Asymmetric cyanohydrin synthesis using heterobimetallic catalysts obtained from titanium and vanadium complexes of chiral and achiral salen ligands

Titanium(IV)(salen) and vanadium(V)(salen) complexes are both known to form catalysts for asymmetric cyanohydrin synthesis. When a mixture of titanium and vanadium complexes derived from the same or different salen ligands is used for the asymmetric addition of trimethylsilyl cyanide to benzaldehyde, the absolute configuration of the product and level of asymmetric induction can only be explained by in situ formation of a catalytically active heterobimetallic complex, and is not consistent with two monometallic species acting cooperatively. Combined use of complexes containing chiral and achiral salen ligands demonstrates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to the vanadium rather than the titanium ion. The titanium complexes also catalyse the asymmetric addition of ethyl cyanoformate to aldehydes, a reaction in which vanadium(V)(salen) complexes are not active. For this reaction, use of a mixture of titanium and vanadium(salen) complexes results in a complete loss of catalytic activity, a result which again can only be explained by in situ formation of a heterometallic complex. Both the titanium and vanadium based catalysts also induce the asymmetric addition of potassium cyanide/acetic anhydride to aldehydes. For this reaction, combined use of chiral and achiral complexes indicates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to titanium rather than vanadium, a result which contrasts with the observed results when trimethylsilyl cyanide is used as the cyanide source.     

Laser photolysis studies of β-bond cleavage of phenacyl phenyl sulfide in acetonitrile

Photo-induced β-bond dissociation of phenacyl phenyl sulfide (PPS) has been investigated in acetonitrile by laser photolysis techniques. Direct excitation of PPS at 295 K provided the acetylmethyl and phenylthiyl radicals with a quantum yield (Φ_rad) of 0.18, whereas triplet sensitization using xanthone revealed an efficiency for β-cleavage of triplet PPS (α _rad) of ≥0.64. From disagreement between the Φ_rad and α _rad values, it was concluded that both the lowest excited single and triplet states are reactive for β-bond dissociation in PPS. The photochemical processes of excited PPS, including β-cleavage, are discussed in detail.     

Studies on vanadium(IV) complexes of azoxine s dyes

Comparative studies of the reaction of 7-arylazo-8-hydroxyquinoline-5-sulphonic acid (Azoxine S) dyes with vanadium(IV), show that 2:1 yellow, water-soluble complexes are formed over the pH range 2.5–6, and that the phenyl derivative is the most suitable for spectrophotometric determination of 0.2–1.4 ppm of vanadium(IV). The colour is formed instantaneously and is stable for about 8 hr. The molar absorptivity at λ_max, 400 mμ;, is 1.15 × 10⁴, and the equilibrium constant for complex formation is of the order of 10². These dyes can also be used as indicators in the direct complexometric determination of vanadium(IV). The interference of a number of anions and cations is reported.     

Synthesis and electrochemistry of vanadium(IV) Schiff base complexes

The electrochemical properties of vanadyl(IV) derivatives, namely salen Schiff base complexes of the type [VO(Salen)] (5-BrSalen, 5-NO₂Salen, 5-MeOSalen, salpn (bis(salicylaldehyde)-1,3-propanediamine, 5-BrSalpn, 5-NO₂Salpn, 5-MeOSalpn, Me₂Salen, Salophen, 5-BrSalophen, and 5-MeOSalophen) were investigated. The equatorial Schiff base ligands affect the oxidation potentials via interaction with the d-orbitals of the vanadyl metal ion. The cathodic peak potential (E_pc) becomes less negative according to the sequence MeO- < H- < Br- < NO_2?.     

Asymmetric hydrogenation using readily prepared chiral sugar diphosphiniterhodium complexes

Asymmetric hydrogenation of N-acetamidoacrylic acid derivatives has been achieved using a rhodium complex in the presence of a series of readily made chiral diphosphinites, small changes in the stereochemistry of which lead to significant difference in the optical yield of reduced product.     

Spectrophotometric determination of vanadium(IV) in the presence of vanadium(V) using Br-PADAP

In the present paper, a simple and sensitive method is proposed for vanadium(IV) determination in the presence of vanadium(V). This is based on the oxidation of vanadium(IV) present in the sample to vanadium(V) by addition of iron(III) cation, followed by a complexation reaction of iron(II) with the spectrophotometric reagent 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP). The iron(II) reacts with Br-PADAP immediately, forming a stable complex with a large molar absorptivity. The vanadium(IV) determination is possible, with a calibration sensitivity of 0.549 g ml^–1, for an analytical curve of 18.8 ng ml^–1 to 2.40 g ml^–1, molar absorptivity of 2.80 × 10⁴ 1 mole^–1 cm^–1 and a detection limit of 5.5 ng ml^–1. Selectivity was increased with the use of EDTA as a masking agent. The proposed method was applied for the vanadium(IV) determination in the presence of several amounts of vanadium(V). The results revealed that 200 g of vanadium(V) do not interfere with determination of 5.00 g of vanadium(IV). The precision and the accuracy obtained were satisfactory (R. S. D.<2%).     

Highly-oxidised, sulfur-rich, mixed-valence vanadium(IV/V) complexes

The reactions of [V(2)(micro-S(2))(2)(S(2)CNR(2))(4)] (R = alkyl) with NOBF(4) produce highly-oxidised, sulfur-rich, V(iv/v) complexes, [V(2)(micro-S(2))(2)(S(2)CNR(2))(4)]BF(4), that exhibit 15-line EPR spectra and structures consistent with Class III mixed-valence behaviour.     

Asymmetric diethylzinc addition and phenyl transfer to aldehydes using chiral cis-cyclopropane-based amino alcohols

A new series of amino alcohols with a chiral cyclopropane backbone have been developed and used in the catalytic asymmetric diethylzinc addition and phenyl transfer to various types of aldehydes. These cyclopropane-based chiral amino alcohols show high enantioselectivity in the addition of organozincs to aromatic and aliphatic aldehydes. For diethylzinc addition to aromatic and aliphatic aldehydes, up to 97% ee and 93% ee are obtained, respectively. For the phenyl transfer to aromatic aldehydes, the best enantioselectivity was 89% ee.     

Binuclear vanadium(V) and vanadium(IV,V) complexes of dihydrocaffeic,caffeic and ferulic acids

Summary Binuclear complexes of dihydrocaffeic, caffeic and ferulic acids with vanadium were prepared and studied. The suggested square-pyramidal structures with catecholic-type coordination are supported by various spectroscopic, magnetic and thermogravimetric data.     

The preparation and characterization of new eight-coordinate vanadium(IV) dithiocarbamate complexes

The reaction between oxovanadium(IV) and the potassium salts of the four dithiocarbamates derived from indole, indoline, carbazole and pyrrole in a mixed acetonitrile-water solvent and subsequent recrystallization of the product gave pure VL₄ complexes. An IR, optical and electron spin resonance spectral characterization is reported. The data are consistant with a VS₈ chromophore with an approximate D_2d geometry.