Asymmetric Epoxidation Of Allylic Alcohols Research Articles

On the Search for NNO-Donor Enantiopure Scorpionate Ligands and Their Coordination to Group 4 Metals

The preparation of new chiral bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the lithium derivatives [Li(bpzb)(THF)] [1; bpzb = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide] and [Li(bpzte)(THF)] [2; bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-p-tolylethoxide] or the alcohol ligands (bpzbH) (3) and (bpzteH) (4) has been carried out by 1,2-addition reactions with trimethylacetaldehyde or p-tolualdehyde. The separation of a racemic mixture of the alcohol ligand 3 has been achieved and gave an enantiopure NNO alcohol-scorpionate ligand in three synthetic steps: (i) 1,2-addition of the appropriate lithium derivative to trimethylacetaldehyde, (ii) esterification and separation of diastereoisomers 5, (iii) saponification. Subsequently, the enantiopure scorpionate ligand (R,R)-bpzmmH {6; R,R-bpzmmH = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethanol} was obtained with an excellent diastereomeric excess (>99% de) in a one-pot process utilizing the aldehyde (1R)-(-)-myrtenal as a chiral substrate to control the stereochemistry of the newly created asymmetric center. These new chiral heteroscorpionate ligands reacted with [MX(4)] (M = Ti, Zr; X = NMe(2), O(i)Pr, OEt, O(t)Bu) in a 1:1 molar ratio in toluene to give, after the appropriate workup, the complexes [MX(3)(kappa(3)-NNO)] (7-18). The reaction of Me(3)SiCl with [Ti(NMe(2))(3)(bpzb)] (7) or [Ti(NMe(2))(3)(R,R-bpzmm)] (11) in different molar ratios gave the halide-amide-containing complexes [TiCl(NMe(2))(2)(kappa(3)-NNO)] (19 and 20) and [TiCl(2)(NMe(2))(kappa(3)-NNO)] (21 and 22) and the halide complex [TiCl(3)(kappa(3)-NNO)] (23 and 24). The latter complexes can also be obtained by reaction of the lithium compound 1 with TiCl(4)(THF)(2) and deprotonation of the alcohol group of 6 with NaH, followed by reaction with TiCl(4)(THF)(2) in a 1:1 molar ratio, respectively. Isolation of only one of the three possible diastereoisomers of the complexes 19 and 22 revealed that chiral induction from the ligand to the titanium center took place. The structures of these complexes were elucidated by (1)H and (13)C{(1)H} NMR spectroscopy, and the X-ray crystal structures of 3-7, 12, and 24 were also established. Finally, we evaluated the influence that the chiral center of the new heteroscorpionate complexes has on the enantioselectivity of the asymmetric epoxidation of allylic alcohols.

Vanadium-Catalyzed Asymmetric Epoxidation of Allylic Alcohols in Water

Asymmetric V-catalyzed epoxidation of allylic alcohols can be carried out in water with chiral ligands, which incorporate sulfonamide and hydroxamic acid fragments. Furthermore, the reaction, notorious for its ligand-deceleration effect, in water turned into the ligand-accelerated process. By using this aqueous protocol, a range of allylic alcohols were epoxidized with up to 94% ee.

Enantiomeric separation of glycidyl tosylate by CE: Application to the study of catalytic asymmetric epoxidation of allyl alcohol

A CE method using CDs as chiral selectors was developed and validated to achieve the separation of glycidyl tosylate enantiomers originated by in situ derivatization of glycidol enantiomers obtained in asymmetric epoxidation of allyl alcohol with chiral titanium-tartrate complexes as catalysts. The effects of the nature, pH and concentration of the buffer, the nature and concentration of chiral selector, the addition of SDS, methanol, ethanol or 2-propanol, the capillary temperature, the effective capillary length and the applied voltage on the chiral resolution of glycidyl tosylate enantiomers were investigated. The best separation conditions were achieved using a Tris-borate buffer mixture (50 and 25 mM, respectively) at pH=9.3 with a dual CD system consisting of 2.5% succinyl-beta-CD and 1.0% beta-CD w/v at 15 degrees C. A baseline separation (resolution approximately 2.0) of the glycidyl tosylate enantiomers was obtained in a relatively short time (less than 12 min). Satisfactory results were obtained in terms of linearity (r>0.99) and intermediate precision (RSD below 8.5%). The LOD and LOQ were 3.0 and 10.0 mg/L, respectively, and the recoveries ranged from 99.8 to 108.8%. Finally, the method was applied to the determination of the enantiomeric excess and the yield obtained in the asymmetric epoxidation of allyl alcohol employing chiral titanium-tartrate complexes as catalysts after an in situ derivatization of glycidol enantiomers to glycidyl tosylate.

Nb(salan)‐Catalyzed Asymmetric Epoxidation of Allylic Alcohols with Hydrogen Peroxide

Bridging the catalyst: The dimeric complex [(μ-oxo){Nb(salan)}2] catalyzes asymmetric epoxidation of allylic alcohols by hydrogen peroxide to give epoxy alcohols with good to high enantioselectivities irrespective of the olefin geometry (see scheme).

Development and validation of a chiral HPLC method for rapid screening of allylic alcohol asymmetric epoxidation processes

A simple and rapid HPLC method has been developed using a polysaccharide chiral stationary phase (Chiralpak AD-H) for the resolution of glycidyl tosylate enantiomers. These compounds were obtained by asymmetric epoxidation of allyl alcohol with chiral titanium–tartrate complexes as catalyst after in situ derivatization of the intermediate glycidols. Separations were achieved using two types of mobile phase: a normal-phase ( n-hexane), and a polar-phase (methanol or acetonitrile). The influence of the type and concentration of organic modifier in the mobile phase (ethanol or 2-propanol), the flow rate and the column temperature was investigated. In normal-phase mode, the optimized conditions were: n-hexane/ethanol 70/30 (v/v) at a flow rate of 1.2 mL min −1 and 40 °C. In polar-phase mode, the optimized conditions were: methanol at a flow rate of 0.8 mL min −1 and 20 °C. In both cases, analysis time was ≤11 min and the chiral resolution was ≥2. Nevertheless, due to the better Rs obtained in normal-phase mode, only this method was validated to avoid peaks overlapping in real samples. This method was found to be linear in the 5–300 μg mL −1 range ( R 2 > 0.999) with an LOD of 1.5 μg mL −1 for both glycidyl tosylate enantiomers. Repeatability and intermediate precision at three different concentrations levels were below 0.5 and 7.2% R.S.D. for retention time and area, respectively. This method was applied successfully for the determination of glycidyl tosylate enantiomers after in situ derivatization of glycidols obtained in allylic alcohol asymmetric epoxidation processes with chiral titanium–tartrate complexes as catalysts.

Development and application of versatile bis-hydroxamic acids for catalytic asymmetric oxidation

In this article, we describe the development and preliminary results of our new designed C 2-symmetric bis-hydroxamic acid (BHA) ligands and the application of the new ligands for vanadium-catalyzed asymmetric epoxidation of allylic alcohols as well as homoallylic alcohols. From this success we demonstrate the versatile nature of BHA in the molybdenum catalyzed asymmetric oxidation of unfunctionalized olefins and sulfides.

Novel titanocene–tartrate complexes as catalysts for the asymmetric epoxidation of allylic alcohols

This communication presents the synthesis of novel titanocene-based chiral complexes and their application as catalysts for asymmetric oxidation reactions. These new complexes drive the catalytic asymmetric oxidation of allylic alcohols without the requirement of low temperature conditions, a usual drawback in asymmetric synthesis. Results from catalytic studies show that the enantioselectivity of these chiral complexes strongly depends on the steric hindrances caused by the chiral ligand and the alkyl hydroperoxide oxidant around the titanium centre.

Enantioselective synthesis of (−)-cytoxazone and (+)-epi-cytoxazone, novel cytokine modulators via Sharpless asymmetric epoxidation and l-proline catalyzed Mannich reaction

Abstract A short and efficient enantioselective synthesis of (−)-cytoxazone and its stereoisomer (+)-epi-cytoxazone, novel cytokine modulators, has been described with good yield and enantioselectivity. Ti-catalyzed Sharpless asymmetric epoxidation of allyl alcohol and l -proline catalyzed three-component Mannich reaction constitute the key steps in introducing stereogenicity into the molecule.

Vanadium‐Catalyzed Asymmetric Epoxidation of Allylic Alcohols Mediated by (+)‐Norcamphor‐Derived Hydroperoxide.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Vanadium‐Catalyzed Asymmetric Epoxidation of Allylic Alcohols Mediated by (+)‐Norcamphor‐Derived Hydroperoxide

AbstractA protocol for the asymmetric epoxidation of allylic alcohols has been established that employs VO(acac)2 as catalyst, the commercial achiral hydroxamic acid, N‐hydroxy‐N‐phenylbenzamide, and optically pure (+)‐norcamphor‐derived hydroperoxide as oxygen atom donor and chiral source. Variation of the reaction parameters has a significant effect on the level of asymmetric induction. Under optimized conditions, the epoxy alcohols have been isolated in good yields with up to 67 % ee and complete control of the diastereoselectivity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Renewable Camphor‐Derived Hydroperoxide: Synthesis and Use in the Asymmetric Epoxidation of Allylic Alcohols.

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Epoxide Migration (Payne Rearrangement) and Related Reactions

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Highly Efficient Catalytic Asymmetric Epoxidation of Allylic Alcohols by an Oxovanadium‐Substituted Polyoxometalate with a Regenerative TADDOL‐Derived Hydroperoxide.

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Asymmetric Epoxidation of Allylic Alcohols Catalyzed by New Chiral Vanadium(V) Complexes.

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Highly efficient catalytic asymmetric epoxidation of allylic alcohols by an oxovanadium-substituted polyoxometalate with a regenerative TADDOL-derived hydroperoxide.

The oxovanadium(IV) sandwich-type POM catalyzes the chemo-, regio-, and stereoselective epoxidation of allylic alcohols by chiral hydroperoxides with very high catalytic efficiency (up to 42 000 TON), a potentially valuable oxidation for the development of sustainable processes. By using the sterically demanding, TADDOL-derived hydroperoxide TADOOH as the chiral oxygen source, enantiomeric ratios (er) of up to 95:5 have been achieved.

Asymmetric epoxidation of allylic alcohols catalyzed by (≡SiO)xTa(OEt)3-x (dialkyl tartrate diolate): influence of the reaction conditions

Silica-supported chiral tantalum alkoxides are active catalysts for the asymmetric epoxidation of propenol and trans hex-2-en-1ol. The influence of different parameters on their catalytic performance was followed: the impregnation duration by a tartrate (step in their preparation); the nature of the solvent (CH2Cl2, pentane, toluene) and of the oxidant (TBHP, CHP, H2O2); poisoning effects by water or t-butanol; the reaction temperature and the substrate concentration.

Asymmetric epoxidation of allylic alcohols catalyzed by new chiral vanadium(V) complexes

Vanadium catalysts bearing (+)-ketopinic acid-based chiral hydroxamic acids as constituent ligands are investigated in the asymmetric epoxidation of allylic alcohols. Chiral ligands lacking an N-substituent and those having a bulkier aryl group in the bornane skeleton provided better selectivity.

Catalytic Asymmetric Epoxidation of Allylic Alcohols Using a Renewable Hydroperoxide.

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Catalytic asymmetric epoxidation of allylic alcohols using a renewable hydroperoxide

Sharpless modified asymmetric epoxidation of allylic alcohols can be successfully performed using an easily accessible and renewable tertiary furyl hydroperoxide in the presence 20 mol% of the chiral Ti(O- i-Pr) 4/ l-DIPT/MS 4 Å system.

ChemInform Abstract: Chiral Hydroxamic Acids as Ligands for the Vanadium Catalyzed Asymmetric Epoxidation of Allylic Alcohols.

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