Naphthyl-l-α-amino acids via chemo-enzymatic dynamic kinetic resolution

A double catalyst system (protease + base) was applied to the dynamic kinetic resolution (DKR) of isomeric 1- and 2-α-naphthyl-glycines and -alanines exploiting the in situ racemization of their thioesters. Due to the different C-acidity of the two sets of compounds, different experimental conditions have been devised to perform the simultaneous resolution/racemization process.In all cases, the racemic N-Boc-thioesters were converted into the aminoacids with an l-configuration almost quantitatively and with complete enantioselectivity.     

Sterically encumbered chiral amino alcohols for the titanium catalyzed asymmetric alkylation of benzaldehyde

Sterically encumbered chiral l-amino alcohols with secondary amines and tertiary alcohols catalyze the enantioselective alkylation of benzaldehyde with diethyl zinc. Using 2 mol % of amino alcohol catalyst predominantly gave (R)-1-phenylpropanol with enantiomeric excesses of up to 61%. Using the in situ prepared titanium complex at 2 mol % as catalyst also favored the (R)-enantiomer with enantiomeric excesses of up to 73%. In almost all cases, the addition catalyzed by the titanium complex exhibited higher enantioselectivity than that of the amino alcohol ligand alone.     

Ruthenium and enzyme-catalyzed dynamic kinetic resolution of alcohols

Ruthenium acts as a good catalyst for the racemization reaction of secondary alcohols and amines. Ruthenium-catalyzed racemization is coupled with enzymatic kinetic resolution to prepare chiral compounds in 100% theoretical yield. Ten ruthenium complexes (1–10) act as a good catalyst the for racemization reaction and are also compatible with DKR process. Two other ruthenium complexes [RuCl₂(PPh₃)₃] and [Cp^*RuCl(COD)] are active for racemization reaction but their successful compatibility with DKR has not yet been reported. Ru/γ-Al₂O₃ and Ru–HAP are the heterogeneous catalysts used for the racemization reaction. They have also not been employed for DKR process. Polymer supported ruthenium is employed as a reusable racemization catalyst for aerobic DKR of alcohols.     

Simple amphiphilic isosteviol–proline conjugates as chiral catalysts for the direct asymmetric aldol reaction in the presence of water

Two novel amphiphilic catalysts 3 and 4 were synthesized by the condensation of isosteviol with l-proline in a one-pot process. With only 1 mol % loading, the catalyst 3 showed excellent activity (up to >99% yield) and stereoselectivity (up to 99:1 dr, >99% ee) for the direct aldol reaction of cyclohexanone and substituted benzaldehydes at room temperature in the presence of water. In addition, solvent effects, catalyst loading, substrate scope, temperature, and the influence of water on the reactions were investigated. These results demonstrate that the catalysts with a chiral concave and hydrophobic substituent in the 4-position of l-proline furnished high activity and stereoselectivity for the reaction.     

Memory of chirality trapping of low inversion barrier 1,4-benzodiazepin-2-one enolates

We have previously demonstrated that chiral, enantiopure 3-substituted 1,4-benzodiazepin-2-ones undergo retentive deprotonation/trapping at −78 °C, if the N1-substituent is sufficiently large (e.g., i-Pr). Stereocontrol in this reaction is attributed to the formation of an enantiopure, conformationally chiral enolate; at −78 °C a large N1 substituent (e.g., i-Pr) is needed to impart a sufficient barrier to enolate racemization. Herein, we report strategies to achieve high enantiomeric excess in deprotonation/alkylation of low inversion barrier 1,4-benzodiazepin-2-ones featuring small N1 substituents.     

Reduction of aromatic compounds with Al powder using noble metal catalysts in water under mild reaction conditions

In water, Al powder becomes a powerful reducing agent, transforming in cyclohexyl either one or both benzene rings of aromatic compounds such as biphenyl, fluorene and 9,10-dihydroanthracene under mild reaction conditions in the presence of noble metal catalysts, such as Pd/C, Rh/C, Pt/C, or Ru/C. The reaction is carried out in a sealed tube, without the use of any organic solvent, at low temperature. Partial aromatic ring reduction was observed when using Pd/C, the reaction conditions being 24 h and 60 °C. The complete reduction process of both aromatic rings required 12 h and 80 °C with Al powder in the presence of Pt/C.     

Highly efficient dynamic kinetic resolution of secondary aromatic alcohols with low-cost and easily available acid resins as racemization catalysts

A new and efficient dynamic kinetic resolution (DKR) process of secondary aromatic alcohols was developed with acid resins as racemization catalysts. Acid resin CD8604 was shown to have excellent racemization activity and good biocompatibility. When employing CD8604 and complex acyl donors as racemization catalyst and acyl donor, respectively, enantiomerically pure aromatic acetate was obtained with excellent yield and ee values through the DKR process. It is noteworthy that the system could be reused more than 10 times with little loss of yield and ee value.     

Asymmetric nitroaldol reaction catalyzed by copper–diamine complexes: selective construction of two contiguous stereogenic centers

Chiral copper(II) complexes of secondary bisamines derived from 1,2-diaminocyclohexane were successfully used in the diastereoselective nitroaldol reaction. The reactions were carried out in the presence of 10 mol % of the Cu(II) complex and 7.7 mol % of i-Pr₂NEt in 2-propanol at ?30 °C. Good to excellent yields, enantioselectivities of up to 99%, and moderate to excellent diastereoselectivities were obtained for both aromatic or aliphatic aldehydes and various prochiral nitrocompounds forming the corresponding β-nitroalcohols with two contiguous stereocenters.     

Pyrolysis of waste tyres: The influence of USY catalyst/tyre ratio on products

In this paper, an ultrastable Y-type (USY) zeolite was investigated with two-staged pyrolysis–catalysis of waste tyres. Waste tyres were pyrolysed in a fixed bed reactor and the evolved pyrolysis gases were passed through a secondary catalytic reactor. The main objective of this paper was to obtain high concentration of certain aromatic hydrocarbons suitable to be used as a chemical feedstock rather than a liquid fuel, and the influence of catalyst/tyre ratio on the product yield and composition of derived oils. The light fraction (boiling point < 220 °C) was distilled from the derived oil prior to be analyzed with gas chromatography/mass spectrometry (GC/MS). It showed that the increase of catalyst/tyre ratio resulted in high yield of gas at the expense of the oil yield. The high catalyst/tyre ratio favored to increase the concentration of light fraction (<220 °C) in oil. Increasing the catalyst/tyre ratio resulted in significant changed in the concentration of benzene, toluene, xylenes and the alkyl aromatic compounds. For benzene and toluene, the highest concentration was obtained at the catalyst/tyre ratio of 0.5. The concentration of xylenes increased with the increasing of catalyst/tyre ratio.     

Albumin-mediated asymmetric nitroaldol reaction of aromatic aldehydes with nitromethane in water

We succeeded in the asymmetric nitroaldol (Henry) reaction of aromatic aldehydes with nitromethane using human serum albumin (HSA) in water at neutral pH. The reaction of 4-nitrobenzaldehyde smoothly proceeded for 24 h at 30 °C to afford the corresponding (R)-2-nitro-1-(4-nitrophenyl)ethanol (27% ee). Lowering the reaction temperature to 0 °C improved the enantioselectivity (53% ee). Although the denatured HSA also catalyzed the coupling reaction, no enantioselectivity was observed. The reaction was also applicable to other substrates bearing various substitutions on the benzene ring, and the ee of (R)-1-(biphenyl-4-yl)-2-nitroethanol was up to 79% ee.     

Catalytic fast pyrolysis of jatropha wastes

In this work, the decomposition behaviors of jatropha wastes (husk, seed shell and branch) have been examined in order to get desired liquid organic compounds, but not undesired inorganic compounds such as CO, CO₂, water and coke. The jatropha wastes exhibit a stepwise degradation pathway which has a slight difference in between samples before and after milling. In the preliminary pyrolysis using quartz reactor and H-ZSM-5(30) catalyst, the liquid products selectivity was seed shell > blanch > husk > seed shell (no catalyst). In the absence of catalyst, the Py-GC/MS analyses for pyrolysis of jatropha wastes show a range of aromatic hydrocarbons, phenols, alcohols and ketones, acids and esters, ethers and aldehydes. Aromatics are predominantly formed above 90% of area percentage by use of catalyst. Of aromatic compounds, xylenes, naphthalenes and toluene are mainly produced. The product selectivity is dependent on both the size of the catalyst pores and the nature of the active sites and one candidate is H-ZSM-5 and the other candidate is β-zeolite. The reaction pathway involves dehydrogenation and dehydroaromatization of aliphatic oxygenates such as alkylcyclohexanol and higher carboxylic acids to form phenol derivatives, which undergo hydrodeoxygenation into toluene and xylenes, followed by dehydroaromatization to give naphthalenes.     

Synthesis of 1,2-azidoalcohols from epoxides using a task-specific protic ionic liquid: 1-hydrogen-3-methylimidazolium azide

A task-specific ionic liquid as an environmentally eco-friendly green catalyst has been synthesized and used in the ring opening of epoxides under green conditions. In order to use protic ionic liquids (PIL), we decided to synthesize 1,2-azidoalcohols via a ring opening reaction of epoxides with 1-hydrogen-3-methylimidazolium azide ([Hmim]N₃), which actually acts as a solvent, a reagent and an activator of the epoxide ring. The reaction was carried out in short times (50–70 min) at 70 °C to give 1,2-azidoalcohol in 80–94% isolated yields.     

Propriétés catalytiques de la montmorillonite intercalée au titane dans l'oxydation de l'alcool allylique (E)-hex-2-én-1-ol

Catalytic properties of Ti-pillared montmorillonite for the epoxidation of (E)-hex-2-en-1-ol allylic alcohol. The present work is relative to the epoxidation of allylic alcohols by a well-characterised Ti-pillared montmorillonite (Ti-PILC) prepared by intercalating polymeric Ti cations formed by partial hydrolysis of TiCl₄ with HCl. The interlayer distance, the specific surface area and thermal stabilities of the resulting pillared clays depend mainly on the pH of the pillaring solution. For a ratio H⁺/Ti = 0.24, the Ti-PILC shows a basal spacing of 2,6 nm and a specific surface area of 316 m²·g^–1, stable up to 773 K. In the conversion of isopropanol, this Ti-PILC shows a Lewis type acidity and a low redox activity. It catalyses the oxidation of allylic alcohol (E)-hex-2-en-1-ol with tert-butyl hydroperoxide in the presence of diethyl tartrate. The yield and nature of the final products depend on the reaction temperature and catalyst activation: no epoxidation could be obtained with non calcined samples. With a solid calcined in air at 773 K and then converted to a Ti-PILC, the main product is hexenoic acid for a reaction below 273 K, while the epoxide of the allylic alcohol is formed with 50 % yield at 298 K. This yield is comparable to that reported for the same substrate for polymer supported Ti(IV) catalysts.     

One catalyst for both enantiomers: uncovering the inversion of enantioselectivity in cinchona-mediated desymmetrization of glutaric meso-anhydrides

A puzzling inversion of enantioselectivity dependent on catalyst loading was observed during the quinine-mediated desymmetrization of glutaric meso-anhydrides. This study presents the improvement of the catalytic path by the inclusion of carboxylic acid additives up to synthetically useful levels. The novel protocol utilizing 0.1 equiv of alkaloid and xanthene-9-carboxylic acid at room temperature (rt) was found comparable to the protocol requiring 1.1 equiv of alkaloid at ?30 °C. Thus, by altering the protocol the same catalyst produces the opposite enantiomer.This occurrence was rationalized by an extensive computational study of the interactions governing the molecular complexes formed by quinine, methanol, 3-methylglutaric anhydride, and the acetic acid. It was found that in a quinine catalyzed reaction the alcohol and the anhydride were directly hydrogen bonded to the catalyst. On the other hand, in the reaction with additive the acid intercalates between the alcohol and quinine. Due to this insertion the alcohol approaches the anhydride from the opposite face, in agreement with the observed inversion of enantioselectivity     

Enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines catalyzed by β-aminoalcohols with the prolinol skeleton

Several β-aminoalcohols with the prolinol framework are shown to be very efficient catalysts for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines. The use of 0.5 equiv of the catalyst leads to the expected addition products in good yields and with ee up to 94% in a reaction time of only 4 h at room temperature. This ee is the highest value reported so far using 0.5 equiv of an aminoalcohol as a promoter. High enantioselectivities are obtained in the addition of dialkylzincs to both aromatic and aliphatic imines. The amount of the catalyst can be reduced to 0.25 equiv with a slight decrease in the ee. A very interesting effect of the addition rate and temperature on the enantioselectivity was also observed.     

FeAgMo2O8 — A novel oxygen evolution catalyst material for alkaline metal–air batteries

This paper reports about FeAgMo₂O₈ — a novel oxygen evolution catalyst material for secondary (rechargeable) metal–air batteries. Bifunctional air electrodes were made using FeAgMo₂O₈ as a charging catalyst for oxygen evolution reaction (OER) and silverized carbon black (Ag/C) was employed as a discharging catalyst for oxygen reduction reaction (ORR). Corresponding air electrodes were investigated using 10 M KOH as an electrolyte. At current densities between 20 and 50 mA per cm² we observed discharging and charging voltages of 1.20 to 1.15 V and 1.96 to 2.05 V, respectively.     

Ring contraction mechanisms in substituted cyclohexylium cations

The effect of an electron donating or withdrawing group on the ring contraction mechanism of a cyclohexylium (cyclohexane-derived) cation has been studied using density functional theory. The barrier to rearrangement of the parent cyclohexane cation (1) was previously calculated to be 7.7 kcal/mol using PBE/6-311++G(2d, 2p). We show in this work that addition of an electron withdrawing group (CF₃) lowers the average barrier to ring contraction, while an electron donating group (CH₃) increases the average barrier, relative to the parent, unsubstituted, cyclohexane cation. Calculated barrier heights for going from a 6-membered to 5-membered ring range from 4.3 to 23.3 kcal/mol for methyl-cyclohexylium (2), but only from 0.6 to 14.0 kcal/mol for trifluoromethyl-cyclohexylium (3). The lower barriers for 3 can be explained by (a) the starting structures involved, and (b) the use of dative bonding as a catalyst in stabilizing intermediates and transition states. For 1 and 3 the reaction involves starting from secondary cations and going downhill in energy through secondary intermediates to a tertiary product. However, 2 does not benefit from such favorable energetics since it more likely starts from a tertiary cation and has to first rearrange to secondary intermediates to derive the tertiary methyl-cyclopentane-type product.     

Asymmetric addition of diethylzinc to aldehydes catalyzed by a camphor-derived β-amino alcohol

The asymmetric addition of diethylzinc to aromatic and aliphatic aldehydes, including linear aliphatic ones, catalyzed by 2 mol % of β-amino alcohol (1S, 2R)-7,7-dimethyl-1-morpholin-4-yl-bicyclo[2.2.1]heptan-2-ol 10 gave the corresponding secondary alcohols in high yields and with up to 94% ee at ambient temperature after 15 min.     

Efficient diastereo- and enantioselective synthesis of α,β-disubstituted γ-phosphono sulfonates

The first asymmetric synthesis of α,β-disubstituted γ-phosphono sulfonates is reported. The key step is the Michael addition of a lithiated enantiopure sulfonate bearing an inexpensive chiral sugar auxiliary to α,β-unsaturated phosphonates in good diastereoselectivities. After chromatographic purification, the cleavage of the chiral sugar auxiliary proceeds without any epimerization or racemization to form the corresponding isopropyl sulfonate in very good overall yield (75%) and excellent diastereomeric and enantiomeric excess (de, ee ? 95%).     

Vapor pressures and sublimation enthalpies of seven heteroatomic aromatic hydrocarbons measured using the Knudsen effusion technique

The vapor pressures of seven heteroatom-containing cyclic aromatic hydrocarbons, ranging in molecular weight from (168.19 to 208.21) g · mol^?1 were measured over the temperature range of (301 to 486) K using the isothermal Knudsen effusion technique. The compounds measured include: anthraquinone, 9-fluorenone, 9-fluorenone oxime, phenoxazine, phenoxathiin, and 9H-pyrido[3,4-b]indole. These solid-state sublimation measurements provided values that are compared to vapor pressures of parent aromatic compounds (anthracene and fluorene) and to others with substituent groups in order to examine the effects of alcohol, ketone, pyridine, and pyrrole functionality on this property. The enthalpies and entropies of sublimation for each compound were determined from the Clausius–Clapeyron equation. Though there is no consistent trend in terms of the effects of substitutions on changes in the enthalpy or entropy of sublimation, we note that the prevalence of enthalpic or entropic driving forces on vapor pressure depend on molecule-specific factors and not merely molecular weight of the substituents.