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A series of tin(II) complexes 2 – 5 bearing 1,3,5-triazapentadienyl ligands are reported: Sn[N(Ph)C(R′)NC(R′)N(SiMe 3 )] 2 [R′ = 1-piperidino, 2 ; NMe 2 , 3 ], [N(Ph)C(NMe 2 )NC(NMe 2 )N(H)]SnCl ( 4 ) and [{N(SiMe 3 )C(R′)NC(R′)N} 2 Sn 2 ] 2 (R′ = 1-piperidino) ( 5 ). Their single-crystal X-ray diffraction studies are presented. Compounds 3 and 4 are efficient catalysts for the catalytic addition of phenylamine to N,N' -diisopropylcarbodiimide giving guanidine. [ABSTRACT FROM AUTHOR]

Methanol-assisted supercritical fluid deposition of TiO 2 (TiO 2 -SCFD), which had previously provided uniformly thick films on deep trenches with an aspect ratio (AR) above 30, was kinetically analyzed. To assess the overall reaction kinetics, a novel analytical scheme exploiting micro/macrocavities and flow/closed reactors was developed. Two reaction paths were thereby revealed: surface reaction of the precursor and consecutive reactions starting from homogeneous reactions of the precursor in the fluid (fluid-phase reaction), followed by the surface reaction of the intermediate species. Compared with the original TiO 2 -SCFD, methanol addition enhanced the surface reaction rate constant and thus growth rate, although the step coverage was slightly deteriorated. Nevertheless, it still had good step coverage on the high-aspect-ratio features ( e.g. , a step coverage of 85% on a trench with an AR of 50). Methanol addition is an effective way to promote the growth rate insofar as conformal deposition has been ensured on the targeted structure. [ABSTRACT FROM AUTHOR]

Computational modelling provides a powerful tool for predicting the possibility of a reaction and permits an important quantitative evaluation of the chemo‐ and enantioselectivities in reactions. Here we present the results of DFT calculations of the chemo‐ and enantioselectivities of the 1,4‐addition of phenylboronic acid to β,γ‐unsaturated α‐ketoamides or α‐ketoesters catalyzed by Rh ligated to a sulfinyl‐phosphine ligand. For the three substrates studied, the predicted chemoselectivity and absolute configuration agree with experimental observations. The ee values calculated at the SMD/M06/DGDZVP level of theory were more accurate than those determined by the IEFPCM/PBE0/DGDZVP method. The high steric pressure exercised by the tBu and diphenylphosphine groups are the origin of the chemo‐ and enantioselectivities of the 1,4‐addition reactions. [ABSTRACT FROM AUTHOR]

The organic superbase P4-t-Bu catalyzes the direct anti-Markovnikov addition of alcohols to aryl alkenes to access valuable β-phenethyl ethers. A diverse substrate scope of aryl alkenes and alcohols is demonstrated, including heterocyclic systems and unprotected aminoalcohols. Mechanistic studies reveal that the reaction is under equilibrium control and extensive comparisons to common inorganic bases indicate that the broad reaction scope is uniquely enabled through the use of the organic superbase. [ABSTRACT FROM AUTHOR]

Selective conversion of guaiacol over MoO 3 to produce various alkylphenols, including ethylphenols, isopropylphenols, butylphenols ( tert -, sec -) and tert -amylphenol is investigated in ethanol without the addition of gaseous hydrogen. A high conversion of 99% is achieved at 280 °C for 4 h and the total alkylphenols account for up to 94% in the quantified products. Six molecules, i.e. 2,5-diethylphenol, 2,6-diisopropylphenol, 2,4-diisopropylphenol, 2,6-ditertbutylphenol, 2,4-ditertbutylphenol and 2,6-ditertbutyl-4-ethylphenol, are the main outcomes. The higher alkylphenols in the aforesaid products are verified to form via a novel reaction step in which hydrogen atom at the α-carbon of the lower alkylphenol is substituted consecutively with methyl or ethyl groups. Further examination reveals that catechol is formed as the intermediate via demethylation of guaiacol and followed by direct conversion to low alkylphenols without the formation of phenol. Post-catalytic analysis indicates that an oxycarbohydride phase (MoO x C y H z ) with Mo 5+ developed in the supercritical alcohol batch system played the role of active species. Ethanol is found to be the most effective solvent for the conversion. The MoO 3 catalyst undergoes a gradual deactivation resulted from the consumption of Mo 5+ and carbon deposition but can be regenerated with a simple calcination. [ABSTRACT FROM AUTHOR]

The rate coefficients of H abstraction and OH addition reactions of 2,4,4-trimethyl-1-pentene with OH were determined by both canonical variational transition state theory and conventional transition state theory. The potential energy surfaces were calculated at CCSD(T)/6-311++G(d, p)//BHANDHLYP/6-311(d, p) level and quantum effects were evaluated by the one-dimensional Wigner method and the multidimensional zero-curvature and small-curvature tunneling methods. 2,4,4-Trimethyl-1-pentene contains primary and secondary allylic H abstraction, alkyl, and vinylic H abstraction channels and OH addition into central and terminal carbon atoms channels. The results show that allylic H abstraction channels dominate the overall H abstractions resulting from the lower barriers, in which, primary H abstraction involves indirect channel via a reactant complex and direct channel. Moreover, OH addition reactions into central and terminal carbon atoms all contain Van der Waals reactant complexes, while the latter channel exhibits negative temperature dependence as a result of its submerged barrier. The branching ratio is more than 75% for addition reactions below 500 K and more than 80% above 1100 K for H abstraction reactions. Rate coefficients of all channels were fitted in terms of a quasi-Arrhenius expression. [ABSTRACT FROM AUTHOR]

In this research, nano-SiO-HBO is introduced as a novel environmentally benign and recyclable heterogeneous catalyst for the synthesis of aminonaphthols under solvent-free conditions without an additional co-catalyst or additive in air. This synthesis method offers several advantages, such as a green and eco-friendly protocol, operational simplicity, mild reaction conditions, short reaction times, higher yields and easy work-up procedures. The work-up of these very clean reactions involves only recrystallization of the product from ethanol and recovery of the catalyst by filtration. The catalyst can be reused several times without losing activity. Graphical Abstract: [ABSTRACT FROM AUTHOR]

The oxidation chemistry of the simplest conjugated hydrocarbon, 1,3-butadiene, can provide a first step in understanding the role of polyunsaturated hydrocarbons in combustion and, in particular, an understanding of their contribution toward soot formation. On the basis of our previous work on propene and the butene isomers (1-, 2-, and isobutene), it was found that the reaction kinetics of Ḣ-atom addition to the C-C double bond plays a significant role in fuel consumption kinetics and influences the predictions of high-temperature ignition delay times, product species concentrations, and flame speed measurements. In this study, the rate constants and thermodynamic properties for Ḣ-atom addition to 1,3-butadiene and related reactions on the Ċ4H7 potential energy surface have been calculated using two different series of quantum chemical methods and two different kinetic codes. Excellent agreement is obtained between the two different kinetics codes. The calculated results including zero-point energies, single-point energies, rate constants, barrier heights, and thermochemistry are systematically compared among the two quantum chemical methods. 1-Methylallyl (Ċ4H71-3) and 3-buten-1-yl (Ċ4H71-4) radicals and C2H4 + Ċ2H3 are found to be the most important channels and reactivity-promoting products, respectively. We calculated that terminal addition is dominant (>80%) compared to internal Ḣ-atom addition at all temperatures in the range 298-2000 K. However, this dominance decreases with increasing temperature. The calculated rate constants for the bimolecular reaction C4H6 + Ḣ → products and C2H4 + Ċ2H3 → products are in excellent agreement with both experimental and theoretical results from the literature. For selected C4 species, the calculated thermochemical values are also in good agreement with literature data. In addition, the rate constants for H atom abstraction by Ḣ Atos have also been calculated, and it is found that abstraction from the central carbon atoms is the dominant channel (>70%) at temperatures in the range of 298-2000 K. Finally, by incorporating our calculated rate constants for both Ḣ Ato addition and abstraction into our recently developed 1,3-butadiene model, we show that laminar flame speed predictions are significantly improved, emphasizing the value of this study. [ABSTRACT FROM AUTHOR]

The free radical addition reaction is very important in UV curing. The benzoyl radical is the most commonly observed radical. In the addition process, the benzoyl radical adds to an acrylate monomer, forming a primary radical that has great value for subsequent research. In this article, a quantum chemical method was used to study the microscopic progression from the reactive complex to the saddle point. The reactions of three monomers (amylene, allyl methyl ether and methyl acrylate) with a benzoyl radical were evaluated in terms of geometry and energy. The results were also interpreted with an expanded version of the Polanyi rules and the interaction/deformation theory. The deformation energy of methyl acrylate was found to be the smallest, and the bond formation index showed that the transition state in the methyl acrylate system forms early, and can easily reach the saddle point. The activity of the monomer was ascertained by charge analysis and was further confirmed by the reaction rate. Mayer bond order curves depicted the constantly changing chemical bonds during formation and dissociation. Reduced density gradient analysis showed a weak interaction between the monomer and the benzoyl radical. [ABSTRACT FROM AUTHOR]

Herein we report that simple α-chloro thioesters undergo soft enolization and direct aldol addition to aldehydes in the presence of MgBr 2 ·OEt 2 and i -Pr 2 NEt. At −78 °C the reaction proceeds in a kinetically controlled manner giving good diastereoselectivity. Significantly, the transformation is applicable to both enolizable and nonenolizable aldehydes. Moreover, excellent stereoselectivity results when a chiral nonracemic α-hydroxy aldehyde derivative is used. To our knowledge, this is the first report of a kinetically controlled soft enolization-based aldol addition. [ABSTRACT FROM AUTHOR]

The oxygen reduction reaction in an ionic liquid (IL) consisting of N , N -diethyl- N -methyl- N -(2-methoxyethyl)ammonium (DEME) cation and the bis(trifluoromethanesulfonyl)imide (TFSI) anion was investigated in the anhydrous and water saturated conditions respectively. The IL was demonstrated to be stable in the operating range of a lithium air cell in both the anhydrous and water saturated conditions. The addition of water was found to lower the overpotential for the oxygen reduction reaction by 40 mV. Finally, the reaction rate constant was found to increase by an order of magnitude with the addition of water hence demonstrating the tendency of water to promote/catalyze the oxygen electrochemistry in non-aqueous systems. [ABSTRACT FROM AUTHOR]

In this study, the properties and growth kinetics of boride layers, generated on the surface of TC21-DT alloy using appropriate amount of powders of boron carbide (B4C) and rare earth oxide (CeO2) were investigated. By conducting a series of experiments at different temperatures of 1123, 1223 and 1273 K for periods up to 10 h, the effects of rare earth (RE) addition on the growth kinetics of boride layers were studied. The characteristics of the boride layers were examined by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction and micro-Vickers hardness tester. The results showed that the boron diffusion in boride layers was obviously accelerated by RE addition and the activation energy for RE addition-boriding in TC21-DT alloy was greatly decreased to 58.13kJ/mol, which was approximately 40% lower than that of conventional one, without RE addition. [ABSTRACT FROM AUTHOR]

Density functional theory, along with the ωB97XD and UM06-2x exchange-correlation functional, has been used to study the reaction mechanisms and kinetics of the atmospheric oxidation of the main (kinetically dominant) thiophene-OH adduct [C4H4S-OH]• (R1) by molecular oxygen in its triplet electronic ground state. Kinetic rate constants and branching ratios under atmospheric pressure and in the fall-off regime have been calculated by means of transition state theory (TST), variational transition state theory (VTST) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. In line with the computed energy profiles, the dominant process under both the thermodynamic and kinetic control of the reaction is O2 addition at the C5 position in syn mode. The computed branching ratios indicate that the regioselectivity of the reaction decreases with increasing temperature and decreasing pressure. [ABSTRACT FROM AUTHOR]

A novel transformation involving phosphinediazo ester zwitterions (generated from dialkyl azodicarboxylates with Ph3P) and α-(alkoxycarbonyl)imidoyl chlorides (prepared from α-addition of acyl chlorides to alkyl isocyanides) to afford dialkyl 2-[3-alkoxy-1-(alkylimino)-1-chloro-3-oxopropan-2-ylidene]hydrazine-1,1-dicarboxylates in moderate yields, is described. [ABSTRACT FROM AUTHOR]

A highly efficient addition of alkyl α-bromocarboxylates to en amines by visible light-induced photoredox catalysis is reported. Compared with traditional methods, the reaction described here provided an alternative route for the construction of valuable c-ketoesters in generally good yields. [ABSTRACT FROM AUTHOR]

Producing hydrogen-rich chemicals, such as methane, ammonia or methanol, from renewable energy may foster the integration of renewables into the current energy system. Here, a flexible ammonia synthesis concept is introduced, which is then compared to the widely discussed power-to-gas concepts on a technical and economic level. The current ammonia prices result in comparably high hydrogen-specific revenues, which imply the ability to operate the system more profitable under fluctuating electricity prices and thereby increase the plant capacity factor. [ABSTRACT FROM AUTHOR]

Highlights: [•] Quantification effect/interaction among factors in polyol synthesis of Ag nanowires. [•] More significant effect of Ag+ addition rate on aspect ratio than Cl− concentration. [•] Very low aspect ratio at Cl− concentration <1.05mM regardless of Ag+ addition rate. [•] Predicting optimal conditions by RSM, achieving nanowires with aspect ratio of ∼100. [Copyright &y& Elsevier]

BACKGROUND Heterogeneous hydrogenation catalysts for fine chemical synthesis are a convenient alternative to homogeneous catalysts because of the ease of separation and reuse. In order to be good catalysts they must have high activity and selectivity and good mechanical properties. Appropriate kinetic models should also be available for reactor design. Novel composite supported Pd catalysts were synthesized and tested in the liquid-phase selective hydrogenation of 2,3-butanedione to 3-hydroxy-2-butanone (acetoin). The composite support comprised a mixture of an organic polymer and γ- Al2O3. The support and the Pd catalyst were further characterized by XRD, SEM, EMPA and XPS spectroscopy. Catalytic tests at various conditions were performed in order to elucidate the kinetics of the system. RESULTS The composite had better mechanical properties (resistance to radial and axial compression) in comparison with other commercial supports. Good activity and high selectivity to acetoin, a product of partial hydrogenation, were obtained at different reaction conditions. A Langmuir-Hinshelwood chemical rate expression useful for reactor design was regressed from the kinetic data. CONCLUSIONS The experimental results could be explained by a Horiuti-Polanyi mechanism in which the addition of an H atom to the carbonyl group in the adsorbed state is the rate limiting step. © 2013 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Thetemperature and pressure dependence of the addition reactionof ethyl methyl ketone (EMK) with HȮ2radical hasbeen calculated using the master equation method employing conventionaltransition state theory estimates for the microcanonical rate coefficientsin the temperature range of 600–1600 K. Geometries, frequencies,and hindrance potentials were obtained at the B3LYP/6-311G(d,p) levelof theory. A modified G3(MP2,CC) method has been used to calculateaccurate electronic energies for all of the species involved in thereactions. The rigid-rotor harmonic oscillator approximation has beenused for all of the vibrations except for the torsional degrees offreedom which are being treated as 1D hindered rotors. AsymmetricEckart barriers were used to model tunneling effect in a one-dimensionalreaction coordinate through saddle points. Our calculated resultsshow that the four reaction channels forming 1-buten-2-ol + HȮ2radical (R5), 2-buten-2-ol + HȮ2radical (R10), acetic acid + ethylene+ ȮH radical (R13), and 2-methyl-2-oxetanol+ ȮH radical (R15) are the dominant channels.When the temperature is below 1000 K, the reaction R15 forming the cyclic ether, 2-methyl-2-oxetanol, is dominantwhile the reaction R13 forming acetic acid +ethylene + ȮH radical becomes increasingly dominant at temperaturesabove 1000 K. The other two channels forming 1-buten-2-ol, 2-buten-2-ol,and HȮ2radical are not dominant but are still importantproduct channels over the whole temperature range investigated here.No pressure dependence has been found for the reaction channels forming2-methyl-2-oxetanol + ȮH radical and acetic acid + ethylene+ ȮH radical. A slightly negative pressure dependence has beenfound for the reaction channels producing the two butenols. Rate constantsfor the four important reaction channels at 1 atm (in cm3mol–1s–1) are kR5= 2.67 × 1015× T–1.32exp(−16637/T), kR10= 1.62 × 108× T0.57exp(−13142/T), kR13= 2.29 × 1017× T–1.66exp(−18169/T), and kR15= 6.17 × 10–2× T3.35exp(−10136/T). A comparison of the total rate constants for the additionof HO2̇ radical to EMK and that for H-atom abstractionby HO2̇ radical from EMK has also been carried out.We find that the abstraction reaction channels are dominant over theentire temperature range of 600–1600 K. [ABSTRACT FROM AUTHOR]

The lithium enolate of tert-amylacetate solvated by N,N,N′,N′-tetramethylethylenediamine (TMEDA) is shown to be a doubly chelated dimer. Adding the dimeric enolate to 4-fluorobenzaldehyde-N-phenylimine affords an N-lithiated β-amino ester shown to be monomeric using 6Li and 15N NMR spectroscopies. Rate studies using 19F NMR spectroscopy reveal reaction orders consistent with a transition structure of stoichiometry [(ROLi)2(TMEDA)2(imine)]‡. Density functional theory computations explore several possible dimer-based transition structures with monodentate and bidentate coordination of TMEDA. Supporting rate studies using trans-N,N,N′,N′-1,2-tetramethylcyclohexanediamine showing analogous rates and rate law suggest that TMEDA is fully chelated. [ABSTRACT FROM AUTHOR]

In this work, addition of OH- to one-electron oxidized thymidine (dThd) and thymine nucleotides in basic aqueous glasses is investigated. At pHs ca. 9-10 where the thymine base is largely deprotonated at N3, one-electron oxidation of the thymine base by Cl2•- at ca. 155 K results in formation of a neutral thyminyl radical, T(-H)·. Assignment to T(-H)· is confirmed by employing 15N substituted 5'-TMP. At pH ⩾ ca. 11.5, formation of the 5-hydroxythymin-6-yl radical, T(5OH)·, is identified as a metastable intermediate produced by OH- addition to T(-H)· at C5 at ca. 155 K. Upon further annealing to ca. 170 K, T(5OH)· readily converts to the 6-hydroxythymin-5-yl radical, T(6OH)·. One-electron oxidation of N3-methyl-thymidine (N3-Me-dThd) by Cl2• at ca. 155 K produces the cation radical (N3-Me-dThd•+) for which we find a pH dependent competition between deprotonation from the methyl group at C5 and addition of OH- to C5. At pH 7, the 5-methyl deprotonated species is found; however, at pH ca. 9, N3-Me-dThd•+ produces T(5OH)· that on annealing up to 180 K forms T(6OH)·. Through use of deuterium substitution at C5' and on the thymine base, that is, specifically employing [5',5?-D,D]-5'-dThd, [5',5?-D,D]-5'-TMP, [CD3]-dThd and [CD3,6D]-dThd, we find unequivocal evidence for T(5OH)· formation and its conversion to T(6OH)·. The addition of OH- to the C5 position in T(-H)· and N3-Me-dThd•+ is governed by spin and charge localization. DFT calculations predict that the conversion of the "reducing" T(5OH)· to the "oxidizing" T(6OH)· occurs by a unimolecular OH group transfer from C5 to C6 in the thymine base. The T(5OH)· to T(6OH)· conversion is found to occur more readily for deprotonated dThd and its nucleotides than for N3-Me-dThd. In agreement, calculations predict that the deprotonated thymine base has a lower energy barrier (ca. 6 kcal/mol) for OH transfer than its corresponding N3-protonated thymine base (14 kcal/mol). [ABSTRACT FROM AUTHOR]

An alternative method to copper-catalyzed conjugate addition followed by enolate silylation for the synthesis of β-disubstituted silyl enol ether products (R1(R2)HCCH=C(OSiR34)R3) is presented. This method uses haloarenes instead of nucleophilic aryl reagents. Nickel ligated to either neocuproine or bipyridine couples an α,β-unsaturated ketone or aldehyde (R2HC=CHC(O)R3) with an organic halide (R1-X) in the presence of a trialkylchlorosilane reagent (Cl-SiR34). Reactions are assembled on the benchtop and tolerate a variety of functional groups (aldehyde, ketone, nitrile, sulfone, pentafluorosulfur, and N-aryltrifluoroacetamide), electron-rich iodoarenes, and electron-poor haloarenes. Mechanistic studies have confirmed the first example of a catalytic reductive conjugate addition of organic halides that proceeds via an allylnickel intermediate. Selectivity is attributed to (1) rapid, selective reaction of LNi0 with chlorotriethylsilane and enone in the presence of other organic electrophiles, and (2) minimization of enone dimerization by ligand steric effects. [ABSTRACT FROM AUTHOR]

Bromine radical-mediated cyclopropylcarbinyl-homoallyl rearrangement of alkylidenecyclopropanes was effectively accomplished by C-C bond formation with allylic bromides, which led to the syntheses of 2-bromo-1,6-dienes. A three-component coupling reaction comprising alkylidenecyclopropanes, allylic bromides, and carbon monoxide also proceeded well to give 2-bromo-1,7-dien-5-ones in good yield. [ABSTRACT FROM AUTHOR]

Despite their formal relationship to alkynes, Ar'GeGeAr', Ar'SnSnAr', and Ar*SnSnAr* [Ar' = 2,6-(2,6-iPr2C6H3)2C6H3; Ar* = 2,6-(2,4,6-iPr3C6H2)2-3,5-iPr2C6H] exhibit high reactivity toward H2, quite unlike acetylenes. Remarkably, the products are totally different. Ar'GeGeAr' can react with 1-3 equiv of H2 to give mixtures of Ar'HGeGeHAr', Ar'H2GeGeH2Ar', and Ar'GeH3. In contrast, Ar'SnSnAr' and Ar*SnSnAr* react with only 1 equiv of H2 but give different types of products, Ar'Sn(μ-H)2SnAr' and Ar*SnSnH2Ar*, respectively. In this work, this disparate behavior toward H2 has been elucidated by TPSSTPSS DFT computations of the detailed reaction mechanisms, which provide insight into the different pathways involved. Ar'GeGeAr' reacts with H2 via three sequential steps: H2 addition to Ar'GeGeAr' to give singly H-bridged Ar'Ge(μ-H)GeHAr'; isomerization of the latter to the more reactive Ge(II) hydride Ar'GeGeH2Ar'; and finally, addition of another H2 to the hydride, either at a single Ge site, giving Ar'H2GeGeH2Ar', or at a Ge–Ge joint site, affording Ar'GeH3 + Ar'HGe:. Alternatively, Ar'Ge(μ-H)GeHAr' also can isomerize into the kinetically stable Ar'HGeGeHAr', which cannot react with H2 directly but can be transformed to the reactive Ar'GeGeH2Ar'. The activation of H2 by Ar'SnSnAr' is similar to that by Ar'GeGeAr'. The resulting singly H-bridged Ar'Sn(μ-H)SnHAr' then isomerizes into Ar'HSnSnHAr'. The subsequent facile dissociation of the latter gives two Ar'HSn: species, which then reassemble into the experimental product Ar'Sn(μ-H)2SnAr'. The reaction of Ar*SnSnAr* with H2 forms in the kinetically and thermodynamically more stable Ar*SnSnH2Ar* product rather than Ar*Sn(μ-H)2SnAr*. The computed mechanisms successfully rationalize all of the known experimental differences among these reactions and yield the following insights into the behavior of the Ge and Sn species: (I) The active sites of Ar'EEAr' (E = Ge, Sn) involve both E atoms, and the products with H2 are the singly H-bridged Ar'E(μ-H)EHAr' species rather than Ar'HEEHAr' or Ar'EEH2Ar'. (II) The heavier alkene congeners Ar'HEEHAr' (E = Ge, Sn) cannot activate H2 directly. Instead, Ar'HGeGeHAr' must first isomerize into the more reactive Ar'GeGeH2Ar'. Interestingly, the subsequent H2 activation by Ar'GeGeH2Ar' can take place on either a single Ge site or a joint Ge–Ge site, but Ar'SnSnH2Ar' is not reactive toward H2. The higher reactivity of Ar'GeGeH2Ar' in comparison with Ar'SnSnH2Ar' is due to the tendency of group 14 elements lower in the periodic table to have more stable lone pairs (i.e., the inert pair effect) and is responsible for the differences between the reactions of Ar'EEAr' (E = Ge, Sn) with H2. Similarly, the carbene-like Ar'HGe: is more reactive toward H2 than is Ar'HSn:. (III) The doubly H-bridged Ar'E(μ-H)2EAr' (E = Ge, Sn) species are not reactive toward H2. [ABSTRACT FROM AUTHOR]

Quantum mechanical calculations reveal the origin of diastereo- and enantioselectivities of aldol reactions between aldehydes catalyzed by histidine, and differences between related reactions catalyzed by proline. A stereochemical model that explains both the sense and the high levels of the experimentally observed stereoselectivity is proposed. The computations suggest that both the imidazolium and the carboxylic acid functionalities of histidine are viable hydrogen-bond donors that can stabilize the cyclic aldolization transition state. The stereoselectivity is proposed to arise from minimization of gauche interactions around the forming C-C bond. [ABSTRACT FROM AUTHOR]

Abstract: The Michael addition is one of the most common and versatile methods for making carbon–carbon bonds, however little is known about the potential of utilizing ionic liquids as solvents in these reactions. Initial work from our laboratory is presented, showing that model imidazolium- and phosphonium-based ionic liquid solvents can be used as effective reaction media in the Michael addition. Kinetic data are also reported and the results indicate that the use of ionic liquids as reaction media resulted in an observed rate enhancement when compared with more common organic solvents such as toluene, THF, and DMF. Observed rates were comparable to those observed in DMSO. [Copyright &y& Elsevier]

BphI, a pyruvate-specific class II aldolase, catalyzes the reversible carbon-carbon bond formation of 4-hydroxy-2-oxoacids up to eight carbons in length. During the aldol addition catalyzed by BphI, the S-configured stereogenic center at C4 is created via attack of a pyruvate enolate intermediate on the si face of the aldehyde carbonyl of acetaldehyde to form 4(S)-hydroxy-2-oxopentanoate. Replacement of a Leu-87 residue within the active site of the enzyme with polar asparagine and bulky tryptophan led to enzymes with no detectable aldolase activity. These variants retained decarboxylase activity for the smaller oxaloacetate substrate, which is not inhibited by excess 4-hydroxy-2-oxopentanoate, confirming the results from molecular modeling that Leu-87 interacts with the C4-methyl of 4(S)-hydroxy-2-oxoacids. Double variants L87N;Y290F and L87W;Y290F were constructed to enable the binding of 4(R)-hydroxy-2-oxoacids by relieving the steric hindrance between the 5-methyl group of these compounds and the hydroxyl substituent on the phenyl ring of Tyr-290. The resultant enzymes were shown to exclusively utilize only 4(R)- and not 4(S)-hydroxy-2-oxopentanoate as the substrate. Polarimetric analysis confirmed that the double variants are able to synthesize 4-hydroxy-2-oxoacids up to eight carbons in length, which were the opposite stereoisomer compared to those produced by the wild-type enzyme. Overall the kcat/Km values for pyruvate and aldehydes in the aldol addition reactions were affected =10-fold in the double variants relative to the wild-type enzyme. Thus, stereocomplementary class II pyruvate aldolases are now available to create chiral 4-hydroxy-2-oxoacid skeletons as synthons for organic reactions. [ABSTRACT FROM AUTHOR]

Herein we report an efficient synthesis of the C1-C13 fragment of bistramide A from (S)-1,2,4-butanetriol and Roche ester in 14 steps and 13% overall yield. [ABSTRACT FROM AUTHOR]

This study investigated the formation of DNA adducts of polybrominated diphenyl ethers (PBDEs) and the possible mechanisms. DNA adduction was conducted by in vitro reaction of deoxyguanosine (dG) and DNA with PBDE-quinone (PBDE-Q) metabolites, and DNA adducts were characterized by using electrospray ionization tandem mass spectrometry. The results suggested DNA adduction involved Michael Addition between the exocyclic NH2 group at the N-2 position of dG and the electron-deficient carbon of quinone, followed by reductive cyclization with loss of (bromo-)1-hydroperoxy-benzene or water to form a type I or type II adduct. PBDE-Q with substituted bromine on the quinone ring was proven to be a favorable structure to form a type I adduct, while the absence of bromine on the quinone ring resulted in a type II adduct. Lower reactivity of adduction was also observed with increasing the number of bromine atoms on the phenoxyl ring. Our data clearly demonstrated PBDEs could covalently bind to DNA mediated by quinone metabolites, depending on the degree of bromine substitution. This study opened a new view on the mechanism of toxicity of PBDEs and reported the structure of PBDE-DNA adducts, which might be valuable for the evaluation on potential in vivo formation of PBDE-DNA adducts. [ABSTRACT FROM AUTHOR]

Ethyl 2-(1-phenylpiperidin-2-yl) acetate was formed in a spontaneous cyclization from (E)-ethyl 7-oxohept-2-enoate whereas ethyl [1-(2-bromophenyl)-2-pyrrolidinyl] acetate could be synthesized in good overall yield when employing a stoichiometric amount of base to facilitate the intramolecular aza-Michael reaction. [ABSTRACT FROM AUTHOR]

An unprecedented stereoselective organocatalytic one-pot 1,4-/1,6-/1,2-addition sequence between β-dicarbonyl compounds, β-nitroalkenes and 4-nitro-5-styrylisoxazoles sequentially catalyzed by low loading of a squaramide catalyst and an achiral base has been developed. The protocol opens an efficient entry to isoxazole bearing cyclohexanes with six consecutive stereogenic centers including one tetrasubstituted carbon in good yields and excellent diastereo- and enantioselectivities. [ABSTRACT FROM AUTHOR]

Abstract: Amino acid‐derived chiral imidazolium salts, each bearing a pyridine ring, are developed as N—heterocyclic carbene ligands. [ABSTRACT FROM AUTHOR]

Abstract: A new kind of podand‐based dimeric salen ligand is synthesized, and its association with potassium cations is investigated by 1H NMR spectroscopy. [ABSTRACT FROM AUTHOR]

Abstract: A new strategy to access chiral tertiary diarylmethanols through copper‐catalyzed direct alkylation of (di)(hetero)aryl ketones by using Grignard reagents is developed. [ABSTRACT FROM AUTHOR]

Abstract: A chiral guanidine catalyst is successfully employed for 1,4‐ and 1,6‐addition reactions of oxazolones (I). [ABSTRACT FROM AUTHOR]

The author discusses research on gold catalysis, with a focus on how nanosized gold clusters efficiently catalyze the addition of water molecules to alkynes and the potential industrial applications. Topics include how the turnover number (TON) signifies the amount of substrate molecules able to be converted to a product before deactivation, high efficiency in heterogeneous gold catalysis, and gold-catalyzed phenol synthesis. Additional information is presented on turnover frequency and stabilizing ligands.