Isotopic fractionation of mercury induced by reduction and ethylation

Isotope ratio measurements characterizing ²⁰²Hg/²⁰⁰Hg in NIST SRM 3133 Mercury Standard Solution were undertaken by multicollector inductively coupled plasma mass spectrometry employing NIST SRM 997 Tl for mass bias correction by use of the slope and the intercept obtained from a natural logarithmic plot of each session of measurements of ²⁰²Hg/²⁰⁰Hg against ²⁰⁵Tl/²⁰³Tl. The calculated value of 1.285333 ± 0.000192 (mean and one standard deviation, n = 40) for the mass bias corrected ²⁰²Hg/²⁰⁰Hg was then used for mass bias correction of other Hg isotope pairs. Ratios of 0.015337 ± 0.000011, 1.68770 ± 0.00054, 2.3056 ± 0.0015, 1.3129 ± 0.0013, 2.9634 ± 0.0038, and 0.67937 ± 0.0013 (expanded uncertainty, k = 2) were obtained for ¹⁹⁶Hg/¹⁹⁸Hg, ¹⁹⁹Hg/¹⁹⁸Hg, ²⁰⁰Hg/¹⁹⁸Hg, ²⁰¹Hg/¹⁹⁸Hg, ²⁰²Hg/¹⁹⁸Hg, and ²⁰⁴Hg/¹⁹⁸Hg, respectively. Reduction of Hg(II) to Hg⁰ in solutions of SRM 3133 was then undertaken using SnCl₂, NaBH₄, UV photolysis in the presence of formic acid, and ethylation of Hg(II) using NaBEt_4. These reactions induced significant isotope fractionation with maximum values of 1.17 ± 0.07, 1.08 ± 0.09, 1.34 ± 0.07, and 3.59 ± 0.09‰ (one standard deviation, 1SD, n = 5) for δ ^202/198Hg relative to the initial isotopic composition in the solution following 85–90% reduction of the Hg by SnCl₂, NaBH₄, UV photolysis, and ethylation with NaBEt₄, respectively. Mass-dependent fractionation was found to be dominant for all reduction processes. Figure Mass dependence of fractionation for all samples from Hg fractionation experiments using NaBEt₄. Solid lines are the theoretically predicted MDF based on δ′ ^202/198 Hg using equation 7. Error bars displayed are one standard deviation of the mean of 5 measurements of each sample     

Study on the “induction period” of toluene ethylation performed on HZSM-5 zeolite catalysts

Formation and consumption of activated toluene species results in an “induction period” in the toluene ethylation on HZSM-5 zeolite, and toluene pretreatment can enhance the activity of HZSM-5 in the reaction by producing activated toluene species.     

Tributyltin analysis in seawater by GC FPD after direct aqueous-phase ethylation using sodium tetrathylborate

Using sodium tetraethylborate as an alkylation reagent, one-step ethylation and extraction of organotin compounds in seawater was performed directly in the aqueous phase to obtain optimal rection conditions. No further purification or concentration of the extract was required before GC FPD analysis. The detection limit for monobutyl-dibutyl-and tributyl-tin was approximately 0.4 ng dm^?3, which is adequate for any currently know toxicity effect. The linearity range (0–200 ng dm^?3) was solely dependent on the detector response curve. Compared with the Grignard reaction after extraction in an organic solvent, this new ethylation procedure reduces the number of analytical steps, thus saving time and improving reliability.     

Catalytic Photochemical Deracemization via Short-Lived Intermediates

Upon irradiation in the presence of a suitable chiral catalyst, racemic compound mixtures can be converted into enantiomerically pure compounds with the same constitution. The process is called photochemical deracemization and involves the formation of short-lived intermediates. By opening different reaction channels for the forward reaction to the intermediate and for the re-constitution of the chiral molecule, the entropically disfavored process becomes feasible. Since the discovery of the first photochemical deracemization in 2018, the field has been growing rapidly. This review comprehensively covers the research performed in the area and discusses current developments. It is subdivided according to the mode of action and the respective substrate classes. The focus of this review is on the scope of the individual reactions and on a discussion of the mechanistic details underlying the presented reaction.     

Highly stereoselective allylic ethylation with alkoxytitanacyclopropane reagents. Synthesis of (1R/S,7R)-1,7-dimethylnonyl propanoate, the Western corn rootworm sex attractant

Allylic ethylation of 2-((E)-dodec-2-en-4-yloxy)tetrahydro-2H-pyran with ethylmagnesium bromide in the presence of titanium(IV) isopropoxide proceeds via a S_N2′ pathway to afford (E)-3-methyltridec-4-ene with excellent syn-diastereoselecivity. This transformation is used as a key step in the synthesis of (1R/S,7R)-1,7-dimethylnonyl propanoate, the Western corn rootworm (Diabrotica virgifera virgifera) sex attractant.     

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.     

Catalytic ammonia oxidation on platinum: mechanism and catalyst restructuring at high and low pressure

Catalytic ammonia oxidation over platinum has been studied experimentally from UHV up to atmospheric pressure with polycrystalline Pt and with the Pt single crystal orientations (533), (443), (865), and (100). Density functional theory (DFT) calculations explored the reaction pathways on Pt(111) and Pt(211). It was shown, both in theory and experimentally, that ammonia is activated by adsorbed oxygen, i.e. by O(ad) or by OH(ad). In situ XPS up to 1 mbar showed the existence of NH(x)(x= 0,1,2,3) intermediates on Pt(533). Based on a mechanism of ammonia activation via the interaction with O(ad)/OH(ad) a detailed and a simplified mathematical model were formulated which reproduced the experimental data semiquantitatively. From transient experiments in vacuum performed in a transient analysis of products (TAP) reactor it was concluded that N(2)O is formed by recombination of two NO(ad) species and by a reaction between NO(ad) and NH(x,ad)(x= 0,1,2) fragments. Reaction-induced morphological changes were studied with polycrystalline Pt in the mbar range and with stepped Pt single crystals as model systems in the range 10(-5)-10(-1) mbar.     

Selective catalytic carbanionic ethylation of methylphenols: influence of catalyst and substitution pattern

Addition of ethylene to the carbanions formed by the metallation of the lithium salts of di- and trimethylphenols by the strongly basic system, n-BuLi-LiK(OCH₂CH₂NMe₂)₂ provides a useful synthetic route to a range of alkylphenols. The ease of alkylation of the methyl groups decreases in the order ortho>meta>para while the inclusion of Mg(OCH₂CH₂OEt)₂ in the catalyst restricts alkylation to the methyl groups ortho to the hydroxy group. Dialkylation occurs only at the ortho-methyl groups and only if the adjacent meta-position is unsubstituted. The potential of these products for the synthesis of sterically hindered ligands is outlined.     

环钯化合物合成及其在催化中的应用

环钯化合物由于丰富的结构、高度的稳定性和卓越的催化性能,已成为钯化学研究的热点之一。迄今已开发出了C-H键活化、氧化加成、转金属化、亲核加成和配体交换等多种方法,可制备出从三元环到十一元环的CY型环钯化合物和多种YCY型环钯化合物。环钯化合物目前已应用于偶联、烯烃氢化和不对称催化等反应中。本文简单介绍了环钯化合物的种类,重点介绍了环钯化合物的合成方法和催化应用情况,最后提出了环钯化合物在今后合成研究和催化应用中的发展建议。     

Catalytic Asymmetric Ring-Opening Reactions of Unstrained Heterocycles Using Cobalt Vinylidenes

Cobalt catalysts promote highly enantioselective ring-opening reactions of 2,5-dihydrofurans using vinylidenes. The products are acyclic organozinc compounds that can be functionalized with an electrophile. The proposed mechanism involves the generation of a cobalt vinylidene species that adds to the alkene by a [2+2]-cycloaddition pathway. Ring-opening then occurs via outer-sphere β-O elimination assisted by coordination of a ZnX₂ Lewis acid to the alkoxide leaving group. DFT models reveal that competing inner-sphere syn β-H and β-O elimination pathways are suppressed by the geometric constraints of the metallacycle intermediate. These models rationalize the observed stereochemical outcome of the reaction.     

Catalytic Properties of High Nitrogen Content Carbonaceous Materials

The influence of structural modifications on the catalytic activity of carbon materials is poorly understood. A collection of carbonaceous materials with different pore networks and high nitrogen content was characterized and used to catalyze four reactions to deduce structure–activity relationships. The CO₂ cycloaddition and Knoevenagel reaction depend on Lewis basic sites (electron-rich nitrogen species). The absence of large conjugated carbon domains resulting from the introduction of large amounts of nitrogen in the carbon network is responsible for poor redox activity, as observed through the catalytic reduction of nitrobenzene with hydrazine and the catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine using hydroperoxide. The material with the highest activity towards Lewis acid catalysis (in the hydrolysis of (dimethoxymethyl)benzene to benzaldehyde) is the most effective for small molecule activation and presents the highest concentration of electron-poor nitrogen species.     

Stereoselective C-Aryl Glycosylation by Catalytic Cross-Coupling of Heteroaryl Glycosyl Sulfones

Stereoselective C-glycosylation reactions are increasingly gaining attention in carbohydrate chemistry because they enable glycosyl precursors, readily accessible as anomeric mixtures, to converge to a single diastereomeric product. However, controlling the stereochemical outcome through transition-metal catalysis remains challenging, and methods that leverage bench-stable heteroaryl glycosyl sulfone donors to facilitate glycosylation are rare. Herein, we show two complementary nonprecious metal catalytic systems, based on iron or nickel, which are capable of promoting efficient C−C coupling between heteroaryl glycosyl sulfones and aromatic nucleophiles or electrophiles through distinct mechanisms and modes of activation. Diverse C-aryl glycosides were secured with excellent selectivity, scope, and functional-group compatibility, and reliable access to both α and β isomers was possible for key sugar residues.     

催化动力学法测定化探样品中的痕量铬

本对在弱酸性介质中铬（ＶＩ）强烈催化Ｈ２Ｏ２氧化铬蓝黑Ｒ褪色反应进行了一系列研究，拟定了用催化动力学法测定铬（ＶＩ）的最佳条件。该法检出限为３．２×１０＾－９ｇ／ｍＬ，测定线性范围０～１．５μｇ／２５ｍＬ铬（ＶＩ），用于化探样品中痕量铬的测定，结果良好。     

Asymmetric ethylation of aromatic aldehyde by diethylzinc in the presence of chiral auxiliaries derived from (S)-[4-HOC6H4CH2CH(NH2)CPh2OH]

A comparative study has been carried out on enantioselective ethylation of aromatic aldehydes (ArCHO) by diethylzinc (Et₂Zn) in the presence of catalytic amounts of (S)-[4-HOC₆H₄CH₂CH(NH₂)CPh₂OH] (1) or chiral auxiliaries derived from (1). Various procedures have been adopted; these involved (i) different sequences of addition of the reagents ArCHO, Et₂Zn and (1), and (ii) use of titanium(IV)isopropoxide [Ti(OPrⁱ)₄] or the boranetetrahydrofuran addition compound (BH₃ · THF) as additional reagents. In all but one of the procedures adopted, (S)-ArCH(OH)Et was selectively formed; enantiomeric excess values up to 87% were obtained.     

Catalytic reductive homocoupling of 9-bromofluorene

A number of organometallic compounds and inorganic salts, including the group 8 metal carbonyls M₃(CO)₁₂ and salts MCl₃ (M = Fe, Ru, Os), were tested for their catalytic activity in the reductive coupling of 9-bromofluorene. Among them, FeCl₃ was found to show excellent activity (TOF = 1960). The reaction is believed to proceed via a radical mechanism.     

Catalytic properties of molybdena-alumina catalysts pretreated at high temperatures

The dependence of the catalytic properties of molybdena-alumina catalysts reduced in hydrogen on the degree of reduction were followed in a model catalytic reaction of deuterium exchange in benzene. The reaction rate constant and the multiplicity factor decrease as the degree of reduction and the degree of dehydroxylation of the catalyst's surface increase.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1204–1207, July, 1994.     

Catalytic Biorefining of Natural Oils to Basic Olefinic Building Blocks of Proven Chemical Valorization Schemes

Catalytic transformation of renewable plant oils including microalgae and waste oil into industrially relevant α-olefins in the C₃ to C₁₀ regime is demonstrated. The biorefinery concept is comprised of a catalytic sequence of ethenolysis, double bond isomerization, and a subsequent ethenolysis, thereby cutting and rearranging the fatty acid chains into valuable chemical building blocks. A benign extraction and reaction solvent, supercritical carbon dioxide (scCO₂), is utilized.