Formation Of Alkoxides Research Articles

Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Chiral recognition of monomeric Me2MOR units resulting in the formation of homochiral dimeric species [Me2M(μ-OR)]2(M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization ofrac-lactide (rac-LA).

ChemInform Abstract: The Domestication of ortho‐Quinone Methides

AbstractReview: 63 refs.

The domestication of ortho-quinone methides.

ConspectusAn ortho-quinone methide (o-QM) is a highly reactive chemical motif harnessed by nature for a variety of purposes. Given its extraordinary reactivity and biological importance, it is surprising how few applications within organic synthesis exist. We speculate that their widespread use has been slowed by the complications that surround the preparation of their precursors, the harsh generation methods, and the omission of this stratagem from computer databases due to its ephemeral nature.About a decade ago, we discovered a mild anionic triggering procedure to generate transitory o-QMs at low temperature from readily available salicylaldehydes, particularly OBoc derivatives. This novel reaction cascade included both the o-QM formation and the subsequent consumption reaction. The overall transformation was initiated by the addition of the organometallic reagent, usually a Grignard reagent, which resulted in the formation of a benzyloxy alkoxide. Boc migration from the neighboring phenol produced a magnesium phenoxide that we supposed underwent β-elimination of the transferred Boc residue to form an o-QM for immediate further reactions. Moreover, the cascade proved controllable through careful manipulation of metallic and temperature levers so that it could be paused, stopped, or restarted at various intermediates and stages. This new level of domestication enabled us to deploy o-QMs for the first time in a range of applications including diastereocontrolled reactions.This sequence ultimately could be performed in either multipot or single pot processes. The subsequent reaction of the fleeting o-QM intermediates included the 1,4-conjugate additions that led to unbranched or branched ortho-alkyl substituted phenols and Diels–Alder reactions that provided 4-unsubstituted or 4-substituted benzopyrans and chroman ketals. The latter cycloadducts were obtained for the first time with outstanding diastereocontrol. In addition, the steric effects of the newly created stereocenters in subsequent reactions of chroman ketals and acetals were studied and proved predictable. Through the use of a chiral auxiliary, Diels–Alder products were deployed in numerous enantioselective reactions including several complex natural products syntheses. In this Account, we summarize our efforts, which we hope have contributed to the synthetic renaissance for this venerable species.

Improvement of bias-stability in amorphous-indium-gallium-zinc-oxide thin-film transistors by using solution-processed Y2O3 passivation

We demonstrate back channel improvement of back-channel-etch amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors by using solution-processed yttrium oxide (Y2O3) passivation. Two different solvents, which are acetonitrile (35%) + ethylene glycol (65%), solvent A and deionized water, solvent B are investigated for the spin-on process of the Y2O3 passivation—performed after patterning source/drain (S/D) Mo electrodes by a conventional HNO3-based wet-etch process. Both solvents yield devices with good performance but those passivated by using solvent B exhibit better light and bias stability. Presence of yttrium at the a-IGZO back interface, where it occupies metal vacancy sites, is confirmed by X-ray photoelectron spectroscopy. The passivation effect of yttrium is more significant when solvent A is used because of the existence of more metal vacancies, given that the alcohol (65% ethylene glycol) in solvent A may dissolve the metal oxide (a-IGZO) through the formation of alkoxides and water.

Enthalpies of formation of europium alkoxides: What lessons can be drawn from them

The synthesis and characterization of two europium alkoxides, Eu(OCH3)2 and Eu(OC2H5)2, were described. For the first time the enthalpies of formation of divalent lanthanide alkoxides were determined by using reaction-solution calorimetry. The values obtained are ΔfH0 [Eu(OCH3)2,cr]=−850.5±5.0kJ/mol and ΔfH0 [Eu(OC2H5)2,cr]=−902.5±5.5kJ/mol, respectively. Since these compounds have a large use as catalysts or catalysts precursors, the first step of the reaction of them with CO2 was addressed, which permits to have an idea of the kind of bond involved in those compounds. Moreover, insertion of CO2 in the europium oxygen bond and formation of metal carboxylate complexes, is in both cases presumably bidentate.

Transition-metal clusters as catalysts for chemoselective transesterification of alcohols in the presence of amines

Abstract Acylation is one of the most abundant organic transformations of alcohols (esterification) and amines (amidation). Because of the greater nucleophilicity of the amino group compared to the hydroxyl group and the stability of amides compared to esters, N-acylation occurs predominantly in organic synthetic reactions. We reported that the μ-oxo-tetranuclear zinc cluster Zn4(OCOCF3)6O efficiently catalyzes highly chemoselective acylation of hydroxyl groups in the presence of primary and secondary alkyl amino groups to afford the corresponding esters in high yields. Not only zinc carboxylate complexes but also various carboxylate complexes of first-row late transition metals, such as Mn, Fe, Co, and Cu, become catalysts for such the hydroxy group-selective acylation in the presence of amines. Among these carboxylate compounds, we found that the combination of an octanuclear cobalt carboxylate cluster [Co4(OCOR)6O]2 (R = CF3, CH3, and t Bu) with nitrogen-containing ligands such as 2,2′-bipyridine show sufficient catalytic activity toward O-selective transesterification. Notably, an alkoxide-bridged dinuclear complex, Co2(OCO t Bu)2(bpy)2(μ2-OCH2-C6H4-4-CH3)2, was successfully isolated as a key intermediate that proceeds with Michaelis–Menten behavior through an ordered ternary complex mechanism similar to dinuclear metallo-enzymes, suggesting that the formation of alkoxides, followed by coordination of the ester, is responsible for the unique O-selective acylation.

Ytterbium and erbium derivatives of 2-methoxyethanol and their use in the thin film deposition of Er-doped Yb3Al5O12

We have proved different methods for yttrium and erbium 2-methoxyethoxides preparation. Our aim was to prepare a solution applicable at the subsequent sol–gel preparation of Yb3Al5O12 garnet thin films. We tested the direct reaction of a metal with 2-methoxyethanol in the presence of HgCl2, the electrolytic dissolution of metals, an alcohol interchange using isopropoxides and, finally, the exchange reaction of acetates with 2-methoxyethanol. Because our demand was to prepare a solution without the necessity of purification, we omitted chlorides as a source of cations. The formation of the metal alkoxides was examined by IR spectroscopy in each case. Both exchange reactions were fully successful; only in the case of acetate use, the arising acetic acid forms ester immediately. The direct reaction of metals with the alcohol did not achieve a full yield; on the other hand, after electrolysis application, the metals dissolved readily. However, in both cases, the filtration of either unreacted metal (direct reaction) or eroded metal pieces (electrolysis) was necessary. Because the filtration has to be processed under inert atmosphere these methods are less convenient for the subsequent use in the sol–gel deposition. Er:Yb3Al5O12 thin films deposited on silicon substrates were also prepared using the two appropriate intermediates—the solutions prepared either from acetates or isopropoxides. Both layers were monophase; however, the microstructure and luminescent properties were influenced by the solution used. The Er:YbAG layers exhibited the sharp and discreet luminescence peaks of the Er3+ transition 4I13/2 → 4I15/2.

Friction and wear properties of αFeSi2–Si alloy, ReSi1.8 and MoSi2 in ethyl alcohol

In this study, Fe–Xat% Si alloy (X=70.5, 80.0 and 96.0), Re–64.3at% Si and Mo–66.7at% Si disk specimens were prepared by spark plasma sintering, and their friction and wear properties were investigated when they were slid against Si3N4 ball specimens in ethyl alcohol. The friction and wear properties of Si ingots were also examined. Fe–70.5at% Si, Fe–80.0at% Si, Fe–96.0at% Si and Re–64.3at% Si disk specimens exhibited friction coefficients as low as 0.15. It is considered that the low friction of the Fe–70.5at% Si, Fe–80.5at% Si and Fe–96.0at% Si disk specimens was due to the formation of low friction silicon alkoxide and polyoxysilane on the worn surfaces of the disk specimens and the paired ball specimens. Re–64.3at% Si disk specimens exhibited the highest microvickers hardness of all the disk specimens prepared in this study. In addition, the microvickers hardness of the Fe–Xat% Si (X=70.5, 80.0, 96.0 and 100) disk specimen increased with increasing the Si content. Moreover, it was difficult to obtain dense Fe–90.0at% Si disk specimens by sintering the annealed and crushed Fe–90.0at% Si powder. However, dense Fe–96.0at% Si disk specimens could be obtained by sintering the Fe–90.0at% Si powder at 1403K.

Role of ReOx in Re-modified Rh/ZrO2 and Ir/ZrO2 catalysts in glycerol hydrogenolysis: Insights from first-principles study

The thermodynamics of glycerol hydrogenolysis to produce 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) over Ru/ZrO 2 , Rh/ZrO 2 , ReO x -Rh/ZrO 2 , and ReO x -Ir/ZrO 2 were studied using density functional theory calculations, with a special focus on the mechanism controlling the activity and selectivity of the reactions. It is found that the decomposition of glycerol on Ru/ZrO 2 and Rh/ZrO 2 proceeds through a dehydration-hydrogenation mechanism. The formation of 1,2-PDO is thermodynamically favored, and the activity of the Ru-based catalyst is higher than that of the Rh-based one. In contrast, a direct hydrogenolysis mechanism is proposed for the Re-modified Rh and Ir catalysts, in which a dissociated H atom on the Rh(Ir) metal surface attacks the C–O bond neighboring the alkoxide species on the ReO x cluster. In the presence of ReO x -Rh/ZrO 2 , the modified catalyst favors the production of 1,2-PDO, and 1,3-PDO production becomes competitive. However, the ReO x -Ir/ZrO 2 catalyst significantly improves 1,3-PDO selectivity. The direct hydrogenolysis pathway, as opposed to the indirect hydrogenolysis mechanism for monometallic catalysts, may be the key to the high 1,3-PDO selectivity on the modified catalysts, where the hydroxylated Re group facilitates the formation of terminal alkoxide species rather than secondary alkoxides. Steric effects are important in preferential terminal alkoxide formation on the ReO x -Ir/ZrO 2 catalysts because of the growth of large Ir-Re clusters, resulting in high selectivity for 1,3-PDO. The decisive role of ReO x present in Re-modified bifunctional catalysts in promoting the activity and 1,3-propanediol selectivity, compared with monometallic clusters, is highlighted.

Alcohol Dispersions of Calcium Hydroxide Nanoparticles for Stone Conservation

Alcohol dispersions of Ca(OH)2 nanoparticles, the so-called nanolimes, are emerging as an effective conservation material for the consolidation of stone, mortars, and plasters present in old masonry and/or mural paintings. To better understand how this treatment operates, to optimize its performance and broaden its applications, here we study the nano and microstructural characteristics, carbonation behavior, and consolidation efficacy of colloidal alcohol dispersions of Ca(OH)2 nanoparticles produced by both homogeneous (commercial nanolime) and heterogeneous phase synthesis (aged slaked lime and carbide lime putties). We observe that the alcohol not only provides a high colloidal stability to Ca(OH)2 particles, but also affects the kinetics of carbonation and CaCO3 polymorph selection. This is due to the pseudomorphic replacement of Ca(OH)2 particles by calcium alkoxides upon reaction with ethanol or 2-propanol. The extent of this replacement reaction depends on Ca(OH)2 size and time. Hydrolysis of alkoxides speeds up the carbonation process and increases the CaCO3 yield. The higher degree of transformation into calcium alkoxide of both the commercial nanolime and the carbide lime fosters metastable vaterite formation, while calcite precipitation is promoted upon carbonation of the aged slaked lime due its lower reactivity, which limits calcium alkoxide formation. A higher consolidation efficacy in terms of strength gain of treated porous stone is achieved in the latter case, despite the fact that the carbonation is much faster and reaches a higher yield in the former ones. Formation of alkoxides, which has been neglected in previous studies, needs to be considered when applying nanolime treatments. These results show that the use Ca(OH)2 nanoparticle dispersions prepared with either aged slaked lime or carbide lime putties is an economical and effective conservation alternative to commercial nanolimes produced by homogeneous phase synthesis. Ultimately, this study contributes to show that nanotechnology can help saving the built and sculptural heritage.

APCVD of ZnO:Al, insight and control by modeling

Atmospheric pressure chemical vapor deposition (APCVD) of ZnO from diethyl zinc (DEZn) and t-butanol was performed using an industrial reactor design. Deposition profiles were recorded to gain insight in the position dependent variations in layer thickness in such a reactor. We observed that for a deposition temperature below 400°C most of the deposition took place close to the exit of the gasses, while for increasing temperatures the deposition shifts towards the gas inlet. This trend can be explained by the reaction mechanism through an intermediate alkoxide species from DEZn and t-butanol, which in turn leads to ZnO deposition through a surface reaction. The deposition profile is dependent on the local alkoxide concentration. With increasing temperature, the formation rate increases. This translates in an earlier formation, i.e. in a shift upstream towards gas inlet because this alkoxide formation takes place during the transport through the reactor. Chemical vapor deposition (CVD) is a highly complex system with many interacting physical and chemical processes. Modeling was used to gain insight on the local variations of the concentration of reactive species inside a reactor and was shown to predict the deposition profiles. Moreover, the impact of changes in reactor design on the deposition is discussed.

Mechanistic Aspects on Cyclopentadienylruthenium Complexes in Catalytic Racemization of Alcohols

Cyclopentadienylruthenium complexes commonly serve as efficient transition metal catalysts in the racemization of alcohols. The combination of the racemization reaction with enzymatic resolution leads to dynamic kinetic resolution (DKR). In DKR, a theoretical yield of 100% is possible, making it a powerful tool for enantioselective synthesis. In this Account, we summarize the most important mechanistic aspects of racemization of alcohols reported over the past decade based on both experimental and computational results. Precatalyst activation is often necessary, either by heating the reaction or by adding an alkoxide-type base. The subsequent alcohol-alkoxide exchange is rapid and introduces the substrate into the catalytic cycle. This exchange requires a free coordination site, which may be created via several different mechanisms. Following alkoxide formation, racemization occurs via β-hydride elimination and subsequent readdition. In cyclopentadienyldicarbonylruthenium alkoxide complexes, which are 18-electron complexes, researchers originally considered two mechanisms for the creation of the free coordination site required for β-hydride elimination: a change in hapticity of the cyclopentadienyl ligand from η5 to η3 and dissociation of a CO ligand. Based on computational and experimental results, we have found strong support for the pathway involving CO dissociation. Researchers had also wondered if the substrate remains coordinated to the metal center (the inner-sphere mechanism) during the hydrogen transfer step(s). Using competition and crossover experiments, we found strong evidence for an inner-sphere mechanism. In summary, we have obtained a detailed picture of the racemization of alcohols by cyclopentadienylruthenium catalysts, leading to the development of more efficient catalytic systems for racemization.

Enzyme-Like Catalysis via Ternary Complex Mechanism: Alkoxy-Bridged Dinuclear Cobalt Complex Mediates Chemoselective O-Esterification over N-Amidation

Hydroxy group-selective acylation in the presence of more nucleophilic amines was achieved using acetates of first-row late transition metals, such as Mn, Fe, Co, Cu, and Zn. Among them, cobalt(II) acetate was the best catalyst in terms of reactivity and selectivity. The combination of an octanuclear cobalt carboxylate cluster [Co4(OCOR)6O]2 (2a: R = CF3, 2b: R = CH3, 2c: R = (t)Bu) with nitrogen-containing ligands, such as 2,2'-bipyridine, provided an efficient catalytic system for transesterification, in which an alkoxide-bridged dinuclear complex, Co2(OCO(t)Bu)2(bpy)2(μ2-OCH2-C6H4-4-CH3)2 (10), was successfully isolated as a key intermediate. Kinetic studies and density functional theory calculations revealed Michaelis-Menten behavior of the complex 10 through an ordered ternary complex mechanism similar to dinuclear metallo-enzymes, suggesting the formation of alkoxides followed by coordination of the ester.

Switchable polarity solvent (SPS) systems: probing solvatoswitching with a spiropyran (SP)–merocyanine (MC) photoswitch

The switchable polarity solvent (SPS) of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and an alcohol (e.g. 1-propanol) reversibly switches to a higher polarity ionic liquid, [DBUH(+)][RCO3(-)], when treated with CO2. A long-lived species with unique properties was detected in an investigation into the use of SPS to control the lifetime of the merocyanine (MC) form in a spiropyran (SP)-MC molecular photoswitch. Irradiation of SP in 1-propanol (PrOH) in the presence of DBU generates a new species (λmax = 420 nm). This species converts to MC upon bubbling with CO2, which produces [DBUH(+)][PrOCOO(-)]. It is proposed that a mixture of 1,2 and 1,4 alkoxide addition products form as a result of nucleophilic attack on the conjugated diene system of MC, where alkoxide formation arises from equilibration of highly basic DBU and the alcohol. These adducts revert to MC upon application of CO2 or addition of acid. Determination of the overall equilibrium constant for alkoxide adduct formation involving DBU was afforded through Benesi-Hildebrand analysis.

Roles of Acetone and Diacetone Alcohol in Coordination and Dissociation Reactions of Uranyl Complexes

Combined collision-induced dissociation mass spectrometry experiments with DFT and MP2 calculations were employed to elucidate the molecular structures and energetics of dissociation reactions of uranyl species containing acetone and diacetone alcohol ligands. It is shown that solutions containing diacetone alcohol ligands can produce species with more than five oxygen atoms available for coordination. Calculations confirm that complexes with up to four diacetone alcohol ligands can be energetically stable but that the effective number of atoms coordinating with uranium in the equatorial plane does not exceed five. Water elimination reactions of diacetone alcohol ligands are shown to have two coordination-dependent reaction channels, through formation of mesityl oxide ligands or formation of alkoxide and protonated mesityl oxide species. The present results provide an explanation for the implausible observation of "[UO(2)(ACO)(6,7,8)](2+)" in and observed water-elimination reactions from purportedly uranyl-acetone complexes (Rios, D.; Rutkowski, P. X.; Van Stipdonk, M. J.; Gibson, J. K. Inorg. Chem. 2011, 50, 4781).

Symmetry Aspects of H2 Splitting by Five-Coordinate d6 Ruthenium Amides, and Calculations on Acetophenone Hydrogenation, Ruthenium Alkoxide Formation, and Subsequent Hydrogenolysis in a Model trans-Ru(H)2(diamine)(diphosphine) System

The potential energy surface (PES) of H(2) addition to the Ru═N bond of a model five-coordinate ruthenium amide (Ru═N), leading to an octahedral trans-Ru(H)(2)(diamine)(diphosphine) (HRu-NH) and subsequent acetophenone hydrogenation, is studied using M06 density functional theory methods. A qualitative molecular orbital analysis reveals that H(2) addition to the ground state of Ru═N (which has a distorted trigonal-bipyramidal geometry) fits the criterion of a symmetry-forbidden reaction. A transition state (TS) for H(2) heterolytic splitting by Ru═N corresponds to the reaction taking place on an excited state of the Ru═N having a square-pyramidal geometry and gives ΔG°(⧧) = 19.5 kcal/mol. The reaction between HRu-NH and acetophenone proceeds by a localized hydride-transfer TS with ΔG°(⧧) = 11.5 kcal/mol. This TS leads to an ion pair between a square-pyramidal d(6) ruthenium amino cation and the alkoxide and is uphill from the separated reactants by 3.5 kcal/mol. Subsequent abstraction of the amino proton by the alkoxide within the ion pair is barrierless, but it also lacks any thermodynamic driving force. In contrast, reorientation of the alkoxide within the ion pair to form an octahedral ruthenium alkoxide is calculated to be exoergic by 7.1 kcal/mol. These features of the PES suggest that the known rapid production of ruthenium alkoxides when stoichiometric amounts of acetophenone and HRu-NH are reacted at low temperatures proceeds by a simple direct route following hydride transfer. For the simplified model complex, ruthenium alkoxide is calculated to be the thermodynamic product of the hydrogenation reaction (exoergic by 3.6 kcal/mol). A TS for H(2) heterolytic splitting across the Ru-alkoxide bond is calculated to have ΔG°(⧧) (16.0 kcal/mol), slightly smaller than that of H(2) addition to the five-coordinate Ru═N.

A review and a fundamental theory of silicon nitride tribochemistry

The present work aims at providing a better understanding of the tribology of silicon nitride, an engineering ceramic with many applications in the automotive and aerospace industry. The tribological properties of silicon nitride depend on tribochemical reactions, which modify both surface composition and morphology. In air or humid conditions, hydroxylated silicon oxide is the main product of tribochemical wear, which flattens surface decreasing stresses on asperities and reducing wear rate and friction level. Severe wear is dominated by fracture wear modes. Boundary lubrication with n-alcohols is also characterized by tribochemical wear, which leads to the formation of silicon alkoxides and polysiloxanes. The working model is based on the tribo-emission process of low-energy electrons during friction with generation of positively charged silicon sites and nitrogen free radicals.

Friction and wear properties of Fe–Si intermetallic compounds in ethyl alcohol

In this study, Fe–Si intermetallic compound disk specimens with some different compositions were prepared by spark plasma sintering and annealing, and their friction and wear properties were investigated when they were slid against SiC and Si 3N 4 ball specimens in ethyl alcohol. The Fe-70.5at% Si/Si 3N 4 and Fe-66.7at% Si/Si 3N 4 tribopairs exhibited the lowest and most stable friction coefficients of all the Fe-Xat% Si/SiC and Fe-Xat% Si/Si 3N 4 ( X = 0, 33.3, 50.0, 66.7, 70.5) tribopairs. When the paired ball materials were the SiC, the Fe-70.5at% Si disk specimens exhibited the lowest friction coefficients 5 min after starting the friction and wear tests. The low friction coefficients of the Fe-70.5at% Si/Si 3N 4, Fe-66.7at% Si/Si 3N 4 and Fe-70.5at% Si/SiC tribopairs are thought to be due to the formation of silicon alkoxide on the worn surfaces of the disk specimens. In addition, the Fe-50.0at% Si disk specimen showed the highest microvickers hardness of all the Fe–Si intermetallic compound disk specimens.

Liquid-phase glycerol hydrogenolysis to 1,2-propanediol under nitrogen pressure using 2-propanol as hydrogen source

2-Propanol was studied as a hydrogen donor molecule in the transfer hydrogenation process to selectively convert glycerol into 1,2-propanediol under N 2 pressure and using Ni or/and Cu supported on Al 2O 3 catalysts. The results were compared to those obtained under the same operating conditions but under H 2 pressure. The results of the activity tests and catalyst characterization techniques (N 2-physisorption, H 2-chemisorption, TPD of NH 3, TPR, TPO and XPS) suggest that glycerol hydrogenolysis to yield 1,2-propanediol occurred through a different mechanism regarding the origin of the hydrogen species. When atomic hydrogen came from dissolved molecular hydrogen dissociation, glycerol was first dehydrated to acetol and then hydrogenated to 1,2-propanediol. On the other hand, when the hydrogen atoms were produced from 2-propanol dehydrogenation, glycerol was directly converted to 1,2-propanediol through intermediate alkoxide formation.

Reaction of lanthanide(II) and lanthanide(III) phenylethynyl cuprates with acetyl chloride

Praseodymium and ytterbium phenylethynyl cuprates [(PhC≡C)3Cu]3Pr2(THF)6 and {[(PhC≡C)3Cu]·Yb(THF)2}2 react with acetyl chloride in tetrahydrofuran with elimination of phenylethynylcopper and formation of alkoxides [PhC≡C-CCl(CH3)O]nLn (n = 3, Ln = Pr; n = 2, Ln = Yb). Then praseodymium alkoxide forms ester [methyl (phenylethynyl)chloromethylethanoate] and praseodymium chloride, alkoxy derivative. Itterbium alkoxide is oxidized to unsymmetrical dialkoxyitterbium chloride PhC≡C-CH(CH3)-O-Yb(Cl)-O-CCl(CH3)C≡CPh·2THF.