Acetaldehyde Enolate Research Articles

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex.

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3- fragment, which undergoes subsequent deoxygenation.

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex

AbstractAddition of CO to a tetrametallic magnesium hydride cluster results in both carbon‐carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]− which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]− fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3− fragment, which undergoes subsequent deoxygenation.

Interstellar Enolization-Acetaldehyde (CH3 CHO) and Vinyl Alcohol (H2 CCH(OH)) as a Case Study.

Owing to the unique conditions in cold molecular clouds, enols-the thermodynamically less stable tautomers of aldehydes and ketones-do not undergo tautomerization to their more stable tautomers in the gas phase because they cannot overcome tautomerization barriers at the low temperatures. Laboratory studies of interstellar analog ices have demonstrated the formation of several keto-enol tautomer pairs in astrochemically relevant ice mixtures over the last years. However, so far only one of them, acetaldehyde-vinyl alcohol, has been detected in deep space. Due to their reactivity with electrophiles, enols can play a crucial role in our understanding of the molecular complexity in the interstellar medium and in comets and meteorites. To study the enolization of aldehydes in interstellar ices by interaction with galactic cosmic rays (GCRs), we irradiated acetaldehyde ices with energetic electrons as proxies of secondary electrons generated in the track of GCRs while penetrating interstellar ices. The results indicate that GCRs can induce enolization of acetaldehyde and that intra- as well as intermolecular processes are relevant. Therefore, enols should be ubiquitous in the interstellar medium and could be searched for using radio telescopes such as ALMA. Once enols are detected and abundances are established, they can serve as tracers for the non-equilibrium chemistry in interstellar ices thus eventually constraining fundamental reaction mechanisms deep inside interstellar ices.

DFT study on the mechanistic details of the hydrolysis of dicyan using acetaldehyde as the first organocatalyst

The possible mechanistic pathways for acetaldehyde-catalyzed hydrolysis of dicyan have been investigated. Five routes, starting from simultaneous addition of acetaldehyde and water to dicyan (a), conversion of acetaldehyde to its hydrated form (b), addition of acetaldehyde (only) to dicyan (c), water-assisted hydrolysis of dicyan, without using acetaldehyde (d) and enolization of acetaldehyde (e) have been studied with details. The order of the overall barriers of examined routes is: a < b < d < e < c.

Correction to: Simulated Temperature Programmed Desorption of Acetaldehyde on CeO2(111): Evidence for the Role of Oxygen Vacancy and Hydrogen Transfer

The temperature programmed desorption of acetaldehyde adsorbed on partially reduced CeO2(111) has been studied in detail using microkinetic modeling based on self-consistent, periodic density functional theory calculations at the GGA-PW91+U level. Previous experimental studies (Chen et al. J. Phys. Chem. C 115: 3385, 2011; Calaza et al. J. Am. Chem. Soc. 134: 18034, 2012) have shown that, whereas on fully oxidized CeO2(111) acetaldehyde desorbs molecularly with a peak temperature of 210 K, the polymerization and enolization of acetaldehyde dominate the surface reactivity on partially reduced CeO2(111), resulting in acetaldehyde desorption at higher temperatures. Here we propose a comprehensive reaction mechanism that is consistent with the spectroscopic evidence of the identities of the surface intermediates and with the observed desorption activities, including the formation of ethylene and acetylene. Besides acetaldehyde (CH3CHO) and its enolate (CH2CHO), several other C2H x O species are proposed as key intermediates which are not seen spectroscopically, including ethoxy (CH3CH2O), ethyleneoxy (CH2CH2O), and formylmethylene (CHCHO). Our study suggests that oxygen vacancies play the critical role of activating the carbonyl bond and thereby facilitating β C–H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling reactions.

Simulated Temperature Programmed Desorption of Acetaldehyde on CeO2(111): Evidence for the Role of Oxygen Vacancy and Hydrogen Transfer

The temperature programmed desorption of acetaldehyde adsorbed on partially reduced CeO2(111) has been studied in detail using microkinetic modeling based on self-consistent, periodic density functional theory calculations at the GGA-PW91+U level. Previous experimental studies (Chen et al. J. Phys. Chem. C 115: 3385, 2011; Calaza et al. J. Am. Chem. Soc. 134: 18034, 2012) have shown that, whereas on fully oxidized CeO2(111) acetaldehyde desorbs molecularly with a peak temperature of 210 K, the polymerization and enolization of acetaldehyde dominate the surface reactivity on partially reduced CeO2(111), resulting in acetaldehyde desorption at higher temperatures. Here we propose a comprehensive reaction mechanism that is consistent with the spectroscopic evidence of the identities of the surface intermediates and with the observed desorption activities, including the formation of ethylene and acetylene. Besides acetaldehyde (CH3CHO) and its enolate (CH2CHO), several other C2H x O species are proposed as key intermediates which are not seen spectroscopically, including ethoxy (CH3CH2O), ethyleneoxy (CH2CH2O), and formylmethylene (CHCHO). Our study suggests that oxygen vacancies play the critical role of activating the carbonyl bond and thereby facilitating β C–H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling reactions.

Why Do Enolate Anions Favor O-Alkylation over C-Alkylation in the Gas Phase? The Roles of Resonance and Inductive Effects in the Gas-Phase SN2 Reaction between the Acetaldehyde Enolate Anion and Methyl Fluoride.

Contributions by resonance and inductive effects toward the net activation barrier were determined computationally for the gas-phase SN2 reaction between the acetaldehyde enolate anion and methyl fluoride, for both O-methylation and C-methylation, in order to understand why this reaction favors O-methylation. With the use of the vinylogue extrapolation methodology, resonance effects were determined to contribute toward increasing the size of the barrier by about 9.5 kcal/mol for O-methylation and by about 21.2 kcal/mol for C-methylation. Inductive effects were determined to contribute toward increasing the size of the barrier by about 1.7 kcal/mol for O-methylation and 4.2 kcal/mol for C-methylation. Employing our block-localized wave function methodology, we determined the contributions by resonance to be 12.8 kcal/mol for O-methylation and 22.3 kcal/mol for C-methylation. Thus, whereas inductive effects have significant contributions, resonance is the dominant factor that leads to O-methylation being favored. More specifically, resonance serves to increase the size the barrier for C-methylation significantly more than it does for O-methylation.

ChemInform Abstract: Concise Synthesis of Functionalized Benzocyclobutenones.

AbstractA modified access to the benzocyclobutene motif via [2 + 2] cycloaddition between benzynes generated from dehydrohalogenation of aryl halides and acetaldehyde enolate in situ generated from THF is presented.

Proton shuffling in acid/base-catalyzed enolizations: a computational study

Enolization of acetaldehyde catalyzed by the combined action of a general base (ammonia) and a general acid (formic acid) was examined by density functional theory at the B3LYP/6-311 + G(3df,2p) level while manipulating distance relationships among the reactants. Computations were carried out in the gas phase, in the presence of four water molecules, and with a dielectric constant of 78.4. Enolization involves an early transition state where general-base catalysis is more developed than general-acid catalysis. Although formic acid does not promote enolization by itself, it does facilitate α-proton transfer from acetaldehyde to the general base by several orders of magnitude. Formic acid accomplishes this feat via a hydrogen bond at a van der Waals distance to the carbonyl oxygen as opposed to forming a low-barrier hydrogen bond. A low-barrier hydrogen bond would indeed be capable of accelerating the enolization were it not for the energy cost of generating it. Formic acid may also facilitate enolization by internal solvation of the ammonium ion that is partially formed in the transition state via carbon-to-nitrogen proton transfer. General-base catalysis by trimethylamine, which is out of position to coordinate with the formic acid carboxyl, actually has lower activation energy than that of ammonia catalysis, possibly owing to basicity/shielding effects. Computations also demonstrate that the proton removed by the ammonia nitrogen remains on the nitrogen throughout rather than being transferred via low-energy rotation processes and secondary proton transfers to an oxygen atom of formic acid or the enol itself. Finally, stepwise and concerted mechanisms for enolizations have been proposed in the literature, with experimental evidence being provided for both. The concerted/non-concerted disagreement seems to stem from the continuum of organic mechanisms that Nature bestows onto organic chemistry. Thus, acid/base catalysis varies from stepwise at one extreme to synchronous at the other extreme with an infinite number of concerted mechanisms in between. Since the degree of concertedness undoubtedly depends upon the particular acid, base, substrate, and solvent, disparate enolization models are to be expected. Copyright © 2012 John Wiley & Sons, Ltd.

Continuous-Flow Synthesis of Monoarylated Acetaldehydes Using Aryldiazonium Salts

Anilines and ethyl vinyl ether can be used as precursors for a process that is the synthetic equivalent of the α-arylation of acetaldehyde enolate. The reaction manifests a high level of functional group compatibility, allowing the ready preparation of a number of synthetically valuable compounds.

Carboxylate-Directed Kumada Coupling of an Acetaldehyde Synthon with 2-Bromobenzoates Used towards the Synthesis of Isochromanes

This letter describes the Kumada coupling of bromobenzoic acid derivatives with the acetaldehyde enolate synthon (1,3-dioxolan-2-ylinethyl)magnesium bromide. The lithium carboxylate salt shows the highest reactivity in the coupling reaction while addition of t-BuOLi affords optimal selectivity. During this work. we have observed a strong directing effect of the sodium carboxylate function which can be useful in differentiating electronically similar diaryl bromides.

Nucleophilic attack on ketene, 6‐methylene‐2,4‐cyclohexadienylideneketene, 6‐oxo‐2,4‐cyclohexadienylideneketene and 4‐oxo‐2,5‐cyclohexadienylideneketene: an ab initio study

AbstractNucleophilic attack of cyanide anion at the carbonyl carbon was studied computationally at the MP2/6–311+G** level for ketene and at the MP2/6–31+G* and MP2/6–31+G*//HF/6–31+G* levels for all four title compounds. Heats of addition were computed for formation of in‐plane adducts and perpendicular adducts where the terms ‘in‐plane’ and ‘perpendicular’ indicate the direction of nucleophilic attack relative to the ketene plane. Heats of activation were also computed for the two modes of attack. For ketene (1) the in‐plane adduct is favored by 37.6 kcal mol−1 and the in‐plane transition state by 18.8 kcal mol−1. For the 6‐methylene compound (2) the in‐plane adduct is favored by 20.1 kcal mol−1 and the in‐plane transition state by 7.3 kcal mol−1. For the 6‐oxo compound (3) the in‐plane adduct is more stable by 11.2 kcal mol−1 whereas the in‐plane transition state is preferred by 8.8 kcal mol−1. For the 4‐oxo compound (4) the in‐plane adduct is favored by 19.3 kcal mol−1 but its transition state by only 2.8 kcal mol−1. The in‐plane adduct for ketene is the enolate of acetyl cyanide while the perpendicular adduct is a pyramidalized carbanion attached to a cyanocarbonyl group. Calculated NPA charges support this conclusion. The perpendicular adduct is actually the transition state for rotation about the CC bond of the ketene moiety in the in‐plane adduct, and the 37.6 kcal mol−1 enthalpy difference is closely matched by the calculated activation enthalpy for rotation about the CC bond of acetaldehyde enolate: ΔH‡ = 33.6 kcal mol−1. The smaller differences found for in‐plane vs perpendicular adduct formation for the 6‐methylene, 6‐oxo and 4‐oxo compounds are correlated with decreases in bond length alternation within the six‐membered rings which occur upon addition of the nucleophile. Less alternation may be associated with an approach to aromaticity, hence an increase in stability. Both modes of attack lead to reduced bond length alternation, but the amount of alternation is lowered more for the perpendicular adducts than for the in‐plane adducts. Changes in the CO bond lengths are also diagnostic. For all four ketenes the ketenyl CO length in the in‐plane adducts is greater than in the perpendicular adducts reflecting more single bond (enolate) character in the former and more double bond (carbonyl) character in the latter. On the other hand the lengths of the 6‐oxo and 4‐oxo CO bonds in their in‐plane adducts are smaller than in the perpendicular adducts, consistent with increased conjugation with the oxo substituent in the latter. The greater NPA charge on the 6‐oxo and 4‐oxo oxygens of the perpendicular adducts supports this conclusion. Calculated enthalpies of activation are in the order ketene &gt; 6‐methyleneketene &gt; 6‐oxoketene, in qualitative agreement with experimentally determined rates of hydration. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Solvent-assisted catalysis in the enolization of acetaldehyde radical cation

The keto–enol isomerization of neutral and ionized acetaldehyde, both isolated and catalyzed by solvent molecules, has been studied using the B3LYP and MP2 levels of theory. Single-point calculations at the CCSD(T) level have also been performed. The results show that ionization favors the enolization of acetaldehyde, both thermodynamically and kinetically. Solvent molecules produce an important catalytic effect, especially for the methanol-solvated system, for which the transition state of the isomerization lies below the ground-state asymptote.

ChemInform Abstract: Formation of Acetaldehyde Enolate from Vinyl Acetate and Its Reaction with Aromatic and Heterocyclic Aldehydes: An Efficient Synthesis of Enals and Polyenals.

Two carbon homologation of aromatic and heterocyclic aldehydes is achieved by reacting them with acetaldehyde enolate anion generated in situ by cleaving vinyl acetate in the presence of barium hydroxide. cross aldol condensation - two carbon homologation - dienals - bismethylthioacrolein - ethyl 3,3-bis(methylthio)-2-propenoate.

Formation of Acetaldehyde Enolate from Vinyl Acetate and its Reaction with Aromatic and Heterocyclic Aldehydes: An Efficient Synthesis of Enals and Polyenals

Two carbon homologation of aromatic and heterocyclic aldehydes is achieved by reacting them with acetaldehyde enolate anion generated in situ by cleaving vinyl acetate in the presence of barium hydroxide.

Intrinsic barriers for imbalanced versus balanced transition structures

Ab initio calculations were carried out on the identity gas-phase proton and methyl cation transfers from carbon acids (acetaldehyde and propene) to conjugate bases (acetaldehyde enolate and allyl anions) at the MP2/6–311+G**//MP2/6–311+G** level. The percentage bond length changes at the transition structure (TS) show that in the imbalanced TS the exoergic bond contraction is always ahead of the proton transfer and concurrent endoergic bond stretching with a net gain of resonance stabilization. This is achieved by a lag in charge delocalization into the π-acceptor group in the TS. The reaction through an imbalanced TS is in accord with a basic natural law of the minimum energy path in physical or chemical changes. Copyright © 1999 John Wiley & Sons, Ltd.

Nonlocal (Pair Site) Reactivity from Second-Order Static Density Response Function: Gas- and Solution-Phase Reactivity of the Acetaldehyde Enolate as a Test Case

A nonlocal (pair site) reactivity scheme is developed and tested. The theory is cast in terms of the first-order Fukui response function f(r,r‘), previously proposed by Fuentealba and Parr [J. Chem. Phys. 1991, 94, 5559]. A change of variables is introduced by using the softness s(r) and t(r) = [∂s(r)/∂N]υ(r) (the variation of softness with respect to the changes in the total number of electrons N at constant external potential υ(r)) that leads to a simple expression for the variation of the Fukui function at site k, namely = − for an electrophilic attack. The first term describes a local contribution, proportional to the variation of the electrostatic potential that can be induced, for example, by the approach of an electrophilic agent, with a variable global softness coefficient thereby incorporating higher (third) order derivatives of the electronic energy. The second term contains nonlocal information implicitly involving a second reactive site that is expressed in terms of the variations to the electronic chemical potential Δμ, accounting for the charge transfer related to the electrophile/nucleophile interaction. New reactivity indexes , , are derived. They are associated with nucleophile, electrophile, and radical attacks. The theory applies to gas-phase reactivity. Solvent effects are introduced in the continuum approach to the surrounding medium. The resulting model is applied to study the chemistry of the enolate ion in the gas and solution phases. In the gas phase, an inversion of f(r) in the vicinity of the transition structure is observed that leads to alkylation at the oxygen atom. Incorporation of two water molecules to mimic solvent effects changes the gas-phase pattern of reactivity, and alkylation at the α-carbon is predicted in solution, in agreement with the experiments.

Hydroxide Attack on Acetylene: Theoretical Structures and Energies

Several C2H3O- species resulting from the interaction of acetylene and hydroxide have been characterized with ab initio calculations. MP2 geometry optimizations were performed with augmented, correlation-consistent, polarized-valence, double-ζ basis sets. The total energies of the most crucial structures were also recalculated at the QCISD(T) level with MP2-optimized geometries. Nine structures corresponding to local minima have been found at the MP2 level, their stability decreasing in the following order: acetaldehyde enolate anion > acetyl anion ≈ ethynide−water complex > ethenyloxy anion ≈ vinyloxy anion. The ethynide−water complex is either the most stable product of the reaction of hydroxide with acetylene or at least an initial stable intermediate.

Charge-Localizing Effect in Alkali-Metal Enolates and Phenolates. Structure and Aromaticity of the Phenolate Anion

Alkali-metal counterion interactions influence the enolate and phenolate anions considerably. Like the acetaldehyde enolate anion, the stabilization energies of the phenolate anion (vs OH-) are very much reduced in the corresponding alkali-metal ion pairs. The stabilization energies (relative to OH-) of 34.9 kcal/mol for the free enolate anion and 40.9 kcal/mol for the phenolate ion (computed with the Becke3LYP density functional on the MP2(full)/6-31+G* optimized structures) reproduce the experimental gas-phase data well. The metal gegenion induced negative charge localization on the enolate and phenolate oxygens varies only somewhat along the Li−Cs series but reduces the stabilization energies enormously: for the enolates to 8−14 kcal/mol and to 9−15 kcal/mol for the phenolates (vs MOH). These ion pair interactions influence the geometries, the charge distributions, the 13C chemical shifts, and the reaction energies of alkali-metal enolates and phenolates but do so to similar extents in both systems. Charge localization is still effective at long metal−oxygen distances, since electrostatic interaction energies decrease only with the inverse of the distance. The phenolate anion has considerable quinoid character, and judging from the geometry and the magnetic criteria (NICS and Λ, the magnetic susceptibility exaltation) its aromaticity is reduced to about 60% of that of phenol and the alkali-metal ion-paired phenolates.

The degenerate Payne rearrangement of the 2,3-epoxypropoxide anion in the gas phase. A joint theoretical and experimental study

Ab initio calculations [at G2 level] indicate that an energised 2,3-epoxypropoxide anion should undergo two competing cyclisation processes, i.e. (i) the degenerate Payne rearrangement (attack of O– at the more substituted carbon of the ethylene oxide ring, to open that ring, and to form another ethylene oxide ring) (the computed barrier to the transition state is 45 kJ mol–1), and (ii) attack of O– at the less substituted carbon of the ethylene oxide ring. This cyclisation forms a more stable oxetane species, but the barrier (from the reactant to transition state) is 122 kJ mol–1. Experimental results are in accord with this prediction. The major fragmentation of energised 2,3-epoxypropoxide is loss of CH2O to yield a product anion identified as the acetaldehyde enolate anion (CH2CHO)–. This cleavage can be used as a probe to investigate the relative extents of the two possible cyclisation processes. Comparison of the spectra of the 2,3-epoxypropoxide anion and the (M – H)– ion from 3-hydroxyoxetane, together with studies of labelled (2H), and doubly labelled (2H, 18O) analogues, demonstrate (a) that 40% of CH2O loss occurs by simple cleavage before any rearrangement of the 2,3-epoxypropoxide anion , (b) 25% of CH2O loss follows Payne equilibration, and (iii) 35% of CH2O loss occurs following equilibration of the Payne product and an oxetane intermediate.