Enolate Complexes Research Articles

Gas-Phase Alkali-Metal Cation Affinities of Stabilized Enolates.

The chemistry of alkali-metal enolates is dominated by ion pairing. To improve our understanding of the intrinsic interactions between the alkali-metal cations and the enolate anions, we have applied Cooks' kinetic method to determine relative M+ (M=Li, Na, K) affinities of the stabilized enolates derived from acetylacetone, ethyl acetoacetate, diethyl malonate, ethyl cyanoacetate, 2-cyanoacetamide, and methyl malonate monoamide in the gas phase. Quantum chemical calculations support the experimental results and moreover afford insight into the structures of the alkali-metal enolate complexes. The affinities decrease with increasing size of the alkali-metal cations, reflecting weaker electrostatic interactions and lower charge densities of the free M+ ions. For the different enolates, a comparison of their coordinating abilities is complicated by the fact that some of the free anions undergo conformational changes resulting in stabilizing intramolecular interactions. If these complicating effects are disregarded, the M+ affinities correlate with the electron density of the chelating functionalities, that is, the carbonyl and/or the nitrile groups of the enolates. A comparison with the known association constants of the corresponding alkali-metal enolates in solution points to the importance of solvation effects for these systems.

Revealing the mechanism of high water resistant and excellent active of CuMn oxide catalyst derived from Bimetal-Organic framework for acetone catalytic oxidation

Environmental H2O is an influential factor in the low-temperature catalytic oxidation of volatile organic compounds (VOCs), and it significantly impacts the reaction process and mechanism. Here, a series of rod-like Cu-Mn oxides were synthesised by pyrolysing Cu/Mn-BTC for acetone oxidation. The results confirm that the formation of multiphase interfaces have more excellent catalytic performance compared to single-phase catalysis. This phenomenon can be attributed to the formation of multiphase interfaces, which resulted in the synthesized catalysts with more active oxygen species and defective sites. The CuMn2Ox catalyst exhibited superior catalytic performance (T90 = 150 °C), high water resistance and long-term stability. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy and thermal desorption-gas chromatography-mass spectrometry results indicated that the degradation pathway of acetone was as follows: acetone ((CH3)2CO*) → enolate complexes ((CH2) = C(CH3) O*) → acetaldehyde ((CH3CHO*) → acetate (CH3COO*) → formate (HCOO*) → CO2 and H2O. At a low-temperature, water vapour dissociated a large number of activated hydroxyl groups on the multiphase interface, which promoted the dissociation of enolate complexes and acetaldehyde species. This composite oxide is a promising catalyst for removing oxygenated VOCs at high humidity.

Taming Shapeshifting Anions: Total Synthesis of Ocellatusone C.

Guided by a synthetic design aimed at late-stage diversification, we report the preparation of unusual shapeshifting anions and their subsequent application to the total synthesis of the polyketide natural product ocellatusone C. Site-selective core functionalization of a readily accessible bicyclo[3.3.1]nonane architecture sets the stage for shape-selective side chain installation via a nonfluxional π-allyl Pd-complex derived from a barbaralyl-type anion. Several interesting chemical findings, including substituent-dependent bridged bicycloisomerism and the isolation of a stabilized, 3° carbon-bound Pd-ketone enolate complex, are reported.

A density functional theory study on the mechanism of the allylpalladium-catalyzed dehydrogenation of aldehydes and cyclic ketones

The results of density functional theory calculations at the APFD/SDD level are detailed herein in order to study the main steps in the α,β-dehydrogenation of aldehydes and cyclic ketones in the presence of an allylpalladium complex catalyst. The mechanism is believed to proceed via an allylpalladium enolate complex (A) in equilibrium with the carbon-bonded complex (B), followed by β-hydride elimination to yield the allylpalladium hydride coordinated to the α,β-unsaturated carbonyl (complex C). The optimized structures and detailed energy profiles of these intermediates and their corresponding transition states are presented herein. The results indicate that the intermediates and their transition states are more stable in THF solution than in the gas phase. In detail, the energy barriers for the two steps are found to be 25.22 and 11.13 kcal/mol, respectively, in THF, and 29.93 and 9.77 kcal/mol, respectively, in the gas phase.

Synthesis, Structure, and Reactivity of Monoguanidinate Rare‐Earth Metal Aminobenzyl Enolate Complexes

Two monoguanidinate rare‐earth metal aminobenzyl enolate complexes LRE(CH2C6H4NMe2‐o)(OCH=CH2)(THF) (L = (PhCH2)2NC(NC6H3iPr2‐2,6)2; RE = Y (1a), Lu (1b)) were synthesized via two pathways of the ring‐opening of THF, and their reactivities with some molecules were also explored. Reaction of complex 1a with iPrNCNiPr gave the insertion product LY[(NiPr2)2CCH2C6H4NMe2‐o](OCH=CH2)(THF) (2). Whereas, in the case of PhNCS, LY[SC(CH2C6H4NMe2‐o)NPh](OCH=CH2)(THF) (3) and the ligands redistribution binuclear yttrium enolate complex [LY(µ‐OCH=CH2)(OCH=CH2)]2 (4) were obtained. The protonolysis products LY(NHPh)(OCH=CH2)(THF)2 (5), LY(NHPh)2(THF)2 (6) and LY(NPh2)(OCH=CH2)(THF)2 (7) were achieved by reaction of 1a with arylamine. Moreover, the THF ring‐opening insertion products with unique structures {LY[O(CH2)4PPh2][µ‐O(CH2)4PPh2]}2 (8) and {LY(SCH2C6H4NMe2‐o)[µ‐O(CH2)4SCH2C6H4NMe2‐o]}2 (9) were afforded through reactions of complex 1a with diphenylphosphine and elemental sulfur. It was the first example of THF activation promoted by rare‐earth metal thiolate complex. And some interesting chemical transformations were observed in our work. Complexes 1 could also serve as efficient initiators in the ring‐opening polymerization of rac‐lactide.

Potential contribution of Amadori compounds to antioxidant and angiotensin I converting enzyme inhibitory activities of raw and black garlic

Black garlic extracts had higher antioxidant and angiotensin I converting enzyme (ACE) inhibitory potential than raw garlic extracts. Black garlic extract contained large amounts of Amadori compounds (ACs), especially N-(1-deoxy-D-fructos-1-yl)-l-arginine (Fru-Arg) (1.0–5.0 mg/g), while no ACs were detected in raw garlic extracts. The antioxidant potential of eight individual ACs present in black garlic was characterized in FRAP, ABTS and ORAC assays. ACs had moderate FRAP values, 2–5 times lower than ascorbic acid (AA). The Fe3+ reducing mechanism of ACs probably includes 2, 3-enolization of the carbohydrate of ACs and formation of the enolate complex with Fe3+. Fru-Arg and N-(1-deoxy-D-fructos-1-yl)-l-phenylalanine could effectively scavenge ABTS radical, nearly the same scavenging activities as AA or Trolox. N-(1-deoxy-D-fructos-1-yl)-l-methionine (Fru-Met) followed by N-(1-deoxy-D-fructos-1-yl)-l-histidine had almost the same ORAC power with AA and at low concentrations, Fru-Met even showed higher ORAC value than AA. These results suggest that ACs probably contributed to antioxidant and ACE inhibitory activities of black garlic extract.

Visible‐Light‐Driven Enantioselective Aerobic Oxidation of β‐Dicarbonyl Compounds Catalyzed by Cinchona‐Derived Phase Transfer Catalysts in Batch and Semi‐Flow

AbstractThe direct asymmetric α‐hydroxylation of β‐dicarbonyl compounds has been developed using cinchona‐derived phase‐transfer catalysts in batch and semi‐flow processes. Using visible light as the driving force and air as oxidant, the corresponding products were obtained in excellent yields (up to 96%) and good enantioselectivities (up to 90% ee). The catalyst acts as a chiral center and a catalytic center and forms a chiral enolate complex with the substrate to act as a photosensitive center in the reaction. The use of semi‐flow photochemical processes allowed a reduction in reaction time (24 h to 0.89 h) and good yields (up to 93%) and enantioselectivities (up to 88% ee) were obtained.magnified image

Reaction mechanism between Cu(II)-enolate complex and O2 as a test case for methodology used in DFT computational studies

The reaction mechanism of an intricate oxidation reaction of chlorodiketonate ligand of mononuclear Cu(II) complex was studied computationally employing five different models that differ in: a) basis set, b) the way that solvent corrections are included, and c) DFT functional. Qualitative and quantitative comparison of structures and enthalpy reaction profiles enabled us to assess how sensitive they are to the changes in computational methodology.Graphical abstractComparison of enthalpy reaction profiles and molecular structures demonstrate how the qualitative picture on Cu(II)-catalyzed reaction changes upon variation of computational methodology

Revisiting the Synthesis and Nucleophilic Reactivity of an Anionic Copper Superoxide Complex.

The addition of 1 equiv of KO2 and Kryptofix222 (Krypt) in CH3CN to a solution of LCu(CH3CN) [L = N, N'-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at -80 °C yielded [K(Krypt)][LCuO2], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO2] reacts with wet 2-PPA to form [LCuOH]-, which then deprotonates 2-PPA to yield the copper(II) enolate complex [LCu(OC═C(Me)Ph)]-. Acetophenone was observed upon workup of this complex or mixtures of KO2 and 2-PPA alone, in support of an alternative mechanism(s) to the one proposed previously involving an initial nucleophilic attack at the carbonyl group of 2-PPA.

Gas-Phase Reactions of the Group 10 Organometallic Cations, [(phen)M(CH3)]+ with Acetone: Only Platinum Promotes a Catalytic Cycle via the Enolate [(phen)Pt(OC(CH2)CH3)]+

Abstract Electrospray ionisation of the ligated group 10 metal complexes [(phen)M(O2CCH3)2] (M = Ni, Pd, Pt) generates the cations [(phen)M(O2CCH3)]+, whose gas-phase chemistry was studied using multistage mass spectrometry experiments in an ion trap mass spectrometer with the combination of collision-induced dissociation (CID) and ion-molecule reactions (IMR). A new catalytic cycle has been discovered. In step 1, decarboxylation of [(phen)M(O2CCH3)]+ under CID conditions generates the organometallic cations [(phen)M(CH3)]+, which react with acetone to generate the [(phen)M(CH3)(OC(CH3)2)]+ adducts in competition with formation of the coordinated enolate for M = Pt (step 2). For M = Ni and Pd, the adducts regenerate [(phen)M(CH3)]+ upon CID. In the case of M = Pt, loss of methane is favored over loss of acetone and results in the formation of the enolate complex, [(phen)Pt(OC(CH2)CH3)]+. Upon further CID, both methane and CO loss can be observed resulting in the formation of the ketenyl and ethyl complexes [(phen)Pt(OCCH)]+ and [(phen)Pt(CH2CH3)]+ (step 3), respectively. In step 4, CID of [(phen)Pt(CH2CH3)]+ results in a beta-hydride elimination reaction to yield the hydride complex, [(phen)Pt(H)]+, which reacts with acetic acid to regenerate the acetate complex [(phen)Pt(O2CCH3)]+ and H2 in step 5. Thus, the catalytic cycle is formally closed, which corresponds to the decomposition of acetone and acetic acid into methane, CO, CO2, ethene and H2. All except the last step of the catalytic cycle are modelled using DFT calculations with optimizations of structures at the M06/SDD 6-31G(d) level of theory.

Direct enolization chemistry of 7-azaindoline amides: A case study of bis(tetrahydrophosphole)-type ligands

7-Azaindoline amides are particularly useful in direct enolization chemistry due to their facilitated enolization in soft Lewis acid/Brønsted base cooperative catalysis. The Cu(I) complex of (R,R)-Ph-BPE, a bis(tetrahydrophosphole)-type chiral bisphosphine ligand, exhibits consistently high catalytic performance and stereoselectivity irrespective of the nature of the α-substituent of the amides. Unexpectedly, however, alkyl-substituted bis(tetrahydrophosphole)-type ligands have substantially inferior catalytic performance. Evaluation of the optimized structures of Cu(I)/amide and Cu(I)/enolate complexes provided clues to dissecting the diverted reaction outcomes.

An Overview of Metal Acetylacetonates: Developing Areas/Routes to New Materials and Applications in Organic Syntheses

In this review, we discuss the nature and bonding, role of metal acetylacetonates as precursors for nanoparticle research, polymer science, catalysis and their mechanistic aspects, which we believe to provide an illustrative overview of current research directions in this rapidly developing field. The chosen examples within this review reflect the remarkable broadness of nanoparticle research, polymer and applications of metal acetylacetonates as catalysts in organic syntheses. The purpose of this review is to provide recent advancements of several catalytic reactions employing well defined metal enolate complexes. Metal enolates are used in a plethora of catalysed reactions. Several catalytic reactions have been highlighted in this review to show where metal enolate complexes have played a significant role in recent years. The ultimate goal of this effort is towards the development of catalysts that impart the selectivities, reactivity, and atom economy needed for cost-effective approaches to a variety of critical chemical transformations.

Reactivity of Bridged and Nonbridged Zirconocenes toward Biorenewable Itaconic Esters and Anhydride

This work investigates the reactivity of neutral and cationic complexes of both bridged ansa-zirconocenes, rac-[C2H4(Ind)2]ZrMe[OC(OiPr)═CMe2] (1) and rac-[C2H4(Ind)2]Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− (1+), and nonbridged zirconocenes, Cp*(nPrCp)ZrMe[OC(OiPr)═CMe2] (13) and Cp*(nPrCp)Zr(THF)[OC(OiPr)═CMe2]+[MeB(C6F5)3]− (13+), toward biorenewable itaconic dialkyl esters (itaconates) and anhydride (IA). Behaving similarly, both cationic complexes 1+ and 13+ react readily with itaconates to form cleanly single monomer addition products, eight-membered-ring metallacycles 2 and 15, respectively, and neutral enolate complexes 1 and 13 insert 1 equiv of IA to afford single-IA-addition products 5 and 17. Behaving differently, eight-membered-ring chelates 2 derived from the bridged metallocene framework undergo slow isomerization at room temperature via ligand exchange between the coordinated and uncoordinated ester groups to form thermodynamically favored seven-membered-ring chelates 4, while eight-membered c...

Asymmetric [4+2] Annulation of C1 Ammonium Enolates with Copper-Allenylidenes.

An asymmetric catalytic decarboxylative [4+2] annulation of 4-ethynyl dihydrobenzooxazinones and carboxylic acids has been established by cooperative copper and nucleophilic Lewis base catalysis. A C1 ammonium enolate and copper-allenylidene complex, each catalytically generated from different substrates, underwent a cascade asymmetric propargylation and lactamization process to yield optically active 3,4-dihydroquinolin-2-one derivatives with excellent levels of stereoselectivity (up to 99 % ee, 95:5 d.r.).

Asymmetric Catalysis with Organic Azides and Diazo Compounds Initiated by Photoinduced Electron Transfer.

Electron-acceptor-substituted aryl azides and α-diazo carboxylic esters are used as substrates for visible-light-activated asymmetric α-amination and α-alkylation, respectively, of 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium-based Lewis acid in combination with a photoredox sensitizer. This novel proton- and redox-neutral method provides yields of up to 99% and excellent enantioselectivities of up to >99% ee with broad functional group compatibility. Mechanistic investigations suggest that an intermediate rhodium enolate complex acts as a reductive quencher to initiate a radical process with the aryl azides and α-diazo carboxylic esters serving as precursors for nitrogen and carbon-centered radicals, respectively. This is the first report on using aryl azides and α-diazo carboxylic esters as substrates for asymmetric catalysis under photoredox conditions. These reagents have the advantage that molecular nitrogen is the leaving group and sole byproduct in this reaction.

Gas-Phase and Computational Study of Identical Nickel- and Palladium-Mediated Organic Transformations Where Mechanisms Proceeding via M(II) or M(IV) Oxidation States Are Determined by Ancillary Ligands.

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH2CO2-C,O)M(CH3)](-) (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give M(IV) species [(CH2CO2-C,O)M(CH3)(η(1)-CH2-CH═CH2)(O2CCH3-O,O')](-) (6) that reductively eliminate 1-butene, to form [(CH2CO2-C,O)M(O2CCH3-O,O')](-) (4). The mechanism contrasts with the M(II)-mediated pathway for the analogous reaction of [(phen)M(CH3)](+) (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly dz(2)) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz(2) to the C═C π* orbital is the key interaction leading to oxidative addition of allyl acetate to M(II). Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for Ni(IV) and Pd(IV) in organic synthesis.

Synthesis, characterization, and unique catalytic activities of a fluorinated nickel enolate.

We have synthesized a new nickel enolate [(PhCOCF2)Ni(dcpe)][FB(C6F5)3] featuring fluorine atoms on the enolate moiety via B(C6F5)3-promoted C-F bond activation of α,α,α-trifluoroacetophenone. X-ray diffraction study of [(PhCOCF2)Ni(dcpe)][FB(C6F5)3] revealed that the complex had adopted an η(3)-oxallyl coordination mode in the crystal lattice. The reaction of (t)BuNC with [(PhCOCF2)Ni(dcpe)][FB(C6F5)3] resulted in the coordination of isocyanide to the nickel center to form a C-bound enolate complex. The reactions of [(PhCOCF2)Ni(dcpe)][FB(C6F5)3] with aldehydes gave insertion products quantitatively which were fully characterized by NMR spectroscopy. Furthermore, we established unique catalytic applications for [(PhCOCF2)Ni(dcpe)][FB(C6F5)3] toward a Tishchenko reaction, along with a highly selective crossed-esterification of α,α,α-trifluoroacetophenones with aldehydes.

CuI-Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron Enolate Intermediates

Copper(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary reaction products are isolated and employed in order to deduce catalytic cycles on an experimental basis. Employing an NHC CuI complex as a model catalyst, it is possible to perform efficient catalytic transformations as well as to isolate and characterize the formed copper enolate complexes as the key intermediates. It is shown that for this catalytic system the nature of this enolate—O- or C-enolate—is crucial for the catalytic process. For α,β-unsaturated aldehydes and ketones the O-enolate is formed predominantly, while for α,β-unsaturated esters the C-enolate is the major product. Catalytic turnover is only facile for copper O-enolates, as they react efficiently with the silyl boronic ester under (re)formation of the catalytically active Cu–Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper C-enolates is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy.

ChemInform Abstract: Mild Heterogeneous Palladium‐Catalyzed Cleavage of β‐O‐4′‐Ether Linkages of Lignin Model Compounds and Native Lignin in Air.

A mild and robust heterogeneous palladium-catalyzed CO bond cleavage of 2-aryloxy-1-arylethanols using formic acid as reducing agent in air was developed. The cleaved products were isolated in 92–98 % yield; and by slightly varying the reaction conditions, a ketone, an alcohol, or an alkane can be generated in near-quantitative yield. This reaction is applicable to cleaving the b-O-4’-ether bond found in lignin polymers of different origin. The reaction was performed on a lignin polymer model to generate either the monomeric aryl ketone or alkane in a quantitative yield. Moderate depolymerization was achieved with native lignin at similar reaction conditions. Mechanistic studies under kinetic control indicate that an initial palladium-catalyzed dehydrogenation of the alcohol is followed by insertion of palladium to an enol equivalent. A palladium–formato complex reductively cleaves the palladium–enolate complex to generate the ketone.

Mild Heterogeneous Palladium‐Catalyzed Cleavage of β‐O‐4′‐Ether Linkages of Lignin Model Compounds and Native Lignin in Air

AbstractA mild and robust heterogeneous palladium‐catalyzed CO bond cleavage of 2‐aryloxy‐1‐arylethanols using formic acid as reducing agent in air was developed. The cleaved products were isolated in 92–98 % yield; and by slightly varying the reaction conditions, a ketone, an alcohol, or an alkane can be generated in near‐quantitative yield. This reaction is applicable to cleaving the β‐O‐4′‐ether bond found in lignin polymers of different origin. The reaction was performed on a lignin polymer model to generate either the monomeric aryl ketone or alkane in a quantitative yield. Moderate depolymerization was achieved with native lignin at similar reaction conditions. Mechanistic studies under kinetic control indicate that an initial palladium‐catalyzed dehydrogenation of the alcohol is followed by insertion of palladium to an enol equivalent. A palladium–formato complex reductively cleaves the palladium–enolate complex to generate the ketone.