Aldol Condensation Products Research Articles

Diastereodivergent α-Homoallylation of Cyclic Enones.

α,β-Unsaturated carbonyls are essential structural motifs for nucleophiles of disease-related proteins. Methods for stereoselective functionalizations at the α-position include the Morita-Baylis-Hillman, Negishi, Sonogashira, Stille, and Rauhut-Currier reactions. Described here is a method for the diastereodivergent α-homoallylation of cyclic enones via a sequence of conjugate addition, aldol condensation, and diastereoselective [3,3]-sigmatropic rearrangement. Mechanistic investigations revealed that the [3,3]-sigmatropic rearrangement proceeds with transfer of chirality. These inspire a photocatalyzed olefin isomerization of the aldol condensation product leading to a highly diastereoselective [3,3]-sigmatropic rearrangement to furnish the α-homoallylation of cyclic enones. Importantly, this photocatalyzed olefin isomerization/diastereoselective [3,3]-sigmatropic rearrangement reaction sequence permits a full stereocontrol of the exo-β-position featuring an allyl group as a synthetic functional handle.

Chromium-Catalyzed Cross-Coupling of Methyl Ketones with Cyclic Ketones toward the Selective Synthesis of β-Branched β,γ-Unsaturated Ketones.

Chromium-catalyzed cross-coupling of methyl ketones with cyclic ketones to β-branched β,γ-unsaturated ketones are reported. Interestingly, single-crossed aldol condensation products are formed, even in reactions in which a mixture of products is possible. The reaction is highly chemoselective and regioselective. This catalytic route gives a unique opportunity to integrate the chemistry of the synthetic challenge cross-coupling reaction of ketones and the alkene migration reaction into a reaction pot.

1-Butyl-3-methylimidazolium tetrafluoroborate as suitable solvent for BF3: the case of alkyne hydration. Chemistry vs electrochemistry.

In order to replace the expensive metal/ligand catalysts and classic toxic and volatile solvents, commonly used for the hydration of alkynes, the hydration reaction of alkynes was studied in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4) adding boron trifluoride diethyl etherate (BF3·Et2O) as catalyst. Different ionic liquids were used, varying the cation or the anion, in order to identify the best one, in terms of both efficiency and reduced costs. The developed method was efficaciously applied to different alkynes, achieving the desired hydration products with good yields. The results obtained using a conventional approach (i.e., adding BF3·Et2O) were compared with those achieved using BF3 electrogenerated in BMIm-BF4, demonstrating the possibility of obtaining the products of alkyne hydration with analogous or improved yields, using less hazardous precursors to generate the reactive species in situ. In particular, for terminal arylalkynes, the electrochemical route proved to be advantageous, yielding preferentially the hydration products vs the aldol condensation products. Importantly, the ability to recycle the ionic liquid in subsequent reactions was successfully demonstrated.

Tuning aldol condensation product selectivity by controlling diffusion and reaction rates in zeolites

This study presents a method for quantifying mass-transfer limitations in microporous acid zeolites that combines a titration technique and a reaction–diffusion model. The method involves partially exchanging H+ sites with Na+ sites to adjust the site density and observing the resulting reaction rates for the aldol condensation of acetone and cyclopentanone. When the intrinsic activity per site is uniform throughout the zeolite and in the absence of mass-transport limitations, a linear decrease in the reaction rate is expected as the density of sites is systematically lowered by gradual titration. Conversely, if mass-transport limitations are present, the initial decrease in rate with increasing number of titrated sites is much lower than that observed when a large fraction of the active sites have been titrated. This non-linear variation of observed rate as a function of the density of titrated sites indicates the presence of mass-transport limitations in the system. Near the limit where the density of remaining active sites is low enough to avoid generating a concentration gradient inside the zeolite crystallite, the variation in rate becomes linear. The titration results can be coupled with reaction–diffusion modeling to fit and calculate the Thiele modulus, which provides quantitative information on the extent of mass-transport limitations in the studied porous acidic domains. In the case of acetone-cyclopentanone cross-condensation reaction, the selectivity towards the different products was found to be influenced by the degree of mass transfer limitations. That is, kinetically driven systems favor the cyclopentanone self-condensation product while diffusion-limited environments favor products of the reactant with faster diffusion rates. Accordingly, the manipulation of site density was found to be useful in tuning product selectivity towards specific desired products, which demonstrates the potential of this method for developing and optimizing microporous catalysts with controllable acid site density and provide valuable insight into the relationship between mass-transfer limitations and product selectivity.

Tandem Hydroformylation-Aldol Condensation Reaction Enabled by Zn-MOF-74.

The tandem hydroformylation-aldol condensation (tandem HF-AC) reaction offers an efficient synthetic route to the synthesis of industrially relevant products. The addition of Zn-MOF-74 to the cobalt-catalyzed hydroformylation of 1-hexene enables tandem HF-AC under milder pressure and temperature conditions than the aldox process, where zinc salts are added to cobalt-catalyzed hydroformylation reactions to promote aldol condensation. The yield of the aldol condensation products increases by up to 17 times that of the homogeneous reaction without MOF and up to 5 times compared to the aldox catalytic system. Both Co2(CO)8 and Zn-MOF-74 are required to significantly enhance the activity of the catalytic system. Density functional theory simulations and Fourier-transform infrared experiments show that heptanal, the product of hydroformylation, adsorbs on the open metal site (OMS) of Zn-MOF-74, thereby increasing the electrophilic character of the carbonyl carbon atom and facilitating the condensation.

Influence of Support Structure on Catalytic Performance of Supported Liquid-Phase (SLP) Catalysts in Hydroformylation of 1-Butene

Several supported liquid-phase (SLP) catalysts with immobilized Rh-biphephos complexes on monolithic supports were prepared and applied for continuous gas-phase hydroformylation (HyFo) of 1-butene. The support comprised macroporous monolithic silicon carbide (SiC) with deposited silica nanoparticles (NPs) in order to provide mesopores with enhanced capillary forces to retain the liquid-phase. Variable parameters were examined for the monolithic SiC supports, including size and loading of deposited silica NPs and intermediate calcination between silica deposition steps to obtain the most efficient support configuration for the SLP system. The SLP catalysts with larger deposited silica NPs gave higher catalytic activity (i.e. 1-butene conversion and turnover frequency) compared to the supports with smaller sized silica NPs. However, the selectivity towards the preferred linear aldehyde was higher in the SLP catalysts with supports containing less silica with small silica NPs. Importantly, the prepared SLP catalyst systems showed long-term stability in HyFo with negligible formation of high boiling aldol condensation products.

Vapor-phase oxidation of propylene glycol-methanol mixture to methyl lactate on CeO2/Al2O3 catalyst

The vapor-phase oxidation of mixtures of propylene glycol with methanol and ethanol to methyl and ethyl lactate, respectively, on supported CeO2/Al2O3 catalyst at 210-2500С was investigated. Air oxygen and a flow reactor with a fixed catalyst bed were used. A 20% solution of propylene glycol in alcohol was supplied to the reactor inlet. Oxidation of propylene glycol in the presence of methanol occurs according to the overall reaction CH3CHOHCH2OH +O2 + СН3OH = CH3CHOHCOOСН3 +2H2O. First, hydroxyacetone is formed, which is further oxidized to pyruvic aldehyde, which adds alcohol to form a hemiacetal. Next, this hemiacetal is rearranged according to Cannizzaro into methyl lactate. At 2200C and catalyst load < 2 mmol PG/gcat/h, the selectivity for methyl lactate reached 70 wt.% at 100% propylene glycol conversion. In the presence of ethanol, the formation of a significant amount of acetaldehyde and its aldol condensation products, as well as the formation of diethoxyethane, was observed. Therefore, the selectivity for ethyl lactate at 100% propylene glycol conversion did not exceed 45%.

On the role of Zr to facilitate the synthesis of diesel and jet fuel range intermediates from biomass-derived carbonyl compounds over aluminum phosphate

Heterogeneous zeolite/zeotype catalysts are attractive for converting renewable biomass-derived chemicals into transportation fuels by aldol condensation reactions, which provide a simple strategy to improve carbon numbers. Herein, Zr-doped aluminum phosphate (ZrAPO-5) were applied for efficient syntheses of diesel and jet fuel range C11, C12 and C16 intermediates from biomass-derived furfural (FF) and cyclohexanone (CH). Incorporation of Zr in the APO-5 framework facilitated the adsorption of the CO groups of FF and CH and created more and stronger acid and base sites, which co-catalyzed the aldol condensation and facilitated the product yields. ZrAPO-5 (Zr/Al = 0.20) yielded up to 79.3% single aldol condensation product C11 or 80.7% double aldol condensation product C16 in the cross-aldol condensation of FF and CH under appropriate reaction conditions, and also realized 83.3% C12 yield in the self-aldol condensation of CH. In addition, the catalytic system displayed good to excellent yields of other value-added chemicals in aldol condensations of alternative carbonyl compounds.

Catalytic aldol condensation of bio-derived furanic aldehydes and acetone: Challenges and opportunities

Bio-derived furfural and 5-hydroxymethylfurfural can be combined with acetone to yield aldol condensation products that may serve as biofuel and polymer precursors. We have explored different catalytic systems to obtain and purify each product in the most efficient way. The results of the catalytic tests of the cross-condensations and of the self-condensation of acetone allowed the comparison of the different reactivity of the two aldehydes. Online and in situ/operando ATR-IR was used to monitor the reaction over time and to study the interaction of the reaction species with the solid catalyst, especially the formation of deactivating organic matter that covers the surface, which is a major issue in heterogeneous condensation processes. In situ NMR was used to study the ongoing reaction, assessing its stereoselectivity, and to study the behavior of deuterated species in the catalytic system. Finally, the preparation of C14, a hetero-double-condensation product, was also explored.

Rap‐Stoermer Reaction: TEA Catalyzed One‐Pot Efficient Synthesis of Benzofurans and Optimization of Different Reaction Conditions

AbstractThe reaction of substituted salicylaldehydes with α‐haloketones to obtain benzofuran‐2‐yl (Alkyl/aryl) ketones, which is a Dickman type aldol condensation product in base, solvent, and temperature environment, is called as Rap‐Stoermer reaction. In this study, a series of Benzofuran‐2‐yl (Alkyl/ aryl) ketones was synthesized by heating substituted salicylaldehydes and α‐haloketones under solvent‐free sealed (closed vessel) conditions by using triethyleamine (TEA) as a novel base catalyst in good to excellent yields (81–97 %) at 130 °C and the obtained results compared with the existing literature. The reaction conditions were optimized in terms of yield, time, base, solvent temperature, stoichiometric ratios, and synthesis environments (open or closed vessels). The optimized protocol offers promising advantages such as solvent‐free and clean reaction media, short reaction time, no byproducts, and appreciable high yields for the prospective studies. Characterization of the synthesized compounds was performed by using spectroscopic methods such as ATR‐FTIR, 1H‐NMR, 13C‐NMR and LC‐MS.

Cyanidin and rutin inhibit the formation of heterocyclic aromatic amines in chemical modeling systems and smoked chicken drumsticks

This study aims to evaluate the heterocyclic aromatic amine (HAA) formation inhibited by cyanidin and rutin in chemical modeling systems and smoked chicken drumsticks. In the PhIP and β-carboline chemical modeling systems, cyanidin inhibited the PhIP, Harman, and Norharman formation while rutin inhibited the PhIP formation but promoted the Harman and Norharman formation compared with control sample (P < 0.05). A mechanistic investigation confirmed that inhibiting the PhIP formation by cyanidin was mainly through trapping phenylalanine, creatine, creatinine, glucose, phenylacetaldehyde, and an aldol condensation product. In the smoked chicken drumsticks, cyanidin inhibited the PhIP, Harman, and Norharman formation dose-dependently compared to the control sample (P < 0.05), with 2.0 % (w/v) cyanidin having the highest inhibitory effect. Moreover, cyanidin inhibited the formation of PhIP precursors and intermediates in the smoked chicken drumsticks. These results provide evidence for using phenolic compounds to reduce HAA formation in smoked meat products.

The supported forest-like structure of PtSn as an effective deterrent for acetaldehyde formation during the electrocatalytic oxidation of ethanol

The activity and antipoisoning property are critical for commercialization of direct ethanol fuel cells (DEFCs) so must be proceeded with extreme caution while designing the new catalysts. Herein, a Pt-based catalyst is designed by templating method and employed as the electrocatalyst in the alkaline oxidation of ethanol. The comparative study exhibits the superior performance of the optimized structure; viz. Meso PtSn-1_NF than state of the art commercial Pt/C mainly related to the increasing in the electroactive surface area and the control over the morphology in the templating method. The ethanol oxidation reaction (EOR) mechanism is studied using electrochemical methods and indicates the roles of Sn in the inhibition of acetaldehyde formation during EOR resulting in the accumulation of large amounts of aldol condensation products and blockage of the active sites.

Production of Jet Fuel Range Alkanes by Oriented Synthesis of Single Aldol Condensation Product and Subsequent Hydrodeoxygenation

Production of Jet Fuel Range Alkanes by Oriented Synthesis of Single Aldol Condensation Product and Subsequent Hydrodeoxygenation

Effects of the Deacetylation Degree of Chitosan on 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) Formation in Chemical Models and Beef Patties

The effects of the deacetylation degree (DD) of chitosan on heterocyclic aromatic amine formation were investigated in chemical models and beef patties. The results in model systems showed that at lower addition levels (10 mg), chitosan with 85% DD showed the strongest inhibitory effect against 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) formation, while chitosan with a higher DD (95%) or a lower DD (72 and 50%) did not show any significantly inhibitory effect. Further mechanism study showed that chitosan addition reduced the content of Maillard reaction intermediates including phenylacetaldehyde and the aldol condensation product but increased the PhIP precursor creatinine residue in the chemical model, indicating that chitosan at least partially competed with creatinine to react with phenylacetaldehyde to inhibit PhIP formation. In roast beef patties, 0.15% (w/w) chitosan (85% DD) significantly reduced the formation of PhIP, MeIQx, 4,8-DiMeIQx, Harman, and Norharman by 56.21, 33.32, 31.35, 25.14, and 28.12%, respectively. Moreover, chitosan significantly inhibited the formation of aldehydes in roast beef patties, further confirming the above-mentioned inhibition mechanism. However, the addition of chitosan might promote fatty acid oxidation. In addition, chitosan addition below 0.15% (w/w) had no significant effect on the textural properties of the roast samples.

Formation of Secondary and Tertiary Volatile Compounds Resulting from the Lipid Oxidation of Rapeseed Oil

The lipid oxidation of fats and oils leads to volatile organic compounds, having a decisive influence on the sensory quality of foods. To understand formation and degradation pathways and to evaluate the suitability of lipid-derived aldehydes as marker substances for the oxidative status of foods, the formation of secondary and tertiary lipid oxidation compounds was investigated with gas chromatography in rapeseed oils. After 120 min, up to 65 compounds were detected. In addition to secondary degradation products, tertiary products such as alkyl furans, ketones, and aldol condensation products were also found. The comparison of rapeseed oils, differing in their initial peroxide values, showed that the formation rate of secondary compounds was higher in pre-damaged oils. Simultaneously, a faster degradation, especially of unsaturated aldehydes, was observed. Consequently, the formation of tertiary products (e.g., alkyl furans, aldol adducts) from well-known lipid oxidation products (i.e., propanal, hexanal, 2-hexenal, and 2-nonenal) was investigated in model systems. The experiments showed that these compounds form the new substances in subsequent reactions, especially, when other compounds such as phospholipids are present. Hexanal and propanal are suitable as marker compounds in the early phase of lipid oxidation, but at an advanced stage they are subject to aldol condensation. Consequently, the detection of tertiary degradation products needs to be considered in advanced lipid oxidation.

Influence of the initial state of ZrO2 on genesis, activity and stability of Ni/ZrO2 catalysts for steam reforming of glycerol

The effect of the initial state of ZrO2 on properties of Ni/ZrO2 catalysts for hydrogen production in steam reforming of glycerol was investigated. The catalysts were synthesized by impregnating the supports obtained by varying the treatment temperature of ZrO2‧nH2O and introducing Y2O3 as a promoter. All materials were characterized by thermal analysis, X-ray diffraction, N2 physisorption, scanning electron microscopy, H2-TPR, NH3-TPD and transmission electron microscopy. The mutual influence of NiO and ZrO2 on the genesis of the phase composition, pore structure and reducibility was demonstrated. Different catalytic behavior is explained by influence of the initial form of the support on the size, morphology of Ni particles, and the support thermal stability. The initial activity of Ni/ZrO2is proportional to the monoclinic phase content. The catalysts based on tetragonal ZrO2 displayed the best stability. For the first time, the presence of the aldol condensation products in glycerol steam reforming was demonstrated.

Aqueous-phase behavior of glyoxal and methylglyoxal observed with carbon and oxygen K-edge X-ray absorption spectroscopy

Abstract. Glyoxal (CHOCHO) and methylglyoxal (CH3C(O)CHO) are well-known components of atmospheric particles and their properties can impact atmospheric chemistry and cloud formation. To get information on their hydration states in aqueous solutions and how they are affected by the addition of inorganic salts (sodium chloride (NaCl) and sodium sulfate (Na2SO4)), we applied carbon and oxygen K-edge X-ray absorption spectroscopy (XAS) in transmission mode. The recorded C K-edge spectra show that glyoxal is completely hydrated in the dilute aqueous solutions, in line with previous studies. For methylglyoxal, supported by quantum chemical calculations we identified not only C–H, C=O and C–OH bonds, but also fingerprints of C–OH(CH2) and C=C bonds. The relatively low intensity of C=O transitions implies that the monohydrated form of methylglyoxal is not favored in the solutions. Instead, the spectral intensity is stronger in regions where products of aldol condensation and enol tautomers of the monohydrates contribute. The addition of salts was found to introduce only very minor changes to absorption energies and relative intensities of the observed absorption features, indicating that XAS in the near-edge region is not very sensitive to these intermolecular organic–inorganic interactions at the studied concentrations. The identified structures of glyoxal and methylglyoxal in an aqueous environment support the uptake of these compounds to the aerosol phase in the presence of water and their contribution to secondary organic aerosol formation.

Vapor-phase oxidation of propylene glycol-methanol mixture to methyl lactate on CeO2/Al2O3 catalyst

The vapor-phase oxidation of mixtures of propylene glycol with methanol and ethanol to methyl and ethyl lactate, respectively, on supported CeO2/Al2O3 catalyst with 10 wt.% CeO2 content was studied. The steel flow reactor with a fixed catalyst bed (4 cm3) was used. 20 wt.% solution of propylene glycol in alcohol was fed to the reactor inlet by Waters 950 pump at LHSV= 0.5-0.8 h-1. Reaction temperature and pressure were varied in the interval of 190-250 0C and 1.3-1.8 bars respectively. Compressed air was given to the reactor inlet at the molar ratio of propylene glycol/O2 = 1. The reaction products were analyzed using gas chromatography (Agilent 7820A) and 3C NMR (Bruker Avance 400) methods. Studied oxidation of propylene glycol in the presence of methanol describes by total reaction CH3CHOHCH2OH +O2 + СН3OH = CH3CHOHCOOСН3 +2H2O At first, hydroxyacetone is formed that is further oxidized to pyruvic aldehyde, which attaches alcohol to form hemiacetal. Then, hemiacetal of methyl glyoxal rearranges into methyl lactate by Cannizzaro. At 220 0C and load on a catalyst of &lt; 2 mmol PG/gcat/h, the selectivity towards methyl lactate reaches 70 wt.% at 100 % propylene glycol conversion. The main by-products are formed as the result of acetaldehyde transformation. Acetaldehyde could be formed at hydroxyacetone aldol decondensation. In the presence of ethanol, the formation of a significant amount of acetaldehyde and its aldol condensation products as well as the formation of diethoxyethane are observed. Therefore, ethyl lactate selectivity at 100 % propylene glycol conversion does not exceed 45 %. Supported CeO2/SiO2 contact was tested in this oxidation reaction also. However, CeO2/SiO2 provides the low, up to 25%, selectivity towards methyl lactate at full propylene glycol conversion. It was shown that at the same conditions methyl lactate is formed with higher selectivity then ethyl lactate. The high methyl lactate yield up to 70 wt.% could be obtained via vapor-phase oxidation of 20% mixture of propylene glycol with methanol by air oxygen on supported CeO2/Al2O3 catalyst at 210 - 220°С and at time contact of 3-4 seconds.

Synthesis of MgO and MgO–CeO2 by co-precipitation for the catalytic conversion of acetone by aldol condensation

This study deals with the synthesis of catalytic samples containing MgO and 10, 15 and 25 wt% of CeO2 by co-precipitation method for aldol condensation of acetone. The obtained powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, temperature programmed desorption of CO2, X-ray photoelectron spectroscopy, nitrogen physisorption, and atomic force microscopy. The materials exhibited XRD patterns corresponding with cubic MgO and fluorite phase of ceria, medium and weak strength basic sites, characteristic electronic properties and distinctive trends regarding textural properties. The aldol condensation of acetone was carried out using the prepared catalyst samples; a decrease in acetone conversion as function of time-on-stream was noted for all catalyst, being the best performance exhibited by the sample containing 10 wt% ceria. The selectivity toward the different products of aldol condensation of acetone was dependent on catalytic sample, which was related to the unique properties of those solids. Diacetone alcohol was best produced by MgO-25 wt% ceria; for mesityl oxide yield, an increase in selectivity as time-on-stream was observed being the best material the sample MgO-10wt% ceria; whereas for phorone, the profiles indicate almost yield reduction as time-on-stream function, indicating the best performance the sample containing 15 wt% of CeO2. Isophorone was optimally produced by using pure MgO which was attributed to the presence of medium strength basic sites and adequate textural properties.

Phosphonic Acid Modified ZnO Nanowire Sensors: Directing Reaction Pathway of Volatile Carbonyl Compounds.

Surface molecular transformations on nanoscale metal oxides are inherently complex, and directing those reaction pathways is still challenging but important for designing their various applications, including molecular sensing, catalysts, and others. Here, a rational strategy to direct a reaction pathway of volatile carbonyl compounds (nonanal: biomarker) on single-crystalline ZnO nanowire surfaces via molecular modification is demonstrated. The introduction of a methylphosphonic acid modification on the ZnO nanowire surface significantly alters the surface reaction pathway of nonanal via suppressing the detrimental aldol condensation reaction. This is directed by intentionally decreasing the probability of two neighboring molecular activations on the nanowire surface. Spectrometric measurements reveal the correlation between the suppression of the aldol condensation surface reaction and the improvement in the sensor performance. This tailored surface reaction pathway effectively reduces the operating temperature from 200 to 100 °C while maintaining the sensitivity. This is because the aldol condensation product ((E)-2-heptyl-2-undecenal) requires a higher temperature to desorb from the surface. Thus, the proposed facile strategy offers an interesting approach not only for the rational design of metal oxide sensors for numerous volatile carbonyl compounds but also for tailoring various surface reaction pathways on complex nanoscale metal oxides.