Schiff Base Ester Research Articles

α‐Alkylation of Mono‐Carbonyl Compounds with N‐Allylamines through Cooperative Actions of Lewis Acids and a Brønsted Base

AbstractWe present a strategy for the union of N‐allylamines and carbonyl compounds by the concerted action of B(C6F5)3 and a N‐based Brønsted base under redox‐neutral conditions. The reaction of in situ generated conjugated iminium ions and enolates affords δ‐aminocarbonyl compounds without waste generation. Furthermore, we describe that the cooperative action of B(C6F5)3, a chiral Zn‐based complex, and an amine facilitates the stereoselective reaction of a glycinate Schiff base ester and an N‐allylamine to afford an α‐substituted α‐amino ester.

Synthesis, phase transition behaviour and dielectric properties of smectogenic palm-based liquid crystals containing schiff base ester and phenyl ring

Smectogenic liquid crystals (LCs) play an important role in various technological applications due to their interesting morphologies and defects. With incessant effort to add value to palm oil, another two series of palm-based liquid crystals (PBLCs), namely PB1a-PB3a and PB1b-PB3b, utilizing palm fatty acids as flexible alkyl chain have been synthesized. The appearance of smectic A phase for all PBLCs was authenticated by Polarizing Optical Microscopy (POM) and further verified by Differential Scanning Calorimetry (DSC) and Small Angle X-ray Scattering (SAXS). Results revealed that the mesophase range of all synthesized LCs is highly dependent on flexible alkyl chain length, the presence of polar terminal substituent as well as the breath to width ratio of mesogenic core. Dielectric measurements show that the dielectric strength is also affected by the length of the flexible alkyl chain. It is worth noting that, as far as mesophase range is concerned, PBLCs were found to be far superior over other structurally related compounds that can be found in the literature.

Laboratory Scale Continuous Flow Systems for the Enantioselective Phase Transfer Catalytic Synthesis of Quaternary Amino Acids.

The use of stereoselective phase-transfer catalysis as a reliable method for the enantioselective synthesis of optically active α-amino acid derivatives using achiral Schiff base esters has been well-developed in batch in the last 40 years. Recently, continuous flow technology has become of great interest in the academy and industry, since it offers safer process operating conditions and higher efficiency compared to a traditional batch processing. Herein, we wish to report the first example of enantioselective phase transfer benzylation of alanine Schiff base ester, under continuous flow conditions. Two different methodologies were investigated: a liquid-solid phase transfer catalytic benzylation using a packed-bed reactor and a liquid-liquid phase transfer catalytic benzylation in continuous stirred-tank reactors. Liquid-liquid phase transfer process in flow showed slightly better productivity than the batch process, while solid-liquid phase transfer benzylation proved much more advantageous in terms of productivity and space-time yield. Furthermore, continuous flow system allowed the isolation of benzylated product without any work up, with a significant simplification of the process. In both cases, phase transfer asymmetric benzylation promoted by Maruoka catalyst demonstrated high enantioselectivity of target quaternary amino ester in flow, up to 93% ee.

Matel complexes of schiff - base ligand; Synthesis, characterization and liquid crystalline properties

In this work a Schiff base ester derived ligand [H2L1] were synthesized in three steps, first Synthesis of the pentanoiyl chloride from reaction thionyl chloride with pentanoic acid.
 in the second step the precursosr (W1), 4-[3-(3-hydroxy-1-methyl- butylideneamino)- 4-methyl-phenylimino]-pentan-2-ol ,was prepared by the reaction of acetylaceton with tolylen2,4-diamin . In the theird step the ligands were synthesized by the reaction of precursor (W1)with pentanoiyl chloride to produce[H2L1] 1:2 mole ratio. The binuclear complexes with Cr3+, Co2+ and Zn2+, were synthesized by the reaction of the ligand with metal salts in 1:1 mole ratio. The compounds were characterized by spectroscopic methods [FT-IR, UV-Vis, CHN, 1H-NMR] elemental analysis, along with conductivity, and melting point measurements. The structure of the complexes were obtained according to the previous studies. Finally the liquid crystalline properties of the prepared compounds were studied by hot stag and DCS.

New (-)-menthol-based blue phase liquid crystals with different polar substituents in the terminal group: Synthesis, mesophase behaviors, and DFT calculations

Eight novel Schiff base ester chiral liquid crystals, MtLC-X (X: -OCH3, -CH3, -F, -H, -CF3, -OCF3, -NO2, -CN), were successfully synthesized and analyzed for their mesophase formation and stability. Each of them was attached to the end of (-)-menthol as a chiral group, and the other end of these molecules was connected with different polar substituent X. The chemical structures of the prepared compounds were characterized via FT-IR, 1H NMR and 13C NMR. The combination differential scanning calorimeter (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) measurements were carried out to systematically study the characterization of mesomorphic properties. The photophysical property was studied by UV–vis spectroscopy. All the prepared CLCs, besides the H derivative, MtLCH, are mesomorphic exhibiting N* and/or SA* mesophase. The results revealed that the mesomorphic behavior and the clearing point of mesophase (Tc) correlate well with polarizability anisotropy (Δα).

Hexafluoroisobutylation of enolates through a tandem elimination/allylic shift/hydrofluorination reaction.

The isobutyl side chain is a highly prevalent hydrophobic group in drugs, and it notably constitutes the side chain of leucine. Its replacement by a hexafluorinated version containing two CF3 groups may endow the target compound with new and advantageous properties, yet this modification remains overlooked due to the absence of a general and practical synthetic methodology. Herein, we report the first general method to introduce the hexafluoroisobutyl group into ketoesters, malonates, 1,3-diketones, Schiff base esters and malononitrile. We demonstrated that the reaction occurs through an elimination/allylic shift/hydrofluorination cascade process which efficiently overcomes the usual fluoride β-elimination observed with α-CF3-vinyl groups. We showed that with alkali metal bases, a pentafluorinated alkene is obtained predominantly, whereas the use of tetrabutylammonium fluoride (TBAF) allows hydrofluorination to occur. This tandem process represents a conceptually new pathway to synthesize bis-trifluoromethylated compounds. This methodology was applied to the multigram-scale synthesis of enantiopure (S)-5,5,5,5′,5′,5′-hexafluoroleucine.

Synthesis and characterization of new Schiff base ester liquid crystals with fatty acids from palm oil as flexible alkyl chain

Palm oil plays an important role in both food and non-food industries. With the aim of further diversifying the non-food applications of palm oil, the viability of using palm fatty acids in the synthesis of liquid crystals (LCs) has been investigated. In this study, three types of palm fatty acid, namely lauric acid, palmitic acid and stearic acid were used as a source of flexible alkyl chain, which is an influential structural feature of LCs. This flexible alkyl chain is usually derived from non-renewable petrochemicals. Three palm-based LCs (PBLCs), namely, PB1 (lauric-based LC), PB2 (palmitic-based LC) and PB3 (stearic-based LC) have been synthesized and the presence of liquid crystallinity was further verified by Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) and Wide Angle X-ray Diffraction (WAXD). Mesophase was observed from POM analysis for PB1-PB3 and based on the scattering pattern of WAXD at small angle region, the texture of the mesophase was Smectic A phase. The synthesized PBLCs can be potential candidate for various practical applications such as engineering of nanostructure and biomaterials due to the presence of curvatures which is a unique characteristic of smectic LCs. This study can be regarded as a continuous effort in realizing the concept of green chemistry by using non-toxic and renewable feedstock in organic synthesis.

Schiff base/ester liquid crystals with different lateral substituents: mesophase behaviour and DFT calculations

ABSTRACTFour new groups of 4-((2ʹ-substitutedphenylimino)methyl)phenyl-4”-alkoxy benzoates, Ina-d, of Schiff base ester liquid crystals, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the lateral attached polar group X in the ortho position for the imine mesogen at terminal benzene ring that alternatively changed from F, Br, NO2 and lateral benzene ring. Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10 and 12. Molecular structures of the prepared compounds were confirmed via elemental analysis, FT-IR, and 1H NMR spectroscopy. Mesomorphic properties were investigated by differential scanning calorimetry (DSC) and the phase identified by polarised light microscopy (PLM). A comparative study was made between the investigated compounds and their previously prepared laterally neat, 4-((4ʹ-phenylimino)methyl)phenyl-4”-alkoxy benzoates (IIn); the result revealed that all lateral substituents not only decrease the melting temperature but also the mesophase stability and shown only nematic phase. Density functional theory (DFT) calculations for new lateral derivatives were discussed.

Synthesis and mesophase behaviour of Schiff base/ester 4-(arylideneamino)phenyl-4″-alkoxy benzoates and their binary mixtures

Four new groups of Schiff base ester liquid crystal compounds, 4-(arylideneamino)phenyl-4″-alkoxy benzoates, Ina–d, were prepared and investigated for their mesophase formation and stability. An alkoxy group of variable chain length (n = 6, and 16 carbons) is attached to the terminal phenylazo benzoate moiety. Each group constitutes two homologous series that differ from each other by the polar substituent X (CH3O, H, NO2, and F). The molecular structure of the prepared compounds was confirmed via FT-IR, 1H NMR, mass spectroscopy and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). For n = 6, all compounds exhibit nematic phase. Amongst them methoxy and nitro derivatives have widest mesophase range. For n = 16, nitro and flouro derivatives exhibit SmA phase, while methoxy and unsubstituted derivative are nematogenic. The change of the mesophase stability is well correlated with van der Waals volume and polarizability of the substituent X and not satisfactory with the dipole moment of the whole molecule. The mesophase behavior of binary mixtures made from each individual derivatives bearing the same substituent X but varies with alkoxy chains length is presented. Results provide insights into the role of substituent X and alkoxy chain length on the mesophase behavior within the whole composition range of the mixtures.

Mesophase stability of new Schiff base ester liquid crystals with different polar substituents

ABSTRACTFour new groups of Schiff base ester liquid crystal compounds, 4-((4ʹ-substituted phenylimino) methyl)phenyl–4″-alkoxybenzoates, Ina–d, were prepared and investigated for their mesophase formation and stability. Each group constitutes four homologous series that differ from each other by the polar substituent X (CH3O, CH3, H, and Cl). Within each homologous series, the number (n) of carbons in the alkoxy chain varies between 6, 8, 10, and 12. Molecular structures of the prepared compounds were confirmed via FT-IR, 1H NMR, mass spectroscopy, and elemental analysis. The mesomorphic properties were investigated by differential scanning calorimetry and polarised light microscopy. Comparative studies between the present series and previously investigated 4-(4-substituted phenylazo)phenyl 4″-alkoxybenzoates revealed that the phenylimino mesogenic core increases the mesophase stability rather than the azo analogues.

Schiff Base liquid Crystals with Terminal Alkoxy Group Synthesis and Thermotropic Properties

A new homologous series of Schiff base esters comprising a terminal alkoxy substituent N-{(n-alkoxy-4``-bezoyloxy)-4`- benzylidene}-4-butyl aniline was studied. The alkyloxy substituent contributes to the molecular polarizability, thus, affecting intermolecular interactions, hence, resulting in nematic polymorphism. The mesomorphic properties were studied using polarizing optical microscope (POM). The lower members, n-methoxy and n-ethoxy derivatives were non-mesogens, enantiotropic nematogenic phase was observed on further lengthening of the alkyl chain from C3 to C6 and smectogenic phase was observed for compounds C7 and C8. FT-IR, 1HNMR, 13CNMR, EI-MS and elemental analysis were employed to elucidate the molecular structure of the title compounds whereas the liquid crystal properties were determined by POM analysis. The mesomorphic behaviors of the homologous compounds are rationalized based on the varying lengths of the alky chain.

The effect of position of tert-butyl tail group on the formation of liquid crystal in Schiff base ester based homologous series

GRAPHICAL ABSTRACT

New Homologues Series of Heterocyclic Schiff Base Ester: Synthesis and Characterization

A homologous series of liquid crystal bearing with heterocyclic thiophene Schiff base ester with alkanoyloxy chain (CH3(CH2)nCOO–, where n=4, 6, 8, 10, 12, 14, 16) was successfully synthesized through the modification of some reported methods. The structural information of these compounds was isolated and characterized through some spectroscopic techniques, such as FTIR, 1H, and 13C NMR and elemental analysis. Textural observation was carried out using a polarizing optical microscope (POM) over heating and cooling cycles. It was found that all synthesized compounds (3a–g) exhibited an enantiotropic nematic phase upon the heating and cooling cycle with high thermal stability. Moreover, a characteristic bar transition texture was observed for compounds 3f and 3g which have shown transition of nematic-to-smectic C phase. This has been further confirmed by obtaining relative phase transition temperature using the differential scanning calorimetry (DSC).

Study of the effect of a heterocyclic ring end group and an azomethane linkage on mesomorphism

ABSTRACTA novel homologous series RO-C6H4-COO-C6H4-N = CH-C5H4N of liquid crystalline Schiff base esters has been synthesized and studied with a view to correlating molecular structure to the liquid crystal properties on the basis of molecular rigidity and flexibility. All 12 members of series are either monotropic (C1, C3, C5) or enantiotropic (C2, C4, C6 to C16) nematic with the absence of smectogenic character throughout the series. Transition temperatures and LC behavior of the series were determined using an optical polarizing microscope equipped with a heating stage. Transition curves (Cr-N or Cr-I and N-I or I-N) behave in the normal established manner as depicted in a phase diagram. An odd–even effect is exhibited by the N-I transition curve up to the C8 homologue.

Capacity of thiourea Schiff base esters as multifunctional additives: synthesis, characterization and performance evaluation in polyol

Novel thiourea Schiff base esters evaluated by a four ball test as multifunctional additives in polyol base oil showing excellent antifriction activity and moderate anti-wear properties.

Derivatizing l-histidine to develop a novel additive for a polyol-based biolubricant

A novel histidine Schiff base ester, HSE-A, with excellent solubility in a polyol was used as a multifunctional additive for biolubricants.

Development of new ecofriendly detergent/dispersant/antioxidant/antiwear additives from l-histidine for biolubricant applications

Novel histidine Schiff base ester molecule containing a detergent like-framework used as multifunctional additive in a polyol base fluid.

Study of cystine schiff base esters as new environmentally benign multifunctional biolubricant additives

Two additives CySBE-A and B were prepared via a two step synthesis. First, the cystine schiff base (CySB) was synthesized utilizing 3,5-di-t-butyl-4-hydroxybenzaldehyde. In the second step, its esterification with lauroyl alcohol and 2-ethyl hexanol results the final products CySBE-A and B respectively. Both additives were evaluated as multifunctional additive in polyol base oil for antioxidant, antifriction, antiwear and anticorrosion property. Universal oxidation test (IP-306) was used for evaluating antioxidant property. Antifriction and antiwear properties in terms of average friction coefficient and wear scar diameter (WSD) were evaluated using four ball test. The CySBE-A was found to be effective than CySBE-B.

Mesomorphic Properties and X-Ray Diffraction Studies of 4-Alkanoyloxybenzylidene-4'-fluoroaniline

A new homologous series of Schiff base esters comprising a terminal fluoro substituent was studied. Molecular polarizability is enhanced by the presence of fluoro substituent, subsequently, affecting intermolecular interactions and resulting in smectic molecular arrangement. All members can be differed from the number of carbons at the alkanoyloxy chain (Cn-1H2n-1COO-, n = 10, 12, 14, 16, 18). The structure of the title compound was confirmed by spectroscopic (IR and NMR) techniques along with mass spectrometric analysis. Their mesomorphic properties were studied by using differential scanning calorimetry and optical polarizing microscopy techniques. Early members (n = 10, 12 and 14) exhibited monotropic smectic A phase while the longer chain members (n = 16 and 18) are non mesogenic compounds. X-ray diffraction analysis was used and confirmed monolayer of smectic A phase in n-dodecanoyloxy derivative.

Effect of the lateral substituent on the mesomorphic behavior of side-chain liquid-crystalline polymers containing a Schiff base ester

New polymerizable macromers containing a Schiff base ester linkage have been synthesized. These macromers were used to synthesize side-chain liquid-crystalline polymers (SCLCPs) by radical polymerization. The chemical structures of the macromers and polymers were characterized by spectroscopic techniques (FTIR, 1H NMR, and 13C NMR). The mesomorphic properties of the macromers as well as the polymers were studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC) and confirmed by powder X-ray diffraction (PXRD). POM and DSC studies revealed that all of the macromers and polymers showed liquid crystal behavior, with their nematic phases occurring in different mesophase ranges. The macromer without a lateral unit in the mesogen (M1) showed a thermal transition at a relatively high temperature (181.4 °C), whereas the macromer with a lateral unit in the mesogen (M2) showed a thermal transition at a relatively low temperature (101.8 °C). The SCLCP from macromer M1 (HPM1) also showed a comparatively high-temperature thermal transition compared to that of the polymer from macromer M2 (HPM2). The thermal properties of the polymers were studied by thermogravimetric analysis (TGA). The polymer from the macromer with a lateral unit in the mesogen (M2) showed higher thermal stability than the polymer from the macromer without a lateral unit (M1).