Eugenol trans - ... Allylbenzene trans - methylstyrene Eugenol trans -isoeugenol Microwave assisted

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Transcript of Eugenol trans - ... Allylbenzene trans - methylstyrene Eugenol trans -isoeugenol Microwave assisted

  • Allylbenzene trans-β methylstyrene

    trans-isoeugenolEugenol

    Allylbenzene trans-β methylstyrene

    trans-isoeugenolEugenol

     Microwave assisted isomerization of alkenyl aromatics over MgAl LDHs – Beneficial and green over conventional thermal heating is

    demonstrated

     99% conversion of estragole to anethole is achieved under energy and material efficient conditions

     pKa of the protons of alkenyl aromatics governs the activity

     Theoretical calculations validate the experimental data

    Microwave assisted isomerization of alkenyl

    aromatics over hydrotalcite-like materials

    Manuscript under review with RSC Advances

  • Microwave assisted isomerization of alkenyl

    aromatics over hydrotalcite-like materials

    3.1 Introduction

    3.2 Experimental

    3.2.1 Catalyst synthesis

    3.2.2 Physicochemical characterization

    3.2.3 Catalytic isomerization of alkenyl aromatics

    3.2.4 Hammett indicator studies

    3.2.5 Computational methods

    3.3 Results and discussion

    3.3.1 Physicochemical characterization

    3.3.2 Catalytic studies

    3.3.2.1 Estragole isomerization-parametric optimization

    3.3.2.2 Basicity measurements

    3.3.2.3 Effect of solvent on isomerization of estragole

    3.3.2.4 Influence of substrate:catalyst weight ratio

    3.3.2.5 Isomerization of different alkenyl aromatics

    3.3.2.6 Scale up studies over MgAl4

    3.4 Conclusions

    3.5 References

  • Chapter 3 Microwave assisted isomerization

    Ph.D Thesis

    71

    3.1 Introduction

    Isomerization of alkenyl aromatics has high industrial demand as

    intermediates for various perfumery chemicals [1, 2]. Among alkenyl aromatics trans-

    anethole has significant application in food and beverage industry, formulation of oral

    sanitation products, pharmaceutical compounds, and perfumery chemicals [3-6].

    Anethole, also known as isoestragole occurs in nature as both cis and trans forms,

    wherein trans-isomer being more abundant. Anethole is a major component of several

    essential oils, including anise seed oil (80-90%), star anise oil (>90%) and sweet

    fennel oil (80%). The total global production of trans-anethole is approximately 0.75

    million metric tonne per annum. Increasing demand for anethole led to development

    of new synthetic routes other than isolation from essential oils. The most viable one is

    simple base catalyzed isomerization of estragole (methyl chavicol) to its propenyl

    derivative, anethole (Scheme 3.1). Conventionally alkalis such as KOH in alcoholic

    solutions (most often in higher alcohols) at high temperatures were used for

    isomerization of alkenyl aromatics [7, 8]. Kameda and Yoneda [9] had reported

    homogeneous isomerization of estragole over [RhH2(Ph2N3)(PPh3)2] in dimethyl

    sulfoxide (DMSO) under hydrogen atmosphere (1 atm) at 30 o C. Later on solid base

    catalysts were tried for this isomerization reaction which provides better effluent

    control, facile separation and easy handling [10, 11]. Recent works on isomerization

    of estragole to anethole comprises using K2CO3 on alumina [12] and Ru-complexes

    [13, 14]. It was reported that under homogeneous conditions using Ru-complexes

    highly trans selective products were obtained. We have reported earlier the utility of

    as-synthesized layered double hydroxides (LDHs) as promising heterogeneous solid

    base catalysts for such double bond isomerization of alkenyl aromatics [15-17].

    Detailed literature survey over these types of isomerization reactions is given in

    Chapter 2.

    Our group here at CSMCRI’s previous report on isomerization of estragole to

    anethole over binary hydrotalcite revealed MgAl4 was the most active catalyst and the

    conversion increased slightly upon incorporation of Ru and Cs under conventional

    heating [18]. Under thermal conditions draw backs are high reaction temperature (200

    o C), larger solvent volume (20 ml) and longer reaction time (6 h). Microwave

    irradiation (MWI) which involves dielectric heating is simple fast and advantageous,

    thereby provide significant enhancement in reaction rates. MWI processes are more

  • Chapter 3 Microwave assisted isomerization

    Ph.D Thesis

    72

    economical by minimizing the energy consumption and it also reduces the reaction

    times (comparison of MWI and conventional heating is given in Table 3.1) [19, 20].

    MWI had also been used in organic transformation involving hydrotalcite as catalyst

    [21, 22]. Thach and Strauss [23] previously used microwave batch reactor for

    isomerization of estragole under aqueous conditions using 0.2M NaOH and obtained a

    conversion of 81% at 230 o C. Recently Crochet and co-workers reported 99% yields

    of anethole with high trans selectivity from estragole within 15 min under microwave

    irradiation using Ru-complexes as homogeneous catalyst [14].

    Table 3.1 Comparison of microwave heating vs. conventional heating

    Microwave Conventional

    Direct coupling of energy - internal

    heating Conduction/convection- external heating

    Volumetric (whole material heated

    simultaneously) Superficial heating (surface)

    Selective absorption of radiation by polar

    substances Non selective

    Rapid heat transfer Slow heat transfer

    With this knowledge, in this Chapter, we disclose isomerization of alkenyl

    aromatics using MgAl series of hydrotalcite as heterogeneous catalyst under

    microwave and theoretical studies to evaluate the variation in isomerization activity of

    different alkenyl aromatics which will validates earlier reports. Our aim was to

    overcome challenges like higher temperature, larger solvent volume, longer reaction

    time, and unproductive recylability. Attempts were done to correlate the activity with

    basicity using Hammett indicators studies. The cause of variation in isomerization

    activity of different alkenyl aromatics over same base catalyst was proved through

    theoretical study. The alkenyl aromatics had similar reaction sites on the MgAl4

    surface; however, the conversion highly depends on the aromatic substitution. To

    examine the difference in the isomerization of these systems, computational studies

    have been carried out.

  • Chapter 3 Microwave assisted isomerization

    Ph.D Thesis

    73

    3.2 Experimental

    3.2.1 Catalysts synthesis

    The MgAl series of hydrotalcites were prepared by low supersaturation

    technique whose details were given as in section 2.2.1 in Chapter 1 [24]. Samples thus

    prepared were named as M(II)M(III)x where x stands for M(II)/M(III) atomic ratio for

    binary systems and A n-

    stands for interlayer anions. The samples are represented here

    either as M(II)M(III)x where ‘x’ stands for the M(II)/M(III) atomic ratio for binary

    systems For all samples, the atomic ratio of M(II):M(III) was kept between 2 to 4.

    3.2.2 Physicochemical characterization

    Physicochemical characterization of all the samples synthesized was done

    using various analytical techniques as given in section 2.2.2 (Chapter 1). The samples

    were characterized by Powder X-ray diffraction (PXRD; Rigaku-MiniFlex) system

    using Cu K radiation ( = 1.5406 Å). Identification of the crystalline phases was

    made by comparison with the JCPDF files [25].

    3.2.3 Catalytic isomerization of alkenyl aromatics

    Isomerization of alkenyl aromatics was conducted in a microwave synthesis

    work station (Sineo, MAS-II) equipped with direct sensing; microwave power could

    be adjusted between 0-1000W in order to achieve the desired reaction temperature.

    The substrate, solvent and catalyst were charged at once and subjected to reaction

    temperature using 100% MWI. The products were analyzed by gas chromatography

    (Varian 450-GC) with a capillary column (Factor Four VF-1) and FID detector.

    Scheme 3.1 Isomerization of alkenyl aromatics

    Identification of the products was also further verified using GC-MS

    (Shimadzu QP 2010). Quantification of the products was done using tetradecane as

  • Chapter 3 Microwave assisted isomerization

    Ph.D Thesis

    74

    internal standard. For conventional heating studies, isomerization was conducted in a

    batch reactor (50 ml), wherein the substrate, solvent and catalyst were charged at once

    and kept in a preheated oil bath at desired reaction temperature [25]. The

    isomerization of the alkenyl aromatics along with the geometries are given in scheme

    3.1.

    3.2.4 Hammett indicator studies

    In order to evaluate the Bronsted basicity associated with the hydroxyl groups,

    Hammett indicator measurements were carried out. 25 mg of catalyst was taken along

    with 2.5 ml of dry methanol and to that calculated amount of indicators with different

    pKa ranges were added separately and kept in a shaker for 3 h under N2 atmosphere

    [27]. The solution was then titrated against 0.02 M benzoic acid in dry methanol to

    determine the basicity. The indicators used w