Pd-Au-Y as Efficient Catalyst for C-C Coupling Reactions, Benzylic C-H Bond Activation, and Oxidation of Ethanol for Synthesis of Cinnamaldehydes

Article abstract of DOI:10.1021/acscatal.9b01197

Pd-Au nanoalloy supported on zeolite-Y (Pd-Au-Y) matrix was found to be an effective catalyst for C-Cl bond activation and oxidative coupling of 2-naphthol, leading to the formation of various biaryl products and 1,1′-bi-2-naphthol, BINOL. The same catalyst was also highly efficient for selective oxidation of benzylic alcohols to benzaldehydes. Cinnamaldehydes were obtained directly from benzaldehydes by aldol condensation with acetaldehyde generated in situ by partial oxidation of ethanol in the presence of Pd-Au-Y catalyst at 120 °C under basic condition. The biaryl products were also obtained directly from benzylic alcohols in a one-pot system by reacting with phenylboronic acid. The formation of biaryls from benzylic alcohols was believed to occur via one-pot benzylic C-H and C-Cl bond activation. A high % yield of biaryls, BINOL, aldehydes, and cinnamaldehydes was obtained by performing different reactions using the single Pd-Au-Y catalyst. The strong interaction of chloro-benzylic alcohol was predominantly located at active gold species. X-ray photoelectron and diffuse reflectance spectroscopic studies revealed the strong interaction between Pd and Au particles. Electrochemical studies provided proper evidence for the individual role of the nanoparticles (NPs) in one-pot synthesis of biaryls from benzylic alcohols.

Full text of DOI:10.1021/acscatal.9b01197

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Article
Pd-Au-Y as Efficient Catalyst for C-C Coupling Reactions, Benzylic C-H
Bond Activation and Oxidation of Ethanol for Synthesis of Cinnamaldehydes
Mukesh Sharma, Biraj Das, Manash J Baruah, Subir Biswas, Subhasish
Roy, Anil Hazarika, Suresh K. Bhargava, and Kusum K. Bania
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 May 2019
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Pd-Au-Y as Efficient Catalyst for C-C Coupling Reactions, Benzylic C-H Bond
Activation and Oxidation of Ethanol for Synthesis of Cinnamaldehydes
a
a
a
a
b
Mukesh Sharma, Biraj Das, Manash J. Baruah, Subir Biswas, Subhasish Roy, Anil
c
d
a,d
Hazarika, Suresh K. Bhargava and Kusum K. Bania *
a*
Department of Chemical Sciences, Tezpur University, Assam, India, 784028
b
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Department of Chemistry, Indian Institute of Technology, Guwahati, India, 781039
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Department of Electronics and Communication Engineering, Tezpur University, Assam
India, 784028
d
Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT
University, G.P.O. Box 2476, Melbourne 3001, Australia
ABSTRACT:
Pd-Au nanoalloy supported on zeolite-Y (Pd-Au-Y) matrix was found to be an
effective catalyst for C-Cl bond activation and oxidative coupling of 2-naphthol leading to
the formation of various biaryl products and 1,1′-bi-2-naphthol, BINOL. The same catalyst
was also highly efficient for selective oxidation of benzylic alcohols to benzaldehydes.
Cinnamaldehydes were obtained directly from benzaldehydes by aldol condensation with
acetaldehyde generated in situ by partial oxidation of ethanol in presence of Pd-Au-Y catalyst
at 120 ᵒC under basic condition. The biaryl products were also obtained directly from
benzylic alcohols in a one-pot system by reacting with phenylboronic acid. The formation of
biaryls from benzylic alcohols was believed to occur via one-pot benzylic C-H and C-Cl
bond activation. A high % yield of biaryls, BINOL, aldehydes and cinnamaldehydes were
obtained by performing different reactions using the single Pd-Au-Y catalyst. The strong
interaction of chloro-benzylic alcohol was predominantly located at active gold species. X-
ray photoelectron and diffuse reflectance spectroscopic studies revealed for the strong
interaction between Pd and Au particles. Electrochemical studies provided a proper evidence
for the individual role of the nanoparticles (NPs) in one-pot synthesis of biaryls from
benzylic alcohols.
Keywords: Pd-Au NPs, Zeolite-Y, Biaryls, Aldehydes, BINOL, Cinnamaldehydes
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. INTRODUCTION
C-Cl bond activation, oxidative coupling of 2-naphthols, selective oxidation of
alcohols and aldol-type condensation reaction leading to the formation of C-C coupled
product, aldehyde and cinnamaldehyde are considered to be some of the important classes of
organic reactions.1-14 Out of the different form of Suzuki-Miyaura cross-coupling (SMCC)
reactions, the formation of biaryls by C-Cl bond activation is highly appreciable due to the
low cost of chloro-derivatives compounds.7,8,15-19 Pd-based catalysts either in the
homogeneous or heterogeneous form are best known for SMCC reaction.5,8,20-22 However, it
has been found that the C-Cl bond activation process with single Pd metal is less
effective.5,7,8 Recent studies suggested that alloying of Pd with other metals like Ni, Au, Co
etc. led to successful activation of C-Cl bond in SMCC reaction.5,7,8,23 We also found that Pd-
NiO nanocatalyst supported on the zeolite-Y matrix to be an effective heterogeneous catalyst
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for C-Cl bond activation in SMCC reaction. As the reports on C-Cl bond activation for
biaryl product formation is limited, therefore finding a highly active and reusable
heterogeneous catalyst for C-Cl bond activation would be highly beneficial.
Apart from the coupling of aryl chlorides, the oxidative coupling of 2-naphthol to
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,1′-bi-2-naphthol commonly known as BINOL is considered to be an important oxidative
coupling reaction due to the high applicability of BINOL in pharmaceutical product and also
as chiral auxiliary.9-11,24 The oxidative coupling of 2-naphthol is rather a difficult process as
the yield is often very poor. Fe (III)-Schiff base catalysts are better known for such catalytic
conversion.10,11,24 Most of the catalytic oxidation of 2-naphthol to BINOL are performed
using air or molecular oxygen (O2).9-11,24 O as oxidant is considered to be the most
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preferential approach.25-27 The oxidation process with hydrogen peroxide (H O ) as oxidant is
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restricted by the formation of several other byproducts reducing the BINOL formation or its
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selectivity. The reaction with O as oxidant is often time-consuming as it requires several
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hours for producing 70-80% of BINOL.25,28 Therefore, the current interest in such process
has been driven towards the synthesis of a new catalyst for obtaining BINOL using H O as a
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suitable oxidant. But the challenges remain in getting BINOL without any other hydroxyl or
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quinone derivatives of 2-naphthol.
Another academically and industrially important reaction is the benzylic C-H bond
activation for the oxidation of benzylic alcohols to benzaldehydes eliminating the free radical
mechanism.1-4,29-32 The reaction is although quite common, but limiting the oxidation route
strictly to benzaldehyde in the presence of oxidants like H O or tetra-butyl hydrogen
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peroxide (TBHP) is still a difficult task.29,33 Hence, in recent times, a large number of works
has been devoted for such conversion accompanied by benzylic C-H bond activation.1-4,29,33
One more challenging approach that has not been well recognized in the literature is the
direct conversion of benzaldehyde to cinnamaldehyde by in situ generation of acetaldehyde
(CH CHO) by ethanol (C H OH) oxidation. To the best of our knowledge, cinnamaldehyde
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is mostly produced by cross-aldol condensation of benzaldehyde with CH CHO in presence
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of a base like sodium hydroxide (NaOH) or potassium hydroxide (KOH). Several other
processes include Horner-Wadsworth-Emmons reaction,35,36 Peterson reaction and Wittig
reaction.38 Nordqvist et al. synthesized several derivatives of cinnamaldehyde by an
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oxidative Heck reaction between acrolein and phenylboronic acids. However, the synthesis
of cinnamaldehyde by in situ formation of CH CHO through partial oxidation of ethanol has
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therefore the preparation of cinnamaldehyde through C H OH oxidation would be a highly
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economically viable process.
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The above-stated reactions are completely different from one another and till now
different types of catalysts are employed for individual reactions.1,5,7,11,14 However, it would
have been more advantageous if all the four reactions mentioned herein could have been
promoted in the presence of a single catalyst. This, in turn, will reduce the time, cost and
consumption of various transition metal-based catalysts during the synthesis of different
catalysts for separate reactions. Apart from the synthesis of the catalyst with high activity, it
is also an important aspect to look into the regeneration of the catalyst. In this regard,
inorganic materials like zeolite-Y appeared as a suitable host-matrix for supporting metal-
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based nanocatalyst, providing a high surface area, alternating the acidity/basicity of the
catalyst and more importantly, preserves the true heterogeneity of the nanocatalyst.7,29,40,41 It
also provides a high surface area framework and possesses a confinement effect, which are
key factors in heterogeneous catalysis.42-44 Therefore, looking at the need of a common
heterogeneous catalyst, herein we report for a single Pd-Au-Y catalyst that can activate
multiple reactions under different reaction conditions.
2
. EXPERIMENTAL SECTION
2.1. Materials. Palladium chloride (PdCl ) and gold (III) chloride (AuCl ) were purchased
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from Merck and Sigma Aldrich and used as sources of palladium and gold, respectively. All
the starting materials for SMCC reaction, 2-naphthol oxidation, oxidation of benzylic alcohol
(OBA) and cinnamaldehyde synthesis were obtained from Sigma Aldrich. Tetra-n-butyl
ammonium bromide (TBAB) was procured from E-Merck. C H OH and CH CN which were
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employed throughout the one-pot OBA-SMCC were purified prior to their use and deionized
water was used for synthesis. All other solvents used in different reactions were purchased
from E-Merck. All the bases for cinnamaldehyde synthesis were also procured from Merck
and Sigma Aldrich.
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.2. Physical Measurements. X-ray diffraction (XRD) of the sample was measured in a
Philips PANalytical Empyrean instrument, enabling low angle measurement from 0-60 with
a minimum step size (2) of 0.0001. A UV-visible diffuse reflectance spectrum (UV-DRS)
was obtained in a DR apparatus equipped with an assimilating sphere having 60 mm inner
diameter, (Hitachi U-3400 spectrophotometer). To minimize the effect caused by
fluorescence, a monochromatic light was employed in the full spectral range. The finely
ground sample was then placed in a 10 mm diameter and 3 mm deep sample holder. The
obtained film was considered to be infinitely thick as required in the Kubelka-Munk (K-M)
theory of reflectance. Reflectance mode was set up in order to record the optical spectra and
the KM study was achieved on the reflectance data providing the necessary data to perform
the K-M analysis. Scanning electron microscopy (SEM) and electron-dispersive X-ray
spectroscopy (EDS or EDX) mapping were done in JEOL, JAPAN, Model: JSM 6390LV.
The transmission electron microscopy (TEM) images and the TEM-EDS were recorded on a
JEOL (JEM-2010) instrument equipped with a CCD camera (slow-scan) with a 200 kV
accelerating voltage. The X-ray photoelectron spectroscopy (XPS) of the sample was
measured using Mg Kα (1253.6 eV) radiation as a source on a KRATOS (ESCA AXIS 165)
spectrometer. The oven-dried samples (finely ground) were dusted on a graphite sheet
(double stick) and mounted over the regular sample holder, before being transferred to an
analysis chamber. Before recording the XPS, the material was outgassed overnight in a
vacuum oven. The Raman analysis of the material was obtained in an EZRaman-N (Enwave
Optronics) Raman Spectrophotometer, having laser light of 150 mW, 785 nm incident
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wavelength through 100x (0.3 N.A) objective lens. The N adsorption-desorption data was
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obtained at 77 K in a NOVA 1000e, (Quantachrome Instruments) and the surface area was
calculated by using the Brunauer–Emmett–Teller (BET) equation. Barret, Joyner, and
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Halenda (BJH) pore diameter method was achieved from the desorption branch of the
isotherm of the material. Catalytic conversions of alcohols to aldehydes and the product for
SMCC reaction were confirmed via nuclear magnetic resonance (NMR) analysis using a 400
MHz NMR from JEOL, ECS-400. Fourier-transform infrared spectra (FT-IR) of the organic
compounds were recorded on a Perkin-Elmer Spectrum 2000 FT-IR spectrometer in the
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range of 500-4000 cm . The percentage of conversion and selectivity of benzylic alcohol
oxidation were obtained from gas chromatography (GC) analysis with a CiC gas
chromatography, 2010. The cyclic voltammetry (CV) studies were executed in CH
Instruments potentiostat (CHI-600E meter). The high-performance liquid chromatography
(HPLC) was recorded in instrument having specifications; model no. a) UV/Visible detector-
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489, b) refractive index detector-2414, c) HPLC pump-525, Software: Empower-2, column:
Symmetry ® C18 5 µm, 4.6 x 250 mm, column. Inductively Coupled Plasma (ICP) analyses
were performed in PERKIN ELMER, USA, OPTIMA 2100 DV.
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.3. Synthesis of gold and palladium exchanged Zeolite-Y, Au-Y and Pd-Y. For synthesis
of gold-exchanged zeolite-Y (Au-Y), a colloidal solution of gold (Au) was prepared
following the reported procedure using amino acids as capping agent.45,46 To a 100 mL
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solution of 10 M AuCl prepared in Millipore water, 10 mg of sodium borohydride (NaBH )
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was added as a reducing agent in order to obtain a ruby-red coloured colloidal mixture. As
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used by Selvakannan et al., 0.5 mmol of lysine was added as a capping agent for stabilizing
the Au-colloidal particles. From this stock colloidal solution of Au, 50 mL was pipetted out
and mixed with 1 g of zeolite-NaY in a round bottom (RB) flask and refluxed for 20 h at 90
ᵒC. The obtained material appearing pale violet in colour was consequently washed several
times with hot distilled water. The AgNO test was also performed in order to check the
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obtained. Soxhlet extraction followed by centrifugation was performed to remove the excess
lysine. The resultant material was then dried under argon for 6 h at 300 ᵒC to obtain the final
Au-Y material. The Pd-exchanged zeolite-Y (Pd-Y) was prepared as reported in our previous
work.7,29
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.4. Synthesis of Pd-Au-Y in Zeolite-Y as support. The bimetallic Pd-Au-Y catalyst was
prepared by drop-wise addition of 20 mL lysine-capped Au NPs colloidal solution (as
synthesized in Au-Y) to the Pd-Y (Scheme 1). The subsequent mixture was continuously
refluxed under nitrogen atmosphere for 24 h at 100 ᵒC. The excess uncoordinated lysine was
separated by performing a number of centrifugation cycles. The resultant Pd-Au-Y material
was then dried under an argon atmosphere at 300 ᵒC for about 12 h prior to its application.
From the ICP analysis, the weight percentage (wt %) of Pd and Au in Pd-Au-Y were found to
be 13.5 and 3.3 (mg/100 mg) respectively. This indicated that the wt % ratio of Pd:Au in the
synthesized Pd-Au-Y catalyst was ~4:1.
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.5. Procedure for Suzuki-Miyaura Cross-Coupling (SMCC) Reaction. In a 50 mL RB
flask containing 10 mL of water/ethanol (1:3), 3.2 mmol of phenylboronic acid (Ar-B(OH)2)
and 3 mmol of aryl chloride was added initially. To this mixture, 30 mg of Pd-Au-Y catalyst
was added followed by the addition of 3 equivalents (equiv.) of cesium carbonate (Cs CO )
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and was further stirred for 10 minutes to dissolve the substrates and the base. The whole
reaction mixture was then stirred for another 75 min at 80 ᵒC to see the formation of biaryl.
Hot filtration test was performed at different time intervals to determine whether there was
any Pd leaching. Finally, the product was confirmed with H and 13C NMR and FT-IR
analysis.
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2.6. Procedure for Oxidation of 2-Naphthol. Oxidation of 2-naphthol was carried out by
dissolving 2 mmol of substrate (2-naphthol) in 10 mL of distilled toluene. The solution was
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then transferred to a 50 mL two-necked RB flask equipped with a condenser. After few
minutes, 25 mg of Pd-Au-Y catalyst was added followed by the drop-wise addition 0.5 mmol
of 30% H O . After the set-up, the whole reaction mixture was refluxed at 60 ᵒC for 4 h. The
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progress of the reaction was monitored after an interval of 30 min with the help of thin layer
chromatography. After 4 h of continuous stirring, the reaction was quenched and the catalyst
was recovered by simple filtration. The recovered catalyst was then washed several times
with ethyl acetate/hexane mixture, and dried in an oven at 80 ᵒC for 12 h for further use. The
same reaction was also carried out in a 50 mL two-necked RB flask equipped with a
condenser and an air pump. The reaction was performed at 60 ᵒC, with toluene as solvent, 25
mg of the catalyst for 11 h in reactor by passing a continuous flow of dry air. The flow of
oxygen was controlled at the rate of 80 mL/min through a flow meter.
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.7. General Procedure for Oxidation of Benzylic Alcohol (OBA). All the OBA reactions
were performed in a 50 mL RB flask under refluxing condition. At first, the RB flask was
loaded with 30 mg of Pd-Au-Y catalyst followed by the addition of 2 mmol of benzylic
alcohol (BA), 6 mL of HPLC grade CH CN as solvent and 10 mmol of H O as oxidant. The
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temperature of the reaction mixture was maintained at 80 ᵒC for 75 min over a digitally
controlled magnetic stirrer. The catalyst was recovered by filtration technique. The %
selectivity and % conversion was calculated from GC analysis. The product formation was
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further confirmed via H and C NMR analysis.
.8. Procedure for One-pot OBA-SMCC Reaction. In a typical reaction, 5 mmol of p-
chlorobenzyl alcohol, 4 mmol of Ar-B(OH) was added in 50 mL RB flask containing 10 mL
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C H OH/CH CN as solvent system followed by H O (20 mmol). To this mixture, 30 mg of
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Cs CO ) and was further stirred for 75 minutes. The product was further confirmed via
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NMR and FT-IR analysis.
2.9. Procedure for Cinnamaldehyde Synthesis from Benzaldehyde. In a 50 mL RB flask,
a mixture of 4 mmol of benzaldehyde and 5 mL of C H OH and 25 mg of Pd-Au-Y catalyst,
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.1 M NaOH solution was added slowly until the pH reached above 8. The whole reaction
mixture was then refluxed at 120 ᵒC for 4 h. After 4 h, the Pd-Au-Y was easily recycled by
filtration. The catalyst was then washed many times with ethyl acetate/hexane mixture and
dried overnight at 80 ᵒC prior to its reuse.
2.10. Electrochemical Measurements. To understand the roles of Pd and Au in the multi-
component reaction, a three-electrode system was employed using Ag/AgCl as a reference
electrode, Pt wire as a counter electrode and glassy carbon as the working electrode,
respectively. With the Pd-Au-Y catalyst drop cast over glassy carbon electrode, no
electrochemical peaks were observed, so consequently, the CV studies were conducted in
stirring conditions in C H OH/CH CN applying a scan rate of 100 mV/s.
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Scheme 1. Graphical representation of the synthesis method for the bimetallic Pd-Au-Y.
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. RESULTS AND DISCUSSION
After synthesizing the bimetallic catalyst, it was characterized by various analytical
tools prior to its use in the catalytic application. Comparison of the XRD pattern of neat
zeolite-Y and the Pd-Au loaded zeolite-Y suggested for the retention of the high crystallinity
of zeolite-Y (Figure 1a). The appearance of the peaks at 2 values of 38.2 and 44.3 in the
XRD pattern (Figure 1a, red) corresponding to the (111) and (200) plane of bimetallic Pd-Au,
clearly indicated the formation of a Pd-Au nanoalloy at the crystallite of zeolite-Y.47,48 The
peak intensities of the Pd-Au-Y were observed to be much higher than those of the pristine
zeolite-Y. Such enhancement might result from surface modification caused by the
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deposition of Pd and Au NPs. The reversal of the peak intensities at 2 values of 10 and 12
were ascribed to the diffusion of very low dimension Pd-Au NPs to the interior cavity of
zeolite-Y during synthesis (Figure 1a, red).7,29 The DRS of Pd-Au-Y was dominated by Pd
NPs in the region of 300-400 nm (Figure 1b).49,50 Usually, Au NPs show an LSPR band at
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~
520 nm, however, in the present case, this characteristic peak was found to be broad and
weak.49 This might result from the alloying of Au atom with Pd resulting in the
transformation of the material from the original grey colour to light brown, (Figure 1b,
inset).49-51 A parallel observation was also found by Wang et al. with Pd-Au NPs supported
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on Mg-Al-layered double hydroxides (LDH). Apart from these, two broad signals were
observed above 600 nm. Pd NPs <10 nm are known to show their characteristic band below
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00 nm (i.e. in the UV-region).49,51 In this particular case, the shifting of the band to the
visible range clearly indicated for NP-support interactions leading to the shifting of the band
from the UV to the visible region. Hussain et al. also recently reported that Pd-particles
below 10 nm supported over TiO exhibited a band above 600 nm and showed activity as a
2
water-splitting photo-catalyst.53
Figure 1. XRD pattern of a) Pd-Y (black) and Pd-Au-Y (red), b) DRS of Pd-Au-Y (inset
shows the enlarged view from 625-700 nm).
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Figure 2. a-d) TEM images showing the interlinked zeolite-Y matrix of Pd-Au NPs (inset of
Figure b shows the lattice fringe pattern of Pd-Au NPs).
Figure 3. a) TEM image of Pd-Au nanoalloy considered for EDS elemental mapping of b)
Si, c) O, d) Al, e) Pd, and f) Au. g) EDX spectra showing the presence of all the elements in
Pd-Au-Y catalyst. The Cu and C contents were from the grid used for TEM analysis. The
elemental amounts are given in wt %, sigma () represents the error in elemental analysis.
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The results obtained from DRS-spectral analysis were found to be in agreement with
our TEM analysis showing the high dispersion of Pd-Au NPs <10 nm on the zeolite-Y
crystallite (Figure 2 a-d). It was interesting to observe from the TEM analysis that the Pd-Au
nanoalloys apart from forming at the zeolite-Y crystallite, the two zeolite crystallites were
also found to be interlinked by these NPs (Figure 2a and 2b). Furthermore, the appearance of
well-resolved lattice fringe having an inter-planar d-spacing value of ~0.23 nm also further
ascertained the formation of Pd-Au nanoalloys (inset of Figure 2b). Similar lattice patterns
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were also observed by Hong et al. during the synthesis of Pd-Au nanowire. The Pd-Au
nanoalloys were mostly isolated without being agglomerated and the separation between
them was within the range of 5-7 nm. The presence of such a small separation between the
nanoalloys might result in a strong NP-NP interaction. This was also supported by a high red
shift of Pd NPs absorption band and weakening of the characteristic LSPR band of Au NPs
(Figure 1b). The presence of isolated nanoalloys of smaller dimension <10 nm further
emphasize that the zeolite-Y acted as a good template for the decoration of small size Pd-Au
nanoalloys. Previously also, other researchers including our group found that silicon-based
matrices like MCM-41, zeolites and SBA-15 as suitable hard templating agent for preventing
the agglomeration of small-sized NPs.29,55,56 But to the best of our knowledge, Pd-Au
nanoalloys are so far not synthesized in the zeolite-Y crystallite using the solid state
dispersion (SSD) approach leading to highly ordered nanostructures. Therefore, SSD can be
considered as a new greener approach for the synthesis of such nanoalloys.
Pd-Au nanoalloy formation on zeolite-Y was also ascertained from both TEM and
SEM EDS/EDX analysis. The TEM-EDS mapping was performed selecting a single Pd-Au
nanoalloy bound to the zeolite-Y support (Figure 3a). From this mapping analysis as shown
in Figure 3 (b-f) and also from the EDX spectra (Figure 3g), it was confirmed that the Pd and
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Au were present within the nanoalloys with a higher amount of Pd content. The presence of
surface bound Pd-Au nanoalloy was also confirmed from the SEM-EDS analysis and are
provided in supporting information, SI (Figure S1). In addition to this elemental analysis,
XPS analysis on the sample was also performed to understand the nature of Pd and Au
species supported on zeolite-Y. The binding energy (BE) region of Pd (3d) core level of Pd-
Au-Y is shown in Figure 4a while that of Pd-Y is provided in SI (Figure S2a). The BE values
for Pd in Pd-Au-Y were found to be ~335.0 eV and ~340.2 eV (spin orbit separation, =5.2
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and 341.3 eV with =5.3eV (Figure S2a).5,47,57 The decrease in Pd (3d) BE in the Pd-Au-Y
catalyst was attributed to the influence of Au on the electronic structure of Pd during the
alloying process.4 Similar BE values were also reported by various other researchers and
was attributed to the formation of Pd (0) NPs.5,47,57 In addition to these peaks, a very weak
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plasmon or satellite peak for PdO was observed at 337.8 eV. The appearance of the satellite
peak might result from the oxidation of Pd during X-ray irradiation. A similar plasmon band
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for PdO species was also observed by Holade et al. with Pd nanomaterial’s. The Au (4f) BE
region in Pd-Au-Y and in Au-Y are shown in Figure 4b and Figure S2b, respectively. It
showed a pair of peaks at 83.8 and 87.5 eV for Au 4f and Au 4f , respectively which were
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found to be less in comparison to the Au-Y. These values were comparable with those of the
Au (0) species observed in different forms of Au NPs.59-62 Moreover, the spin orbit
separation (3.7 eV) for Au was exactly matching with the reported results.59-61 The XPS
analysis thus signified that the Pd and Au in Pd-Au-Y were mostly in zerovalent oxidation
state. The BE for Si 2p and Al 2p were found to be at 102.1, 103.1 and 74.5 eV indicating
that the formation of Pd-Au nanaoalloy on zeolite-Y did not hampered the electronic
configuration of Si and Al (Figure 4c and Figure 4d). No any peak for C 1s and N 1s was
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detected in the XPS analysis suggesting the complete removal of capping agent (lysine)
during the synthesis of the Pd-Au-Y catalyst. The ratio of Pd to Au (Pd:Au) from XPS
analysis was found to be 4:1. As this ratio was matching with that of the ICP analysis, so it
was considered to be the correct ratio.
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Figure 4. XPS spectra of a) Pd 3d, b) Au 4f, c) Si 2p and d) Al 2p in Pd-Au-Y catalyst.
Apart from the above analysis, the strong alloying of Au with Pd was also tested by
treatment with acetic acid (CH COOH). The Pd-Y, Au-Y and Pd-Au-Y were treated
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individually with 0.1 M solution of CH COOH. The brown and yellow coloured solution of
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Pd and Au acetate respectively was observed on centrifuging the Pd-Y and Au-Y with
CH COOH at 8000 rpm (Figure S3a and S3b, provided in SI). No any colour formation was
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non-leaching property of Pd-Au nanoalloy in solution but also provided an experimental
evidence for strong alloying of Au with Pd. Based on these experimental analyses, a
plausible path for the formation of Pd-Au nanoalloy at the zeolite-Y surface is shown in
Scheme 2.
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Scheme 2. Schematic representation demonstrating the probable interaction of Pd-Au NPs
with the zeolite-Y (NaY) matrix.
Raman spectroscopy is one of the useful techniques for the investigation of metal
oxide-based surfaces. Hence to further confirm the formation of Pd-Au nanoalloy at the
zeolite-Y crystallite, Raman analysis was performed. In the Raman spectrum, a band at 228
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metal cations.63 The breathing mode vibration of four-membered rings of zeolite-Y was
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Figure 5a).7,63 In general, zeolite-Y exhibits a very weak breathing mode at 502 cm in the
Raman spectrum.7,29,63 The splitting of bands was supposed to be caused by Fermi resonance.
The result was found to resemble with those obtained with bimetallic Pd-NiO over zeolite-Y
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matrix. The band at 888 cm could be attributed for the formation of Pd oxide surface due to
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some unalloyed Pd species. Weak bands appearing at 1090 and 1354 cm are ascribed to
the Si-O stretching vibration associated with Al-O-Si bonds.63
The surface area of the synthesized material was determined from BET analysis and
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was found to be 420 m /g (Figure 5b). The drop in the surface area from 659 (neat zeolite-Y)
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region (1.8 to 2.6 nm) after modification with bimetallic Pd-Au NPs clearly provides
supporting evidence of the presence of Pd-Au NPs at the zeolite-Y crystallite.7,29,65
Figure 5. a) Raman spectra of Pd-Au-Y, (b) N adsorption-desorption isotherm for Pd-Au-Y,
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.1. Catalytic study: The main objective of this present work was to synthesize a suitable
single catalyst that could manifest different types of catalytic reactions. In order to
understand the ability of synthesized Pd-Au-Y catalyst, herein we have considered four
important reactions namely, C-Cl bond activation, oxidation of 2-naphthol to BINOL with
H O , benzylic C-H bond activation and direct conversion of aldehydes to cinnamaldehydes.
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activation and selective synthesis of BINOL with H O . Further, the development of new
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and convenient synthetic approach for benzaldehydes and cinnamaldehydes preparation is
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highly demanding in fine chemical industries. The reactions performed using the catalytic
amount of Pd-Au-Y is discussed one by one in the following sections.
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.2. Suzuki-Miyaura Cross-Coupling (SMCC) Reaction. The SMCC reaction is
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considered as an important process for the construction of C-C bonds.5,7,8,66,67 Nowadays,
various metals like Au and Ni are amalgamated to Pd NPs to improve their catalytic activity
owing to their synergistic effect.5,7,8,67 Verma et al. demonstrated the efficiency of
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mesoporous silica supported Pd-Au NPs in carbon-halogen bond activation. Therefore, Pd-
Au-Y was used as a catalyst for SMCC reaction via activation of the C-Cl bond. The scope of
the reaction was targeted only to chloro-substituted aryl halide as C-Br and C-I bonds are
easy to activate even with Pd-salts.20-22 From our previous studies, it was found that C-Cl
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bond activation occurs effectively in water/ethanol (H O/C H OH) mixture (1:3). Therefore,
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without going for solvent optimization, SMCC reaction was performed using ArB(OH) (3.2
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0 mg of the Pd-Au-Y catalyst and 3 equiv. of Cs CO as base at 80 ᵒC. The results obtained
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under these conditions are depicted in Table 1. Thus, it was revealed that the Pd-Au-Y
catalyst was able to activate the C-X (X=Cl) bond giving moderate to high percentage yield
of biaryl products. The biaryl products were characterized by different spectroscopic analysis
(NMR and FT-IR) and are provided in the SI. Under the present reaction condition, the C-Cl
bond activation reaction was found to be sluggish with most of the substrates on performing
the reactions with Pd-Y and Au-Y (Table 1). This implied that the alloying of Pd with Au
was vital to improve its activity for effective C-Cl bond activation (Table 1). This was also in
agreement with previous work where Pd-Y was found to be inactive for the activation of the
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synergistic influence with respect to alloying of Pd-Au NPs for enhancing SMCC reaction.67
Song et al. also showed the superior catalytic ability of Pd-Au core-shell NPs in Suzuki
reaction.68
Table 1. SMCC reaction of phenylboronic acid with different aryl chloride using Pd-Au-Y
catalyst conducted at 80 ᵒC in H O/C H OH as the solvent.
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2
3
H O/C H OH (1:3)
2
2 5
Entry Substrate
Time
Yield (%)
(min) Pd-Y
Au-Y
Pd-Au-Y
1
2
3
4
5
6
7
8
R=H, X=Cl, Y=H
R=OCH X=Cl, Y=NO
2
75
75
75
75
75
75
75
75
-
18
15
-
-
70
12
14
-
67
72
73
71
76
70
66
3
,
R=H, X=Cl, Y=NO2
R=CHO, X=Cl, Y=H
R=CN, X=Cl, Y=H
-
-
R=CHO, X=Cl, Y=NO2
R=OCH X=Cl, Y=H
16
-
16
-
3
,
R=CH CO X=Cl, Y=H
-
-
3
,
Reactions were run using derivatives of ArB(OH) (3.2 mmol), 3 mmol of aryl chloride, 30
2
mg of Pd-Au-Y and 3 equiv. of Cs CO in a mixture solvent of H O/C H OH (1:3).
2
3
2
2
5
3
.3. Oxidation of 2-Naphthol to BINOL. As the Pd-Au-Y catalyst was found to be effective
for C-C coupling reaction, the same catalyst was therefore employed for the C-C coupling of
-naphthol to BINOL. Usually, iron or copper-based catalysts are used in presence of air or
2
molecular oxygen (O2).10,11,25,26,28 There are several other catalysts that are also reported for
synthesis of such chiral auxillaries.10,11,25-28 The difficult part of this particular reaction is the
controlled oxidation of 2-naphthol to BINOL, as the reaction is always hampered by the
existence of other side products like 1,2-dihydroxynaphthalene and 1,2-naphthoquinone.26,28
1
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2
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Further, the reaction is very selective to solvent and oxidant.11,26,28 The coupling reaction of
2
-naphthol was found to be effective in presence of air or O as an oxidant in chlorinated
2
solvent or toluene.11,26,28 Hence, challenges remains in the selective conversion of 2-naphthol
to BINOL using H O as an oxidant. Naya et al. reported for the selective oxidation of 2-
2
2
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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8
9
0
1
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9
0
1
2
3
4
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8
9
0
9
naphthol to BINOL with Au/SrTiO -H O based catalyst. The catalyst was although
3
2
2
selective towards BINOL formation but they studied the catalytic performance in micromolar
(mol) level. Narute et al. also performed asymmetric coupling of 2-naphthol using chiral Fe
(
III) catalyst with tertiary butyl peroxide (t-BuOOt-Bu).10 Therefore, the search for a
particular catalytic system that would allow a high yield of BINOL in presence H O is still
2
2
an ongoing process. Intrigued by the problem and challenges associated with this reaction in
the presence of H O the catalytic oxidation of 2-naphthol was performed with Pd-Au-Y
2
2,
catalyst.
From the literature, it was evident that the reaction preceded well in dichloromethane
1
1,26,28
(
CH Cl ) or toluene.
Considering CH Cl (10 ml) as the suitable solvent, the reaction
2 2
2
2
was performed using 2 mmol of 2-naphthol, 10 mg of Pd-Au-Y and 0.5 mmol of H O at
2
2
room temperature. No conversion was observed up to 1 h, however, there was a slight
1
1
decrease (3%) in 2-naphthol concentration due to surface adsorption. The reaction was then
run for another 3 h and fortunately, BINOL was obtained selectively without any other
byproduct. The extracted yield of BINOL was 28% and increasing the reaction time did not
improve the product yield and hence was not continued further. The C-C coupling of 2-
naphthol is known to be influenced by catalyst amount, solvent system, temperature, and
oxidant.11,26,28 Therefore, keeping the reaction time to be fixed for 4 h and H O amount as
2
2
0
.5 mmol, the other reaction parameters were varied. On variation of the catalyst amount
from 10 to 30 mg, the % yield of BINOL was found to be 46% on using 25 mg of Pd-Au-Y.
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Under the same condition, BINOL was achieved up to 49% in toluene and 42% in CCl4.
Hence CH Cl , toluene and carbon tetrachloride (CCl ) were found to be the suitable solvents
2
2
4
for 2-naphthol oxidation with H O as oxidant. The reaction was very non-selective in a polar
2
2
solvent like C H OH, methanol (CH OH) and water leading to a mixture of products and was
2
5
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
henceforth discarded. On varying the temperature from room temperature (20 ᵒC) to 90 ᵒC
and keeping all other conditions same, the yield was improved to 58% in toluene, 56% in
CH Cl and 52% in CCl at 60 C (Figure S4a). Increasing the temperature above 60 ᵒC
2
2
4
dropped the selectivity of the reaction (Figure S4a) and at 90 ᵒC mixtures of BINOL and 1,2-
dihydroxynaphthalene and 1,2-naphthoquinone was obtained. The isolated % yield of
BINOL, 1,2-dihydroxynaphthalene and 1,2-naphthoquinone found after performing the
reaction in various solvents using these conditions are presented in Table 2. The reaction was
1
1,26
also found to be highly dependent on H O amount.
On increasing the H O amount from
2 2
2
2
0
.1 mmol to 0.5 mmol at 60 ᵒC in toluene with 25 mg of Pd-Au-Y, % of BINOL formation
also increases (Figure S4b). The reaction was well maintained up to 0.65 mmol, but
overdosing of H O up to 1 mmol reduced the % yield from 58% to 30% along with other
2
2
side products (Figure S4b). Thus, from the above discussion, it was found that oxidation of 2-
naphthol to BINOL was possible up to 58% yield using Pd-Au-Y catalyst in the presence of
H O . However, there remained a large scope to improvise this particular reaction to achieve
2
2
a better yield of BINOL.
2
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Catalytic oxidation of 2-naphthol using Pd-Au-Y with H O /O at 60 ᵒC.
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Solvent
Product yield (%) in H O
Product yield (%) in O2
2
2
A
36
35
42
-
B
44
48
37
-
BINOL
A
-
-
-
-
B
-
-
-
-
BINOL
17
Ethanol
16
10
14
56
58
52
Methanol
Acetonitrile
CH Cl
12
21
65
72
2
2
Toluene
CCl4
-
-
-
-
-
-
-
-
62
Reactions were carried out using 25 mg of Pd-Au-Y, 0.5 mmol H O for 4 h at 60 ᵒC, and
2
2
with O for 11 h with a flow rate of 80 mL/min. A=1,2-dihydroxynaphthalene; B=1,2-
2
naphthoquinone
Although the oxidative coupling of 2-naphthol occurred in presence of Pd-Au-Y-
H O system the reaction was very sensitive to solvent, temperature and peroxide amount
2
2
,
and the yield was not so significant. The reaction was therefore again performed using O as
2
oxidant considering 25 mg of Pd-Au-Y and at 60 ᵒC in toluene. The reaction was monitored
as a function of the rate of O flow. It was observed that the constant increase in O flow
2
2
from 10 mL/min up to 80 mL/min resulted in the successful formation of BINOL up to 72%
without any byproduct (Figure S5). Previously, we also observed such dependency on the
rate of oxygen flow or partial pressure of O in the enantioselective oxidation of 2-naphthol
2
6
9
using vanadium-Schiff base complex. The reaction completion time with O was more (11
2
h) in comparison to that in the presence of H O (4 h). Considering this reaction time and the
2
2
rate of oxygen flow as standard, the oxidation process was monitored in various solvents and
the results are depicted in Table 2. It was observed that the reaction also proceeded without
any byproduct in solvents like C H OH, CH OH and CH CN. The % yield of BINOL in the
2
5
3
3
polar solvents was less in comparison to those in toluene and other chlorinated solvents
2
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(
Table 2). From the above analysis, it was thus ascertained that reaction with O as oxidant
2
was more advantageous in comparison to that with H O viz. the product yield was high and
2
2
the reaction was less sensitive to solvent. In the presence of H O , a small increment in H O
2
2
2
2
amount might change the whole story producing very less amount of BINOL. However,
controlled oxidation of 2-naphthol to BINOL was achieved with moderate to good yield
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
within a short period of time using 30% H O as the green oxidant. Since the reports on
2
2
9,28
such oxidation process with H O as oxidant is very sparse,
so we believe that Pd-Au-Y
2
2
can be an effective catalyst for such non-preferential oxidation using H O . No BINOL
2
2
formation was observed with Pd-Y and Au-Y either with H O or O as oxidant up to 72 h.
2
2
2
The overall process of oxidation of 2-naphthol to BINOL can be represented as shown in
Scheme 3.
non-selective oxidation in polar solvents,
EtOH, MeOH and H O
2
O
OH
O
+
OH
OH
OH
+
2
o
O
6
0 C
2
Toluene
CH Cl
H
OH
H2O2
OH
OH
2
2
O , 80 mL/min,
OH
OH
2
o
o
6
0 C
60 C, toluene
CCl4
72 % in 11 h
52-58 % in 4 h
Scheme 3. Catalytic oxidation of 2-naphthol to BINOL in presence of H O and O with Pd-
2
2
2
Au-Y catalyst.
3
.4. Oxidation of Benzylic Alcohol (OBA). The OBA reaction was initially explored using
1
0 mg of the Pd-Au-Y catalyst, 2 mmol of the benzylic alcohol (as a test substrate), and 10
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2
2
2
2
2
2
2
3
3
3
3
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3
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4
4
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5
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5
5
5
5
6
mmol of H O at 40 ᵒC in C H OH as the solvent for 1 h. Unfortunately, the obtained %
2
2
2
5
conversions were very low and therefore the reaction was performed in different solvents
keeping the other condition to be the same (Figure S6a). On the variation of the solvent, it
was found that the reaction proceeded well in CH CN compared to many other solvents
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Figure S6a). As the conversions and the selectivities were not satisfactory at 40 ᵒC, therefore
the reaction temperature was varied from 40 to 100 ᵒC using CH CN as a solvent and all
3
other conditions to be fixed (Figure S6b). On monitoring the temperature effect, the highest
o
activity was found at 80 C. As the catalyst amount also plays a dominant role in such
reaction, consequently the catalyst amount was also varied from 10 to 50 mg and the results
were found to be satisfactory with 30 mg of the catalyst (Figure S6c). Increase in the catalyst
amount above 30 mg didn’t alter the conversion and selectivity which might be due to the
fact that 30 mg was the saturation level for the catalyst for this particular reaction. The effect
of reaction time was also monitored at 80 ᵒC using 30 mg of the catalyst in CH CN. The
3
maximum conversion was observed on running the reaction for 75 min and no further
increment was observed on increasing the reaction time (Figure S6d). Therefore, CH CN, 80
3
ᵒC, 30 mg catalyst and 75 min was considered to be the standard conditions for the selective
OBA to benzaldehyde. As different solvent responds differently to temperature and catalyst
o
amount, so the OBA was again tested in various solvents, at 80 C, and 30 mg of Pd-Au-Y
o
(Table S1). The results were slightly different in comparison to that performed at 40 C with
1
0 mg of the catalyst (Table S1). C H OH was found to be much comparable with CH CN,
2 5 3
but the conversion and selectivity were slightly better in CH CN and hence was considered
3
as the most suitable solvent system for OBA.
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Table 3. Results for the OBA derivatives using CH CN as a solvent with Pd-Au-Y catalyst.
3
HO
O
Catalyst
o
80 C, H O , CH CN
2
2
3
X
X
Y
Y
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Entry Substrate
Time
(%)
(%)
(min)
Conversion
Selectivity
Pd-Y Au-Y Pd-Au-Y Pd-Y Au-Y Pd-Au-Y
1
2
3
4
5
6
7
8
X, Y= H
75
75
75
75
75
75
75
75
21
25
31
27
26
20
21
17
24
28
36
32
34
39
28
19
73
75
80
78
76
71
74
68
>99
>98
>98
>98
>98
>98
>98
>98
>99
>98
>98
>98
>98
>98
>98
>98
>99
>98
>98
>98
>98
>98
>98
>98
X=H, Y=Ar
X=H, Y=NO2
X=H, Y=MeO
X= NO , Y=H
2
X=MeO, Y=H
X=H, Y=Cl
X=H, Y=F
Reactions were implemented on a 2 mmol scale of benzylic alcohol derivative, 6 mL of
CH CN, 10 mmol of H O and 30 mg of Pd-Au-Y catalyst. GC-analysis was used to obtain
3
2
2
the percentage conversion and selectivity.
After standardizing all the reaction parameter, the reaction was carried out with other
alcohol substrates and the outcomes are presented in Table 3. For a better understanding of
the reaction conditions, the bimetallic Pd-Au-Y catalyst was also compared with the
monometallic zeolite-Y exchanged counterparts, Pd-Y and Au-Y. The catalytic conversions
with all the substrates under identical conditions were found to be much improved with the
Pd-Au-Y in comparison to the monometallic counterparts (Table 3). The obtained results
clearly validated that the catalytic activity of the Pd towards selective OBA was influenced
3
2
by Au and vice versa. It is pertinent to mention herein that the percentage of conversion
obtained herein with Pd-Y was less compared to the other reported results; however, the
2
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
5
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results from the Au-Y were found to be in good agreement with other Au catalysts.31,32 The
difference in the catalytic performance of Pd-Y was attributed to the variation in reaction
conditions and catalyst preparation technique.32
o
Table 4. Results for the one-pot OBA-SMCC reaction with Pd-Au-Y catalyst at 80 C.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Y
Y
Pd-Au-Y
X + (HO) B
2
o
HO
80 C, H O , Cs CO , O
2
2
2
3
CH CN/C H OH
3
2 5
Entry
Time
Yield (%)
Au-Y
Substrate
(min)
Pd-Y
Pd-Au-Y
1
2
3
4
5
6
X=Cl, Y= H
X=Cl, Y=NO2
X=Br, Y=H
X=Br, Y=NO2
X=I, Y=H
75
-
-
76
75
75
75
75
75
4
-
79
85
87
90
92
10
22
26
32
7
16
21
29
X=I, Y=NO2
Reactions were run on a 5 mmol scale of X-benzylic alcohols and 4 mmol of phenylboronic
acid, 10 mL of CH CN/ C H OH, 20 mmol of H O , Cs CO (4 equiv.) and 30 mg of Pd-Au-Y
3
2
5
2
2
2
3
catalyst.
3
.5. One-pot OBA-SMCC Reaction. With the success in individual reaction and standard
reaction parameters in hand, the Pd-Au-Y catalyst was employed for the synthesis of biaryl
aldehyde derivative directly from X-benzylic alcohols (X=Cl, Br, I). The reactions were
performed taking identical parameters i.e. 30 mg of catalyst, p-chloro benzylic alcohol (5
mmol), ArB(OH) (4 mmol), H O (20 mmol) as oxidant and Cs CO (4 equiv.) in
2
2
2
2
3
C H OH/CH CN solvent system at 80 ᵒC. The results for the one-pot reaction are listed in
2
5
3
Table 4. The mixture of C H OH /CH CN (3:1) solvent was employed in this reaction as this
2
5
3
combination was found to be optimum and the results are shown in Table S2.
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The percentage yield of biaryl derivative directly obtained from bromo/iodo
substituted benzylic alcohols was almost comparable with those obtained with the chloro-
substituted benzylic alcohol (Entries 4 and 6 in Table 4). Hence, it can be considered that
biaryl aldehydes could be synthesized by activation of the benzylic C-H bond followed by
successive C-X (X=Br, I, Cl) bond activation and C-C bond formation reactions, presenting
an effective route for the synthesis of biaryl aldehydes starting from halo-substituted benzylic
alcohols. In addition, it also provides a convenient approach to achieve multi-bond activation
in a single-pot without the use of multistep work-up processes. The formation of biaryl
aldehydes from benzylic alcohols was believed to occur through in situ oxidation of X-
benzylic alcohols (X=Cl, Br, I) to corresponding aldehydes and its coupling with
phenylboronic acid. To have better information on the assumption of in situ oxidation of X-
benzylic alcohols, the samples were collected after an interval of 15 min to observe the
presence of benzaldehyde, biaryl and also to see the change in the concentration of Cl-
benzylic alcohol (Cl-BA). As the main objective of the present work was on C-Cl bond
activation, so, this conversion was monitored taking Cl-BA as the test substrate. From the
graph shown in Figure S7, it was observed that as the reaction progresses, there was a
decrease in the concentration of Cl-BA. Up to 15-30 min, there was a certain concentration of
Cl-benzaldehyde, but after 30 min, the concentration of biaryl aldehyde was sufficiently high
indicating the coupling of the Cl-benzaldehyde resulted from the oxidation of Cl-BA with
ArB(OH)2.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
This one-pot approach of synthesizing biaryl aldehydes from X-benzylic alcohols was
expected to have certain advantages, i) aldehydes, especially the benzaldehydes are very
sensitive to air producing the benzoic acids. So, this form of SMCC reaction that is selective
for aldehyde formation would be beneficial in restricting the formation of over-oxidized
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products, ii) usually the cost of benzylic alcohols is slightly higher in comparison to that of
X-substituted benzaldehyde, therefore one might use directly the aldehydes for preparation of
biaryl aldehydes. But in another way, starting with benzylic alcohol would be beneficial as it
will reduce the cost and the time that will be spent for preparation of benzaldehyde from
benzylic alcohols, one of the common techniques use for benzaldehyde preparation.1-4
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Recently, a sufficient number of studies has been made on the selective oxidation of benzylic
alcohol to benzaldehyde using different transition metal-based catalyst.1-4,29,33,40 As the Pd-
Au-Y catalyst was highly selective for benzylic alcohol oxidation, therefore, from the
industrial point of view it will be quite an effective process to look into such one-pot
synthetic approach in SMCC type reaction.
3
.6. Synthesis of Cinnamaldehyde from Benzaldehyde.
Cinnamaldehydes are considered to be one of the classical compounds that have high
biological and pharmacological significance.38,39 It has also found many applications as a
flavoring agent, antimicrobial, antifungal compounds.34,35,38 However, the synthesis of
cinnamaldehydes is rather typical and its biosynthesis process involves a number of multiple
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steps using coenzymes. Commonly known procedures for the synthesis of cinnamaldehydes
are shown schematically in Scheme 4 along with the new and unknown approach adopted in
this current study.
The % yield of cinnamaldehydes produced through condensation of CH CHO with
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benzaldehyde is comparatively not so prominent. The process depends on how one adds the
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CH CHO to the benzaldehyde and also the pH of the media.
From the economic and
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industrial point of view, the synthesis of cinnamaldehyde by cross-aldol condensation is not a
cost effective method as the price of CH CHO is reasonably high. Further, it is also difficult
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to have control over the oxidation of CH CHO to CH COOH. Looking at the price of the
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CH CHO and its industrial importance, a large number of studies are now going on to obtain
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CH CHO from C H OH. Various types of catalysts like Au-CuOx, Cu/ZrO and
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Au/MgCuCr O are reported to be active for complete or partial oxidation of C H OH.
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Pd-Au catalysts are also known to be active for the oxidation of C H OH.
Thus, taking
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the advantage of Pd-Au-Y surface, herein we opted to synthesize various cinnamaldehydes
by reaction of ortho and para substituted benzaldehyde with CH CHO formed by in situ
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partial oxidation of C H OH over Pd-Au-Y surface.
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Horner Wadsworth Emmons reaction, Peterson reaction, Wittig reaction,
Oxidative Heck reaction, aldol condensation etc.
Known
X
X
O
Pd-Au-Y,
O
Y
base
Y
yield: 80-86%
o
1
20 C
X= H, F, Cl
Y= H, F, Cl, Br,
O
OH
NO , OMe, CHO,
X
2
+
Ph
insitu ethanol
oxidation
O
This work
Unknown
Y
Scheme 4. Schematic representation showing known and unknown protocols for the
synthesis of cinnamaldehydes.
The reaction was initially performed at 120 ᵒC using 25 mg of Pd-Au-Y catalyst, 4
mmol of benzaldehyde and the cinnamaldehydes were obtained with very good yield (~80%)
within 240 min (Entry 1, Table 5). The reaction was believed to be affected by temperature
and catalyst amount, so the oxidation of C H OH and condensation with benzaldehyde was
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monitored at different temperatures and also by varying the catalyst amount. Taking 10 mg of
Pd-Au-Y as constant catalyst amount, the reaction was performed at different temperatures
and the results are plotted graphically (Figure S8a). The temperature was varied from 60 to
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