Fe(HSO4)3/SiO2: An efficient and heterogeneous catalyst for one-pot synthesis of 2-aryl-chromene-4-ones (flavanones)

Article abstract of DOI:10.1080/14786419.2013.872103

Silica ferric hydrogensulphate is an efficient heterogeneous catalyst for cyclisation of 2-hydroxychalcones to their corresponding flavanones (chromanone). This intramolecular oxa-Michael reaction was carried out in high yields in the presence of electron donating and electron withdrawing groups in the chalcone structure. Also, we found that aniline can act as an organic co-catalyst in direct synthesis of flavanones from 2-hydroxyacethophenone and aldehydes in the presence of Fe(HSO₄)₃/SiO₂. Flavanones were prepared in high yields in a new catalytic system with similar substituted effect in the indirect cyclisation method. The catalyst in all the above reactions is reusable without significant decreases in its activity after four times of recycling.

Full text of DOI:10.1080/14786419.2013.872103

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Fe(HSO4 )3 /SiO2: an efficient and
heterogeneous catalyst for one-pot
synthesis of 2-aryl-chromene-4-ones
(flavanones)
Hossein Eshghia, Mohammad Rahimizadeha & Seyed Mohsen
Mousavia
a Department of Chemistry, School of Sciences, Ferdowsi
University of Mashhad, Mashhad 91775-1436, Iran
Published online: 16 Jan 2014.
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To cite this article: Hossein Eshghi, Mohammad Rahimizadeh & Seyed Mohsen Mousavi (2014)
Fe(HSO4 )3 /SiO2: an efficient and heterogeneous catalyst for one-pot synthesis of 2-aryl-
chromene-4-ones (flavanones), Natural Product Research: Formerly Natural Product Letters, 28:7,
438-443, DOI: 10.1080/14786419.2013.872103
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Natural Product Research, 2014
Vol. 28, No. 7, 438–443, http://dx.doi.org/10.1080/14786419.2013.872103
Fe(HSO4 )3 /SiO2: an efficient and heterogeneous catalyst for one-pot
synthesis of 2-aryl-chromene-4-ones (flavanones)
Hossein Eshghi*, Mohammad Rahimizadeh and Seyed Mohsen Mousavi
Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran
(Received 5 August 2013; final version received 1 December 2013)
This paper is dedicated to Professor Hashem Sharghi on the occasion of his 65th birthday and his ongoing
35 years research and teaching in organic synthesis.
Silica ferric hydrogensulphate is an efficient heterogeneous catalyst for cyclisation of 2-
hydroxychalcones to their corresponding flavanones (chromanone). This intramolecular
oxa-Michael reaction was carried out in high yields in the presence of electron
donating and electron withdrawing groups in the chalcone structure. Also, we found
that aniline can act as an organic co-catalyst in direct synthesis of flavanones from
2-hydroxyacethophenone and aldehydes in the presence of Fe(HSO4)3/SiO2. Flavanones
were prepared in high yields in a new catalytic system with similar substituted effect in
the indirect cyclisation method. The catalyst in all the above reactions is reusable without
significant decreases in its activity after four times of recycling.
Keywords: Fe(HSO4)3; heterogeneous; flavanones; chalcones; oxa-Michael; organo-
catalyst; silica ferric hydrogensulphate
1. Introduction
The flavanones are a group of natural products which are widely distributed in plants (Siddikov
et al. 2008) and also available from synthetic sources. The major reported biological activities of
these compounds include neuron protection (Vauzour et al. 2007), antitumour (Cabrera et al.
2007), anti-metastasis (Hsiao et al. 2007), antimicrobial (Ward et al. 1981), anti-oxidant (Jung
et al. 2008), anti-inflammatory (Njamen et al. 2004) and antiviral activities (Paredes et al. 2003).
A commonly reported synthetic method for flavanones involve the Claisen–Schmidt
reaction of o-hydroxyacetophenones with benzaldehydes to produce chalcone intermediates
using different catalysts (Wang & Cheng 2006), and then cyclisation of chalcone with various
bases (Chen et al. 2011), acids (Chaturvedi et al. 1992) and other catalysts (Climent et al.
1989).
Most of the existing procedures for the preparation of flavanone derivatives suffer from some
drawbacks such as the use of corrosive reagents, limited synthetic scope, low yields, long
reaction times, needs of large amount of catalyst, solvents and harsh reaction conditions.
Therefore, the development of new methods that lead to a convenient procedure and better yield
is still desirable.
The aim of this study was to investigate the efficiency of Fe(HSO4)3/SiO2 as a catalyst in the
synthesis of flavanones through cyclisation of 2-hydroxychalcones. Also, we wish to report the
one-pot synthesis of flavanones via three-component Mannich reaction in the presence of silica
ferric hydrogensulphate (SFHS) and aniline as an organic co-catalyst.
*Corresponding author. Email: heshghi@um.ac.ir
q 2014 Taylor & Francis

Natural Product Research
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2. Results and discussion
The first application of SFHS in oxa-Michael reaction was demonstrated by intramolecular
cyclisation of 2-hydroxychalcones (Scheme 1).
The effects of catalyst loading and solvent screening on the rate of cyclisation reactions are
shown in Table S1. All reactions were carried out with 1 mmol of 2-hydroxychalcone (1a) in
2.5 mL solvent at reflux conditions or in solvent-free condition at room temperature. No reaction
was observed without catalyst. The yield with unsupported ferric hydrogensulphate (FHS) was
low. So the optimum conditions for the conversion of 2-hydroxylchalcones to the corresponding
2-aryl flavanones are 5 mol% Fe(HSO4)3/SiO2 under reflux condition in EtOH.
Reusability of the catalyst in these reactions was also investigated. After each reaction, the
catalyst was recovered by filtration and rinsed by CH2Cl2 and absolute EtOH. The recovered
catalyst did not show a significant decrease in its activity at least after five new runs (Table S1,
entries 14–18).
In order to show the generality of the intramolecular oxa-Michael reactions, various
substituted chalcones were subjected to cyclisation using SFHS as catalyst. The results of these
reactions are shown in Table S2. Both electron-donating and electron-withdrawing substituted
chalcones undergo cyclisation, but the latters were more reactive. The results of Table S2
demonstrate that the reaction is chemoselective and further oxidation of flavonone to flavone
was not observed. Other functional group like hydroxyl was not oxidised under these conditions.
Generally, ortho-substituted chalcones are cyclised slower than meta- or para-isomer because of
1
large steric effect. All the isolated products were well characterised by their IR, H NMR
1
spectral analysis. The H NMR (100 MHz, CDCl3) spectra of compounds 2a–2i showed a
doublet of doublet in 5.1–5.9 and a multiplet in 2.7–3.3 region as characteristic bands of
flavanones (Supplementary data).
In a possible mechanism for these conversions, the chalcone is activated by coordination
with catalyst followed by hydroxyl attack to electrophilic carbon atom and regeneration of the
catalyst.
On the other hand, in continuation to our previous investigations on the three-component
Mannich reaction of aldehyde, amine and enolisable ketone to prepare the b-aminocarbonyl
compounds (Eshghi et al. 2011, 2013), we found that there is no report in using 2-hydroxy
acetophenone as enolisable ketone component in these reactions. So we decided to carry out the
Mannich reaction of benzaldehyde, aniline and 2-hydroxyacetophenone in the presence of SFHS
as catalyst in ethanol under reflux condition. Surprisingly, only flavanone 2a was obtained
instead of b-aminocarbonyl compound. So, we try to optimise the reaction condition for this
novel one-pot flavanones synthesis (Table S3). As can be seen, the aniline acts as an organic
co-catalyst in this reaction and the reaction does not proceed in the absence of catalyst or
co-catalyst.
O
O
O
Fe(HSO4)3/SiO2
EtOH
reflux
OH
R
1
2
R
1a, 2a) R=H
1f, 2f) R=p–Cl
1b, 2b) R=p–Me 1g, 2g) R=o–Cl
1c, 2c)
1d, 2d)
1h, 2h)
1i, 2i)
R=p–i–Pr
R=p–OMe
R=p–NO2
R=m–OH
1e, 2e) R=m–OMe
Scheme 1. Intramolecular oxa-Michael reaction catalyzed with SFSH.

440
H. Eshghi et al.
Fe(HSO4)3/SiO2 was recovered by simple filtration and aniline was removed by acid
treatment of organic phase. Reusability of the catalyst in this reaction was also examined. After
five times recycling of catalyst, no significant decrease in reaction yields or rate was observed.
After the reaction condition was optimised, the reaction was performed with different
substituted benzaldehydes (Scheme 2), and the obtained results are shown in Table S4.
Electron-withdrawing group increases the rate of reaction, while the electron-donating group
decreases the rate. The steric effect of bulky groups in ortho-position are also decreases the rate
of reaction.
While writing this manuscript, we noticed a similar protocol has been reported by Kavala
et al. (2012) but stoichiometric rather than catalytic amount of aniline was used. They also
screened the effect of different amines and concluded that aniline was the most suitable amine
for this conversion. A possible mechanism for this transformation is suggested in Scheme S3.
The initially formed imine and 2-hydroxyacetophenone are both activated by coordination with
SFHS catalyst. The enol forms of 2-hydroxyacetophenone attacks the electrophilic carbon of
coordinated imine, to form the intermediate (I). Coordination of this intermediate with SFHS
gave intermediate (II) which may easily be cyclised with either SN1 or SN2 mechanism,
or eliminated to chalcone and then cyclised to product. TLC monitoring of the reaction mixture
did not show any chalcone formation and substitution effects ruled out the SN1 mechanism
(Table S4, entry 8). So SN2-type intramolecular cyclisation or nucleophilic participation
borderline mechanisms are more probable for this reaction. Aniline and SFHS regenerate and
initiate a new catalytic cycle.
3. Experimental
3.1. General
Chemicals were either prepared in our laboratories or purchased from Merck (Germany), Fluka
(Switzerland) and Aldrich (USA) Chemical Companies. All yields refer to the isolated products.
Melting points are recorded on an Electro thermal type 9100 melting point apparatus. IR spectra
1
were recorded on a Thermo Nicolet AVATAR-370-FT-IR spectrophotometer (USA). The H
NMR spectra were recorded on a Bruker AC 100 spectrometer (Germany) at 100 MHz.
Chemical shifts are reported in ppm downfield from TMS as internal standard; coupling constant
J is given in Hertz. Elemental analyses were obtained on a Thermo Finnigan Flash EA micro-
analyser. Silica-supported FHS was prepared as we have previously reported (Eshghi et al. 2012,
2013). Chalcone 1 was prepared according to the reported procedure (Lin et al. 2002).
3.2. General procedure for the synthesis of flavanones 2a–2i
Fe(HSO4)3/SiO2 (5 mol%) was added to a stirred solution of 2-hydroxychalcones (1) (1.0 mmol)
in ethanol (2.5 mL). The reaction mixture was heated with stirring at reflux conditions for the
appropriate time. After completion of the reaction as monitored by TLC, the mixture was cooled,
filtered and the solvent was evaporated in vacuum. Products 2a–2i were purified by silica gel
O
O
O
O
Fe(HSO4)3 / SiO2 (10 mole %)
H
CH3
Aniline (20 mole%)
EtOH/ reflux
OH
R
R
Scheme 2. Direct synthesis of flavanones catalyzed with SFSH in the presence of aniline as an organo
catalyst.

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441
column chromatography using hexane–diethyl acetate (9:1 to 4:1) as eluent. The separated
catalyst was rinsed with CH2Cl2 and absolute ethanol, dried and re-used.
3.3. General procedure for the one-pot synthesis of flavanones 2a–2i in the presence of Fe
(HSO4)3/SiO2/aniline catalytic system
A mixture of Fe(HSO4)3/SiO2 (10 mol%) and aniline (20 mol%) was added to a stirred solution
of 2-hydroxyacetophenone (1.0 mmol) and aldehydes (1.0 mmol) in ethanol (3 mL). The
reaction mixture was heated with stirring at reflux conditions for the appropriate time. After
completion of the reaction as indicated by TLC, the mixture was cooled, filtered and the organic
phase was evaporated in vacuum. Crude products dissolved in CH2Cl2 (10 mL), washed with 5%
hydrochloric acid (10 mL) and water (10 mL), dried over MgSO4 and solvent was evaporated in
vacuum.
Products 2a–2i were purified by silica gel column chromatography using hexane–diethyl
acetate (9:1 to 4:1) as eluent. The separated catalyst was rinsed with CH2Cl2 and absolute
ethanol, dried and re-used.
3.4. Spectral data of the prepared compounds
3.4.1. 2-Phenyl-4-chromanone (2a)
Yield 95%; m.p. 75–768C, lit. (Kumar et al. 2009) m.p. 75–768C; 1H NMR (CDCl3, 100 MHz)
d: 7.95 (dd, 1H, J1 ¼ 7.7, J2 ¼ 1.5 Hz), 6.9–7.8 (m, 8H), 5.5 (dd,1H, J1 ¼ 12.3, J2 ¼ 4.6 Hz),
2.7–3.3 (m, 2H). IR (KBr, cm21) n: 3063, 3035, 1690 (CvO), 1637, 1605.
3.4.2. 2-(4-Methylphenyl)-4-chromanone (2b)
Yield 93%; m.p. 80–828C, lit. (Kavala et al. 2012) m.p. 81–838C; 1H NMR (CDCl3, 100 MHz)
d: 8.0 (dd, 1H, J1 ¼ 7.7, J2 ¼ 1.5 Hz), 7.0–7.67 (m, 7H), 5.5 (dd,1H, J1 ¼ 12.3, J2 ¼ 3.0 Hz),
2.7–3.3 (m, 2H), 2.4 (s, 3H). IR (KBr, cm21) n: 3063, 2960, 1694 (CvO), 1606, 1576.
3.4.3. 2-(4-Isopropylphenyl)-4-chromanone (2c)
Yield 83%; m.p. 778C, lit. (Hoshino & Takeno 1986) m.p. 76–778C; 1H NMR (CDCl3,
100 MHz) d: 7.9–8.1 (m, 2H), 7.20–7.67 (m, 3H), 7.0–7.2(m, 3H), 5.5 (dd, 1H, J1 ¼ 12.3,
J2 ¼ 3.0 Hz), 4.2–4.6 (m, 1H), 2.76–3.3 (m, 2H), 1.38 (m, 6H). IR (KBr, cm21) n: 3345, 3055,
2921, 1690 (CvO), 1602, 1576.
3.4.4. 2-(4-Methoxyphenyl)-4-chromanone (2d)
Yield 92%; m.p. 75–778C, lit. (Chimenti et al. 2010) m.p. 75–778C; 1H NMR (CDCl3,
100 MHz) d: 7.95 (d, 1H, J ¼ 8.3 Hz), 7.3–7.76 (m, 2H), 6.9–7.2 (m, 5H), 5.5 (dd, 1H
J1 ¼ 12.9, J2 ¼ 3.7 Hz), 3.9 (s, 3H), 2.74–3.6 (m, 2H). IR (KBr, cm21) n: 3088, 3026, 2961,
2937, 2834, 1693 (CvO), 1641, 1603.
3.4.5. 2-(3-Methoxyphenyl)-4-chromanone (2e)
Yield 80%; m.p. 78–808C, lit. (Jiang et al. 2011) m.p. 79–808C; 1H NMR (CDCl3, 100 MHz) d:
7.95 (dd, 1H, J1 ¼ 8.6, J2 ¼ 2.5 Hz), 7.25–7.63 (m, 2H), 6.88–7.2 (m, 5H), 5.5 (dd, 1H,
J1 ¼ 12.3, J2 ¼ 4.6 Hz), 3.85 (s, 3H), 2.77–3.1 (m, 2H, CH2). IR (KBr, cm21) n: 3060, 3015,
2921, 2831, 1685 (CvO), 1608, 1489.

442
H. Eshghi et al.
3.4.6. 2-(4-Chlorophenyl)-4-chromanone (2f)
Yield 93%; m.p. 92–948C, lit. (Kavala et al. 2012) m.p. 93–958C; 1H NMR (CDCl3, 100 MHz)
d: 8.0 (dd, 1H, J1 ¼ 9.2, J2 ¼ 1.5 Hz), 7.85 (m, 1H), 7.2–7.7 (m, 4H), 6.9–7.2 (m, 2H), 5.5 (dd,
1H, J1 ¼ 12.3, J2 ¼ 3.0 Hz), 2.8–3.2 (m, 2H). IR (KBr, cm21) n: 3064, 2925, 2847, 1707
(CvO), 1606, 1490.
3.4.7. 2-(2-Chlorophenyl)-4-chromanone (2g)
Yield 85%; m.p. 95–978C, lit. (Jiang et al. 2011) m.p. 96–978C; 1H NMR (CDCl3, 100 MHz) d:
8.0 (dd, 1H, J1 ¼ 9.2, J2 ¼ 1.5 Hz), 6.97–7.9 (m, 7H), 5.9 (dd, 1H, J1 ¼ 12.3, J2 ¼ 4.6 Hz),
2.7–3.0 (m, 2H). IR (KBr, cm21) n: 3068, 2921, 1691 (CvO), 1646, 1607.
3.4.8. 2-(4-Nitrophenyl)-4-chromanone (2h)
1
Yield 91%; m.p. 156–1578C, lit. (Kavala et al. 2012) m.p. 156–1578C; H NMR (CDCl3,
100 MHz) d: 7.95 (dd, 1H, J1 ¼ 7.7, J2 ¼ 1.5 Hz), 6.8–7.7 (m, 7H), 5.5 (dd, 1H, J1 ¼ 12.3,
J2 ¼ 4.6 Hz), 2.8–3.2 (m, 2H). IR (KBr, cm21) n: 3019, 2921, 2847, 1720 (CvO), 1519,
1348 (NO2).
3.4.9. 2-(3-Hydroxyphenyl)-4-chromanone (2i)
1
Yield 90%; m.p. 143–1448C, lit. (Furlong & Nudelman 1985) m.p. 141–1448C; H NMR
(CDCl3, 100 MHz) d: 6.86–8.04 (m, 8H), 5.5 (dd, 1H, J1 ¼ 11.4, J2 ¼ 4.6 Hz), 5.12 (s, 1H, OH),
2.75–3.25 (m, 2H). IR (KBr, cm21) n: 3370, 3047, 3023, 2921, 1637 (CvO), 1613, 1580.
4. Conclusions
In conclusion, this study demonstrates the application of silica-supported FHS as a catalyst for
the oxa-Michael cyclisation of 2-hydroxychalcones to the corresponding flavanone derivatives.
Also, this efficient catalyst can catalysed the one-pot direct flavanone synthesis from
2-hydroxyacetophenone and aldehydes in the presence of aniline as organic co-catalyst.
Therefore, the novelty of this method is the simplicity of the procedure, the mildness of the
reaction conditions, high yields, the ease of catalyst separation and its reusability.
Supplementary material
Supplementary material relating to this article is available online, alongside Tables S1–S4,
Schemes S1–S3 and spectral data.
Acknowledgement
The authors are grateful to Ferdowsi University of Mashhad Research Council for their financial support of
this work (Grant No. 3/20273).
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