Synthesis of ortho-hydroxybenzophenones catalyzed by magnetically retrievable Fe3O4 nanoparticles under ligand-free conditions

Article abstract of DOI:10.1002/ejoc.201300909

ortho-Benzoylation of phenols with aryl aldehydes to afford the substituted 2-hydroxybenzophenones can be catalyzed efficiently by Fe₃O ₄ nanoparticles under ligand-free conditions. This method is useful for the preparation of xanthones in good yields. The catalyst can be magnetically removed and recycled easily over four cycles without a significant decrease in activity. The applicability of magnetically retrievable Fe ₃O₄ nanoparticles in the oxidative coupling of aryl aldehydes with phenols under ligand free conditions is demonstrated.

Full text of DOI:10.1002/ejoc.201300909

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300909
Synthesis of ortho-Hydroxybenzophenones Catalyzed by Magnetically
Retrievable Fe3O4 Nanoparticles under Ligand-Free Conditions
Thekkathu Ramani,*[a] Paspulati Umadevi,[a] K. Leon Prasanth,*[b] and Bojja Sreedhar*[a]
Keywords: Iron / Nanoparticles / Benzoylation / Lewis acids / Heterogeneous catalysis
ortho-Benzoylation of phenols with aryl aldehydes to afford
the substituted 2-hydroxybenzophenones can be catalyzed
efficiently by Fe3O4 nanoparticles under ligand-free condi-
tions. This method is useful for the preparation of xanthones
in good yields. The catalyst can be magnetically removed
and recycled easily over four cycles without a significant de-
crease in activity.
are not suitable substrates because of the formation of unfa-
vorable four-membered metallacycles.[12] In this context,
Miura and co-workers showed the benzoylation reaction of
unprotected 1-naphthol by using aldehydes as benzoylating
agents for the first time. They synthesized 2-benzoyl-1-
naphthol in the presence of Pd(OAc)2 as the catalyst and
PPh3 as the ligand.[13] Later, the Wang group reported the
ortho-benzoylation of phenols with arenecarbaldehydes by
using CuCl2 as a catalyst in combination with PPh3 as the
ligand under basic conditions.[14] Although the above re-
sults are promising, for eco-friendly reasons, it is necessary
to develop a ligand-free and reusable catalytic system for
the synthesis of o-hydroxybenzophenones.
Magnetic nanoparticles have been widely used as hetero-
geneous catalysts in various organic transformations.
Among the various magnetic nanoparticles under investiga-
tion, Fe3O4 nanoparticles (NPs) are arguably the most ex-
tensively studied.[15] Our group has paid much attention to
nanoparticle-catalyzed organic reactions such as Hiyama
cross-couplings, the amination of aryl iodides, the synthesis
of propargylamines and tetrazoles, and so on.[16] In contin-
uation of our work on magnetic nanoparticles, we herein
report the synthesis of o-hydroxybenzophenones under li-
gand-free conditions (Scheme 1). The experimental pro-
cedure is very simple, and the catalyst can be removed easily
by an external magnet to be recycled.
Introduction
ortho-Hydroxybenzophenones are prevalent motifs in nu-
merous natural products[1] and in biologically active phar-
maceuticals.[2] They have been used as light stabilizers, UV
absorbers in cosmetics and textiles, and as antianaphylactin
in medicines.[3] The most promising methods for their syn-
thesis include Friedel–Crafts acylation and esterification
(O-acylation) of phenols followed by Fries rearrangement.[4]
However, these methods often suffer from harsh reaction
conditions and require the handling of air-/water-sensitive
Lewis acids; moreover, the reactions result in the formation
of a mixture of (ortho and para) products, the separation
of which is complicated.[5] To circumvent these drawbacks,
various alternative methods have been developed in which
transition-metal-catalyzed cross-coupling reactions provide
an excellent way to convert a C–H bond into C–O,[6] C–
N,[7] C–X,[8] C–S,[9] and C–C bonds.[10] Recently, remark-
able efforts were made in cross-dehydrogenative coupling
(CDC) reactions and oxidative cross-coupling reactions by
utilizing two different hydrocarbons as the reagents in a
green, atom-economic, and sustainable manner.[11] The de-
velopment of highly efficient and selective catalytic systems
in the oxidative coupling of an aldehyde C–H bond with
other C–H bonds, and especially the unprotected Csp2–H
bonds in phenols, is still a great challenge, as simple phenols
[a] Inorganic and Physical Chemistry Division, Indian Institute of
Chemical Technology,
Hyderabad 500607, India
E-mail: ramaniiict@gmail.com
sreedharb@iict.res.in
Homepage: www.iictindia.org
[b] Department of Chemistry, The Zamorin’s Guruvayurappan
College,
Calicut 673014, India
E-mail: leonprasanthk@gmail.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300909.
Scheme 1. Synthesis of o-hydroxybenzophenones.
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T. Ramani, P. Umadevi, K. L. Prasanth, B. Sreedhar
SHORT COMMUNICATION
talline nature of both the fresh and used magnetic nanopar-
ticles.
Results and Discussion
The Fe3O4 nanoparticles were prepared according to a
previously established procedure[17] and were characterized
by X-ray diffraction (XRD), transmission electron micro-
scopy (TEM), and X-ray photoelectron spectroscopy
(XPS). The acidity measurements of the Fe3O4 nanopar-
ticles were performed by temperature-programmed desorp-
tion (TPD) of NH3 and pyridine adsorption followed by
FTIR spectroscopy. The XRD patterns of fresh and used
Fe3O4 nanoparticles are shown in Figure 1.
Elemental analysis and the relative composition of Fe3+
and Fe2+ were determined by using XPS. Fe3O4 is a mixed-
valence compound with a conventional notation of [Fe3+]tet-
[Fe2+Fe3+
]octO4. It is of cubic inverse spinel crystal struc-
ture; the oxygen anions (O2–) form a closely packed face-
centered cubic (fcc) sublattice in which the iron cations are
located in the interstitial sites. There are two different kinds
of cation sites: tetrahedrally coordinated sites occupied by
Fe3+ and octahedrally coordinated sites occupied by Fe3+
and Fe2+ in equal numbers.[18] The Fe2+cation can be con-
sidered to be Fe3+ plus an “extra” electron, and it un-
dergoes rapid valence oscillation between the FeIII and FeII
octahedral sites. The survey scan of the Fe3O4 nanoparticles
shows peaks characteristic of C 1s (284.6 eV), O 1s
(529.9 eV), and Fe 2p (710 eV)[19] (Figure 3). As can be seen
in the inset of Figure 3, high-resolution narrow scans corre-
sponding to Fe 2p of both the fresh and used catalyst could
be deconvoluted into two peaks. The peaks centered at 710
and 723.6 eV correspond to Fe 2p3/2 and Fe 2p1/2, respec-
tively, and are due to Fe in the +2 oxidation state.[19] The
peaks centered around 711.9 and 725.8 eV, corresponding
to Fe 2p3/2 and Fe 2p1/2, respectively, are consistent with
typical values for ferric oxides.[19]
Figure 1. XRD spectra of (a) fresh and (b) used Fe3O4 nanopar-
ticles.
The XRD pattern recorded with Cu-Kα radiation shows
characteristic peaks at 2θ = 30.25, 35.50, 43.38, 53.69,
57.23, and 62.77°, which correspond to (220), (311), (400),
(422), (511), and (440) Bragg reflections, respectively. The
XRD pattern agrees with standard magnetite (Fe3O4)
(JCPDS: 65-3107), which identifies that the Fe3O4 nanopar-
ticles adopt a cubic spinel structure.[18] The size of the par-
ticles was calculated by the Debye–Scherrer formula on the
basis of the full width at half-maximum of the high-inten-
sity diffraction peak at 2θ = 35.50° and was found to be
approximately 19 nm. This is in agreement with TEM
analysis (Figure 2), which shows mainly spherical nanopar-
ticles with a particle size in the range of 10–30 nm. Notably,
the morphology and size of the particles were unaltered
even after five cycles, which correlates well with the reten-
tion of catalytic activity after recycling. The selected-area
electron diffraction (SAED) images also confirm the crys-
Figure 3. XPS survey scans for (a) fresh and (b) used Fe3O4 nano-
particles [inset shows the high-resolution narrow scans for Fe 2p
(a) fresh and (b) used Fe3O4 nanoparticles].
The total acidity of the Fe3O4 nanoparticles was mea-
sured by TPD of ammonia as well as pyridine adsorption
followed by FT-TR spectroscopy. The desorption tempera-
ture and amount of ammonia desorbed are considered as
indexes of acid strength and the total number of acid sites,
respectively. A TPD peak at about 400 °C arises from de-
sorption of ammonia from moderately strong acid sites
(Figure S5, Supporting Information).[19c] The FTIR spec-
trum of the pyridine adsorbed Fe3O4 nanoparticles shows
a band at 1440 cm–1, which is commonly assigned to Lewis
acid sites. A trace amount of Brønsted acid sites is also
present, which is indicated by the broad peak at 1560 cm–1.
However, the main contribution of acidity is due to Lewis
acid sites (Figure S4, Supporting Information).[19d]
Figure 2. TEM and SAED images of Fe3O4 nanoparticles (a) be-
fore and (b) after use.
2
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Synthesis of ortho-Hydroxybenzophenones
The numerous advantages of iron make it highly attract- dehydes. Phenol substituted with an electron-donating tert-
ive as a catalyst or reagent for chemical synthesis.[20] The butyl group (i.e., 2b) offered a higher yield than unsubsti-
challenge of selective functionalization of phenols tuted phenol (i.e., 2a); conversely with electron-deficient 4-
prompted us to investigate the reaction of phenol (1a) with nitrophenol (2c), no reaction was detected with aldehydes
4-nitrobenzaldehyde (2a) in the presence of an iron catalyst containing either an electron-withdrawing group (Table 2,
(Table 1). It was found that Fe3O4 nanoparticles gave the Entry 10) or an electron-donating group (Table 2, En-
best result as a catalyst in the reaction (Table 1, Entry 7), try 11). This is not unexpected, as electron-donating substit-
whereas anhydrous (anhy.) FeCl3, Fe(acac)3 (acac = acetyl- uents in the para position of phenol decrease the bond
acetonate), Fe powder, and Fe3O4 showed low catalytic ac- dissociation energies (BDEs) of the O–H bonds of substi-
tivity (Table 1, Entries 3–6). Fe(OAc)2 and FeCl3·6H2O tuted phenols by raising the ground-state energies. Alterna-
failed to give any oxidative coupling product (Table 1, En- tively, electron-withdrawing substituents in the para posi-
tries 1 and 2).
tion interact with the O–H dipoles and cause a lowering of
the ground-state energies, which thereby increases the
BDE(O–H) values.[21]
Table 1. Optimization of reaction conditions.[a]
Table 2. ortho-Benzoylation of phenols with aryl aldehydes.[a]
[a] Reaction conditions: 1a (1.3 mmol), 2a (1 mmol), catalyst
(20 mol-%), base (2.2 mmol), solvent (3 mL). [b] Isolated yield.
[c] In the absence of a catalyst. [d] In the absence of base.
[a] Reaction conditions: Fe3O4 (20 mol-%), aldehyde (1 mmol),
phenol (1.3 mmol), K2CO3 (2.2 mmol), toluene (3 mL), 110 °C, air,
24 h. [b] Isolated yield.
Later, we optimized the reaction conditions further by
When 2-substituted arenecarbaldehydes were treated
screening different solvents, bases, and temperatures. Fi- with phenols, xanthones were obtained in good yields in
nally, the optimal conditions were obtained, that is, catalyst one step (Table 3). Xanthones constitute the core of many
(20 mol-%), 1a (1.3 equiv.), 2a (1 equiv.), K2CO3 natural products and biologically active compounds. They
(2.2 equiv.), toluene (3 mL), and the reaction was per- exhibit diverse physicochemical and pharmacological prop-
formed at 110 °C for 24 h.
erties, including antitumoral, antibacterial, and anti-inflam-
Under these optimized conditions, the reaction scope matory activities.[22] 2-Nitrobenzaldehyde (1h) produced the
was explored with various arenecarbaldehydes and phenols corresponding xanthone in good yields (Table 3, Entries 1
(Table 2). Arenecarbaldehydes with electron-withdrawing and 4), but 2-chloro- and 2-bromobenzaldehydes gave lower
and electron-donating substituents gave the desired prod- yields, which is likely due to a cross-coupling reaction be-
ucts in almost similar yields when treated with the same tween the halogen and the hydroxy groups (Table 3, En-
phenol [40–64% (Table 2, Entries 1–7) vs. 50–67% (Table 2, tries 2 and 3). It is believed that, first, the 2-substituted al-
Entries 8 and 9)]. However, the results showed that substitu- dehyde reacts with phenol to give the corresponding ortho-
ents on the phenol played an important role, which is in benzoylation product and, second, under basic conditions,
contrast to what was observed for substituted arenecarbal- the ortho-substituent of the aldehydes serves as a leaving
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T. Ramani, P. Umadevi, K. L. Prasanth, B. Sreedhar
SHORT COMMUNICATION
group, which leads to the final product. Unfortunately, the
present method is not amenable to aliphatic aldehydes.
Table 3. One-step synthesis of xanthones.[a]
Scheme 2. A plausible mechanism.
Conclusions
We have developed a simple method for the synthesis of
2-hydroxybenzophenones through ortho-benzoylation by
using magnetically retrievable Fe3O4 nanoparticles as the
catalyst under ligand-free conditions. This method is also
useful for the preparation of xanthones in good yields. The
simple procedure for the preparation of the catalyst and its
easy recovery and reusability are expected to contribute to
its utilization in the development of benign chemical pro-
cesses and products.
[a] Reaction conditions: Fe3O4 (20 mol-%), aldehyde (1 mmol),
phenol (1.3 mmol), K2CO3 (2.2 mmol), toluene (3 mL), 110 °C, air,
24 h. [b] Isolated yield. [c] 1-Naphthol.
To check the recyclability of the catalyst, as can be seen
from Table 4, the reaction was performed with 4-nitrobenz-
aldehyde and phenol under the optimized reaction condi-
tions. After completion of the reaction, the catalyst was eas-
ily separated from the reaction mixture with an external
magnet and could be reused in the next cycle. The Fe3O4
nanoparticles were used over five runs, and no obvious loss
in the catalytic activity was observed. This indicated that
the Fe3O4 nanoparticle catalyst had great recyclability in
this reaction.
Experimental Section
Preparation of Fe3O4 Nanoparticles:[17] FeSO4·7H2O (13.9 g) and
Fe2(SO4)3 (20 g) were dissolved in H2O (500 mL) in a 1000 mL bea-
ker. NH4OH (aq., 25%) was added slowly to adjust the acidity of
the solution to pH = 10. The reaction mixture was then continually
stirred at 60 °C for 1 h. The precipitated nanoparticles were sepa-
rated magnetically, washed with water until pH = 7, and then dried
under vacuum at 60 °C for 2 h.
Table 4. Recycling of the catalyst.[a]
General Procedure for ortho-Benzoylation of Phenols with Arenecar-
baldehydes: Phenol (1.3 equiv.), arenecarbaldehyde (1 equiv.),
K2CO3 (2.2 equiv.), and the Fe3O4 nanoparticles (20 mol-%) were
added to toluene (3 mL), and the reaction mixture was stirred at
110 °C for 24 h. Upon completion of the reaction, the catalyst was
easily separated from the reaction mixture with an external magnet.
Then, the reaction mixture was extracted with dichloromethane
(3ϫ 10 mL) and washed with brine, and the combined organic lay-
ers were dried with anhydrous Na2SO4. The organic phase was con-
centrated under vacuum, and the residue was purified by flash col-
umn chromatography on silica gel to afford the pure product.
Run
Yield [%][b]
Run
Yield [%][b]
1
2
3
55
52
51
4
5
50
48
[a] Reaction conditions: 4-nitrobenzaldehyde (1 mmol), phenol
(1.3 mmol), Fe3O4 (20 mol-%), K2CO3 (2.2 mmol), toluene (3 mL).
[b] Isolated yield.
Regarding the mechanism, it is generally believed that
this reaction is a Friedel–Crafts-type reaction that involves
the nucleophilic addition of phenols to aldehydes under ba-
sic conditions, and this is then followed by dehydrogenative
oxidation in air to give the ortho-benzoylation prod-
ucts.[14,23] During this process, the Fe3O4 nanoparticles act
as a Lewis acid to activate the carbonyl group and stabilize
the reactive intermediates. The presence of Lewis acidic
sites on the surface of Fe3O4 nanoparticles is confirmed
from FTIR and TPD of NH3 studies. A plausible mecha-
nism is shown in Scheme 2. However, for aldehydes having
good leaving groups as ortho substituents, ring-closed xan-
thones will form automatically.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, data for the ortho-benzoylation
1
products and the xanthones, and H and 13C NMR spectra for all
compounds.
Acknowledgments
T. R. thanks the Department of Science and Technology (DST) for
a WOS-A fellowship [DST No: SR/WOS-A/CS-115/2011(G)].
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Synthesis of ortho-Hydroxybenzophenones
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Job/Unit: O30909
/KAP1
Date: 20-08-13 11:33:17
Pages: 7
T. Ramani, P. Umadevi, K. L. Prasanth, B. Sreedhar
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Received: June 21, 2013
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30909
/KAP1
Date: 20-08-13 11:33:17
Pages: 7
Synthesis of ortho-Hydroxybenzophenones
Oxidative Coupling
The applicability of magnetically retriev-
able Fe3O4 nanoparticles in the oxidative
coupling of aryl aldehydes with phenols
under ligand free conditions is demon-
strated.
T. Ramani,* P. Umadevi, K. L. Prasanth,*
B. Sreedhar* ..................................... 1–7
Synthesis of ortho-Hydroxybenzophenones
Catalyzed by Magnetically Retrievable
Fe3O4 Nanoparticles under Ligand-Free
Conditions
Keywords: Iron / Nanoparticles
/ Ben-
zoylation / Lewis acids / Heterogeneous ca-
talysis
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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