Cu(OTf)2 catalyzed synthesis of bis(5-methyl-2-furyl)methanes by condensation of 2-methylfuran with carbonyl compounds under solvent free conditions

Article abstract of DOI:10.1002/cjoc.201180269

A facile and efficient one-pot three-component synthesis of bis(5-methyl-2-furyl)methanes has been achieved via the reaction of 2-methylfuran with a series of aliphatic and aromatic aldehydes and aliphatic ketones in presence of copper(II) triflate under solvent free conditions. The bis(5-methyl-2-furyl)methanes were obtained in 34%-72% yields and the catalyst was recycled up to four successive cycles without much loss in catalytic activity. Copyright

Full text of DOI:10.1002/cjoc.201180269

FULL PAPER
Cu(OTf)2 Catalyzed Synthesis of Bis(5-methyl-2-furyl)methanes
by Condensation of 2-Methylfuran with Carbonyl
Compounds under Solvent Free Conditions
Muthyala, Manoj Kumar
Rao, V. Kameswara
Kumar, Anil*
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, India
A facile and efficient one-pot three-component synthesis of bis(5-methyl-2-furyl)methanes has been achieved
via the reaction of 2-methylfuran with a series of aliphatic and aromatic aldehydes and aliphatic ketones in presence
of copper(II) triflate under solvent free conditions. The bis(5-methyl-2-furyl)methanes were obtained in 34%— 72%
yields and the catalyst was recycled up to four successive cycles without much loss in catalytic activity.
Keywords Lewis acid, copper(II) triflate, 2-methylfuran, bis(5-methyl-2-furyl)methane, multicomponent con-
densation, solvent free condition
Introduction
relatively long reaction time, costly catalysts, harsh re-
action conditions and toxic aqueous waste resulting
from the catalyst. The quest for cheap, environmentally
friendly catalysts and mild reaction conditions is still a
major challenge for the synthesis of bis(furyl)methanes.
Thus, development of a simple and more efficient
method is still desirable for synthesis of bis(furyl)-
methanes. Recently, there has been a growing interest in
solvent free reactions in organic synthesis.17 The ad-
vantages associated with solvent free reactions are
safety, economy, short reaction time, easy work up pro-
cedure, and greener nature.
Recent work in our laboratories has shown that
metal triflates are effective reusable catalysts for the
development of useful synthetic methods.18 Herein, we
report our efforts towards the development of environ-
mentally benign synthesis of bis(5-methyl-2-furyl)-
methanes by the condensation of 2-methylfuran with a
series of aliphatic and aromatic aldehydes and aliphatic
ketones using Cu(OTf)2 as catalyst under solvent free
conditions (Eq. 1).
Bis(furyl)methanes are industrially important com-
pounds.1 They have been used as an intermediate for the
synthesis of tetraoxaquaterenes2 and other macromole-
cules which are used as metal ions carriers.3,4 Some
compounds of this class are of interest in food industry,5
dye chemistry,6 copy engineering7 and agricultural
chemistry.8 Their synthesis is an important and chal-
lenging goal. A variety of catalytic systems have been
introduced for the synthesis of bis(2-furyl)methanes by
condensation of 2-methylfuran with carbonyl com-
pounds9 such as glacial acetic acid/phosphoric anhy-
dride, sulfonic acid-functionalized mesoporous silicas,10
MCM-41 supported Mo/Zr mixed oxides,11 AuCl3,
Hg(ClO4)2, Tl(ClO4)3 and p-TSA,12 and zeolites.13 They
have also been synthesized by acid catalyzed self-con-
densation of furfuryl alcohols, but acid-promoted
self-condensation of furfuryl alcohol gives low yield
along with a mixture of products.14,15 Recently, Geno-
vese et al.16 has reported Yb(OTf)3 catalyzed solvent
free synthesis of triaryl- and triheteroaryl-methanes
from substituted aldehydes and 2-methylfuran or meth-
oxybenzene. However, reaction of 2-methylfuran with
hexadeuteroacetone in presence of 0.7 mol% Yb(OTf)3
was reported to give only 5% conversion to
bis(2-furyl)hexadeuteropropane after 2 d.11
Due to high reactivity of furan ring towards electro-
philic aromatic substitution large amounts of tarry oli-
gomers and unidentified decomposition materials are
generally produced during the reactions of furan het-
erocycles under conventional Lewis acids. In addition to
this, these methodologies are associated with one or
more problem from environmental point of view such as
Results and discussion
In our first attempt, we selected 2-methylfuran (1)
and benzaldehyde (2a)/cyclopentanone (2k) as model
substrates for the condensation reaction catalyzed by
different metal triflates under solvent free conditions.
*
E-mail: anilkumar@bits-pilani.ac.in; Tel. 0091-1596-245073-276; Fax 0091-1596-244083
Received November 4, 2010; revised March 9, 2011; accepted March 23, 2011.
Project supported by the Council of Scientific and Industrial Research (CSIR), New Delhi (No. 01(2214)/08/EMR-II).
Chin. J. Chem. 2011, 29, 1483— 1488
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Muthyala, Rao & Kumar
FULL PAPER
Table 1 Optimization of reaction conditions for the synthesis of
Among the metal triflates used Cu(OTf)2, Yb(OTf)3,
Zn(OTf)2 and Sc(OTf)3 were found to give the product
3a or 3k in good yield and Cu(OTf)2 was found to be
most efficient among all catalysts studied to give 3a in
61% yield (Table 1, Entry 2). After finding Cu(OTf)2 as
the best catalyst for this condensation reaction, the ef-
fect of catalyst loading, solvent and reaction tempera-
ture was investigated. We explored the reaction in the
presence of 1, 5, 10, 20 and 30 mol% of Cu(OTf)2 (Ta-
ble 1, Entries 5—9). It was found that 10 mol% of
Cu(OTf)2 was enough to accomplish the reaction at
room temperature and increasing the amount of catalyst
did not obviously improve the yield. When model reac-
tion was carried out in various solvents such as water,
MeOH, PEG-400, DCM, CHCl3, DMF, acetonitrile, and
ionic liquid [bmim][BF4] using Cu(OTf)2 as catalyst, the
yield was lower than under solvent free condition. Only
CHCl3 and DCM were found to be effective reaction
media to give 2a in 18% and 22% yields, respectively,
at room temperature after 6 h. Reaction of 1 with 2a at
higher temperature resulted in sluggish reaction mixture.
It should also be pointed out here that the reaction did
not proceed in the absence of Cu(OTf)2, confirming the
effectiveness of the catalyst.
bis(furyl)methanesa
Entry Substrate
1
Catalyst (mol%)
Zn(OTf)2 (10)
Time/h Yieldb/%
6
6
6
8
45
61
40
34
2
3
4
Cu(OTf)2 (10)
Yb(OTf)3 (10)
In(OTf)3 (10)
5
6
Cu(OTf)2 (1)
Cu(OTf)2 (5)
Cu(OTf)2 (10)
Cu(OTf)2 (20)
Cu(OTf)2 (30)
Zn(OTf)2 (10)
Sc(OTf)3 (10)
Yb(OTf)3 (10)
In(OTf)3 (10)
La(OTf)3 (10)
Eu(OTf)3 (10)
6
6
21
36
51
48
45
36
33
30
16
1
The product 3a was characterized by H NMR, 13C
7
6
NMR, and mass spectral data. It showed a peak at m/z
291.0156 for [M＋K]＋ in MS and a characteristic
1
singlet peak at δ 5.33 in H NMR for ethane proton,
8
6
peak at δ 45.27 in 13C NMR for methane carbon along
with other aromatic and aliphatic protons and carbons.
After success of the model reaction, the condensa-
tion reaction of 1 was carried out with other aldehydes
and aliphatic ketones (2b—2n) to give corresponding
bis(furyl)methanes (3b—3n) in moderate to good yield
(Table 2). The structures of 3a—3n were confirmed by
spectral analysis and all the products showed satisfac-
tory spectral data (see supporting information). Aro-
matic aldehydes with electron withdrawing substituent
gave lower yield than aromatic aldehydes with electron
donating groups (Table 2, Entry 7). This may be due to
stabilization of intermediate 5 by the electron releasing
substituents in aryl ring. It is also noteworthy to mention
that acetophenone and benzophenone did not result in
the formation of expected products. This is in accor-
dance to the literature reports that furan reacts with
some methyl ketones in the presence of hydrochloric
acid, to give anhydrotetramers, but neither acetophe-
none nor pinacolone reacts.19
9
6
10
11
12
13
14
15
6
6
8
8
c
36
36
—
12
a
Reaction conditions: 2-methylfuran (2.0 mmol), carbonyl com-
pound (1.0 mmol), and catalyst. Isolated yield. c No product
b
It is expected that the reaction mechanism is similar
to the previously reported acid catalyzed mechanism.9 A
plausible mechanistic pathway for the reactions that
form the bis(5-methyl-2-furyl)methanes is shown in
Scheme 1.
The reaction proceeds through furfuryl alcohol de-
rivative followed by condensation with another mole-
cule of carbonyl compound to give the desired product.
However, we did not succeed in isolating (5-methyl-
formation was observed, starting material recovered.
furan-2-yl) (phenyl)methanol (4a), instead we observed
a peak at m/z 171.0717 in mass analysis of reaction
mixture for reaction of 1 with 2a which corresponds to
C12H11O＋ ion (5a) (calculated 171.0810) generated
from 4a and thus confirmed formation of 4a as interme-
diate. Similar observations were also made in the reac-
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Chin. J. Chem. 2011, 29, 1483— 1488

Cu(OTf)2 Catalyzed Synthesis of Bis(5-methyl-2-furyl)methanes
Table 2 Cu(OTf)2-promoted reaction of 2-methylfuran (1) with carbonyl compounds 2a— 2n at room temperature
Entry
1
Carbonyl compound
Substrate 2
Producta
Product 3
Time/h
Yieldb/%
2a
3a
6
61
2
3
2b
2c
3b
3c
6
6
63
65
4
2d
3d
6
57
5
6
2e
2f
3e
3f
6
6
8
8
8
6
66
59
57
60
58
75
7
2g
2h
2i
3g
3h
3i
8
9
10
2j
3j
Chin. J. Chem. 2011, 29, 1483— 1488
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Muthyala, Rao & Kumar
Continued
FULL PAPER
Entry
Carbonyl compound
Substrate 2
Producta
Product 3
Time/h
Yieldb/%
11
2k
3k
6
51
12
13
14
2l
2m
2n
3l
3m
3n
6
6
8
61
55
34
a All products showed satisfactory 1H NMR, 13C NMR and mass spectroscopic data. b Isolated yield.
Scheme 1 Proposed mechanism
dehyde (2a) as model substrates. After extracting the
product 3a in organic layer, the aqueous layer was con-
centrated and dried to recover the catalyst. This recov-
ered catalyst was again used for the reaction of 1 and 2a
and this was repeated three times to give 3a in 56%,
53% and 52% yields, respectively. Thus, the catalyst
showed good catalytic activity without noticeable de-
crease in yield of the product up to four successive cy-
cles.
tion of 1 with 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2k and 2l with
peaks at m/z 201.0871 (calculated 201.0916), 201.0872
(calculated 201.0916), 231.1030 (calculated 231.1021),
205.0409 (calculated 205.0420), 185.0918 (calculated
185.0966), 123.0883 (calculated 123.0804), 137.0752
(calculated 137.0961), 149.0752 (calculated 149.0961),
and 123.0883 (calculated 123.0810), respectively, sug-
gesting formation of corresponding furfuryl alcohols as
intermediate. Further, evidence for the intermediate 4a
in this pathway was gained by independent synthesis of
4a from 2-methylfuran and benzaldehyde in the pres-
ence of butyl lithium followed by reaction of 4a with
2-methylfuran catalyzed by Cu(OTf)2 to give 3a.
Conclusions
In summary, we have developed a facile and effi-
cient method for the synthesis of bis(5-methyl-2-furyl)-
methanes from the reaction of 2-methylfuran and car-
bonyl compounds catalyzed by Cu(OTf)2 under solvent
Finally, the reusability of the recovered catalyst was
investigated by using 2-methylfuran (1a) and benzal-
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Chin. J. Chem. 2011, 29, 1483— 1488

Cu(OTf)2 Catalyzed Synthesis of Bis(5-methyl-2-furyl)methanes
free conditions. The main advantages of this method are
mild, clean and solvent-free reaction conditions, moder-
ate to good yields and an environmentally benign cata-
lyst. This reaction system not only provides a novel
method for the synthesis of bis(furyl)methanes but also
is an environmentally friendly chemical process.
45.26, 13.86; HRMS calcd for C18H18O3 282.125＋6,
found 201.0871 [M－C5H5O]＋, 305.1178 [M＋Na] ,
321.0967 [M＋K]＋.
2-[(2-Methoxyphenyl)(5-methylfuran-2-yl)meth-
yl]-5-methylfuran (3d) 1H NMR (CDCl3, 400 MHz)
δ: 7.27—7.24 (m, 1H), 7.16—7.14 (m, 1H), 6.92—6.90
(m, 2H), 5.88—5.84 (m, 5H), 3.84 (s, 3H), 2.26 (s, 6H);
13C NMR (CDCl3, 100 MHz) δ: 156.67, 153.00, 151.22,
129.31, 128.25, 128.11, 120.51, 110.70, 108.03, 106.00,
55.70, 37.48, 13.74; MS (ESI) calcd for C18H18O3
282.1256, found 201.0872 [M－C5H5O]＋ , 305.1177
[M＋Na]＋, 321.0968 [M＋K]＋.
Experimental section
Physical measurements and materials
The 1H NMR and 13C NMR spectra were obtained at
400 and 100 MHz, respectively, and recorded on a
Bruker-400 instrument with TMS as an internal stan-
dard and CDCl3 as the solvent. Mass spectra (ESI-MS)
were recorded using a QSTAR® Elite LX/MS/MS mass
spectrometer from applied biosystems. Column chro-
matography was carried out over silica gel (100—200
mesh, S. D. Fine, India) and TLC was performed using
silica gel GF254 (Merck) plates. The chemicals, re-
agents and solvents were purchased either from Spec-
trochem, India or Sigma-Aldrich and used as received.
2-[(3,4-Dimethoxyphenyl)(5-methylfuran-2-yl)-
methyl]-5-methylfuran (3e) 1H NMR (CDCl3, 400
MHz) δ: 6.83—6.79 (m, 3H), 5.89—5.87 (m, 4H), 5.29
(s, 1H), 3.87 (s, 3H), 3.84 (s, 3H), 2.26 (s, 6H); 13C
NMR (CDCl3, 100 MHz) δ: 153.06, 151.44, 147.95,
132.51, 120.42, 111.72, 111.02, 108.36, 106.07, 55.85,
44.72, 13.68; MS (ESI) calcd for C19H20O4 312.1362,
found 231.1030 [M—C5H5O]＋, 335.1145 [M＋Na]＋,
647.2627 [2M＋Na]＋.
2-Methyl-5-[(5-methylfuran-2-yl)(p-tolyl)methyl]-
furan (3f) 1H NMR (CDCl3, 400 MHz) δ: 7.17—7.12
(m, 4H), 5.88—5.87 (s, 4H), 5.31 (s, 1H), 2.34 (s, 3H),
2.25 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ: 153.25,
151.54, 137.22, 136.70, 129.32, 128.43, 108.21, 106.21,
44.93, 21.28, 13.82; MS (ESI) calcd for C18H18O2
266.1307, found 185.0918 [M－C5H5O]＋ , 289.1176
[M＋Na]＋, 305.0962 [M＋K]＋.
General procedure for the synthesis of bis(5-methyl-
2-furyl)methanes
A mixture of 2-methylfuran (2.0 mmol), carbonyl
compound (1.0 mmol) and Cu(OTf)2 (10 mol%) was
vigorously stirred at room temperatures for 6—36 h (as
mentioned in Tables 1 & 2) under solvent free condi-
tions. After completion of the reaction, water (5.0 mL)
was added to the reaction mixture and resulting solution
was extracted with ethyl acetate (5 mL×3). The com-
bined organic phase was dried with anhydrous sodium
sulfate and concentrated under reduced pressure. The
crude residue was purified by passing through a bed of
silica gel using ethyl acetate and hexane as eluent.
Spectral data of selected bis(furyl)methanes are
given below.
2-[(3-Chlorophenyl)(5-methylfuran-2-yl)methyl]-
5-methylfuran (3g) 1H NMR (CDCl3, 400 MHz) δ:
7.24—7.19 (m, 3H), 7.16—7.10 (m, 1H), 5.88 (s, 4H),
5.30 (s, 1H), 2.24 (s, 6H); 13C NMR (CDCl3, 100 MHz)
δ: 151.99, 151.77, 142.06, 129.72, 128.58, 127.26,
126.68, 108.51, 106.20, 99.64, 44.77, 13.68; MS (ESI)
calcd for C17H15ClO2 286.0761, found 105.0409 [M－
C5H5O]＋, 309.0598 [M＋Na]＋.
2-Methyl-5-[(5-methylfuran-2-yl)(phenyl)meth-
yl]-furan (3a) 1H NMR (CDCl3, 400 MHz) δ: 7.31—
7.29 (m, 2H), 7.26—7.24 (m, 3H), 5.88—5.86 (m, 4H),
5.33 (s, 1H), 2.25 (s, 6H); 13C NMR (CDCl3, 100 MHz)
δ: 153.00, 151.61, 140.17, 128.59, 128.55, 127.12,
108.34, 106.22, 45.27, 13.80; HRMS calcd for C17H16O2
252.1150, found 291.0156 [M＋K]＋.
2-Methyl-5-[1-(5-methylfuran-2-yl)propyl]furan
(3h) 1H NMR (CDCl3, 400 MHz) δ: 5.94—5.84 (m,
4H), 3.85 (t, J＝6.71 Hz, 1H), 2.27 (s, 6H), 2.04—1.95
(m, 2H), 0.93 (t, J＝6.64 Hz, 3H); 13C NMR (CDCl3,
100 MHz) δ: 156.39, 153.04, 108.57, 108.28, 43.09,
28.56, 16.00, 14.49; MS (ESI) calcd for C13H16O2
204.1150, found 123.0883 [M－C5H5O]＋ , 205.0081
[M＋Na]＋.
2-[(4-Methoxyphenyl)(5-methylfuran-2-yl)meth-
yl]-5-methylfuran (3b) 1H NMR (CDCl3, 400 MHz)
δ: 7.13 (d, J＝7.46 Hz, 2H), 6.81 (d, J＝7.46 Hz, 2H),
5.91—5.86 (m, 4H), 5.32 (s, 1H), 2.26 (s, 6H); 13C
NMR (CDCl3, 100 MHz) δ: 157.43, 153.06, 151.72,
138.24, 128.41, 128.11, 126.56, 109.84, 108.01, 56.26,
45.27, 13.83; MS (ESI) calcd for C18H18O3 282.1256,
found 289.1176 [M＋Li]＋, 305.1162 [M＋Na]＋.
2-Methyl-5-[1-(5-methylfuran-2-yl)butyl]furan (3i)
1H NMR (CDCl3, 400 MHz) δ: 5.94 (s, 2H), 5.88 (s,
2H), 3.96 (t, J＝6.71 Hz, 1H), 2.27 (s, 6H), 1.94 (t, J＝
6.71 Hz, 2H), 1.34—1.33 (m, 2H), 0.93 (t, J＝6.64 Hz,
3H); 13C NMR (CDCl3, 100 MHz) δ: 154.19, 150.63,
106.02, 105.90, 38.73, 35.15, 20.63, 13.90, 13.64; MS
(ESI) calcd for C14H18O2 218.1307, found 137.0752
[M－C5H5O]＋, 241.1987 [M＋Na]＋.
2-[(3-Methoxyphenyl)(5-methylfuran-2-yl)meth-
yl]-5-methylfuran (3c) 1H NMR (CDCl3, 400 MHz) δ:
7.25—7.21 (m, 1H), 6.86—6.79 (m, 3H), 5.88 (s, 4H),
5.31 (s, 1H), 3.77 (s, 3H), 2.55 (s, 6H); 13C NMR
(CDCl3, 100 MHz) δ: 159.80, 152.84, 151.61, 141.73,
129.54, 121.01, 114.47, 112.40, 108.36, 106.25, 55.34,
2-Methyl-5-(1-(5-methylfuran-2-yl)cyclohexyl)fu-
ran (3j) 1H NMR (CDCl3, 400 MHz) δ: 5.85 (s, 4H),
2.25 (s, 6H), 2.12 (t, J＝6.40 Hz, 4H), 1.53—1.47 (m,
6H); 13C NMR (CDCl3, 100 MHz) δ: 157.83, 150.42,
105.94, 105.75, 41.77, 34.14, 26.07, 22.70, 13.82; MS
Chin. J. Chem. 2011, 29, 1483— 1488
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Muthyala, Rao & Kumar
FULL PAPER
(ESI) calcd for C16H20O2 244.1463, found 185.1218
[M＋Na－C5H6O]＋ , 283.1164 [M＋K]＋ , 306.0962
[M＋Na＋K]＋.
7
8
9
Fukuda, M.; Fukunishi, A.; Mori, M. JP 06171214, 1994.
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2-Methyl-5-(1-(5-methylfuran-2-yl)cyclopentyl)-
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(m, 4H), 2.24 (s, 6H), 1.72 (t, J＝6.74 Hz, 4H), 1.26 (t,
J＝6.74 Hz, 4H); 13C NMR (CDCl3, 100 MHz) δ:
157.36, 150.78, 105.90, 105.65, 47.77, 37.08, 24.16,
13.84; MS (ESI) calcd for C15H18O2 230.1307, found
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100 MHz) δ: 159.22, 150.57, 105.87, 104.67, 26.64,
13.79; MS (ESI) calcd for C13H16O2 204.1150, found
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2-Methyl-5-(2-(5-methylfuran-2-yl)butan-2-yl)fu-
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(m, 4H), 2.25 (s, 6H), 1.98 (q, J＝6.81 Hz, 2H), 1.53 (s,
3H), 0.79 (t, J＝6.82 Hz, 3H); 13C NMR (CDCl3, 100
MHz) δ: 157.96, 150.64, 105.79, 105.62, 41.38, 31.86,
22.58, 13.82, 9.04; HRMS calcd for C14H18O2 218.1307,
found 241.1925 [M＋Na]＋, 257.0939 [M＋K]＋.
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methyl)-furan (3n) 1H NMR (CDCl3, 400 MHz) δ:
8.14 (m, 2H), 7.62—7.60 (m, 2H), 7.52—7.48 (m, 2H),
5.95—5.93 (m, 4H), 5.45 (s, 1H), 2.27 (s, 6H); 13C
NMR (CDCl3, 100 MHz) δ: 152.12, 151.16, 142.25,
134.64, 129.37, 123.48, 122.21, 108.86, 106.32, 44.70,
13.64.
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