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Ru(II) complexes bearing tertiary phosphine ligands a novel and efficient homogeneous catalyst for one-pot synthesis of dihydropyrano[3 2-c]chromene and tetrahydrobenzo[b]pyran derivatives.

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Full Paper
Received: 23 August 2011
Revised: 6 November 2011
Accepted: 25 November 2011
Published online in Wiley Online Library: 10 January 2012
(wileyonlinelibrary.com) DOI 10.1002/aoc.1866
Ru(II) complexes bearing tertiary phosphine
ligands: a novel and efficient homogeneous
catalyst for one-pot synthesis of
dihydropyrano[3,2-c]chromene and
tetrahydrobenzo[b]pyran derivatives
Khalil Tabatabaeian*, Hannaneh Heidari, Manouchehr Mamaghani and
Nosrat O. Mahmoodi
A number of ruthenium complexes were prepared and their catalytic activities in three-component one-pot condensation of
aldehydes, malononitrile and 4-hydroxycoumarin or dimedone was considered to afford dihydropyrano[3,2-c]chromenes and
tetrahydrobenzo[b]pyran derivatives under optimum reaction conditions. We found that a catalytic amount of RuBr2(PPh3)4
efficiently promotes the reaction in a short time (3–15 min) and with high yield (75–88%). Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: ruthenium; one-pot synthesis; dihydropyrano[3,2-c]chromene; tetrahydrobenzo[b]pyran
Introduction
56
Multicomponent reactions (MCRs) are among the most efficient
synthetic strategies for the production of highly functionalized
heterocyclic and biologically active compounds and have attracted
the attention of researchers in organic, combinatorial and
medicinal chemistry. In addition, dihydropyrano[3,2-c]chromenes
and tetrahydrobenzo[b]pyran derivatives have received considerable attention because of numerous biologically important
and pharmacological activities.[1] Several synthesis methods
for the preparation of these heterocyclic compounds have
been reported in the presence of different catalysts such as
diammonium hydrogen phosphate,[2] K2CO3 under microwave
irradiation,[3] tetrabutylammonium bromide (TBAB),[4] ion liquids,[5,6]
heteropolyacids,[7] hexamethylenetetramine,[8] KF/Al2O3,[9] sodium
dodecyl sulfate (SDS),[10] 1,8-diazabicyclo[5.4.0] undec-7-ene
(DBU),[11] trisodium citrate,[12] high surface area MgO,[13] K3PO4,[14]
Na2SeO4,[15] NaBr,[16] sulfonic acid functionalized silica,[17]
hexadecyldimethylbenzyl ammonium bromide (HDMBAB),[18]
(S)-proline,[19] or by electrochemical reactions.[20]
However, each of the above procedures has its own
disadvantages such as long reaction time, low yield, tedious
workup and so forth. Because of their wide applications, we would
like to further develop an efficient and useful method and
introduce a new homogeneous catalyst to construct such
significant heterocyclic compounds.
On the other hand, many organic transformations which
involve organometallic ruthenium species as catalyst are
known and well documented,[21] and the investigation of the
chemistry of ruthenium continues to be an active area of
organometallic chemistry.
Therefore, as part of our continued studies on ruthenium compound catalyzed organic reactions, this present study focused on
Appl. Organometal. Chem. 2012, 26, 56–61
a comparison of catalytic activity among several ruthenium
(oxidation states 0, +2, +3) compounds such as Ru3(CO)12, RuBr2
(PPh3)4, RuCl2(PPh3)3 and RuCl3.xH2O. The effect of amount of
catalyst, solvent and reaction temperature on the yield of
reaction is also considered too. To the best of our knowledge,
this is the first report of using ruthenium compounds as
homogeneous catalysts to catalyze this type of MCR. It can
perform the reaction to produce the main products in good purity
and high yields.
Experimental
All reactions were followed by thin-layer chromatography (TLC)
with detection by UV light. IR spectra were obtained in KBr
discs on a Shimadzu FT-IR-8400S spectrometer. 1H NMR spectra
were obtained on a Bruker DRX-400 Avance spectrometer
and 13C NMR were obtained on a Bruker DRX-100 Avance
spectrometer. Samples were analyzed in DMSO-d6, and chemical
shift values are reported in ppm relative to tetramethylsilane
(TMS) as the internal reference. Melting points were measured on an Electrothermal apparatus and are uncorrected.
Elemental analyses were made using a Carlo-Erba EA1110
CNNO-S analyzer and agreed with the calculated values.
UV–visible spectra were recorded on a Shimadzu UV–visible
UV-2100 spectrophotometer.
* Correspondence to: Khalil Tabatabaeian, Department of Chemistry, Faculty
of Sciences, University of Guilan, PO Box 41335–1914, Rasht, Iran.
E-mail: taba@guilan.ac.ir
Department of Chemistry, Faculty of Sciences, University of Guilan, Rasht, Iran
Copyright © 2012 John Wiley & Sons, Ltd.
Ru(II)-catalyzed synthesis of dihydropyrano[c]chromene derivatives
Materials
4-Hydroxycoumarin, aldehydes, malononitrile, RuCl3.xH2O, Ru3
(CO)12 and solvents were purchased from Merck and used
without further purification. The metal complexes RuCl2(PPh3)3
and RuBr2(PPh3)4 were prepared and characterized as described
previously.[22]
General Procedure
Figure 2. 2-Amino-4-(pyridin-4-yl)-5-oxo-4H,5H-pyrano[3,2-c]chromene3-carbonitrile (3j)
The catalytic process for the synthesis of dihydropyrano[3,2-c]
chromene derivatives was performed in liquid phase. In a typical
reaction, 4-hydroxycoumarin (1 mmol), aldehyde (1 mmol),
malononitrile (1.5 mmol) and Ru complex (5.0 mol%) in 5 ml
methanol were placed in a 50 ml round-bottom flask. The reaction mixture was refluxed and stirred magnetically. After completion of the reaction, as was shown by TLC using petroleum ether
and ethyl acetate as eluent (6:4), the mixture was cooled. The
precipitate was filtered, washed with methanol and for further
purification this product was recrystallized from methanol or a
1:1 chloroform–ethanol solution to afford product 3. In order to
evaluate the reusability of the catalyst, the product was separated
from the reaction mixture by filtration. The same substrates were
added to the filtrate and tested again.
For the typical preparation of tetrahydrobenzo[b]pyran,
the reaction of aldehyde (1.0 mmol), malononitrile (1.5 mmol),
dimedone (1.0 mmol) and Ru complex (5.0 mol%) in 5 ml
methanol was carried out as described above for appropriate
times, given in Table 2. The solvent was removed under
reduced pressure and the desired product 7 was obtained after
washing and recrystallization with methanol. The corresponding
products were identified by IR, 1H NMR, 13C NMR and physical
data (Mp), which are in agreement with those reported in the
literature. The spectral data of selected compounds are
given below.
Spectral and Analytical Data for New
Compounds
2-Amino-4-(3-phenoxyphenyl)-5-oxo-4H,5H-pyrano[3,2-c]
chromene-3-carbonitrile (3e) (Fig. 1)
Yellowish solid; IR (KBr): 3380, 3193, 2198, 1709, 1605, 1483, 1379,
1240 cm 1; 1H NMR dH 4.49(s, 1H, CH), 6.84 (dd, J = 8.0, 1.6 Hz, 1H,
ArH, H18), 6.98–7.05 (m, 4H, NH2, ArH, H22,26); 7.13 (t, J = 7.2 Hz, 1H,
ArH, H24); 7.30–7.39 (m, 3H, ArH, H16,19,20); 7.42–7.52 (m, 4H, ArH,
H9,11,23,25); 7.74 (t, J = 7.2 Hz, 1H, ArH, H10), 7.89 (d, J = 7.2 Hz, 1H,
ArH, H12); 13C NMR dC 37.2 ( C4), 58.1 (C3), 104.1 (C5), 113.4 (C18),
Appl. Organometal. Chem. 2012, 26, 56–61
2-Amino-4-(pyridin-4-yl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (3j) (Fig. 2)
White solid; IR (KBr): 3455, 2973, 2193, 1717, 1639, 1598, 1381,
1273 cm 1; 1H NMR dH 4.53 (s, 1H, CH, H4), 7.33 (d, J = 6.0 Hz,
2H, ArH, H16,19), 7.48–7.55 (m, 4H, NH2, ArH, H9,11); 7.75 (dt,
J = 7.8,1.2 Hz, 1H, H10); 7.92 (dd, J =7.8,1.2 Hz, 1H, ArH, H12); 8.52
(d, J = 6.0 Hz, 2H, ArH, H17,18); 13C NMR dC 36.9 (C4), 56.9 ( C3),
103.0 (C5), 113.4 (CN), 117.1 (C13), 119.4 (C9), 123.0, 123.4, 125.5
(C11, C12, C16, 19), 133.6 (C10), 150.2, 152.2, 152.8 (C8, C15, C17, 18),
154.6, 158.6 ( C2, C14, ), 160.1 (C6); anal. calcd for C18H11N3O3: C,
68.14; H, 3.49; N, 13.24; found: C, 68.11; H, 3.51; N, 13.27.
2-Amino-3-cyano-4-(3-phenoxyphenyl)-7,7-dimethyl-5-oxo4H-5,6,7,8-tetrahydrobenzo[b]pyran (7b) (Fig. 3)
White solid; IR (KBr): 3377, 3211, 2958, 2190, 1685, 1593, 1491,
1370, 1262 cm 1; 1H NMR dH 0.92 (s, 3H, CH3), 1.04 (s, 3H, CH3),
2.12 (d, J = 16.0 Hz, 1H, -CH2); 2.27 (d, J = 16.0 Hz, 1H, -CH2); 2.44
(d, J = 17.6 Hz, 1H, -CH2), 2.54 (d, J = 18.4 Hz, 1H, -CH2); 4.19
(s, 1H, CH, H4); 6.77 (s, 1H, ArH, H12); 6.82 (dd, J = 8.0, 1.6 Hz, 1H,
ArH, H14); 6.95 (d, J = 7.6 Hz, 1H, ArH, H16); 6.99 (d, J = 8 Hz, 2H,
ArH, H18,22); 7.05 (br s, 2H, NH2); 7.14 (t, J = 8.0 Hz, 1H, ArH, H20);
7.31 (t, J = 8.0 Hz, 1H, ArH, H15); 7.38 (t, J = 8.0 Hz, 2H, ArH, H19,21);
13
C NMR dC 27.2 (CH3), 28.9 (C8), 32.2 (C4), 35.9 (C9), 50.4 (C7),
58.4 (C3), 112.8 (C5), 117.0, 117.6, 119.1, 120.1 (CN, C12, C14,
C18, 22), 122.7, 123.9 (C16, C20), 130.4, 130.5 (C15,C19,21), 147.5
(C11), 156.8, 157.1, 158.9 (C10, C13, C17), 163.2 (C2), 196.1 (C6); anal.
calcd for C24H22N2O3: C, 74,59; H, 5.74; N, 7.25; found: C, 74.58; H,
5.78; N, 7.22.
Figure 3. 2-Amino-3-cyano-4-(3-phenoxyphenyl)-7,7-dimethyl-5-oxo-4H-5,6,
7,8-tetrahydrobenzo[b]pyran (7b)
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc
57
Figure 1. 2-Amino-4-(3-phenoxyphenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3carbonitrile (3e)
117.1, 117.3, 118.2 (CN, C13, C16), 119.1, 119.6 (C9, C22,26), 123.0
(C24), 123.0 (C20), 124.0 (C11), 125.2 (C12), 130.5 (C10), 130.6 (C19),
133.5 (C23,25), 146.1 (C15), 152.6 (C8), 154.0, 156.8, 157.0 (C2, C17,
C21), 158.5 (C14), 160.0 (C6); anal. calcd for C25H16N2O4: C, 73.52;
H, 3.95; N, 6.86; found: C, 73.49; H, 3.93; N, 6.82.
K. Tabatabaeian et al.
(1a), malononitrile and 4-hydroxycoumarin (2a) as the model
reaction in the presence of RuBr2(PPh3)4, Ru3(CO)12, RuCl3.xH2O
or RuCl2(PPh3)3 as catalysts to compare their catalytic activities
in methanol as solvent for the synthesis of 2-amino-4(phenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (3a).
The corresponding results are summarized in Table 1.
Among the catalysts tested, RuBr2(PPh3)4 was found to be the
most suitable one in terms of the isolated product yield and
Results and Discussion
We herein present a simple and highly efficient method for the
synthesis of substituted dihydropyrano[3,2-c]chromenes catalyzed by commercially available or easily synthesized ruthenium
complexes in mild reaction conditions.
Initially, for optimization of the reaction conditions we investigated the one-pot, three-component reaction of benzaldehyde
Table 1. Optimization of conditions for the reaction of benzaldehyde (1a), malononitrile and 4-hydroxycoumarin (2a)a
NH2
O
O
O
H
NC
1a
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
Catalyst
CN
O
Ru Catal.
CN +
O
OH
2a
O
3a
Amount (mol%)
Solvent
Time (min)
Yieldb (%)
5
5
5
5
—
5
5
5
5
1
3
7
5
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3CN
n-Hexane
EtOH
H2O
CH3OH
CH3OH
CH3OH
CH3OH
30
45
45
3
45
45
45
45
60
45
45
10
250
80%
40%
45%
80%
33%
45%
Trace
45%
Trace
40%
42%
80%
47%c
RuCl2(PPh3)3
RuCl3.xH2O
Ru3(CO)12
RuBr2(PPh3)4
None
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
RuBr2(PPh3)4
a
Reaction conditions: benzaldehyde (1a) (1 mmol), malononitrile (2a) (1.5 mmol), 4-hydroxycoumarin (3a) (1 mmol), reflux.
Isolated yield.
c
At room temperature.
b
Table 2. Synthesis of dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]pyran derivatives using RuBr2(PPh3)4 as catalysta
Entry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Aldehyde
Benzaldehyde
3,4-Dimethoxybenzaldehyde
3-Nitrobenzaldehyde
4-Cyanobenzaldehyde
3-Phenoxybenzaldehyde
2,4-Dichlorobenzaldehyde
4-Methoxybenzaldehyde
4-Nitrobenzaldehyde
2-Chlorobenzaldehyde
Pyridine4-carbaldehyde
Isophthalaldehyde
4-Cyanobenzaldehyde
3-Phenoxybenzaldehyde
Isopropylbenzaldehyde
4-Methylbenzaldehyde
4-Nitrobenzaldehyde
Time ( min)
Productb
3
5
3
5
14
5
15
5
10
5
50
10
6
5
12
12
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
5a
7a
7b
7c
7d
7e
c
Yield
80
75
78
87
80
88
80
85
80
83
80
82
80
85
80
75
(%)
m.p. (lit. m.p.) ( C)
256–258 (255–256)[6]
226–228 (228–230)[10]
260 (261–262)[6]
283–285 (289–290)[8]
238–240
255–257 (257–259)[2]
228–230 (222–224)[6]
256–258 (258–260)[2]
262–264 (266–268)[8]
250–252
278–280 (281–283)[8]
224–226 (224–226)[14]
182–184
198 (198–200)[14]
219–221 (218–220)[18]
177–178 (178–180)[18]
a
Reaction conditions: aldehyde (1 mmol), malononitrile (1.5 mmol), 4-hydroxycoumarin (1 mmol), [Ru] (5 mol%), methanol (5 ml), reflux.
Melting points, IR, 1H NMR and 13C NMR were in accordance with those of authentic samples.
c
Isolated yield.
b
58
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 56–61
Ru(II)-catalyzed synthesis of dihydropyrano[c]chromene derivatives
Scheme 1. Synthesis of dihydropyrano[3,2-c]chromene derivatives
Table 3. Reusability of RuBr2(PPh3)4 in the model reaction
Run No.
Yield (%)
1
2
3
4
80
80
77
76
Table 4. UV–visible spectral data of RuBr2(PPh3)4 in MeOH solution
Entry
1
2
3
a
Complex
Catalyst condition lmax (nm) (a) lmax (nm) (b)
RuBr2(PPh3)4 Fresh catalyst
RuBr2(PPh3)4 Aged catalysta
RuBr2(PPh3)4 After four runs
225
224
233
260
264
270
After 2 weeks.
reaction time in the absence of any additive (base, ligand) for this
MCR. Ru3(CO)12, RuCl3.xH2O and RuCl2(PPh3)3 generated desired
products in lower yields or in longer reaction times. By using
RuCl3.xH2O the reaction did not go to completion and starting
material was always left in the reaction mixture. Even addition of
sodium carbonate as a base or ligands such as triphenylphosphine
or 2,2′-bipyridine did not have any positive effect on the reaction
time and yield. Moreover, in this case the desired product was
separated by chromatography (TLC), petroleum ether–ethyl
acetate (6:4), but with other catalyzed systems the precipitated
product was easily separated and purified by recrystallization
(Table 1, entries 1–4).
It seems that different catalytic activities depend on catalyst
structures and also on the electronic properties and acid/base
strength of the ligands coordinated to ruthenium metal.
As shown in Table 1, When the reaction was performed
in methanol in the absence of catalyst the yield of
desired product 3a was low (Table 1, entry 5). In other efforts, for elucidating the role of the solvents, various solvents,
including nonpolar solvents such as n-hexane, aprotic polar
solvents such as acetonitrile and protic solvents such as
water, ethanol and methanol were examined by using
5 mol% RuBr2(PPh3)4 as catalyst. The results are summarized in
Table 1. After screening different solvents, methanol was found
to be the best solvent system and gave the corresponding
product 3a in short reaction time and high yield (Table 1,
entries 4, 6–9).
Furthermore, in the presence of varying amounts of
RuBr2(PPh3)4, we found that the optimum reaction rate and yield
could be achieved at 5 mol% catalyst concentration (Table 1,
entries 4, 10–12). Also after 250 min of stirring at room
temperature, 47% yield was observed (Table 1, entry 13).
With the optimal reaction conditions in hand, a range of
aldehydes were selected to undergo the condensation reaction
in the presence of 5 mol% RuBr2(PPh3)4 (Scheme 1 and Table 2,
entries 1–10).
As can be seen in Table 2, with regard to the substituents
with different electronic properties, aldehydes containing either
electron-withdrawing or electron-donating groups such as
methoxy, phenoxy, nitro, cyano and chloro in the ortho, meta or
para position (Table 2, entries 1–9) as well as heteroaromatic
aldehyde (entry10) could be employed in the reaction and led
to product in high yield. Notably, the reaction was completed
within 3–15 min and the main product could be obtained
simply by filtration from the reaction medium without any side
product formation.
The reusability investigation in the model reaction showed
that the reused catalyst maintains its activity and only 4%
decrease in the yield of product was found after four runs, as
shown in Table 3.
59
Figure 4. UV–visible absorption spectra of RuBr2(PPh3)4 in MeOH solution
Appl. Organometal. Chem. 2012, 26, 56–61
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc
K. Tabatabaeian et al.
Furthermore, the UV spectra of catalyst in methanol initially, after 2 weeks and after four runs in the model reaction indicate significant similarities in the positions of bands (Fig. 4 and Table 4,
entries 1–3).
In spite of instability of the complex in methanol, according to
UV spectra, presumably ruthenium(II) catalyst species are still active as a Lewis acid in the reaction media even after four runs.
Encouraged by these experimental results, we extended the
methodology to the synthesis of bis-dihydropyrano[3,2-c]chromene (Scheme 2 and Table 2, entry 11). For comparison of our
results with those of Wang et al.,[8] the protocol was applied to
the synthesis of 4,4′-(1,3-phenylene)bis(2-amino-5-oxo-4H,5Hpyrano-[3,2-c]chromene-3-carbonitrile) (5a) with three equiv.
malononitrile and two equiv. 4-hydroxycoumarin under similar
conditions; the product was formed in shorter reaction time.
In order to broaden the scope of the present method,
the replacement of 4-hydroxycoumarin with dimedone under
the same reaction conditions afforded the corresponding
tetrahydrobenzo[b]pyran derivatives in high yields and short reaction time (Scheme 3 and Table 2, entries 12–16).
Inspired by previous work on MCRs,[23] a proposed mechanistic
route for the condensation of aldehydes (1), malononitrile and
4-hydroxycoumarin (2), which rationalizes the formation of
products (3), is exhibited in Scheme 4. As shown, the reaction is
initiated by ruthenium complex assisted Knoevenagel condensation to provide intermediate [A]. Then, elimination of proton in
4-hydroxycoumarin to form intermediate [B] followed by Michael
addition of intermediate [B] on [A] produces [C], which in turn
undergoes cyclization and isomerization, to provide the final
product (3) (Scheme 4).
Scheme 2. Synthesis of bis-dihydropyrano[3,2-c]chromene derivatives
Scheme 3. Synthesis of tetrahydrobenzo[b]pyran derivatives
60
Scheme 4. Proposed mechanism for the formation of dihydropyrano[3,2-c]chromene derivatives 323
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 56–61
Ru(II)-catalyzed synthesis of dihydropyrano[c]chromene derivatives
Conclusions
A novel, simple and convenient method for the synthesis of
dihydropyrano[3,2-c]chromene and tetrahydrobenzo[b]pyran derivatives via one-pot three-component reaction in the presence of
a catalytic amount of Ru(II) as efficient, easily synthesized and
reusable catalyst has been developed. Good to excellent yields,
shorter reaction periods and lower loading of catalyst compared
with the other methods, easy product separation and purification
are the main advantages of this method.
Acknowledgements
The authors thank the Research Council of University of Guilan
for support of this study.
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