Palladium-catalyzed cycloisomerization of (Z)-enynols into furans using green solvents: Glycerol vs. water

Article abstract of DOI:10.1039/c0gc00169d

Heteroannulation reactions of (Z)-2-en-4-yn-1-ol derivatives into furans can be conveniently performed in water and glycerol using cis-[PdCl ₂(DAPTA)₂] as catalyst. Higher activities were observed in an aqueous medium, but catalyst r

Full text of DOI:10.1039/c0gc00169d

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COMMUNICATION
www.rsc.org/greenchem | Green Chemistry
Palladium-catalyzed cycloisomerization of (Z)-enynols into furans using
green solvents: glycerol vs. water†
Javier Francos and Victorio Cadierno*
Received 2nd June 2010, Accepted 12th July 2010
DOI: 10.1039/c0gc00169d
Heteroannulation reactions of (Z)-2-en-4-yn-1-ol deriva-
tives into furans can be conveniently performed in water
Metal-catalyzed cycloisomerization of (Z)-2-en-4-yn-1-ol
derivatives represents an attractive and atom-economic route to
furans, which are important structural motifs present in many
biologically active molecules and pharmaceutical substances, as
and glycerol using cis-[PdCl
2
(DAPTA) ] as catalyst. Higher
2
activities were observed in an aqueous medium, but catalyst
recycling was much more effective in glycerol.
11
well as versatile building blocks in synthetic organic chemistry.
Several Group 8–11 metal complexes have been successfully em-
ployed to promote these heteroannulation reactions in organic
media, with palladium-, ruthenium- and gold-based catalysts
Chemical transformations are experiencing a deep change to
meet sustainability criteria imposed by the Green Chemistry
principles. In this sense, one of the crucial points in realizing a
1
12
showing the best performances. However, despite its great
“
green chemical” process involves the choice of a safe, non-toxic
synthetic interest, efforts devoted to develop this process in
alternative media have been scarce. Herein, we would like to
communicate that the cycloisomerization of (Z)-2-en-4-yn-1-ols
can be conveniently performed in glycerol and water using the
2
13
and cheap solvent. Water has been, for a long time, the first
solvent of choice regarding the aforementioned considerations.
Indeed, it is now well-accepted that water is a reliable alternative
to the organic, petroleum-based, solvents commonly used in
hydrophilic Pd(II) complexes cis-[PdCl
Bn (2), DAPTA (3)) as catalysts (see Fig. 1).
2
L
2
] (L = PTA (1), PTA-
2
14,15
chemical laboratories and industry. Innumerable examples
illustrating the utility of water in stoichiometric and catalytic
organic synthesis have been described in the literature during
3
the last two decades.
On the other hand, the large surplus of glycerol, generated
as the main byproduct (ª10% by weight) of the biodiesel
production process, has recently led to a collapse in prices
which has been accompanied by a growing imbalance in
the supply/demand of this chemical. Therefore, finding new
applications for glycerol as a low-cost raw material has become
4
an urgent necessity. One possible application of glycerol is its
use as a green reaction medium. In fact, as recently suggested
by J e´ r oˆ me and co-workers, glycerol can be considered as
5
Fig. 1 Structure of the hydrophilic Pd(II) catalysts used in this study.
“
organic water” since, like water, it is abundant, biodegradable,
inexpensive, non-toxic, highly polar, immiscible with hydro-
carbons, able to form strong hydrogen bonds and to dissolve
inorganic compounds (salts, acids, bases and transition metal
complexes). In addition, compared to water, it has the advantage
of its higher boiling point, lower vapor pressure, and that it
is able to dissolve organic compounds usually immiscible with
water. However, despite its great potential as an environmentally
friendly solvent, there are currently very few synthetic processes
described in the literature where glycerol is used as the reaction
Firstly, using the cycloisomerization of commercially available
Z)-3-methyl-2-penten-4-yn-1-ol into 2,3-dimethylfuran as a
(
model reaction, we compared the efficiency and selectivity
of these Pd-catalysts in glycerol and water. Experiments were
performed at room temperature, employing 2 mmol of the (Z)-
enynol (1 M solution) and a metal loading of 0.2 mol%. The
results obtained are collected in Table 1.
As shown in the table, regardless of the solvent employed, all of
the compounds tested led to the selective and almost quantitative
formation (≥96% GC yield) of the desired furan after 3–9 h of
stirring at r.t. (entries 1–6). However, the reactions proceeded
in all cases significantly faster in water than in glycerol (entry 2
vs. 1, 4 vs. 3 and 6 vs. 5). From this catalyst screening, complex
5,6
medium. In particular, concerning the field of metal-catalysis,
7
the only known examples relate to Heck and Suzuki couplings,
8
9
hydrogenation and transfer-hydrogenation reactions, and the
10
ring closing metathesis of diolefins.
cis-[PdCl
2
(DAPTA) ] (3) emerged as the top choice due to its
2
high efficiency (99% GC-yield after 5 or 3 h; entries 5–6). In
addition, the reaction rate could be significantly improved by
increasing the working temperature to 75 C. Under these new
Departamento de Qu ´ı mica Org a´ nica e Inorg a´ nica. IUQOEM,
Universidad de Oviedo, Juli a´ n Claver ´ı a 8, 33006, Oviedo, Spain.
E-mail: vcm@uniovi.es; Fax: +34 985103446; Tel: +34 985102985
◦
†
Electronic supplementary information (ESI) available: Copies of the
H NMR and GC-MS spectra of all isolated furans, as well as
gas chromatographs of the reactions collected in Table 2. See DOI:
reaction conditions, using the same metal loading (0.2 mol% of
1
3), 2,3-dimethylfuran was formed in 99% GC-yield after only
1
0.1039/c0gc00169d
15–20 min of heating (entries 7–8).
1
552 | Green Chem., 2010, 12, 1552–1555
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Table 1 Pd-catalyzed cycloisomerization of (Z)-3-methyl-2-penten-4-
The lifetime of a catalytic system and its level of reusability are
very important factors. In this sense, the hydrophilic character
a
yn-1-ol in environmentally friendly media
17
of cis-[PdCl
2
(DAPTA) ] allows it to be recycled easily after
2
simple extraction of the final reaction product with immiscible
3
diethyl ether (3 ¥ 10 cm ). Comparative results in glycerol and
water using the cycloisomerization of (Z)-3-methyl-2-penten-4-
yn-1-ol into 2,3-dimethylfuran as a model reaction are shown in
Fig. 2. Thus, we have found that, while no appreciable loss of
activity occurs in glycerol during five consecutive runs (96–99%
GC-yields after 20 min), the efficiency of the aqueous solution
decreases considerably after each recycling cycle, 24 h of heating
being required in the fifth one to attain 91% of conversion. This
b
c
Entry
Catalyst
Solvent
Temp. Time
Yield
TOF
1
1
1
1
1
1
2
3
4
5
6
7
8
1
1
2
2
3
3
3
3
glycerol r.t.
water r.t.
glycerol r.t.
water r.t.
glycerol r.t.
water r.t.
9 h
5 h
9 h
5 h
5 h
3 h
96%
98%
97%
97%
99%
99%
53 h-
-
98 h
54 h-
-
97 h
99 h-
165 h-
1
different behavior of cis-[PdCl
2
(DAPTA)
2
] is not only related to
◦
◦
-1
-1
glycerol 75 C
water
20 min 99%
15 min 99%
1485 h
1980 h
-3
it being more highly soluble in glycerol (3.1 mg cm ) than water
75 C
-
3
18
(
1.7 mg cm ), but also to its stability in both media. In fact,
a
2
Reactions performed under N atmosphere using 2 mmol of sub-
strate (1 M solution in glycerol or water). [enynol : Pd] ratio =
00 : 1. Determined by GC. Turnover frequencies ((mol product/mol
although the homogeneity of the aqueous solution is apparently
maintained after each cycle, decomposition of the catalyst via
decoordination of the DAPTA ligand has been observed by
b
c
5
Pd)/time) were calculated at the indicated time.
3
1
1
19
P{ H} NMR spectroscopy.
Table 2 Cycloisomerization of (Z)-2-en-4-yn-1-ols catalyzed by cis-
a
[
2 2
PdCl (DAPTA) ] (3) in environmentally friendly media
1
2
b
c
Entry Substrate (R /R )
Solvent Time Yield
TOF
1
1
2
3
4
5
6
7
8
9
1
H/H
H/H
glycerol 20 min 99% (84%) 1485 h-
-1
water
glycerol 2 h
water 2 h
glycerol 2 h
water 2 h
/H glycerol 2 h
/H water 2 h
glycerol 24 h
water 24 h
15 min 99%
1980 h
n
-1
Bu/H
Bu/H
99% (86%) 247 h
n
-1
99%
247 h
1
1
1
1
Ph/H
Ph/H
95% (83%) 237 h-
-
97%
242 h
CH
CH
2
C(Me) CH
C(Me) CH
2
2
99% (90%) 247 h-
-
2
99%
247 h
d
-1
-1
H/Ph
H/Ph
88% (77%) 3 h
d
0
92%
4 h
a
◦
Reactions performed at 75 C under N
2
atmosphere using 2 mmol of
substrate (1 M solution in glycerol or water). [enynol : Pd] ratio = 500 : 1.
b
c
Determined by GC (isolated yields in brackets). Turnover frequencies
(
(mol product/mol Pd)/time) were calculated at the indicated time.
Reactions performed using 1 mol% of complex 3. [enynol : Pd] ratio =
00 : 1.
Fig. 2 Catalyst recycling in glycerol and water: reactions were per-
formed as described in Table 2.
d
1
Complex cis-[PdCl
2
(DAPTA)
2
] (3) was also effective in the
selective cycloisomerization of other (Z)-enynols both in glycerol
and water, thus confirming the generality of the process. As
shown in Table 2, replacement of the hydrogen atom (entries 1–
The excellent capability of the palladium/glycerol system to
be recycled was further studied in depth. As shown in Table 3, the
system could be recycled up to 17 times, leading to a cumulative
TON value of 8190, the highest reported to date for this catalytic
transformation.
In summary, we have demonstrated that, like water, glycerol
can be employed as an environmentally friendly solvent for
metal-catalyzed cycloisomerization reactions of (Z)-2-en-4-yn-
1-ols into furans. The use of this low-cost and renewable
feedstock, together with the highly effective catalyst recycling
observed in glycerol, opens great prospects for developing more
sustainable processes in the organic synthesis field. Further
studies aimed at broadening the scope of this atom-economic
transformation are currently in progress in our lab and will be
presented in due course.
16
2
) by alkyl (entries 3–4), aryl (entries 5–6) or alkenyl (entries 7–8)
groups at C-1 of the 3-methyl-2-penten-4-yn-1-ol skeleton was
compatible with the cyclization conditions, the corresponding
furans being generated in excellent GC yields (≥95%) after 2 h
◦
of heating at 75 C. Heteroannulation of (Z)-3-methyl-5-phenyl-
2
-penten-4-yn-1-ol, bearing an internal C C bond, was also
conveniently accomplished in glycerol and water (entries 9–
0). However, a higher catalyst loading (1 mol%) was required
1
in these cases. Remarkably, the high boiling point of glycerol
allowed the isolation of the final products (77–90% yield) by
K u¨ gelrohr distillation of the crude reaction mixtures, thus
circumventing the use of organic solvents.
This journal is © The Royal Society of Chemistry 2010
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Table 3 Isomerization of (Z)-3-methyl-2-penten-4-yn-1-ol catalyzed
Jacob and G. Perin, J. Braz. Chem. Soc., 2009, 20, 93; (f) F. He, P.
Li, Y. Gu and G. Li, Green Chem., 2009, 11, 1767; (g) C. C. Silveira,
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Catal., 2010, 352, 519; (i) J.-N. Tan, M. Li and Y. Gu, Green Chem.,
a
2 2
by cis-[PdCl (DAPTA) ] (3) in glycerol: catalyst recycling.
2
010, 12, 908.
7
(a) A. Wolfson and C. Dlugy, Chem. Pap., 2007, 61, 228; (b) A.
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Douliez, S. Camy, J.-S. Condoret, Y. Pouilloux, J. Barrault and F.
J e´ r oˆ me, Green Chem., 2010, 12, 804.
8 (a) K. Tarama and T. Funabiki, Bull. Chem. Soc. Jpn., 1968, 41, 1744;
(b) T. Funabiki and K. Tarama, Bull. Chem. Soc. Jpn., 1971, 44, 945.
9 (a) A. Wolfson, C. Dlugy, Y. Shotland and D. Tavor, Tetrahedron
Lett., 2009, 50, 5951; (b) E. Farnetti, J. Ka sˇ par and C. Crotti, Green
Chem., 2009, 11, 704; (c) D. Tavor, O. Sheviev, C. Dlugy and A.
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E. Farnetti, Green Chem., 2010, 12, 1295.
b
c
b
c
Cycle Time
Yield
TON
Cycle Time
Yield
TON
1
2
3
4
5
6
7
8
9
20 min 99%
20 min 98%
20 min 99%
20 min 97%
20 min 96%
20 min 95%
30 min 95%
30 min 98%
30 min 96%
495
985
10
11
12
13
14
15
16
17
30 min 96%
40 min 95%
50 min 96%
4845
5320
5800
6275
6750
7235
7720
8190
1480
1965
2445
2920
3395
3885
4365
1 h
95%
95%
97%
97%
94%
1.5 h
3 h
9 h
24 h
1
0 N. Bakhrou, F. Lamaty, J. Martinez and E. Colacino, Tetrahedron
Lett., 2010, 51, 3935.
a
◦
Reactions performed at 75 C under N
2
atmosphere using 2 mmol
of substrate (1 M solution in glycerol). [enynol : Pd] ratio = 500 : 1.
11 For recent reviews on furans synthesis, see: (a) X. L. Hou, H. Y.
Cheung, T. Y. Hon, P. L. Kwan, T. H. Lo, S. Y. Tong and H. N. C.
Wong, Tetrahedron, 1998, 54, 1955; (b) B. A. Keay, Chem. Soc. Rev.,
b
c
Determined by GC. Cumulative TON values.
1
999, 28, 209; (c) A. Jeevanandam, A. Ghule and Y.-C. Ling, Curr.
Org. Chem., 2002, 6, 841; (d) R. C. D. Brown, Angew. Chem., Int. Ed.,
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Acknowledgements
2
(
(
f) D. M. D’Souza and T. J. J. M u¨ ller, Chem. Soc. Rev., 2007, 36, 1095;
This work was supported by the Spanish MICINN (Projects
CTQ2006-08485/BQU and CSD2007-00006) and the Gobierno
del Principado de Asturias (FICYT Project IB08-036). J. F.
thanks MICINN and the European Social Fund for the award
of a PhD grant.
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1
2 For representative examples, see: (a) D. V e´ gh, P. Zalupsky and J.
Kov a´ cˇ , Synth. Commun., 1990, 20, 1113; (b) B. Seiller, C. Bruneau
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1
2
3
See, for example: (a) P. T. Anastas and J. C. Warner, in Green
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1
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1
998; (b) A. S. Matlack, in Introduction to Green Chemistry, Marcel
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(
1
b) J. H. Clark and S. J. Taverner, Org. Process Res. Dev., 2007, 11,
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´
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4
(a) M. Pagliaro and M. Rossi, in The Future of Glycerol: New Usages
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3,7
1
0, 13; (f) F. J e´ r oˆ me, Y. Pouilloux and J. Barrault, ChemSusChem,
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3,7
2
azonia-7-phosphatricyclo[3.3.1.1 ]decane chloride; DAPTA = 3,7-
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Gon c¸ alves, Quim. Nova, 2009, 32, 639.
5
6
(a) Y. Gu, J. Barrault and F. J e´ r oˆ me, Adv. Synth. Catal., 2008, 350,
2
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3
043; (b) C. Dlugy and A. Wolfson, Bioprocess Biosyst. Eng., 2007,
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of cis-[PdCl (COD)] (50 mg, 0.175 mmol) and PTA-Bn (99 mg,
2
3
2
009, 2, 34; (e) E. J. Lenord a˜ o, D. O. Trecha, P. C. Ferreira, R. G.
0.35 mmol) in methanol (10 cm ) was stirred overnight at r.t. The
1
554 | Green Chem., 2010, 12, 1552–1555
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yellow solid precipitate was then filtered, washed twice with methanol
K u¨ gelrohr distillation of the crude reaction mixtures generated in
glycerol provided analytically pure samples of the furans, whose
identity was assessed by comparison of their H and C{ H} NMR
data with those previously described in the literature, and by their
fragmentation in GC-MS.
3
(
5 cm ), and dried in vacuo. Yield: 109 mg, 84% (Found: C, 41.82;
1
13
1
H, 5.33; N, 11.40%. C26 Pd requires C, 41.93; H, 5.24; N,
H
38Cl
4
N
6
P
2
◦
-1
2
-1
1
1.28%). Conductivity (water, 20 C) 207 X cm mol . d
) -16.98 (s); d (DMSO-d ) 4.35–4.44 (m, 12H, PCH N), 4.74–4.87
N), 5.33 (br, 4H, NCH Ph), 7.50–7.55 (m, 10H,
) 49.9 and 52.3 (br, PCH N), 64.0 (s, NCH Ph),
8.3 and 78.2 (s, NCH N), 125.4 (s, C of Ph), 128.8, 130.2 and 132.6
s, CH of Ph).
6 General procedure for the catalytic reactions: The corresponding
P
(DMSO-
d
(
6
H
6
2
m, 12H, NCH
(DMSO-d
2
2
17 Recoverable and Recyclable Catalysts, ed. M. Benaglia, John Wiley &
Ph);d
6
(
C
6
2
2
Sons, Chichester, 2009.
2
18 Due to its lower solubility in water, small amounts of the catalyst
dissolve in the organic phase.
1
19 (a) This fact was clearly evidenced by the appearance, among other
3
(
Z)-enynol (2 mmol) and the appropriate solvent (2 cm ) were
introduced into a sealed tube under nitrogen atmosphere. Complex
(DAPTA) ] (2.5 mg, 0.004 mmol; 0.2 mol% of Pd) was
unidentifiable signals, of two singlet resonances at -75 and 9 ppm in
31
1
the P{ H} NMR spectrum of the aqueous solution recovered after
the fifth cycle, attributable to free DAPTA and its oxide, respectively:
D. J. Darensbourg, C. G. Ortiz and J. W. Kamplain, Organometallics,
2004, 23, 1747; (b) Such a decoordination process was not observed
cis-[PdCl
2
2
then added at room temperature, and the resulting solution heated
◦
at 75 C for the indicated time (the course of the reaction was
31
1
monitored by regular sampling and analysis by GC). Fractional
by P{ H} NMR in the glycerol solution.
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Green Chem., 2010, 12, 1552–1555 | 1555