JandaJel as a polymeric support to improve the catalytic efficiency of immobilized-1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) under solvent-free conditions

Article abstract of DOI:10.1039/c1gc15790f

JandaJel, with its greater spacing between the linear polymeric chains compared to that of polystyrene matrices, is a very efficient support for improving the catalytic efficiency of TBD under SolFC. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15790f

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PAPER
JandaJel as a polymeric support to improve the catalytic efficiency of
immobilized-1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) under solvent-free
conditions†
Daniela Lanari,*a Roberto Ballini,b Simona Bonollo,a Alessandro Palmieri,b Ferdinando Pizzoa and
Luigi Vaccaro*a
Received 4th July 2011, Accepted 10th August 2011
DOI: 10.1039/c1gc15790f
JandaJel, with its greater spacing between the linear polymeric chains compared to that of
polystyrene matrices, is a very efficient support for improving the catalytic efficiency of TBD
under SolFC.
as in the case of Tentagel and Argogel, minimizing thus the
Introduction
swelling problems when polar solvents are used.1b,1c,1f However,
Among the factors that regulate the activity of a polymer-bound
even in this case, it has been frequently observed that not-inert
catalyst, the nature of the support is undoubtedly fundamental
and conformationally-unstable linkers can negatively influence
and in several cases can define the success or the failure of the
the process and, sometimes, these resins are difficult to separate
catalyst itself.1 Immobilization on a polymeric matrix produces
from a polar reaction medium because of the PEG hygroscopic
steric and electronic modifications of the catalyst framework,
nature. Another interesting solution consists of the replacement
generally but not always, resulting in a consistent reduction of
its efficiency.1
of 1,4-divinylbenzene (DVB) with alternative cross-linkers char-
acterized by larger dimensions, flexibility and compatibility with
Swelling of the resin by the reaction medium is considerably
several solvent classes.1b,1c,3,4 In this context, excellent swelling
influenced by the structure and the conformation of the support
properties have been reached by preparing polystyrene polymers
facilitating or hampering the access of the reactants to the active
using PEG1b,1c,3 or polytetrahydrofuran-based (JandaJel resins)
sites.1
cross-linkers.1b,4
To minimize these possible drawbacks, many polymeric
In the last few years the attention has been principally turned
modifications have been proposed, so that a more “solution-
to the JandaJel (JJ) resins, that are chemically more stable
like” microenvironment around the catalyst could be created.
and ease to prepare than polyethylene glycol-based ones. These
Interesting results have been obtained, for example, by using
polymers have been successfully used for solid supported organic
linear soluble supports as PEG or NCPS,1b,1e,2 but in some cases,
synthesis (SPOS)5 and as supports for catalysts in a variety of
the catalytic efficiency that results is strictly dependent on the
synthetic transformations,6 showing a better propensity to be
spatial folding of the polymeric chains that may interact with
swelled by the solvent and a greater diffusion value than the
the reaction partners influencing the process course.
classical Merrifield resins, with remarkable advantages in terms
Styrene-based resins are the most widely used supports,
of reactivity.
thanks to their easy availability and high loading capacity.
We are interested in the development of environmentally and
Access impediments can be reduced by increasing the distance
chemically efficient methodologies by exploiting the unique
between the catalytic site and the polymeric matrix, through the
properties of water as reaction medium7 and solvent-free
introduction of spacers (generally PEG) of several dimensions,
conditions (SolFC)8 to realize multigram continuous-flow
processes8a,b,e characterized by minimal waste production (low
E-factor).9
aLaboratory of Green Synthetic Organic Chemistry, CEMIN -
Some years ago we have proposed an alternative solution to
Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8,
resin swelling problems that concerns the adoption of SolFC.8l
Perugia, Italia. E-mail: luigi@unipg.it; Fax: +39 075 5855560; Tel: +39
075 5855541
Since then, we have contributed in this direction proving that in
several cases the efficiency of an anchored catalyst under SolFC
is higher than in the presence of a reaction medium, where often
the process cannot even be realized satisfactorily.8a-e,i,l Besides
the high chemical efficiency, the protocols reported feature
low E-factors.8a,b,e To further reduce waste, our approach was
b“Green Chemistry Group” School of Science and Technology, Chemistry
Division, Via S. Agostino 1, 62032, Camerino, Italy
† Electronic Supplementary Information (ESI) available: Characteriza-
tion data for catalysts 1a–g, characterization data and copies of the 1H
and 13C NMR spectra for compounds 10, 11, 12, 18, 19, 20, 29. See DOI:
10.1039/c1gc15790f/
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Table 1 Michael addition of 1-butanethiol (3a) to (E)-benzyli-
deneacetone (2a)a
also exploited in the setting of cyclic continuous-flow protocols
operating in SolFC or highly concentrated conditions and
running on a multigram scale.8a,b,e
We are interested in studying novel polymer supports more
suitable for being used under SolFC and in continuous-flow
processes than the classic Merrifield resins. The resulting immo-
bilized catalysts should be recoverable, reusable and sufficiently
efficient to allow the use of equimolar amounts of reactants
and minimize the use of organic solvent for the recovery of the
product.
With this goal in mind, we have evaluated the influence of
the polymer mesh size and/or the cross-linking nature and
percentage on the catalyst activity under SolFC. We have
selected the representative base 1,5,7-triazabicyclo[4.4.0]dec-5-
ene (TBD) that in its polystyrene-supported version is a well-
known catalyst.
Entry
Catalyst
Conversionb (%)
Yieldc (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
1g
1h
1a
1a
>99
>99
94
99
<1
2
<1
21
—
98
98
96d
98
—
—
—
8
9e
10f
—
—
—
a Reaction conditions: 2a (1.0 mmol), 3a (1.0 mmol), catalyst (0.5 mol%),
30 ◦C. b Determined by GC analyses. c Yield of the isolated pure product.
d Determined after completion occurred in 80 min. e Reaction conducted
in CH2Cl2 (2.0 mL). f Reaction conducted in CH3CN (2.0 mL).
Results and discussion
To this purpose, JJ-TBD 1a–d (1,4-bis(vinylphenoxy)-butane-
cross-linked) and PS-TBD 1e–g (DVB-cross-linked) have been
prepared and characterized by elemental analysis, IR spec-
troscopy and scanning electron microscopy (SEM) (see support-
ing information†).
Table 2 Thiolysis of 2,3-epoxypropyl-phenylether (5a) by thiophenol
(3b)a
Their efficiency has been evaluated and compared also to
commercially available Si-TBD (silica gel-supported TBD) 1h
(Scheme 1) in representative C–S bond forming processes as
the sulfa-Michael addition (SMA) of (E)-benzylideneacetone
(2a) and 1-butanethiol (3a) (Table 1) and the thiolysis of 2,3-
epoxypropyl-phenylether (5a) by thiophenol (3b) (Table 2).
Entry
Catalyst
Conversionb (%)
Yieldc (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
1g
1h
1a
1a
94
96d
97
97
97
—
—
—
—
—
—
>98
>98
>98
28e
25e
30e
88
8
9f
10g
—
20
a Reaction ◦conditions: 5a (1.0 mmol), 3b (1.0 mmol), catalyst (0.5
mol%), 30 C. b Determined by GC analyses. c Yield of the isolated pure
product. d Determined after completion occurred in 140 min. e Complete
conversion of 5a to 6 was reached after 10 h. f Reaction conducted in
CH2Cl2 (2.0 mL). g Reaction conducted in CH3CN (2.0 mL).
performances are strongly determined by the nature of the cross-
linker and, in particular, as its size increases, the reactants’ access
to the active sites is greatly facilitated. Silica gel-supported TBD
1h gave only a slightly better result than PS-TBD 1e–g (Table 1,
entry 8).
It should be noted that the reactions mediated by PS-TBD 1e–
g and silica-TBD 1h were extremely slow and, in all cases, the
complete conversion was not reached at all after long reaction
times.
In the case of JJ-TBD 1a–d, a substantially identical catalytic
efficiency is observed, suggesting that neither the bead sizes
variation (Table 1, entries 1–3), nor the slightly different cross-
linking percentage used in this study (Table 1, entries 2 vs. 4),
markedly influenced the catalyst behaviour. Moreover, in all
cases, the product was isolated in high yields (96–98%) and
purity (≥ 98%) simply by filtration of the reaction mixture.
Scheme 1 Polymer-supported-TBD 1a–g and silica gel-supported-
TBD 1h.
We found that highly promising JJ polymers were in line
with our expectations when used under SolFC. JJ-supported
TBD 1a–d was far more efficient than the PS-supported parents
1e–g (Table 1, entries 1–4 vs. 5–7). Under SolFC catalytic
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Table 3 Michael addition of thiol 3a–d to enones 2a–d catalyzed by polymer-supported-TBD 1ba
Entry
Enone
Thiol
Product
t (h)
Yieldb (%)
1
2
3
3a: n-BuSH
4
0.7
1
1
8
2
4
6
2
0.3
0.5
1
6
4
0.5
1
8
98
97
93
91
96
99
96
95
91
94
97
93
88
98
97
90
3b: C6H5SH
7
3c: 4-Me-C6H4SH
3d: C6H5CH2SH
8
9
4
5c
6
3a
3b
3c
3d
3a
3b
3c
3d
3a
3b
3c
3d
10
11
12
13
14
15
16
17
18
19
20
21
7
8c
9
10
11
12
13c
14
15
16
a Reaction conditions: enone (1.0 mmol), thiol (1.0 mmol), catalyst (0.5 mol%), 30 ◦C. b Yield of the isolated pure product. c 2 mol% of catalyst were
used.
By using 0.5 mol% of catalyst 1b in the presence of an organic
medium (dichloromethane and acetonitrile) a dramatic drop
of the catalytic efficiency was observed (Table 1, entries 9–10
respectively).
Catalyst 1b was also reused in subsequent three runs obtaining
the same results in terms of reaction time and isolation yield of
the Michael adduct 4.
JJ-TBD 1a–d was again much more efficient than the PS-
catalysts 1e–g, allowing a complete conversion in 100–140 min,
compared to the 10 h needed for the PS-TBD 1e–g analogues
(Table 2, entries 1–4 vs. 5–7). Besides, the cross-linking percent-
age variation (Table 2, entries 2 vs. 4) and the mesh sizes (Table
2, entries 1–3) seem to not affect noticeably the catalytic activity,
confirming thus the same trend observed in the previously
illustrated Michael addition. Again, the reactions performed
in the presence of an organic medium gave discouraging results
(Table 2, entries 9 and 10). Obviously, PS-catalysts 1e–g gave
no trace of product in the presence of a reaction medium being
always less efficient that JJ-catalysts.
in five subsequent runs on a 5 mmol scale, and we observed
that its efficiency was unchanged. Product 6 was isolated in 95–
97% yield after an identical reaction time (100 min). The only
problem observed is related to the recovery of the solid catalyst
that requires a filtration and therefore on a small scale some loss
may occur.
To prove the efficiency of our approach that is based on the
combination of a novel JandaJel-TBD catalyst and SolFC we
have also created a continuous-flow reactor. In according with
our previous reports in this field, the reactor has been designed to
optimize the recovery and reuse of the catalyst, minimize waste
and in particular the amount of organic solvent needed to isolate
the final products.8a–c The schematic of the reactor is presented
in Scheme 2 (thermostated box is not showed for clarity).
The equimolar mixture (50 mmol) of 5a and 3b was charged
into a glass column functioning as a reservoir. Catalyst 1b
(0.5 mol%, 0.25 mmol of TBD) was charged into a glass column
and the◦reaction mixture was continuously pumped through
it at 30 C for 100 min, necessary for the complete conversion
to 6.
Also in this case the product was isolated in very satisfactory
yields (96–97%) and purities (≥ 98%).
At this point, the pump was left to run in order to recover the
reaction mixture into the reservoir. Then EtOAc was added (3 ¥
2 mL) to wash the catalyst and to isolate product 6 in 97% yield.
The same protocol was repeated for ten consecutive runs and
the efficiency of the catalyst was unchanged. After 100 min the
conversion of 5a and 3b to 6 was always complete and the final
product was recovered always in very high yields (97–98%).
The scope and applicability of the representative JJ-TBD
catalyst 1b has been further investigated (Table 3 and Table
4). The results are very positive confirming the efficiency of JJ-
TBD as a basic catalyst. All the reactions were conducted with
equimolar amounts of substrates under SolFC at 30 ◦C, with
0.5 mol% of catalyst (with only a few exceptions, see footnotes).
The corresponding products were obtained, by simple filtration
of the reaction mixture, in short times and with high yields and
purities (≥ 98%).
Conclusions
Finally, we have evaluated the catalyst recycling. The repre-
In conclusion, taking the C–S bond forming reactions of thiols
sentative base 1b was tested in the ring opening of 5a with 3b
3 with Michael acceptors 2 and with epoxides 5 as purely
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Table 4 Thiolysis of epoxides 5a–d by thiols 3a–d catalyzed by polymer-supported-TBD 1ba
b
Entry
Epoxide
Thiol
Product
t (h)
b/a
Yieldc (%)
1
2
3
3a: n-BuSH
22
6
1
1.7
1
2
5
7
5
14
22
9
8
9
26
24
30
1
>99/<1
>99/<1
>99/<1
>99/<1
77/23
63/37
67/33
66/34
>99/<1
>99/<1
>99/<1
>99/<1
—
96
97
99
98
90
93
96
94
90
93
96
90
98
87
88
85
3b: C6H5SH
3c: 4-Me-C6H4SH
3d: C6H5CH2
23
24
25
26
27
28
29
30
31
32
33
34
35
36
4
5d
6
3a
3b
3c
3d
3a
3b
3c
3d
3a
3b
3c
3d
7
8
9e
10
11
12d
13e
14
15
16e
—
—
—
◦
a Reaction conditions: epoxide (1.0 mmol), thiol (1.0 mmol), catalyst (0.5 mol%), 30 C. b Determined by GC analyses. c Yield of the isolated pure
product. d 2 mol% of catalyst were used. e Reaction conditions: epoxide (2.0 mmol), thiol (1.0 mmol), catalyst (5 mol%), 60 ◦C.
realizing highly efficient processes featuring a minimal waste
production.
Experimental Section
Compounds 4,7a 6,10 7,11 8,12 9,13 13,7a 14,14 15,12 16,15 17,16
21,7a 22,10 23,10 24,17 25 (a and b regioisomers),18 26 (a and
21
b regioisomers),19 27 (a and b regioisomers),20 28 (a and
22
b
regioisomers), 30,19 31,22 32,23 33,24 34,19 35,20 36,25 are
known and their spectroscopic data are in agreement with those
reported in literature.
Compounds 10, 11, 12, 18, 19, 20, 29 are new compounds.
Characterization data for catalysts 1a–g, characterization
data and copies of the 1H and 13C NMR spectra for compounds
10, 11, 12, 18, 19, 20, 29 are in the supporting information.†
Representative experimental procedure for the synthesis of
JJ-TBD catalysts. Preparation of 1a
Scheme 2 Schematic of the cyclic continuous-flow reactor in the
reaction of 5a and 3b catalyzed by 1b.
A two-necked round-bottomed flask (25 ml) equipped with a
magnetic stirrer, reflux condenser, and inert gas in- and outlet
was charged with 7 ml of anhydrous THF and TBD (0.134 g, 0.96
mmol). The stirred solution was cooled to -78 ◦C, and n-BuLi in
hexane (0.6 ml, 0.96 mmol) was added drop wise under nitrogen.
After the addition was complete, the solution was stirred for
an additional 2 h and JandaJel-Cl (200–400 mesh, 2% cross-
linked, 1.163 g, 0.55 mmol(Cl) g-1, 0.64 mmol) was added to
the solution. The mixture was allowed to warm slowly to room
temperature and stirred for 48 h under nitrogen. Then 2 ml
of methanol were added to the reaction mixture, the polymer
beads were isolated by filtration and washed thoroughly with
THF-MeOH (1 : 1), methanol, methanol-water (1 : 1), acetone,
THF, and dichloromethane. The catalyst was subsequently dried
representative processes, in this communication we have showed
that JandaJel resins 1a–d ensure an excellent catalytic efficiency
under SolFC compared to that of polystyrene matrices. Thanks
to the greater spacing between the linear polymeric chains,
reactants can access the active sites more easily without the
help of a swelling medium. In addition the use of an organic
medium led to a dramatic drop of the catalytic efficiency
confirming our approach to the use of polymer-supported
catalyst under SolFC. To optimize the recovery and reuse
of the catalyst, a very efficient continuous-flow procedure
operating under SolFC has been representatively realized. The
results obtained prove the efficiency of this approach for
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◦
under vacuum for 15 h at room temperature and 4 h at 50 C
3 M. E. Wilson, K. Paech, W.-J. Zhou and M. J. Kurth, J. Org. Chem.,
1998, 63, 5094–5099.
furnishing JJ-TBD 1a as a yellow solid. Elemental analysis: C,
4 (a) C. Gambs, T. Dickerson, S. Mahajen, L. B. Pasternack and K.
D. Janda, J. Org. Chem., 2003, 68, 3673–3678; (b) A. R. Vaino and
K. D. Janda, J. Comb. Chem., 2000, 2, 579–596; (c) A. R. Vaino,
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(d) P. H. Toy and K. D. Janda, Tetrahedron Lett., 1999, 40, 6329–
6332.
5 As representative examples see (a) H. Matsushita, S.-H. Lee, M.
Joung, B. Clapham and K. D. Janda, Tetrahedron Lett., 2004, 45,
313–316; (b) S.-H. Lee, H. Matsushita, G. Koch, B. Clapham and
K. D. Janda, J. Comb. Chem., 2004, 6, 822–827; (c) J. Tois and A.
Koskinen, Tetrahedron Lett., 2003, 44, 2093–2095; (d) T. Fujimori,
P. Wirsching and K. D. Janda, J. Comb. Chem., 2003, 5, 625–
631.
6 As representative recent examples see (a) H.-L. Liu and H.-F. Jiang,
Tetrahedron, 2008, 64, 2120–2125; (b) H.-L. Liu, H.-F. Jiang, L. Xu
and H.-Y. Zhan, Tetrahedron Lett., 2007, 48, 8371–8375; (c) T. Y.
S. But, Y. Tashino, H. Togo and P. H. Toy, Org. Biomol. Chem.,
2005, 3, 970–971; (d) L.-J. Zhao, C. K.-W. Kwong, M. Shi and
P. H. Toy, Tetrahedron, 2005, 61, 12026–12032; (e) L.-J. Zhao,
H. S. He, M. Shi and P. H. Toy, J. Comb. Chem., 2004, 6, 680–
683.
87.76; H, 7.69; N, 1.45; 0.34 mmol TBD g-1.
Representative experimental procedure for the Michael addition:
reaction of 1-butanethiol (3a) to (E)-benzylideneacetone (2a)
catalyzed by polymer-supported-TBD 1b under solvent-free
conditions
In a screw-capped vial equipped with a magnetic stirrer JJ-
TBD 1b (0.021 g, 0.48 mmol TBD g-1, 0.01 mmol), (E)-
benzylideneacetone (2a) (0.292 g, 2.0 mmol) and 1-butanethiol
(3a) (0.215 ml, 2.0 mmol) were consecutively added and the
resulting mixture was left under stirring at 30 ◦C. After 40 min,
ethyl acetate was added, the catalyst recovered by filtration, and
the organic solvent evaporated under vacuum to give ≥ 98% pure
4-(butylthio)-4-phenylbutan-2-one (4) as a yellowish oil (0.463
g, 98% yield).
7 As representative examples see: (a) S. Bonollo, D. Lanari, F. Pizzo
and L. Vaccaro, Org. Lett., 2011, 13, 2150–2152; (b) S. Bonollo,
D. Lanari and L. Vaccaro, Eur. J. Org. Chem., 2011, 2587–2598;
(c) S. Bonollo, D. Lanari, A. Marrocchi and L. Vaccaro, Curr. Org.
Synth., 2011, 8, 319–329; (d) D. Lanari, R. Ballini, A. Palmieri,
F. Pizzo and L. Vaccaro, Eur. J. Org. Chem., 2011, 2874–2884;
(e) S. Bonollo, F. Fringuelli, F. Pizzo and L. Vaccaro, Synlett, 2008,
1574–1578; (f) S. Bonollo, F. Fringuelli, F. Pizzo and L. Vaccaro,
Synlett, 2007, 2683–2686; (g) R. Ballini, L. Barboni, F. Fringuelli,
A. Palmieri, F. Pizzo and L. Vaccaro, Green Chem., 2007, 9, 823–
838; (h) R. Girotti, A. Marrocchi, L. Minuti, O. Piermatti, F. Pizzo
and L. Vaccaro, J. Org. Chem., 2006, 71, 70–74; (i) S. Bonollo, F.
Fringuelli, F. Pizzo and L. Vaccaro, Green Chem., 2006, 8, 960–
964.
8 As representative examples see: (a) A. Zvagulis, S. Bonollo, D. Lanari,
F. Pizzo and L. Vaccaro, Adv. Synth. Catal., 2010, 352, 2489–2496;
(b) F. Fringuelli, D. Lanari, F. Pizzo and L. Vaccaro, Green Chem.,
2010, 12, 1301–1305; (c) T. Angelini, F. Fringuelli, D. Lanari, F.
Pizzo and L. Vaccaro, Tetrahedron Lett., 2010, 51, 1566–1569; (d) F.
Fringuelli, D. Lanari, F. Pizzo and L. Vaccaro, Curr. Org. Synth.,
2009, 6, 203–208; (e) R. Ballini, L. Barboni, L. Castrica, F. Fringuelli,
D. Lanari, F. Pizzo and L. Vaccaro, Adv. Synth. Catal., 2008, 350,
1218–1224; (f) L. Castrica, F. Fringuelli, F. Pizzo and L. Vaccaro,
Lett. Org. Chem., 2008, 5, 602–606; (g) F. Fringuelli, R. Girotti, F.
Pizzo and L. Vaccaro, Org. Lett., 2006, 8, 2487–2489; (h) F. Fringuelli,
R. Girotti, F. Pizzo, E. Zunino and L. Vaccaro, Adv. Synth. Catal.,
2006, 348, 297–300; (i) L. Castrica, F. Fringuelli, L. Gregoli, F.
Pizzo and L. Vaccaro, J. Org. Chem., 2006, 71, 9536–9539; (j) G.
D’Ambrosio, F. Fringuelli, F. Pizzo and L. Vaccaro, Green Chem.,
2005, 7, 874–877; (k) F. Fringuelli, F. Pizzo, S. Tortoioli and L.
Vaccaro, J. Org. Chem., 2004, 69, 8780–8785; (l) F. Fringuelli, F.
Pizzo, C. Vittoriani and L. Vaccaro, Chem. Commun., 2004, 2756–
2757.
Representative experimental procedure for the thiolysis of
epoxides: reaction of 2,3-epoxypropyl-phenylether (5a) by
thiophenol (3b) catalyzed by polymer-supported-TBD 1b under
solvent-free conditions
In a screw-capped vial equipped with a magnetic stirrer JJ-TBD
1b (0.031 g, 0.48 mmol TBD g-1, 0.015 mmol), 2,3-epoxypropyl-
phenylether (5a) (0.410 ml, 3.03 mmol) and thiophenol (3b)
(0.311 ml, 3.03 mmol) were consecutively added and the resulting
mixture was left under stirring at 30 ◦C. After 100 min, ethyl
acetate was added, the catalyst recovered by filtration, and the
organic solvent evaporated under vacuum to give ≥ 98% pure 1-
phenoxy-3-(phenylthio)propan-2-ol (6) as a colourless oil (0.764
g, 97% yield).
Acknowledgements
We gratefully acknowledge the Ministero dell’Istruzione,
dell’Universita` e della Ricerca (MIUR) within the projects
PRIN 2008 and “Firb-Futuro in Ricerca” and the Universities
of Camerino and Perugia for financial support. We also wish
to thank FONDAZIONE Cassa di Risparmio di Perugia for
financial support within the project “Nuovi sistemi catalitici per
la realizzazione di processi a flusso continuo” prot. 2008.031.
0356.
9 (a) R. A. Sheldon, Chem. Commun., 2008, 3352–3365; (b) J. Auge´,
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Notes and references
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