Poly(ethylene glycol) as a reaction matrix in platinum- or gold-catalyzed cycloisomerization: A mechanistic investigation

Article abstract of DOI:10.1002/chem.201203800

Design for diversity: A new catalytic system based on PEG-3400 and a metal salt (Pt or Au) was designed to efficiently perform a cycloisomerization reaction under microwave irradiation, which gave diverse heterocycles in good to excellent yields, after a

Full text of DOI:10.1002/chem.201203800

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DOI: 10.1002/chem.201203800
Poly(ethylene glycol) as a Reaction Matrix in Platinum- or Gold-Catalyzed
Cycloisomerization: A Mechanistic Investigation
Rosella Spina, Evelina Colacino, Jean Martinez, and Frꢀdꢀric Lamaty*[a]
Transition-metal-catalyzed cycloisomerization has recently
become a fundamental chemical transformation for the syn-
thesis of a large variety of organic molecules including het-
erocycles.[1] Usually, the reaction is performed under homo-
geneous conditions: a starting material is stirred in an or-
ganic solvent with a metallic salt, often under conventional
heating in an oil bath. Treatment of the reaction including
aqueous work-up, followed by purification by silica gel chro-
matography, provides the expected product. These reactions
are carried out with noble-metal-based catalysts such as
platinum[1i,j,m,2] and gold,[1i,k,m,2,3] which are becoming rare
and expensive. So far, efforts to recover and recycle these
salts in the frame of these transformations have been
scarce.[4] Furthermore most of these reactions are carried
out in “classical” organic solvents, often volatile, toxic and
detrimental to the environment, whereas solvent-consuming
purification methods are required, with the formation of
waste. Herein, we report on an alternative and new highly
efficient catalytic system for the cycloisomerization of alkyn-
yl diols and amino alcohols to provide furan and pyrrole de-
rivatives[4f,5] based on late carbophilic transition-metals such
as platinum or gold mixed in eco-friendly poly(ethylene
glycol) matrix (PEG), under microwave activation. The
elaboration of this system was based on the postulate that
the presence of PEG would stabilize and encapsulate metal-
lic species, preventing aggregation and deactivation. PEG
could also play the role of solvent[6] when used at a tempera-
ture above its melting point, efficiently heated by micro-
waves, thus providing a safe reaction environment. Finally,
recovery of the product and the catalytic system was per-
formed by a simple, fast, and effective precipitation/filtra-
tion work-up to provide the pure product after evaporation
of the filtrate and the catalytic system as PEG-based solid.[7]
The results presented herein confirm the validity of this ap-
proach. We report unprecedented results with platinum and
gold catalysts, by studying the efficiency of the PEG-based
catalytic system, its recycling, its improvement by using an
oxidant, and provide an insight into the nature of the metal-
lic species in the PEG matrix. These new systems compare
favorably to other catalytic systems in terms of efficiency
and handling of the work-up procedure.
The hydroxyalkoxylation was studied in which the Pt or
Au catalyst is activating an alkyne towards nucleophilic
attack by an oxygen or nitrogen atom. The reaction of al-
kynyl diol 1a was used as a test reaction to explore various
catalysts that could catalyze the 5-endo-dig cycloisomeriza-
tion to afford furan 2a when embedded in the PEG 3400-
matrix (Table 1). Temperatures higher than 558C (corre-
sponding to the melting point of PEG-3400) were used.
Since microwave heating is focused on the reaction mixture
and because PEG polymers are effective microwave absorb-
ers, this technique is practical because it ensures the rapid
melting of the polymer during the reaction. In a typical ex-
periment, compound 1a, a transition-metal salt (2 mol%),
and PEG-3400 were charged in a microwave vessel and
heated under microwave irradiation above 508C for 15–
60 min. At the end of the allotted time, the mixture was di-
Table 1. Screening of reaction conditions for the cycloisomerization of
1a.
Entry
Catalyst/Additive[a]
T
t
Yield
[8C]
[min]
[%][b,c]
1
2
3
4
5
6
7
8
AuCl
AuCl(PPh3)/AgOTf
AuCl(PPh3)/AgSbF6
AuCl(PPh3)/AgSbF6
AuCl(PPh3)/AgSbF6
AuCl3
AuCl3
AuCl3/AgOTf
AuCl3/AgSbF6
AuCl3
AuCl3
PtCl2
PtCl2
PtCl2
PtCl2
PtCl2
U
50
50
50
50
50
50
50
50
50
50
50
50
50
80
80
80
60
60
60
15
15
15
60
60
60
30
30
15
30
15
30
30
5 (30)
50 (100)
54 (100)
81 (100)
98[d] (100)
60 (80)
53 (100)
52 (100)
52 (100)
85 (100)
9
10
11
12
13
14
15
16
79[d] (100)
0[d] (0)
15[d] (15)
70 (100)
80 (100)
83[d] (100)
[a] Dr. R. Spina, Dr. E. Colacino, Prof. J. Martinez, Dr. F. Lamaty
Institut des Biomolꢀcules Max Mousseron (IBMM)
CNRS-Universitꢀ Montpellier I et 2, Place E. Bataillon
34095 Montpellier Cedex 5 (France)
[a] Typically: compound 1a (0.13 mmol), PEG-3400 (300 mg), catalyst/ad-
ditive (2 mol%, 1:1 for AuI and 1:3 for AuIII). [b] Yields were determined
by 1H NMR spectroscopic analysis versus internal standard. [c] HPLC
conversion is given in parenthesis. [d] Cooling option was used during mi-
crowave irradiation.
Fax : (+33)4-67-14-48-66
E-mail: frederic.lamaty@univ-montp2.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203800.
Chem. Eur. J. 2013, 19, 3817 – 3821
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3817

luted with a small amount of CH2Cl2, precipitated in Et2O
and filtered. The filtrate was concentrated in vacuo and ana-
lyzed by 1H NMR spectroscopy with an internal standard.
Gold and platinum salts were tested in this reaction and the
results obtained are presented in Table 1. AuClACTHUNRGTNEUNG(PPh3) was
first tested at 508C for 1 h (Table 1, entry 1) but provided
low conversion and product yield. Full conversion of sub-
strate was possible by addition of a silver salt (AgOTf or
AgSbF6), providing a more efficient activation of the sub-
strate through the formation of a more electrophilic cationic
gold(I) species by ligand exchange (Table 1, entries 2 and 3).
However, the expected product was still obtained in moder-
ate yield, probably due to a long reaction time, which
caused its degradation. Adding AgSbF6 and reducing the re-
action time to 15 min maintained the full conversion and im-
proved the yield dramatically (Table 1, entry 4). Silver catal-
ysis was excluded after performing a control experiment in
the absence of gold(I): the final product was obtained in
traces and extensive substrate degradation was observed. By
using the same reaction conditions (AuClACTHUNRTGNEUNG(PPh3)/AgSbF6)
Figure 1. Yields of the isolated substituted furans by cycloisomerization
in PEG/metal/MW conditions.
under microwave irradiation with a simultaneous cooling
system, a quantitative yield of furan 2a was obtained
(Table 1, entry 5). This technique provided sufficient heating
for the reaction to occur without producing concomitant
degradation. A similar study was performed with AuCl3 but
15 min was too short a reaction time (Table 1, entry 6); ex-
tending to one hour provided full conversion but with a
moderate yield (Table 1, entry 7) even in the presence of
silver additives (entries 8 and 9). Finally, furan 2a was ob-
tained in a good yield by reducing the reaction time to
30 min, with or without the cooling technique (Table 1, en-
tries 10 and 11). PtCl2, which to our knowledge was never
used in this cycloisomerization, was also tested. Temperature
adjustment with respect to gold-catalyzed processes was nec-
essary: at 508C (Table 1, entry 12) the reaction did not pro-
ceed after 15 min, whereas only 15% of conversion was ob-
HPLC) and did not require further purification such as re-
crystallization or column chromatography. Both metals, Au
and Pt were very satisfying in this cycloisomerization, espe-
cially when PEG-3400 was heated by microwave irradiation.
The synthesis of furane 2b was also explored under the
same reaction conditions developed for each PEG/metal
catalytic system, but by heating up with an oil bath. Full
conversion of substrate could be obtained only for PEG/PtII
catalytic system, but the final product was obtained in lower
yield (81%). In the case of the catalytic system PEG/AuI/
AgI, incomplete conversion of 1b led to 2b in a lower yield
(80%). In the case of PEG/AuIII system, heating by using an
oil-bath was detrimental, and compound 2b could not be
obtained because of extensive degradation of the substrate.
Compared with other catalytic systems and procedures,[5]
the present process, activated by microwave irradiation, rep-
resents a very practical, efficient, and fast access to furans,
especially with the AuIII catalyst, which is more efficient in
terms of product yields and metal-loading.
By using these general conditions, a similar transforma-
tion, heterocyclization of alkynylamino alcohols into pyr-
roles, was also performed. Di- and trisubstituted pyrroles
4a–e were obtained from linear substrates 3a–e in good to
excellent yields, after separation by filtration from the PEG-
catalytic system, because the conversions were always com-
plete (Scheme 1). Next, we investigated the nature of the
electrophilic catalytic system and its recycling ability. Recy-
cling was tested on the transformation of diol 1b in furan
2b. After the precipitation/filtration step, which provided
furan 2b, the recovered solid consisting of PEG and metal
catalyst was used again in another cycloisomerization of 1b.
This procedure was repeated successively until the catalytic
activity dropped, resulting in an incomplete conversion. This
assay was performed with the three different metallic sys-
tained after 30 min (entry 13).
A higher temperature
(808C), compound 2a was obtained in a very good yield
after only 30 min (Table 1, entries 15 and 16). Reactions per-
formed in the absence of catalyst, that is, in the presence of
AgSbF6 or TfOH only, did not yield any product. To delin-
eate the scope of the reaction, various alkynyl diols were
prepared and cyclized with the three catalytic systems ex-
plored on the model reaction (Figure 1). Generally, a reac-
tion time of only 15–30 min was needed to complete the re-
action. Various substituents could be accepted on the start-
ing material.
It is noteworthy that a monosubstituted furan (2e), in
some cases difficult to isolate and purify,[5c,h] could be effec-
tively prepared by this method starting from a molecule
bearing an unsubstituted alkyne; in addition, no hydration
of the unsubstituted alkyne was observed in this case. The
reaction proved to be general with the different catalysts. In
all cases, the conversions were complete and the yields were
very good to excellent, or very often quantitative. The pre-
cipitation/filtration procedure was performed and afforded
the expected product with an excellent purity (>99% by
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Poly(ethylene glycol) in Platinum- or Gold-Catalyzed Cycloisomerization
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only twice before losing their activity. One possible explana-
tion for the decrease of activity could be the degradation of
the catalyst into various homogeneous species and their loss
during the precipitation/filtration process. This was evaluat-
ed by performing inductively coupled plasma-mass spec-
trometry (ICP-MS) analyses of the organic product obtained
in the filtrate after each run. In the case of platinum, 22%
of the initial amount of catalyst was lost after one run. A
further 11% was lost in the next run, whereas after four
runs, about half of the catalyst was retained in the polymer
matrix. However, even if the amount of catalyst decreased
dramatically, the catalytic activity remained high for each of
these four cycles. In sharp contrast, the amount of catalyst
recovered in the PEG matrix for AuI or AuIII salts was much
higher since in each cycle less than 1% of the initial quanti-
ty catalyst was lost. This probably arises from the oxophilici-
ty of gold, which generates strong metal–polymer interac-
tions. Nevertheless, the catalytic activity dropped dramati-
cally after two runs. Even if the quantity of catalyst retained
in the PEG matrix is the major factor that governs the effi-
cient recycling, another crucial parameter is the oxidation
state of the metal. The cycloisomerization deACTHNURTGNEUsGN cribed herein
Scheme 1. Synthesis of substituted furans by cycloisomerization.
is considered to occur through p-bond activation by ionic
complexes[4e] (Pt2+, Au1+, or Au3+). Microwave irradiation
of a mixture of AuCl3 (or PtCl2) in the presence of PEG re-
sulted in the formation of PEG-stabilized nanoparticles of
Au (or Pt, respectively), as confirmed by performing spec-
troscopic studies such as transmission electronic microscopy
(TEM) and X-ray photoelectron spectroscopy (XPS). These
spherical particles have even, narrow size-distribution, and
arose from the reduction of PtCl2 (ca. 5–7 nm)[8] or AuCl3
(and ca. 3–5 nm), with PEG under microwave activation,
similar to the polyol method[9] used to generate such spe-
cies.[10] This was further confirmed by XPS analysis of the
samples of catalytic systems, which detected a mixture of M0
(M=Pt, Au) and Mn+ (n=2, 3, respectively), the major
metallic species being the reduced metal, resulting in a deac-
tivation of the catalyst.[8,11] The catalytic cycle describing
this situation in the case of gold[12] is depicted in Scheme 2.
Figure 3 presents the XPS spectrum of gold nanoparticles
after the first run followed by a precipitation/filtration
work-up. After deconvolution, the Au 4f spectrum is ob-
served presenting a doublet signal centered on 87.84 and
84.20 eV, respectively, corresponding to the Au 4f7/2 and
tems used in this study and the results are presented in
Figure 2. The Pt-catalyzed cycloisomerization was per-
formed four times without a loss of catalytic activity, each
time resulting in a complete conversion and a very good
yield ranging from 82 to 92%. This was not the case for the
AuClACHTUNGTRENNUNG(PPh3)/AgSbF6 or AuCl3 systems, which could be used
Figure 2. Recycling experiments of PEG/metal system and oxidative
modified conditions. Bottom: purple PEG/PtII; yellow PEG/AuI/AgI;
white PEG/AuIII ; red PEG/AuIII +5.
Scheme 2. Redox competitive pathways in gold-catalyzed cyclizations.
Chem. Eur. J. 2013, 19, 3817 – 3821
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www.chemeurj.org
3819

F. Lamaty et al.
Figure 3. XPS spectrum of the PEG/AuCl3 catalytic system after precipi-
tation/filtration work-up.
Figure 4. XPS spectrum of the PEG/Au0 catalytic system after recycling
in the presence of oxidant 5.
Au 4 f5/2 spin-orbit components and satisfying the character-
istic separation of the Au 4f (3.6 eV) of Au0[13] (Figure 3).
This confirms the generation of inactive metallic species
during the course of the cyclization. Because of the better
retention of gold by the polymer matrix, we focused our at-
tention to the study of this metal. To overcome the reduc-
tion problem, an oxidant, which could bring back the oxida-
tion state of Au from 0 to +3 (or another higher oxidation
state), was used concomitantly to activate the catalytic cycle
again and produce the organic molecule.[4e,14] Initial results
obtained by XPS analysis of the platinum catalyst[12] showed
that oxidants such as benzoquinone 5, PhICl2, and PhI-
In summary, we have designed a new catalytic system
made of PEG and a transition metal (Pt or Au) and illus-
trated its efficiency in the cycloisomerization of alkynyl
diols and amino alcohols under microwave irradiation. The
cyclization is very effective with all the substrates and cata-
lysts tested. The heterocycles thus obtained can be efficient-
ly separated from the reaction mixture by a precipitation/fil-
tration technique and obtained with a high purity without
purification by column chromatography. In the case of gold,
the electrophilicity of the nanoparticle catalytic system can
be maintained by adding a catalytic amount of an oxidant
such as benzoquinone. This resulted in the successful recy-
cling of the catalytic system in five consecutive runs.
ACHTUNGTRENNUNG
(OAc)2, could bring the oxidation state of Pt0 (after the first
reaction) back to +2.[4e,12]
Worth noticing is the use of an oxidation method in which
stoichiometric quantities of heavy and toxic metals are
avoided. We investigated the structural validity of this ap-
proach by XPS analysis of the most efficient catalyst based
on gold, AuCl3, in the presence of benzoquinone 5 and PEG
after reaction and precipitation/filtration (Figure 2). The oxi-
dant was employed in a catalytic amount (3 catalytic equiva-
lents, which corresponds to 6 mol%). Despite its high oxida-
tion potential, Au0 could be efficiently reoxidized to a cata-
lytically active species. The complete disappearance of the
Au0 species and the exclusive formation of AuI species with
two signals at 84.52 eV (Au 4f7/2) and 88.18 eV (Au 4f5/2)[13]
is demonstrated by XPS spectra through a shift of the dou-
blet to a higher binding energy.
This can be clearly seen on the comparative spectra in
Figure 4. With the exception of palladium,[14] XPS insight on
the catalytic behavior of PEG-stabilized nanoparticles have
never been reported so far. The Au0 nanoparticles were not
transformed back to the Au +3 complexes but to Au +1,
species which are electrophilic enough to carry out the cycli-
zation efficiently. This correlates with the excellent results
obtained in recycling the catalytic system initially made of
AuCl3 benzoquinone, and PEG. The catalytic system was re-
cycled five times without appreciable loss of activity, where-
as only one recycle was possible working in the absence of
oxidant (Figure 2).
Experimental Section
A typical experimental procedure for the heterocyclization of 1,2-diols:
Substrate 1b (28.3 mg, 0.13 mmol) was added to a mixture of AuCl3
(0.0026 mmol) and PEG-3400 (300 mg). The resulting mixture was
heated by using microwave irradiation at 508C (initial power 400 W and
cooling ON) for 15 min. The reaction mixture was solubilized in CH2Cl2
(1.5 or 2.0 mL) and was added slowly at RT to Et2O (150 mL) for precip-
itation. After 3 h at À188C, filtration of PEG-3400/catalyst and evapora-
tion of ether afforded 2-butyl-4-phenyl-furan ACTHNUTRGNE(NUG 2b) (24.2 mg, 93%) as a
1
pure oil. H NMR (CDCl3, 300 MHz): d=7.51 (s,1H), 7.40–7.37 (m, 2H),
7.27–7.25 (m, 2H), 7.18–7.13 (m, 1H), 6.23 (s, 1H), 2.58 (t, 2H, J=
7.7 Hz), 1.61–1.54 (m, 2H), 1.34–1.27 (m, 2H), 0.87 ppm (t, 3H, J=
7.3 Hz);13C NMR (CDCl3, 75 MHz): d=157.8, 136.6, 133.0, 128.8, 127.6,
127.0, 126.7, 125.7, 104.0, 30.1, 27.9, 22.3, 13.9 ppm; IR (neat): n˜ =2957,
2930, 2863, 1770, 1601, 1551, 1449, 1128, 928, 739 cmÀ1; MS (ESI): m/z
201.1 [M+H]+; HMRS (ESI): m/z calcd for C14H17O: 201.1253
AHCTUNGTRENNUNG
[M+H]+; found: 201.1279.
Acknowledgements
We thank the CNRS and the MERT for financial support. R. S. is grate-
ful to Universitꢂ della Calabria (Italy) for Ph.D. fellowship. We thank
Valꢀrie Flaud (Institut des Matꢀriaux Charles Gerhardt, Montpellier) for
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Poly(ethylene glycol) in Platinum- or Gold-Catalyzed Cycloisomerization
COMMUNICATION
the XPS analyses and for her help in interpreting the results and M.
Franck Godiard (Service Commun de Microscopie Electronique et Ana-
lytique, Universitꢀ Montpellier II) for TEM analyses.
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Keywords: cycloisomerization
·
gold
·
heterocycles
·
nanoparticles · oxidation · platinum
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Received: October 24, 2012
Revised: January 9, 2013
Published online: February 18, 2013
Chem. Eur. J. 2013, 19, 3817 – 3821
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3821