Sol–Gel Immobilized N-Heterocyclic Carbene Gold Complex as a Recyclable Catalyst for the Rearrangement of Allylic Esters and the Cycloisomerization of γ-Alkynoic Acids

Article abstract of DOI:10.1002/cctc.201600632

The synthesis of a bis-silylated (NHC)AuCl (NHC=N-heterocyclic carbene) complex and the formation of a hybrid silica material by the sol–gel process by cogelification with tetraethylorthosilicate under fluoride catalysis are described. This material was characterized by using ²⁹Si solid-state NMR spectroscopy, N₂ sorption measurements, electron microscopy, and elemental analysis. It was tested as a reusable catalyst in the rearrangement of allylic esters in conjunction with a silver salt under microwave conditions and it displayed a much better performance than a homogeneous analogue. This catalyst is active and recyclable in the cycloisomerization of γ-alkynoic acids to five-membered enol-lactones at room temperature in two reaction media, a toluene/water biphasic system and a deep eutectic solvent.

Full text of DOI:10.1002/cctc.201600632

DOI: 10.1002/cctc.201600632
Full Papers
Sol–Gel Immobilized N-Heterocyclic Carbene Gold
Complex as a Recyclable Catalyst for the Rearrangement
of Allylic Esters and the Cycloisomerization of g-Alkynoic
Acids
Meritxell Ferrꢀ,[a] Xavier Cattoꢁn,[b] Michel Wong Chi Man,[c] and Roser Pleixats*[a]
Dedicated to the memory of Professor Robert J. P. Corriu
The synthesis of a bis-silylated (NHC)AuCl (NHC=N-heterocy-
clic carbene) complex and the formation of a hybrid silica ma-
terial by the sol–gel process by cogelification with tetraethylor-
thosilicate under fluoride catalysis are described. This material
was characterized by using 29Si solid-state NMR spectroscopy,
N2 sorption measurements, electron microscopy, and elemental
analysis. It was tested as a reusable catalyst in the rearrange-
ment of allylic esters in conjunction with a silver salt under mi-
crowave conditions and it displayed a much better per-
formance than a homogeneous analogue. This catalyst is
active and recyclable in the cycloisomerization of g-alkynoic
acids to five-membered enol-lactones at room temperature in
two reaction media, a toluene/water biphasic system and
a deep eutectic solvent.
Introduction
AuI and AuIII species have gained an increasing popularity in
homogeneous catalysis[1] because of their excellent p-coordi-
nating properties, which allow the activation of unsaturated
substrates, such as alkynes,[2] alkenes,[3] and allenes,[4] towards
the addition of a wide variety of nucleophiles. The stability, re-
activity, and selectivity of Au catalysts can be fine-tuned by the
proper choice of ancillary ligands. In this sense, N-heterocyclic
carbenes (NHC) combine strong s-donating properties with
a steric profile that leads to both the electronic and steric sta-
bilization of the metal center and enhancement of its activity,
which lead to significant advantages over classical complexes
that bear phosphine or phosphite ligands.[5] NHC-Au catalysis
encompasses a wide variety of organic transformations such as
skeletal rearrangements, cycloisomerizations, the addition of
water to alkynes and nitriles, and CÀH bond activation.[5–7]
In particular, (NHC)AuCl complexes have been involved in the
catalytic cycloisomerization of g-alkynoic acids into enol-lac-
rearrangement of allylic acetates.[10] In addition, they have
been decisive for the systematic isolation of vinyl- and aryl-
gold(I) intermediates in catalytic cycles.[11]
However, homogeneous catalysts present several drawbacks
such as deactivation through decomposition pathways and
difficult recycling, which can be overcome by heterogenization.
To the best of our knowledge, very few examples of immobi-
lized (NHC)AuI catalysts have been reported. (NHC)AuCl
complexes supported on porous organic polymers (POP) have
been used as heterogeneous catalysts for alkyne hydration.[12]
Corma et al. have described the grafting of a (NHC)AuCl com-
plex onto mesoporous silica MCM-41 and zeolite ITQ-2 for the
hydrogenation of diethyl citraconate.[13] Several (NHC)-dioxo-
lane AuI complexes grafted to MCM-41 have been reported as
reusable catalysts in a three-component coupling reaction of
aldehydes, amines, and terminal alkynes to afford
propargylamines.[14]
Following our studies on NHC-Pd,[15] NHC-Ru,[16] and NHC-
Rh[17] recyclable catalysts prepared by sol–gel methodolo-
gies,[18] we present herein our results on the synthesis of
a silica-supported (NHC)AuCl catalyst obtained by a sol–gel
procedure from a silylated NHC-gold(I) precursor and its activi-
ty and reusability in the rearrangement of allylic esters and the
cycloisomerization of g-alkynoic acids.
tones[8,9] and, in conjunction with
a
silver salt, in the
[a] Dr. M. Ferrꢀ, Prof. R. Pleixats
Department of Chemistry and Centro de Innovaciꢁn en Quꢂmica Avanzada
(ORFEO-CINQA)
Universitat Autꢃnoma de Barcelona
08193-Cerdanyola del Vallꢄs (Barcelona) (Spain)
E-mail: roser.pleixats@uab.cat
[b] Dr. X. Cattoꢅn
Inst. Nꢀel, CNRS and Univ Grenoble-Alpes
38042 Grenoble (France)
Results and Discussion
[c] Dr. M. Wong Chi Man
The synthesis of the bis-silylated (NHC)AuCl precursor 3 was
undertaken as summarized in Scheme 1. meso-Diallyl dihydroi-
midazolium chloride (1)[16b,17] was treated with silver oxide in
dichloromethane at room temperature. Further transmetalla-
tion of the corresponding [NHC-AgI] species in the presence of
Institut Charles Gerhardt Montpellier UMR 5253
CNRS-Universitꢀ de Montpellier-ENSCM
34296-Montpellier (France)
Supporting information and ORCID number from the author for this arti-
cle can be found under http://dx.doi.org/10.1002/cctc.201600632.
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Scheme 1. Synthesis of the bis-silylated AuI complex 3.
an equivalent of chloro(dimethyl sulfide)gold(I)[19] in dichloro-
methane at room temperature, afforded the gold(I) complex 2
in 88% yield after chromatographic purification. Hydrosilyla-
tion with trichlorosilane in the presence of the Karstedt cata-
lyst followed by treatment with ethanol/triethylamine afforded
3 in 26% isolated yield.
The organosilica M1 was prepared from 3 by cogelification
with tetraethyl orthosilicate (TEOS; 3/TEOS molar ratio of 1:30).
The reaction was performed under Ar in anhydrous and de-
gassed DMF at room temperature using a stoichiometric
amount of water (with respect to the ethoxy groups) and
tetrabutylammonium fluoride as
a
nucleophilic catalyst
To improve the overall yield of 3 an alternative synthesis
was attempted, namely, through a free carbene route from the
bis-silylated dihydroimidazolium chloride 4 obtained from 1 as
described previously[16b] (Scheme 2).
(1 mol% with respect to Si; Scheme 3). The solution gelified
after 30 min and was aged for six days at room temperature
under Ar. Then, the solid was collected by filtration and
washed successively with dry and degassed ethanol, acetone,
and diethyl ether. Residual DMF entrapped in the material was
removed by Soxhlet extraction with dry and degassed chloro-
form. The resulting white powder was dried overnight at 408C
under vacuum.
This material was characterized by 29Si solid-state NMR spec-
troscopy, N2 adsorption–desorption measurements, TEM, SEM,
energy-dispersive X-ray spectroscopy (EDX), and elemental
analysis. The 29Si cross-polarization magic-angle spinning (CP-
MAS) NMR spectrum of M1 (Supporting Information) con-
firmed the covalent bonding of the organic moiety to the silica
matrix by the presence of a T3 signal (CÀSiO3 environments) at
d=À66.0 ppm in addition to the characteristic Q3 and Q4 sig-
nals that correspond to the condensed TEOS at d=À104.0
Although the treatment of 4 with chloro(dimethylsulfide)-
gold(I) in acetone at 608C[20] or chloro(tetrahydrothiophene)-
gold(I) in dichloromethane at room temperature[21] allowed the
formation of 3 in respectable 69 and 72% yields (after the fil-
tration of the crude mixture through Celite) on small scale
(0.1 mmol), the reactions failed if they were scaled up to
0.3 mmol. Moreover, the solid obtained in the second case de-
composed and turned from white to black after two days. The
transmetallation route from the silylated salt 4 was also unsuc-
cessful and led to decomposition and the formation of a pink-
ish solution. Notably, in our hands, the syntheses of the analo-
gous NHC complexes of Ru[16b] and Rh[17] from the bis-silylated
salt 4 were successful.
Scheme 2. Alternative synthesis of the bis-silylated AuI complex 3.
Scheme 3. Synthesis of hybrid silica material M1.
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and À110.0 ppm, respectively. The high dilution of the organic
moiety in the inorganic matrix precluded the observation of
the corresponding signals in the 13C solid-state NMR and IR
spectra.
As observed by TEM (Figure 1), the material is formed as an
agglomerate of primary nanoparticles (50–100 nm). This should
enable the easy diffusion of the reaction mixtures within the
material through the interstices. Furthermore, an important
mesoporosity (total pore volume 0.62 cm3 gÀ1 and BET surface
Scheme 4. Rearrangement of allylic esters under AuI catalysis.
during the formation of the material by the sol–gel process. It
should be emphasized that the material is a white solid (Fig-
ure S1) and neither TEM nor SEM showed clear evidence of the
presence of Au nanoparticles that would result from the partial
decomposition of the precursor 3 during the sol–gel process.
First, as a model and for comparison, the homogeneous AuI
complex 2 (3 mol%) was tested in the allylic isomerizations of
ester 5a in conjunction with a silver salt (2 mol% of AgBF4) in
1,2-dichloroethane (DCE) (Scheme 4 and Table 1). As a very low
conversion was achieved with 2 as the catalyst after 2 days of
the reaction of 5a in 1,2-dichloroethane heated to reflux, two
experiments with this homogeneous catalyst were performed
under microwave heating (200 W, 808C; Table 1, entries 1 and
2). Although the complete conversion was observed by
1H NMR spectroscopy, the isolated yields of 6a were only 17
and 15% at 30 and 12 min of reaction, respectively, because of
extensive oligomerization. Strikingly, under the same micro-
wave conditions, the desired product 6a was obtained in 93%
yield after 40 min by using the supported Au complex M1,
without the formation of any byproducts (Table 1, entry 3,
cycle 1). Moreover, the reusability of the catalyst was
demonstrated, and two more runs could be performed with
Figure 1. TEM image of hybrid silica material M1. Scale bar=500 nm.
area 818 m2 gÀ1) is present within the particles, as deduced
from N2 sorption experiments (Figure 2). The isotherm is of
Type IV, representative of a mesoporous material with a sharp
pore diameter distribution centered at 35 ꢃ. Notably, there is
no noticeable microporosity, as deduced from the t-plot. The
Au content was determined by inductively coupled plasma
optic emission spectroscopy (ICP-OES) analysis (2.99% w/w,
0.15 mmol Au/g material). As the elemental analysis revealed
an Au/Si ratio of 1:66 and the theoretical value should be 1:32
(if we assume complete condensation), it is likely that incom-
plete condensation and the partial loss of the metal occur
Figure 2. N2-sorption isotherm of M1. Inset: pore size distribution (BJH on desorption branch).
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interphase, can be in contact with both water and toluene. To
our delight, after 2 days, a complete conversion was observed,
and 8a was isolated in 75% yield. Moreover, the catalyst M1
could be recycled successfully in up to six runs and suffered
no damage to remain as a white solid. In all cases, the conver-
sion was complete and good isolated yields of 8a were
obtained (Scheme 5).
Table 1. Rearrangement of allylic esters catalyzed by AuI.
Entry[a]
Cat/5
Cycle
t [min]
6 [%][b]
1
2
3
4
5
6
7
8
2/5a
2/5a
1
1
1
2
3
1
2
3
1
2
3
1
2
3
1
2
30
12
40
40
40
40
40
40
40
40
40
75
60
60
40
40
6a (17)
6a (15)
6a (93)
6a (94)
6a (83)
6b (83)
6b (79)
6b (71)
6c (71)
6c (72)
6c (59)
6d (69)
6d (74)
6d (74)
6e (40)
6e (57)
M1/5a
M1/5a
M1/5a
M1/5b
M1/5b
M1/5b
M1/5c
M1/5c
M1/5c
M1/5d
M1/5d
M1/5d
M1/5e
M1/5e
Encouraged by these results, other alkynoic acids 7b–d
were subjected to the reaction with M1 under the same
conditions (Scheme 6).
9
10
11
12
13
14
15
16
[a] 808C under microwave activation (200 W maximum), 0.25 mmol of 5
in 0.6 mL 1,2-dichloroethane. [b] Isolated yield.
the same batch of catalyst recovered by filtration (Table 1, en-
tries 4 and 5, cycles 2 and 3). Other secondary allylic acetates
5b–c (Table 1, entries 6–8 and 9–11), secondary allylic benzoate
5d (Table 1, entries 12–14), and tertiary allylic acetate 5e
(Table 1, entries 15–16) were also isomerized successfully using
M1 under microwave conditions, and the catalyst was recycled
in each case. The benzoate 5d required longer reaction times.
The need for a Ag salt as the chloride abstractor to generate
the true catalytic species [(NHC)Au]+ has been avoided in the
cycloisomerization of g-alkynoic acids catalyzed by water-solu-
ble (NHC)AuCl complexes in a water/toluene biphasic sys-
tem.[8a] The presence of water facilitates the liberation of the
chloride ligand. This prompted us to assay our supported cata-
lyst M1 in this process under these conditions. In a first set of
experiments, we tested the homogeneous catalyst 2 and M1
(2.5 mol%) in the cycloisomerization of 4-pentynoic acid (7a;
[7a]=0.15m) to the lactone 8a in a 1:1 water/toluene mixture
at room temperature (Scheme 5). Whereas 2 led to an isolated
Scheme 6. Cycloisomerization of 7b–d catalyzed by M1 in water/toluene.
Compounds 7b and 7c, which bear two and one terminal
alkyne units, respectively, afforded the five-membered enol-lac-
tones 8b and 8c selectively. Furthermore, M1 was recycled in
both cases. By contrast, internal diyne 7d was transformed
into a 4:1 mixture of five- and six-membered enol-lactones 8d
and 8d’ (5-exo-dig vs. 6-endo-dig)[8] in an overall 98% yield.
The selectivity decreased upon recycling to a 2:1 mixture in
the second run, although no loss of activity was found. In par-
ticular, the chemoselectivity offered by the catalyst in this bi-
phasic medium is noteworthy as byproducts derived from
alkyne hydration were not observed despite the presence of
water.
The use of green and bio-renewable solvents is a lasting
challenge. In this sense, deep eutectic solvents (DES) have
been used as environmentally friendly solvents in a variety of
applications, which include biological transformations, metal
processing, the purification of biodiesel, organic synthesis, ma-
terials chemistry, and catalytic processes.[22] Several advantages,
such as low toxicity, high availability, low flammability, high re-
cyclability, low volatility, biodegradability, and low price, make
them attractive alternatives for use in academia and industry.
DESs are mostly obtained by mixing a low-cost and readily
available quaternary ammonium salt (such as choline chloride;
ChCl) with a bio-renewable and environmentally benign hydro-
gen-bond donor (such as glycerol (Gly), lactic acid, urea, or
water). Recently, Garcꢄa-Alvarez et al. have reported the cycloi-
somerization of g-alkynoic acids in eutectic mixtures of 1ChCl/
2Urea and 1ChCl/2Gly catalyzed by several AuI species.[23]
Prompted by these precedents, we decided to investigate
the use of the mixture of 1ChCl/2Urea as a reaction medium
in the cycloisomerization reactions of g-alkynoic acids 7b–d
Scheme 5. Cycloisomerization of 4-pentynoic acid under AuI catalysis.
80% yield of 8a after 4 h, no conversion was achieved with
M1 after 18 h under magnetic stirring, and the material
became pinkish. At this point, we thought that the type of stir-
ring could be crucial for the evolution of the reaction and we
changed the magnetic stirrer to a wrist-type shaker (rocking
mixer vibromatic). This enables the good mixing of the
immiscible layers, and the insoluble M1, which remains at the
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dry solvents, which were degassed by the freeze–thaw method
and transferred with a cannula or syringe. Commercial reagents
were used directly as received except for trichlorosilane, which was
purified first by distillation under vacuum and gaseous HCl was
eliminated in a second step by performing two freeze–thaw cycles
(this compound was always used with a secondary cold trap).
Na2SO4 and MgSO4 used to adsorb water from organic layers were
anhydrous. Dry solvents were obtained by using two instruments:
PureSolv (Innovative Technologies: THF, CH2Cl2, and pentane) and
in some experiments MBraun SPS-800 (pentane, CHCl3, CH2Cl2, THF,
Et2O, and toluene). Other dry solvents were prepared using stan-
dard methods: ClCH2CH2Cl, NEt3, and DMF were distilled over CaH2.
Toluene, benzene, and Et2O were heated to reflux with Na/benzo-
phenone, whereas ethanol and methanol were distilled with Mg/I2.
Acetone was distilled with K2CO3. If needed, deuterated NMR sol-
vents, such as CDCl3, were dried by distillation over CaH2. For the
preparation of hybrid silica materials, distilled and deionized water
was used (MilliQ H2O). NMR spectra were recorded at the Servei de
Ressonꢅncia Magnꢆtica Nuclear of the Universitat Autꢇnoma de
Barcelona. 1H NMR, 13C NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C
HMBC, and selective TOCSY spectra were recorded by using Bruker
instruments (DRX-250, DPX-360, and AVANCE-III 400). Chemical
shifts (d) are given in ppm using the residual nondeuterated sol-
vent as the internal reference. 29Si CP-MAS NMR spectra were re-
corded at the Universitꢀ de Montpellier by using a Varian VNMRS
400 MHz instrument. IR spectra were recorded by using a Bruker
Tensor 27 spectrometer using a Golden Gate ATR module with a di-
amond window. Low- and high-resolution mass spectra were ob-
tained by the direct injection of the sample with electrospray tech-
niques by using Hewlett–Packard 5989 A and microTOF-Q instru-
ments, respectively. These analyses were performed by the Servei
d’Anꢅlisi Quꢄmica (SAQ) of the Universitat Autꢇnoma de Barcelona.
Elemental analysis of C, N, H, and Si was performed by the Serveis
Cientꢄfico-Tꢆcnics of the Universitat de Barcelona (SCT-UB). The per-
centages of C, N, and H were determined by combustion by using
an EA-1108 C.E. elemental analyzer from Thermo Scientific with
(bis(5-tert-butyl-2-benzoxazol-2-yl) thiophene (BBOT) as the internal
standard. The content of Si and Au was determined by ICP-OES by
using a multichannel PerkinElmer Optima 3200RL instrument. TLC
was performed with 0.25 mm plates (Alugram Sil G/UV254). Flash
Chromatography was performed under compressed air pressure
with Macherey–Nagel GmbH & Co KG silica gel, which had a parti-
cle size of 230–400 mesh and a pore volume of 0.9 mLgÀ1. The
specific surface area of M1 was determined by the BET method
from N2 adsorption–desorption isotherms obtained by using a Mi-
cromeritics ASAP2020 analyzer after degassing samples for 30 h at
558C under vacuum. The pore size distribution was determined
from the desorption branch using the Barrett–Joyner–Halenda
(BJH) method.[24] TEM images were obtained by using a JEOL 1200
EX II instrument equipped with a SIS Olympus Quemesa 11 Mpixel
camera at the Universitꢀ de Montpellier. SEM images were ob-
tained at the Institut Europꢀen des Membranes in Montpellier by
using a Hitachi S4800 apparatus after Pt metallization. Microwave-
assisted reactions were performed by using a CEM Discoverꢈ
Microwave instrument, which operates between 0 and 300 W
(200 W maximum power used). The reactions were conducted in
a 10 mL sealed reactor. The temperature was measured by using
an IR sensor placed under the reactor. During the experiments
compressed N2 was used to cool the reactor Compounds 1 and 4
were synthesized as reported previously.[16b,17] Acetates 5a–c and
e,[10] benzoate 5d,[10] diynes 7b and d,[25] and 7c[26] were prepared
following literature procedures. 4-Pentynoic acid (7a) was supplied
by Sigma–Aldrich.
(0.15 mmol of 7 in 0.150 g of DES) catalyzed by M1 (2.5 mol%).
Complete conversions were achieved, and the reuse of the cat-
alyst in a second run was successful in all cases, but the re-
quired reaction times were considerably longer (6 days) than
that in the biphasic toluene/water system. The selectivity in
the case of 7d was the same as that attained in the biphasic
medium. Remarkably, no loss of selectivity was observed upon
recycling, and the preference for the formation of the five-
membered enol-lactone 8d over 8d’ (4:1) was maintained in
the second cycle. Notably, magnetic stirring was suitable for
the reactions performed in this medium, and no decomposi-
tion of the catalyst occurred.
Conclusions
A AuI complex 3 that bears an N-heterocyclic carbene (NHC)
ligand functionalized with two silylated groups was synthe-
sized from the 4,5-diallyldihydroimidazolium salt 1 by a route
that involved the formation of (NHC)AgI species, transmetalla-
tion to the corresponding (NHC)AuCl complex, and the subse-
quent hydrosilylation of the two olefinic moieties. The reversal
of the order of steps was unsatisfactory. From this precursor,
the hybrid silica material M1 was prepared by sol–gel cogelifi-
cation with tetraethyl orthosilicate under nucleophilic catalysis.
This mesoporous material was stable towards air and moisture.
M1 was characterized by using standard solid-state techniques,
and the Au content was determined by using ICP-OES. This
material was evaluated as a recyclable catalyst in two different
reactions. It was efficient, in conjunction with a Ag salt, for the
rearrangement of allylic esters under microwave activation.
Interestingly, M1 exhibited a much better performance than its
homogeneous analogue 2. Thus, polymerization byproducts
formed under homogeneous conditions were avoided by the
use of the silica-supported catalyst, which was reused success-
fully in up to three runs. Moreover, M1 displayed a good activi-
ty in the cycloisomerization of g-alkynoic acids to enol-lactones
in a toluene/water biphasic system at room temperature with-
out the need to add Ag salts. The employment of an appropri-
ate stirring method (wrist-type shaker) was a key factor for the
success of the reaction and the preservation of the catalyst,
which was reused in up to six cycles. No alkyne hydration by-
products were observed. The activity and recyclability of M1 in
the cycloisomerization reaction was also tested successfully in
a deep eutectic solvent (1choline chloride/2urea) as a green
reaction medium at room temperature. To the best of our
knowledge, this is the first report of a silica-supported
(NHC)AuI complex prepared by a sol–gel method from a well-
defined silylated NHC-Au precursor, and silica-supported
(NHC)AuI complexes have not been used previously for the
reactions studied here.
Experimental Section
General considerations
If required, experiments were performed using standard high-
vacuum and Schlenk techniques under N2 or Ar atmosphere using
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mixture was stirred at RT for 15 min, then the stirring was stopped.
A gel formed within 20 min and was left to age at RT under Ar and
protected from light for 6 days. At this time, the gel was pulver-
ized, filtered, washed through a cannula with dry and degassed
EtOH (3ꢉ15 mL), dry and degassed acetone (3ꢉ15 mL), and dry
and degassed diethyl ether (3ꢉ15 mL). Then the solid was washed
with dry and degassed CHCl3 in a Soxhlet apparatus for 48 h to
remove the residual DMF entrapped in the material. After drying
overnight at 408C under vacuum, M1 was obtained (1.419 g) as
a white solid. 29Si-CP-MAS NMR (79.5 MHz): d=À66.0 (T3), À104.0
(Q3), À110.0 ppm (Q4); BET SBET: 818 m2 gÀ1; pore diameter distribu-
tion centered around 35 ꢃ; pore volume: 0.62 cm3/g; elemental
analysis: calcd (%) for C27H36N2ClAuSi32O63 (which assumes com-
plete condensation): C 12.8, H 1.44, N 1.11, Au 7.80, Si 35.4; found:
C 17.72, H 2.91, N 1.06, Au 2.99, Si 28.0 (0.15 mmol[(NHC)Au] gM1À1).
Preparation of (meso-4,5-diallyl-1,3-dimesitylimidazolidin-2-
yl)gold(I) chloride (2)
In a Schlenk tube protected from light, a mixture of silver(I) oxide
(0.318 g, 1.37 mmol) and 1 (0.983 g, 2.32 mmol) in dry CH2Cl2
(200 mL) was stirred overnight at RT under Ar. The mixture was fil-
tered through a cannula into another light-protected Schlenk tube,
and (dimethylsulfide)gold(I) chloride (0.669 g, 2.27 mmol) was
added. The resulting mixture was stirred for 24 h at RT under Ar.
Activated carbon was added, and the mixture was filtered through
Celite. The filtrates were concentrated under vacuum. The residue
was purified by flash chromatography using dichloromethane as
1
the eluent to afford 2 as a white solid (1.235 g, 88% yield). H NMR
(CDCl3, 400 MHz): d=6.92 (s, 2H. Ar), 6.91 (s, 2H, Ar), 5.47 (m, 2H,
ÀCH=), 4.97 (m, 4H, CH2=), 4.42 (m, 2H, CH), 2.62–2.57 (m, 2H,
ÀCH2), 2.39 (s, 6H, ÀCH3), 2.36–2.32 (m, 2H, ÀCH2), 2.28 (s, 6H,
ÀCH3), 2.27 ppm (s, 6H, ÀCH3); 13C NMR (CDCl3, 100.6 MHz):
d=195.2, 139.05, 136.8, 135.5, 133.8, 130.3, 130.25, 118.05, 85.0,
31.5, 20.7, 19.06, 17.9 ppm; IR (ATR): n˜ =2918, 1639, 1610, 1376,
1316, 1260, 1091, 994, 913, 852, 799, 642 cmÀ1; HRMS (ESI): m/z
calcd for [12C27H34Au35ClN2+Na]+: 641.1968; found: 641.1974.
Procedure for the rearrangement of allylic acetate 5a cata-
lyzed by 2
AgBF4 (3.9 mg, 0.02 mmol, 2 mol%) was added to a dry 1,2-di-
chloroethane (DCE) solution (1.7 mL) of 2 (0.019 g, 0.03 mmol,
3 mol%) in a microwave vial under protection from light and an
inert atmosphere. The solution became cloudy instantly, and the
mixture was stirred for 1 min before a dry DCE solution (0.6 mL) of
the allylic ester (1 mmol, 1 equiv.) was added. The vial was then
placed in a microwave reactor and heated at 808C for the time in-
dicated in Table 1. The resulting mixture was dissolved in pentane,
filtered through Celite, and the solvent from the filtrate was evapo-
rated. The crude product 6a was purified by flash chromatography
on silica gel.
Preparation of (meso-1,3-dimesityl-4,5-bis(3-(triethoxysilyl)-
propyl)imidazolidin-2-yl)gold(I) chloride (3)
In a sealable Schlenk tube protected from light, 2 (0.961 g,
1.55 mmol) was dissolved in dry CH2Cl2 (20 mL) under Ar. To this
solution, freshly distilled HSiCl3 (3.2 mL, 1.34 gmLÀ1, 31.6 mmol)
and Karstedt’s catalyst (1.0 mL of a solution of 2 wt% Pt in xylene,
0.855 gmLÀ1, 0.08 mmol Pt) were added. The reaction mixture was
stirred under Ar at 408C for 3 h. After this time, excess HSiCl3 was
removed by distillation and collected in a secondary cold trap. The
residue was redissolved in dry CH2Cl2 (20 mL), and the mixture was
cooled to 08C with an ice bath. Then, anhydrous EtOH/NEt3 (1:1,
12 mL) was added slowly, and the mixture was stirred at RT for 2 h.
The volatiles were removed under vacuum, and the residue was
treated with dry toluene and filtered to separate the ammonium
salt. The filtrates were concentrated under vacuum, and the result-
ing brown solid was digested with dry pentane (3ꢉ20 mL) and fil-
tered through a cannula each time to another Schlenk tube. The
filtrates were concentrated under vacuum to afford 3 as a white
solid (0.386 g, 26% yield). The product was stored under Ar and
protected from light at RT. 1H NMR (CDCl3, 400 MHz): d=6.92 (s,
2H, Ar), 6.90 (s, 2H, Ar), 4.25 (m, 2H, CH), 3.71 (q, 12H, J=6.9 Hz,
ÀCH2), 2.39 (s, 6H, ÀCH3), 2.28 (s, 12H, ÀCH3), 1.79 (m, 2H, ÀCH2),
1.63 (m, 2H, ÀCH2), 1.27 (m, 4H, ÀCH2), 1.17 (t, 18H, J=6.9 Hz,
ÀCH3), 0.51 ppm (m, 4H, ÀCH2Si); 13C NMR (CDCl3, 100.6 MHz):
d=194.8, 138.7, 136.9, 135.4, 134.9, 130.25, 130.2, 65.25, 58.2,
31.65, 30.7, 29.4, 26.8, 20.7, 20.45, 18.95, 17.97, 17.88, 10.35 ppm;
IR (ATR): n˜ =2963, 2923, 1482, 1389, 1259, 1166, 1075, 1022, 954,
793 cmÀ1; HRMS (ESI): m/z calcd for [12C39H66Au35ClN2O6Si2+Na]+:
969.3706; found: 969.3706.
General procedure for the rearrangement of allylic esters
catalyzed by M1
AgBF4 (3.9 mg, 0.02 mmol, 2 mol%) was added to a dry DCE solu-
tion (1.7 mL) of M1 (0.197 g, 0.03 mmol, 3 mol%) in a microwave
vial under protection from light and an inert atmosphere. The mix-
ture was stirred for 1 min before a dry DCE solution (0.6 mL) of the
corresponding allylic ester 5 (1 mmol, 1 equiv.) was added. The vial
was then placed in a microwave reactor and heated at 808C for
the time indicated in Table 1. The crude mixture was diluted with
CH2Cl2 (2 mL) and filtered. The insoluble catalytic material M1 was
washed several times with CH2Cl2 (3ꢉ3 mL), and the combined fil-
trates were concentrated under vacuum to yield the desired prod-
uct 6. The recovered catalytic material M1 (which had a pinkish
color) was dried under vacuum and used directly in the next cycle
(a new loading of AgBF4 was needed). Spectral data of compound-
s 6a–e were in accordance with those described in the
literature.[10]
Procedure for the cycloisomerization of 4-pentynoic acid
(7a) catalyzed by 2
Preparation of supported gold(I) catalyst M1 by sol–gel co-
gelification
To a biphasic system composed of toluene (1 mL) and distilled
water (1 mL), 7a (0.3 mmol) and catalyst 2 (2.5 mol% Au) were
added. The mixture was stirred magnetically under air at RT until
the complete conversion of the alkynoic acid was observed by TLC
(hexane/AcOEt 9:1). The organic phase was then separated, the
aqueous layer was extracted with Et2O (3ꢉ1.5 mL), and the com-
bined organic extracts were dried over anhydrous MgSO4 and fil-
tered through a short pad of silica gel using dichloromethane as
the eluent. The volatiles were removed under vacuum to yield 8a.
A
solution of 3 (0.772 g, 0.815 mmol) and TEOS (5.5 mL,
0.94 gmLÀ1, 24.8 mmol) in dry and degassed DMF (20 mL) was pre-
pared in a round-bottomed Schlenk flask under Ar. Under stirring,
a solution of tetrabutylammonium fluoride (TBAF) (0.26 mL, com-
mercial solution 1m in anhydrous THF, 0.26 mmol, 1 mol% F with
respect to Si) and MilliQ water (1.9 mL, 105 mmol, H2O/EtO=1) in
dry and degassed DMF (5 mL) was added to the first solution. The
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1998, 110, 1475–1478; c) A. S. K. Hashmi, T. M. Frost, J. W. Bats, J. Am.
Chem. Soc. 2000, 122, 11553–11554.
[3] For the first gold-catalyzed reactions of alkenes, apart from the inter-
mediates after the first nucleophilic additions to alkynes, see: C. G.
Yang, C. He, J. Am. Chem. Soc. 2005, 127, 6966–6967.
[4] For the first gold-catalyzed reactions of allenes, see: A. S. K. Hashmi, L.
Schwarz, J. H. Choi, T. M. Frost, Angew. Chem. Int. Ed. 2000, 39, 2285–
2288; Angew. Chem. 2000, 112, 2382–2385.
[5] a) N. Marion, S. P. Nolan, Chem. Soc. Rev. 2008, 37, 1776–1782; b) S. P.
Nolan, Acc. Chem. Res. 2011, 44, 91–100.
[6] S. Gaillard, C. S. J. Cazin, S. P. Nolan, Acc. Chem. Res. 2012, 45, 778–787.
[7] For the latest, modular template syntheses of NHC-gold(I) complexes,
see: a) D. Riedel, T. Wurm, K. Graff, M. Rudolph, F. Rominger, A. S. K.
Hashmi, Adv. Synth. Catal. 2015, 357, 1515–1523; b) A. S. K. Hashmi, C.
Lothschꢊtz, K. Graff, T. Hꢋffner, A. Schuster, F. Rominger, Adv. Synth.
Catal. 2011, 353, 1407–1412; c) A. S. K. Hashmi, C. Lothschꢊtz, C. Bçh-
ling, T. Hengst, C. Hubbert, F. Rominger, Adv. Synth. Catal. 2010, 352,
3001–3012.
General procedure for the cycloisomerization of g-alkynoic
acids catalyzed by M1 in toluene/water
To a biphasic system composed of toluene (0.5 mL) and distilled
water (0.5 mL), the alkynoic acid (0.15 mmol) and catalyst M1
(2.5 mol% Au) were added. The mixture was stirred by using a labo-
ratory wrist-type shaker apparatus under air at RT until the
complete conversion of the alkynoic acid was observed by using
TLC (hexane/AcOEt 9:1). The organic phase was then separated,
the aqueous layer was extracted with Et2O (3ꢉ1.5 mL), and the
combined organic extracts were dried over anhydrous MgSO4. The
volatiles were removed under vacuum to yield the corresponding
lactone 8. Recyclability tests were performed by recharging the
system (water+white solid M1) with toluene and the correspond-
ing alkynoic acid. The identity of lactones 8a–c and 8d/8d’ was as-
1
sessed by comparison of their H and 13C NMR spectra with those
reported in the literature.[8a,26]
[8] a) E. Tomꢌs-Mendivil, P. Y. Toullec, J. Borge, S. Conejero, V. Michelet, V.
Cadierno, ACS Catal. 2013, 3, 3086–3098; b) K. Belger, N. Krause, Org.
Biomol. Chem. 2015, 13, 8556–8560.
General procedure for the cycloisomerization of g-alkynoic
acids catalyzed by M1 in DES
[9] a) For a general review that covers the transition-metal-catalyzed addi-
tion of carboxylic acids onto alkynes, see: N. T. Patil, R. D. Kavthe, V. S.
Shinde, Tetrahedron 2012, 68, 8079–8146; for a specific review of gold-
catalyzed OÀH bond addition to unsaturated organic molecules, see: N.
Huguet, A. M. Echavarren, Top. Organomet. Chem. 2013, 43, 291–324.
[10] N. Marion, R. Gealageas, S. P. Nolan, Org. Lett. 2007, 9, 2653–2656; addi-
tion and correction: N. Marion, R. Gealageas, S. P. Nolan, Org. Lett. 2008,
10, 1037.
[11] a) A. S. K. Hashmi, A. M. Schuster, F. Rominger, Angew. Chem. Int. Ed.
2009, 48, 8247–8249; Angew. Chem. 2009, 121, 8396–8398; b) A. S. K.
Hashmi, I. Braun, M. Rudolph, F. Rominger, Organometallics 2012, 31,
644–661.
[12] W. Wang, A. Zheng, P. Zhao, C. Xia, F. Li, ACS Catal. 2014, 4, 321–327.
[13] A. Corma, E. Gutiꢀrrez-Puebla, M. Iglesias, A. Monge, S. Pꢀrez-Ferreras, F.
Sꢌnchez, Adv. Synth. Catal. 2006, 348, 1899–1907. Corma et al. have
also been able to extend heterogeneous Au nanoparticles to homoge-
neous enyne-cycloisomerization-type reactions: S. Carrettin, M. C.
Blanco, A. Corma, A. S. K. Hashmi, Adv. Synth. Catal. 2006, 348, 1283–
1288.
Alkynoic acid
7 (0.15 mmol) was dissolved in 1ChCl/2Urea
(0.150 g) in a vial and then heterogeneous catalyst M1 (2.5 mol%
Au) was added. The mixture was stirred magnetically under air at
RT until the complete conversion of the alkynoic acid was ob-
served by using TLC (hexane/AcOEt 9:1). The reaction mixture was
then extracted with Et2O (3ꢉ1.5 mL), and the combined organic
extracts were dried over anhydrous MgSO4. The volatiles were re-
moved under vacuum to yield the corresponding lactone 8. Recy-
clability tests were performed by recharging the system (DES+solid
M1) with the corresponding alkynoic acid.
Acknowledgements
Financial support from the CNRS, ENSCM, Ministerio de Ciencia
e Innovaciꢁn (MICINN) of Spain (Project CTQ2011-22469), Minis-
terio de Economꢂa y Competitividad (MINECO) of Spain (Projects
CTQ2014-51912-REDC and CTQ2014-53662-P), and DURSI-Gener-
alitat de Catalunya (SGR2014-1105) is gratefully acknowledged.
M.F. thanks the Universitat Autꢃnoma de Barcelona for a predoc-
toral scholarship. Chiara Mauriello Jimenez (ENSCM) is gratefully
acknowledged for preliminary experiments. We thank Joaquꢂn
Garcꢂa-Alvarez for providing a sample of 1ChCl/2Urea.
[14] G. Villaverde, A. Corma, M. Iglesias, F. Sꢌnchez, ACS Catal. 2012, 2, 399–
406.
[15] G. Borja, M. Monge-Marcet, R. Pleixats, T. Parella, X. Cattoꢁn, M. Wong
Chi Man, Eur. J. Org. Chem. 2012, 3625–3635.
[16] a) G. Borja, R. Pleixats, R. Alibꢀs, X. Cattoꢁn, M. Wong Chi Man, Molecules
2010, 15, 5756–5767; b) A. Monge-Marcet, R. Pleixats, X. Cattoꢁn, M.
Wong Chi Man, J. Mol. Catal. A 2012, 357, 59–66; c) A. Monge-Marcet,
R. Pleixats, X. Cattoꢁn, M. Wong Chi Man, Tetrahedron 2013, 69, 341–
348; d) A. Monge-Marcet, R. Pleixats, X. Cattoꢁn, M. Wong Chi Man, J.
Sol–gel Sci. Technol. 2013, 65, 93–103.
[17] M. Fernꢌndez, M. Ferrꢀ, A. Pla-Quintana, T. Parella, R. Pleixats, A. Ro-
glans, Eur. J. Org. Chem. 2014, 6242–6251.
[18] A. Monge-Marcet, R. Pleixats, X. Cattoꢁn, M. Wong Chi Man, Catal. Sci.
Technol. 2011, 1, 1544–1563.
[19] P. de Frꢀmont, N. M. Scott, E. D. Stevens, S. P. Nolan, Organometallics
2005, 24, 2411–2418.
Keywords: carbene ligands · gold · immobilization · sol–gel
processes · supported catalysts
[20] A. Collado, A. Gꢍmez-Suꢌrez, A. R. Martꢄn, A. M. Z. Slawin, S. P. Nolan,
Chem. Commun. 2013, 49, 5541–5543.
[21] R. Visbal, A. Laguna, M. C. Gimeno, Chem. Commun. 2013, 49, 5642–
5644.
[1] For some reviews, see: a) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180–
3211; b) D. J. Gorin, F. D. Toste, Nature 2007, 446, 395–403; c) Z. Li, C.
Brouwer, C. He, Chem. Rev. 2008, 108, 3239–3265; d) A. Arcadi, Chem.
Rev. 2008, 108, 3266–3325; e) E. Jimꢀnez-NuÇez, A. M. Echavarren,
Chem. Rev. 2008, 108, 3326–3350; f) A. Fꢊrstner, Chem. Soc. Rev. 2009,
38, 3208–3221; g) A. S. K. Hashmi, Angew. Chem. Int. Ed. 2010, 49,
5232–5241; Angew. Chem. 2010, 122, 5360–5369; h) A. Corma, A.
Leyva-Pꢀrez, M. J. Sabater, Chem. Rev. 2011, 111, 1657–1712; i) M. Ru-
dolph, A. S. K. Hashmi, Chem. Soc. Rev. 2012, 41, 2448–2462; j) D. Ga-
rayalde, C. Nevado, ACS Catal. 2012, 2, 1462–1479; k) A. S. K. Hashmi,
Acc. Chem. Res. 2014, 47, 864–876; l) W. Yang, A. S. K. Hashmi, Chem.
Soc. Rev. 2014, 43, 2941–2955.
[22] a) Q. Zhang, K. de Oliveira Vigier, S. Royer, F. Jꢀrꢎme, Chem. Soc. Rev.
2012, 41, 7108–7146; b) D. Carriazo, M. C. Serrano, M. C. Gutiꢀrrez, M.-L.
Ferrer, P. Del Monte, Chem. Soc. Rev. 2012, 41, 4996–5014; c) C. Ruß, B.
Kçnig, Green Chem. 2012, 14, 2969–2982; d) E. L. Smith, A. P. Abbott,
K. S. Ryder, Chem. Rev. 2014, 114, 11060–11082; e) J. Garcꢄa-ꢏlvarez, Eur.
J. Inorg. Chem. 2015, 5147–5157; f) D. A. Alonso, A. Baeza, R. Chinchilla,
G. Guillena, I. M. Pastor, D. J. Ramꢍn, Eur. J. Org. Chem. 2016, 612–632;
g) N. Guajardo, C. R. Mꢊller, R. Schrebler, C. Carlesi, P. Domꢄnguez de Ma-
rꢄa, ChemCatChem 2016, 8, 1020–1027; h) C. Vidal, J. Garcꢄa-Alvarez, A.
Hernꢌn-Gꢍmez, A. R. Kennedy, E. Hevia, Angew. Chem. Int. Ed. 2014, 53,
5969–5973; Angew. Chem. 2014, 126, 6079–6083.
[2] For the first gold-catalyzed reactions of alkynes, see: a) Y. Fukuda, K. Uti-
moto, J. Org. Chem. 1991, 56, 3729–3731; b) J. H. Teles, S. Brode, M.
Chabanas, Angew. Chem. Int. Ed. 1998, 37, 1415–1418; Angew. Chem.
ChemCatChem 2016, 8, 1 – 9
www.chemcatchem.org
7
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[23] a) M. J. Rodrꢄguez-ꢏlvarez, C. Vidal, J. Dꢄez, J. Garcꢄa-Alvarez, Chem.
Commun. 2014, 50, 12927–12929; b) C. Vidal, L. Merz, J. Garcꢄa-Alvarez,
Green Chem. 2015, 17, 3870–3878.
[26] E. Tomꢌs-Mendivil, P. Y. Toullec, J. Dꢄez, S. Conejero, V. Michelet, V. Ca-
dierno, Org. Lett. 2012, 14, 2520–2523.
[24] E. P. Barrett, L. G. Joyner, P. P. Halenda, J. Am. Chem. Soc. 1951, 73, 373–
380.
Received: May 25, 2016
Revised: June 17, 2016
[25] M. Wilking, C. Mꢊck-Lichtenfeld, C. G. Daniliuc, U. Hennecke, J. Am.
Chem. Soc. 2013, 135, 8133–8136.
Published online on && &&, 0000
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FULL PAPERS
M. Ferrꢀ, X. Cattoꢅn, M. Wong Chi Man,
R. Pleixats*
&& – &&
Sol–Gel Immobilized N-Heterocyclic
Carbene Gold Complex as
a Recyclable Catalyst for the
Rearrangement of Allylic Esters and
the Cycloisomerization of g-Alkynoic
Acids
Golden gels: The first silica-supported
(NHC)AuCl (NHC=N-heterocyclic car-
bene) complex prepared by a sol–gel
process from a well-defined bis-silylated
precursor is an active and recyclable
catalyst for rearrangement and cycloiso-
merization reactions.
ChemCatChem 2016, 8, 1 – 9
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