Rhodium-NHC hybrid silica materials as recyclable catalysts for [2+2+2] cycloaddition reactions of alkynes

Article abstract of DOI:10.1002/ejoc.201402649

Bis-silylated dihydroimidazolium salt 1 and monosilylated imidazolium salt 2 are transformed to (NHC)RhCl(COD) complexes 3 and 4, allowing the preparation of hybrid silica materials either by sol-gel or grafting processes. Full characterization of the materials by means of solid state NMR, N₂-sorption measurements, thermogravimetric analysis (TGA) and elemental analysis was followed by evaluation of catalytic activity in the [2+2+2] cycloaddition of alkynes. Excellent yields of the cycloadducts are obtained for up to six consecutive cycles with the grafted material, using simple filtration to recover the catalyst. Both conventional and microwave heating prove effective for the process described.

Full text of DOI:10.1002/ejoc.201402649

FULL PAPER
DOI: 10.1002/ejoc.201402649
Rhodium-NHC Hybrid Silica Materials as Recyclable Catalysts for [2+2+2]
Cycloaddition Reactions of Alkynes
Martí Fernández,[a] Meritxell Ferré,[b] Anna Pla-Quintana,[a] Teodor Parella,[b,c]
Roser Pleixats,*[b] and Anna Roglans*[a]
Keywords: Heterogeneous catalysis / Supported catalysts / Sol-gel processes / Rhodium / Cycloaddition / Carbenes
Bis-silylated dihydroimidazolium salt 1 and monosilylated
imidazolium salt 2 are transformed to (NHC)RhCl(COD)
complexes 3 and 4, allowing the preparation of hybrid silica
and elemental analysis was followed by evaluation of cata-
lytic activity in the [2+2+2] cycloaddition of alkynes. Excel-
lent yields of the cycloadducts are obtained for up to six con-
materials either by sol-gel or grafting processes. Full charac- secutive cycles with the grafted material, using simple fil-
terization of the materials by means of solid state NMR, N2-
sorption measurements, thermogravimetric analysis (TGA)
tration to recover the catalyst. Both conventional and micro-
wave heating prove effective for the process described.
Introduction
reactivity and selectivity. Rh-NHC complexes have only
The transition-metal catalyzed [2+2+2] cycloaddition re- been described by our group as efficient catalysts for intra-
action of unsaturated substrates is a very simple and atom- and partially intramolecular [2+2+2] cycloaddition reac-
economic strategy to obtain polysubstituted six-membered tions of alkynes,[6] although NHC complexes of other met-
carbo- and heterocyclic molecules.[1] Many examples of als such as ruthenium, cobalt, nickel and iron have been
[2+2+2] cycloadditions catalyzed by different transition shown to be efficient catalysts in [2+2+2] cycloaddition re-
metals have been described.[1] With regards to the rhodium- actions by other groups.[7]
catalyzed [2+2+2] cycloaddition of three alkynes to afford
highly substituted and/or annulated benzenes, two catalytic
systems have proven particularly efficient: the Wilkinson
complex, which was first used in this reaction by Grigg et
al.[2] and which is now used as a neutral rhodium com-
plex,[3] and the combination of a cationic rhodium complex
and a BINAP-type ligand. This latter system has been ex-
tensively explored by the groups of Tanaka and Shibata[4]
and forms a highly active and versatile cationic catalyst.
Given that phosphine ligands are prone to oxidation
leading to catalyst deactivation and complicated product
purifications, other stabilizing ligands, such as N-heterocy-
clic carbenes (NHC), are increasingly used as alternatives
for stabilizing transition-metal catalysts.[5] Furthermore,
this type of ligand can have a dramatic influence on both
The recovery and reuse of the catalytic system, especially
when based on transition metals, is an important challenge
in organic synthesis both from an economical and environ-
mental point of view. In the field of [2+2+2] cycloaddition
reactions, few examples of the recyclability of catalytic sys-
tems have been reported. Among them, several strategies
have been tested for the recovery of catalysts: i) the anchor-
ing of a metal complex to a polystyrene resin;[8] ii) the use
of molten salts as immobilizing agents;[9] iii) the use of a
water soluble catalyst;[10] iv) the immobilization of the li-
gand at the surface of a dendrimer,[11] and v) the recovery
of air stable complexes by column chromatography.[12] In
this field, the formation of hybrid silica materials is attract-
ive as a means of achieving supported catalysts based on
metal complexes. These materials combine the advantages
of a silica matrix, such as high surface area, thermal and
mechanical stability and chemical inertness, with the prop-
erties of the organometallic precursor.[13] Indeed, the sol-
gel hydrolytic condensation of organoalkoxysilanes[14] is a
convenient method for the preparation of hybrid silica ma-
terials with targeted properties.[15,16] Moreover, the surfac-
tant-assisted sol-gel synthesis or the grafting on a mesos-
tructured silica are commonly employed routes for the syn-
thesis of mesostructured hybrid silicas[17] with high surface
areas and large pore sizes.
[a] Institut de Química Computacional i Catàlisi and Departament
de Química, Universitat de Girona,
Campus de Montilivi, 17071 Girona, Catalonia, Spain
E-mail: anna.roglans@udg.edu
http://iqcc.udg.edu
[b] Departament de Química and Centro de Innovación en
Química Avanzada (ORFEO-CINQA), Universitat Autònoma
de Barcelona,
Cerdanyola del Vallès, 08193 Barcelona, Catalonia, Spain
E-mail: roser.pleixats@uab.cat
http://www.uab.cat/departament/quimica
[c] Servei de RMN, Universitat Autònoma de Barcelona,
Cerdanyola del Vallès, 08193 Barcelona, Catalonia, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402649.
Silica supported catalysts have also been used in [2+2+2]
cycloaddition reactions as a recovery strategy. Yu et al.[18]
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Rhodium-NHC Hybrid Silica Materials for [2+2+2] Cycloadditions
immobilized a PdII complex with a bidentate bipyridyl li-
gand on a range of polysiloxanes with controllable solubil-
ity, affording materials that proved to be efficient and recy-
clable catalysts in [2+2+2] cyclotrimerization of alkynes.
Blümel et al.[19] described the use of carbonyl nickel(0) cata-
lysts with mono- and bidentate phosphine ligands immobi-
lized on hybrid silica materials as recyclable catalysts for the
cycloaddition of phenylacetylene. More recently, Hapke et
al.[20] described that the air-stable cobalt complex they reco-
vered by column chromatography[12a] could be grafted onto
a silica support through a siloxane linker at the Cp-moiety.
The heterogeneous cobalt catalyst showed moderate activity
in the cycloaddition of 1,6-heptadiyne with benzonitrile to
afford a pyridine derivative. However, the catalytic system
was found to undergo inactivation upon recycling.
Scheme 1. Synthesis of Rh-NHC complexes 3 and 4. Mes: 2,4,6-
trimethylphenyl.
Given our interest in the use of Rh-NHC complexes for
the [2+2+2] cycloaddition reaction, we sought to immobi-
lize Rh-NHC complexes onto silica materials. In this study
Recently, Esteruelas and Yus[25] have described an alter-
imidazolium salts conveniently functionalized with trialk- native method using the [Rh(μ-OMe)(COD)]2 complex in
oxysilyl groups either in the carbon backbone or in the ni- the presence of an imidazolium salt to form the correspond-
trogen atom were complexed with Rh and incorporated into ing Rh-NHC species directly. The methoxide anion depro-
hybrid silica materials either by a sol-gel or a grafting pro- tonates the imidazolium ring releasing methanol from the
cess. A few hybrid silica-based Rh-NHC materials have metal center. Subsequent coordination of the resulting
been previously synthesized and shown to be efficient cata- NHC ligand and the chloride anion to the rhodium atom
lysts in Rh-catalyzed reactions.[21] However, to the best of affords the desired Rh-carbene complex.
our knowledge, these systems have not been reported as cat-
To avoid side-etherification reactions with the triethoxy-
alysts for [2+2+2] cycloadditions, the aim of the present silyl groups of our salts 1 and 2, we used the analogous
work.
non-commercially available ethoxy complex, [Rh(μ-OEt)-
(COD)]2, which was prepared from [Rh(μ-Cl)(COD)]2 by a
nucleophilic substitution of the chloride ligands by ethox-
ide. When bis-silylated imidazolium salt 1 was mixed with
[Rh(μ-OEt)(COD)]2 in anhydrous dichloromethane at room
temperature for 15 hours, desired rhodium complex 3 was
Results and Discussion
Synthesis and Characterization of Rh-NHC
In the last few years, we have made a number of advances obtained in a 60% isolated yield. Application of the same
in the area of recyclable catalysts based on hybrid silica ma- procedure to imidazolium salt 2 over the course of 4 hours,
terials. We have described a silica-supported Hoveyda– afforded a 91% yield of monosilylated rhodium complex 4
Grubbs’ type complex in which the NHC ligand played a (Scheme 1).
key tethering role by performing a sol-gel co-gelification
Both complexes 3 and 4 were fully characterized by ESI-
process of a silylated NHC-Ru monomer with tetraethoxy- HRMS, H and 13C-NMR spectroscopy. ESI-HRMS spec-
silane (TEOS). The new material proved to be an efficient tra of complex 3 showed a peak at m/z = 925.4470, corre-
recyclable catalyst for ring-closing metathesis reactions.[22] sponding to [M – Cl]+. The same behaviour was observed
Furthermore, sol-gel co-condensation of a silylated Pd- for complex 4, which showed a peak at m/z = 601.233, cor-
NHC complex with TEOS to afford the corresponding hy- responding to the same chloride loss observed with 3. As
brid silica material was also described. This supported pal- a result of hindered rotation around the carbene carbon–
ladium complex was efficiently used as a recyclable catalyst rhodium bond, two different sets of signals in a 4:1 ratio
1
in Heck, Suzuki and Sonogashira coupling reactions.[23] are observed in the H and 13C NMR spectra of complex
1
Having developed efficient syntheses for both nitrogen and 3. Six different signals corresponding to the methyl groups
backbone functionalized imidazolium salts, we decided to in the aromatic ring can be seen in the 1H-NMR spectrum:
evaluate their complexation ability with rhodium. There- the major isomer presented three signals at δ = 2.26, 2.30
fore, we first studied the ability of silylated imidazolium and 2.63 ppm whereas the signals corresponding to the
salts 1 and 2 to coordinate rhodium (Scheme 1).
minor isomer appeared at δ = 2.29, 2.36, and 2.52 ppm. The
The first attempt to synthesize a Rh-carbene complex 13C-NMR spectrum displays two sets of doublets at δ =
from the imidazolium salt 1 followed the procedure re- 67.1 ppm (JRh-C = 14.5 Hz, major isomer) and 68.3 ppm
ported by Dastgir and Green.[24] This method was based on (JRh-C = 14.0 Hz, minor isomer), corresponding to the
treatment of the imidazolium salt with Ag2O to afford the methylene carbons of the COD ligand, and at δ = 96.8 ppm
NHC-AgI complex followed by transmetallation with (JRh-C = 6.9 Hz, major isomer) and 96.5 ppm (JRh-C
=
[Rh(μ-Cl)(COD)]2. However, in our case, only decomposi- 7.0 Hz, minor isomer), corresponding to the olefinic car-
tion to elemental silver and the recovery of 1 were observed. bons of the COD ligand. The carbene carbon atom of the
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R. Pleixats, A. Roglans et al.
FULL PAPER
NHC ligand coordinated to Rh appears as a doublet at δ = NMR spectrum, two doublets at δ = 96.7 and 96.9 ppm
213.2 ppm with a coupling constant of JRh-C = 48.0 Hz. corresponded to the olefinic carbons of the COD ligand
High-temperature 1H-NMR experiments at temperatures (Rh-C coupling constants of 7.3 Hz and 7.1 Hz, respec-
up to 95 °C were carried out in an unsuccessful attempt to tively). Two additional doublets at δ = 67.4 and 68.3 ppm
observe the interconversion between the two isomers. The were attributed to the methylene carbons of the COD li-
formation of two diastereoisomers has previously been de- gand (Rh-C coupling constants of 14.7 and 14.2 Hz). The
scribed by Köhler et al.[26] in the synthesis of a Rh complex carbene carbon atom of the NHC ligand coordinated to Rh
derived from 4,5-diallyl-1,3-dimesityl-4,5-dihydroimid- appeared at δ = 181.6 ppm as a doublet with a Rh-C cou-
azolium chloride, which, in our case, is a precursor to 1 (see pling constant of 51.5 Hz.
Supporting Information). In this study, the authors also
found that no dynamic process was observed by variable-
1
temperature H NMR experiments at temperatures up to
70 °C.
In an attempt to identify the relative geometry of the
Synthesis and Characterization of Organic–Inorganic
Hybrid Silica Materials M1–M4
two rotamers, NOESY experiments were planned. Since Rh
complex 3 containing triethoxysilyl groups is prone to hy-
drolysis when subjected to air, the NMR study was per-
formed with the already described Rh-carbene complex de-
rived from the allylic precursor of imidazolium salt 1
(named 3-allyl), which analogously gives a double set of
signals in a 4:1 isomer ratio. The square planar rhodium
complexes feature the carbene ligand in a perpendicular ori-
entation relative to the square-plane of the complex and cis
to the chlorine atom. Both diastereoisomers have a plane
of symmetry corresponding to the square-plane of the com-
plex. The NOESY data reveal that the two allyl chains of
the major isomer point to the Cl atom whereas in the minor
isomer they point to the COD ligand (Figure 1). In the
major isomer both the olefin COD proton and the H of the
NHC core give a cross-peak with the same ortho methyl of
the mesityl; in the minor isomer both protons correlate to
a different ortho methyl of the mesityl which is perpendicu-
lar to the dihydroimidazole ring. We expect the same type
of structures to be formed for complex 3.
Two different materials M1 and M2 were prepared from
complex 3 by co-gelification with different amounts of
tetraethoxysilane (molar ratios of 3/TEOS of 1:14 and 1:30,
respectively). The reactions were performed under an atmo-
sphere of N2 in anhydrous DMF at room temperature un-
der nucleophilic conditions using a stoichiometric amount
of water (with respect to the ethoxy groups) and tetrabut-
ylammonium fluoride as the catalyst (1 mol-% with respect
to Si). Both solutions gelified after one hour and were aged
for six days at room temperature under an N2 atmosphere.
The solids were then filtered and washed successively with
ethanol, acetone and anhydrous diethyl ether. The resulting
powders were dried overnight under vacuum at 40 °C to
afford both materials as orange powders (Scheme 2).
Figure 1. Representation of the two isomers of 3-allyl showing the
major NOESY correlations observed.
Scheme 2. Synthesis of hybrid silica materials M1 and M2.
Using complex 4, two different materials were also pre-
In the 1H-NMR spectra of complex 4, three different
methyl and two aromatic signals corresponding to the mes-
ityl ring were observed. The methylene group attached to pared using two different methodologies. Whereas hybrid
the second nitrogen atom of the NHC ligand appeared silica M3 was prepared by co-gelification with tetraethoxy-
overlapped with COD ligand signals at around 2 ppm. silane (molar ratio 1:30), as described previously for com-
Overlapping was also found at 1.5 ppm between the signals plex 3, material M4 was obtained by grafting to the meso-
of the central CH2 unit of the silylated chain and protons structured silica SBA-15 under standard conditions (in re-
of the cyclooctadiene ligand. The CH2 group next to the Si fluxing anhydrous toluene under N2 atmosphere for 24 h)
atom appeared at δ = 0.77 ppm as a triplet. In the 13C (Scheme 3).
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Rhodium-NHC Hybrid Silica Materials for [2+2+2] Cycloadditions
N of 0.4 for material M4 from the analytical data, which
was close to the expected 0.5 value.
Lower experimental molar ratios for the other materials
M1–M3 were found, indicating that partial decomplexation
had occurred during the formation of materials by the sol-
gel process. The covalent incorporation of the organosilane
into the hybrid materials was ascertained by 29Si CP MAS
solid-state NMR spectroscopy. The 29Si spectra of M1–M3
showed two sets of chemical shifts: T units at around –55
to –67 ppm, resulting from the hydrolysis-condensation of
monomers 3 and 4, and Q units ranging from –90 to
–112 ppm, corresponding to the condensed TEOS, as exem-
plified by the 29Si solid-state NMR of M1 (Figure 2, a).
Only the solid-state 13C NMR of M1 was performed (Fig-
ure 2, b). Significant absorptions appeared at δ = 13.3 ppm
(CH2-Si), confirming the covalency of the ligand to silica
association, and at δ = 213.3 ppm, attributable to the carb-
enic carbon of the NHC ligand coordinated to the rhodium
Scheme 3. Synthesis of hybrid silica materials M3 and M4.
The materials were characterized by solid state 29Si (C-Rh). In the case of the other materials, M2–M4, the high
NMR and 13C NMR (only for M1), N2-sorption measure- dilution of the organic moiety in the inorganic matrix pre-
ments, thermogravimetric analysis (TGA) and elemental cluded the observation of the corresponding absorptions in
analysis, and the amount of rhodium was determined by 13C solid-state NMR spectroscopy. N2-sorption measure-
inductively coupled plasma (ICP). Some analytical and tex- ments revealed a significant porosity for all the materials
tural data are summarized in Table 1. The TGA curves of and high surface areas ranging from 325 to 512 m2 g–1 were
all materials (Supporting Information) showed a weight de- obtained. Functionalized mesostructured silica M4 pre-
crease of less than 5% below 200 °C attributed to the loss pared by grafting exhibited the highest surface area, with a
of the physisorbed water and the remaining uncondensed rather sharp pore size distribution centered at around 59 Å
ethoxy groups. A more significant weight loss was found in and a type IV isotherm typical for mesoporous materials
the 250–500 °C range, assigned to the decomposition of the (Figure 3). Powder X-ray diffraction of M4 confirmed that
organometallic constituent. We obtained a molar ratio Rh/ the original 2D hexagonal mesostructure of the parent silica
Table 1. Selected analytical and textural data of materials M1–M4.
[b]
29Si CP MAS NMR
TGA[a]
[%]
Rh
[%]
Rh
SBET
Øpore
[Å]
Vpore
T2
T3
Q2
Q3
Q4
[mmolg–1] [m2 g–1]
[cm3 g–1]
[c]
M1
M2
M3
M4
–56.3
–55.7
–
–66.2
–65.6
–65.1
–
–93.4
–92.5
–91.6
–
–101.9 –111.4
–101.7 –110.2
–101.8 –111.3
77.06
80.66
75.98
84.30
4.47
2.93
3.15
1.03
0.435
0.285
0.306
0.100
453
493
325
512
0.230 (0.209)
0.280 (0.088)
0.232 (0.071)
0.774 (0.044)
20–30[d]
[e]
–
–
–
58.8
[a] Residual mass measured in the TGA analysis. [b] Total pore volume at p/p0 0.99 (p/p0 = 0.8 for M3); in brackets contribution of
micropores to the Vpore. [c] Micropores, type I isotherm. [d] Type IV isotherm, see ESI. [e] Type II isotherm, see text and ESI.
Figure 2. Solid state NMR spectra for M1.
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R. Pleixats, A. Roglans et al.
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Figure 3. N2 adsorption–desorption isotherm and pore volume distribution of M4.
had not been affected by the grafting (see Supporting Infor- Table 2. Catalytic performance of hybrid silica material M2 in the
[2+2+2] cycloaddition of triyne 5.
mation). However, the BET surface area decreased signifi-
cantly from 732 m2 g–1 for parent SBA-15-type silica to
Entry Solvent
Temp.
[°C]
Cycle Reaction time Yield of 6
[h]
[%][a]
512 m2 g–1 for M4, in a clear indication that precursor 4
had been successfully grafted to the parent silica and had
partially filled the pores. For the case of materials derived
from sol-gel methodologies, M1 was found to be a micro-
porous solid (type I isotherm), whereas M2 exhibited a type
IV isotherm and the mesoporous material M3 presented a
type II isotherm according to IUPAC rules.[27] In the case
of M3, the large amount of nitrogen adsorbed at p/p0 Ͼ 0.8
appears to arise from nitrogen that condenses in the voids
between particles (see Supporting Information).
1
2
3
4
5
6
7
8
toluene
DCE
EtOH
toluene
toluene
DCE
25
25
25
80
80
80
80
80
80
80
80
1
1
1
1
2
1
2
1
2
3
4
3
3
3
5
30
3
30
3
22
28
33
0
0
0
61
16[b]
69
10[b]
98
84
92
95
DCE
EtOH
EtOH
EtOH
EtOH
9
10
11
1
[a] Isolated yield. [b] Calculated by H-NMR spectroscopy.
Catalytic Activity of the Supported Catalysts in [2+2+2]
Cycloaddition Reactions
(Table 2, Entries 1–3 ). When the reaction temperature was
increased to 80 °C for all solvents, the reactions were com-
pleted after 3–5 hours (Table 2, Entries 4, 6, 8). It can be
seen that ethanol was clearly the best solvent as cyclized
product 6 was obtained in almost quantitative yield
(Table 2, Entry 8). In addition, using ethanol as the solvent,
it was possible to recover the catalyst by filtration and
highly pure cycloadduct 6 was afforded by evaporation of
the organic phase, eliminating the need for column
chromatography. The heterogeneous Rh complex was then
washed with dichloromethane and diethyl ether and used
in a subsequent cycle. With this simple methodology, the
catalytic system could be reused for four cycles affording
good isolated yields of 6, although longer reaction times
were needed (Table 2, Entries 8–11 ). Reusability was also
tested using DCE and toluene as reaction solvents. How-
ever, the reaction yield dropped in the second cycle (Table 2,
Entries 5, 7). When catalyst loading was reduced to 5 mol-
% under the same conditions as shown in Entry 8 [(EtOH,
80 °C) (Table 2, Entry 8)], incomplete conversion was ob-
served even after 30 hours of reaction. Consequently, we
decided to use EtOH as the solvent at 80 °C and 10 mol-
% of catalyst for further studies with the other supported
Once the rhodium hybrid silica materials were prepared
and characterized, their catalytic activity in the [2+2+2] cy-
cloaddition reactions of alkynes and their amenabilities to
recovery and reuse were evaluated.
The supported catalysts were tested in the completely in-
tramolecular version of the [2+2+2] cycloaddition reaction
of triynes, using O-tethered triyne 5 (Scheme 4) as a bench-
mark reaction to establish the best operating conditions.
Accordingly, different solvents and temperatures were
tested with this substrate in the presence of putative catalyst
M2 (Table 2).
Scheme 4. Rh-catalyzed [2+2+2] cycloaddition reactions of triynes.
First, the catalyst loading was set at 10 mol-% and reac- catalysts (Table 3).
tions were carried out at room temperature using three dif-
When the other materials were tested in the cycload-
ferent solvents (toluene, dichloroethane and ethanol), but dition reaction of triyne 5, M1 gave the lowest yield
no evolution was observed after three hours of reaction (Table 3, Entry 1) whereas M3 and M4 showed similar ac-
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Rhodium-NHC Hybrid Silica Materials for [2+2+2] Cycloadditions
Table 3. Catalytic performance of hybrid silica materials M1, M3 Table 4. Catalytic performance of hybrid silica material M4 in the
and M4 in the [2+2+2] cycloaddition of triyne 5.[a]
[2+2+2] cycloaddition of triynes 7 and 9.[a]
Entry
Material
Cycle
Reaction time
[h]
Yield of 6
Entry
Substrate
Cycle
Reaction time
[h]
Product
[%][b]
(yield [%][b]
)
1
2
3
4
5
6
7
8
M1
M3
M4
M4
M4
M4
M4
M4
1
1
1
2
3
4
5
6
4
3
77
85
85
85
92
1
2
3
4
5
6
7
8
9
10
7
7
7
7
7
9
9
9
9
9
1
2
3
4
5
1
2
3
4
5
4
8 (99)
16
23
25
55
3
19
24
31
42
8 (96)
2.5
16
19
23
24
48
8 (98)
8 (98)
8 (81)[c]
10 (99)
10 (96)
10 (97)
10 (99)
10 (98)
93
89[c]
95
[a] All reactions were carried out in EtOH at 80 °C. [b] Isolated
yield. [c] Yield calculated by H-NMR spectroscopy.
1
[a] All reactions were carried out in EtOH at 80 °C. [b] Isolated
1
yield. [c] Yield calculated by H-NMR spectroscopy.
tivities (Table 3, Entries 2, 3). The recyclability of M4 was
then assessed. Material M4 was reused successfully for 6
cycles, although longer reaction times were required upon
recycling to achieve full conversions (Table 3, Entries 3–8).
The extent of rhodium leaching was determined for the
four materials. After complete conversion of 5 in the first
cycle, the rhodium content in the crude product 6 was de-
termined by ICP-MS analysis and losses of 5.7% for M1,
12.2% for M2, 8.4% for M3 and 12.15% for M4 were found
with respect to the initial amount of rhodium added as cat-
alyst. In the case of M4 the rhodium content was also mea-
sured after the second cycle and the loss was found to be
9.76%. A hot filtration test was performed with catalyst M4
in the cycloaddition of triyne 5. Catalyst M4 was filtered
off from the hot reaction mixture under the conditions de-
tailed in Table 3 after 4 min of reaction (17% GC conver-
sion of 5) and the remaining filtrate was made to react un-
der the same conditions. After 2.5 h, the conversion, as de-
termined by GC analysis, increased to 56% suggesting a
role for a homogeneous pathway in addition to the M4-
Table 5. Catalytic performance of hybrid silica material M4 under
microwave irradiation.
Cycle
Reaction time [min]
Yield of 6 [%][a]
1
2
3
4
15
45
80
77
84
88
80
120
[a] Isolated yield.
As shown in Table 5 microwave heating efficiently pro-
catalyzed reaction. We hypothesize that homogeneous Rh moted the cycloaddition reaction of triyne 5 giving excellent
species released from the immobilized rhodium system are, yields of 6 in significantly shorter reaction times relative to
at least in part, responsible for catalytic generation of 6 conventional heating. For instance, when comparing Entry
from 5.
6 of Table 3 with Entry 4 of Table 5, we observe that the
To explore the scope of the reaction, the cycloaddition of reaction time for cycle four is reduced from 1 day to just
N-tosyl-tethered triyne 7 and triyne 9 bearing two different 120 min. In contrast, rhodium leaching after the first and
tethers (N-Ts and O) were tested with M4 under the opti- second run was 14.94% and 12.57% respectively, slightly
mized reaction conditions (EtOH, 80 °C). The results are higher than with conventional heating.
shown in Table 4.
Finally, we wanted to test the catalytic activity of mate-
For both substrates 7 and 9, M4 proved to have high rial M4 in the partially intramolecular version of the
activity and good recyclability. Almost quantitative yields [2+2+2] cycloaddition reaction between several diynes 11
of 8 and 10 were obtained, respectively, in five successive and monoalkynes 12. We used the same optimized reaction
cycles (Table 4, Entries 1–5 for 7 and Entries 6–10 for 9), conditions as for the cycloaddition of triynes. Both terminal
although reaction times did require substantial increases af- (Table 6, Entries 1–4) and non-terminal (Table 6, Entries 5,
ter the first run.
6) diynes with different tethers were active in this process.
In order to shorten the reaction times, especially after the However, the use of non-terminal diynes made it necessary
first cycle, we carried out cycloadditions under microwave to heat the reaction to 110 °C using nBuOH as the solvent.
irradiation conditions. The application of microwaves in or- Non-terminal diynes are clearly less reactive than terminal
ganic synthesis and catalysis as an alternative to conven- diynes under these reaction conditions. Both mono- and di-
tional heating has increased considerably in recent years.[28] substituted alkynes 12 gave desired cycloadducts in good
We used this heating system in the cycloaddition of triyne yields. In addition, the reusability of M4 was assessed in
5 to afford tricyclic derivative 6 using catalyst M4 in EtOH the Entry 1 reaction (Table 6, Entry 1); cycloadduct 13aa
at 80 °C (Table 5).
was generated in 85% yield after 4.5 h.
Eur. J. Org. Chem. 2014, 6242–6251
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6247

R. Pleixats, A. Roglans et al.
FULL PAPER
ods. Details for the synthesis of triyne 9, performed here for the
first time, are given in the Supporting Information. SBA-15-type
mesostructured silica was synthesized as described previously.[31]
Table 6. Catalytic performance of hybrid silica material M4 in the
[2+2+2] cycloaddition between diynes 11 and monoalkynes 12.
Synthesis of Bis-silylated Rh-NHC Complex 3: Imidazolium salt 1
(2.32 mmol) and [Rh(μ-OEt)(COD)]2 (1.17 mmol) were transferred
into a Schlenk tube under N2 atmosphere and dissolved in anhy-
drous CH2Cl2 (50 mL). The reaction mixture was stirred at room
temperature for 15 h (TLC monitoring). The solvent was evapo-
rated under vacuum and the residue was treated with anhydrous
hexanes until the filtrate was colorless. The combined filtrates were
then concentrated under vacuum to give Rh complex 3 as an
orange gum (1.33 g, 60% yield). 1H NMR (400 MHz, CDCl3): δ =
0.49 (t, 3JH,H = 8.2 Hz, 4 H, major + minor, CH2Si), 1.142 (t, 3JH,H
= 7.0 Hz, 18 H, major, OCH2CH3), 1.147 (t, 3JH,H = 7.0 Hz, 18 H,
Entry 11 (X, R1)
12 (R2, R3)
Reaction time Yield of 13
[h]
[%][a]
1
11a (O, H)
12a (CH2OH, H)
12a (CH2OH, H)
12b (Ph, H)
2
4.5
1
13aa (100)
13aa (85)[b]
13ab (50)
11a (O, H)
2
11a (O, H)
3
4
11b (NTs, H)
11b (NTs, H)
11c (O, CH3)
11c (O, CH3)
12a (CH2OH, H)
12b (Ph, H)
12a (CH2OH, H)
12c (CH2OH,
CH2OH)
3
5
2.5
5
13ba (100)
13bb (72)
minor, OCH2CH3), 1.37–1.76 (m, 16 H, major + minor,
CH2CH2CH2Si, CH2CH2CH2Si, COD-CH2), 2.26 (s, 6 H, major,
CH3), 2.30 (s, 6 H, minor, CH3), 2.30 (s, 6 H, major, CH3), 2.36 (s,
6 H, minor, CH3), 2.53 (s, 6 H, minor, CH3), 2.63 (s, 6 H, major,
CH3), 3.02 (br. signal, 2 H, major, COD-CH), 3.32 (br. signal, 2
5[c]
6[c]
13ca (100)
13cc (71)[d]
3
[a] Isolated yield. [b] Second round of catalyst use. [c] Reaction run
in nBuOH at 110 °C. [d] Yield calculated by 1H-NMR spec-
troscopy.
H, minor, COD-CH), 3.683 (q, JH,H = 7.0 Hz, 12 H, major,
3
OCH2CH3), 3.687 (q, JH,H = 7.0 Hz, 12 H, minor, OCH2CH3),
4.01–4.09 (m, 2 H, major + minor, NCH), 4.43 (br. signal, 2 H,
major + minor, COD-CH), 6.90 (s, 2 H, major, Ar), 6.93 (s, 2 H,
minor, Ar), 6.94 (s, 2 H, minor, Ar), 7.00 (s, 2 H, major, Ar) ppm.
13C NMR (75 MHz, CDCl3): δ = 10.6 (CH2Si, major), 10.7 (CH2Si,
minor), 18.2 (OCH2CH3, major + minor), 19.1 (CH3, major), 19.8
(CH3, minor), 20.2 (CH3, minor), 20.6 (CH2, minor), 20.7 (CH2,
major), 20.9 (CH3, major), 21.0 (CH3, minor), 21.7 (CH3, major),
27.9 (CH2, major), 28.0 (CH2, minor), 30.8 (COD-CH2, minor),
31.2 (COD-CH2, major), 32.5 (COD-CH2, major), 32.7 (COD-
CH2, minor), 58.2 (OCH2CH3, major + minor), 65.3 (NCH,
Conclusions
Rhodium(I) complexes bearing an N-heterocyclic carb-
ene ligand functionalized with either two silylated groups at
the saturated carbon backbone (complex 3) or one silylated
group at the nitrogen (complex 4) were synthesized by re-
acting the corresponding imidazolium salts with [Rh(μ-
OEt)(COD)]2. Hybrid silica materials were obtained by co-
gelification of complexes 3 and 4 with different amounts of
tetraethoxysilane, effectively affording hybrid silica materi-
als M1–M3. Monosilylated rhodium complex 4 was also
grafted onto mesostructured silica SBA-15 giving material
M4. The materials obtained were fully characterized with
the standard solid state techniques, making it possible to
verify that the structure of the catalytic site was maintained
in the solid. Furthermore, the rhodium content of the mate-
rials was determined by ICP. Catalytic activities of the new
materials were evaluated in [2+2+2] cycloaddition reactions
of triyne substrates, which afforded tricyclic polysubstituted
benzene derivatives. After screening the reaction conditions,
ethanol at 80 °C was found to be optimal for the catalytic
system giving excellent yields of cycloadducts. It is possible
to separate the catalytic system from the reaction mixture
by simple filtration affording analytically pure product. The
catalyst can be reused up to six times without loss of yield
of the cycloadducts. The catalytic activity of material M4
was also tested in the cycloaddition reaction between diynes
and monoalkynes resulting in good yields of the corre-
sponding cycloadducts. Cycloadducts were obtained in
good purity after simple filtration of the catalyst and evapo-
ration of the solvent.
1
minor), 66.1 (NCH, major), 67.1 (d, JRh,C = 14.5 Hz, COD-CH,
1
major), 68.3 (d, JRh-C = 14.0 Hz, COD-CH, minor), 96.5 (d,
1JRh-C = 7.0 Hz, COD-CH, minor), 96.9 (d, 1JRh,C = 6.9 Hz, COD-
CH, major), 128.3 (CH-Ar, minor), 128.5 (CH-Ar, major), 129.9
(CH-Ar, minor), 130.0 (CH-Ar, major), 134.9 (C-Ar, major), 136.1
(C-Ar, minor), 136.1 (C-Ar, major), 136.5 (C-Ar, minor), 137.2 (C-
Ar, major + minor), 138.2 (C-Ar, minor), 139.3 (C-Ar, major),
1
213.2 (d, JRh,C = 48.0 Hz, Rh-Ccarbene, minor + major) ppm. ESI-
HRMS (m/z): calculated for [C47H78O6N2RhSi2]+: 925.4448, found
925.4470.
Synthesis of Monosilylated Rh-NHC Complex 4: Prepared accord-
ing to the method described for complex 3 with the following spe-
cific conditions: Imidazolium salt 2 (3.87 mmol), [Rh(μ-OEt)-
(COD)]2 (1.95 mmol), anhydrous CH2Cl2 (25 mL), work up with
anhydrous diethyl ether. Yellow solid (2.24 g, 91% yield). 1H NMR
3
(400 MHz, CDCl3): δ = 0.77 (t, JH,H = 8.4 Hz, 2 H, CH2Si), 1.25
3
(t, JH,H
= 7.0 Hz, 9 H, OCH2CH3), 1.46–1.70 (m, 2 H,
CH2CH2CH2 + 2 H, COD-CH2), 1.82 (s, 3 H, CH3), 1.96–2.20 (m,
4 H, COD-CH2 + 2 H, NCH2), 2.37 (s, 3 H, CH3), 2.43 (s, 3 H,
CH3), 2.96 (m, 1 H, COD-CH2), 3.43 (m, 1 H, COD-CH2), 3.86
3
(q, JH,H = 7.0 Hz, 6 H, OCH2CH3), 4.19 (m, 1 H, COD-CH),
4.76–4.88 (m, 2 H, COD-CH), 5.29 (m, 1 H, COD-CH), 6.73 (d,
3JH,H = 1.8 Hz, 1 H, imidazole), 6.90 (br. s, 1 H, Ar), 7.03 (d, 3JH,H
= 1.8 Hz, 1 H, imidazole), 7.09 (br. s, 1 H, Ar) ppm. 13C NMR
(75 MHz, CDCl3): δ = 7.83 (CH2Si), 17.7, 18.3 (OCH2CH3), 19.7,
21.0, 24.6, 27.9 (COD-CH2), 29.1 (COD-CH2), 31.5 (COD-CH2),
1
34.0 (COD-CH2), 54.0 (NCH2), 58.5 (OCH2CH3), 67.4 (d, JRh,C
1
= 14.7 Hz, COD-CH), 68.3 (d, JRh,C = 14.2 Hz, COD-CH), 96.7
Experimental Section
1
1
(d, JRh,C = 7.3 Hz, COD-CH), 96.9 (d, JRh,C = 7.1 Hz, COD-
CH), 120.9 (CH-imidazole), 122.8 (CH-imidazole), 128.0 (CH-Ar),
129.5 (CH-Ar), 134.3 (C-Ar), 136.2 (C-Ar), 137.2 (C-Ar), 138.5 (C-
General: Imidazolium chlorides 1 (see Supporting Information)
and 2[23] were prepared as described previously. [Rh(μ-OEt)-
(COD)]2,[29] triynes 5[2b] and 7[30] were prepared by published meth-
1
Ar), 181.6 (d, JRh,C = 51.5 Hz, Rh-Ccarbene) ppm. ESI-HRMS
6248
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 6242–6251

Rhodium-NHC Hybrid Silica Materials for [2+2+2] Cycloadditions
(m/z): calculated for [C29H46O3N2RhSi]+: 601.2327, found
601.2333.
transferred into the reaction tubes of the reactor and the reaction
mixtures were stirred at 80 °C (external temperature). When there
was no presence of starting material (TLC or GC monitoring), the
stirring was stopped and the reaction mixture was cooled down.
The solution was then filtered. The recovered catalyst was washed
with CH2Cl2 (3ϫ 3 mL) and Et2O (2ϫ 3 mL), dried under vacuum
and directly used in the next cycle. The filtrates were concentrated
under reduced pressure to afford the corresponding product.
1,3,6,8-Tetrahydro-2,7-dioxa-as-indacene (6):[2b] Colourless solid.
1H NMR (400 MHz, CDCl3): δ = 5.03 (br. s, 4 H, CH2O), 5.12
(br. s, 4 H, CH2O), 7.14 (s, 2 H, Ar) ppm.
Preparation of Materials
Material M1: A mixture of TBAF (0.16 mL of 1 m solution in an-
hydrous THF, 0.160 mmol) and doubly deionized water (milliQ)
(0.745 mL, 41.3 mmol) in anhydrous DMF (3 mL) was added to a
stirred solution of complex 3 (0.64 g, 0.6 mmol) and TEOS
(1.70 mL, 8.57 mmol) in anhydrous DMF (10 mL) under N2 atmo-
sphere. The reaction mixture was stirred at room temperature for
5 min. A gel was formed within 1 h and was left to age at room
temperature under N2 atmosphere for 6 days. This gel was pulver-
ized, filtered and washed with EtOH (3ϫ 10 mL), acetone (3ϫ
10 mL) and anhydrous Et2O (3ϫ 10 mL). In order to completely
remove residual DMF, the powder obtained was left in a soxhlet
apparatus for 48 h with chloroform. The powder was dried over-
night at 40 °C under vacuum to afford material M1 as an orange
powder (1.25 g). 29Si-CP-MAS NMR (79.5 MHz): δ = –111.4 (Q4),
–101.9 (Q3), –93.4 (Q2), –66.2 (T3), –56.3 (T2) ppm. 13C-CP-MAS
NMR (100.6 MHz): δ = 13.3, 20.2, 30.7, 60.0, 67.9, 97.0, 98.6,
130.0, 138.5, 213.3 ppm. BET SBET: 453 m2/g; type I sorption iso-
therm; TGA (air, 30 to 700 °C) residual mass 77.06%. EA calcu-
lated for C35H48N2ClRh·2SiO1.5·14SiO2 (considering complete con-
densation): 1.77% N, 26.60% C, 3.06% H, 6.51% Rh; found 1.01%
N, 15.45% C, 2.65% H, 4.47% Rh.
2,7-Bis(p-tolylsulfonyl)-1,3,6,8-tetrahydro-2,7-diaza-as-indacene
1
(8):[30] Colourless solid. H NMR (300 MHz, CDCl3): δ = 2.40 (s,
6 H, CH3), 4.45 (s, 4 H, CH2), 4.57 (s, 4 H, CH2), 7.04 (s, 2 H,
3
3
Ar), 7.31 [d, J(H,H) = 8.0 Hz, 4 H, Ts], 7.74 (d, JH,H = 8.0 Hz,
4 H, Ts) ppm.
7-(p-Tolylsulfonyl)-2-oxa-7-aza-1,3,6,8-tetrahydro-as-indacene (10):
Colourless solid, m.p. 142–144 °C (dec.). 1H NMR (300 MHz,
CDCl3): δ = 2.40 (s, 3 H, CH3), 4.50 (br. s, 2 H, CH2-NTs), 4.62
(br. s, 2 H, CH2-NTs), 4.97 (br. s, 2 H, CH2-O), 5.06 (br. s, 2 H,
3
CH2-O), 7.05–7.12 (m, 2 H, CH-Ar), 7.31 (d, JH,H = 8.1 Hz, 2 H,
3
Ar-Ts), 7.76 (d, JH,H = 8.1 Hz, 2 H, Ar-Ts) ppm. 13C NMR
(75 MHz, CDCl3): δ = 21.8 (CH3-Ts), 52.6 (CH2-NTs), 53.7 (CH2-
NTs), 72.3 (CH2-O), 73.7 (CH2-O), 120.7 (CH-Ar), 121.9 (CH-Ar),
127.9 (CH-Ts), 129.7 (C-Ar), 130.2 (CH-Ts), 133.9 (C-Ar), 134.0
(C-Ar), 135.9 (C-Ts), 139.5 (C-Ar), 144.1 (C-Ts) ppm. IR (ATR):
Material M2: Prepared according to the method described for ma-
terial M1 with the following specific conditions: TBAF (0.215 mL,
0.215 mmol), milliQ water (1.5 mL, 83.3 mmol), complex 3 (0.64 g,
0.6 mmol), TEOS (4.6 mL, 20.1 mmol). M2 was obtained as an
orange powder (1.91 g). 29Si-CP-MAS NMR (79.5 MHz): δ =
–110.2 (Q4), –101.7 (Q3), –92.5 (Q2), –65.6 (T3), –55.7 (T2) ppm.
BET SBET: 493 m2/g; type IV sorption isotherm; TGA (air, 30 to
ν = 2857, 1340, 1156 cm–1. ESI-MS (m/z): 316 [M + H]+, 338
˜
[M + Na]+, 653 [2M + H]+. ESI-HRMS (m/z): calculated for
[C17H17O3NS + Na]+: 338.0809, found 338.0821.
5-Hydroxymethylphthalan (13aa):[32] Colourless solid. 1H NMR
(300 MHz, CDCl3): δ = 4.69 (s, 2 H, CH2OH), 5.09 (s, 5 H, CH2O
+ OH), 7.19–7.27 (m, 3 H, CH-Ar) ppm.
700 °C)
residual
mass
80.66%.
EA
calculated
for
C35H48N2ClRh·2SiO1.5·30SiO2 (considering complete condensa-
tion): 1.10% N, 16.54% C, 1.90% H, 4.05% Rh; found 0.83% N,
10.65% C, 2.33% H, 2.93% Rh.
1
5-Phenylphthalan (13ab):[32] Colourless solid. H NMR (300 MHz,
CDCl3): δ = 5.17 (s, 4 H, CH2O), 7.29–7.60 (m, 8 H, CH-Ar) ppm.
5-Hydroxymethyl-2-(p-tolylsulfonyl)-1,3-dihydroisoindole (13ba):[33]
Colourless solid. 1H NMR (300 MHz, CDCl3): δ = 2.39 (s, 3 H,
CH3), 4.56 (s, 4 H, CH2-NTs), 4.64 (s, 2 H, CH2OH), 7.11–7.23
(m, 3 H, CH-Ar), 7.30 (d, JH,H = 8.1 Hz, 2 H, Ts), 7.74 (d, JH,H
= 8.1 Hz, 2 H, Ts) ppm.
Material M3: Prepared according to the method described for ma-
terial M1 with the following specific conditions: TBAF (0.350 mL,
0.350 mmol), milliQ water (2.5 mL, 83.3 mmol), EtOH as solvent,
complex 4 (0.72 g, 1.13 mmol), TEOS (7.7 mL, 33.9 mmol). M3
was obtained as a pale-yellow powder (2.64 g). 29Si-CP-MAS NMR
3
3
(79.5 MHz): δ = –111.3 (Q4), –101.8 (Q3), –91.6 (Q2), –65.1 5-Phenyl-2-(p-tolylsulfonyl)-1,3-dihydroisoindole (13bb):[32] Colour-
(T3) ppm. BET SBET: 325 m2/g; type II sorption isotherm; TGA
(air, 30 to 700 °C) residual mass 75.98%. EA calculated for
C23H31N2ClRh·SiO1.5·30SiO2 (considering complete condensa-
tion): 1.20% N, 11.86% C, 1.34% H, 4.42% Rh; found 1.18% N,
12.19% C, 2.29% H, 3.15% Rh.
less solid. 1H NMR (300 MHz, CDCl3): δ = 2.43 (s, 3 H, CH3),
4.69 (br. signal, 4 H, CH2-NTs), 7.25 (d, 3JH,H = 8.1 Hz, 1 H, CH-
3
Ar), 7.30–7.56 (m, 9 H), 7.80 (d, JH,H = 8.3 Hz, 2 H, Ts) ppm.
5-Hydroxymethyl-4,7-dimethylphthalan (13ca):[2b] Colourless solid.
1H NMR (300 MHz, CDCl3): δ = 2.21 (s, 6 H, CH3), 4.69 (s, 3 H,
CH2OH), 5.11 (s, 4 H, CH2O), 7.07 (s, 1 H, CH-Ar) ppm.
Material M4: A mixture of complex 4 (0.21 g, 0.34 mmol) and me-
sostructured silica SBA-15 (1.96 g, 32.70 mmol) was transferred
into a Schlenk tube equipped with a Dean–Stark apparatus in an-
hydrous toluene (40 mL). The reaction mixture was stirred and re-
fluxed for 24 h. The resulting suspension was filtered and the pow-
der obtained was washed with EtOH (3ϫ 20 mL), acetone (3ϫ
20 mL) and anhydrous Et2O (3ϫ 20 mL). The powder was dried
overnight at 40 °C under vacuum. M4 was obtained as a white
powder (1.76 g). BET SBET: 512 m2/g; pore diameter (BJH): 58.8 Å
(desorption); type IV sorption isotherm; pore volume (BJH):
0.692 cm3/g (desorption). TGA (air, 30 to 700 °C) residual mass
84.30%. EA found: 0.55% N, 5.39% C, 1.00% H, 1.03% Rh.
5,6-Dihydroxymethyl-4,7-dimethylphthalan (13cc):[2b] Colourless so-
1
lid. H NMR (400 MHz, [D6]DMSO): δ = 2.18 (s, 6 H, CH3), 4.56
3
3
(d, JH,H = 5.2 Hz, 4 H, CH2OH), 4.71 (t, JH,H = 5.2 Hz, 2 H,
OH), 5.01 (s, 4 H, CH2O) ppm.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures for the synthesis of 1, 3-allyl and 9;
NMR spectra for all of the products synthesized; characterization
of materials M1–M4.
Acknowledgments
General Procedure for [2+2+2] Cycloaddition Reactions: The reac-
tions were carried out in a Carousel multireactor. The alkynes Financial support from the Spanish Ministerio de Ciencia e Innov-
(0.1 mmol), the material (0.01 mmol Rh) and EtOH (3 mL) were ación (MICINN) (grant numbers CTQ2011-23121, CTQ2009-
Eur. J. Org. Chem. 2014, 6242–6251
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6249

R. Pleixats, A. Roglans et al.
FULL PAPER
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Received: May 28, 2014
Published Online: August 20, 2014
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