A highly active, heterogeneous catalyst for alkyne metathesis

Article abstract of DOI:10.1002/anie.200502840

(Chemical Equation Presented) An alkylidyne molybdenum amide complex is attached to nontoxic, amorphous silica to form a highly active, recyclable heterogeneous catalyst for alkyne metathesis. The catalyst does not undergo alkyne polymerization, can be utilized at a loading of < 1 mol% at room temperature, and has shown unprecedented metathesis activity for the homodimerization of 2-propynylthiophene, a substrate that was previously problematic for alkyne metathesis.

Full text of DOI:10.1002/anie.200502840

Angewandte
Chemie
Synthetic Methods
DOI: 10.1002/anie.200502840
A Highly Active, Heterogeneous Catalyst for
Alkyne Metathesis**
Haim Weissman, Kyle N. Plunkett, and
Jeffrey S. Moore*
Highly active alkyne-metathesis catalysts have been realized
by exchanging the amide ligands on molybdenum(vi) alkyli-
dyne trisamide complexes 1 with simple phenols,[1,2] such as 4-
nitrophenol, or by use of chlorotrisamide Mo(iv) complexes.[3]
In spite of this practical utility, these catalysts can become
deactivated over time, apparently through a bimolecular
dimerization pathway.[4] The polymerization of 2-butyne, the
metathesis by-product obtained from propynyl substrates,[1a,b]
is another interfering reaction which occurs when the catalytic
reaction is preformed at room temperature. Herein, we
describe a heterogeneous, alkyne-metathesis catalyst that
avoids the use of phenolic ligands. The supported catalyst
exhibits very high activity at room temperature while showing
exceptional selectivity for catalyzing alkyne metathesis rather
than alkyne polymerization, even on a preparative scale.
We investigated the coordination of the molybdenum
center to amorphous silica to avoid bimolecular decomposi-
tion with minimal changes to the electronic structure of the
catalyst. The similarity of the pKa values of the phenolic
hydroxy and silanol groups was the basis for this choice.[5,6]
Mortreux et al. have pioneered similar, although less well-
defined, approaches by impregnating [Mo(CO)6]on sili-
ca.[2e,7a] Alumina- and titania-supported molybdenum species
have also been reported.[7] However, none of these previous
systems were particularly efficient; all required high temper-
atures (160–3508C) to obtain turn-over frequencies (TOFs) of
several turnovers per hour.[7,8] Utilization of organic silanols
as cocatalysts with molybdenum hexacarbonyl for alkyne
metathesis were somewhat successful, yet high catalyst
[*] Dr. H. Weissman, Dr. K. N. Plunkett, Prof. J. S. Moore
Department of Chemistry
University of Illinois
470B Roger Adams Lab
600 South Mathews Avenue, Urbana, IL 61801 (USA)
Fax: (+1)217-244-8024
E-mail: jsmoore@uiuc.edu
[**] This work is supported by the National Science Foundation (Grant
No. 0345254) and the US Department of Energy, Division of
Materials Sciences (Award No. DEFG02-91ER45439), through the
Frederick Seitz Materials Research Laboratory at the University of
Illinois at Urbana. We thank Wei Zhang for fruitful discussions and
Dr. Paul Molitor for his help recording the 13C-MAS NMR spectra.
We would also like to thank Richard Haasch for help with the X-ray
photoelectron spectroscopy (XPS) measurements, which were
performed in the Center for Microanalysis and Materials, University
of Illinois, which is partially supported by the US Department of
Energy Award No. DEFG02-91ER45439.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 585 –588
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585

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loading and temperatures above 1508C were again requir-
ed.[8a,9] Basset and co-workers reported the impregnation of
silica surfaces with well-defined molybdenum, tungsten, and
rhenium alkylidyne complexes for olefin metathesis.[10] It was
suggested that a-hydrogen elimination was responsible for
metal-carbene formation and thus, possibly, for deactivation
of possible alkyne metathesis.[9e] Only the silica-supported
À
=
À
rhenium catalyst [(SiO)Re(C tBu)( CH tBu)(CH2Bu)]has
been mentioned briefly as a well-defined precursor for
heterogeneous alkyne metathesis.[10d]
In a typical procedure, a solution of precursor 1 in toluene
is added to a suspension of silica[11] with an approximately
equimolar ratio of silanols and molybdenum. After stirring
for one minute at room temperature, the honey-brown color
of the solution transfers onto the silica (Scheme 1). The
Figure 1. X-ray photoelectron spectroscopy data for the peak that
corresponds to the Mo 3d electrons of crystalline precursor 1 and the
supported catalyst 2.
center, a likely result of an exchange between an amide and
an oxide ligand. A similar increase of 1.2 eV in the N 1s
binding energy is also evident (see the Supporting Informa-
tion). This energy change is consistent with amido ligands
bound to a more Lewis acidic Mo center (e.g., a Mo center
bound to an oxide ligand).
To prove the existence of the carbyne[12] on the silica
surface of 2, we measured the 13C-MAS NMR spectrum at
natural abundance (Figure 2). A peak at d = 330 ppm corre-
Scheme 1. Reaction of Mo precursor 1 with a silica surface. The
images show the capture of 1 from solution onto silica to produce an
active catalyst.
amount of residual molybdenum in the washings was
determined to be 0.7% of the starting material, thus indicat-
ing > 99% of 1 reacted with the silica. Based on the
characterization data described below, the resulting structure
is consistent with catalyst 2 shown in Scheme 1.
Elemental analysis of the impregnated silica indicated
that the surface contained 1.56 Æ 0.04 wt.% molybdenum and
4.2 Æ 0.6 wt.% carbon, thus suggesting that the average Mo/
aniline ratio on the surface was approximately 1:1.35
(Scheme 1). This ratio was independently verified by measur-
ing the quantity of aniline released and collected in the
washings. Further evidence for the formation of 2 was
Figure 2. 13C-MAS NMR spectrum of 2.
À
ꢀ
obtained from IR spectroscopy. The SiO H stretching band
sponding to a typical chemical shift for Mo C in the liquid
at 3747 cmÀ1 observed in unmodified silica was absent in the
spectrum of 2, thus indicating consumption of the surface
silanols groups. Bands at 3020, 1592, and 1460 cmÀ1 in the
spectrum of 2 indicate the presence of a substituted aromatic
moiety, and the bands at 2971–2871 and 1390–1361 cmÀ1
indicate the existence of an aliphatic moiety (see the
Supporting Information).
phase[14] and other silica-impregnated molybdenum alkyli-
dyne complexes is evident.[15] Resonances at 100–145 ppm
and 0–40 ppm are consistent with aromatic and aliphatic
carbon atoms on the surface, respectively.
Alkyne-metathesis reactions [Eq. (1)]were performed
differently to test the metathesis activity of 2. In a typical
To further confirm the immobilization of the complex
onto the silica surface, X-ray photoelectron spectra of catalyst
2 were obtained and compared to the X-ray photoelectron
spectra of the crystalline precursor 1. A 1.4-eV shift in the
peak that corresponds to the Mo 3d electrons is evident for 2
(Figure 1), which is consistent with a significant change in the
coordination sphere of the molybdenum center (Figure 1).
Similar trends have been observed for other high-oxidation
state Mo species when ligands are changed by the introduc-
tion of more electron-withdrawing groups.[12] This shift
suggests the reduction of electron density around the Mo
experiment, 2 was preweighed into an NMR tube and the
substrate was introduced in a solution of [D8]toluene. In
experiments testing the recycling capability of the silica
support, a suspension of 2 was centrifuged, the solvent
decanted, and the silica was rinsed once with [D8]toluene
before the addition of the next batch of substrate.
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Alkyne-metathesis experiments with catalyst 2 are sum-
marized in Table 1. The scope of metathesis activity was
probed with 2-propynylthiophene (3), 1-phenyl-1-butyne (4),
1-phenyl-1-propyne (5), 3-butynyl methyl benzoate (6), 3-
system.[1,2] More significantly, high catalytic activity is ach-
ieved for 2, even though its molybdenum alkylidyne moieties
likely contain a mixture of nitrogen- and oxygen-containing
ligands.
No oligomerization of the alkyne substrates or the
products was observed by 1H NMR spectroscopic analysis
for all the reactions, even after several days. A mass balance
higher than 93% was established with an internal standard.
The fact that 2 was able to metathesize propynyl derivatives,
including the unprecedented homodimerization of 2-propynyl
thiophene,[3c,d] demonstrates the success of the heterogeneous
approach in constricting the coordination sphere around the
molybdenum center. Good metathesis activity with 2 is
observed under conditions that were impossible in the past
because of problems with polymerization. Recyclability was
tested with 3 as the substrate and an initial catalyst loading of
4.0 mol% of 2. Cycles 1–3 produced conversions of 45, 52, and
32%, respectively, thus suggesting the recyclability of the
catalyst. The activity with 4 as the substrate at a catalyst
loading of 0.2 mol% was measured to monitor the stability of
solid 2. The catalytic activity of 2 stored at room temperature
in an argon-filled dry box remained relatively unchanged for
at least two weeks.
We tested the homodimerization of 1-phenyl-1-propyne at
room temperature under vacuum-driven conditions to dem-
onstrate the applicability of this system for preparative
synthesis. The yield of the isolated and purified homodimer
on a 7.5-mmol scale was 88%.
A series of experiments was preformed to confirm
catalysis takes place on the support rather than in solution.
First, we confirmed that a solution of precursor 1 is catalyti-
cally inactive at room temperature. Second, filtrates obtained
from suspensions of catalyst 2 exhibit no residual metathesis
activity. Similarly, when a substrate suspended with 2 is
removed before reaching equilibrium, metathesis activity
ceases. Finally, activity resumes as soon as the filtrate is
combined with recovered 2.
Table 1: Values of t1/2 for metathesis reactions[a] of 3–10.
Entry
Substrate
R1
R2
Catalyst
loading
[mol%][b]
t1/2
[min][c]
1
2
3
4
2-thiophenyl
Ph
Me
Et0.8
4.0
<10[d]
<5[e]
<8[f]
11[g]
3
4
5
6
7
8
9
10
0.4
0.2
0.2
4.0
0.8
0.4
0.2
0.8
5
6
Ph
Me
Et
20[g]
3-methyl benzoate
<5[d]
<10[e]
40[f]
7
3-methyl benzoate
Et
Me
nPr
14[g]
8[h]
<5[e]
9
EtEt
nPr
11
0.8
<5[e]
10[h]
nPr
[a] Conditions: 248C, [D8]toluene (600 mL). [b] Load of 2 relative to
substrate. [c] t1/2 =time for the reaction to proceed to half of the final
equilibrium concentrations. [d] Substrate amount=62.5 mmol. [e] Sub-
strate amount=62.5 mmol. [f] Substrate amount=125 mmol. [g] Sub-
strate amount=250 mmol. [h] Carried outin m-xylene (600 mL).
propynyl methyl benzoate (7), 3-heptyne (8), 3-hexyne (9),
and 3-octyne (10), and four different catalyst loadings were
tested. Half-lives at room temperature of less than 1 h were
generally observed, even with as little as a catalyst loading of
0.8 mol% (based on Mo). Table 2 shows k2,obs and TOF values
Table 2: Values of k2,obs and TOF for the homodimerization of 4–7, 11,
and 12.[a]
Entry Substrate R1
R2
k2,obs
TOF
[mÀ1 minÀ1
]
[molp molcÀ1 sÀ1
]
We have developed a readily accessible heterogeneous
system that exhibits high catalytic activity and good stability,
probably because bimolecular deactivation is prevented.[1a,b]
The catalyst can be utilized with a loading of less than 1% at
room temperature. Several other benefits of the catalytic
system have also been found. The system is resistant to alkyne
polymerization, which is problematic for similar homoge-
neous catalysts.[1a,b] Catalyst 2 is recyclable, shows activity
over at least three cycles, and has unprecedented metathesis
activity for the homodimerization of 2-propynylthiophene, a
substrate unable to undergo metathesis with any other known
catalyst system.[16] The study of ring-closing alkyne-metathe-
sis[2a,17] activity and other types of metal oxides in this catalyst
system are under way. Our results for these studies will be
reported elsewhere.
1
2
3
4
5
6
11
12
4
5
6
MeO
MeO
H
Et0.24
Me 0.058
Et0.19
0.31
0.33
0.13
0.087
0.051
0.046
H
Me 0.031
m-MeO2C Et0.068
m-MeO2C Me 0.023
7
[a] The alkyne substrate (167 mmol) was added to
containing 2 (0.30 mol%) in [D8]toluene (600 mL).
a NMR tube
from the alkyne homodimerization reactions of 4–7, 4-(1-
butynyl)anisole (11), and 4-(1-propynyl)anisole (12)
[Eq. (2)]. The steric and electronic trends seen in Tables 1
and 2 parallel those observed for the analogous 1/phenol
Received: August 10, 2005
Published online: December 15, 2005
Keywords: alkynes · heterogeneous catalysis · metathesis ·
.
molybdenum · silica
Angew. Chem. Int. Ed. 2006, 45, 585 –588
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
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;
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