Multidentate triphenolsilane-based alkyne metathesis catalysts

Article abstract of DOI:10.1002/adsc.201201105

A series of triphenolsilane-coordinated molybdenum(VI) propylidyne catalysts has been developed, which are resistant to small alkyne polymerization and compatible with various functional groups (including phenol substrates). The catalysts remain active in

Full text of DOI:10.1002/adsc.201201105

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DOI: 10.1002/adsc.201201105
Multidentate Triphenolsilane-Based Alkyne Metathesis Catalysts
a
b
a,
Haishen Yang, Zhenning Liu, and Wei Zhang *
a
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA
Fax : (+1)-303-492-5894; phone: (+1)-303-492-0652; e-mail: wei.zhang@colorado.edu
Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, Peopleꢀs Republic of
b
China
Received: December 18, 2012; Published online: March 15, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201105.
Abstract: A series of triphenolsilane-coordinated
molybdenum(VI) propylidyne catalysts has been
developed, which are resistant to small alkyne poly-
merization and compatible with various functional
groups (including phenol substrates). The catalysts
remain active in solution for days at room tempera-
ture (months at À308C). The catalysts are also com-
patible with 5 ꢁ molecular sieves (small alkyne
scavengers), and have enabled the homodimeriza-
tion of small alkyne substrates at 40–708C in
a closed system, with dimer products being ob-
and complete inhibition of the undesired polymeri-
[
4j]
zation of the 2-butyne by-product. Highlights of its
applications include the syntheses of shape-persistent
2D macrocycles, 3D molecular cages, as well as conju-
[
4j,k,5b]
gated polymers (1D molecular wires).
However,
there exists a potential drawback with such triphenol-
amine-based catalysts: strong coordination of the
ligand nitrogen to molybdenum, which lowers its
Lewis acidity, thus the catalyst activity (no activity ob-
[4k]
served without the nitro substituents, i.e., 2b). Al-
though methylation of the nitrogen atom in the ligand
prevents its metal coordination and restores the cata-
lyst activity (2c, Figure 1), the synthetic difficulty
(time-consuming, low-yielding synthesis) and poor
solubility of the charged triphenolammonium ligand
tained in 76–96% yields.
A
shape-persistent
aryleneethynylene macrocycle (11) was also pre-
pared on a gram scale with 0.5 mol% catalyst load-
ing, in almost quantitative yield.
[
4k]
represent big disadvantages. Herein, we report the
design and synthesis of a series of multidentate cata-
lysts (3a–3c) consisting of triphenolsilane ligands (L1,
L2 and L3, Scheme 1), which are readily accessible
Keywords: alkyne metathesis; macrocycle synthesis;
multidentate ligands
Alkyne metathesis represents an emerging organic
transformation that has attracted significant research
interest. It has been utilized in efficient syntheses of
[
1]
a variety of organic compounds and materials, in-
[
2]
[3]
cluding natural products, organic polymers , shape-
[
4]
persistent 2D macrocycles and rigid 3D covalent or-
[
5]
ganic polyhedrons (COPs, i.e., molecular cages). In
the past two decades, numerous well-defined catalytic
systems based on Schrock-type molybdenum(VI) and
tungsten(VI) alkylidyne complexes have been devel-
oped, consisting of various monodentate ligands, such
[6]
[4j,k,7]
[4f,8]
as alkoxide, aryloxide,
amido,
imidazolin-2-
or silanolate li-
gands. Recently, our group reported a multidentate
[9]
[10]
iminato,
phosphoraneiminato,
[11]
[
4j]
molybdenum(VI) carbyne catalyst (2a, Figure 1).
The properties of this multidentate catalyst surpass
those of many existing monodentate alkyne metathe-
sis catalysts (e.g., 1), showing high catalytic activity, Figure 1. Molybdenum(VI) carbyne catalysts with phenol li-
improved functional group tolerance, longer lifetime, gands.
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Scheme 1. Syntheses of multidentate ligands (L1–L3) and their coordinated molybdenum(VI) propylidyne catalysts. Condi-
tions: a) PPh , Br , imidazole, CH CN, Et O (67%) for 4a; NBS, PPh , CH Cl (77–86%) for 4b and 4c; b) Mg, THF, room
3
2
3
2
3
2
2
temperature, then MeSiCl (72–90%); c) Et N·HF, THF (39%) for L1; PPTS, i-PrOH (63%–66%) for L2 and L3.
3
3
and compatible with various substrates. The catalysts 408C, ~10 h, dynamic vacuum), the reactions cata-
remain active in solution phase for days at room tem- lyzed by triphenolic silane-based catalysts 3a–3c all
perature (months at À308C). Such catalysts also ena- showed good yields (54%, 58%, and 54%, respective-
bled the metathesis of challenging phenol-based sub- ly). It is interesting to note that the catalytic activity
strates in good yield. A gram-scale preparation of of this new family of catalysts (3a–3c) seems not to be
a phenyleneethynylene macrocycle was also accom- sensitive to the sterics of the ligands. Since catalysts
plished starting from simple dipropynyl-substituted 3a–3c exhibit similar activity, we used 3b as the cata-
monomers in a closed system.
lyst in the catalysis studies described below.
Triphenolsilane ligands (L1–L3) were prepared
Next, we explored the solvent effect on the catalyst
from the protected salicyclic alcohols (4a–c). In order activity. The pre-generated catalyst 3b in carbon tetra-
to study the steric effect on catalytic activity, we de- chloride (20 vol%) was used in this study. We used 4-
signed and synthesized ligands with different ortho formylpropynylbenzene (8) as the substrate and the
substituents (H, Me, i-Pr). Conversion of benzylic al- reaction was carried out at 408C with 3 mol% catalyst
cohols (4a–c) to benzylic bromides (5a–c), generation loading [Eq. (1)]. It should be noted that propynylat-
of Grignard reagents, followed by coupling with
MeSiCl3 and subsequent deprotection provided li-
gands L1–L3. The active catalysts 3a–3c were generat-
ed by mixing the molybdenum(VI) trisamide precur-
sor (7) with a triphenolic silane ligand in 1:1 ratio in
carbon tetrachloride. The complete displacement of
the amine ligands with the multidentate ligand L2
1
was confirmed by H NMR analysis (see the Support-
13
ing Information, Figure S1). Further C NMR char-
acterization clearly showed the downfield shift of the
carbyne carbon from 302.3 ppm to 310.1 ppm upon ed benzaldehyde (e.g., 8) derivatives are reported to
ligand exchange, indicating the formation of the mul- be difficult substrates to metathesize, destroying some
tidentate catalyst 3b (see the Supporting Information, alkyne metathesis catalysts, such as monodentate
[
11c]
Figure S2 and Figure S3).
siloxy-based molybdenum catalyst.
To our great
Triphenolic silane ligands share similar geometrical delight, the multidentate triphenolsilane catalyst
features with our previous multidentate triphenol- showed 60–74% substrate conversion as well as good
ACHTUNGTRENNUNGa mine ligands, in which the effective coordination of solubility in a variety of commonly used solvents as
the three phenol moieties to molybdenum forms shown in Table 1. Considerably lower conversions in
a cage-shaped metal center and blocks the extra sub- THF and hexane were observed, presumably due to
strate-binding site. However, unlike nitrogen, which the coordinating nature of THF and lower solubility
can easily coordinate to molybdenum and reduce its of the catalyst in hexane. It appears that carbon tetra-
Lewis acidity, the silicon atom has no lone pair avail- chloride is a preferred solvent for the generation of
able for the metal-ligand coordination. We therefore the catalyst, which could be a potential disadvantage
expected catalysts 3a–3c to possess the advantages of of this catalyst system. We attempted to generate the
robust multidentate catalysts without sacrificing their catalyst in other solvents, such as toluene. However,
catalytic activity. A well-known relatively inert com- we observed reduced catalytic activity (conversion
pound, 4-nitropropynylbenzene, was used as the sub- 73% vs. 65%).
strate to test the activity. As expected, under the typi-
It is well-known that the polymerization of the 2-
cal alkyne metathesis condition (3 mol% cat. loading, butyne by-product, one of the commonly-observed
886
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Adv. Synth. Catal. 2013, 355, 885 – 890

Multidentate Triphenolsilane-Based Alkyne Metathesis Catalysts
Table 1. Metathesis reactions in various solvents.
sieves can be used as an efficient small alkyne scaven-
[
11c]
ger.
However, only few catalysts have been report-
Solvent
Conversion
%]
Solvent
Conversion
[%]
ed to be effective for catalytic alkyne metathesis reac-
[
[10,11c,13]
tions in the presence of 5 ꢁ molecular sieves.
n-C H
t-BuOMe
26
69
chlorobenzene
dichloroethane
70
74
To further improve the efficiency and the synthetic
utility of our newly developed multidentate triphenol-
silane catalysts, and make them more user-friendly,
we explored the feasibility of conducting metathesis
reactions in a closed system by using 5 ꢁ molecular
sieves. To our delight, comparable or much improved
conversions of metathesis reactions catalyzed by this
class of silane-based catalysts were observed in the
presence of 5 ꢁ molecular sieves. Such conditions
work particularly well for those challenging substrates
(e.g., 4-nitropropynylbenzene) which usually require
longer reaction time and higher catalyst loading
under conventional dynamic vacuum conditions. In
6
14
THF
50
66
CH Cl2
61
71
79
2
CHCl3
toluene
63
73 (65)
CCl4
major side reactions in alkyne metathesis, could entry 5 (Table 2), the conversion was improved from
poison the metathesis catalyst through the “ring-ex- 49% to 86% when 5 ꢁ molecular sieves were used as
[
7b]
pansion” mechanism. Previously, we demonstrated 2-butyne scavenger. Such closed system conditions
that triphenolamine-based catalyst (2a), with the NÀ can also be applied to the synthesis of shape-persis-
Mo coordination, can completely inhibit the small tent macrocycles in almost quantitative yields on
[
4j]
alkyne polymerization,
Now without an SiÀMo a multigram scale (2.6 g, entry 10). It should also be
bonding interaction, the question arises as to whether noted that for the same multigram macrocycle synthe-
such a multidentate silane-based catalyst can still in- sis (entry 10), the catalyst loading could be reduced to
hibit small alkyne polymerization. As a model study, as little as 0.5 mol% without sacrificing the yield, at
the efficiency of catalyst 3b on inhibiting alkyne poly- slightly elevated temperature (558C, 4.31 g, 98%).
merization was tested with a large excess of 2-butyne The carbazole-based cyclic tetramer was also pre-
(
>100 equiv.). Catalyst 3b showed no polymerization pared in one step from a simple propynyl-substituted
even after 24 h. Furthermore, the catalyst 3b, with or monomer in quantitative yield (entry 9). Thus our cat-
without 2-butyne treatment, showed similar metathe- alyst system holds promise for the convenient access
sis activity even after exposure to 2-butyne for one to shape-persistent 2D or 3D molecular architectures
week (4-formylpropynylbenzene 8 as the substrate). which have been recognized as important building
This result further supports the notion that the cata- blocks for the future of nanotechnology.
lyst with multidentate ligands can completely inhibit
Given these encouraging findings, we next investi-
the small alkyne polymerization, presumably because gated their substrate scope. The reactions were per-
the cage-shaped catalyst effectively blocks the access formed either under dynamic vacuum or in the pres-
of butyne to the extra open binding site on the ence of 5 ꢁ molecular sieves. Table 2 summarizes re-
Mo(VI) center.
actions of different substrates catalyzed by in situ gen-
Another complication with alkyne metathesis is the erated catalyst 3b. Catalyst 3b is compatible with all
efficient removal of one of the alkyne products in the different substrates tested, providing the corre-
order to drive the reaction to completion. Since the sponding products in good to excellent yields. The
most widely used common substrates contain methyl- substrates include: (i) compounds containing electron-
substituted alkynes, a typical alkyne metathesis reac- donating/electron-withdrawing substituents, (ii) heter-
tion is driven to completion by the removal of the 2- ocyclic molecules, (iii) the ring closing alkyne meta-
butyne by-product. Continuous dynamic vacuum is thesis (RCAM) of diyne to cycloalkyne, (iv) com-
commonly applied to remove 2-butyne. However, pounds containing free phenolic hydroxy groups
such an approach usually requires solvent refill, and (entry 6), and (v) the precipitation-driven cyclooligo-
often does not work well for catalysts that are highly merization reaction of the carbazole diyne substrate
sensitive to air and moisture. Although the later de- (entry 9). Even those challenging substrates, contain-
velopment of precipitation-driven alkyne metathesis ing nitro or aldehyde functional groups are compati-
by Moore et al. significantly improved shape-persis- ble with our conditions to give the corresponding
tent macrocycle synthesis, such an approach requires dimers (86–96%). Half-lives of less than 1 hour were
more synthetic efforts (e.g., installment of large aro- generally observed for those reactions, even at 408C
matic end groups) and generates a large amount of di- with a catalyst loading as low as 3 mol% (based on
arylacetylene by-product, thus it is not atom-econom- Mo). It should be noted that the metathesis reaction
[12]
ic.
Recently Fꢃrstner showed that 5 ꢁ molecular works surprisingly well with the phenol substrate
Adv. Synth. Catal. 2013, 355, 885 – 890
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Haishen Yang et al.
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[a,b]
Table 2. Homodimerization, RCAM and cyclooligomerization reactions of propynyl substrates.
Entry Substrate
Product
Method Temperature
A or B [8C]
Time
[h]
Yield
[%]
1
2
A
A
40
40
4.5
5
91
94
3
A
40
5
92
A
B
B
A
B
40
40
40
40
40
5
16
16
5 (15)
20
79
4
5
6
94[
c]
96
47 (49)
86
A
B
70
70
19
20
64
76
A
B
B
70
70
70
4
84
84 (88)
90
7
8
4 (7)
4
[c]
A
40
3.5
95
[
d]
2
40
0
0.5
5
93
9
B
>99
10
B
40
16
>99
[
[
[
[
a]
Catalyst 3b and solvent CCl were used unless stated otherwise.
4
b]
3
mol% catalyst loading for entries 1–5 and 7–10; 10 mol% for entry 6.
c]
Catalyst 3c was used.
d]
Precipitation-driven cyclooligomerization. Method A: vacuum with 30 min-interval without the addition of molecular
sieves. Method B: 150 mg MS 5 ꢁ/0.1 mmol for entries 4–7 and 300 mg MS 5 ꢁ/0.1 mmol for entries 9 and 10.
(
entry 6), which, to the best of our knowledge, repre- widely used by chemists in various disciplines, the
sents the first successful alkyne metathesis of a sub- analogous alkyne metathesis lags far behind in popu-
[
14]
strate containing free phenolic hydroxy groups.
larity. A significant disadvantage of alkyne metathesis
This finding would enable the synthesis and applica- is the availability of a user-friendly catalyst. Previous
tions of other phenol-based substrates, including catalysts for alkyne metathesis are usually generated
shape-persistent macrocycles.
in situ from relatively more stable precatalysts, and
Compared to olefin metathesis that has become the catalytically active species possess very limited
a powerful tool in organic synthesis and has been lifetimes in solution. For example, as shown in Fig-
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Adv. Synth. Catal. 2013, 355, 885 – 890

Multidentate Triphenolsilane-Based Alkyne Metathesis Catalysts
and also for industrial processes where catalyst stabili-
ty is often of paramount importance.
In summary, a class of triphenolsilane-based, un-
charged, multidentate alkyne metathesis catalysts has
been developed. The good functional group tolerance,
fast reaction rate and long lifetime (remaining active
in solution for months) represent major advantages.
These catalysts are compatible with 5 ꢁ molecular
sieves that serve as small alkyne by-product scaveng-
ers. A variety of tough substrates (e.g., pyridine,
phenol, benzaldehyde, nitrobenzene) were successful-
ly cross-metathesized. Moreover, shape-persistent ary-
leneethynylene macrocycles were prepared in almost
quantitative yields on a multi-gram scale in a closed
system, highlighting the feasibility of achieving a con-
venient access to a variety of novel 2D and also 3D
molecular architectures targeting various potential ap-
plications (e.g., carbon capture, artificial photosynthe-
sis, catalysis, etc.).
Figure 2. The conversion of 4-formylpropynylbenzene in the
catalytic runs (3 mol% loading, 408C) at different time in-
tervals after the generation of the catalysts. (a): catalyst 1,
stored at room temperature after its generation; (b): catalyst
3
b, stored at room temperature after its generation; (c) cata-
lyst 3b, stored at À308C after its generation.
ure 2a, the catalyst 1 with monodentate ligands, in the
absence of any alkyne substrates, shows a progressive
measurable decrease in the catalytic activity, and is Experimental Section
completely inactive after 4 h of the generation under
argon at room temperature. In great contrast, catalyst General Procedure for Alkyne Metathesis Reactions
3
b showed good stability in solution. Its metathesis listed in Table 2
activity at different time intervals after its generation
in the absence of any alkyne substrates) was tested,
The ligand (L1, L2 or L3) (0.003 mmol) and the precursor 7
2.0 mg, 0.003 mmol) were premixed in dry carbon tetra-
(
(
using the 4-formylpropynylbenzene (8) as the model
compound. Complex 3b showed no loss in catalytic
chloride (3 mL for all entries except for entry 8, Table 2,
where the solvent volume was doubled to ensure intramo-
activity after 24 h storage at room temperature and lecular alkyne metathesis) and stirred for 5 min to generate
remained active even after one week at room temper- the catalyst in situ. Subsequently, the substrate (0.1 mmol)
ature. When stored in solution at À308C, catalyst 3b and 5 ꢁ molecular sieves (for removing 2-butyne by-prod-
uct) were added, and the stirring was continued with regular
did not show any noticeable decrease in activity for
monitoring of the reaction by NMR. For the vacuum-driven
alkyne metathesis reactions, the solution was exposed to
a period of more than 3 months. The presence of only
1
mol% of 100-day aged catalyst 3b was sufficient to
vacuum with 30 min intervals to remove the metathesis by-
product 2-butyne and the loss of solvent during the applica-
tion of vacuum was compensated by adding fresh solvent
catalyze the cyclooligomerization of monomer 10 to
form macrocycle 11 in 5 h at 408C in 90% yield [Eq.
(
2)]. Our findings here represent one step forward for
1
each time. The yields were determined with H NMR by
commercializing a user-friendly stable alkyne meta-
thesis catalyst. Such a long lifetime is desired particu-
larly for the alkyne metathesis of tough substrates,
using 1,4-dimethoxybenzene as an internal standard.
Acknowledgements
We thank Prof. Richard Shoemaker for assistance with NMR
spectroscopy, Dr. Yinghua Jin for help with the manuscript
preparation, and National Science Foundation (DMR-
1
055705) for funding support. Acknowledgements is also
made to the donors of the American Chemical Society Petro-
leum Research Fund for partial support of this research.
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