Efficient ruthenium metathesis catalysts containing carborane ligands

Article abstract of DOI:10.1016/j.jorganchem.2013.09.005

A first generation Hoveyda-Grubbs ruthenium complex with a closo-1,2-C ₂B₁₀H₁₁ tag has been prepared. This new catalyst was tested in ring-closing metathesis (RCM) reactions and proved to be quite efficient. In addition, t

Full text of DOI:10.1016/j.jorganchem.2013.09.005

Journal of Organometallic Chemistry 749 (2014) 13e17
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Efficient ruthenium metathesis catalysts containing carborane ligands
,
a
a
b,
Guiyan Liu a , Huizhu Zhang , Xia Zhao , Jianhui Wang
*
**
a College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory of InorganiceOrganic Hybrid
Functional Material Chemistry, Ministry of Education, Tianjin Normal University, Tianjin 300387, PR China
b Department of Chemistry, College of Science, Tianjin University, Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A first generation HoveydaeGrubbs ruthenium complex with a closo-1,2-C2B10H11 tag has been prepared.
This new catalyst was tested in ring-closing metathesis (RCM) reactions and proved to be quite efficient.
In addition, the recovery and reuse of a previously reported second generation HoveydaeGrubbs
Received 11 May 2013
Received in revised form
23 August 2013
ruthenium carbene complex bearing an ionic [nido-7,8-C2B9H10]
ꢀ tag was further studied. When used in
Accepted 5 September 2013
an ionic liquid, the catalyst can be conveniently recycled and reused with only a very slight loss of activity
for the RCM of oxygen-containing and N-protected substrates with di- or trisubstituted double bonds.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Olefin metathesis
Ruthenium
Grubbs catalyst
Immobilization
Ionic liquid
1. Introduction
2. Results and discussion
In the past decade, ruthenium catalysts 1e4 (Fig. 1) have been
widely used in organic synthesis for olefin metathesis [1,2]. The
robustness of 3 and 4 has made it desirable to make these catalysts
more recyclable [3]. To do this, various strategies like immobiliza-
tion have been employed [4,5]. Introducing an ion-appended ligand
onto the ruthenium metal center is a very effective method of
immobilizing transition metal catalysts [6]. However, most of the
reported catalysts of this type have a cation-appended ligand on
the ruthenium metal center. Previously, we reported catalysts 5 and
6 (Fig. 2) which have a carborane-tagged isopropoxy styrene ligand
bound to a Ru carbine [7]. Ruthenium carbene complex 5 with a
closo-1,2-C2B10H11 tag has a high level of activity which is similar to
that of 4 in the ring-closing metathesis (RCM) of a wide variety of
di-, tri-, and even tetrasubstituted diene anꢀd enyne substrates.
Scheme 1 illustrates the synthetic routes for the first generation
HoveydaeGrubbs ruthenium complex 8 with a closo-1,ꢀ2-C2B10H11
tag and complex 9 with an ionic [nido-7,8-C2B9H11
]
tag. The
treatment of 7 with 1 in the presence of CuCl in CH2Cl2 at 50 ꢁC, as
described by Hoveyda and co-workers [1 h], resulted in the com-
plete exchange of the styrene group to give the air-stable brownish
powder 8 in good yield (75.1%). This complex was fully character-
ized by 1H and 13C NMR spectroscopy as well as by elemental
analysis. Reaction of compound 7 with tetrabutylammonium fluo-
ride (TBAF) in a mixed solvent of THF and water opened the car-
borane cage to give an intermediate, followed by a ligand exchange
with 1 to afford a brownishꢀsolid. This product is believed to be the
anionic [nido-7,8-C2B9H11
]
tagged ruthenium complex 9. How-
ever, complex 9 is unlike complex 6 which contains an N-hetero-
cyclic carbine (a strong electron-donating ligand). The N-hetero
cyclic carbine enables the Ru metal center in 6 to bear more elec-
tron density than the Ru center in 9 which contains tricyclohexyl
phosphine. The higher electron density in 6 prevents the Ru metal
from coordinating with an electron-donating ligand or an anionic
Complex 6 with an ionic [nido-7,8-C2B9H11
]
tag was found to
exhibit an extremely high level of recyclability in the RCM reactions
of N,N-diallyltosylamide. In this paper, a first generation Ru com-
plex with a closo-1,2-C2B10H11 tag is reported. In addition, an
extension of our research on complex 6 was done.
group. In contrast, the interaction between the anionic [nido-7,8-
ꢀ
C2B9H11
]
group and the Ru metal center in 9 is strong enough to
accelerate the decomposition of 9 and so complex 9 was unstable
and could not be characterized and no catalytic activity for it could
be detected.
* Corresponding author. Fax: þ86 022 2376653.
** Corresponding author. Fax: þ86 22 2740 3475.
E-mail addresses: guiyanliu2013@163.com (G. Liu), wjh@tju.edu.cn (J. Wang).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.09.005

14
G. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 13e17
3. Conclusions
PCy3
Cl
MesN
Cl
NMes
Cl
PCy3
Cl
MesN
Ru
NMes
Cl
Ru
In conclusion, complex 8 with a closo-1,2-C2B10H11 tag is an
efficient catalyst for the RCM of di- and trisubstituted diene sub-
strates. In addition it has been shown that ruthenium carbene
Ru
Ru
O
Cl
Cl
Ph
PCy3
O
Cl
Ph
PCy3
ꢀ
complex 6 which has an ionic [nido-7,8-C2B9H10
]
tag is a robust
1
2
3
4
and recyclable RCM catalyst for the RCM of di- and trisubstituted
diene substrates in an ionic solvent system. This catalyst has the
combined advantages of high reactivity and a high level of recy-
clability and reusability. This work is useful for the continued
development of catalyst systems that are suitable for organic
transformations in ionic liquid.
Fig. 1. Ru catalysts for olefin metathesis.
Complexes 5 and 8 were tested as catalysts for RCM reactions
and the results are shown in Table 1. Like ruthenium carbene
complex 5, complex 8 is also an active RCM catalyst for the ring
closures of the N-protected substrates 10, 12, and 14 and the
oxygen-containing substrates 16 and 18. These reactions resulted in
the formation of five-, six-, or seven-membered rings with di-
substituted double bonds. The products were obtained in high
conversions with a low catalyst loading (0.2 mol %) (see Table 1).
For the di-substituted diene 20 and the tri-substituted dienes (22
and 24), both complexes 5 and 8 gave low conversions with a low
catalyst loading (0.2 mol %). However RCM was achieved in good
conversions with a longer reaction time (12 h) when the catalyst
loading was increased to 1.0 mol %. So, like complex 5, 8 is an active
catalyst for a large scope of RCM reactions, and it is tolerant to many
functional groups. This behavior is similar to that of its parent
compound 3.
4. Experiment
4.1. General
1H NMR, 13C NMR and 11B NMR spectra were acquired in CDCl3
on Varian 400 spectrometers. If not otherwise noted, the chemical
shift values are reported in ppm relative to residual CHCl3 (d 7.26)
for the 1H NMR spectra, relative to CDCl3 (77.16 ppm) for the 13C
NMR spectra, and relative to BF3.OEt2 (0.00 ppm) for the 11B NMR
spectra. Multiplicities are described using the following abbrevia-
tions: s ¼ singlet, d ¼ doublet, m ¼ multiplet. Coupling constants (J)
are quoted in Hz at 400 MHz for 1H NMR.
Infrared spectra were recorded on a PerkineElmer Model 1600
FT-IR spectrophotometer. Elemental analyses were determined at
Nankai University using a PerkineElmer-2400C instrument. Con-
versions were obtained by HPLC analysis of the sample on an Agi-
lent 1100 system using an Eclipse XDB-C8 (4.6 ꢂ 150 mm) column.
The mobile phase was CH3OH/H2O ¼ 4:1.
The recyclability and reusability of complex 6 for substrate 10 in
an ionic liquid (IL, BMI$PF6) has previously been established [7]. In
order to further investigate the reusability and recyclability of 6,
other di- and tri-substituted diene substrates were selected. The
results are shown in Table 2. These reactions led to the formation of
various carbocyclic olefins. Rutꢀhenium carbene complex 6 which
4.2. Materials
has an ionic [nido-7,8-C2B9H10
] tag can be recycled and reused 8
times for substrate 14 and 5 times for substrate 12 with 2.5 mol %.
For the oxygen-containing diene 18, catalyst 6 is also recyclable and
high conversions were obtained. Even for the trisubstituted sub-
strate 22, catalyst 6 could be recycled and reused 6 times with
2.5 mol% loading. All the reactions proceeded smoothly and gave
the corresponding cyclic olefin products in excellent yields. In
contrast, a dramatic decrease in the conversion of substrates was
found after the first run when 8 was used to promote RCM in ILs.
Therefore, 6 is a recyclable catalyst that can be used in an IL for
many different diene substrates.
Importantly, catalyst 6 is the first catalyst with an anion-
appended ligand on the ruthenium metal center that has good
recyclability and is easily recovered. In 2003, Guillemin and co-
workers [3p,3q] and Yao [6e,6f] independently reported the syn-
thesis of IL-supported catalysts with a cation-appended ligand for
the ring-closing metathesis (RCM) of olefins. The recycling and
recovery capability of complex 6 is similar to those catalysts.
Unless otherwise noted, all reactions were performed under an
atmosphere of dry N2 with oven-dried glassware and anhydrous
solvents. THF, benzene, hexanes and diethyl ether were distilled
with sodium/benzophenone under a N2 atmosphere. CH2Cl2 was
dried over CaH2, and distilled prior to use. All other solvents were
ꢀ
dried over 4e8 A mesh molecular sieves (Aldrich) and were either
saturated with dry argon or degassed before use. Reactions were
monitored by analytical thin layer chromatography (TLC) on
0.20 mm Anhui Liangchen silica gel plates and spots were detected
by UV-absorption. Silica gel (200e300 mesh) (from Anhui Liang-
chen Chem. Company, Ltd.) was used for flash chromatography. The
ionic liquid (BMI$PF6) was prepared and purified as reported pre-
viously and dried under high vacuum at 70 ꢁC overnight to remove
trace amounts of water. Compound 7 was prepared according to the
literature method [7]. All other chemicals or reagents were ob-
tained from commercial sources.
4.2.1. Preparation of catalyst 8
Grubbs catalyst I (411 mg, 0.50 mmol) was added to a flask with
CuCl (50 mg, 0.50 mmol) under N2. A solution of 1-(3-isopropoxy-
Mes
N
Mes
N
Cl
Cl
4-vinyl-phenoxy)-methyl-1,2-closo-1,2-carborane
7
(124 mg,
O
Ru
Cl
Ru
O
0.4 mmol) in dry dichloromethane (25 mL) was then poured into
the reaction mixture. The resulting mixture was then stirred for 1 h
at 50 ꢁC. After cooling to room temperature, the reaction mixture
was filtered to collect the filtrate. The solvent from the filtrate was
evaporated under vacuum to give a residue. The residue was pu-
rified by flash column chromatography on silica using pentanes/
CH2Cl2 (2:1) as the eluent to give the desired product 8 as a
brownish powder (203 mg, 0.30 mmol, 75.1%). Analytical Data.
Calcd (found) for: C31H57B10Cl2PO2Ru C, 48.18 (48.23); H, 7.43
N
Cl
N
Mes
Mes
O
O
= CH or C
= BH
H
(n-Bu)4N+
5
6
Fig. 2. Carbollide tagged ruthenium catalysts.
(7.42). 1H NMR (400 MHz, CDCl3)
d (ppm): 1.27 (m, 11H, CH3 and

G. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 13e17
15
Cl
Ru PCy3
Cl
PCy3
Ru
O
Cl
Cl
O
1
1 ) TBAF, THF
(n-Bu)4N+
O
CH2Cl2, reflux
2 ) 1, CH2Cl2, reflux
O
O
H
O
9
7
8
= CH or C
= BH
Scheme 1. Synthesis of ruthenium catalysts 8 and 9.
PCy3), 1.70 (m, 23H, PCy3), 1.27 (br, 10H, BH), 2.17e2.34 (m, 5H,
PCy3), 4.45 (s, 2H, CH2), 5.20 (m, 1H, CH), 6.50 (d, 3J(H, H) ¼ 7.6 Hz,
H, CHar), 7.59 (d, 3J(H, H) ¼ 10.0 Hz, 1H, CHar), 17.22 (s, 1H, Ru]CH).
under nitrogen. The reaction mixture was stirred for 1.0e12 h
at 40 ꢁC. At the end of the reaction (monitored by thin-layer
chromatography (TLC)), the catalysts were separated by
silica gel chromatography using CH2Cl2 as the eluent to remove
any trace amounts of Ru residue. Conversions were estimated
by HPLC analysis. The starting materials and products
were separated by silica gel chromatography. Yields were ob-
tained by comparing the weight of the products of substrate
with the weight of the starting substrate. The catalytic activities
of complex 5 and 8 for a variety of substrates are shown in
Table 1.
13C NMR (100 MHz, CDCl3)
d (ppm): 22.0, 26.2, 27.6, 30.1, 35.6, 35.8,
57.8, 69.5, 71.0, 102.0,106.8,123.8,140.5,154.5,158.2, 279.2. IR (KBr)
v: 2931, 2852, 2593 (Ru]C), 1596, 1445, 1387, 1324, 1261, 1165,
1125, 911, 846, 730, 679 cmꢀ1
.
4.2.2. General procedure for RCM reactions
A weighed amount of ruthenium catalyst 5 or 8 and a solution
of substrate in dry CH2Cl2 were mixed in a reaction flask
Table 1
Application of catalysts 5 and 8 to different substrates.a
Entry
1
Substrate
Product
Cat. (mol %)
Conditions
Conv. (Yield)
Ts
N
TsN
11
5 (0.2)
8 (0.2)
0.5 h
2.5 h
>98% (98%)
>98% (97%)
10
N
PhH2CON
2
3
5 (0.2)
8 (0.2)
0.3 h
2.0 h
>98% (97%)
>98% (97%)
OCH2Ph
12
13
Ts
N
5 (0.2)
8 (0.2)
0.5 h
2.5 h
>98% (97%)
>98% (98%)
TsN
14
15
O
O
4
5
5 (0.2)
8 (0.2)
0.5 h
3.5 h
>98% (96%)
>98% (96%)
16
17
O
O
5 (0.2)
8 (0.2)
0.5 h
4.0 h
>98% (96%)
>98% (97%)
18
19
O
O
O
O
O
O
6
7
5 (1.0)
8 (1.0)
8.0 h
12 h
>98% (96%)
>98% (96%)
O
O
21
20
Ts
N
TsN
23
O
5 (1.0)
8 (1.0)
0.5 h
3.0 h
>98% (97%)
>98% (98%)
22
O
8
5 (1.0)
8 (1.0)
8.0 h
12 h
>98% (96%)
>98% (95%)
25
24
a
Reaction was conducted in CH2Cl2 at 40 ꢁC; conv. was obtained by HPLC; yields are isolated yields.

16
G. Liu et al. / Journal of Organometallic Chemistry 749 (2014) 13e17
Table 2
Recycling and reuse of 6 and 8 in BMI$PF6.
Substrate
Product
Catalyst
Cycle
Conv.a (%) (time)
Cycle
Conv.a (%) (time)
6
1
2
3
4
1
2
>98 (1.0 h)
>98 (1.0 h)
>98 (1.0 h)
>98 (1.0 h)
>98 (1.0 h)
<10 (12 h)
5
6
7
8
>98 (1.0 h)
>98 (1.0 h)
>98 (1.0 h)
>98 (5.0 h)
Ts
N
TsN
15
14
8
6
O
1
2
3
4
1
2
>98 (5.0 h)
>98 (5.0 h)
>98 (5.0 h)
>98 (5.0 h)
>98 (8.0 h)
<10 (12 h)
5
6
7
>98 (5.0 h)
>98 (5.0 h)
>98 (12 h)
O
8
18
19
6
8
1
2
3
1
2
>98(1.0 h)
>98 (1.0 h)
>98 (1.0 h)
>98 (2.0 h)
<10 (12 h)
4
5
6
>98 (1.0 h)
>98 (1.0 h)
>98 (12 h)
TsN
TsN
22
23
6
8
1
2
3
1
2
>98 (0.5 h)
>98 (0.5 h)
>98 (0.5 h)
>98 (1.0 h)
<10 (12 h)
4
5
>98 (0.5 h)
>98 (5.0 h)
OCPh
N
PhCON
12
13
a
Reactions were conducted at 35 ꢁC using 2.5 mol % of catalyst in CH2Cl2; conv. obtained by HPLC; yields are isolated yields.
(g) S. Gessler, S. Randl, S. Blechert, Tetrahedron Lett. 41 (2000) 9973;
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4.2.3. Recycle study of ruthenium carbene 6 and 8 in an IL
The general procedure for RCM with 6 or 8 in an IL was per-
formed as follows: catalyst 6 or 8 (2.5 mol % Ru), and 0.04 mL
BMI$PF6 in dry CH2Cl2 (0.5 mL) were mixed in a reaction flask under
N2. After the complete transfer of the catalyst from the organic
phase to the ionic phase, a substrate (0.13 mmol) was introduced to
the catalyst solution. The reaction mixture was stirred at 35 ꢁC until
the conversion of substrate was completed (monitored by TLC).
Then the solvent was removed under vacuum and petroleum ether
was added. The mixture was subjected to centrifugation to separate
the ionic layer (containing the catalyst) and the product. The ionic
layer (containing the catalyst) was then subjected to another cycle.
The solvent from the organic solution was evaporated to afford the
crude cyclized product. After purification by short column chro-
matography to remove any trace amounts of Ru residue, the sample
was subjected to HPLC. Conversions were obtained by comparing
the ratios of the peak areas of the starting materials with those of
the products. The recycle data for the catalysts is shown in Table 2.
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