Ruthenium-based complexes containing a benzimidazolium tag covalently connected to N-heterocyclic carbene ligands: Environmentally friendly catalysts for olefin metathesis transformations

Article abstract of DOI:10.1039/c2dt32856a

Two ruthenium-based pre-catalysts containing an ionic tag covalently connected to a N-heterocyclic carbene (NHC) ligand are reported. These novel complexes, bearing a polar benzimidazolium group, are air-stable and can be easily prepared from commercially available reagents. The quaternary benzimidazolium tag allows the efficient separation of ruthenium waste from the metathesis product after reaction. Application in several olefin metathesis transformations leads to desired products of high purity, which exhibit ruthenium contamination levels as low as 1 ppm after simple filtration through a pad of silica gel.

Full text of DOI:10.1039/c2dt32856a

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Rutheniumꢀbased complexes containing benzimidazolium tag covalently
connected to Nꢀheterocyclic carbene ligands: environmentally friendly
catalysts for olefin metathesis transformations
Marek Klučiar,a Karol Grela*a and Marc Mauduit*b
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two Rutheniumꢀbased preꢀcatalysts containing ionic tag covalently connected to a Nꢀheterocyclic carbene (NHC) ligand are reported.
These novel complexes, bearing a polar benzimidazolium group, are airꢀstable and can be easily prepared from commercially available
reagents. The quaternary benzimidazolium tag allows efficient separation of ruthenium wastes from the metathesis product after reaction.
10 Application in several olefin metathesis transformations leads to desired products of high purity, which exhibit as low ruthenium
contamination levels as 1 ppm after simple filtration through a pad of silica gel.
promising results. Nevertheless, all these tag containing preꢀ
catalysts operate on leaving ligands and active dichlororuthenium
methylene species are logically disconnected from the support
45 leading to the undesired Ruꢀleaching. Therefore, introduction of a
tag to nonꢀdissociative Nꢀheterocyclic carbene ligand[14] (NHC) in
second generation of Hoveyda type catalysts 2b should allow
enhanced separation and recovering of metathesis catalyst. This
should promise environmentally friendly metathesis process with
Introduction
In the past two decades, olefin metathesis has become an
15 ineluctable powerful tool for synthetic organic chemistry[1].
Among the different metalꢀbased complexes enabling to catalyse
this transformation, homogeneous ruthenium preꢀcatalyst such as
Grubbs 1a,b[2] and Hoveyda–Grubbs 2a,b[3] have widely
contributed to the intensive use of this technology to make
20 valuable molecules[4] (Scheme 1).
50 low ruthenium contamination. Surprisingly, only few examples of
introduction of ionic tag on this ancillary ligand were reported[15]
.
In continuation of our program to develop ‘greener’ metathesis
complexes, we report herein a new class of stable ruthenium
homogeneous preꢀcatalysts that contain
a
Nꢀmethyl
55 benzimidazolium tag covalently connected to the NHC fragment.
Results and discussion
Scheme 1 Selected ruthenium precatalysts for alkene metathesis
Synthesis of ionicallyꢀtagged NHC ligand precursors and
their related Ruꢀcomplexes
On the other hand, rutheniumꢀbased complexes share common
liability of all homogeneous transition metalꢀcatalysed reactions,
25 which is difficult separation of toxic metal side products[5].
Indeed, considering a catalytic process that involves 5 mol % of
Ru preꢀcatalyst loading, the ruthenium content in the finalproduct
exceeds 2000 ppm[6] after purification by silica gel
chromatography. As olefin metathesis reaction is commonly used
30 in pharmaceutical processes and production of fine chemicals,
ruthenium concentration levels have to be drastically decreased
below 5 ppm[5]. To solve this issue and decrease ruthenium
wastes, water extraction, scavengers[7], supported phosphines[8] or
treatment with activated charcoal in combination with few cycles
35 of chromatography[9] were proposed. However, the efficiency of
these removal protocols remains moderate, giving around 100 to
1200 ppm of ruthenium contamination in the best cases. Other
concepts based on catalysts immobilisation on various supports
were also studied with more or less success.[10] Among them,
40 strategies involving the introduction of polar or nonꢀpolar tags on
The general routes to Nꢀheterocyclic carbene ligands (NHC)
60 synthesis are well established[16]. However, as the NHC ligand
precursor is intrinsically an (di)azolium function, the introduction
of an additional ionic group is difficult to manage in a synthetic
manner. Indeed, generation of the free carbene ligand requires
strong bases (KHMDS for instance) that are incompatible with
65 the presence of the spectator ionic tag. Irreversible sideꢀreactions
could occur such as the tag loss or, in some cases, the formation
of corresponding bisꢀcarbene species when the spectator ionic tag
is an imidazolium function. Furthermore, certain reactions
involved to build the heterocyclic framework of NHC precursors
70 require generally the use of strong reducing reagents (LiAlH4) or
hard nucleophiles (Grignard and organozinc reagents) that are
also incompatible with the ionic tag spectator. Taking in
consideration all these problematic issues, we decided to develop
a
more suitable synthetic pathway providing the targeted
,
phosphine[11] pyridine[12] or benzylidene[13] ligands showed
75 ionicallyꢀtagged NHC based Ruꢀcomplexes. As depicted in the
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scheme 2, we started from the costless Nꢀallyl benzimidazoles 3aꢀ
b. The bromination under acidic conditions followed by the direct
substitution using an excess of mesitylamine gave the desired
performed under standard conditions (CH2Cl2 at 0.1M, 30 °C, 1
mol%)[19] showed that both the rate of metathesis reaction and the
conversion were slightly lower for all new taggedꢀcomplexes,
compared to the nonꢀmodified Hoveyda complex 2b. Similar
diamines 4aꢀb in 53% and 55% of yield respectively[17]
.
5
Subsequent treatment with ethylorthoformate in the presence of 40 trends were also observed during the RCM of 8b performed at
NH4BF4 yielded the NHC ligand precursors 5aꢀb in 75% and
76%. The deprotonation of the imidazolium proton with
potassium tꢀamylate in toluene followed by the addition of
Hoveyda complex 2a afforded the corresponding benzimidazoleꢀ
60 °C (figure 2). Considering the complexes 6aꢀb, we suspect
the presence of the free tertiary amine of the benzimidazole unit
that is susceptible to chelate the Ruꢀactive species. For the preꢀ
catalysts 7aꢀb, the lower reactivity is probably due to the
10 grafted preꢀcatalysts 6aꢀb in excellent isolated yields of 90 and 45 presence of diodo anionic ligands that minimizes the catalytic
95%, respectively. Any attempts of quarternization with alkyl
bromides, chlorides, tosylates and sultones up to 200°C under
conventional heating or microwave conditions were unsuccessful.
Hopefully, microwave irradiation of catalysts 6aꢀb in neat methyl
15 iodide at 120 °C led to the desired preꢀcatalysts 7aꢀb in very good
isolated yields (90% and 88%, respectively). However, this
process resulted in a halogen exchange at ruthenium centre from
dichloride to diodide, as witnessed the shift of benzylidene
protons of 16.43 and 16.25 ppm for catalysts 6aꢀb to 15.50 and
performance of the resulting I2(NHC)Ruꢀmethylene species
compared to their parent Cl2(NHC)Ruꢀmethylene counterparts[20]
.
R
R
1 mol% catalyst
solvent (0.1M)
O
O
O
O
O
O
O
O
8a, R = H
8b, R = Ph
9a,b
Scheme 3 RCM reaction of DEDAM 8a and DEMAM 8b
20 15.45 ppm for catalysts 7aꢀb, respectively[18a]
. It is well
100
90
established that halides and more generally various anionic
ligands are labile in solution, and these complexes undergo
anionic ligand exchange even in nonꢀprotic media at room
temperature [18b]. Apparently, elevated temperatures facilitate this
25 exchange. All attempts to optimize the quarternization conditions
to prevent the halogen exchange afforded inseparable mixture of
dichloro, chloroiodo and diodo ruthenium complexes.
80
70
2b
6b
6a
7b
7a
60
100
50
90
80
40
70
Therefore we decided to use stable diodide complexes 7a and 7b.
60
30
20
10
0
50
40
30
20
10
0
N
N
1. Br2 / HCl / CH2Cl2 / RT
R
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Tim e
h
N
N
2. 2,4,6-trimethylaniline / 125 °C
0
2
4
6
8
NH HN
R
Time h
50
Figure 1 Kinetic profile of RCM of DEDAM 8a, 1 mol % catalyst,
3a, R = Me
3b, R = Ph
4a (55% yield)
4b (53% yield)
CD2Cl2 (0.1M), 30 °C
N
N
100
90
80
70
60
R
NH4BF4 / HC(OEt)3
80 - 100 °C
t-amylate / 2b / 80 °C
N
N+
-
BF4
5a (75% yield)
5b (76% yield)
I-
N
R
N
R
2b
50
N
N
6b
6a
7b
7a
40
30
20
10
0
neat CH3I / microwave
N
N
N
I
N
I
Cl
Ru
O
Ru
O
Cl
0
5
10
15
20
25
30
35
40
45
50
55
60
6a (93% yield)
6b (95% yield)
7a (90% yield)
7b (88% yield)
Time [min]
30
Scheme 2 Synthesis of desired complexes 7aꢀb
Catalytic Performances
Figure 2 Kinetic profile of RCM of Diethyl 2ꢀallylꢀ2ꢀmethallylmalonate
8b, 1 mol % catalyst, tolueneꢀD8 (0.1M), 60°C
55
To further compare catalysts 7aꢀb and evaluate the possibility to
remove easily residual Ruꢀwastes from the product, various
olefin metathesis transformations were investigated. We selected
ringꢀclosing or enyne metathesis reactions involving substrates
To compare relative activities of preꢀcatalysts 2b, 6aꢀb and 7aꢀb,
the ringꢀclosing metathesis (RCM) of diallyl malonate
(DEDAM) 8a and diethylꢀmethallylallyl malonate (DEMAM)
35 8b were investigated. As depicted in figure 1, RCM of 8a
60 10aꢀb, 12, 14 (Scheme 4) and crossꢀmetathesis reactions
2
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involving olefins 16 and 19 (Scheme 5). Results are summarized
in Table 1.
Regarding Crossꢀmetathesis transformation, the steroid like
25 substrate 16 and functionalisedꢀhomoallyl amine 19 were reacted
at 80°C in toluene over 12h with acrylic acid 17 and nꢀ
butylacrylate 20, respectively. To obtain conversions >95% and
excellent isolated yields (90ꢀ93%) in desired crossꢀmetathesis
products 18 and 21, a catalyst loading of 5 mol% and 4
30 equivalents of electronꢀdeficient olefin partners were necessary.
R
O
O
a: R = H, b: R = CH3
R
10a,b
11a,b
Ruthenium contamination in metathesis products
O
O
O
O
O
N
N
O
N
N
In order to remove the residual Ruꢀwastes, the reaction mixture
was deposited on dry silica (2 g per 0.0025 mmol of catalyst
35 used) and the metathesis product was eluted with appropriate
solvent (Figure 3). The ruthenium content in selected metathesis
products was quantified by ICPMS analyses[22] (table 2).
N
N
O
O
12
13
14
15
Scheme 4 Selected enyne and ringꢀclosing metathesis reactions
Table 2 Residual Ru content in selected productsa
O
O
OH
Eluentb
Ru content
O
Product
Catalyst
Loading
(mol%)
O
O
OH
(ppm)c
O
O
O
16
17
18
11a
13
15
18
21
7a
7b
7a
7a
7b
1
1
1
5
5
Toluene
cHex/EA 2/1
cHex/EA 1/1
EA
6.2
5.6
196
30
O
N
N
O
O
O
O
O
O
O
Toluene
<1
19
20
21
a Reaction mixture filtrated through pad of silica (2g per 0.0025 mmol of
5
40 catalyst) b EA = Ethyl Acetate, cHex = Cyclohexane c Determined by
Scheme 5 Selected crossꢀmetathesis reactions
ICPꢀMS analysis[22]
.
For enyne metathesis, the lessꢀhindered substrate 10a gave
full conversion with 1 mol% loading of catalyst after 12 h at
80°C under conventional heating or after 30 min at 120°C using
10 microwave irradiation. On the other side, despite the use of 5
mol % of 7aꢀb in refluxing toluene over 24 h, only the startingꢀ
material 10b was recovered. It should be noted, that this
substrate is particularly difficult even for the most active
catalysts [21]. Regarding RCM reactions, substrates 12 and 14
15 were tested. Cyclization proceeds smoothly with 1 mol% of
catalysts at 40°C for 12h, affording respectively the
corresponding metathesis products 13 and 15 in 87 to 95% of
isolated yields.
Table 1 Representative metathesis reactions
Substratea
10a
Catalyst
Loading
(mol%)
Conditions Conv.b/Yieldc
(%)
Figure 3 Separation of residual Ru catalyst/side products: a) deposition of
7a
7a
7b
7b
7a
7b
7a
7b
7a
7b
7a
7b
7a
7b
1
1
1
1
5
5
1
1
1
1
5
5
5
5
12 h, 80 °C
0.5 h, 123 °Cd
12 h, 80 °C
0.25 h, 123 °Cd
24 h, 110 °C
24 h, 110 °C
12 h, 40 °C
12 h, 40 °C
12 h, 40 °C
12 h, 40 °C
12 h, 80 °C
12 h, 80 °C
12 h, 80 °C
12 h, 80 °C
98/95
99/95
98/95
93/89
0/96e
45
reaction mixture on silica b) elution of product
As expected, the presence of highly polar Nꢀmethyl
benzimidazolium tags on the NHC ancillary ligand in catalysts
7aꢀb allowed very efficient separation of residual ruthenium side
products as low as 1 ppm (product 21). To obtain good separation
10b
12
0/95e
92/87
99/94
94/90
99/95
95/90f
98/93g
97/90h
99/93i
50 and residual Ru content as low as possible, the choice of the
eluent appears crucial but remains closely dependent on the
nature of the organic product. Toluene, ethyl acetate (EA) or
mixtures of cyclohexane/EA are tolerated. Remarkably,
metathesis products 11, 13 and 21 were isolated in excellent
55 purity, containing only few ppm traces of metallic ruthenium.
However, application of such flashꢀchromatography protocol for
certain highly polar products such as 15 appeared more
problematic despite the reduction of the eluent polarity
(cyclohexane/EA 1/1; 196 ppm). On contrary, while pure ethyl
14
16 + 4eq. 17
19 + 4eq. 20
20 a 1 mmol, toluene (0.1M).b Conversion monitored by 1H NMR
spectroscopy. c Isolated yields. d Microwave irradiation, 300 W. e Starting
material recovered. f E/Z ratio= 3.5/1 g E/Z ratio= 3.7/1 h E/Z ratio= 2.4/1 i
E/Z ratio= 2.7/1
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5
6
Information concerning EMEA regulations can be found at
http://www.emea.europa.eu/pdfs/human/swp/444600en.pdf
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A. E. Holliday, C. M. Crudden, Org. Lett. 2006, 8, 2663ꢀ2666 c)
Crude untreated product 8a RCM catalyzed by 5 mol % of 1a, the
theoretical amount of Ru is 90 ꢁg per 5 mg of product (18 000 ppm).
After filtration of the crude reaction mixture, the Ru level was
reduced to ±60 ꢁg per 5 mg (12 000 ppm). Further purification of
such crude metathesis products usually reduces Ru levels below 2000
ppm.
acetate was necessary for recovering efficiently the lipidic
product 21, the filtration protocol remained suitable as the
ruthenium content was drastically reduced down to 30 ppm.
65
70
Conclusions
5
Two new Hoveydaꢀtype complexes bearing Nꢀmethyl
benzimidazolium tag covalently connected to the NHC ligand
were synthetized and fully characterized. The involved strategy
using the quaternarisation of tertiary amine directly on the Ruꢀ
complex opens a promising way to generate new class of ionicꢀ
7
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10 tag containing metalꢀNHC complexes. The catalysts 7aꢀb were
efficient for RCM, crossꢀ and enyne metathesis of moderately
hindered substrates. Nevertheless, they exhibited low activity
towards sterically hindered olefins. In order to reach more
efficient catalysts, further optimizations of the key quarternisation
15 reaction on Ruꢀcomplex to avoid exchange of anionic coꢀligand
from chloride to iodide are currently underway. Moreover, thanks
to the covalent grafting of highly polar benzimidazolium tag, easy
and highly efficient separation of ruthenium wastes from the
metathesis product through the simple filtration over small
20 quantity of SiO2 allowed decreasing residual metal concentration
as low as 1 ppm. Therefore, compared to other Ruꢀremoval
protocols[23], ionicꢀtagged complexes 7aꢀb might prove suitable
olefin metathesis catalysts for pharmaceutical and other
technologically advanced applications demanding low Ru
25 content.
75
80
8
9
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85
10 For a review dealing with different concepts used for supported Ruꢀ
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90
95
Notes and references
a Department of Chemistry, Warsaw University, Zwirki i Wigury Street
101, 02-089 Warsaw, Poland. Fax: (+)48 -22 -632-66-81; E-mail:
klgrela@gmail.com, Web: http://karolgrela.eu/
30 b Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226,
OMC Team, Av. du Général Leclerc, CS 50837, 35708 Rennes Cedex 7,
France. Fax : (+)33-223-238-108, E-mail: marc.mauduit@ensc-rennes.fr
† Electronic Supplementary Information (ESI) available: [Experimental
procedures, characterization data and 1H and 13C NMR spectra for all
35 previously unreported compounds.]. See DOI: 10.1039/b000000x/
‡ M.K. thanks the Foundation for Polish Science for the MPD Program
coꢀfinanced by the EU European Regional Development Fund. MM tanks
the the CNRS, the Ministère de la Recherche et de la Technologie and
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16 For a special review dealing with the synthesis of NHC ligands, see:
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17 This procedure is really useful as alkene substituted (benz)imidazoles
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20 22 The ICPꢀMS measurements were performed by UT2A Company,
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23 Georgios C. Vougioukalakis, Chem. Eur. J. 2012, 18, 8868ꢀ8880.
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