Cycloalkyl-based unsymmetrical unsaturated (U2)-NHC ligands: Flexibility and dissymmetry in ruthenium-catalysed olefin metathesis

Article abstract of DOI:10.1039/c4dt00142g

Air-stable Ru-indenylidene and Hoveyda-type complexes bearing new unsymmetrical unsaturated N-heterocyclic carbene (U₂-NHC) ligands combining a mesityl unit and a flexible cycloalkyl moiety as N-substituents were synthesised. Structural features, chemical stabilities and catalytic profiles in olefin metathesis of this new library of cycloalkyl-based U₂-NHC Ru complexes were studied and compared with their unsymmetrical saturated NHC-Ru homologues as well as a set of commercially available Ru-catalysts bearing either symmetrical SIMes or IMes NHC ligands.

Full text of DOI:10.1039/c4dt00142g

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ARTICLE TYPE
Cycloalkyl–based Unsymmetrical Unsaturated (U2)-NHC ligands:
flexibility and dissymmetry in ruthenium-catalysed olefin metathesis
Mathieu Rouena, Etienne Borréa, Laura Faliveneb, Loic Toupetc, Mikaël Berthod d, Luigi Cavallob,e,
Hélène Olivier-Bourbigoud and Marc Mauduit*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Airꢀstable Ruꢀindenylidene and Hoveydaꢀtype complexes bearing new Unsymmetrical Unsaturated NꢀHeterocyclic Carbene (U2ꢀNHC)
ligands combining a mesityl unit and a flexible cycloalkyl moiety as Nꢀsubstituents were synthesised. Structural features, chemical
stabilities and catalytic profiles in olefin metathesis of this new library of cycloalkylꢀbased U2ꢀNHC Ru complexes were studied and
10 compared with their unsymmetrical saturated NHCꢀRu homologues as well as a set of commercially available Ruꢀcatalysts bearing either
symmetrical SIMes or IMes NHC ligands.
potassium hexamethyldisilazane (KHMDS) in toluene followed
by the addition of commercially available (PCy3)2Cl2Ruꢀ
Introduction
indenylidene complex 10 (M1)12.
NꢀHeterocyclic Carbenes (NHCs) have become powerful
15 ancillary ligands in TransitionꢀMetal (TM) based catalysis,
affording beneficial properties to the metal center, thanks to their
remarkable σꢀdonor character.1 In the area of rutheniumꢀbased
olefin metathesis,2 the involvement of this class of ligands
represents certainly the more significant breakthrough, affording
20 improved stability and activity3 as well as selectivity4 of the
corresponding complexes (for instance preꢀcatalysts 1ꢀ4,5 Fig. 1).
Over the past two decades, considerable efforts were focused on
the NHC design,6 with the main goal to extend the application
window of olefin metathesis, notably for industrial applications.7
25 In order to bring improved selectivities to the reactive metallic
species, the quest for original scaffolds was intensified notably
through the development of NHCs showing a high level of
dissymmetry (for instance complex 5, Fig. 1).8 The highly Zꢀ
selective complex 6 reported by Grubbs in 2011 illustrates well
30 this statement (Fig. 1).9 In this context, we envisioned the design
of a new library of indenylidene as well as Hoveydaꢀtype Ruꢀ
complexes 7 bearing unsaturated unsymmetrical (U2)ꢀNHCs
having a flexible cycloalkyl moiety10 and a mesityl group as Nꢀ
substituents (Fig. 1). Structural features, chemical stabilities and
35 catalytic profiles in olefin metathesis of these new NHCꢀRu
complexes 7 were fully examined. Furthermore, they were
compared with their saturated homologues 8 as well as a set of
commercially available Ruꢀcatalysts 2ꢀ4 bearing symmetrical
N,Nꢀbisꢀmesityl ꢀimidazolinꢀ2ꢀylidene (SIMes) or ꢀimidazolꢀ2ꢀ
40 ylidene (IMes) ligands.
Ru-complexes bearing symmetrical NHCs
Ru-complexes bearing dissymmetrical NHCs
N
N
L
L
Cy
Cl
Cl
Ph
Cl
Ru
Ru
Ru
O
Cl
Cl
Cl
PCy3
PCy3
2 L = SIMes (Umicore M2)
3 L = IMes (Evonik)
1 L = SIMes
(Grubbs II)
5 (Verpoort)
L
Cl
N
N
Ru
Cl
O
R
Ru
O
t-BuCOO
4a L = SIMes; R = H (Hoveyda-Grubbs)
4b L = SIMes; R = NHCOCF3 (Umicore M71)
4c L = IMes; R = NHCOCF3
6 (Grubbs Z-selective)
N
N
N
N
SIMes =
IMes =
N
N
Cl
Ru
L
Cl
Ar
7
Flexible group
This work
50 Fig. 1 Selected examples of (un)symmetrical (un)saturated NHC Ruꢀ
based complexes 1ꢀ6 and targeted dissymmetrical unsaturated flexible
cycloalkylꢀNHC Ruꢀcomplexes 7.
However, the desired Ruꢀcomplexes 7aꢀb were isolated in low
yields (30ꢀ45%) due to a competitive formation of bisꢀNHC
55 complex (up to 21%). Fortunately, the use of potassium tꢀamylate
as base resulted in improved yields (Eq. 1), up to 43% and 62%
respectively,13 with reduced amount of bisꢀNHC Ruꢀcomplexes
(5ꢀ13%). Furthermore, 7a and 7b were easily converted into their
corresponding phosphineꢀfree Hoveydaꢀtype precatalysts 7c-d
60 and 7e in respectively 42%, 76% and 45% isolated yield by
reacting styrenylethers 11 and 12 in presence of copper chloride
Results and discussion
Our study started with the synthesis of 1ꢀmesitylꢀ3ꢀcycloalkylꢀ
imidazolꢀ2ꢀylidene Ruꢀcomplexes 7, disclosed in scheme 1. The
oneꢀstep
available
tetrafluoroborate
cyclopentylꢀ
and
45 cyclododecylꢀ imidazolium salts 9aꢀb11 were deprotonated with
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(Eq. 2 and 3).
isolated in 23% and 65% of yield, respectively (Eq. 4, scheme 2).
The low yield observed for 8a was mainly due to the partial
25 degradation of the complex occurring during the silica gel
purification. Furthermore, treatment of 8b with the styrenylether
11 in presence of CuCl gave the corresponding phosphineꢀfree
Hoveydaꢀtype complex 8c in 63% isolated yield. The structures
of complexes 7d, 7e and 8b were confirmed by singleꢀcrystal Xꢀ
30 ray diffraction (Fig. 2).‡ Based on these solidꢀstate structures, we
then decided to study the steric properties15 of the newly
developed unsymmetrical (un)saturated NHCs derived from salts
BF4
N
N
t-AmylOK
Eq. (1)
N
N
n
Cl
toluene, rt, 30 min.
then 10, 80 °C, 2 h
Ph
n
Ru
9a n = 0
9b n = 7
Cl
PCy3
7a n = 1 (43%)a + 5% of bis-NHC complex
7b n = 8 (62%)a + 13% of bis-NHC complex
PCy3
Cl
Ph
Ru
Cl
PCy3
10
N
N
9a-b and 18b (Fig. 3). The percent buried volume (%VBur
mesitylꢀ3ꢀcyclododecylꢀimidazolꢀ2ꢀylidene, 1ꢀmesitylꢀ3ꢀ
)
16 of 1ꢀ
n
Cl
11
O
NH
O
Ru
O
O
CF3
Cl
CF3
Eq. (2)
7a or 7b
NH
35 cyclopentylꢀimidazolꢀ2ꢀylidene and 1ꢀmesitylꢀ3ꢀcyclododecylꢀ
imidazolinꢀ2ꢀylidene were calculated from the corresponding Ruꢀ
complex 7d, 7e and 8b using the Xꢀray structures. The
corresponding %VBur of these three NHCs, 30.7% and 29.9% and
29.3% respectively, indicated that the steric hindrance was only
40 poorly dependant on both the size of the cyclododecyl group and
the saturation degree of the NHC.
CH2Cl2, CuCl
40 °C, 5 h
7c n = 1 (42%) 7d n = 8 (76%)
N
N
12
O
Cl
Ru
O
Eq. (3)
7a
Cl
CH2Cl2, CuCl
40 °C, 5 h
7e (45%)
Scheme 1 Synthesis of 1ꢀmesitylꢀ3ꢀcycloalkylꢀimidazolꢀ2ꢀylidene Ruꢀ
a
complexes 7a-e. A mixture of two rotamers was observed in 31P NMR
5
spectroscopy for 7a and 7b with a ratio of 83/17 and 93/7, respectively.13
In order to fully evaluate structural features and the catalytic
behaviour of these new unsaturated unsymmetrical (U2)ꢀNHC
complexes, we decided to synthetise their saturated analogues 8a-
c (Scheme 2).8c Unsymmetrical imidazolinium salts 18 were
7d
7e
10 easily synthesised through the wellꢀknown and efficient fourꢀstep
synthetic route,14 which involves ethyloxalyl chloride 13,
mesitylamine 14 and cycloalkylamine 16. Good overall yields of
30% and 38% were obtained for azolium salts 18aꢀb,
respectively. Curiously, the use of KHMDS to afford the
15 corresponding carbene species was less problematic in
comparison with their unsaturated analogues 9a-b, as lower
amounts of undesired bisꢀNHC Ruꢀcomplexes (5ꢀ8%) were
detected in the crude mixtures.
8b
Fig. 2 Solidꢀstate structures of 1ꢀmesitylꢀ3ꢀcycloalkylꢀimidazolꢀ2ꢀylidene
Hoveydaꢀtype complexes 7d-e and 1ꢀmesitylꢀ3ꢀcycloalkylꢀimidazolinꢀ2ꢀ
45 ylidene Ruꢀindenylidene complex 8b from single crystal Xꢀray
diffraction.‡ Hydrogen atoms have been partially omitted for clarity.
O
O
Cycloalkyl-NH2 16
EtO
O
O
O
EtO
O
toluene, MW
Mes-NH2 14
NH HN
140 °C, 20 min.
Et3N, CH2Cl2
rt, 12 h
n
Cl
NHMes
We further compared the steric hindrance of the NHC in 7d, 7e
and 8b with that obtained by replacing the unsymmetrical ligand
with both the classical IMes and SIMes ligands. Since no Xꢀray
50 structure was available for all the complexes, we decided to use
DFT optimized structures for the %VBur calculation. To this end,
the calculations were first performed on complexes 7d, 7e and 8b
to verify that DFT based %VBur are consistent with %VBur from
Xꢀray structures. As the DFT based %VBur (30.0%, 29.7% and
55 28.9%) are reasonably close to those reported above from
analysis of the Xꢀray structures, the DFT optimized structures
were used to measure the steric hindrance of SIMes and IMes
NHC ligands. Analogous SIMesꢀbased complexes (4b, 4a and 2)
resulted in %VBur of 32.8%, 32.9% and 29.9% respectively,
60 whereas replacing the unsymmetrical NHC ligand with the
classical IMes NHC (complexes 4c, 4d and 3) results in %VBur of
31.7%, 31.9% and 28.9%. Finally, DFT based %VBur of complex
7a, with the small cyclopentyl moiety, is only 27.8%, the lowest
value of this series. This indicates that the steric hindrance of the
17a n = 1 (83%)
17b n = 8 (79%)
13
15 (quant.)
1) LiAlH4, THF
80 °C, 12 h
2) NH4BF4, HC(OMe)3
120 °C, 2 h
N
N
BF4
KHMDS
n
toluene, rt, 30 min.
Cl
N
N
Ph
Eq. (4)
Ru
then 10, 60 °C, 2 h
n
Cl
18a n = 1 (36%)
18b n = 8 (48%)
PCy3
8a n = 1 (23%) + 5% of bis-NHC complex
8b n = 8 (65%) + 8% of bis-NHC complex
N
N
8
Cl
11
Ru
O
O
Eq. (5)
8b
8c (63%)
Cl
CF3
(CH2)2Cl2, CuCl
60 °C, 15 min.
NH
20 Scheme 2 Synthesis of 1ꢀarylꢀ3ꢀcycloalkylꢀimidazolinꢀ2ꢀylidene Ruꢀ
complexes 8aꢀc.
Therefore, the expected indenylidene Ruꢀcomplexes 8a-b were
2
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unsymmetrical NHCs we developed is somewhat reduced relative
to the classical IMes and SIMes ligands. These structural features
in the case of unsaturated NHC complexes, as unsymmetrical
cycloalkylꢀNHC based complexes (7a and 7b) showed better
were confirmed by the analysis of the steric maps17 reported in 45 activity profile than their symmetrical IMes homologue 3.
Fig. 3. The slightly lower value of the %Vbur in 7a, 7d, 7e and
100%
5
8b is clearly due to the unsymmetrical steric hindrance around the
metal. In fact, the cycloalkyl moiety (left side in the maps of Fig.
100%
3) can be folded away from the metal center, thus reducing its
50%
steric impact in proximity of the metal, minimising repulsion with
80%
other ligands. This is different from the analogous complexes
0%
10 bearing an IMes or a SIMes ligand, due to the more rigid nature
of the mesityl Nꢀsubstituent.
0
1
2
3
4
5
6
60%
40%
20%
0%
Time (h)
2
3
From X-ray
From DFT-calculation
4b
7a
7c
8a
8c
4c
7b
7d
8b
7e
N
N
N
N
8
Cl
Ph
Ru
Cl
Ph
Cl
Ru
8b
2
Cl
PCy3
N
PCy3
N
0
20
40
60
80
100
120
140
N
N
Time (h)
8
Cl
Cl
Ru
O
Ru
O
Cl
CF3
Cl
CF3
O
NH
Fig. 4 Chemical stability in tolueneꢀD8 (10mM) at 60 °C of Ruꢀcomplexes
7-8 and 2ꢀ4. Preꢀcatalyst decomposition was monitored by 1H NMR
spectroscopy with anthracene as internal standard.†
O
NH
7d
4c
N
N
N
N
Cl
Cl
50 Astonishingly, while they led to similar steric hindrance (vide
supra), the cyclopentyl substituent afforded an improved activity
profile than the cyclododecylꢀmoiety. And this trend was more
pronounced for the unsaturated NHC’s as a complete conversion
was reached within 6h with 7a whereas 24h were needed for
55 cyclododecylꢀcomplex 7b. Concerning the Hoveydaꢀtype preꢀ
catalysts bearing a cyclododecylꢀ or a cyclopentylꢀ unsaturated
NHC (7d, 7c and 7e respectively), we were quite disappointed by
their activity profile in comparison with their (S)IMes
homologues 4aꢀc (fig. 6). Indeed, complex 7e required 14h to
60 reach a maximum of 90% of conversion while the original
Hoveyda 4a completed the reaction within 7h.
Ru
Ru
Cl
Cl
7e
4d
O
O
N
N
Cl
Ph
7a
Ru
Cl
PCy3
!
Fig. 3 Percentage of buried volume (%VBur) in the single quadrants
around the Ru center, and steric maps of unsymmetrical Ruꢀmetal
15 complexes 8b, 7d, 7e and 7a, and of the corresponding symmetrical
complexes bearing SIMes (2) or IMes ligand (4c, 4d). The orientation of
the complex for the steric maps calculations, and the isocontour scale, in
Å, are reported at the bottom.
EtO2C
Before studying the catalytic efficiency in olefin metathesis of
20 this small library of new Ruꢀcomplexes 7 and 8, we examined
their chemical stability in tolueneꢀD8 (10mM) at 60 °C in
comparison with their symmetrical SIMes and IMes analogs 2ꢀ4
(Fig. 4). Considering the indenylideneꢀbased complexes, the
newly developed unsymmetrical 7a and 8a-b were fully
25 decomposed after 5ꢀ6h, as the M2 catalyst 2 was. The less stable
member of this series was the unsaturated cyclododecylꢀbased
NHCꢀcomplex 8b, which was fully decomposed within 1h, while
IMesꢀcomplex 3 appeared the most stable in solution, up to 40h.
On the other hand, Hoveydaꢀtype complexes 7cꢀe and 8c, which
30 are wellꢀknown to be more stable than their phosphine analogues,
showed a slower thermal decomposition (ranging from 48h to 5
days), close to complexes 4aꢀc.
Having all these new unsymmetricalꢀNHC based Ruꢀ
complexes 7 and 8 in hands and their respective structural
35 features and chemical stabilities, we next started their evaluation
in olefin metathesis transformations. Firstly, we studied their
activity profiles in RingꢀClosing Metathesis (RCM) of stericallyꢀ
demanding metallylallyl diethylmalonate 19 (Scheme 3) in
homogeneous standard conditions (i.e. CD2Cl2 0.1M, 30°C, 1
40 mol%).18 As depicted in Fig. 5, saturated unsymmetricalꢀNHC
Ruꢀindenylidene complexes 8aꢀb were less active than
symmetrical SIMesꢀRu complex 2. This behaviour was inversed
EtO2C CO2Et
CO2Et
catalyst (1 mol%)
Eq. (6)
CD2Cl2 (0.1M), 30 °C
20
19
Scheme 3 RCM model reaction selected for evaluation of preꢀcatalysts 2-
8.
100%
90%
80%
70%
2
60%
3
50%
8a
40%
30%
20%
10%
0%
8b
7a
7b
0
5
10
15
20
25
30
Time (h)
65
Fig. 5 Catalytic activity profiles of Ruꢀindenylidene complexes 7aꢀb, 8aꢀ
b and 2ꢀ3 for RCM of metallylallyl diethylmalonate 19. Conversion was
monitored by 1H NMR spectroscopy with mesitylene as internal
standard.†
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Surprisingly, this trend was also observed with complexes 7c and
7d despite the presence of the electronꢀwithdrawing (EWG)
trifluoroacetamide activating function. The reaction progressed
a 1:1 mixture of the expected CM product 43 and the undesired
selfꢀmetathesis product 44 (entry 11). Interestingly, catalyst 7a
was quite efficient in neat condition at 80 °C for the selfꢀ
slowly reaching a maximum of 85% conversion after 24h while 50 metathesis of allylbenzene 45, affording after 5 min. of reaction
5
their SIMes or IMes counterparts 4bꢀc afforded >90% conversion
within only 2h.5e This lack of reactivity was more pronounced
with the saturated cyclododecylꢀNHC Hoveyda catalyst 8c
leading only to 50% of conversion after 20h. All these catalytic
behaviours indicate that the positive effect of the EWG function
82% of the desired product 46 in 84/16 E/Z ratio (entry 12). More
importantly, despite the absence of solvent, no trace of
isomerised byꢀproducts was detected in the crude mixture.20
Ts
Ts
Ts
N
N
N
Eq. (7)
N
Ts
Eq. (8)
10 on the styrenylether leaving ligand is not always ensured but
closely dependant on synergy effects taking into account steric
and electronic properties of the NHC ligand.19 Therefore, the
introduction of unsaturated cylcoalkylꢀfunctionalized NHCs
appeared more beneficial for phosphineꢀindenylidene based
15 complexes than for Hoveydaꢀtype complexes. Moreover, 7a
bearing the cyclopentyl moiety was the most efficient preꢀ
catalyst.
21
25
22
23
27
24
Ts
Ts
O
O
N
O
N
Eq. (9)
Eq. (10)
Eq. (12)
Ph
Ph
26
30
28
O
Eq. (11)
Ph
Ph
29
31
32
36
100%
90%
80%
Eq. (13)
Eq. (15)
Eq. (14)
Eq. (16)
Ph
Ph
Ph
Ph
33
34
35
70%
60%
Ph Ph
O
Ph
Ph
4a
4b
N
N
Ts
O
Ts
4c
7c
7d
7e
8c
50%
40%
30%
20%
10%
0%
37
38
39
40
O
Ph
MeO2C
42
O
Ph
O
CO2Me
2
Eq. (17)
Eq. (18)
43
Ph
O
2
O
Ph
OCO2Ph
41
Ph
O
Ph
46
2
2
45
44
0
2
4
6
8
10
12
14
16
18
20
Conv.a (Yield)b (%)
entry
Substrates
Products
Time (h)
Time (h)
1
2
21
23
22
24
2
>98 (92)
60 (45)
Fig. 6 Catalytic activity profiles of Hoveydaꢀtype complexes 7c-e, 8c and
20 4a-c for RCM of metallylallyl diethylmalonate 19. Conversion was
monitored by 1H NMR spectroscopy with mesitylene as internal
standard.†
5
3
25
26
2
88 (75)
4
27
28
2
>98 (95)
30 (29)
5
29
30
5
Next the scope of metathesis transformations was investigated
using 1 mol% of 7a in dichloromethane (0.1M) at 30 °C (Fig. 7).
25 RCM involving dienes bearing various functional groups were
firstly examined taking in consideration the effect on the ring size
formed as well as the influence of double bond substitution. In
the case of tosylamides, the formation of the 5ꢀmembered ring
was achieved easily (2h, >98%, entry 1) while RCM leading to 6ꢀ
30 and 7ꢀmemberedꢀring led to lower conversions (entries 2 and 3,
60 and 88% of conv. respectively) despite a substantial increase
of reaction time. A similar trend was observed for ethers as a
complete formation of 6ꢀmembered ether ring 28 occurred within
2h and only 30% of yield was reached for the 7ꢀmembered ring
35 30 after 5h reaction (entries 4 and 5). Catalyst 7a appeared quite
competent for hydrocarbon dienes (entries 6ꢀ8), as it allowed us
to decrease the catalyst loading down to 0.05 mol%, without
detrimental effect on the conversion (93% for 32, entry 6).
Interestingly, the stericallyꢀdemanding diene 37 required only 2
40 mol% of 7a at subambient temperature to produce 54% of
tetrasubstituted tosylamide 38 (entry 9). Catalyst 7a was also
efficient regarding the enyne cyclisation of 39 as the expected
diene 40 was formed in >98 % of yield after only 30 min (entry
10). Lastly, we examined the crossꢀmetathesis (CM) reactions of
45 terminal alkenes (entries 11 and 12). The reaction of homoallyl
benzoate 41 with an excess of methylacrylate 42 yielded 51% of
6
31
32
0.15c/1.5d
>98c/93d
98 (87)
7
33
34
1.5
1
8
35
36
82 (70)
9e
10
11
12j
37
38
3
54 (50)
39
40
0.5
20
>98 (97)
55g (27/27)
82 (75)
41/42f
45
43h/44i
46k
5 min.
a Conversion were determined by 1H NMR spectroscopy with mesitylene as internal standard.†
b Isolated yield after purification on silicagel
c 0.5 mol% of 7a were used.d 0.05 mol% of 7a were used.e 2 mol% of 7a were used.
f 5 equiv. of 42 were used.g Ratio 43/44: 1/1.† h E/Z = 100/0.† i E/Z = 80/20.†
j Neat at 80 °C. k E/Z = 84/16.†
55
Fig. 7 Olefin metathesis reactions catalysed by 7a. Reaction conditions: 1
mol% catalyst, CD2Cl2 (0.1M), 30 °C (excepted for entries 6, 9 and 12).
Conclusions
In summary, we have synthesised a small library of original Ruꢀ
60 based olefin metathesis complexes bearing unsaturated
unsymmetrical (U2)ꢀNHC ligands, which combine a Nꢀmesityl
unit and a flexible Nꢀcycloalkyl moiety. Interestingly, the
merging of the unsaturation and the cycloalkyl fragment on the
NHC lead to improve catalytic efficiency of PCy3ꢀbased Ruꢀ
65 indenylidene complexes. Among this new designed library, the
4
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8
a) For a review dealing with unsymmetrical NHCs in catalysis, see: J.
Tornatzky, A. Kannenberg, S. Blechert, Dalton Trans., 2012, 41,
indenylidene complex 7a bearing a 1ꢀmesitylꢀ3ꢀcyclopentyl
imidazolꢀ2ꢀylidene as NHC ligand appeared the most powerful
one, catalysing with efficiency a wide range of metathesis
transformations, even at 500 ppm of catalyst loading.
Noteworthy, this lowꢀcost complex, thanks to the straightforward
access of cycloalkylꢀbased U2ꢀNHCs, appears quite useful in selfꢀ
metathesis (SM) of terminal alkenes in neat condition. Further
studies to extend the scope in challenging SM reactions are
currently underway and will be reported soon.
8215; b) For
a special review on unsymmetrical NHCsꢀRu
70
75
complexes, see: F. B. Hamad, T. Sun, S. Xiao, F. Verpoort, Coord.
Chem. Rev., 2013, 257, 2274; For Ruꢀbenzylidene complexes bearing
saturated unsymmetrical NHCs, see: c) N. Ledoux, A. Linden, B.
Allaert, H. V. Mierde, F. Verpoort, Adv. Synth. Catal., 2007, 349,
1692; d) A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W.
Lehmann, R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J., 2001, 7,
3236; e) K. Vehlow, S. Maechling, S. Blechert, Organometallics,
2006, 25, 25.; f) G. C. Vougioukalakis, R. H. Grubbs,
Organometallics, 2007, 27, 2469; g) G. C. Vougioukalakis, R. H.
Grubbs, Chem. Eur. J., 2008, 14, 7545; h) For Ruꢀindenylidene
5
10 Notes and references
80
complexes bearing saturated unsymmetrical NHCs, see:
O.
a Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226,
OMC Team, 11 allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7,
France. Fax : (+)33-223-238-108, E-mail: marc.mauduit@ensc-rennes.fr
b Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo,
15 I-84084 Fisciano (SA), Italy.
Ablialimov, M. Kedziorek, C. Toborg, M. Malinska, K. Wozniak, K.
Grela, Organometallics, 2012, 31, 7316.
a) K. Endo, R.H. Grubbs, J. Am. Chem. Soc., 2011, 133, 8525; b) B.
K. Keitz, K. Endo, P. R. Patel, M. B. Herbert, R. H. Grubbs, J. Am.
Chem. Soc., 2012, 134, 693.
9
85
c Institut de Physique de Rennes - Université Rennes 1, CNRS, UMR 6251
Campus de Beaulieu Bâtiment 11A, 263 av. Général Leclerc, 35042
Rennes Cedex, France.
10 The flexibility around the metal brought by cycloalkyl Nꢀsubstituents
on symmetrical diaminocarbene ligands has proved to be highly
beneficial in crossꢀcoupling reactions involving stericallyꢀdemanding
substrates, see for instance: a) G. Altenhoff, R. Goddard, C. W.
Lehmann, F. Glorius, Angew. Chem. Int. Ed., 2003, 42, 3690. b) G.
Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, J. Am. Chem.
Soc., 2004, 126, 15195; c) S. Würtz, C. Lohre, R. Frölich, K.
Bergander, F. Glorius, J. Am. Chem. Soc., 2009, 131, 8344; d) P. De
Frémont, N. M. Scott, E. D. Stevens, T. Ramnial, O. C. Lightbody, C.
L. B. Macdonald, J. A. C. Clyburne, C. D. Abernethy, S. P. Nolan,
Organometallics, 2005, 24, 6301.
11 This low cost process leading to various (a)chiral Unsymmetrical
Unsaturated (U2)ꢀNHC precursors and their conversion to various
TransitionꢀMetal NHCꢀcomplexes were recently reported by our
group, see: P. Queval, C. Jahier, M. Rouen, J.ꢀC. Legeay, I. Artur, P.
Querard, L. Toupet, C. Crévisy, L. Cavallo, O. Baslé, M. Mauduit,
Angew. Chem. Int. Ed., 2013, 52, 14103.
d IFP Energies nouvelles, Rond Point de l’échangeur de Solaize, BP3,
20 69360 Solaize, France.
90
e KAUST Catalysis Center, 4700 King Abdullah University of Science and
Technology, Thuwal 23955-6900, Saudi Arabia
† Electronic Supplementary Information (ESI) available: [Experimental
25 procedures, characterization data and 1H and 13C NMR spectra for all
previously unreported compounds.]. See DOI: 10.1039/b000000x/
‡ CCDC 843466 (8b); 890262 (7d); 876822 (7e). For crystallographic
data in CIF or other electronic format, see DOI: 10.1039/b000000x/
‡ This work was financed by the European Community through the
30 seventh framework program (CPꢀFP 211468ꢀ2 EUMET, grant to MR, LF
and EB). MM thanks the Agence Nationale de la Recherche (Grant ANRꢀ
12ꢀCD2Iꢀ0002 CFLOWꢀOM), Rennes Métropole and the Régionꢀ
Bretagne for their financial supports concerning the development of Ruꢀ
based complexes. Drs. O. Baslé and C. Crévisy are gratefully
35 acknowledged for their helpful discussion.
95
100
105
110
115
12 a) A. Fürstner, A. F. Hill, M. Liebl, J. D. E. T. WiltonꢀEly, Chem.
Commun., 1999, 601; b) A. Fürstner, J. Grabowski, C. W. Lehmann,
J. Org. Chem., 1999, 64, 8275; c) A. Fürstner, O. Guth, A. Düffels,
G. Seidel, M. Liebl, B. Gabor, R. Mynott, Chem. Eur. J., 2001, 7,
4811.
1
a) For a recent book on NHCs, see: S. DíezꢀGonzález, N-
13 31P chemical shifts for complex 7a: major rotamer (δ=30.06 ppm),
minor rotamer (δ=16.85 ppm); for complex 7b: major rotamer
(δ=29.06 ppm), minor rotamer (δ=17.05 ppm). Based on the 31P
chemical shift observed for complex 8b (δ=27.11 ppm), we
suggested that both major rotamers of 7a and 7b correspond to the
complex having the mesityl substituent pointing toward indenylidene
moiety.† These conformational isomers were propably favorised by a
slipped πꢀπ stacking interaction between the indenylidene moiety and
the Nꢀmesityl group (see ref. 8f).
Heterocyclic Carbenes, RSC Catalysis series, RSC Publishing:
Cambridge, 2011; b) For a review dealing with the synthetic routes of
NHC precursors, see: L. Benhamou, E. Chardon, G. Lavigne, S.
BelleminꢀLaponnaz, V. César, Chem. Rev., 2011, 111, 2701.
Selected comprehensive reviews on olefin metathesis : a) Handbook
of Metathesis, (Ed.: R. H. Grubbs), WileyꢀVCH, Weinheim, 2003; b)
G. C. Vougioukalakis, R. H. Grubbs, Chem. Rev., 2010, 110, 1746–
1787.
For instance, see: D. J. Nelson, P. Queval, M. Rouen, M. Magrez, L.
Toupet, F. Caijo, E. Borré, I. Laurent, C. Crévisy, O. Baslé, M.
Mauduit, J. M. Percy, ACS Catalysis, 2013, 3, 259.
Selected examples: a) R. M. Thomas, B. K. Keitz, T. M. Champagne,
R. H. Grubbs, J. Am. Chem. Soc., 2011, 133, 7490; b) N. Ledoux, B.
Allaert, S. Pattyn, H. V. Mierde, C. Vercaemst, F. Verpoort, Chem.
Eur. J., 2006, 12, 4654.
Complex 1: a) M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org.
Lett., 1999, 1, 953; Complex 2: b) H. Clavier, C. A. UrbinaꢀBlanco,
and S. P. Nolan, Organometallics, 2009, 28, 2848; Complex 4a: c)
S. B. Garber, J. S. Kingsbury, B. L. Gray, A. H. Hoveyda, J. Am.
Chem. Soc., 2000, 122, 8168; Complex 4b: d) D. Rix, F. Caïjo, I.
Laurent, F. Boeda, H. Clavier, S. P. Nolan, M. Mauduit, J. Org.
Chem., 2008, 73, 4225; e) H. Clavier, F. Caïjo, E. Borré, D. Rix, F.
Boeda, S. P. Nolan, M. Mauduit, Eur. J. Org. Chem., 2009, 25, 4254.
C. Samojłowicz, M. Bieniek, K. Grela, Chem. Rev., 2009, 109, 3708
For perspectives, see: a) O. Briel, C. S. J. Cazin, N-Heterocycle
Carbene Complexes in Industrial Processes, in N-Heterocycle
Carbenes in Transition Metal Catalysis and Organocatalysis (C. S. J.
Cazin Vol. Ed.) pp 315ꢀ325 (2010) Springer; b) S. Chikkali, S.
Mecking, Angew. Chem. Int. Ed., 2012, 51, 5802.
40
45
50
55
60
65
2
3
4
14 For a review dealing with the synthetic routes of NHC precursors,
see: L. Benhamou, E. Chardon, G. Lavigne, S. BelleminꢀLaponnaz,
V. César, Chem. Rev., 2011, 111, 2701.
120 15 For a seminal review dealing with electronic and steric characters of
NHCs, see: T. Dröge, F. Glorius, Angew. Chem. Int. Ed., 2010, 49,
6940.
16 a) A. Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V.
Scarano, L. Cavallo, Eur. J. Inorg. Chem., 2009, 1759; b) R. A. Kelly
5
125
130
135
III, H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens, J. Bordner, I.
Samardjiev, C. D. Hoff, L. Cavallo, S. P. Nolan, Organometallics,
2008, 27, 202; c) A. Poater, F. Ragone, S. Giudice, C. Costabile, R.
Dorta, S. P. Nolan, L. Cavallo, Organometallics, 2008, 27, 2679. D)
L. Cavallo, A. Correa, C. Costabile, H. Jacobsen, J. Organomet.
Chem., 2005, 690, 5407.
17 a) F. Ragone, A. Poater, L. Cavallo, J. Am. Chem. Soc., 2010, 132,
4249; b) A. Poater, F. Ragone, R. Mariz, R. Dorta, L. Cavallo, Chem.
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2870.
6
7
18 T. Ritter, A. Hejl, A. G. Wenzel, T. W. Funk, R. H. Grubbs,
Organometallics, 2006, 25, 5740.
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DOI: 10.1039/C4DT00142G
19 The synergy effect between the NHC and the chelating benzylideneꢀ
ether leaving ligand on the initiation step was recently studied by our
group, see ref. 3.
20 Linear αꢀolefins are more prompt to isomerise during the metathesis
reaction due to the premature decomposition of the metal alkylidene
species, see for instance: a) S. H. Hong, M.W. Day, R. H. Grubbs, J.
Am. Chem. Soc. 2004, 126, 7414; b) B. Schmidt, J. Mol. Catal. A:
Chem., 2006, 254, 53; c) M. Jordaan, H. C. M. Vosloo, Adv. Synth.
Catal. 2007, 349, 184.
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