Ruthenium-catalyzed olefin metathesis accelerated by the steric effect of the backbone substituent in cyclic (alkyl)(amino) carbenes

Article abstract of DOI:10.1039/c3cc45823g

Three ruthenium complexes bearing backbone-monosubstituted CAACs were prepared and displayed dramatic improvement in catalytic efficiency not only in RCM reaction but also in the ethenolysis of methyl oleate, compared to those bearing backbone-disubstitut

Full text of DOI:10.1039/c3cc45823g

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Ruthenium-catalyzed Olefin Metathesis Accelerated by Steric Effect of
Backbone Substituent in Cyclic (Alkyl)(Amino)Carbenes
Jun Zhang,*,† Shangfei Song,† Xiao Wang,† Jiajun Jiao,† and Min Shi*,†,‡
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Three
ruthenium
complexes
bearing
backbone-
monosubstituted CAACs were prepared and displayed
considerable improvement on catalytic efficiency not only in
RCM reaction but also in the ethenolysis of methyl oleate,
10 compared to those bearing backbone-disubstituted CAACs.
Over the last decade, N-heterocyclic carbenes (NHCs) have
received much attention mainly due to their widespread and
spectacular applications as organocatalysts and as ligands for
organometallic catalysis.1 The number, electronic nature,
15 steric effect and position of the substituents on both the
nitrogen atoms and the backbone of NHCs have played
important roles in NHC-promoted reactions.2 In 2005,
Bertrand etc. synthesized cyclic(alkyl)(amino) carbenes
(CAACs), in which one amino group derived from an NHC
20 imidazolidin-2-ylidene has been replaced by a quaternary
alkyl group (Figure 1).3 Due to the presence of a quaternary
carbon atom in a position α to the carbene center, CAACs
featured peculiar electronic and steric properties that are
dramatically different from NHCs. Grubbs and Bertrand etc.
25 found that the CAAC-ligated Hoveyda-Grubbs catalysts 1 (1a:
Figure 1. Known Ru complexes 1 ligated by backbone-
disubstituted CAACs and targeted ruthenium complexes 2
ligated by backbone-monosubstituted CAACs
Scheme 1. Synthesis of ruthenium complexes 2a~2c
R1 = Pr; R2-R2 = (CH2)5; 1b: R1 = Pr; R2 = Me; 1c: R1 = Et;
R2 = Me) were very reactive in ring-closing metathesis, and a
dramatic increase in catalytic activity was observed after
slightly decreasing the steric bulkiness of the N-aryl group
30 from 2,6-diisopropylphenyl group (1a, 1b) to 2,6-
diethylphenyl group (1c).4 Another study showed 1a~1c also
exhibited high efficiency for the formation of terminal olefins
in the ethenolysis of methyl oleate.5 At 100ppm catalyst
loading, 1b displayed higher selectivity (92%) and TON
i
i
35 (5600) than 1c (73% of selectivity and 5300 of TON) did. At a
low catalyst loading of 10ppm, only complex 1c is reactive,
though lower selectivity of 83% obtained, and achieves TONs
of 35000, the highest TON recorded so far for this reaction.5
Most modifications in CAAC ligands are made on their N-
40 substituents and/or C-substituents adjacent to the carbene
center, and almost CAACs employed are those bearing two
methyl groups in their backbone.3~5 To the best of our
knowledge, the effect of backbone substitution of CAAC in
catalytic performance is not yet investigated. As part of our
45 studies on the design and synthesis of backbone-substituted
carbenes and their application in catalysis,6 we herein report a
series of Ru complexes 2 ligated by CAACs bearing a bulky
isopropyl group in their backbone. We also demonstrate that,
compared to 1, ruthenium complexes 2 displayed considerable
50 improvement on catalytic efficiency not only in RCM reaction
but also in the ethenolysis of methyl oleate. With 10 ppm
a Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, 130 Mei Long Road, Shanghai
200237, China.
E-mail: zhangj@ecust.edu.cn
Tel: +086 021 64252995
b State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Road, Shanghai 200032 China
i
catalyst loading, complex 2b (R1 = Pr; R2 = Me) displayed
E-mail: mshi@mail.sioc.ac.cn
Tel: +086 021 64252995
high selectivity (97%) in the ethenolysis of methyl oleate and
achieved 26000 TONs.
55 We first synthesized three new pyrrolidinium salts 3a~3c
according to a published procedure reported by Bertrand
† Electronic Supplementary Information (ESI) available: Experimental
and spectroscopic data for all compounds. CCDC 887373 (2a). For ESI
and crystallographic data in CIF or other electronic format See
DOI: 10.1039/b000000x/
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Table 1. Comparison of Ruthenium Catalysts in the RCM
Reaction of 5~7a
View Article Online
DOI: 10.1039/C3CC45823G
Cat. Run Conversion (5 to 8)b
Run Conversion (6 to 9)b
3 % (23 h at 30 oC)
Figure 2. Left: Molecular structure of 2a. Right: side-view of
2a (cyclohexyl group omitted for clarity).
1
60% (20 h at 30 oC)
9
1b
10 11 % (5 h at 60 oC)
11c 95 % (20 h at 60 oC)
12c 96 % (48 h at 60 oC)
13 52 % (23 h at 30 oC)
14 65 % (5 h at 60 oC)
15 90 % (23 h at 30 oC)
16 90 % (5 h at 60 oC)
etc.3,4 The corresponding CAACs were generated in situ by
treatment of 3a~3c with potassium hexamethyldisilazide
(KHMDS) and then reacted with RuCl2(PCy3)(=CH–o-
OiPrC6H4) (4) to afford the desired Ru complexes 2a~2c
(Scheme 1). These complexes were isolated as air- and
moisture-stable solids by column chromatography. Both mass
and NMR spectra, and in particular the X-ray diffraction
analyses of 2a confirmed the formation of these CAAC-Ru
complexes (Figure 2). Similar to 1a~1c,4 the solid-state
2c 97 % (3.3 h at 60 oC)
3c 95 % (10 h at 60 oC)
1a
2a
4
5
6
77 % (20 h at 30 oC)
5
85 % (20 h at 30 oC)
2b
98 % (15 min at 30 oC) 17 95 % (50 min at 30 oC)
7c 95 % (15 min at 30 oC) 18c 95 % (60 min at 30 oC)
78 % (15 min at 30 oC) 19 98 % (50 min at 30 oC)
2c
1c
10 structure of 2a adopts a square-pyramidal geometry. The
benzylidene moiety is located at the apical position, and the
N-aryl ring is situated above the benzylidene moiety.
8
11
a
General conditions: Diolefin (0.1 M), catalyst (1.0 mol%), 30
o
b
c
oC in CD2Cl2 or 60 C in C6D6. Determined by NMR. Taken
RCM is the first widely used olefin metathesis reaction in
organic synthesis and has been used as the standard reaction
15 to evaluate the catalytic efficiency of most ruthenium-based
complexes.1 We initially investigated the catalytic activity of
2a~2c in the RCM of diethyl diallylmalonate 5, diethyl
allylmethallyl malonate 6, and diethyl dimethallylmalonate 7,
and the catalytic performances are summarized in Table 1. A
20 study reported by Grubbs etc. showed Ru complexes 1a, 1b
from Ref. 4
For 1a and 1b bearing bulky N-aryl group, this process may
be sterically unfavorable, thus resulting in poor initiation. A
dramatic increase in catalytic activity was observed using 1c
bearing less bulky N-aryl group (Table 1, runs 7 and 18).4 In
50 the case of 2a and 2b, due to the repulsion between the
backbone isopropyl group and one ortho-isopropyl substituent
of N-aryl group in right side, another ortho-isopropyl
substituent in left side is rotated away from the metal reactive
centre (Figure 2), which could release more space in the left
55 side of CAAC for the dissociation of the ether moiety and
subsequent rotation of the benzylidene ring, thus resulting in
accelerated catalyst initiation. Complex 2c, which differs from
2b only by replacement of ortho-isopropyl substituent of N-
aryl group with ortho-ethyl substituent, exhibited dramatically
60 increased catalytic activity in the RCM reaction of 5 and 6. 2c
achieved 98% conversion of 5 to cyclopentene 8 in 15 min at
o
are highly active in RCM of 5 at 60 C, but a longer reaction
time is required for 1a (Table 1, runs 2 and 3).4 The RCM of 6
is more sterically demanding than the corresponding RCM of
5. At 60 oC, 1b converted 6 to 95% of trisubstituted olefin 9 in
o
25 20 h at 60 C, while 1a achieved 96% conversion after 48 h
(Table 1, runs 11 and 12). We found that, at lower reaction
temperature of 30 oC, complex 1b afforded moderate
conversion of 60% after 20 h for the RCM of 5 (Table 1, run
1), and very low conversion of 3% after 23 h for the RCM of
30
6 (Table 1, run 9). 2a and 2b bearing a backbone-
monosubstituted CAAC were identified to be much more
efficient in these RCM reactions than their congeners 1a and
1b bearing a backbone-disubstituted CAAC. 2a and 2b
achieved 77% and 85% conversion of 5 at 30 oC in 20 h
o
30 C, and 95% conversion of 6 to trisubstituted olefin 9 at 30
oC in 50 min (Table 1, entries 6 and 17), which is comparable
to the catalytic performance of 1c (Table 1, entries 7 and 18)
65 and NHC-Ru catalyst 11 (Table 1, entries 8 and 19).
Unfortunately, like 1c, 2c showed no catalytic activity in the
RCM of sterically bulky 7 to tetrasubstituted olefin 10.
35 (Table 1, runs 4 and 5), and 52% and 90% conversion of 6 at
30 oC in 23 h (Table 1, runs 13 and 15), respectively. At
o
higher reaction temperature of 60 C, 2a and 2b achieved 65%
and 90% conversion of 6 in 5 h (Table 1, runs 14 and 16),
while 1b achieved 11% conversion in 5 h (Table 1, run 10).
40 The reasonable explanation for the improvement on
catalytic activities of 2a and 2b can be given by invoking
steric factors. Grubbs etc. postulated that, in catalyst 1,
catalyst initiation requires dissociation of the ether moiety and
rotation of the benzylidene ring into a plane parallel to the N-
45 aryl group to open a coordination site for incoming olefin.4
We next examined the ethenolysis of methyl oleate (12), a
process that produces terminal olefin from internal olefins
70 derived from renewable seed oils. Most of catalysts for
ethenolysis reactions generally suffered from poor to
moderate selectivity, turnover numbers and/or conversion, due
to lower stability of catalysts and competing processes taking
place.7 Self-metathesis of ethenolysis products of terminal
2
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selectivity (97% of selectivity for 2a and 96% of selectivity
Table 2. Comparison of Ruthenium Catalysts in the
for 2b) in 4 h, respectively, and moderate conversions were
obtained (50% conversion achievedDOIb: 1y0.120Va3i9e/wCaA3nrCdtCic4le56O80n2l%3inGe
25 conversion by 2b) (Table 2, runs 1 and 3). At 15 ppm catalyst
loading, complex 2b exhibits high selectivity (96%) and
obtains higher conversion (30%) than 2a does (16%) (Table 2,
runs 2 and 4). Complex 1a showed no active toward the
ethenolysis reaction at 15 ppm catalyst loading (Table 2, run
30 7). Complex 2c with a less sterically bulky N-aryl substituent
exhibited much lower selectivity of 80% compared with that
of 2a and 2b (Table 2, runs 5, 1, and 3). The same trend on
selectivity is also observed in 1a~1c (Table 2, runs 9, 10, and
11). Using methyl oleate with 99% purity and at 10 ppm
35 catalyst loading, 2b displayed highest selectivity of 97% and
achieved high TONs of 26000 (Table 2, Run 8).
In conclusion, compared to their congeners 1a, 1b bearing
backbone-disubstituted CAACs, 2a, 2b ligated by CAACs
bearing a sterically bulky isopropyl group on the backbone are
40 featured with fast catalyst activation, and displayed
considerable improvement on catalytic efficiency not only in
ring-closing metathesis but also in the ethenolysis of methyl
oleate. With 10 ppm catalyst loading, complex 2b displayed
high selectivity (97%) in the ethenolysis of methyl oleate for
45 the formation of terminal olefins with highest TONs (26000).
Financial support from Shanghai Pujiang Talent Program
(11PJ1402500), the Fundamental Research Funds for the
Central Universities (WK1114014), the Shanghai Municipal
Committee of Science and Technology (08dj1400100-2),
50 National Basic Research Program of China (973)-
2010CB833302, and the National Natural Science Foundation
of China for financial support (21171056 and 21072206) is
greatly acknowledged.
Ethenolysis of 12a
Cat.
Run
Time Conv. Selectivity Yield TON
(h)
(ppm)
(%)b
(%)c
(%)d
(%)e
1
2a (100)
2a (15)
2b (100)
2b (15)
2c (50)
4
4
4
4
1
6
6
4
6
50
16
60
30
53
36
-
97
96
96
96
80
95
-
48 4800
15 10000
58 5800
29 19300
42 8000
35 3500
2
3
4
5
6
1a (100)
1a (15)
2b (10)
1a (100)
7
8f
-
-
27
46
61
73
42
97
94
93
73
83
26 26000
43 4200
56 5600
53 5300
35 35000
9f,g
10f,g
11f,g
12f,g
a
1b (100) 22
1c (100) < 0.5
1c (10) < 0.5
General conditions: neat 12 (96%, 3 g), 150 psi ethylene,
b
40 °C.
100/(initial moles of 12)].
ethenolysis products 13 + 14) × 100/(moles of total
Conversion = 100 – [(final moles of 12) ×
c
Selectivity = (moles of
d
products 13 + 14 + 15 + 16).
Yield = (moles of
ethenolysis products 13 + 14) × 100/(initial moles of 12) =
e
conversion × selectivity/100. TON = yield × [(moles of
12)/(moles of cat.)]. neat 12 (99%, 3 g). Taken from
Ref. 5
Notes and references
f
g
55
1
For reviews, see: (a) G. C. Vougioukalakis and R. H. Grubbs,
Chem. Rev., 2010, 110, 1746; (b) S. Díez-González, N. Marion
and S. P. Nolan, Chem. Rev., 2009, 109, 3612; (c) T. Dröge and
F. Glorius, Angew. Chem. Int. Ed., 2010, 49, 6940; (d) D. Enders,
O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5606; (e)
E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Angew.
Chem. Int. Ed., 2007, 46, 2768.
olefins 9-methyldecenoate (13) and 1-decene (14) is the
primary competing process, which leads to the formation of
undesired internal olefins, 1,18-dimethyl 9-octadecenoate (15)
and 9-octadecene (16). Several pioneer studies pointed out
high reagent purity is essential to ensure good conversions
with a low catalyst loading, and in most cases, >99% purity of
methyl oleate was used.5 Considering that purifications are
cost-consuming and time-consuming, which is not desired for
industrial application, we focused our study on the
10 development of the efficient catalyst under a low catalyst
loading using commercially available methyl oleate with 96%
purity, which is much cheaper than 99% purity methyl oleate.
Grubbs etc. reported, among 1a~1c, 1b achieved the highest
TON of 5600 with high selectivity of 93% at 100 ppm catalyst
15 loading, but extended reaction time of 22 h was necessary for
1b to reach the maximum conversion of 61% (Table 2, runs 9,
10, and 11).5 Complexes 2a and 2b bearing backbone-
monosubstituted CAAC were also found to be more efficient
in the ethenolysis reaction than their congeners 1a,1b. At 100
20 ppm catalyst loading and using methyl oleate with 96%
purity, 2a and 2b achieved TONs of 4800 and 5800 with high
60
65
70
75
80
2
3
4
5
6
K. M. Kuhn, J.-B. Bourg, C. K. Chung, S. C. Virgil and R. H.
Grubbs, J. Am. Chem. Soc., 2009, 131, 5313.
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D. R. Anderson, V. Lavallo, D. J. O’Leary, G. Bertrand and R.
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Grubbs and Y. Schrodi, Organometallics, 2008, 27, 563.
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Qin, M. Zhao and M. Shi, Chem. Commun., 2012, 48, 9625; (d) J.
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