Simple and highly Z-selective ruthenium-based olefin metathesis catalyst

Article abstract of DOI:10.1021/ja311505v

A one-step substitution of a single chloride anion of the Grubbs-Hoveyda second-generation catalyst with a 2,4,6-triphenylbenzenethiolate ligand resulted in an active olefin metathesis catalyst with remarkable Z selectivity, reaching 96% in metathesis homocoupling of terminal olefins. High turnover numbers (up to 2000 for homocoupling of 1-octene) were obtained along with sustained appreciable Z selectivity (>85%). Apart from the Z selectivity, many properties of the new catalyst, such as robustness toward oxygen and water as well as a tendency to isomerize substrates and react with internal olefin products, resemble those of the parent catalyst.

Full text of DOI:10.1021/ja311505v

Communication
pubs.acs.org/JACS
Simple and Highly Z‑Selective Ruthenium-Based Olefin Metathesis
Catalyst
Giovanni Occhipinti, Fredrik R. Hansen, Karl W. Tornroos, and Vidar R. Jensen*
̈
́
Department of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway
S
* Supporting Information
Scheme 1. Synthesis of Complex 2
ABSTRACT: A one-step substitution of a single chloride
anion of the Grubbs−Hoveyda second-generation catalyst
with a 2,4,6-triphenylbenzenethiolate ligand resulted in an
active olefin metathesis catalyst with remarkable Z
selectivity, reaching 96% in metathesis homocoupling of
terminal olefins. High turnover numbers (up to 2000 for
homocoupling of 1-octene) were obtained along with
sustained appreciable Z selectivity (>85%). Apart from the
Z selectivity, many properties of the new catalyst, such as
robustness toward oxygen and water as well as a tendency
to isomerize substrates and react with internal olefin
products, resemble those of the parent catalyst.
Single-crystal X-ray diffraction revealed that the molecular
structure of 2 (Figure 1) is typical for Grubbs−Hoveyda
lefin metathesis (OM) has evolved to become one of the
2,3 This change has
Omost flexible ways to make C−C bonds.
to a large extent been spurred by the development of well-defined
and tunable homogeneous catalysts.1 In general, OM results in a
mixture of Z and E isomers, usually with the thermodynamically
preferred E isomer as the major component. Frequently,
however, the target product is a single isomer. For example,
syntheses of many natural products and pharmaceuticals require
selective formation of the Z isomer.4 Thus, the development of
Z-selective catalysts has been an important and challenging goal
in OM during the last 10−15 years.
Highly Z-selective catalysts based on Mo and W were
obtained5,6 by following a straightforward and intuitive strategy,
namely, using two monoanionic, monodentate ligands with very
different sizes to give monoalkoxide−pyrrolide (MAP) cata-
lysts.6 These complexes are otherwise similar to the underlying
class of “nonselective” catalysts. In contrast, the same design
strategy has led to only modest selectivity for Ru.7 Highly Z-
selective Ru catalysts were recently reported, but these are based
on a different design involving a distinct bidentate N-heterocyclic
carbene (NHC)−adamantyl ligand.8−10 Thus, it is not
straightforward to draw upon the known structure−property
relationships and the great design flexibility of the underlying
class of nonselective Ru-based catalysts.3,11
Herein we show that the highly Z-selective Ru-based OM
catalyst 2 can be obtained in high yield (81%) in one step from
the five-coordinate second-generation Grubbs−Hoveyda-type
catalyst precursor 1 by substituting a single chloride anion with
the sterically demanding 2,4,6-triphenylbenzenethiolate ligand
(Scheme 1). This strategy gives the highly stereoselective catalyst
2, in accordance with predictions from DFT calculations,12 while
largely conserving the properties of 1.
Figure 1. X-ray structure of 2 with displacement ellipsoids drawn at the
50% probability level. H atoms and solvent molecules (dichloromethane
and pentane) have been omitted for clarity. Selected geometrical
parameters: Ru1−C7 = 1.8343(15) Å, Ru1−C11 = 2.0026(13) Å, Ru1−
Cl1 = 2.3948(4) Å, Ru1−O1 = 2.2369(11) Å, Ru1−S1 = 2.3125(4) Å,
Ru1−S1−C32 = 113.30(5)°, C11−Ru1−S1 = 91.25(4)°, C11−Ru1−
Cl1 = 91.65(4)°.
catalysts, featuring two anionic ligands oriented trans to each
other. The regularity and lack of deviation from the structure of
the underlying class of catalysts suggest that the thiolate ligand,
although much larger than a chloride, does not impose much
steric congestion in 2. For example, the orientation of the NHC
ligand (SIMes) is almost perfectly symmetric with respect to the
Received: November 23, 2012
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Table 1. Metathesis Homocoupling of Terminal Olefins with Catalyst 2
a
cat. loading
(mol %)
DMAN loading
(mol %)
substrate
conv.
b
c
b
entry substrate
P (bar)
solvent
THF
conc. (M)
t (h) T (°C)
(%)
yield (%) % Z
1
1-octene
0.25
1
0.12
4
0.5
1.5
3
40
17
33
45
64
75
67
>99
5
14
86
83
81
75
85
66
23
93
87
86
87
−
27
37
6
53 (42)
64 (55)
42
d
2
3
1-octene
1-octene
0.25
5
1·10−5
1
0.12
2.5
THF
THF
5.4
0.5
5.5
3
50
40
10
66
4
5
1-octene
1-octene
1-octene
1-octene
1-octene
0.01
0.01
0.01
0.01
0.01
0.25
0.25
1·10−5
1·10−5
1·10−5
1
0
neat
neat
neat
neat
neat
THF
THF
−
−
−
−
−
4
0.5
4
60
60
60
40
40
40
40
4
0.05
0
19
24
8
17 (12)
20 (15)
8 (7)
0
6
4
7
0.05
0.05
0.12
0.12
4
e
8
1
0.5
1.5
1.5
6.0
3
0
9
1-hexene
1-hexene
1
55
15
55
76
>99
12
7
36 (6)
12
76
85
67
64
26
90
80
60
96
95
91
89
82
45
80
39
83
83
52
72
82
74
93
81
89
87
74
45
10
1·10−5
4
42 (10)
49
11
1-hexene
5
1
2.5
THF
0.5
40
10
54
12
13
1-hexene
0.04
0.25
1·10−5
1
0.2
neat
−
4
2
2
60
40
11 (3.5)
7
f
Me undecenoate
0.12
THF
8
46
6
41 (40)
3 (2.5)
12 (11)
11
g
14
15
16
allyl-TMS
0.25
0.25
0.25
1
1
1
0.12
0.12
0.12
THF
THF
THF
4
4
4
35
18
40
60
40
d
g
allyl-TMS
22
16
73
44
>99
38
>99
26
22
96
75
16
19
13
100
6
4-phenyl-1-butene
0.5
2
30 (29)
9
17
18
allylbenzene
0.25
0.25
1
1
0.12
0
THF
THF
4
4
0.5
2
40
40
13
allylbenzene
0.5
2
12
14
19
20
allylbenzene
0.25
0.25
1·10−5
1.10−5
0.12
0.12
THF
THF
4
4
0.5
0.5
2
40
40
7
h
allylbenzene
8
27 (21)
20 (15)
16
i
21
22
allylbenzene
allyl acetate
0.25
0.25
1.10−5
1
0.12
0.12
THF
4
4
2
40
40
THF
4
8
19 (16)
13
d
d
23
24
25
26
27
allyl acetate
1
1
1
1
1
1
0
THF
THF
THF
THF
THF
3
1
60
60
40
60
40
allyl acetate
5
0
0.5
4
22
18
2.5
100
N-allylaniline
N-allylaniline
2-(allyloxy)ethanol
0.25
0.25
0.25
0.12
0.12
0.12
6 (5)
28
d
4
28
7
4
2
8
1
63
1
a
b
c
1,8-Bis(dimethylamino)naphthalene (H+ sponge). Determined by 1H NMR analysis. 1H NMR yields (values in parentheses are isolated yields).
d
e
The reaction conditions were partially optimized. The substrate used was stored for a long time under air, and the atmosphere employed was air
f
g
h
i
rather than Ar. Methyl undecenoate. Allyltrimethylsilane. Cy3PO (1.25 mol %) was added to the reaction mixture. Containing 5 mol % degassed
water.
two anionic ligands, as shown by the nearly identical bond angles
the NHC carbene atom forms with sulfur and chorine (C11−
Ru1−S1 = 91.3°, C11−Ru1−S1 = 91.7°).
is observed, as this bond is slightly shorter in 2 [2.2369(11) Å]
than in 1 [2.256(1) Å].14
Because of the acute Ru−S−C bond angle, the triphenylben-
zene moiety points toward the site opposite the NHC ligand. As a
result of the steric pressure from the relatively large
triphenylbenzene moiety, the substituents of the alkylidene
ligand and of the incoming olefin both prefer to be oriented away
from this “wall”, thus promoting the formation of Z olefins.12
In the absence of an acid (e.g., a carboxylic acid), the stability of
complex 2 is comparable to that of other Ru-based catalysts. In
contrast to the case for acids, the presence of a relatively strong
base such as the proton sponge 1,8-bis(dimethylamino)-
naphthalene (DMAN) is tolerated very well by 2 [see the
Supporting Information (SI) for details].
Despite the presence of two sterically demanding phenyl
substituents at the ortho positions, the arylthiolate ligand forms a
relatively acute bond angle with Ru [Ru1−S1−C32 =
113.30(5)°]. Divalent sulfur prefers narrower bond angles
than, for example, divalent oxygen, as illustrated by the wider
Ru−O−C angles of aryloxy-substituted Ru−alkylidene com-
plexes with insignificant steric congestion (127−130°).13 At the
same time, there is little doubt that the Ru−S−C bond angle
would widen in response to significant steric pressure. In a similar
vein, one might speculate that the presence of severe steric
congestion in 2 would impose a weakening (lengthening) of the
dative Ru−O bond compared with that in 1. In fact, the opposite
B
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Solutions of 2 (e.g., in CD2Cl2) can be stored under Ar for a
few days without signs of decomposition. The temperature
stability of the new complex is good [e.g., it exhibits negligible
decomposition after 24 h in tetrahydrofuran (THF) at 60 °C]
but lower than that of the parent catalyst 1. Complex 2 also
tolerates well the presence of water and to some extent also
oxygen.15 The oxygen tolerance of 2 appears to be better than
that of the carboxylate-coordinated version of the NHC−
adamantyl chelate Ru catalysts and comparable to that of the
improved nitrate version of the latter.10 The substrates tested in
this work were generally used as received or at most degassed.
Complex 2 was tested as a catalyst for metathesis
homocoupling of nine different terminal olefins (see Table 1).
To facilitate comparison, all of the substrates were tested with
reaction conditions identical to those of entry 1. Despite the fact
that the substrates tested displayed considerable variation in
reactivity, most of them were subjected to tests using rather
standard reaction conditions. In only a few cases (where
indicated) were the reaction conditions partially optimized for
a particular substrate.
In general, 2 displayed both high catalytic activity and Z
selectivity, the latter frequently in the range 80−95%, which is
comparable to or slightly lower than those of the best NHC−
adamantyl chelate Ru catalysts.9,10 In fact, the activity is
remarkable for a catalyst as selective as 2, even considering the
fact that many of the catalytic experiments reported for other Z-
selective catalysts were performed at lower temperatures than
those in Table 1. Nonfunctionalized linear substrates such as 1-
octene and 1-hexene (entries 1−12) were easily converted to the
target products, reaching turnover numbers (TONs) of up to
2000 (1500 isolated) in only 4 h in combination with a Z
selectivity higher than 85% (entries 5−7).
The observed catalyst performance was strongly dependent on
both the substrate and the reaction conditions. Low catalyst
loadings, moderately high temperatures, and the presence of
small amounts of coordinating solvents such as THF typically
enhanced the overall performance. Before we turn to the
reactions of the individual substrates, it may be instructive to
consider what may be termed the “inherent selectivity” of 2, that
is, the selectivity theoretically achieved in the absence of
isomerization of the product as well as competing unselective
catalysts (including those potentially generated from decom-
position of 2). The inherent selectivity of 2 is clearly very high, as
demonstrated by the fact that experiments run to only low
conversions (to limit the influence of Z−E isomerization of the
product; see below) normally reached Z selectivities above 85%
and in some cases above 90%. Of course, a more accurate
assessment of the inherent selectivity can be obtained if the
internal olefin product does not react with the catalyst. This is the
case for metathesis of allyltrimethylsilane (allyl-TMS), for which
the two TMS groups in the product effectively shield the double
bond from reaction with the catalyst, resulting in a Z selectivity of
up to 96% (entries 14 and 15). Certainly, the inherent selectivity
varies from substrate to substrate, but it is still clear that in
metathesis experiments run to higher conversions, such as in
entries 3 (1-octene) and 11 (1-hexene), isomerization of the
product explains much more of the deviation from 100% Z
selectivity than does “imperfect” inherent selectivity.
(E)-5-decene was not observed. This problem stems from the
ability of 2 to react with internal olefins and is a side effect of the
high catalytic activity mentioned above.
A similar but weaker tendency was also observed for the
NHC−adamantyl chelate Ru catalysts,9,10 and as was suggested
for the latter catalysts, hydride-induced isomerization and
secondary metathesis events mediated by nonselective meta-
thesis-active decomposition products may be additional causes of
this phenomenon.10 The presence of DMAN appeared to reduce
the Z−E isomerization (e.g., see entries 5 vs 6 and 17 vs 18).
Presumably, the proton sponge inhibits a catalyst decomposition
pathway, thus prolonging the lifetime of 2. The rate of Z−E
isomerization depended on both the nature of the substrate and
the reaction conditions and increased with substrate conversion.
Unfortunately, the optimal reaction conditions also depended on
the substrate. For example, high catalyst loading and high
dilution were favorable for allyl acetate, allowing complete
conversion with a relatively high Z selectivity of 81% (entry 24).
In contrast, for 1-octene and 1-hexene (entries 3 and 11), very
similar conditions led to almost complete loss of Z selectivity at
close to 100% substrate conversion (after 10 h).
Similarly to 1 and other Ru-based OM catalysts, 2 also
isomerizes many substrates, and this tendency appears to
increase with the progress of the reaction.16 This side reaction,
which may be useful for some applications,17 is believed to be
caused by new Ru species generated during the catalytic
metathesis transformations.18 Attempts to suppress it using
acids or quinones led to partial decomposition of 2 and the
1
formation of small amounts of 1 (as detected by H NMR
spectroscopy), while the addition of a small amount of either
tricyclohexylphosphine oxide (Cy3PO) or water reduced the
amount of substrate isomerization, albeit at the expense of lower
catalytic activity (entries 20 and 21). A similar beneficial effect
was obtained using static vacuum, which also appeared to slow
both isomerization and OM but increased the selectivity for the
latter. The rate of substrate isomerization was also influenced by
the presence of the proton sponge. However, this effect was
rather small and appeared to depend on both the nature of the
substrate and the reaction progress (e.g., compare entries 5 vs 6
and 17 vs 18).
Despite the above-described isomerization issues, it was
possible to obtain 7-tetradecene from 1-octene in an isolated
yield of 55% with a Z selectivity of 85% under partially optimized
reaction conditions (entry 2). This result suggests that it should
also be possible to obtain better yields for other substrates.
However, the optimization of each individual process may be
time-consuming and was beyond the scope of this work.
Methyl undecenoate (entry 13) appeared to be less reactive
than 1-octene or 1-hexene and also underwent little isomer-
ization compared with the unfunctionalized substrates above.
Both the lower reactivity and the reduced tendency to isomerize
for this substrate may be caused by its donor function (ester).
The Z selectivity achieved with this substrate was somewhat
lower than those obtained for 1-octene and 1-hexene.
Allyl-TMS (entries 14 and 15) turned out to be the least
reactive of the present substrates, and, as mentioned above, also
provided the highest Z selectivity because isomerization of the
product was negligible. Whereas a conversion of only 6% was
obtained after 35 h at 40 °C, increasing the temperature to 60 °C
resulted in much improved conversion (22% after 18 h) while
retaining a high Z selectivity (95%).
Z−E isomerization of internal olefin products, presumably
proceeding via reversible OM, was quantified for the two
products of 1-hexene metathesis (Table S5 in the SI). It is evident
that the catalyst progressively reduced the Z content, while under
identical reaction conditions, the corresponding isomerization of
The reactivities of 4-phenyl-1-butene and allylbenzene were
very high, comparable to those observed for 1-octene and 1-
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Journal of the American Chemical Society
Communication
hexene. As also seen for the latter olefins, the tendency of the
substrate to isomerize was significant. Addition of a small amount
of either Cy3PO or water was found to suppress this usually
undesirable side reaction to some extent (entries 20 and 21).
Higher Z selectivity was obtained for 4-phenyl-1-butene than for
allylbenzene (91 vs 82% at low conversion).
In contrast, for allyl acetate and N-allylaniline, no isomer-
ization of the starting material was observed. The reactivities of
these substrates were lower than that of 1-octene but higher that
that of allyl-TMS. Under identical reaction conditions, the
reactivity was higher for allyl acetate and the Z content of the
product was higher for N-allylaniline.
Finally, we also tested 2-(allyloxy)ethanol (entry 27), which is
known to be a challenging substrate because of the presence of
the alcohol function. The formation of a small amount of the
homocoupling product (ca. 1%) with a Z selectivity of 73% was
observed after 2 h, along with a considerable fraction of
isomerized substrate (ca. 6%).19 Longer reaction times resulted
only in further isomerization of the starting material and Z−E
isomerization of the product. Although it appears that 2
decomposed after only about four turnovers, the observed
product formation and selectivity demonstrated at least some
degree of tolerance toward this functional group. To date, the
NHC−adamantyl chelate Ru catalysts are the only Z-selective
OM catalysts for which clear robustness toward alcohol
functionalities has been demonstrated.9
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
Vidar.Jensen@kj.uib.no
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(19) The substrate was used as received.
The authors gratefully acknowledge financial support from The
Norwegian Research Council via the KOSK II (Grant 177322/
V30) and FORNY (Grant 203379) Programs and thank Dr.
Bjarte Holmelid for assistance with the HRMS-DART and GC-
HRMS analyses. F.R.H. gratefully acknowledges a Ph.D.
scholarship from the University of Bergen.
D
dx.doi.org/10.1021/ja311505v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX