The catalytic performance of Ru-NHC alkylidene complexes: PCy3 versus pyridine as the dissociating ligand

Article abstract of DOI:10.3762/bjoc.6.136

The catalytic performance of NHC-ligated Ru-indenylidene or benzylidene complexes bearing a tricyclohexylphosphine or a pyridine ligand in ring closing metathesis (RCM), cross metathesis, and ring closing enyne metathesis (RCEYM) reactions is compared. While the PCy₃ complexes perform significantly better in RCM and RCEYM reactions than the pyridine complex, all catalysts show similar activity in cross metathesis reactions.

Full text of DOI:10.3762/bjoc.6.136

The catalytic performance of
Ru–NHC alkylidene complexes:
PCy3 versus pyridine as the dissociating ligand
Stefan Krehl, Diana Geißler, Sylvia Hauke, Oliver Kunz, Lucia Staude
and Bernd Schmidt*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Universität Potsdam, Institut für Chemie, Organische
Synthesechemie, Karl-Liebknecht-Straße 24–25, D-14476 Golm,
Germany
doi:10.3762/bjoc.6.136
Received: 10 September 2010
Accepted: 09 November 2010
Published: 15 December 2010
Email:
Bernd Schmidt* - bernd.schmidt@uni-potsdam.de
Guest Editor: K. Grela
*
Corresponding author
©
2010 Krehl et al; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
homogeneous catalysis; N-heterocyclic carbenes; olefin metathesis;
pyridine ligand; Ruthenium carbene complexes
Abstract
The catalytic performance of NHC-ligated Ru-indenylidene or benzylidene complexes bearing a tricyclohexylphosphine or a pyri-
dine ligand in ring closing metathesis (RCM), cross metathesis, and ring closing enyne metathesis (RCEYM) reactions is compared.
While the PCy3 complexes perform significantly better in RCM and RCEYM reactions than the pyridine complex, all catalysts
show similar activity in cross metathesis reactions.
Introduction
Over the past two decades, the olefin metathesis reaction and moisture, and their comparatively low sensitivity towards
became one of the most important C–C-bond forming reactions functional groups, Ru-carbene complexes have attracted a
in organic synthesis [1]. The elucidation of the crucial role of particularly high degree of attention and “numerous variations
metal carbenes by Chauvin [2] and the development of stable on a theme by Grubbs”, as so accurately phrased by Fürstner
and defined precatalysts for homogeneously catalyzed reac- [7], have been published. In the early stages of catalyst evolu-
tions by Schrock [3] and Grubbs [4] paved the way for this tion improved methods for the introduction of the alkylidene
development. Since then, molybdenum- [5] and ruthenium- ligand were the main focus. Thus, the original version of first
based [6] catalysts have experienced extensive further develop- generation Grubbs’ catalyst (A) [8], in which the alkylidene
ments and improvements. Due to their robustness towards air moiety originates from 2,2-diphenylcyclopropene, was soon
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
replaced by the second version B [9], since the benzylidene pyridine ring is attached to the alkylidene ligand [36], a latent
ligand is more conveniently available from phenyldiazo- catalyst results which might be a particularly useful property for
methane. The obvious disadvantages of handling non-stabilized metathesis polymerization reactions [37]. More recently,
diazo compounds stimulated investigations into the use of Ru-indenylidene complexes bearing one or two pyridine ligands
propargylic alcohols as alkylidene precursors [10], which were also synthesized [38,39], with the commercially available
resulted in the synthesis of a first generation analogue C with an Umicore M31 catalyst H being a particularly interesting
indenylidene ligand [11,12]. A landmark in the evolution of example. This catalyst is available from the M2 complex by
Ru-metathesis catalysts was the introduction of alkylidene ligand substitution [18] and shows high activity in living ROMP
complexes bearing N-heterocyclic carbenes (NHC) [13-15], in [40], whereas the reactivity in some RCM and CM reactions
particular complex D, which became known as second genera- appears to be diminished [18,41].
tion Grubbs’ catalyst [16], and the Umicore M2 catalyst E
[
17,18]. The ligation of strongly σ-donating NHC’s leads to an During our research directed at the development of novel
improvement of catalytic activity, which sometimes equals the metathesis-non metathesis reaction sequences [42] catalyzed by
activity of Mo-based catalysts while maintaining the general a single site catalyst and initiated by organometallic transforma-
robustness and tolerance towards several functional groups [19]. tions in situ, we became interested in the use of phosphine-free
A third major topic in catalyst development has been the varia- catalyst H. Unfortunately, only limited data is available
tion of the dissociating “placeholder”-ligand. In this respect, the concerning the catalytic efficiency of this catalyst in small
introduction and further improvement of hemilabile benzyl- molecule metathesis, in particular in comparison to the estab-
idene ligands by Hoveyda [20], Grela[21] and Blechert [22,23] lished phosphine containing catalysts D and E (Figure 1).
have been important achievements. These phosphine-free cata-
lysts of the general type F sometimes show higher activities due In this contribution, representative ring closing olefin, ring
to enhanced catalyst lifetimes and have often been applied in closing enyne and cross metathesis reactions of indenylidene
cross metathesis reactions [24]. An alternative approach to complexes E and H and benzylidene complex D are compared.
phosphine-free Ru-metathesis catalysts uses pyridines as place-
Results and Discussion
holders. Originally, complex G was synthesized as a precursor
for mixed NHC-phosphine complexes other than D [25-28] or, Effects of solvent and catalyst loading
very recently, for the synthesis of Ru-alkylidenes with two As a test reaction, we initially investigated the ring closing
different NHC ligands [29]. Comparatively little information is metathesis of allyl ether 1 to dihydropyran 2 (Scheme 1). To
available concerning the catalytic activity of these pyridine- this end, conversion to the desired product in the presence of
NHC complexes. They were found to initiate ring opening 5 mol % of catalyst H was determined after a reaction time of
metathesis polymerization reactions very rapidly [30,31] and one hour at a slightly elevated temperature in seven different
show a certain preference for cross metathesis [32-35]. If the solvents, under otherwise identical conditions.
Figure 1: Ru-based metathesis catalysts.
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Next, we were interested to see how the performance of pyri-
dine containing catalyst H compares with the more established
phosphine complexes D and E. The test reaction depicted in
Scheme 1 was therefore repeated in toluene and in acetic acid
with a significantly lower catalyst loading, because we assumed
that the highly active catalysts D and E would otherwise lead to
full conversion in extremely short time (Figure 3).
Scheme 1: RCM of an allyl ether catalyzed by catalyst H.
The results are summarized in Figure 2. Benzene, toluene,
dichloromethane and 1,2-dichloroethane are commonly used
solvents in Ru-catalyzed RCM reactions. However, for catalyst
H only in dichloromethane was high conversion to the desired
RCM product observed. In hexafluorobenzene, which has
recently been shown to give highly impressive results, even in
difficult metathesis reactions catalyzed by D, E, or F [43,44],
the rate of conversion to dihydropyran 2 is quite similar to
benzene or dichloroethane. Polar and, in particular, protic
solvents would normally be considered inappropriate for
metathesis reactions, because catalyst inhibition or degradation
to Ru-hydrides might occur [45-47]. Nevertheless, such
solvents have previously been investigated and useful results
were obtained for esters [48] and – even more surprising – for
acetic acid [49]. Therefore, we used ethyl acetate and acetic
acid for our test reaction. While ethyl acetate gave a conversion
Figure 3: Comparison of catalysts D, E and H in toluene and acetic
acid (standardized conditions as denoted in Scheme 1).
of 75%, which is better than most of the classical solvents, Thus, with 1.0 mol % of catalyst under conditions identical to
nearly quantitative conversion to the RCM product was those given in Scheme 1, a quantitative conversion to the dihy-
observed in acetic acid. Presumably, the pyridine ligand is dropyran 2 was observed for both catalysts D and E in toluene.
protonated under these conditions which would result in a In accord with the results summarized in Figure 2, catalyst H
higher amount of the catalytically active 14-electron species. gave only 34% conversion in this solvent after one hour,
This interpretation is corroborated by the kinetic data obtained suggesting that initiation was rather slow. In acetic acid, the
by Adjiman, Taylor et al. in their original investigation on analogous phosphine containing indenylidene catalyst E
solvent effects [49].
displayed a slightly reduced conversion of 89%, which might be
attributed to slow catalyst deactivation, whereas pyridine com-
plex H showed significantly enhanced activity in acetic acid,
with a conversion of 68% after one hour. This result suggests
that by switching from toluene to acetic acid the balance of
catalyst deactivation and enhanced initiation is shifted to the
deactivation side for phosphine catalyst E, and to the enhanced
initiation side for pyridine catalyst H. The most remarkable
result from this set of experiments, however, is a collapse of
conversion if benzylidene catalyst D is used in acetic acid. With
D, a reproducible conversion of only 9% to the dihydropyran 2
was observed, compared to 89% conversion with the analogous
indenylidene complex E. This result seems to contradict the
observations by Adjiman, Taylor et al. who reported prepara-
tively useful conversions and yields for second generation
Grubbs’ catalyst D in acetic acid [49], however, their studies
were conducted at ambient temperature and for different
substrates, while our experiments were conducted at 40 °C. It is
Figure 2: Solvent screening for the RCM of 1 catalyzed by H (stan-
dardized conditions as denoted in Scheme 1).
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
possible that our result suggests a higher robustness of indenyl-
idene catalyst E compared to benzylidene catalyst D, at least
under these rather unusual conditions.
To further evaluate the catalytic performance of pyridine cata-
lyst H in acetic acid, we next wanted to determine the minimum
amount of catalyst required to obtain a preparatively useful
(
>90%) rate of conversion. Consequently, it was demonstrated
Scheme 2: Catalyst screening for RCM of acrylate 3a.
that, instead of using the conditions noted in Scheme 1, a cata-
lyst loading of 2 mol % was sufficient to achieve a 92% conver-
sion within one hour (Figure 4).
Table 1: RCM of acrylate 3a.
Entry Solvent Catalysta Ratio 4a:3ab
Yield of 4a
1
2
3
4
toluene
D
E
H
H
>15:1
>15:1
10:5
80%
92%
41%
—c
toluene
toluene
acetic acid
10:17
aA catalyst loading of 5 mol % was used in all experiments. bRatio of
product to starting material was determined from the 1H NMR-spec-
trum of the crude reaction mixture after three hours at 80 °C. cYield
was not determined due to low conversion.
Because acetic acid did not lead to the expected improvement in
acrylate metathesis reactions, only toluene was tested as a
solvent in further experiments (Figure 5 and Table 2). Toluene
was preferred over dichloromethane, because reactions are more
conveniently conducted at elevated temperatures in this solvent.
Figure 4: Conversion vs catalyst loading for the RCM of 1 in acetic
acid (standardized conditions as denoted in Scheme 1).
The acrylates 3 investigated in ring closing metathesis reac-
tions with catalysts D, E and H are listed in Figure 5, together
with the resulting unsaturated lactones 4. Results for the ring
Ring closing metathesis of acrylates
Having established the beneficial effect of acetic acid for the closing metathesis of acrylates 3a–g are summarized in Table 2.
ring closing metathesis reaction of allyl ether 1 catalyzed by H, Lactones 4b–f [50] are accessible in preparatively useful yields
we were interested to see if a similar effect exists for other with catalyst loadings of 2.5 mol % to 5.0 mol % if catalysts D
substrates. Therefore, acrylate 3a was subjected to the condi- and E are used. Conversions observed with catalyst H under
tions of a ring closing metathesis reaction (Scheme 2 and otherwise identical conditions are significantly lower and yields
Table 1). Not unexpectedly [50], significantly reduced initial exceed 50% only in few cases such as 4b, 4e and 4f. A particu-
substrate concentrations were required for useful rates of larly difficult substrate for olefin metathesis reactions is acry-
conversion to the desired α,β-unsaturated lactone 4a. It tran- late 3g, which has recently been used by us as an intermediate
spired, that preparatively useful yields could only be obtained in the synthesis of the natural product (–)-cleistenolide [51].
with the phosphine containing catalysts D and E in toluene Acrylate 3g requires very high dilution, the addition of phenol
(
Table 1, entries 1 and 2), whereas pyridine complex H gave a as a rate accelerating additive [52], a rather high catalyst
conversion of approximately 65% and an isolated yield of 41% loading of 10 mol % and an even higher reaction temperature.
of lactone 4a in this solvent (Table 1, entry 3). In contrast to the Under these conditions, the best product to substrate ratio and
ring closing metathesis of allyl ether 1, the rate of conversion the best isolated yield was obtained with the indenylidene com-
decreased significantly when catalyst H was used in acetic acid plex E (Table 2, entry 6).
(
Table 1, entry 4). Notably, the 1H NMR-spectra of the crude
reaction mixtures did not indicate the presence of any decompo- Ring closing enyne metathesis
sition products, thus, even the comparatively labile acrylate 3 Imahori et al. have recently discovered that allylic hydroxy
appears to be quite stable in acetic acid at elevated tempera- groups significantly enhance the rate of ring closing enyne
tures for several hours.
metathesis reactions [53,54]. In these cases, addition of an
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Figure 5: Acrylates 3 and their RCM products 4.
Table 2: RCM reactions of acrylates to unsaturated lactones.a
Entry
3
4
c/mol·L−1
Catalyst
loading
Ratiob 4:3 (isolated yields) for
D
E
H
1
2
3
4
5
3b
3c
3d
3e
3f
4b
4c
4d
4e
4f
0.02
0.02
0.02
0.01
0.02
0.002
2.5 mol %
2.5 mol %
2.5 mol %
5.0 mol %
2.5 mol %
10.0 mol %
> 20:1 (86%)
>20:1 (75%)
7.1:1 (79%)
>10:1 (59%)
>20:1 (80%)
1.1:1 (36%)
>20:1 (90%)
>20:1 (85%)
3.3:1 (41%)
>20:1 (71%)
>20:1 (53%)
2.6:1 (58%)
1.8:1 (52%)
2.1:1 (43%)
0.6:1 (27%)
1.8:1 (56%)
4.5:1 (65%)
0.3:1 (14%)
6c
3g
4g
aReactions were run in toluene at 80 °C for 1 h, unless otherwise stated. bThe substrate to product ratio was determined from the 1H NMR-spectra of
the crude reaction mixtures. cReaction was run in toluene at 110 °C for 3 h in the presence of 0.5 equiv of phenol.
ethylene atmosphere [55] which is normally considered to be are used [57,58], and we were therefore interested to test NHC-
mandatory for good results, is not required. We have recently ligated catalysts in this transformation (Scheme 3).
investigated the highly selective enyne metathesis of substrates
such as 5a,b to dihydropyrans 6a,b in the presence of first The results for ring closing enyne metathesis reactions with
generation catalysts B and C [56]. Notably, no dihydrofurans or indenylidene catalysts E and H are summarized in Table 3.
other isomers were observed. Sometimes, the selectivity Enyne 5a reacted to afford the expected dihydropyran 6a with
decreases when the more reactive second generation catalysts high conversion and high selectivity in toluene at 80 °C and at
Scheme 3: Ring closing enyne metathesis reactions.
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Table 3: Ring closing enyne metathesis reactions catalyzed with E and H.
Entry
5
–R
Catalysta
T
Conversionb
Ratio of 6:7b
Product
Yield)
(
1
2
3
5a
5a
5a
–CH2OBn
–CH2OBn
–CH2OBn
E
E
H
80 °C
110 °C
80 °C
95%
95%
50%
>20:1
>20:1
>20:1
6a (50%)
6a (44%)
6a (29%)
5a (21%)
4
5
6
7
8
5a
5b
5b
5b
5b
–CH2OBn
–CH3
H
E
E
H
H
110 °C
80 °C
80%
100%
100%
100%
100%
>20:1
10:10.7
10:10.3
10:10.2
10:9.6
6a (34%)
n. d.
–CH3
110 °C
80 °C
n. d.
–CH3
n. d.
–CH3
110 °C
6b (24%)
7b (45%)
a2.5 mol % of catalyst, toluene as a solvent and an initial substrate concentration of 0.1 mol/L were used in all experiments. bRates of conversion and
monomer/dimer ratios were determined from the 1H NMR-spectra of the crude reaction mixtures, which showed only signals of starting materials 5,
dihydropyrans 6 and dimers 7.
1
10 °C in the presence of phosphine complex E (Table 3, The results discussed above for the less reactive enyne 5a
entries 1 and 2). The dimerization product 7a was not detected, suggest that pyridine complex H is less active than E in enyne
and although the 1H NMR spectra of the crude reaction metathesis reactions, however, the results for the apparently
mixtures showed only signals of 6a and trace amounts of the more reactive substrate 5b may indicate that the gap in catalytic
starting material 5a, the isolated yields are mediocre which can activity between E and H is much smaller for ring closing
be attributed to a significant loss of material during purification. enyne reactions than for ring closing acrylate metathesis reac-
Significantly lower conversions of 50% and 80% were observed tions. However, the remarkably high amount of homodimeriza-
with the pyridine complex H at 80 °C or 110 °C, respectively. tion product 7b points to considerable activity of H in cross
Again, 1H NMR-spectra of the crude reaction mixtures showed metathesis reactions, a peculiarity which has previously been
only signals for 6a and the starting material 5a and not even noted for the bis(pyridine) complex G [33-35].
trace amounts of a dimerization product 7a (Table 3, entries 3
and 4) were detected. Different results were found for the Cross metathesis reactions
methyl substituted enyne precursor 5b. Remarkably, with both Allylic alcohol 8 [59] was chosen to test the catalysts investi-
catalysts and both reaction temperatures apparently identical gated in this study for cross metathesis activity, because allylic
conversions and product distributions were observed: In all alcohols are known to undergo undesired “redox isomerization”
cases (Table 3, entries 5–8) the starting material was fully in the presence of Ru metathesis catalysts in some cases with
consumed and the 1H NMR-spectra of the crude reaction the formation of ethyl ketones [47,60]. Therefore, 8 can be
mixtures revealed only the presence of dihydropyran 6b and its considered as a rather challenging substrate. As partners in the
dimer 7b in roughly a 1:1 ratio. This ratio is reflected nicely in cross metathesis reactions, three acrylates 9a–c were chosen. In
the isolated yields, which were determined for one example addition, homodimerization of 8 to diol 11 was also investi-
(
Table 3, entry 8). From these results, it can be concluded that gated (Scheme 4).
benzyloxy-substituted enyne 5a is significantly less reactive in
enyne metathesis reactions which becomes more obvious if the The results are summarized in Table 4. With methyl acrylate 9a
pyridine complex H is used as a catalyst. For other examples, in dichloromethane at 40 °C, only the benzylidene catalyst D
we have previously observed that benzyl ether moieties in close showed a satisfactory rate of conversion (Table 4, entry 1). A
proximity to a C–C-multiple bond retard or inhibit metathesis significantly lower conversion was observed under these condi-
reactions [50]. Presumably, partial catalyst deactivation by tions for pyridine complex H (Table 4, entry 3), however, the
coordination of the benzyloxy group to the metal plays a role, isolated yields were similar in both cases. Surprisingly, with
and this might also explain why no dimerization product 7a was catalyst E conversion was incomplete and a considerable
observed, whereas 7b, the self-metathesis product of 6b, was amount of dimer 11 was formed (Table 4, entry 2). Compound
isolated in significant quantities, which might suggest a remark- 11 is an inseparable mixture of diastereomers, because 10a was
able residual activity of the catalysts after completion of the used as a racemate. Raising the temperature to 80 °C in toluene
enyne metathesis.
solved the problems of incomplete conversion and competing
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Scheme 4: Cross metathesis reactions with allylic alcohol 8.
dimerization for all three catalysts (Table 4, entries 4–6) and Therefore, 8 was subjected to olefin metathesis conditions in the
rac-10a was isolated in comparable yields of between 83% and absence of acrylates to check if any isomerization occurred, or
8
6%. Phenyl acrylate (9b) is a less reactive cross metathesis if dimer 11 was the preferred or sole product. With all three
partner, and all three catalysts led to incomplete conversion catalysts similar results were obtained: The starting material
Table 4, entries 7–9). The best result in this series was was rapidly and almost completely consumed, and the two
(
achieved with catalyst H, which gave a 2:1 ratio of product 10b diastereomers of 11 were the only detectable products in the
and starting material 8, and an isolated yield of 60% (Table 4, reaction mixture (Table 4, entries 12–14).
entry 9). Methyl methacrylate (9c) did not react in cross
metathesis reactions with 8 using either D or H. Instead, dimer- These results demonstrate that the novel catalyst H, albeit
ization to compound 11 occurred. We [50] and others [61] have significantly less active than D or E in the ring closing
recently observed a considerable reduction of isomerization side metathesis of acrylates, appears to be competitive in cross
reactions if acrylates are present during a metathesis reaction. metathesis reactions.
Table 4: Cross metathesis reactions catalyzed by D, E, and H.
Entry
Acrylate
Solvent
T
Catalysta
Ratio of
0:8:11b
Product (Yield)c
1
1
2
3
4
5
6
7
8
9
9a
9a
9a
9a
9a
9a
9b
9b
9b
9c
9c
—
—
—
CH2Cl2
CH2Cl2
CH2Cl2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
40 °C
40 °C
40 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
80 °C
D
E
H
D
E
H
D
E
H
D
H
D
E
H
16:1:0
1.4:1.3:1
3.7:1:0
1:0:0
rac-10a (72%)
n. d.
rac-10a (65%)
rac-10a (84%)
rac-10a (83%)
rac-10a (86%)
rac-10b (53%)
rac-10b (47%)
rac-10b (60%)
11 (n. d.)
1:0:0
1:0:0
1.2:1:0
1:1:0
2:1:0
10
11
12
13
14
0:0:1
0:0:1
11 (n. d.)
—:1:30
—:1:9
—:1:16
11 (68%)
11 (n. d.)
11 (n. d.)
aA catalyst loading of 5.0 mol % was used in all experiments. bRatios were determined from the 1H NMR-spectra of crude reaction mixtures. cAll cross
metathesis products were obtained as single E-isomers.
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Beilstein J. Org. Chem. 2010, 6, 1188–1198.
Conclusion
temperature control) for a period of time between 60 and 62
In conclusion, we have evaluated and compared the catalytic min. After this time, the reaction vessel was allowed to cool to
performance of two indenylidene NHC-metathesis catalysts and ambient temperature, the solvent was removed by evaporation,
the well established second generation Grubbs catalyst in and the residue immediately subjected to NMR spectroscopy.
various small molecule metathesis reactions. The activity of the The ratio of dihydropyran 2 to allyl ether 1 was determined by
mixed NHC-phosphine catalysts D and E appears to be similar integration of characteristic, baseline separated signals. Each
in most applications. Some results hint at a somewhat faster experiment was repeated at least two times. The reported rates
reaction with benzylidene complex D, while E apparently of conversion are average values.
performs slightly better in “slow” metathesis reactions, presum-
ably since it is more robust. The novel pyridine complex H was Ring closing metathesis of acrylates
also tested in several olefin metathesis reactions. While this General procedure for the synthesis of furanones 4b–4f by
catalyst gives rather unsatisfactory results in the ring closing RCM: To a solution of the appropriate acrylate 3 (1.0 mmol) in
metathesis of acrylates, its performance in ring closing enyne dry and degassed toluene (50 mL for 0.02 mol·L–1 or 100 mL
metathesis reactions is only slightly lower than the phosphine for 0.01 mol·L–1), was added either catalyst D (21.2 mg for
complexes D and E. However, the activity of H in cross 2.5 mol % or 42.4 mg for 5.0 mol %), E (23.7 mg for 2.5 mol %
metathesis reactions is similar to D and E. Furthermore, initial or 47.4 mg for 5.0 mol %) or H (18.7 mg for 2.5 mol % or 37.4
studies concerning the use of unconventional solvents revealed mg for 5.0 mol %). The solution was heated to 80 °C for 90
that H might be quite active, at least for some applications, in min. The solvent was then removed by evaporation and the
acetic acid.
residue purified by flash column chromatography on silica to
give the corresponding lactones 4. The ratio of lactone 4 to
acrylate 3 was determined by integration of characteristic, base-
Experimental
All experiments were conducted in dry reaction vessels under line separated signals in the 1H NMR-spectrum of the crude
an atmosphere of dry argon. Solvents were purified by standard reaction mixture. Representative example: 5-phenylfuran-
procedures. All yields and conversions reported herein are 2(5H)-one (4f). This compound was obtained as a colourless oil
average values of at least two experiments. 1H NMR spectra from 3f (189 mg, 1.0 mmol) following the general procedure.
were obtained at 300 MHz in CDCl3 with CHCl3 (δ = 7.26 Yield of 4f using catalyst D: 128 mg (0.80 mmol, 80%). Yield
ppm) as the internal standard. Coupling constants (J) are given of 4f using catalyst E: 85 mg (0.53 mmol, 53%). Yield of 4f
in Hz. 13C NMR spectra were recorded at 75 MHz in CDCl3 using catalyst H: 104 mg (0.65 mmol, 65%). 1H NMR (300
with CDCl3 (δ = 77.0 ppm) as the internal standard. IR spectra MHz, CDCl3) δ 7.53 (dd, J = 5.7, 1.7, 1H), 7.45–7.35 (3H),
were recorded as films on NaCl or KBr plates or as KBr discs. 7.30–7.23 (2H), 6.23 (dd, J = 5.7, 2.1, 1H), 6.01 (t (br), J = 1.9,
Wavenumbers (ν) are given in cm–1. Mass spectra were 1H); 13C NMR (75 MHz, CDCl3) δ 173.0 (0), 155.8 (1), 134.4
obtained at 70 eV. Whenever known compounds were used as (0), 129.3 (1), 129.0 (1), 126.5 (1), 121.0 (1), 84.3 (1); HRMS
starting materials, reagents or catalysts, they were either (ESI) calcd for C10H8O2Na [M+Na]+: 183.0422, found:
purchased or were synthesized following literature procedures: 183.0439.
1
[62], 3a [63], 3b–3e [50], 3g [51], 5a,b [56]. Catalyst D was
purchased from Aldrich and used without further purification. Procedure for the synthesis of 6-phenyl-5,6-dihydropyran-2-
Catalysts E and H were donated by Umicore, Hanau, Germany, one (4a): The acrylate 3a (201 mg, 1.0 mmol) was dissolved in
and also used without further purification. The following prod- dry and degassed toluene (100 mL). After adding the catalyst
ucts have previously been synthesized via olefin metathesis (D: 42.4 mg B: 47.4 mg H: 37.4 mg, 0.05 mmol, 5 mol %) the
reactions under different conditions: 2 [62], 4a [63], 4b–4e [50], solution was stirred for 3 h at 80 °C. After cooling the solution
4
f [64], 4g [51], 6a,b [56].
to ambient temperature, all volatiles were evaporated. The
residue was purified by chromatography on silica (hexane/
MTBE 2:1). Yield of 4a using catalyst D: 139 mg (0.80 mmol,
80%). Yield of 4a using catalyst E: 160 mg (0.92 mmol, 92%).
General procedure for the RCM of 1: varia-
tion of solvent, catalyst and catalyst loading
Allyl ether 1 (94.0 mg, 0.5 mmol) was dissolved in the appro- Yield of 4a using catalyst H: 170 mg (0.41 mmol, 41%). The
priate dry and degassed solvent (5.0 mL). Catalyst D (4.2 mg ratios of lactone 4a to acrylate 3a were determined by integra-
for 1.0 mol %), E (4.7 mg for 1.0 mol %) or H (3.7 mg for 1.0 tion of characteristic, baseline separated signals in the 1H NMR-
mol %, 7.4 mg for 2.0 mol %, 11.2 mg for 3.0 mol %, 15.0 mg spectrum of the crude reaction mixture. mp 58–59 °C; 1H NMR
for 4.0 mol % or 18.8 mg for 5.0 mol %) was then added. (300 MHz, CDCl3) δ 7.43–7.33 (5H), 6.97 (ddd, J = 9.7, 5.7,
Immediately after addition of the catalyst, the reaction vessel 2.6, 1H), 6.13 (ddd, J = 9.7, 1.1, 1.1, 1H), 5.45 (dd, J = 11.2,
was immersed in an oil bath preheated to 40 °C (electronic 4.8, 1H), 2.70–2.57 (2H); 13C NMR (75 MHz, CDCl3) δ 164.0
1195

Beilstein J. Org. Chem. 2010, 6, 1188–1198.
(
(
0), 144.8 (1), 138.4 (0), 128.6 (1), 128.6 (1), 126.0 (1), 121.7 882 (s), 827 (s); HRMS (EI) calcd for C18H24O4 [M]+:
1), 79.2 (1), 31.6 (2); IR (neat) ν 3064 (w), 2904 (w), 1716 (s), 304.1675, found: 304.1683; Anal. calcd for C18H24O4: C, 71.0,
1
381 (m), 1242 (s), 1059 (m), 1020 (m), 909 (m); EIMS (%) H, 8.0, found: C, 70.8, H, 7.7.
m/z 174 (M+, 6), 77 (13), 68 (100), 39 (63); HRMS (EI) calcd.
for C11H10O2 [M+]: 174.0675, found 174.0689; Anal. calcd for Cross-metathesis reactions
C11H10O2: C, 75.8, H, 5.8; found: C, 75.6, H, 5.8.
General procedure: A solution of the allylic alcohol rac-8
139 mg, 0.5 mmol) and the corresponding acrylate 9 (5 mmol)
(
Procedure for the synthesis of (R)-2-((2R,3R)-3-(tert-butyl- in dry and degassed toluene (1.0 mL for 0.5 mol·L–1, 5.0 mL for
dimethylsilyloxy)-6-oxo-3,6-dihydro-2H-pyran-2-yl)-2- 0.1 mol·L–1) was warmed to approximately 45 °C. Then the
hydroxyethyl benzoate (4g): The acrylate 3g (330 mg, 0.78 catalyst (0.025 mmol, 5 mol %, D: 42.4 mg E: 47.4 mg H: 18.7
mmol) and phenol (37 mg, 0.39 mmol) were dissolved in dry mg) was added to the solution and stirred for 90 min at 80 °C.
and degassed toluene (390 mL). After adding the catalyst (D: After cooling to ambient temperature all volatiles were evapo-
6
6.1 mg B: 73.9 mg H: 58.3 mg, 0.078 mmol, 10 mol %), the rated and the residue purified by chromatography on silica. The
solution was stirred for 3 h at 80 °C. The solution was cooled to rates of conversion and the ratios of 10 to 8 to 11 were deter-
ambient temperature and all volatiles were evaporated. The mined by integration of characteristic, baseline separated
residue was purified by chromatography on silica. Yield of 4g signals in the 1H NMR-spectrum of the crude reaction mixture.
using catalyst D: 110 mg (0.28 mmol, 36%). Yield of 4g using
catalyst E: 177 mg (0.45 mmol, 58%). Yield of 4g using cata- (4RS,5SR,E)-methyl 5-(tert-butyldimethylsilyloxy)-4-
lyst H: 43 mg (0.11 mmol, 14%). The ratios of lactone 4g to hydroxy-5-phenylpent-2-enoate (rac-10a). Obtained as
acrylate 3g were determined by integration of characteristic, colourless oil from rac-8 (139 mg, 0.5 mmol) and methyl acry-
baseline separated signals in the 1H NMR-spectrum of the crude late (9a) following the general procedure. Yield of rac-10a
reaction mixture. All analytical data were identical to those using catalyst D: 141 mg (0.42 mmol, 84%). Yield of rac-10a
reported previously in the literature [51].
using catalyst E: 139 mg (0.41 mmol, 83%). Yield of rac-10a
using catalyst H: 145 mg (0.43 mmol, 86%). 1H NMR (300
MHz, CDCl3) δ 7.37–7.25 (5 H), 6.73 (dd, J = 15.7, 4.1, 1 H),
Ring closing enyne metathesis reactions
General procedure: To a solution of the corresponding 6.07 (dd, J = 15.7, 1.9, 1 H), 4.47 (d, J = 6.8, 1 H), 4.29 (dddd,
precursor 5 (2.0 mmol) in toluene (40 mL), was added catalyst J = 6.6, 4.1, 3.6, 2.0, 1 H), 3.70 (s, 3 H), 2.90 (d, J = 3.9, 1 H),
E (47.4 mg, 2.5 mol %) or H (37.4 mg, 2.5 mol %). The solu- 0.89 (s, 9 H), 0.03 (s, 3 H), −0.18 (s, 3 H); 13C NMR (75 MHz,
tion was heated to the appropriate temperature (80 °C or 110 CDCl3) δ 166.7 (0), 145.8 (1), 140.2 (0), 128.4 (1), 128.2 (1),
°C) for 20 h, then cooled to ambient temperature and the solvent 126.9 (1), 121.6 (1), 78.4 (1), 75.8 (1), 51.5 (3), 25.7 (3), 18.1
evaporated. The crude product was analyzed by 1H and 13C (0), −4.6 (3), −5.1 (3); IR ν 3492 (w), 2952 (w), 2857 (w), 1724
NMR spectroscopy. Analytically pure samples were obtained (m), 1253 (m), 1068 (m), 835 (s), 777 (s); HRMS (ESI) calcd
by column chromatography on silica. Representative example: for C18H28O4NaSi [M+Na]+: 359.1655, found: 359.1652;
Ring closing enyne metathesis of 5b. Following the general EIMS (%) m/z = 337 (M+, 1), 221 (100), 187 (11), 173 (6), 145
procedure, 5b (330 mg, 2.0 mmol) was treated with catalyst H (9), 115 (11), 91 (6), 73 (15); Anal. calcld C18H28O4:C, 64.3,
(
37.4 mg, 2.5 mol %) at 80 °C. After column chromatography, H: 8.4; found: C: 64.2, H: 8.4.
6
b (80 mg, 0.48 mmol, 24%) and 7b (274 mg, 0.90 mmol,
4
5%) were isolated. The rate of conversion and the ratios of 6b (4RS,5SR,E)-phenyl 5-(tert-butyldimethylsilyloxy)-4-
to 7b were determined by integration of characteristic, baseline hydroxy-5-phenylpent-2-enoate (rac-10b). Obtained as
separated signals in the 1H NMR-spectrum of the crude reac- colourless solid from rac-8 (139 mg, 0.5 mmol) and phenyl
tion mixture. All analytical data of 6b were identical to those acrylate (9b) following the general procedure. Yield of rac-10b
reported previously in the literature.[56] Analytical data for using catalyst D: 105 mg (0.27 mmol, 53%). Yield of rac-10a
(
2R,2'R,3R,3'R,E)-2,2'-(ethene-1,2-diyl)bis(5-(prop-1-en-2- using catalyst E: 94 mg (0.30 mmol, 47%). Yield of rac-10a
yl)-3,6-dihydro-2H-pyran-3-ol (7b): [α]D25: –390.0 (c 0.13, using catalyst H: 119 mg (0.43 mmol, 60%). mp 82–84 °C; 1H
CH2Cl2); 1H NMR (300 MHz, CDCl) δ 6.05 (d, J = 5.4, 2H); NMR (300 MHz, CDCl3) δ 7.41–7.18 (8 H), 7.12–7.06 (2 H),
5
1
1
1
6
2
.96 (d, J = 1.7, 2H); 4.93 (s, 1H); 4.83 (s, 2H); 4.49 (d, J = 6.93 (dd, J = 15.6, 4.0, 1 H), 6.30 (dd, J = 15.6, 1.8, 1 H), 4.55
5.6, 2H); 4.25 (d, J = 15.5, 2H); 4.04 (s, 2H); 3.99 (d, J = 5.5, (d, J = 6.6, 1 H), 4.37 (dddd, J = 6.1, 4.2, 4.0, 1.9, 1 H), 2.96 (d,
H); 3.32 (s, 2H); 1.87 (s, 6H); 13C NMR (75 MHz, CDCl3) δ J = 4.0, 1 H), 0.91 (s, 9 H), 0.06 (s, 3 H), −0.16 (s, 3 H); 13C
39.5 (0), 138.5 (0); 129.1 (2), 122.8 (1); 112.3, (2); 77.3 (1), NMR (75 MHz, CDCl3) δ 164.6 (0), 150.7 (0), 147.9 (1), 140.2
6.2 (2); 64.2 (1); 20.1 (3); IR ν 3297 (s); 3087 (m); 2945 (m); (0), 129.3 (1), 128.5 (1), 128.3 (1), 126.9 (1), 125.7 (1), 121.5
828 (m); 1606 (m); 1371 (w); 1124 (s); 1103 (m); 1055 (m), (1), 121.3 (1), 78.4 (1), 76.0 (1), 25.8 (3), 18.2 (0), −4.5 (3),
1196

Beilstein J. Org. Chem. 2010, 6, 1188–1198.
−
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(
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C23H30O4NaSi [M+Na]+: 421.1811, found: 421.1800; EIMS
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7
7
5 (21) 73 (47); Anal. calcd C, 69.3, H, 7.6; found C, 69.4, H,
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tion mixture. Analytical data for diastereomer 11a: 1H NMR
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J = 2.6, 0.9, 1H), 4.20 (d, J = 7.5, 1H), 4.00 (dm, J = 7.4, 1 H),
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27.7 (1), 127.3 (1), 79.5 (1), 76.7 (1), 25.8 (3), 18.1 (0), −4.5
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5
1
51.2989, found: 551.3010; EIMS (%) m/z = 283 (4), 189 (4),
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diastereomer 11b: 1H NMR (300 MHz, CDCl3) δ 7.33–7.17
1
1
1
1
5.Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L.
(
(
(
5H), 5.52 (dd, J = 2.6, 0.9, 1H), 4.38 (d, J = 6.6, 1 H), 4.03
ddm, J = 6.0, 3.0, 1H), 2.74 (d, J = 3.4, 1H), 0.89 (s, 9H), 0.03
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2
Supporting Information
Supporting Information File 1
2
Experimental procedures and characterization data for
starting materials which have previously not been described
in the literature; representative illustrations for
determination of conversions; copies of spectra for new
compounds.
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