Microwave-assisted ruthenium-catalysed olefin metathesis in fluorinated aromatic hydrocarbons: A beneficial combination

Article abstract of DOI:10.1002/adsc.201100053

High conversions in ruthenium-based olefin metathesis of demanding substrates are commonly forced by using a high loading of a catalyst and conducting reactions at elevated temperature for extended times. However, in many cases this approach is not fully

Full text of DOI:10.1002/adsc.201100053

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DOI: 10.1002/adsc.201100053
Microwave-Assisted Ruthenium-Catalysed Olefin Metathesis in
Fluorinated Aromatic Hydrocarbons: A Beneficial Combination
Cezary Samojłowicz,a Etienne Borrꢀ,b,c Marc Mauduit,b,c,* and Karol Grelaa,*
a
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Fax : (+48)-22-343-2109; phone: (+48)-22-343-2108; e-mail: klgrela@gmail.com
Sciences Chimiques de Rennes, UMR 6226, CNRS – Equipe Chimie Organique et Supramolꢀculaire, Ecole Nationale
b
Supꢀrieure de Chimie de Rennes, Av. du Gꢀnꢀral Leclerc, CS 50837, 35708 Rennes cedex 7, France
Universitꢀ europꢀenne de Bretagne, 35000 Rennes, France
c
Fax : (+33)-2-2323-8108; phone: (+33)-2-2323-8112; e-mail: marc.mauduit@ensc-rennes.fr
Received: January 20, 2011; Revised: March 23, 2011; Published online: July 22, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100053.
Abstract: High conversions in ruthenium-based commercially available ruthenium-based pre-catalyst
olefin metathesis of demanding substrates are com- creates attractive reaction conditions for promoting
monly forced by using a high loading of a catalyst challenging olefin metathesis transformations.
and conducting reactions at elevated temperature for
extended times. However, in many cases this ap-
proach is not fully effective. In this article we present Keywords: fluorinated aromatic hydrocarbons; mi-
that fluorinated aromatic hydrocarbons (FAH) com- crowave irradiation; N-heterocyclic carbenes; olefin
bined with microwave (MW) irradiation using a metathesis; ruthenium
Introduction
takes place to the detriment of stability in the reac-
tion media (catalyst lifetime).[4,5]
Functionalised olefins are important building blocks
in organic synthesis.[1] Catalytic olefin cross-metathe-
sis (CM), ring-closing metathesis (RCM) and en-yne
metathesis are convenient routes to obtain functional-
ised molecules from simple alkene, diene and en-yne
precursors.[1] With the advent of stable, commercially
available 2nd generation ruthenium complexes (1a–e,
g),[1–4] that combine high catalytic activity and excel-
lent functional group tolerance, olefin metathesis re-
actions have emerged as one of the most attractive
and powerful tools for the formation of new carbon-
carbon double bonds (Figure 1). Although a wide
range of functionalised di- or trisubstituted olefins
can be used efficiently in olefin metathesis transfor-
mations,[2–4] some sterically or electronically deactivat-
ed substrates are still challenging and typically require
high loadings of the Ru complexes and extended reac-
tion times.[4] Recently, through an important effort in
the design of N-heterocyclic carbene (NHC) ligands,
Grubbs et al. have reported attractive new Ru com-
plexes (1e, 1f) enabling one to improve significantly
the yield in RCM[4] and CM transformations.[5] How-
ever, in some cases, the efficiency of these catalysts
Figure 1. Ruthenium-based olefin metathesis initiators. Cy=
cyclohexyl; Mes=2,4,6-trimethylphenyl; Tol=2-methyl-
phenyl.
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Very recently, another approach based on modifica- NHC complexes 1e and 1f, the ring closing of 2s was
tions of the reaction conditions, by use of fluorinated unproductive under “classical” thermal conditions
aromatic hydrocarbons (FAH), e.g., C6F6 and (5 mol%, C6D6, 0.1M, 608C, 24 h; entry 1).[4] In the
C6F5CF3, to dope the activity of standard ruthenium meantime, D’Annibal and co-workers have reported
2nd generation commercially available initiators has the isolation of lactone 2p in a low yield of 40% but
been published.[6–8] This approach was found to be requiring
a
large amount of Ru complex 1a
quite successful in the case of a wide range of difficult (20 mol%, entry 2).[10a]
olefin metathesis substrates, including biologically
active compounds.[7]
We started our study by confirming, indeed, the ab-
sence of reactivity of 2s with 2 mol% of 1b in C6D6,
The use of microwave (MW) irradiation is a well where no product was observed at 1008C over 24 h
known practice in modern organic synthesis as an al- (the reaction was conducted in a sealed vial, entry 3).
ternative way to optimise the thermal activation.[9] In Interestingly, with the same catalyst loading, using
many cases, the main profit is a dramatic reduction of hexafluorobenzene, 15% formation of desired product
reaction time limiting the premature degradation of 2p was observed at 1008C after only 15 min (entry 4).
the catalyst and/or formation of by-products and en- However, when the same reaction mixture was heated
hancing the yields for demanding substrates.[9] How- via microwave irradiation (1008C, 200 W), 31% con-
ever, it should be quoted that the beneficial effect of version of 2s has been obtained (entry 5). In the next
microwave irradiation in general, and particularly on step, we used octafluorotoluene at 1208C and after
olefin metathesis reactions is poorly understood.[9b,c] 15 min (entry 6) we observed 33% conversion of 2s.
Nevertheless, in view of these improvements, we de- Octafluorotoluene in combination with microwave ir-
cided to examine if a synergistic effect could be ob- radiation (1208C, 200 W, entry 7) led to improved
served in difficult metathesis transformations promot- conversion up to 57%. Remarkably, under fully opti-
ed by commercially available ruthenium complexes mised conditions (e.g., 2ꢂ2 mol% of 1b, 2ꢂ15 min of
1a–d, g and conducted in FAH solvents under micro- microwave irradiation, entry 8), the expected lactone
wave irradiation.
2p was obtained in 80% isolated yield.[6b]
It is worth noting that while the increase of catalyst
activity in FAH solvents was clearly demonstrated in
practically all cases tested,[6,7] the additional benefit of
microwave irradiation was not observed for all hin-
Results and Discussion
The first demanding substrate, which has been select- dered substrates. As showed in Figure 2, the forma-
ed by us, was diene 2s leading to the formation of six- tion of cyclic olefin 3p bearing a tetrasubstituted C=
À
membered lactone 2p, bearing a tetrasusbtituted C C C double bond at 1008C within 5 min in hexafluoro-
bond (Table 1). The presence of such a lactone as a benzene gave improved results compared to “classi-
structural subunit in numerous natural products cal” solvent (63% vs. 23%, respectively), however,
makes the possibility of accessing it via RCM very at- microwave irradiation has not improved the yield sig-
tractive.[10] As reported by Grubbs and co-workers in nificantly. Interestingly, repeated heating did not
2007, despite the higher reactivity of new modified allow us to increase the conversion, attesting the
Table 1. RCM leading to lactone 2p.
Entry
Solvent (conc.)
Complex (mol%)
Conditions
Conversion [%][a]
1a
1b
2
C6D6 (0.1M)
C6D6 (0.1M)
CH2Cl2 (0.01M)
C6D6 (0.2M)
1e (5)
1f (5)
1a (20)
1b (2)
608C, 24 h
608C, 24 h
408C, 48 h
1008C, 24 h
0[4a]
0[4a]
(40)[10a]
0
3
4
C6F6 (0.2M)
1b (2)
1008C, 15 min
15
5
6
7
8
C6F6 (0.2M)
1b (2)
1b (2)
1008C, 15 min, MW (200 W)
1208C, 15 min
1208C, 15 min, MW (200 W)
1208C, 2ꢂ15 min, MW (200 W)
31
33
57
81 (80)
C6F5CF3 (0.2M)
C6F5CF3 (0.2M)
C6F5CF3 (0.2M)
1b (2)
1b (2ꢂ2)[b]
[a]
1
Determined by H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
MW irradiation has been repeated using 2 mol% of pre-catalysts.
[b]
1994
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Microwave-Assisted Ruthenium-Catalysed Olefin Metathesis in Fluorinated Aromatic Hydrocarbons
[a]
1
[b]
Figure 2. RCM of 3s leading to tetrasubstituted olefin 3p. Determined by H NMR. Reaction protocol has been repeat-
ed.
13% conversion of 3s) at the beginning of the reac-
tion.
In the next experiment we decided to examine fur-
ther the stability of the Ru pre-catalysts and the cata-
lytically active species in the presence of diene 4s.
The straightforward starting material was selected for
the RCM transformation (Table 2) using C6D6 and
C6F6 as solvents. The reaction was performed with
low loadings (0.5 mol% at 508C) of well established
Figure 3. Reaction profile of RCM with 3s. Reaction condi-
tions: 5 mol% of 1b, 0.2M, 408C, 3 days.
Ru complexes of both types of pre-catalyst: Grubbs
2nd generation (1a) and the more stable Hoveyda 2nd
generation (1c). After each 5 min of the reaction its
1
probable rapid deactivation of the Ru active spe- progress has been measured using H NMR spectros-
cies.[1d,e] Therefore, we have attempted to study an copy and a fresh amount of 4s was added into the re-
RCM reaction profile with catalyst 1b in FAH vs. a action mixture.
non-fluorinated aromatic solvent (Figure 3). The reac-
Interestingly, in RCM of 4s more catalytic cycles
tion profiles of RCM of 3s in C6F6 and in toluene at took place in FAHs compared to “classical” solvent
408C promoted by pre-catalyst[11] 1b (5 mol%) differ (Table 2) for both ruthenium complexes. In the case
dramatically, suggesting that the lifetime of the active of 1a we observed 500 and 1000 catalytic turnover
catalytic species is prolonged in the FAH media. numbers (TONs) in C6D6 and C6F6, respectively,
Indeed, the formation of 3p increased continually in whereas for 1c TONs reached 1400 and 1800, respec-
C6F6 until 48 h reaching a maximum of 70% whereas tively. These results can suggest that FAH solvents
a premature degradation of pre-catalyst 1b occurred prevent quick degradation of the Ru propargating
in toluene, stopping the transformation (reaching only species.
Table 2. Stability of 1a and 1c in C6D6 and C6F6.
[a]
Determined by 1H NMR spectroscopy.
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Table 3. RCM to form tetrasubstituted chloro-olefin 5p.
Entry
Solvent (conc.)
Complex (mol%)
Conditions
Conversion [%][a]
1
2
3
4
5
6
C6D6 (0.1M)
C6D5CD3 (0.2M)
C6D6 (0.2M)
C6F6 (0.2M)
C6F5CF3 (0.2M)
C6F5CF3 (0.2M)
1e (5)
1b (5)
1b (2)
1b (2)
608C, 18 h
1208C, 24 h
1008C, 10 min, MW (200 W)
1008C, 10 min, MW (200 W)
1208C, 10 min, MW (200 W)
1208C, 2ꢂ10 min MW (200 W)
24[13b]
35
15
34
50
1b (2)
1b (2ꢂ2)[b]
82 (78)
[a]
1
Determined by H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
[b]
MW irradiation has been repeated using each time 2 mol% of pre-catalyst 1b.
Table 4. RCM leading to lactam 6p.
Entry
Solvent[a]
Complex (mol%)
Conditions
Conversion [%][a]
1
2
3
C6D5CD3
C6F6
C6F6
1b (6)
1008C, 24 h
1008C, 3ꢂ5 min
1008C, 3ꢂ5 min, MW (200 W)
0
29
69 (61)
1b (3ꢂ2)[b]
1b (3ꢂ2)[b]
[a]
1
Determined by H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
[b]
The reaction protocol has been repeated three times using each time a fresh portion (2 mol%) of pre-catalyst 1b.
Having these results in hand, we decided to test our significant doping of activity was observed when octa-
newly developed conditions towards some even more fluorotoluene was combined with microwave irradia-
reluctant dienes, such as 5s, a precursor of the tetra- tion effect (entry 5) giving a 50% conversion of 5s. Fi-
substituted chloro-olefin 5p (Table 3). The reluctance nally, when 4 mol% of the catalyst (added in two con-
of this kind of substrate is well documented,[12] and is secutive portions) was used over 20 min, the desired
probably related to both the electronic and steric de- tetrasubstituted chloro-olefin 5p was isolated in a re-
activating effects of the olefin function. The total syn- markable yield of 78% after purification (entry 6).
thesis of an Elatol (that belongs to chamigrene sub-
class of sesquiterpenes, characterised by a spiro- bearing also a chloro-substituted C=C double bond
ACHTUNGTRENNUNG[5.5]undecane core) involving such a difficult RCM, (Table 4). Its corresponding RCM product, lactam 6p,
Next, we focused our attention on the amide[14] 6s
has been recently executed by Stolz et al. using the is an important fragment for the construction of
more reactive, NHC modified catalyst 1e.[13] In our building blocks through cross-coupling reactions.[15]
model study, we used 5s as a closely related substrate, However, the problem related to this substrate is not
that has been recently described by the same authors only the presence of the electronically deactivating
as even more challenging for catalyst 1e (Table 3, chloro substituent, but also the ability of the amide
entry 1) than the Elatol precursor.[13] The “classical” function to neutralise the 14-electron Ru active spe-
conditions in combination with standard pre-catalyst cies by chelation.[16] Due to the mentioned factors, 6s
1b under both thermal condition (1208C, 24 h) or mi- in toluene remained completely inert towards 6 mol%
crowave irradiation (1008C, 200 W, 10 min), gave only of 1b under “classical” thermal conditions, even after
low conversions of the cyclic product 5p, 35% and a prolonged heating (1008C, 24 h, entry 1). Under
15%, respectively (entries 2 and 3). In hexafluoroben- thermal conditions in C6F6, only 29% of the expected
zene using microwave irradiation, the conversion was lactam 6p was formed after 15 min at 1008C (entry 2).
slightly improved, reaching 34% (entry 4). A more Interestingly, in hexafluorobenzene using microwave
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Microwave-Assisted Ruthenium-Catalysed Olefin Metathesis in Fluorinated Aromatic Hydrocarbons
Table 5. En-yne metathesis leading to endo/exo products.
Entry (Substrate) Solvent (conc.)
Complex (mol%) Conditions
endo/exo Conversion [%][a]
1 (7s)
2 (7s)
3 (7s)
4 (7s)
5 (7s)
6 (7s)
7 (8s)
8 (8s)
9 (8s)
CH2Cl2 (0.01M)
C6H5CH3 (0.1M)
C6F5CF3 (0.1M)
C6F5CF3 (0.1M)
C6F5CF3 (0.1M)
C6F5CF3 (0.1M)
C6H5CH3 (0.02M) 1a (5)
C6F5CF3 (0.2M)
C6F5CF3 (0.2M)
1b (10)
408C, 24 h
1208C, 24 h
–
–
0
0
25
33
1b (10)
1b (2ꢂ5)[b]
1c (2ꢂ5)[b]
1d (2ꢂ5)[b]
1d (2ꢂ5)[b]
1208C, 2ꢂ15 min MW (200 W) 100/0
1208C, 2ꢂ15 min MW (200 W) 100/0
1208C, 2ꢂ15 min MW (200 W) 100/0
1208C, 2ꢂ15 min
708C, 14 h
75 (60)
17
100/0
0/100
0/100
0/100
(73)[19a]
80
1a (5)
1208C, 24 h
1208C, 2ꢂ5 min MW (200 W)
1a (2ꢂ2)[b]
79 (75)
[a]
1
Determined by H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
[b]
The reaction protocol has been repeated two times using each time a fresh portion (2 or 5 mol%) of the pre-catalyst.
irradiation at 1008C, the conversion was significantly FAH/MW conditions, under which similar isolated
improved reaching up to 69% conversion and 61% yields were observed (73% and 75%, respectively).
isolated yield after only 15 min (entry 3).
Another highly reluctant substrate, recently report-
A similar beneficial effect was also observed in the ed by Campagne and Debleds, is 1,5-enyne 9s, afford-
en-yne metathesis of 7s bearing also a chloro-alkene ing the strained cyclobutene 9p (Table 6).[20] Hoveyda
function (Table 5, entries 1–6). Indeed, under “classi- pre-catalyst 1c (20 mol%) employed under microwave
cal” thermal conditions, using 1b in dichloromethane irradiation at 708C in dichloromethane lead to 53%
at 408C or in toluene at 1208C, only the starting ma- of isolated yield (entry 1).[20a] Remarkably, using the
terial was present after 24 h (entries 1 and 2). When same catalyst loading of 1c, under optimised condi-
the reaction was conducted in octafluorotoluene at tions in hexafluorobenzene at 1008C gave full conver-
1208C under microwave irradiation (1b at 2ꢂ5 mol% sion and 76% isolated yield after 35 min of micro-
over 2ꢂ15 min, entry 3), only 25% of the cyclised wave irradiation (entry 2).
product was formed. Interestingly, we have identified
The positive results of the above experiments en-
1
by H NMR that the formed metathesis product was couraged us to examine even more challenging cross-
the six-membered endo-carbocyclic diene 7p instead metathesis reactions. The first type of a reluctant
of the expected five-membered exo-diene 7p’. It is starting material was olefin 10, bearing bulky substitu-
worthy of mention that such a high endo-selectivity is ents in the allylic position (Table 7). As reported by
quite rare in ruthenium-catalysed en-yne reactions,[17] Grubbs and co-workers,[5] CM of 10 with allylbenzene
but typical in those catalysed by molybdenum com- (11) or with 5-acetoxy-1-pentene (12), promoted by
plexes.[18] We have also tried the Hoveyda pre-catalyst pre-catalyst 1c gave the expected products 13 and 14
1c under similar conditions and observed a slight im- in moderate yields after 6 h in refluxing dichlorome-
provement of the conversion (33%, entry 4). Finally, thane (entries 1 and 5). By using the newly developed,
the best activity was obtained with the activated less hindered catalyst 1e, the yields were improved up
“nitro” pre-catalyst 1d allowing us to obtain the exo- to 91% and 89%, respectively (entries 2 and 6).[5] In
diene 7p with a remarkable 75% conversion and in FAH, where the lifetime of the catalysts is extended
60% isolated yield (Table 5, entry 5).
(see Figure 3 and Table 2), 1b showed excellent reac-
The less pronounced, but still beneficial, effect of tivity at 1208C, giving 13 and 14 in 69% and 90%
the FAH/MW combination was also visible in the conversion (entries 3 and 7). Conversion could be im-
case of another challenging substrate, methallyl en- proved up to 91% and 95%, respectively, by using oc-
yne 8s, leading to the expected exo-carbocyclic diene tafluorotoluene in combination with microwave irra-
8p’ (Table 5, entries 7–9). However, except for the re- diation as the optimal conditions (entries 4 and 8,
duction of the reaction time (10 min vs. 14 h) and the 82% and 86% isolated yields, respectively).
catalyst loading (4 mol% vs. 5 mol%),[19] no signifi-
The second type of reluctant CM substrates tested
cant differences were noted between “classical” and were 1,1’-disubstituted olefins, such as compounds 15
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Table 6. 1,5-enyne cycloisomerization.
Entry
Solvent
Complex (mol%)
Conditions
Conversion [%][a]
1
2
CH2Cl2
C6F6
1c (20)
1c (20)
708C, 35 min, MW (200 W)
1008C, 35 min, MW (200 W)
(53)[20a]
100 (76)
[a]
Determined by 1H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
Table 7. Formation of disubstituted olefins in CM with 10.
Entry (Olefin)
Solvent
Complex (mol%)
Conditions
Product[a]/Conversion [%][b]
1 (11)
2 (11)
3 (11)
4 (11)
5 (12)
6 (12)
7 (12)
8 (12)
CH2Cl2
CH2Cl2
C6F5CF3
C6F5CF3
CH2Cl2
CH2Cl2
C6F5CF3
C6F5CF3
1c (5)
408C, 6 h
408C, 6 h
1208C, 3ꢂ10 min
1208C, 3ꢂ10 min MW (200 W)
408C, 6 h
408C, 6 h
1208C, 3ꢂ10 min
1208C, 3ꢂ10 min MW (200 W)
13/(64)[5]
13/(91)[5]
13/69
13/91 (82)
14/(70)[5]
14/(89)[5]
14/90
1e (5)
1b (3ꢂ2)[c]
1b (3ꢂ2)[c]
1c (5)
1e (5)
1b (3ꢂ2)[c]
1b (3ꢂ2)[c]
14/95 (86)
[a]
E/Z>20/1 in all cases.
[b]
[c]
1
Determined by H NMR. In parenthesis are isolated yields of analytically pure product.
Reaction protocol has been repeated three times using each time a fresh portion (2 mol%) of pre-catalyst 1b.
and 16 (Table 8). It has been reported that under
“classical” conditions these olefins in CM with 5-acet- tested in CM reaction involved synthetic ethyl chrys-
ACHTUNGTRENNUNGoxy-1-pentene (12) promoted by 1c, gave products 17 anthemate 22 (Figure 5). The challenge of this trans-
Finally, the last demanding molecule that has been
and 18 in 78% and 32% yield, respectively (entries 1 formation is related to substantial steric hindrance
and 4).[2b,5] We decided to evaluate the phosphine-con- around the geminally disubstituted alkene fragment
taining complex 1b under optimised conditions. Un- of 22, bearing additionally a bulky substituent in the
fortunately, in spite of the higher loading of pre-cata- allylic position. As expected, only a low conversion of
lyst 1b (8 mol% instead of 5 mol%) at elevated tem- 22 in the CM reaction was observed under “classical”
perature (1208C), we obtained only 50% and 41% of conditions (toluene-d8, 1208C, sealed vial). However,
conversion for 17 and 18 in FAH solvent (entries 2 the desired olefin 23 was obtained as a mixture of iso-
and 5; C6F5CF3, 40 min.). The use of microwave irra- mers (E/Z=4.5:1) in 64% NMR yield by applying mi-
diation led to increased conversions in both cases – crowave irradiation in C6F5CF3 at 1208C. The possibil-
up to 61% and 67% (entries 3 and 6).
ity of modifying the prenyl fragment (CH=CMe2) via
Another challenging molecule that has been modi- CM[21] opens a new and promising approach in the
fied in the CM reaction was S-(+)-carvone (20), a ter- elaboration of new building blocks based on the natu-
pene derivative (Figure 4). Regrettably, after an opti- rally occurring compounds.
misation process of the CM between 19 and 20 cata-
These CM reactions show that, indeed, there are
lysed by 20 mol% of 1b in optimal reaction conditions still a number of attractive olefinic building blocks,
(microwave irradiation in C6F5CF3 at 1208C) the reac- which are unfortunately, very challenging substrates
tion led only to a 33% isolated yield of 21.
for metathetical transformations. Therefore, the con-
1998
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Microwave-Assisted Ruthenium-Catalysed Olefin Metathesis in Fluorinated Aromatic Hydrocarbons
Table 8. Formation of trisubstituted olefins in CM with 12.
Entry (Olefin)
Solvent
Complex (mol%)
Conditions
Product/Conversion [%][a]
1 (15)
2 (15)
3 (15)
4 (16)
5 (16)
6 (16)
CH2Cl2
C6F5CF3
C6F5CF3
CH2Cl2
C6F5CF3
C6F5CF3
1c (5)
408C, 24 h
1208C, 4ꢂ10 min
1208C, 4ꢂ10 min MW (200 W)
408C, 24 h
1208C, 4ꢂ10 min
17/(78)[5]
17/50
17/61 (56)
18/(32)[2b]
18/41
1b (4ꢂ2)[b]
1b (4ꢂ2)[b]
1c (5)
1b (4ꢂ2)[b]
1b (4ꢂ2)[b]
1208C, 4ꢂ10 min MW (200 W)
18/67 (62)
[a]
1
Determined by H NMR spectroscopy. In parenthesis are isolated yields of analytically pure product.
[b]
Reaction protocol has been repeated four times using each time a fresh portion (2 mol%) of pre-catalyst 1b.
1
Figure 4. Modification of terpene scaffold. [a] Determined by H NMR spectroscopy. [b] Isolated yields 33%.
[a]
[b]
[c]
Figure 5. Modification of ethyl chrysanthemate scaffold. 1:10 mixture of cis/trans isomers. E/Z=4.5/1 in all cases. De-
termined by H NMR spectroscopy. [d] Isolated yields 51%.
1
tinuous search for new reaction conditions as well as Conclusions
improvements of ruthenium pre-catalyst structures is
of vital importance for the further development of In conclusion, we have demonstrated that fluorinated
olefin metathesis methodology.
aromatic hydrocarbons used as solvent in ruthenium-
catalysed olefin metathesis are able to give higher iso-
lated yields in the challenging metathesis transforma-
tions compared to reactions performed in “classical”
solvents.[28] Combining the activating effects of fluori-
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Cezary Samojłowicz et al.
FULL PAPERS
sumed starting material was detected. All the aliquots were
diluted in C6D6 (400 mL) and the conversion of 4s was mea-
sured by 1H NMR spectroscopy.
nated aromatic hydrocarbons and microwave irradia-
tion, we have developed new attractive reaction con-
ditions for the efficient metathetical transformation of
a wide range of reluctant precursors. Especially, RCM
and en-yne cycloisomerisations of difficult substrates
have been enhanced under FAH/MW conditions. Al-
though we succeeded in showing that the combination
of FAH/MW conditions gives a better outcome, each
reaction needs to be studied carefully to find optimal
reaction conditions. The still existing limitations in
the synthesis of challenging molecules by olefin meta-
thesis, require further research in this field.
General Procedure for the RCM Reactions
Reactions performed using conventional heating: A Schlenk
flask was filled under argon with diene (0.4 mmol) and sol-
vent (2 mL) before the addition of the pre-catalyst 1b
(0.008 mmol) as a solid. Progress of the reaction was moni-
tored by TLC and 1H NMR. The solvent was removed
under vacuum and the crude residue was purified by flash
chromatography.
Microwave heating: A microwave vial was filled with
olefin (0.4 mmol) and solvent (2 mL) before the addition of
the pre-catalyst 1b (0.008 mmol). The vial was flushed with
argon and then placed in a microwave reactor and heated
for the reported time. Progress of the reaction was moni-
tored by TLC and 1H NMR. The procedure was repeated
until disappearance of the starting material. The vial content
was then transferred to a round-bottom flask where the sol-
vent was removed under vacuum and the crude residue was
purified by flash column chromatography to obtain the ana-
lytically pure product.
Ethyl 1-phenylsulfonyl-cyclopent-3-enecarboxylate (4p):
The general procedure for RCM using conventional heating
after flash chromatography on silica gel (c-hexane:EtOAc=
4:1, Rf =0.3) afforded the title compound 4p as a colorless
oil. 1H NMR (500 MHz, CDCl3): d=7.87 (d, J=8.0 Hz,
2H), 7.67 (t, J=7.5 Hz, 1H), 7.55 (dd, J=7.5, 8.0 Hz, 2H),
Experimental Section
General Considerations
Microwave-assisted reactions were performed with a CEM
Microwaves Discovery (200 W) using an infrared tempera-
ture detector. The reactions were performed in sealed vials
using MW as well as conventional heating. The solvents: oc-
tafluorotoluene (FluoroChem), hexafluorobenzene (Fluoro-
Chem), toluene (Fluka), dichloromethane (Merck) and 1,2-
dichloroethane (Aldrich) were dried by distillation over
CaH2 under argon and stored under an argon atmosphere.
Analytical thin-layer chromatography (TLC) of all reactions
was performed on silica gel 60 F254 TLC plates. Visualisation
was performed with standard potassium permanganate
stains or UV light. Flash column chromatographies were
performed using silica gel 60 (230–400 mesh). NMR spectra
were recorded using Bruker or Varian machines. Mass (EI)
spectra were recorded on an AMD 604 Intectra GmbH
spectrometer. Mass (ESI) spectra were recorded on Mariner
Perseptive Biosystems, Inc. instrument. IR spectra were re-
corded on a Perkin–Elmer Spectrum One FT-IR spectrome-
ter with a diamond ATR accessory. Elemental analyses were
performed by Institute of Organic Chemistry, PAS, Warsaw.
Glassware was either oven dried or flame dried, and reac-
tions were performed under an argon atmosphere unless
otherwise noted. All reagents were used as received. Cata-
lysts were purchased and used as received: 1a, 1b, 1c, 1e
(Aldrich), 1d (Apeiron-Catalysts) and 1g (Wcat SYSTEMꢃ).
The following substrates (2s,[10a] 3s,[22] 5s,[13b] 8s,[24] 9s,[25] 10[26]
and 16[27]) were prepared according to the literature proce-
dures. The following products (2p,[10a] 3p,[23] 5p,[13b] 8p’,[19c]
9p,[20a] 13,[5a] 14,[5a] 17[5a] and 18[5a]) have been described pre-
viously and were identified by comparison of their physical
and spectroscopic data (1H and 13C NMR, MS) with those in
the cited references and their purities were confirmed by
GC analysis.
5.62 (s, 2H), 4.13 (q, J=7.1 Hz, 2H), 3.22 (AB, JAB
=
16.0 Hz, 4H), 1.18 (t, J=7.1 Hz, 3H); 13C NMR (125 MHz,
CDCl3): d=168.5 (C), 136.8 (C), 134.0 (CH), 129.9 (CH),
128.7 (CH), 127.5 (CH), 78.0 (C), 62.6 (CH2), 38.8 (CH2),
13.7 (CH3): IR (film): n=3066, 2984, 2939, 1732, 1584, 1478,
1448, 1367, 1309, 1269, 1206, 1147, 1084, 1055, 1016, 952,
861, 759, 722, 689, 632, 595, 556 cmÀ1; HR-MS (ESI+): m/z=
303.0649 [M+Na]+, calcd. for C14H16O4SNa: 303.0662; elem.
anal. calcd. for C14H16O4S: C 59.98, H 5.75, S 11.44; found:
C 60.04, H, 5.58, S 11.33.
N-Benzyl-5-chloro-1,6-dihydropyridin-2(3H)-one
(6p):
The general procedure for RCM using microwave heating
after flash chromatography on silica gel (c-hexane/EtOAc=
9:1; Rf =0.2) afforded the title compound 6p as colorless
1
crystals; mp 64–678C. H NMR (400 MHz, CDCl3): d=7.56–
7.27 (m, 5H), 5.83 (m, 1H), 4.64 (s, 2H), 3.88 (m, 2H), 3.14
(m, 2H); 13C NMR (100 MHz, CDCl3): d=165.5 (C), 135.9
(C), 128.7 (CH), 128.2 (CH), 127.8 (CH), 123.6 (C), 120.1
(CH), 51.9 (CH2), 49.6 (CH2), 32.7 (CH2). IR (KBr): n=
2923, 1650, 1487, 1453, 1408, 1360, 1332, 1289, 1254, 1204,
1174, 1095, 1076, 1029, 1015, 944, 877, 816, 766, 736, 700,
642, 500 cmÀ1; HR-MS (EI+): m/z=221.0600 [M]+C, calcd.
for C12H12NO35Cl: 221.0607.
Procedure for the Stability Studies
General Procedure for the En-Yne Reactions
A
Schlenk apparatus was filled with the pre-catalyst
Microwave heating: A microwave vial was filled with en-
yne precursor (0.5 mmol) and solvent (5 mL) before the ad-
dition of the pre-catalyst (0.01 mmol). The vial was flushed
with argon and then placed in a microwave reactor and
heated for the reported time. Progress of the reaction was
monitored by TLC and 1H NMR. The procedure was repeat-
ed until disappearance of the starting material. The vial con-
(0.002 mmol) and the C6D6 or C6F6 solvent (2 mL) under an
argon atmosphere and then substrate 4s (0.4 mmol) was
added. After 5 min of reaction, a small amount (50 mL) of
the reaction media was taken for NMR monitoring followed
by another loading of 4s (0.4 mmol). This procedure was re-
peated every 5 min until a significant amount of uncon-
2000
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1993 – 2002

Microwave-Assisted Ruthenium-Catalysed Olefin Metathesis in Fluorinated Aromatic Hydrocarbons
tent was then transferred to a round-bottom flask where the
solvent was removed under vacuum and the crude residue
was purified by flash column chromatography to obtain the
analytically pure product.
anal. calcd. for C13H20O4: C 64.98, H 8.39; found: C 64.99, H
8.24.
5-Chloro-3-methylene-2,2-diphenyl-3,6-dihydro-2H-pyran
(7p): The general procedure yielded after flash chromatog-
raphy on silica gel (pentane, Rf =0.1) afforded the title com-
Acknowledgements
1
CS thanks the Foundation for Polish Science for financial
support. The project ꢀVentures/2008–1/5ꢁ is realized within the
ꢀVenturesꢁ program of Foundation for Polish Science, co-fi-
nanced from European Union, Regional Development Fund.
MM and EB gratefully thank the CNRS, the Ministꢂre de la
Recherche, “Bretagne-Valorisation”, Rennes-Mꢃtropole and
the “Rꢃgion Bretagne” for financial support. KG, EB and
MM thank the European Community for its financial support
through the seventh framework program through grant (CP-
FP 211468–2 EUMET). We are grateful to Mr. Dariusz Kra-
pound 7p as a yellow oil. H NMR (400 MHz, CDCl3): d=
7.40–7.30 (m, 10H), 6.20 (s, 1H), 5.83 (s, 1H), 5.32 (s, 1H),
4.46 (s, 2H); 13C NMR (100 MHz, CDCl3): d=142.5 (C),
142.3 (C), 142.2 (C), 128.1 (CH), 128.0 (CH), 127.9 (CH),
117.8 (CH), 111.4 (CH2) , 92.4 (C), 69.9 (CH2). IR (film):
n=3405, 3096, 3071, 2955, 1881, 1782, 1643, 1624, 1595,
1583, 1493, 1461, 1376, 1323, 1279, 1226, 1202, 1192, 1175,
1135, 1093, 1074, 1007, 959, 827, 669, 639, 628, 509 cmÀ1
;
HR-MS (EI+): m/z=282.0806 [M]+C, calcd. for C18H15O35Cl:
282.0811.
´
jewski and Mr. Marcin Sniez˙ek for the preparation of com-
pounds 4s and 9s, respectively.
General Procedure for the CM Reactions
Microwave heating: A microwave vial was filled with olefin References
(1 mmol), deactivated olefin partner (0.5 mmol) and solvent
(2 mL) before the addition of the pre-catalyst 1b
(0.025 mmol). The vial was flushed with argon and then
placed in a microwave reactor and heated for the reported
time. Progress of the reaction was monitored by TLC and
1H NMR. The procedure was repeated until disappearance
of the starting material. The vial content was then trans-
ferred to a round-bottom flask where the solvent was re-
moved under vacuum and the crude residue was purified by
flash column chromatography to obtain the analytically pure
product.
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tate (21): The general procedure yielded after flash chroma-
tography on silica gel (c-hexane:EtOAc=9:1; Rf =0.1) af-
forded the title compound 21 as a brownish oil. 1H NMR
(600 MHz, CDCl3) d=6.76–6.43 (m, 1H), 5.40 (tt, J=1.2,
6.9 Hz, 1H), 4.61 (d, J=6.9 Hz, 2H), 2.73–2.67 (m, 1H),
2.56 (ddd, J=1.6, 3.8, 15.9 Hz, 1H), 2.44–2.28 (m, 3H), 2.06
(s, 3H), 1.79 (s, 3H), 1.72 (s, 3H); 13C NMR (150 MHz,
CDCl3): d=199.4 (C), 170.9 (C), 144.4 (CH), 142.4 (C),
135.5 (C), 119.1 (CH), 61.1 (CH2), 43.9 (CH), 42.8 (CH2),
30.9 (CH2), 20.9 (CH3), 15.7 (CH3), 14.6 (CH3); IR (film):
n=2953, 2924, 1981, 1935, 1738, 1674, 1435, 1368, 1232,
1112, 1024, 958, 903, 799, 706, 607, 494, 444 cmÀ1; HR-MS
(EI+): m/z=222.1250 [M]+C, calcd. for C13H18O3: 222.1256.
ACHTUNGTRENNUNG(E/Z)-Ethyl 3-(3-Acetoxyprop-1-enyl)-2,2-dimethylcyclo-
propanecarboxylate (23): The general procedure yielded
after flash chromatography on silica gel (c-hexane-EtOAc=
20:1; Rf =0.1) afforded the title compound 23 as a brownish
oil. 1H NMR (500 MHz, CDCl3): d=5.75 (td, J=6.5,
15.3 Hz, 1H), 5.53 (dd, J=8.7, 15.3 Hz, 1H), 4.53 (d, J=
6.5 Hz, 2H), 4.18–4.07 (m, 2H), 2.06 (s, 3H), 1.57 (d, J=
5.3 Hz, 1H), 1.29–1.24 (m, 7H), 1.17 (s, 3H); 13C NMR
(125 MHz, CDCl3): d=171.7 (C), 170.8 (C), 133.1 (CH),
125.9 (CH), 64.8 (CH2), 60.4 (CH2), 35.3 (CH3), 34.2 (CH),
28.7 (C), 22.1 (CH3), 21.0 (CH), 20.2 (CH3), 14.3 (CH3); IR
(film): n=2981, 2953, 2876, 1741, 1726, 1668, 1459, 1448,
1430, 1380, 1341, 1277, 1231, 1177, 1148, 1115, 1096, 1075,
1057, 1026, 967, 844, 607 cmÀ1; HR-MS (ESI+): m/z=
263.1264 [M+Na]+, calcd. for C13H20O4Na: 263.1254 ; elem.
Adv. Synth. Catal. 2011, 353, 1993 – 2002
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2001

Cezary Samojłowicz et al.
FULL PAPERS
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asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1993 – 2002