Tandem catalysis of ring-closing metathesis/ atom transfer radical reactions with homobimetallic ruthenium-arene complexes

Article abstract of DOI:10.3762/bjoc.6.133

The tandem catalysis of ring-closing metathesis/atom transfer radical reactions was investigated with the homobimetallic ruthenium-indenylidene complex [(p-cymene)Ru(μ-Cl)₃RuCl(3-phenyl-1-indenylidene)(PCy ₃)] (1) to generate active species in situ. The two catalytic processes were first carried out independently in a case study before the whole sequence was optimized and applied to the synthesis of several polyhalogenated bicyclic γ-lactams and lactones from αω-diene substrates bearing trihaloacetamide or trichloroacetate functionalities. The individual steps were carefully monitored by ¹H and ³¹P NMR spectroscopies in order to understand the intimate details of the catalytic cycles. Polyhalogenated substrates and the ethylene released upon metathesis induced the clean transformation of catalyst precursor 1 into the Ru(II)-Ru(III) mixed-valence compound [(p- cymene)Ru(μ-Cl)₃RuCl ₂(PCy₃)], which was found to be an efficient promoter for atom transfer radical reactions under the adopted experimental conditions.

Full text of DOI:10.3762/bjoc.6.133

Tandem catalysis of ring-closing metathesis/
atom transfer radical reactions with homobimetallic
ruthenium–arene complexes
Yannick Borguet1, Xavier Sauvage1, Guillermo Zaragoza2,
Albert Demonceau1 and Lionel Delaude*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1167–1173.
1Laboratory of Macromolecular Chemistry and Organic Catalysis,
Institut de Chimie (B6a), Université de Liège, Sart-Tilman par 4000
Liège, Belgium and 2Unidade de Raios X, Edificio CACTUS,
Universidade de Santiago de Compostela, Campus Vida, 15782
Santiago de Compostela, Spain
doi:10.3762/bjoc.6.133
Received: 23 September 2010
Accepted: 09 November 2010
Published: 08 December 2010
Email:
Guest Editor: K. Grela
Lionel Delaude* - l.delaude@ulg.ac.be
© 2010 Borguet et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
Grubbs catalyst; indenylidene ligands; Kharasch reaction; microwave
heating; olefin metathesis
Abstract
The tandem catalysis of ring-closing metathesis/atom transfer radical reactions was investigated with the homobimetallic ruthe-
nium–indenylidene complex [(p-cymene)Ru(μ-Cl)3RuCl(3-phenyl-1-indenylidene)(PCy3)] (1) to generate active species in situ.
The two catalytic processes were first carried out independently in a case study before the whole sequence was optimized and
applied to the synthesis of several polyhalogenated bicyclic γ-lactams and lactones from α,ω-diene substrates bearing trihalo-
acetamide or trichloroacetate functionalities. The individual steps were carefully monitored by 1H and 31P NMR spectroscopies in
order to understand the intimate details of the catalytic cycles. Polyhalogenated substrates and the ethylene released upon
metathesis induced the clean transformation of catalyst precursor 1 into the Ru(II)–Ru(III) mixed-valence compound [(p-
cymene)Ru(μ-Cl)3RuCl2(PCy3)], which was found to be an efficient promoter for atom transfer radical reactions under the adopted
experimental conditions.
Introduction
During the course of our investigations on homobimetallic Contrastingly, this catalyst precursor was almost inactive in the
ruthenium–arene complexes, we found that the indenylidene self-metathesis of styrene, as stilbene formation leveled off after
compound [(p-cymene)Ru(μ-Cl)3RuCl(3-phenyl-1-indenyl- a few minutes without going past the 10% threshold. We attri-
idene)(PCy3)] (1) was a very efficient promoter for the ring- buted this negative result to a rapid degradation of the active
closing metathesis (RCM) of diethyl 2,2-diallylmalonate [1]. species via a bimolecular pathway leading to the ethylene com-
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Scheme 1: Reaction of homobimetallic ruthenium–indenylidene complex 1 with ethylene.
plex [(p-cymene)Ru(μ-Cl)3RuCl(η2-C2H4)(PCy3)] (2). Support were applied to olefin metathesis in tandem with ATRA [11],
in favor of this hypothesis came from the observation that com- ATRC [11-14], ATRP [15-18], cyclopropanation [19], dihydro-
plex 1 reacted quantitatively with ethylene at 40 °C to afford xylation [20], hydrogenation [21-23], hydrovinylation [24],
product 2 [1], which is completely devoid of metathetical isomerization [25-28], oxidation [29], or Wittig reactions [30],
activity (Scheme 1) [2]. Moreover, early work from Grubbs and to name just a few [31].
co-workers had shown that bimetallic ruthenium–methylidene
or ethylidene complexes decomposed rapidly to afford an In this contribution, we investigate the tandem catalysis of
unidentified ruthenium–ethylene species [3]. We were able to RCM/ATRC reactions with homobimetallic ruthenium–indenyl-
isolate and characterize this product, which turned out to be idene complex 1 to generate active species in situ. The two
complex 2 [1]. The synthesis of this compound was first catalytic processes were first carried out independently in a case
reported in 2005 by Severin et al. who successfully used it as a study before the whole sequence was optimized and applied to
catalyst for atom transfer radical addition (ATRA) and cycliza- the synthesis of several polyhalogenated bicyclic γ-lactams and
tion (ATRC) reactions [4,5]. In 2007, we further extended its lactones.
application field to the related process of atom transfer radical
polymerization (ATRP) [2].
Results and Discussion
2,2,2-Trichloro-N-(octa-1,7-dien-3-yl)acetamide (4) was chosen
Because the transformation of complex 1 into compound 2 as a model substrate to begin our investigations (Scheme 3).
occurs seamlessly in the presence of ethylene, which is a The RCM of this functionalized α,ω-diene was carried out in
byproduct of many metathesis reactions, we reasoned that it toluene-d8 (0.2 M) at 30 °C in the presence of 5 mol % of cata-
could serve to trigger a change in mechanism, thereby allowing lyst precursor 1 and monitored by 1H NMR spectroscopy.
us to perform two consecutive catalytic cycles in a single proce- Under these conditions, ring closure took place readily and a
dure (Scheme 2). This process, known as assisted tandem ca- full conversion of the substrate into its cyclohexene derivative 5
talysis [6], presents significant advantages over multistep syn- was achieved within 20 minutes. At this temperature, the
thesis for increasing molecular complexity, particularly in terms second step of ATRC did not occur. Previous work had shown
of time- and cost-savings, atom economy, environmental friend- that a significant thermal activation was required to perform the
liness, or applicability to diversity-oriented high-throughput radical cyclization of this cyclohexenyl trichloroacetamide,
synthesis [7-10]. The monometallic ruthenium–benzylidene presumably due to the unfavorable disposition of the
complex [RuCl2(=CHPh)(PCy3)2] (3) and its second- or even trichloromethyl unit and the endocyclic double bond in the most
third-generation analogues developed by Grubbs and stable rotamer of the amido group [11]. Hence, this preliminary
co-workers are prominent examples of catalyst precursors that experiment allowed us to determine the nature of the catalytic
Scheme 2: Schematic illustration of tandem assisted catalysis with complexes 1 and 2.
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Beilstein J. Org. Chem. 2010, 6, 1167–1173.
Scheme 3: Tandem RCM/ATRC of 2,2,2-trichloro-N-(octa-1,7-dien-3-yl)acetamide (4) catalyzed by complex 1.
species present in the reaction mixture after the metathesis step. tively. In this case, however, 31P NMR analysis of the reaction
No meaningful information could be obtained by 31P NMR mixture revealed the presence of at least five different ruthe-
spectroscopy even when acquisition was prolonged overnight to nium–phosphine species in solution. Unless all these species are
compensate for the low catalyst concentration in the sample. able to promote the ATRC reaction, the catalytic switch
Visual inspection of the NMR tube revealed, however, the for- required to complete the tandem process should therefore be far
mation of a phosphorus-containing precipitate. Suitable crystals less efficient with monometallic benzylidene complex 3 than
for X-ray diffraction analysis were obtained by repeating the with bimetallic indenylidene precursor 1.
RCM experiment on a larger scale in toluene at room tempera-
ture. Their structure was solved and assigned to the paramag- Next, we investigated separately the ATRC of 2,2,2-trichloro-
netic complex [(p-cymene)Ru(μ-Cl)3RuCl2(PCy3)] (7). This N-(cyclohex-2-en-1-yl)acetamide (5) with different catalyst
mixed valence Ru(II)–Ru(III) compound had already been precursors. The starting material employed in these experi-
isolated and fully characterized by Severin and co-workers ments was prepared by trichloroacetylation of 2-cyclohexenol
when they investigated the reaction of ethylene complex 2 with with CCl3COCl in the presence of Et3N [34]. This procedure
carbon tetrachloride in toluene [4]. Yet, differences between the guaranteed the absence of any residual metal catalyst coming
molecular structures obtained by the Swiss team and our group from the RCM reaction. A solution of 2-cyclohexenyl trichloro-
indicate that complex 7 can adopt various crystalline structures acetamide 5 in toluene was heated for 2 h at 160 °C in the pres-
(see Supporting Information File 2 for more details on crystal ence of various ruthenium initiators (1 mol %). Conversion into
structures and Supporting Information File 3 for X-ray crystal racemic product 6 was then determined by GC analysis of the
data).
reaction mixture (Scheme 3 and Table 1). Previous work from
the group of Itoh et al. had already established that the ruthe-
The clean transformation of catalyst precursor 1 into compound nium-catalyzed cyclization of N-allyl trichloroacetamides
7 induced by polyhalogenated substrates is in line with the proceeded diastereoselectively, and a mechanism accounting for
general mechanism postulated for ruthenium-catalyzed atom
transfer radical reactions, as it involves a reversible oxidation of
Table 1: ATRC of 2,2,2-trichloro-N-(cyclohex-2-en-1-yl)acetamide (5)
the metal center [32,33]. Under the experimental conditions
adopted for our study, conversion of indenylidene precursor 1
into labile ethylene complex 2 probably occurred rapidly upon
release of ethylene in the reaction mixture by the RCM of sub-
strate 4 (Scheme 3). The low concentration of compound 2 in
solution prevented, however, its instantaneous detection by
NMR spectroscopy. When a longer acquisition time was
applied, only oxidized product 7 was obtained. It should be
noted that when the Grubbs first-generation catalyst 3 was
allowed to react with substrate 4 for 2 h in toluene-d8 at room
temperature, the RCM product 5 was also formed quantita-
catalyzed by various ruthenium complexes.a
Entry
Ru cat.
Conversion (%)b
1
2
3
4
5
–
1
2
3
7
0
94
98
55
97
aExperimental conditions: substrate (0.2 mmol), catalyst (2 μmol),
toluene (1 mL) in a sealed tube under Ar for 2 h at 160 °C.
bDetermined by GC with n-dodecane as internal standard.
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Beilstein J. Org. Chem. 2010, 6, 1167–1173.
the formation of a cis-fused bicyclic system was proposed [35]. sis [11,13,14]. These experiments were conducted on a prepara-
Nuclear Overhauser effects also indicated that the angular H-3a tive microscale in sealed tubes under inert atmosphere. Sub-
and the CHCl H-4 protons were trans to each other.
strate 4 and complex 1 were dissolved in toluene. A color
change from red to orange occurred within a few minutes,
Because radical reactions may occur spontaneously at high which indicated the formation of metathetically active species.
temperature, we first carried out a blank test in the absence of Stirring was prolonged for 2 h at 25 °C. The vessel was then
an initiator (Table 1, entry 1). This experiment confirmed the heated in an oil bath to trigger the ruthenium-catalyzed cycliza-
necessity of mediating the transformation of 5 into 6 with a tion of intermediate 5 into 3,3,4-trichlorohexahydro-1H-indol-
transition metal complex. Unlike the Grubbs benzylidene cata- 2(3H)-one (6) (Scheme 3). This final product was isolated by
lyst 3, bimetallic compound 1 was an efficient catalyst column chromatography. Its identity and purity were confirmed
precursor for this reaction (Table 1, entries 2 and 4). Opstal and by 1H and 13C NMR analyses. Table 2 summarizes the results
Verpoort had already established that monometallic of these experiments.
ruthenium–indenylidene complexes were able to promote the
ATRA and ATRP of vinyl monomers [36,37]. In a tandem When a 5 mol % catalyst loading was employed and the ATRC
RCM/ATRC process, it is, however, very unlikely for the reaction was allowed to proceed for 2 h at 160 °C, bicyclic
indenylidene species to remain unaltered in solution after the lactam 6 was isolated in 71% yield (Table 2, entry 1). We were
metathesis step. Indeed, during the course of our investigations pleased to note that homobimetallic complex 1 slightly outper-
on the RCM of various α,ω-dienes catalyzed by complex 1, 31P formed the Grubbs first-generation catalyst 3, which led to a
NMR monitoring of the reaction media always showed a rapid 61% yield under identical conditions (Table 2, entry 2). We
disappearance of the signal originating from this precatalyst, tried to further optimize the catalytic process by reducing the
and its replacement by a new singlet at ca. 40.5 ppm due to the reaction temperature and the catalyst loading. Performing the
ethylene complex 2. As expected, this compound was highly second step at 110 °C completely inhibited the cyclization as
suitable for catalyzing the ATRC of 5 (Table 1, entry 3). To our evidenced by GC analysis, which revealed a complete conver-
great satisfaction, oxidation product 7 was equally active under sion of substrate 4 into intermediate 5, but did not show any
the experimental conditions adopted for this cyclization and did sign of product 6 formation (Table 2, entry 3). On the other
not require any co-catalyst (Table 1, entry 5). This result hand, it was possible to accomplish the dual catalysis at 160 °C
contrasts with previous observations from Severin and with only 1 mol % of catalyst precursor 1 (Table 2, entry 4).
co-workers, who found that the presence of a radical initiator or The slight increase in isolated yield compared to run #1 should
a reducing agent (typically Mg) was mandatory to activate com- not be over interpreted. It probably reflects the systematic errors
plex 7 for ATRA and ATRC reactions at room temperature [5]. in the weighing of the reagents and in the chromatographic
At 160 °C, a reduction of the mixed Ru(II)–Ru(III) compound purification of the product formed. A control experiment carried
probably takes place under the sole influence of radicals gener- out with [RuCl2(=CHPh)(PCy3)2] 3 confirmed the superiority
ated via thermal dissociation of the substrate.
of the bimetallic system under these conditions (Table 2, entry
5). Attempts to further decrease the molar ratio of complex 1
In order to complete the full sequence of RCM and ATRC reac- remained unsuccessful (see Table 2, entry 6 for a representative
tions, we carried out a third series of catalytic tests based on example). Finally, we were able to significantly shorten and
literature procedures developed for this type of tandem cataly- simplify the whole process through the use of a monomodal
Table 2: Tandem RCM/ATRC of 2,2,2-trichloro-N-(octa-1,7-dien-3-yl)acetamide (4).
Entry
Catalyst precursor
Exp. conditions for RCM
Exp. conditions for ATRCa
Isolated yield of product 6 (%)
1
2
3
4
5
6
7
1 (5 mol %)
3 (5 mol %)
1 (5 mol %)
1 (1 mol %)
3 (1 mol %)
1 (0.5 mol %)
1 (1 mol %)
25 °C, 2 h
25 °C, 2 h
40 °C, 2 h
25 °C, 30 min
25 °C, 2 h
–
Δ, 160 °C, 2 h
Δ, 160 °C, 2 h
Δ, 110 °C, 2 h
Δ, 160 °C, 2 h
Δ, 160 °C, 2 h
Δ, 160 °C, 2 h
μw, 160 °C, 40 min
71
61
0b
76
20
0b
73
–
aΔ: conductive heating in an oil bath, μw: microwave heating in a monomodal reactor.
bOnly RCM product 5 was present.
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microwave reactor (Table 2, entry 7). Such a device is catalyst precursor 1 (see Supporting Information File 1 for
becoming increasingly popular in organic synthesis and has details). As expected, N-benzyl trichloroacetamide 8 underwent
already been used as a convenient heating source for numerous the RCM/ATRC sequence at a lower temperature than its parent
ruthenium-catalyzed reactions [38].
4 (110 °C vs 160 °C), although the reaction time had to be
extended in order to achieve a full conversion into bicyclic
Because thermal degradation of the catalyst is likely to occur at lactam 9 (Table 3, entry 1). The reaction of N-tosyl trichloro-
the high temperature required to promote the ATRC of 2-cyclo- acetamide 10 proceeded faster, but followed a different course,
hexenyl trichloroacetamide 5, we searched for alternative as demonstrated by the isolation of a diastereomeric mixture of
substrates that would allow us to perform the tandem reaction unsaturated bicyclic lactams 11 (Table 3, entry 2). The two
under less drastic conditions. We were guided in this endeavor products were separated by column chromatography. 2D NMR
by Snapper et al. who had shown that adding a benzyl or tosyl spectroscopy and mass spectrometry analyses confirmed that
group to the amide functionality of compound 5 facilitated its dehydrochlorination had occurred during the catalytic process.
radical cyclization mediated by complex 3. Replacing the Further work is underway to rationalize this change of reaction
trichloroacetamide moiety with the corresponding tribromoac- path and to address all its stereochemical implications.
etamide unit was also found to enhance the Kharasch reactivity
[11]. Thus, we synthesized four additional N-protected octadi- With the N-benzyl tribromoacetamide 12, the RCM step
enyl trichloro- or tribromoacetamide substrates and we fol- proceeded swiftly at 25 °C, but ATRC did not occur at this
lowed their transformation under the influence of bimetallic temperature, in sharp contrast with previous results obtained by
Table 3: Reactions of various octadienyl trichloro- or tribromoacetamide substrates catalyzed by complex 1.a
Entry
Substrate
Product(s)
Conditions
Isolated yield (%)
1
μw, 110 °C, 4 h
89%
2
3
4
μw, 110 °C, 2 h
25 °C, 2.5 h
25 °C, 2.5 h
73%b
89%
96%
aExperimental conditions: substrate (0.3 mmol), catalyst (3 μmol), and toluene (1.5 mL) stirred in a pressure vessel under Ar in a monomodal
microwave reactor (μw, 110 °C) or in a thermostated oil bath (25 °C).
bDiastereomeric ratio: 44:56.
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Beilstein J. Org. Chem. 2010, 6, 1167–1173.
Snapper et al. with catalyst precursor 3 [11]. Work-up afforded trichloroacetic acid onto cyclopentadiene followed by a
only cyclohexenyl tribromoacetamide 13 in high yield (Table 3, lactonization into 3,3-dichloro-3,3a,4,6a-tetrahydro-2H-cyclo-
entry 3). Attempts to promote the cyclization of this compound penta[b]furan-2-one (18). These results prompted us to examine
at higher temperatures led to a complex mixture of uncharacter- the cascade RCM/decomposition/ATRA/lactonization of sub-
ized products. A final experiment carried out with the N-tosyl strate 16 into dichloro compound 18 with homobimetallic com-
tribromoacetamide 14 afforded quantitatively the cyclohexenyl plex 1 under mild thermolysis conditions. Thus, the substrate
dibromoacetamide 15 (Table 3, entry 4). We suspected that the and the catalyst precursor (5 mol %) were dissolved in toluene
high Kharasch reactivity of the starting material or the inter- and heated for 1 h at 80 °C under microwave irradiation. Under
mediate RCM product caused a radical transfer to the solvent. these conditions, product 18 was isolated in 52% yield after
However, performing the reaction in benzene or chromatographic purification. 1H NMR data matched those
dichloromethane instead of toluene led to similar outcomes. reported for a sample known to possess a cis stereochemistry
Adding a larger amount of complex 1 (5 mol %) to substrates for its bridgehead hydrogens [39].
12 and 14 did not seem to improve the ATRC step.
Conclusion
To complement the data acquired with trihaloacetamide starting In this study, we have demonstrated that homobimetallic ruthe-
materials, we also investigated the transformation of hepta-1,6- nium–indenylidene complex 1 is a suitable catalyst precursor
dien-3-yl 2,2,2-trichloroacetate 16 in the presence of ruthe- for the tandem RCM/ATRC of polyhalogenated α,ω-dienes 4
nium–indenylidene catalyst precursor 1 (5 mol %). A prelimi- and 8 into the corresponding bicyclic γ-lactam derivatives. A
nary experiment was carried out in toluene-d8 and monitored by more complex cascade sequence involving RCM and ATRA
1H NMR spectroscopy. The RCM of the α,ω-diene occurred reactions afforded γ-lactone 18 starting from acyclic unsatu-
readily at 25 °C and a quantitative conversion into cyclopen- rated ester 16. The individual steps were carefully monitored by
tene derivative 17 was achieved within 1 h (Scheme 4). The 1H and 31P NMR spectroscopies in order to understand the inti-
temperature was then raised to 60 °C in an attempt to initiate an mate details of the catalytic cycles. The RCM of model sub-
ATRC reaction. Under these conditions, 1H NMR analysis strate 4 into cyclohexenyl trichloroacetamide 5 was accompa-
unambiguously revealed the formation of cyclopentadiene nied by a clean transformation of complex 1 into mixed-valence
instead of the expected bicyclic lactone. The decomposition of bimetallic scaffold 7. This well-defined compound was an effi-
cyclopentenyl trichloroacetate 17 into cyclopentadiene and cient promoter for the ATRC of intermediate 5 into the final
trichloroacetic acid was already observed by Quayle et al. under product 6.
similar conditions [13,14]. These authors successfully trapped
the diene via a Diels–Alder reaction with maleic anhydride.
Supporting Information
They also reported that a heterobimetallic catalytic system
derived from the Grubbs second-generation complex
Full experimental procedures and spectral data for the new
[RuCl2(=CHPh)(SIMes)(PCy3)], CuCl, and dHbipy (SIMes is
compounds, detailed crystallographic analysis of
1,3-dimesitylimidazolin-2-ylidene, dHbipy is 4,4'-di-n-heptyl-
[(p-cymene)Ru(μ-Cl)3RuCl2(PCy3)] (7), and a cif file with
2,2'-bipyridine) was able to promote the ATRA of
crystallographic data for complex 7.
Supporting Information File 1
Experimental procedures and spectral data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-133-S1.pdf]
Supporting Information File 2
Detailed crystallographic analysis of complex 7.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-133-S2.pdf]
Supporting Information File 3
X-ray crystal data for complex 7.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-133-S3.cif]
Scheme 4: Ruthenium catalyzed transformation of substrate 16.
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Beilstein J. Org. Chem. 2010, 6, 1167–1173.
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doi:10.1021/jo0525635
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