New indem lidene-schiff base-ruthenium complexes for crossmetathesis and ring-closing metathesis

Article abstract of DOI:10.1002/adsc.200900477

We here report on the stability and catalytic activity of new indenylidene-Schiff base-ruthenium complexes 3a-f through representative cross-metathesis (CM) and ring-closing metathesis (RCM) reactions. Excellent activity of the new complexes was found for the two selected RCM reactions; prominent conversion was obtained compared to the commercial Hoveyda-Grubbs catalyst 2. Moreover, excellent results were obtained for a standard CM reaction. Higher conversions were achieved with one of the indenylidene catalysts compared with HoveydaGrubbs catalyst. Unexpectedly, an isomerization reaction was observed during the CM reaction of allylbenzene. To the best of our knowledge, isomerization reactions in this model CM reaction in closed systems have never been described using first generation catalysts, including the Hoveyda-Grubbs catalyst. The first model CM reactions as well as the RCM reactions have been monitored using ¹H NMR. The course of the CM reaction of 3-phenylprop-1-ene (8) and di-1,4-diacetoxybut-2-ene (9) was monitored by GC. The isomerization reaction was studied by means of GC-mass spectrometry and in situ IR spectroscopy. All catalysts were structurally characterized by means of ¹H, ¹³C, and ³¹P NMR spectroscopy.

Full text of DOI:10.1002/adsc.200900477

FULL PAPERS
DOI: 10.1002/adsc.200900477
New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-
Metathesis and Ring-Closing Metathesis
Ana M. Lozano Vila,a Stijn Monsaert,a Renata Drozdzak,a Stanislaw Wolowiec,b
and Francis Verpoorta,*
a
Department of Inorganic and Physical Chemistry, Laboratory of Organometallic Chemistry and Catalysis, Ghent
University, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
Fax : (+32)-9-264-4983; phone: (+32)-9-264-4436; e-mail: francis.verpoort@UGent.be
Department of General Chemistry and Electrochemistry, Rzeszow University of Technology, Al. Powstancow Warrszawy
b
6, PO Box 85, 35-959 Rzeszow, Poland
Received: July 9, 2009; Revised: September 16, 2009; Published online: October 21, 2009
Abstract: We here report on the stability and catalyt- tems have never been described using first genera-
ic activity of new indenylidene-Schiff base-ruthenium tion catalysts, including the Hoveyda–Grubbs cata-
complexes 3a–f through representative cross-meta- lyst. The first model CM reactions as well as the
1
thesis (CM) and ring-closing metathesis (RCM) reac- RCM reactions have been monitored using H NMR.
tions. Excellent activity of the new complexes was The course of the CM reaction of 3-phenylprop-1-
found for the two selected RCM reactions; promi- ene (8) and cis-1,4-diacetoxybut-2-ene (9) was moni-
nent conversion was obtained compared to the com- tored by GC. The isomerization reaction was studied
mercial Hoveyda–Grubbs catalyst 2. Moreover, ex- by means of GC-mass spectrometry and in situ IR
cellent results were obtained for a standard CM reac- spectroscopy. All catalysts were structurally charac-
1
tion. Higher conversions were achieved with one of terized by means of H, 13C, and 31P NMR spectros-
the indenylidene catalysts compared with Hoveyda– copy.
Grubbs catalyst. Unexpectedly, an isomerization re-
action was observed during the CM reaction of allyl- Keywords: carbene ligands; cross-metathesis; indeny-
benzene. To the best of our knowledge, isomerization lidenes; ring-closing metathesis; ruthenium; Schiff
reactions in this model CM reaction in closed sys- bases
Introduction
lent stability towards air and moisture and could be
recycled several times. Various other efforts have
Olefin metathesis has evolved into a routine and com- been directed towards the modification of the Grubbs
petent method for the construction of carbon-carbon catalysts, such as the introduction of bidentate Schiff
double bonds, causing a stir with its promise of clean- base ligands,[11,12] substituted acetic acid groups,[13–16]
er, cheaper and more efficient processes. This has different alkoxides,[17,18] halides,[19] pyridines,[18,20,21]
been possible by the significant progress in catalyst phenoxides,[22,23] less electron-donating phosphines,[24]
development and applications that has been made es- several pyridinecarboxylates[25] and indenylidene
pecially during the past 15 years.[1–8] Especially ruthe- ligand substituting benzylidene.[26,27] Recently, our
nium-based compounds have shown a versatile and group has reported new robust ruthenium-indenyli-
increasing potential for organic synthesis and polymer dene complexes bearing a saturated N-heterocyclic
chemistry, such as the Grubbs first generation catalyst carbene ligand, showing both high activity and in-
1a and the Grubbs second generation catalyst 1b.[9] creased stability with an excellent application pro-
An important advance in the design of ruthenium- file.[28] In addition, the Ru-indenylidene compounds
alkylidene precatalysts was the incorporation of a bearing salicylaldimine ligands reveal an impressive
bidentate chelating carbene ligand in the Grubbs 1st stability towards air, moisture and heat. They tolerate
generation complex leading to the family commonly storage for months as solids in ambient conditions, in-
known today as the Hoveyda–Grubbs catalysts. Hov- cluding contact with air, without suffering from any
eyda et al. were the first to disclose the preparation of degradation or change in isomer ratio. In solution, no
2 by reaction of (2-isopropoxyphenyl)-diazomethane, decomposition was observed and the isomer ratio was
PCy3 and Cl2RuACHTUNGTRENNUNG
(PPh)3.[10] This catalyst showed excel- preserved over a period of months.[29] O,N-chelating
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Schiff base ligands have proven to be very useful due dene complexes have been successfully applied in
to different characteristics they provide to Ru-based olefin metathesis reactions such as cross metathesis
catalysts. Thus, the catalytic activity and stability of (CM) and ring-closing metathesis (RCM).
the ruthenium complexes can be easily tuned by vary-
ing the steric and electronic configuration of the
Schiff base. Besides, the N and O donor atoms display Results and Discussion
different electronic effects: the phenolate oxygen
(hard donor atom) stabilizes the higher oxidation In the present study, we report on the synthesis and
state of the metallic atom, while the imine nitrogen structure determination of new indenylidene-Schiff
(softer donor atom) does the same with the lower oxi- base-ruthenium complexes as well as their kinetics,
dation state of the ruthenium. Moreover, these types stability and catalytic activity compared to first gener-
of ligands can be synthesized in practically quantita- ation Hoveyda–Grubbs catalyst 2. For this benchmark
tive yields through one-step procedures.[30] Further- study, a set of carefully chosen olefin metathesis reac-
more, some salicylaldiminato-type ruthenium com- tions involving model substrates, either commercially
plexes have been water-adapted (catalysts for aqueous available or prepared by synthetic methods reported
metathesis) and are excellent for biological applica- in the literature, were applied. The main objective
tions and green chemistry.[31] Taking into account the throughout this investigation is to bring into light new
highly desirable attributes of this type of ligands, catalytic systems and their potential towards olefin
some new indenylidene Schiff base Ru-based com- metathesis reactions.
plexes have been conveniently designed and prepared
(3a–f) exhibiting a good tolerance towards organic
functionalities, air and moisture. The new indenyli- Structure of Catalysts
The synthesis of catalysts 3a–f was carried out follow-
ing the general procedure.[12] To dichloro-(3-phenyl-
1H-inden-1-ylidene)bis(tricyclohexylphosphine)ruthe-
nium(II) the appropriate amount of the Schiff base Tl
salt was added resulting in catalysts 3a–f. The synthet-
ic ease of preparation of these complexes renders this
class of ruthenium catalysts cheap alternatives to ex-
pensive commercially available compounds.
Structure Elucidation of 3a and 3d
The assignment of resonances in the 1H, 13C, and
31P NMR spectra was performed using 1-D and 2-D
homonuclear COSY and NOESY and HSQC and
HMBC (1H-13C and 1H-31P) heteronuclear experi-
ments. The NMR spectra of 3a and 3d were recorded
at 295 K in dichloromethane-d2 and CDCl3, respec-
tively.
The most characteristic features of the 1H NMR
spectrum are the presence of a singlet resonance of
indenylidene H-16 (3a) and H-15 (3d) protons at
6.88ppm (3a) and 6.70 ppm (3d), respectively, which
have been assigned via heteronuclear long-range cou-
pling with carbon C-15 (3a) and C-14 (3d) in HMBC
experiments. Interestingly, the proton H-22 of the in-
denylidene ligand in 3a (doublet at 8.38 ppm in
CD2Cl2, 8.29 ppm in CDCl3 and multiplet at 8.97 ppm
in benzene-d6) showed the same coupling with C-15.
In the case of 3d, the most downfield shifted doublet
at 8.27 ppm was assigned to H-21 of the indenylidene
moiety. This proton showed long-range coupling to
the C-14 resonance centered at 299.2 ppm in the
HMBC (1H-13C) spectrum. The H-21 doublet is scalar
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New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis
FULL PAPERS
coupled with the doublet resonance of H-20 centered
at 7.16 ppm. Further scalar coupling with H-19 and
H-18 protons was not found. These allowed assigning
1
the H and 13C signals of the indenylidene part of the
complex through homonuclear COSY/NOESY and
heteronuclear HSQC/HMBC couplings.
The 1H NMR spectrum of Schiff base ligand is
more complicated due to overlapping of the salicyli-
dene proton as well as aromatic o-methylphenyl
proton resonances in the 7.20–7.35 ppm region in 3a.
Nevertheless, the doublet resonance of the azome-
thine proton was found typically at 7.84 ppm (in 3a),
which was confirmed by a heteronuclear 1H-31P
HMBC experiment. The H-2 and H-3 resonances of
the salicylidene residue of 3a were found considerably
shifted upfield as doublet and doublet of doublets at
7.04 and 6.50 ppm, respectively. Whereas in 3d, the
Figure 1. Numbering scheme of ligands in 3a and 3d (PCy3
salicylidene resonances were observed as mutually
coupled H-2 and H-3 doublets at 6.97 and 7.96 ppm,
respectively; the H-3 doublet was further split into a
doublet with H-5. This proton gave the doublet reso-
nance at 8.02 ppm. Moreover, the protons of the in-
denylidene-attached phenyl substituent of 3d were ob-
served as a doublet (intensity 2H, H-24 at 7.30 ppm),
a triplet (intensity 2H, H-25 at 7.30 ppm) and a triplet
(intensity 1H, H-26 at 7.43 ppm), scalar coupled with
a common J=7.5 Hz.
omitted for clarity).
The 31P NMR spectrum consists of a broad singlet
resonance at 37.12 ppm in CDCl3 for 3a and one reso-
nance at 39.7 ppm in CDCl3 for 3d. The HMBC
(1H-31P) spectrum showed a series of cross-peaks be-
tween the signal at 39.7 ppm and the cyclohexyl pro-
tons and additionally, a strong cross-peak with the
proton resonance at 7.76 ppm, which was attributed
to azomethine H-7 proton.
The 13C NMR spectrum in dichloromethane showed
a series of singlet resonances, except for the C-15 and
C-16 resonances (in 3a), which were split into dou-
blets.
Figure 2. Kinetic plots of ring-closing metathesis (RCM) of
DEDAM catalyzed by 3a-f at room temperature. The lines
are only added for better visualization.
1
The complete H and 13C resonance assignments
(Figure 1) of the common ligands are collected in catalysts were 3b and 3d. The more sterically hin-
Table 4 in the Experimental Section.
dered complex, 3f, does not show the highest initia-
tion rate but it appears to be the most stable one to-
gether with 3e. Both of them show an induction
period that could be related with an increase in steric
congestion.
Kinetic Study and Stability Test
A kinetic study was conducted based on the activity
Stability tests of the catalysts were conducted by
of the Schiff base catalysts 3 towards the RCM reac- heating 20 mg of catalyst in benzene-d6 at 808C and
tion of diethyl diallylmalonate (DEDAM). As can be taking a 31P NMR spectrum every hour. As can be ob-
seen in Figure 2, the two initially catalytic more active served in Figure 3, all catalysts studied were potential-
ly more stable than 2. For instance, after the first
hour, all the indenylidene Schiff base catalysts decom-
pose slightly, whereas decomposition of the Hoveyda–
Grubbs complex amounted to 35%. After 3 h of heat-
ing, 2 is totally decomposed. In contrast, the Schiff
base indenylidene catalysts still have 40–50% of initial
catalyst left or even 86% in the case of 3f. The latter
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of this reaction.[32] Due to this enhancing performance
of CM in synthesis of natural and biologically active
products, we submitted our catalysts to CM as well.
To compare the activities of the indenylidene-Schiff
base-ruthenium-based catalysts and first generation
Hoveyda–Grubbs catalyst, we studied two different
standard CM reactions. Both of them were conducted
under identical conditions: 2.5 mol% of catalyst in
different solvents (CHCl3, toluene, CH2Cl2 and
CH2ClCH2Cl) at different temperatures (408C, 60 8C
and 808C) and under air. Important to mention is
that all reactions were carried out in closed systems.
The first standard CM reaction, between 5-hexenyl
acetate and methyl acrylate (Table 1), is a typical CM
reaction between an olefinic partner and an electron-
deficient alkene. This type of CM reaction is one of
the most attractive transformations since it affords
the functionalization of a C=C double bond.[33]
Figure 3. Stability tests of 2 and 3a–f. The lines are only
added for better visualization.
Metathesis reactions involving one substrate with
an electron-withdrawing group has started to revolu-
proved to be the most stable one, exhibiting a half- tionize fine chemical synthesis, complementing other
live of 6 h at 808C. During this stability test the gener- methods such as Wittig, Horner–Wadsworth–Emmons
ation of ruthenium hydride was checked but no peak or Heck reactions.[32] However, it still needs further
could be observed in the typical hydride range of the investigation in order to establish some rules to
1H NMR spectra. Nonetheless, the concentration of permit a good choice of catalysts and conditions for
Ru hydride could be below the detection limit of the CM of electron-deficient olefins. From a practical
NMR spectrometer.
point of view, we selected one model CM reaction
representing a relatively easy case. Comparing the
pre-catalysts, optimized reaction conditions for each
initiator are presented in Table 1. Importantly, the
cross-product 6 has been obtained with only the E-
Cross-Metathesis (CM)
Over the last 10 years, CM has begun to emerge from homodimer of 5-hexenyl acetate 7. This high E-Z se-
the shadow of ring-closing metathesis (RCM) and lectivity is principally due to the low dimerization
ring-opening metathesis polymerization (ROMP) to rate of a,b-unsaturated carbonyl compounds. Grubbs
take its place as a powerful and mild tool for the for- and co-workers were the first to prove that CM reac-
ꢀ
mation of C C bonds. This has been made possible tions between terminal alkenes and a,b-unsaturated
with the evolution of new catalysts, and a study of se- carbonyl compounds, catalyzed by a Grubbs SIMes-
lectivity, efficiency and functional group compatibility containing complex 1b (5 mol%) in CH2Cl2 at 458C,
Table 1. CM reactions catalyzed by 2, 3a–f under air. Optimized reaction conditions.
Entry
Catalyst[a]
Solvent
Temperature [8C]
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
2
DCE
toluene
DCE
toluene
toluene
toluene
toluene
80
80
40
80
60
60
80
24
1
3
24
24
1
62
24
32
53
71
42
26
3a
3b
3c
3d
3e
3f
24
[a]
Catalyst loading: 2.5 mol%.
[b]
Conversions of 5 into 6 determined by 1H NMR spectroscopy. Isolated yields (only E-isomers).
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New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis
FULL PAPERS
worked with excellent E selectivity and good yields.[34]
Because of the total selectivity, the course of the reac-
tion was monitored by NMR spectroscopy, measuring
the conversion of the starting material to the product
over time, using anthracene as internal standard. The
catalyst loading (2.5 mol%) and the concentration of
substrates were kept constant throughout the entire
screening process.
From a practical point of view, it should be noticed
that not only complex 2 can be used for this CM reac-
tion but also some of these indenylidene-Schiff base
catalysts like 3c, 3d and 3e yielded good conversions
(53%, entry 4; 71%, entry 5, 42%, entry 6; respective-
ly). Interestingly, 3d showed even a better efficacy as
compared to the commercially available 2 (71%,
entry 5 vs. 62%, entry 1) after a reaction time of 24 h.
In order to explain the higher yield of 3d, we can con-
sider that the electron-withdrawing group in the aldi-
Figure 5. CM of 4 and 5. Conditions: c4 =0.4M, 2.5 mol% of
catalyst, 608C, 24 h, under air.
mine fragment (chloride substituent) weakens the Figures in combination with the results compiled in
N!Ru chelation facilitating the decoordination of Table 1, it is noteworthy to mention that the catalytic
the N of the imine and, consequently, faster initiating activity of the complexes strongly depends on the ap-
the catalytic cycle. The “one-arm” dissociation of the plied solvent. According to Ledoux, in ROMP of cy-
Schiff base ligand in 3d, leading to the formation of clooctadiene, Grubbs second generation catalyst was
the catalytically active 14-electron species, seems to unambiguosly more active in C6D6 than in CDCl3, at
proceed faster than the release of the aryl ether the same temperature. Two possible explanations for
ligand in Hoveyda–Grubbs complex, required for ini- this observation were proposed.[37] In the present
tiation,[35] and as a result 3d initiates more rapidly study, toluene was found to be the best solvent in
than first generation Hoveyda–Grubbs catalyst. Gen- almost all cases using Ru-indenylidene complexes.
erally, increasing the temperature from 408C to 608C Complex 2 and only one of the Schiff base-indenyli-
or even 808C, due to the high thermal stability of the dene catalysts (3b) preferred dichloroethane (DCE)
ruthenium indenylidene complexes,[36] the conversion instead of toluene (entries 1 and 3, Table 1). Related
of the metathesis product 6 was enhanced. Neverthe- to our observations, it seems that all the catalysts
less, in some cases a prolonged reaction time was nec- studied do not perform well in CH2Cl2 at tempera-
essary to reach good yields (entries 1, 4, 5 and 7, tures higher than 408C and the same holds for CHCl3
Table 1).
at all temperatures studied for this model CM reac-
Figure 4 and Figure 5 illustrate the comparison of tion.
the activities of the pre-catalysts in this model CM re-
The progress of the reaction of 4 and 5 conducted
action using four different solvents after 24 h at 808C in toluene at 608C in the presence of the first-genera-
and 608C, respectively. As can be seen from these tion Schiff base-indenylidene catalysts is shown in
Figure 6. Remarkably, catalyst 3d depicts a high activ-
ity enhancement if the reaction is continued up to
24 h. The observable diminishment after 24 h for cata-
lysts 2 and 3c, although not fully understood, could be
explained by assuming a collateral isomerization pro-
cess. A clear and convincing explanation will be given
further on.
Overall, the desired product was isolated in a mod-
erate to good yield with a high stereoselectivity in the
first standard CM reaction. Special attention should
be given to our very stable (several months at room
temperature in air) indenylidene-Schiff base-Ru-
based catalyst 3d as a highly reactive species for the
model CM reaction with a pronounced high thermal
stability compared to complex 2.
The second standard reaction of interest is the CM
of allylbenzene and cis-1,4-diacetoxy-2-butene
(Table 2). In this model reaction, the strategy (the use
Figure 4. CM of 4 and 5. Conditions: c4 =0.4M, 2.5 mol% of
catalyst, 808C, 24 h, under air.
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ence in CM of allylbenzene and cis-1,4-diacetoxy-2-
butene was complex 2 in DCE at 408C yielding 66%
conversion in only 1 h (entry 1, Table 2). Nonetheless,
the Schiff base-indenylidene-Ru-containing catalysts
gave moderate yields. Again, similar to the first stan-
dard CM reaction studied above, complex 3a showed
the lowest efficacy in all screened conditions, owing
to an observable decomposition in solution. Surpris-
ingly, as opposed to the highest results obtained for
indenylidene catalysts in CM of 4 and 5 (entries 4 and
5, Table 1), these complexes (3c and 3d) failed in this
CM reaction since only poor yields (or no conversion)
were obtained even after 24 h of reaction (entries 4
and 5, respectively, Table 2). As it can be seen from
the data presented in Table 2, DCE was again the pre-
ferred solvent for complex 2, contrary to the results
found for the other catalysts. Regarding to E-Z selec-
tivity, all Schiff base catalysts studied here displayed a
good selectivity in all cases (Table 2). Intriguingly,
Figure 6. CM of 4 and 5. Conditions: c4 =0.4M, 2.5 mol% of
catalyst, 608C, toluene, under air.
of two equivalents of cis-1,4-diacetoxy-2-butene vs. only the reaction conducted with catalyst 3f could be
one equivalent of allylbenzene) is to obtain the pref- accelerated under microwave irradiation conditions (1
erable selectivity and to avoid CM coupling of two hour at 808C; 16%) but the conversion never exceed-
terminal olefins.
ed 16% even at elevated temperature (1008C). In
Since both E and Z dimers of the starting materials contrast, when we carried out the same microwave-as-
and cross-products were found to be reactive, the sisted reaction using Hoveyda–Grubbs catalyst, 2,
course of the reaction was now monitored by GC, only a moderate yield (15%) was obtained compared
measuring the conversion of starting material to prod- with classical heating conditions (entry 1, Table 2).
ucts over time, using tridecane as internal standard.
Unexpectedly, in the course of this investigation we
The catalyst loading (2.5 mol%) and the concentra- found a decrease in yield after 24 h that could indi-
tion of substrates were kept constant throughout the cate some type of isomerization reaction. To clearly
entire screening process.
present this in a graph, we chose the progress of this
Analysis of the conversions and selectivities yielded standard CM reaction performed in CH2Cl2 at 808C
some interesting findings. Comparing the pre-cata- in the presence of the first-generation catalysts 3a–f
lysts, the best reaction conditions for each initiator as an example, see Figure 7.
are presented in Table 2. Here, the catalyst of prefer-
Table 2. CM reactions catalyzed by the ruthenium complexes 2 and 3a–f under air. Optimized reaction conditions.
Entry Catalyst[a] Solvent Temperature Time Yield of 10
Yield of 11
Yield of 12
Yield of 13
Yield of 14
[8C]
[h]
[%][b]
[%][b]
[%][b]
[%][b]
[%][b]
1
2
3
4
5
6
7
2
DCE
40
1
4
14
52
0
0
[c]
3a
3b
3c
3d
3e
3f
Toluene 80
CHCl3 80
1
24
0
0
5
2
14
5
5
6
4
8
CHCl3 40
CH2Cl2 40
24
1
0
0
6
5
17
13
10
3
14
5
[a]
[b]
[c]
Catalyst loading: 2.5 mol%.
Conversions determined by ultrafast GC.
Decomposition of catalyst, no conversion.
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New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis
FULL PAPERS
Figure 7. CM of 8 and 9. Conditions: c8 =0.4M, 2.5 mol% of
catalyst, 808C, CH2Cl2, under air. Y%=yield (11) + yield
(12).
Special attention should be given to the perfor-
mance of complex 2. Although it reached the highest
yield, surprisingly the product conversion decreased
from 54% (after 3 h) to 36% (after 24 h). Due to this
Figure 8. CM of 8 and 9 using complex 2. Conditions: c8 =
0.4M, 2.5 mol% of catalyst, 808C, CHCl3, 24 h, under air.
intriguing fact, we decided to investigate this in more homodimers, respectively, presumably indicating iso-
detail and, at this time, we discovered some new merization side-reactions.
peaks in several chromatograms. Above all, these new
Since cis-1,4-diacetoxy-2-butene seemed not to be
compounds appeared at 808C after 24 h, but they affected by the isomerization reactions, we focused in
were hardly present in chromatograms where the in- the next experiment only on using 8 and the Hovey-
denylidene catalysts were used. Moreover, this collat- da–Grubbs complex 2. To do so, these reactions were
eral reaction was found in all of the tested solvents carried out with 1 mol% and 2.5 mol% of catalyst,
and it seemed to proceed in higher percentage in using CHCl3 as solvent and at 808C and 1008C. Sam-
CHCl3 and CH2Cl2. Figure 8 shows the chromatogram ples were taken after 24 h, 48 h and 72 h and analyzed
of the CM reaction between 8 and 9 using Hoveyda– by ultrafast-GC. Isomer conversions and metathesis
Grubbs catalyst 2, in CHCl3 at 808C and after 24 h. percentages attained applying these conditions are
As can be seen, new peaks, corresponding to new presented in Table 3.
compounds 15, 16 and 17, are clearly observable near
From the results presented in Table 3 we can con-
to 8 (allylbenzene), heterodimers and allylbenzene clude the following: 1) total conversion of allylben-
Table 3. Isomerization percentages (I%) and metathesis percentages (M%) in the reaction of allylbenzene using Hoveyda–
Grubbs catalyst in CHCl3.[a]
Entry Temp. Catalyst load-
I(15)%
I(17)%
I(15)%
I(17)%
I(15)%
I(17)%
M% at M% at M% at
[8C]
ing (mol%)
at 24 h
at 24 h
at 48 h
at 48 h
at 72 h
at 72 h
24 h
48 h
72 h
1
2
3
4
80
2.5
1
2.5
1
36
26
45
49
28
7
55
51
35
26
45
49
30
7
55
51
32
15
46
50
32
9
54
50
34
47
0
33
47
0
35
68
0
100
0
0
0
[a]
I(15)%=isomerization percentage of 15. I(17)%=isomerization percentage of 17. Metathesis percentages are referred to
the homodimerization reactions of 8 into 13 and 14.
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Figure 10. Structural formulae of compounds detected by
GC-MS.
least for 1-(prop-1-enyl)benzene and cis-1,4-diphenyl-
1-butene, the substrate is subjected to a double-bond
migration and thereafter enters the catalytic cycle.[38]
Encouraged by these results we also verified the
isomerization by using in situ IR spectroscopy which
monitors reactions in real-time. This technique is very
appropriate to characterize components that are diffi-
cult to isolate (in situ advantage). The CM reaction of
8 and 9 was performed in the same conditions as used
before: 2.5 mol% of complex 2 in CHCl3 at 808C for
24 h. The course of the reaction was monitored every
minute and after 22 h, when the metathesis reaction
Figure 9. Comparison of catalysts 2 and 3a–f in the isomeri-
zation of allylbenzene. Conditions: c5b =0.4M, 2.5 mol% of
catalyst, 808C, CHCl3, 24 h, under air, closed vessels. In
black colour: isomer of 8 (%); in lighter grey colour: isomer
of product (%); in darker grey colour: M%.
zene into its isomer compound is only reached at had finished, a new IR band appeared at about
1008C, 2) concerning catalyst loading, a significant 800 cmꢀ1 (Figure 11). In accordance with literature
difference in isomerization yield was found at 808C data,[39] this band can be assigned to the wagging vi-
ꢀ
between 2.5 mol% and 1 mol% (entries 1 and 2, re- bration of the C H bond in para-substituted ben-
spectively, Table 3). So, the catalyst loading influences zenes. This means that it is related to the formation of
the conversion factor. 3) With 2.5 mol% catalyst, the 15.
maximum isomerization is obtained after 24 h.
To the best of our knowledge, isomerization reac-
To further extend our investigation, reactions on al- tions in this model CM reaction in closed systems
lylbenzene now using 2.5 mol% of indenylidene- using first generation catalysts 3a–f, including Hovey-
Schiff base catalyst were conducted in CHCl3 at 808C da–Grubbs complex 2 are described here for the first
for 24 h. Thereafter, the isomerization percentages time.
were calculated and presented for comparison with
the commercial Hoveyda–Grubbs catalyst in Figure 9.
We were pleased to see that, except for 3f, most of Ring-Closing Metathesis (RCM)
the indenylidene-Schiff base catalysts here tested
showed low or poor isomers conversions compared to Ring-closing metathesis has become an important
Hoveyda–Grubbs complex. Gratifyingly, the best in- synthetic method in organic chemistry.[40] For this
denylidene-Ru carbene found in the CM reaction of 8 reason, and due to its high degree of reproducibility
and 9 (3e; entry 6, Table 2) displayed the poorest iso-
merization percentage (5%) in the above experiment.
In order to further verify the occurrence of the iso-
merization reaction, it was also confirmed by means
of GC-mass spectra analysis and in situ IR spectrosco-
py. Firstly, the CM of 8 and 9 was conducted using
2.5 mol% of Hoveyda–Grubbs catalyst in CHCl3 at
808C and for 24 h. At this time, a sample was collect-
ed and analyzed by GC-MS. The new products detect-
ed (15 and 17) were identified by means of the
WILEY library linked to the computer as 1-(prop-1-
enyl)benzene (rt=10.271 min; m/z=117) and cis-1,4-
diphenyl-1-butene (rt=22.026 min; mz=208), respec-
tively (see Figure 10). Although we did not manage to
identify product 16 (rt=18.416 min; mz=130), all
products (15, 16 and 17) were found in a 6:5:4 ratio,
6b using catalyst 2. Conditions: c5b =0.4M, 2.5 mol% of cat-
respectively, attending to the area proportion. No
direct observations regarding the mechanism were re-
corded. However, it is reasonable to assume that, at ( , after 22 h).
Figure 11. In situ IR spectrum of the CM reaction of 5b and
^
alyst, 808C, CHCl3, 24 h, under air; ( , after 21 h and
~
1 min), (ꢁ, after 21 h and 6 min), ( , after 21 h and 16 min),
&
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New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis
FULL PAPERS
it can be seen in Figure 14, practically all indenyli-
dene-Schiff base catalysts exceeded the yield afforded
by the Hoveyda–Grubbs catalyst. It should be empha-
sized that whereas commercial catalyst 2 only led to
65% conversion after 24 h, four of our catalysts (3b,
3d, 3e and 3f) gave overall yields being clearly more
efficient. In the case of catalyst 3d, full conversion
was attained after only 3 h, exactly the double com-
pared to Hoveyda–Grubbs catalyst 2 after the same
time.
It should be noted that the results of RCM of
DATA-1 and the results of the stability tests (see
above) are in good agreement. More stable catalysts
allow for higher conversions.
In conclusion, the present investigation in RCM re-
actions has proved that indenylidene-Schiff base-
ruthenium catalysts 3a–f are at least as effective as
the Hoveyda–Grubbs complex, while the rigorous
Figure 12. RCM reactions of DATA and DATA-1.
and ease to perform and monitor over time, this reac-
tion class was selected for elaborate study as well.
Moreover, it has been used extensively to test numer-
ous catalysts. Having the new catalysts described
above in hand, we decided to study their catalytic ac-
tivity by choosing two model RCM substrates, namely
DATA (N,N-diallyltosylamide) and DATA-1 [N-allyl-
N-(2-methyl-2-propenyl)-p-toluenesulfonamide] (Fig-
ACHTUNGTRENNUNG
Figure 13. RCM of DATA. Conditions:
0.5 mol% of catalyst, 608C, CHCl3, under air.
cDATA =0.1M,
the commercial catalyst 2 were evaluated at 608C in
CHCl3. Measurements were performed after 1 h, 3 h
and 24 h. The course of the reaction was monitored
by NMR spectroscopy by measuring the increase in
the amount of product in time. The catalyst loading
(0.5 mol%) and the concentrations of substrates were
kept constant throughout the entire screening process.
RCM of DATA
Interestingly, all catalysts afforded quantitative yields
at 608C after only 1 h (Figure 13). The origin of the
slight decrease in yield obtained with 3f after 24 h re-
mains, however, unclear.
RCM of DATA-1
Even more interesting are the results obtained with
Figure 14. RCM of DATA-1. Conditions: cDATA-1 =0.1M,
DATA-1, a more challenging substrate for RCM. As 0.5 mol% of catalyst, 608C, CHCl3, under air.
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choice of ligand environment enables excellent meta- Experimental Section
thesis activity.
General Remarks
All synthesis reactions involving organometallic compounds
were carried out in oven-dried glassware under an oxygen-
free argon atmosphere using standard Schlenk techniques.
Solvents were dried with appropriate drying agents and dis-
tilled prior to use. All solvents and reagents, except DATA
and DATA-1, were obtained from commercial sources. Sub-
strates DATA and DATA-1 were prepared according to lit-
erature procedure.[45] Starting chemicals were used without
further purification. 1 H, 13C and 31P NMR characterization
of 3a–f and conversion measurements were acquired with a
Varian Unity-300 spectrometer and were obtained at room
temperature. The detailed assignment of the resonances of
3a and 3d in the 1 H, 13C, and 31P NMR spectra was per-
formed using a Bruker 500 Mhz. Elemental analysis was per-
formed using a Perkin–Elmer-2400 CHNS/O analyzer.
Flash column chromatography was performed on silica
gel 60 (grade 7734, 70–230 mesh, Silicycle). Microwave-as-
sisted reactions were carried out using a MultiSYNTH sci-
entific microwave (Microwave Organic Synthesis System)
produced by Milestone. Gas chromatography (GC) was con-
ducted using a Finning Trace GC ultra from Thermo Elec-
tron Corporation equipped with a 10 m capillary column
and with a flame ionization detector. The temperature pro-
gram was as follows: 508C initial temperature followed by a
heating rate of 108C/min up to 1808C and finally an in-
crease up to 1908C at a heating rate of 28C/min. Products
15 and 17 were identified by gas chromatography-mass spec-
troscopy (Hewlett Packard instrument) equipped with a
mass selective detector (HP 5973) and a GC system (HP
6890 series). IR study was possible by using a ReactIR 45 m
equipped with a 1.5 m AgX fiber and a 9.5 mm DiComp
(Diamond) probe.
Conclusions
We have prepared a series of indenylidene-ruthenium
complexes bearing a salicylaldimine ligand, and their
structures were fully confirmed by NMR spectrosco-
py. These catalysts show no sign of decomposition
when stored under ambient conditions. Their robust-
ness also holds at higher temperatures when dissolved
in benzene-d6. With the results presented above, we
have demonstrated the great stability and activity of
these new complexes. They exhibit a particular elec-
tronic and steric configuration and a readily accessible
imine ligand that lend them to catalyst tuning.[41] Be-
sides, these catalysts 3a–f are relatively cheap and
easily produced. Another advantage is that all these
catalysts show negligible olefin metathesis activity at
room temperature. This would be a good choice when
commercial polymerization technology requires a
latent catalyst.[42–44] Most metathesis catalysts are op-
erative at room temperature and are therefore not
well suited for applications where catalyst latency is
beneficial.
Overall, the desired product was isolated in moder-
ate to good yields with high stereoselectivities in the
first standard CM reaction. It is clear that by changing
the electronic configuration of the Schiff base, the cat-
alytic activity and stability of the ruthenium initiators
were tuned. In a second standard CM reaction, unde-
sired products due to isomerization and homodimeri-
zation of starting materials were formed, thereby re-
sulting in a low selectivity for this process. Regarding
RCM reactions, a comparison between the classical
Hoveyda–Grubbs complex 2 and complexes 3a–f
Typical Procedure for the CM reaction between 4 and
5 (Catalyst Loading=2.5 mol%)
A 15-mL, screw-cap, septum tops sealed vial was charged
demonstrates the excellent conversions obtained with with 100 mL (0.62 mmol; 88 mg) of 4, 56 mL (0.62 mmol;
54 mg) of 5, 15–20 mg of anthracene as internal standard
and 1.55 mL of solvent (CHCl3, toluene, CH2Cl2 or DCE).
A first sample was taken and analyzed (initial time), there-
after 0.015 mmol (2.5 mol%) of catalyst was added under
air. The solution was heated in a controlled-temperature sili-
cone-oil bath at 408C, 608C or 808C. Progress of the reac-
tion was monitored by NMR spectroscopy. Measurements
were performed after 1 h, 3 h and 24 h.
the indenylidene-based initiators, in most cases ex-
ceeding the yields achieved using 2. However, the cur-
rent difficulty remains the anticipation of the efficacy
of pre-catalysts with regard to a specific substrate; no
single catalyst outperforms all others in all cases.
Due to the wide possibilities of modifying the struc-
ture of the Schiff base ligand, there is still enough
room for further investigation and improvement of
these catalytic systems. Investigations concerning the
activity of other new Schiff base-Ru-indenylidene
complexes, bearing NHC ligands, are currently under-
way in our laboratory and will be reported in due
course.
Typical Procedure for the CM Reaction between 8
and 9 (Catalyst Loading=2.5 mol%)
A 15-mL, screw-cap, septum tops sealed vial was filled with
28.80 mL (0.22 mmol) of 8, 69.32 mL (0.44 mmol) of 9,
26.84 mL (0.11 mmol) of tridecane as internal standard and
1 mL of solvent (CHCl3, toluene, CH2Cl2 or DCE). A first
sample was taken and analyzed (initial time), thereafter
2.5 mol% of catalyst was added under air. The solution was
heated in a controlled-temperature silicone-oil bath at 408C,
608C or 808C. The reaction mixture was then analyzed by
GC. Measurements were performed after 1 h, 3 h and 24 h.
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New Indenylidene-Schiff Base-Ruthenium Complexes for Cross-Metathesis
FULL PAPERS
1
0.1 mL). The reaction progress was monitored by H NMR
spectroscopy as a function of time at ambient temperature
by integrating the olefinic 1H signals of the formed ring-
closed product and the disappearing diene.
Typical Procedure for the RCM Reaction of DATA
or DATA-1 (Catalyst Loading=0.5 mol%)
A solution of the substrate (DATA or DATA-1) (0.020 mL;
0.1M) and 0.5 mol% of catalyst 3a–f in CHCl3 (0.85 mL)
was prepared in a 15-mL, screw-cap, septum tops sealed vial
under air and heated at 408C or 608C. The reaction mixture
was analyzed by NMR. Measurements were performed after
1 h, 3 h and 24 h.
Elemental Analysis of 3a–f and H, 13C (Table 4) and
1
31P NMR Characterization of 3b, 3c, 3e and 3f
3a: Anal. calcd. (%) for C47H55ClNOPRu (817.45): C 69.06,
H 6.78, N 1.71; found: C 68.87, H 6.54, N 1.97.
1
3b: H NMR (CDCl3): d=8.31, 7.61, 7.58, 7.49, 7.37, 7.23,
General Procedure for Stability Tests
7.08, 7.01, 6.82, 6.74, 6.62, 6.47, 6.21, 5.43, 3.85, 3.14, 2.17,
2.12, 1.63, 1.22, 1.17, 0.85, 0.62, 0.44, 0.12; 13C NMR
(CDCl3): d=166.9, 144.2, 139.9, 136.7, 134.5, 132.4, 132.3,
132.2, 130.8, 129.5, 129.2, 128.8, 128.7, 128.6, 128.4, 128.3,
127.4, 126.7, 125.9, 123.3, 117.7, 114.6, 36.0, 35.2, 33.4, 33.2,
31.6, 30.1, 29.5, 28.3, 28.2, 28.1, 27.9, 27.3, 27.1, 26.7, 26.6,
26.4, 26.1, 20.7, 18.6; 31P NMR (CDCl3): d=48.0, 43.6, 37.1,
32.5; anal. calcd. (%) for C49H59ClNOPRu (845.51): C 69.61,
H 7.03, N 1.66; found: C 69.54, H 7.04, N 1.82.
After charging an NMR tube with 20 mg of catalyst dis-
solved in dry deuterated benzene, the solution was heated at
808C in a controlled-temperature silicone-oil bath for 6 h.
The catalyst stability was monitored as a function of time by
integrating every hour the 31P signal of the decomposing cat-
alyst.
General Procedure for Kinetic Studies
1
3c: H NMR (CDCl3): d=8.37, 7.70, 7.67, 7.51, 7.49, 7.38,
An NMR tube was filled with the precatalyst 3a–f
(0.004 mmol; 1 mol%) and dry CDCl3 (0.4 mL), then
DEDAM (diethyl diallylmalonate) was added (0.414 mmol;
7.19, 7.12, 6.91, 6.86, 6.79, 6.59, 6.33, 5.51, 3.96, 3.27, 2.25,
2.21, 1.76, 1.35, 1.24, 0.95, 0.75, 0.57, 0.25; 13C NMR
(CDCl3): d=296.2, 295.3, 167.0, 143.6, 139.9, 135.9, 135.8,
Table 4. 1H and 13C NMR assignments of the compounds 3a and 3d at 295 K in dichloromethane-d2 and CDCl3, respective-
ly.[a]
Atom label
1H (3a)
13C (3a)
1H (3d)
13C (3d)
1
–
170.5
–
175.0
1
-
170.5
-
175.0
2
7.04
136.0
6.97
123.2
3
6.50
114.4
7.96
127.6
4
5
7.26-7.21
7.20
133.5
122.4
–
8.02
135.9
134.5
6
–
117.5
–
116.5
7
7.84
165.3
7.76
165.4
8
–
154.3
–
150.6; 139.2; 129.0;126.5; 126.0
9
7.26–7.21
7.26–7.21
7.26–7.21
7.30
–
2.42
–
6.88
–
–
7.40
6.81
7.26–7.21
8.38
–
–
7.71
7.36
7.51
128.9; 125.5; 117.3
128.9; 125.5; 117.3
128.9; 125.5; 117.3
129.4
131.2; 130.4
33.1
292.8
136.6
139.6
140.9
129.1
124.7
128.9
128.2
131.8
127.4
126.3
129.0
7.45;7.22; 7.12
7.45;7.22; 7.12
7.45;7.22; 7.12
7.45;7.22; 7.12
–
–
6.70
–
–
6.95
–
7.16
8.27
–
150.6; 139.2; 129.0;126.5; 126.0
150.6; 139.2; 129.0;126.5; 126.0
150.6; 139.2; 129.0;126.5; 126.0
150.6; 139.2; 129.0;126.5; 126.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
PCy3
PCy3
PCy3
150.6; 139.2; 129.0;126.5; 126.0
299.2
136.1
142.0
142.6
117.8
128.6
129.4
128.2
138.8
135.2
126.2
129.2
128.5
–
7.56
7.30
7.43
–
2.40
1.95–1.33
1.2–1.00
128.0
33.1
29,8; 29.3; 27.8
26.6; 26.5
2.52
1.95–1.54
1.30–1.23
33.3
29.5; 27.9
26.6
[a]
The arbitrary numbering is depicted in Figure 1. Chemical shifts are quoted in ppm.
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137.0, 132.2, 132.0, 131.5, 130.5, 129.3, 128.9, 128.5, 128.4,
128.3, 127.4, 126.5, 126.2, 123.0, 118.0, 117.4, 116.7, 35.7,
34.9, 33.5, 33.2, 31.8, 31.4, 29.9, 29.3, 28.0, 27.9, 27.8, 27.6,
27.2, 27.0, 26.8, 26.4, 26.1, 25.8, 20.4, 18.2; 31P NMR
(CDCl3): d=44.2; anal. calcd. (%) for C48H56ClN2O3PRu
(876.48): C 65.78, H 6.44, N 3.20; found: C 65.38, H 6.37, N
3.46.
[3] Y. Schrodi, R. L. Pederson, Aldrichimica Acta 2007, 40,
45–52.
[4] R. H. Grubbs, Tetrahedron 2004, 60, 7117–7140.
[5] K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew. Chem.
2005, 117, 4564–4601; Angew. Chem. Int. Ed. 2005, 44,
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[6] S. Monsaert, A. M. Lozano Vila, R. Drozdzak, P.
Van Der Voort, F. Verpoort, Chem. Soc. Rev. 2009,
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[7] a) R. Drozdzak, N. Ledoux, B. Allaert, I. Dragutan, V.
Dragutan, F. Verpoort, Cent. Eur. J. Chem. 2005, 3,
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I. Dragutan, V. Dragutan and F. Verpoort, Curr. Org.
Synth. 2008, 5, 291–304.
[8] a) B. Allaert, N. Dieltiens, C. Stevens, R. Drozdzak, I.
Dragutan, V. Dragutan, F. Verpoort, in: Metathesis
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Advanced Materials, Vol. 243, (Eds.: Y. Imamoglu, V.
Dragutan), Nato Science Series, Series II: Mathematics,
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[9] a) P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs,
Angew. Chem. 1995, 107, 2179–2181; Angew. Chem.
Int. Ed. 1995, 34, 2039–2041; b) P. Schwab, R. H.
Grubbs, J. W. Ziller, J. Am. Chem. Soc. 1996, 118, 100–
110; c) M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs,
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[10] J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus, A. H.
Hoveyda, J. Am. Chem. Soc. 1999, 121, 791–799.
[11] S. Chang, L. Jones, C. M. Wang, L. M. Henling, R. H.
Grubbs, Organometallics 1998, 17, 3460–3465.
[12] B. Allaert, N. Dieltiens, N. Ledoux, C. Vercaemst, P.
Van der Voort, C. V. Stevens, A. Linden, F. Verpoort, J.
Mol. Catal. A: Chem. 2006, 260, 221–226.
3d: Anal. calcd. (%) for C46H51Cl2N2O3PRu (882.87): C
62.58, H 5.82, N 3.17; found: C 62.24, H 5.78, N 3.42.
1
3e: H NMR (CDCl3): d=8.98, 8.32, 7.88, 7.71, 7.61, 7.31,
7.11, 7.04, 6.93, 6.78, 5.75, 4.22, 3.52, 2.55, 2.18, 1.84, 1.53,
1.16, 1.06, 0.82, 0.80, 0.24; 13C NMR (CDCl3): d=295.9,
295.0, 174.8, 174.4, 167.2, 143.6, 140.0, 135.9, 135.8, 135.4,
135.0, 134.9, 132.2, 132.0, 131.1, 129.1, 128.9, 128.5, 128.4,
128.2, 128.1, 126.5, 123.0, 118.0, 117.5, 116.8, 35.7, 34.9, 33.5,
33.2, 32.0, 31.4, 29.9, 29.3, 28.0, 27.9, 27.8, 27.6, 27.2, 27.0,
26.8, 26.4, 26.1, 25.8, 21.0, 20.3, 18.1; 31P NMR (CDCl3): d=
44.2; anal. calcd. (%) for C49H58ClN2O3PRu (890.50): C
66.09, H 6.56, N 3.15; found: C 65.93, H 6.53, N 3.15.
1
3f: H NMR (CDCl3): d=8.62, 8.59, 7.57, 7.45, 7.36, 7.32,
7.30, 6.94, 6.68, 6.58, 6.41, 6.29, 6.23, 6.07, 5.88, 5.38, 3.38,
3.14, 2.92, 2.39, 2.13, 1.82, 1.53, 1.42, 1.17, 0.98, 0.69, 0.48,
0.36, 0.23, 0.19, 0.14; 13C NMR (CDCl3): d=295.2, 295.0,
293.1, 293.0, 174.8, 168.1, 167.6, 148.8, 148.0, 144.9, 143.6,
141.8, 141.6, 141.1, 140.7, 140.6, 140.3, 140.2, 140.1, 139.4,
136.2, 136.0, 135.2, 135.1, 134.0, 132.2, 132.1, 131.9, 130.0,
129.8, 129.3, 129.2, 129.1, 128.8, 128.6, 128.3, 127.0, 126.4,
126.3, 126.0, 123.4, 123.2, 123.1, 122.9, 122.5, 118.0, 117.3,
116.5, 116.4, 35.9, 35.0, 33.6, 33.5, 33.3, 33.2, 31.6, 30.6, 30.0,
29.5, 29.4, 28.1, 27.9, 27.8, 27.6, 27.4, 27.3, 27.0, 26.9, 26.5,
26.2, 25.6, 25.3, 23.6, 22.9, 22.8, 22.7, 21.3, 14.1; 31P NMR
(CDCl3): d=48.1, 47.22, 46.83; anal. calcd. (%) for
C52H64ClN2O3PRu (932.58): C 66.97, H 6.92, N 3.00; found:
C 66.78, H 7.36, N 2.37.
Acknowledgements
[13] W. Buchowicz, J. C. Mol, M. Lutz, A. L. Spek, J. Orga-
The authors gratefully acknowledge the financial support
from the FWO-Flanders. A. M. Lozano-Vila is indebted to
the Junta de Extremadura for her postdoc contract and the
project PRI08A022. S.M. acknowledges the Institute for the
Promotion of Innovation through Science and Technology in
Flanders (IWT-Flanders) and the Research fund of Ghent
University (BOF) for a research grant. We wish to thank J.
Goode of Mettler Toledo, N. de Coensel of Dep. of Org.
Chem. UGent and O. Grenelle, V. Dhaenens, M. Proot from
Chevron Belgium, for in situ IR analysis, GC-MS analysis
and elemental analysis, respectively. Funding for this project
was generously provided by the EC through the seventh
framework program (Grant CP-FP 211468-2-EUMET).
Umicore AG is gratefully acknowledged for the generous gift
of materials.
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