Activity and stereoselectivity of Ru-based catalyst bearing a fluorinated imidazolinium ligand

Article abstract of DOI:10.2478/s11532-011-0034-6

Grubbs II generation catalyst (3), bearing a fluorinated imidazolinium ligand, was investigated in cross metathesis (CM), ring closing metathesis (RCM) and ring opening polymerization metathesis (ROMP) for a variety of substrates. Kinetic studies showed r

Full text of DOI:10.2478/s11532-011-0034-6

Cent. Eur. J. Chem. • 9(4) • 2011 • 605-609
DOI: 10.2478/s11532-011-0034-6
Central European Journal of Chemistry
Activity and stereoselectivity
of Ru-based catalyst bearing
a fluorinated imidazolinium ligand
Research Article
Vincenzo Siano, Ilaria D’Auria, Fabia Grisi,
Chiara Costabile*, Pasquale Longo
Dipartimento di Chimica, Università di Salerno Via Ponte Don Melillo,
I-84084 Fisciano, Salerno, Italy
Received 12 January 2011; Accepted 23 February 2011
Abstract:
Grubbs II generation catalyst (3), bearing a fluorinated imidazolinium ligand, was investigated in cross metathesis (CM), ring closing
metathesis (RCM) and ring opening polymerization metathesis (ROMP) for a variety of substrates. Kinetic studies showed reduced
stability of the catalyst in methylene chloride following the first 15 minutes of reaction preventing a higher efficiency despite the very
high activity. Beneficial solvent effects on the catalyst stability were observed by performing RCM in C6F6.
Keywords: Metathesis • Ruthenium-based catalyst • Kinetics
© Versita Sp. z o.o.
1. Introduction
In order to depict a more comprehensive analysis
of the behavior of this catalyst, the catalytic activity and
stereoselectivity of 3 was tested in cross metathesis
(CM), ring closing metathesis (RCM) and ring opening
polymerization metathesis (ROMP) for a variety of
substrates.
Olefin metathesis has emerged in past decades as one
of the most useful methods for constructing C=C bonds
[1]. The discovery of Ru alkylidene complexes able to
catalyze olefin metathesis reactions (1 in Chart 1) [2]
inspired intensive research efforts toward the synthesis
of more efficient catalytic precursors. In particular,
after the first isolation in 1991 of a free N-heterocyclic
carbene (NHC) by Arduengo et al. [3], this family of
ligands came to the forefront of coordination chemistry
and Ru-based catalysis leading to the substitution of one
phosphine with an N-heterocyclic carbene (NHC) ligand
(2) [4,5]. Recently, the introduction of fluorine atoms in
the ortho-position of the NHC N-aryl groups (3) led to a
rate acceleration in the ring closing metathesis (RCM)
of diethyldiallylmalonate, probably due to a ruthenium-
fluorine interaction [6]. Despite this interesting result, no
more experimental data, relative to the behaviour of 3
toward other substrates, have been reported.
2. Experimental Procedure
All reactions involving metal complexes were conducted
in oven-dried glassware under nitrogen atmosphere with
anhydrous solvents, using standard Schlenk techniques
and glove box techniques. Toluene was distilled from
sodium/benzophenone. CH2Cl2 was dried on CaH2 and
freshly distilled before use. All chemical products were
purchased from Sigma –Aldrich Company and were
reagent quality. These products were used without
further purification. The synthesis of substrates 8 and 10
were produced by reported procedures [7]. Flash column
chromatography of organic compounds was performed
using silica gel 60 (230-400 mesh). Silica gel for the
purification of organometallic complexes was obtained
from TSI Scientific, Cambridge, MA(60 Å, 230-400 mesh,
pH 6.5-7.0). All procedures relative to RCM, ROMP and
CM in the presence of 3 are described in the supporting
information.
Chart 1
* E-mail: ccostabile@unisa.it
605

Activity and stereoselectivity of Ru-based
catalyst bearing a fluorinated imidazolinium ligand
Compound
4.
Under
inert
atmosphere, TSI Scientific gel dry eluting with pentane:Et2O = 4:1,
t r i s ( d i b e n z y l i d e n e a c e t o n e ) d i p a l l a d i u m ( 0 ) then 1:1, and finally 1:4. The solution was concentrated
(203.3 mg, 0.222 mmol), BINAP (450.2 mg, in vacuo to obtain a brown-red powder of 3 (468.7 mg,
0.723 mmol), sodium t-butoxide (2.129 g, 22.16 mmol), 0.56 mmol, 46.3% yield). NMR data matched with those
2-bromo-1,3-difluorobenzene (3.036 g, 15.73 mmol) and reported in [6] (see supporting information).
ethylenediamine (455.5 mg, 7.48 mmol) were added to
NMR spectra and catalytic test details have been
toluene (50 mL). The solution was heated at 110°C for reported in the supporting information.
24 hours, then cooled to room temperature, diluted with
30 mL of hexanes and filtered through a plug of silica.
Theproductwaselutedwithdichloromethaneandpurified
bychromatographyelutingwithhexane:dichloromethane
3. Results and Discussion
= 7:3. The solution was concentrated in vacuo to obtain
a yellow oil of 4 (818.2 mg, 2.88 mmol, 38.5% yield).
1H-NMR (400 MHz, CDCl3) δ: 3.52 ppm (s, 4H),
6.68 ppm (m, 2H), 6.81 ppm (t, 4H). 13C-NMR (100 MHz,
CDCl3) δ: 46.8; 111.7; 118.1; 125.7; 152.6; 155.0. 19F-
NMR (376 MHz, CDCl3) δ: 129.3 ppm (s) (see supporting
information).
Compound 5. Compound 4 (403.3 mg, 1.42 mmol)
was diluted with 2 mL of diethyl ether and acidified with
HCl/Et2O 2 M. The white powder of diammonium salt was
filtered and washed with HCl/Et2O 2M, then concentrated
in vacuo. The salt was added to triethyl orthoformate
(9.4 mL, 56.5 mmol) and the solution was heated at
135°C for 30 min. The solution was cooled, filtered and
the solid was washed with diethyl ether to obtain a white
solid of the tetrafluorodihydroimidazolinium chloride 5
(390.3 mg, 1.18 mmol, 83% yield). NMR data matched
with those reported in the literature [6] (see supporting
information).
Compound 3. In a glove box, compound 5
(701.1 mg, 2.12 mmol), silver(I) oxide (280.4 mg,
1.21 mmol) and 5Å molecular sieves (822 mg) were
added to dichloromethane (8.5 mL) and the solution
stirred in the dark for 1 hour at room temperature. The
grey suspension was filtered on a glass microfibre
filter and the filter was washed with dichloromethane.
The obtained solid and 1st generation Grubbs catalyst
(1) (995.8 mg, 1.21 mmol) were dissolved in toluene
(15 mL) and the solution was stirred in the dark at
room temperature for 16 hours. The suspension was
purified by chromatography in a column packed with
Synthesis of complex 3 is described in Scheme 1.
Hartwig-Buchwald coupling of the first step [8], for the
synthesis of 4, requires harsh conditions, due to the
strongly deactivated Br-C bond of the Br-aryl derivative
[9]. In fact, the presence of an electron-withdrawing
function on the aromatic moiety significantly reduces
the yield with respect to other substrates. Despite the
moderate yield (around 39%), the Hartwig-Buchwald
coupling as a first step discloses the possibility of
achieving analogue chiral ruthenium catalysts with a
fluorine atom in the ortho-position of the NHC N-aryl
groups. In fact, a chiral diamine could be used instead
of ethylenediamine. Dihydroimidazolium salt 5 was
obtained in high yields (83 %) by salification of 4 with
HCl followed by condensation with triethyl orthoformate.
The overall procedure to obtain 5 leads to lower yields
(32%) with respect to the synthesis reported by Grubbs
et al. [6] (54% yield), which involves the reaction of
oxalyl chloride with 2,6-difluoroaniline. Nevertheless,
the latter procedure does not allow for the production
of chiral ligands. Compound 5 was treated with Ag2O to
afford the carbene tranfer agent that was reacted with
1 to obtain the desired complex 3, by transmetallation
[10], in 47% yield.
The activity of 3 was tested in the RCM of 6, 8 and
10 (Scheme 2). The course of reaction was monitored
by 1H NMR spectroscopy and the conversion of starting
materials to products over time was measured as shown
in Fig. 1. As for the kinetic profile of RCM of substrate
6, the experimental data are in agreement with those
reported in the literature [6]. The activity and the
conversions in the RCM of 8 and 10, which lead to a tri
and a tetrasubstituted olefin, respectively, showed to be
lower. Nevertheless, catalysts of Grubbs type reported in
the literature, are generally unable to promote the RCM
of all three kinds of substrates. Only a few of them, with
particulary low steric hindrance, revealed to be active
toward 6, 8 and 10 [11,12].
RCM of substrate 6 was also performed in the
presence of 0.1% of 3 as shown in the kinetic profile
depicted in Fig. 2 and is compared with the analogue
reaction in the presence of 1% of 3. After the first
Scheme 1. Synthesis of 3.
606

V. Siano et al.
Scheme 2. RCM of 6, 8 and 10.
Figure 3. Log plots for 3 in the RCM of 6 (0.1% catalyst loading),
8 (1% catalyst loading) and 10 (5% catalyst loading).
illustrated by plotting ln([starting material]) vs. time
(Fig. 3). In fact, instead of a linear plot, which would
indicate a pseudo-first kinetic rate over the course
of the reaction, curved plots, consistent with catalyst
decomposition, are observed for all substrates.
Kinetic profiles depicted in Fig. 4 revealed that no
relevant yield improvement in the challenging RCM of
hindered substrate 10 could be obtained in benzene
(Fig. 4A), even at higher temperatures (60°C). The
RCM of 10 was also conducted in C6F6 to reach higher
conversions. In fact, fluorinated aromatic solvents
have been reported to significanlty enhance yields
in the RCM of olefins promoted by Ru catalysts [13].
Significant beneficial solvent effects were, in this case,
observed (Fig. 4B). Catalyst lifetime at 70°C increases
up to 2 hours leading to an 81% yield and, at 30°C, the
catalyst was still working after 40 hours reaching a 92%
yield. The C6F6 can affect the catalyst lifetime either
stabilizing the Ru species or, alternatively, decreasing
the Ru complex solubility that would occur slowly during
the reaction. Indeed, the poor solubility of 3 in C6F6 is
directly observable from the undissolved Ru complex
under the reaction conditions.
Figure 1. Kinetic profiles of RCM of 6, 8, 10 0.1 M, conducted in
CD2Cl2 at 30°C and monitored by 1H NMR.
Afterward, complex 3 was tested in ROMP of 12,
13 and 14. Reactions are sketched in Scheme 3 and
all results are reported in Table 1. The high activity
of 3 toward 12 and 13 were confirmed by the good
conversions achieved after 5 minutes in the presence
of only 0.001% of catalyst [14]. As for 14, longer times
(30 minutes) and higher amounts of catalyst (0.01%)
were required in order to obtain high conversions.
1H and 13C NMR spectroscopy allowed for the
Figure 2. Kinetic profiles of RCM of 6 0.1 M, conducted in CD2Cl2 at
30°C and monitored by 1H NMR.
15-20 min, RCM in the presence of 0.1% of catalyst
reaches a plateau, suggesting that after that time the determinationoftheZfractionofthenewlyformeddouble
catalyst decomposes. The lack of stability of 3 can be bond in the polymer chains. Poly(12) shows about 60%
607

Activity and stereoselectivity of Ru-based
catalyst bearing a fluorinated imidazolinium ligand
Kinetic profiles of ROMP of 14 0.5 M, conducted in CD Cl
Figure 5.
2
2
at 30°C and monitored by 1H NMR.
Table 1. ROMP and CM conducted in the presence of 3.
Run Cat%a) Substrate Product t(h) Yield%b) E:Zc)
1d)
2d)
3d)
4d)
5e)
0.001
0.001
0.001
0.01
12
13
poly(12) 0.08
poly(13) 0.08
68
54
1.5
0.9
-
14
poly(14)
poly(14)
17
1
traces
88
14
0.5
0.5
6.3
2.5
15-16
16
47
a) Percentage of catalyst 3 with respect to the substrate.
b) Yields from isolated products.
c) Ratio based on data from 1H and 13C NMR of isolated products.
d) Reaction conducted in dichloroethane at 57°C, substrate 0.5 M.
e) Reaction conducted in CH2Cl2 at 40°C, 15-16 0.2 M.
of E double bonds along the chain at 68 % of monomer
conversion. The slight prevalence of E is probably related
to an equilibrium-controlled polymerization in which
secondary chain transfer occurs [15]. The ROMP of 13
and of 14 showed a lack of stereoselectivity as well. It is
noteworthy that the 66% of Z double bonds in poly(14)
is not indicative of a selectivity toward the formation of
cis double bonds, which would be confirmed only by an
amount of Z double bonds greater than 75%.
Figure 4. Kinetic profiles of RCM of 10 0.1 M and 5% catalyst
loading, conducted in C6D6 (A) and in C6F6 (B) monitored
by 1H NMR.
The ROMP of 14 in CD2Cl2 was also monitored
by 1H NMR in the presence 0.1 and 0.01% of 3
(see Fig. 5). The comparison between the kinetic profile
and the data reported in literature indicates that the
activity of 3 is similar to that of catalyst 2 and much higher
than the activity of 1 [15]. As in the case of RCM of 6, the
kinetic profile of ROMP of 14, at lower concentrations
of catalyst, underlines that the conversion slows down
after 15-20 min.
We finally investigated the activity of 3 in the
representative cross metathesis (CM) reaction of cis-
1,4-diacetoxy-2-butene (15) and allyl benzene (16),
depicted in Scheme 4 [16]. Conversion to desired
heterocoupled product 17 (47%) showed to be lower
than Grubbs first and second generation catalysts and
E:Z ratio (6.3:1) comparable to that of Grubbs second
generation catalyst (2).
Scheme 3. ROMP of 12, 13 and 14.
Scheme 4. CM of 15 and 16.
608

V. Siano et al.
The synthetic approach for 3, presented in this
paper, discloses the possibility to achieve analogue
chiral ruthenium catalysts with fluorine atom in the ortho-
position of the NHC N-aryl groups.
4. Conclusions
The activity and stereoselectivity of Grubbs II generation
catalyst 3, bearing fluorinated imidazolinium ligand, has
been tested in RCM, ROMP and CM reactions. The
catalyst showed to be highly active in methylene chloride,
with respect to (SIMes)(Cl)2Ru=CHPh(PCy)3 (2), during
the first 15-20 minutes of the reaction. Afterward, the
reaction rate slowed down, independently from the type
of substrate, due to the decomposition of the active
species, as confirmed by kinetic studies. Catalyst
lifetime can be significantly lengthened by performing
RCM in C6F6, reaching over 90% yield in the challenging
RCM of hindered olefins.
Acknowledgements
The authors wish to thank Patrizia Oliva and Dr.
Mariagrazia Napoli of the Department of Chemistry of
University of Salerno for their technical assistance.
References
[1]
[9]
(a) A. Füstner, Angew. Chem. 112, 3140 (2000);
(b) A. Füstner, Angew. Chem. Int. Ed. 39, 3012
(2000); (c) R.H. Grubbs, Handbook of Olefin
Metathesis (Wiley-VCH, Weinheim, Germany,
2003); (d) R.R. Schrock, A.H. Hoveyda, Angew.
Chem. 115, 4740 (2003); (e) R.R. Schrock,
A.H. Hoveyda, Angew. Chem. Angew. Chem. Int.
Ed. 42, 4592 (2003)
(a) S.T. Nguyen, R.H. Grubbs, J.W. Ziller, J. Am.
Chem. Soc. 115, 9858 (1993); (b) P. Schwab,
R.H. Grubbs, J.W. Ziller, J. Am. Chem. Soc. 118,
100 (1996)
L.C. Liang, J.M. Lin, C.H. Hung, Organometallics
22, 3007 (2003)
I.J.B. Lin, C.S. Vasam, Coord. Chem. Rev. 251,
642 (2007)
J.M. Berlin, K. Campbell, T. Ritter, T.W. Funk,
A. Chlenov, R.H. Grubbs, Org. Lett. 9, 1339
(2007)
I.C. Stewart, T. Ung, A.A. Pletnev, J.M. Berlin,
R.H. Grubbs, Y. Schrodi, Org. Lett. 9, 1589 (2007)
(a) R. Kadyrov, M. Bieniek, K. Grela, Pat. Appl.,
DE102007018148.7 (2007); (b) D. Rost, M. Porta,
S. Gessler S. Blechert, Tetrahedron Lett. 49, 5968
(2008); (c) A. Grandbois, S.K. Collins, Chem.–Eur.
J. 14, 9323 (2008); (d) C. Samojowicz, M. Bieniek,
A. Zarecki, R. Kadyrov, K. Grela, Chem. Commun.
6282 (2008); (e) L. Vieille-Petit, X. Luan, M. Gatti,
S. Blumentritt, A. Linden, H. Clavier, S.P. Nolan,
R. Dorta, Chem. Comm. 3783 (2009)
(a) C.W. Bielawski, R.H. Grubbs, Angew.
Chem. 112, 3025 (2000); (b) C.W. Bielawski,
R.H. Grubbs,Angew. Chem. Int. Ed. 39, 2903
(2000)
C.W. Bielawski, R.H. Grubbs, Prog. Polym. Sci. 32,
1 (2007)
[10]
[11]
[12]
[13]
[2]
[3]
[4]
[5]
A.J. Arduengo, R.L. Harlow, M. Kline, J. Am. Chem.
Soc. 113, 361 (1991)
M. Scholl, S. Ding, C.W. Lee, R.H. Grubbs, Org.
Lett. 1, 953 (1999)
(a) M. Scholl, T.M. Trnka, J.P. Morgan,
R.H. Grubbs, Tetrahedron Lett. 40, 2247 (1999);
(b) J. Huang, E.D. Stevens, S.P. Nolan,
J.L. Peterson, J. Am. Chem. Soc. 121, 2674 (1999);
(c) T. Weskamp, F.J. Kohl, W. Hieringer, D. Gliech,
W.A. Herrman, Angew. Chem, 111, 2573 (1999);
(d) T. Weskamp, F.J. Kohl, W. Hieringer,
D. Gliech, W.A. Herrman, Angew. Chem., Int. Ed.
38, 2416 (1999); (e) J.P. Morgan, R.H. Grubbs, Org.
Lett. 2, 3153 (2000)
T. Ritter, M.W. Day, R.H. Grubbs, J. Am. Chem.
Soc. 128, 11768 (2006)
T.A. Kirkland, R.H. Grubbs, J. Org. Chem. 62, 7310
(1997)
B.H. Yang, S.L. Buchwald, J. Organomet. Chem.
576, 125 (1999)
[14]
[15]
[16]
[17]
T. Ritter, A.Hejl, A.G. Wenzel, T.W. Funk,
R.H. Grubbs, Organometallics 25, 5740 (2006)
A.K. Chatterjee, T.–L. Choi, D.P. Sanders,
R.H. Grubbs, J. Am. Chem. Soc. 125, 11360
(2003)
[6]
[7]
[8]
609