Latent sulfur chelated ruthenium catalysts: Steric acceleration effects on olefin metathesis

Article abstract of DOI:10.1016/j.jorganchem.2008.03.028

A series of sulfur chelated dormant ruthenium olefin metathesis catalysts is presented. The catalysts prepared were shown to possess the uncommon cis-dichloro arrangement and were mostly inactive at room temperature. By systematically modifying the size of the substituent groups at the chelating sulfur atom, catalyst activity at different temperatures was significantly affected; more bulky substituents fomented activity at lower temperatures. The catalysts were also shown to be stable in solution and retained their catalytic activity even after being exposed to air for two weeks.

Full text of DOI:10.1016/j.jorganchem.2008.03.028

Journal of Organometallic Chemistry 693 (2008) 2200–2203
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Latent sulfur chelated ruthenium catalysts: Steric acceleration effects on
olefin metathesis
a,
Tamar Kost a, Mark Sigalov a, Israel Goldberg b, Amos Ben-Asuly c, , N. Gabriel Lemcoff
*
*
a Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
b School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel
c Achva Academic College, Shikmim 79800, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of sulfur chelated dormant ruthenium olefin metathesis catalysts is presented. The catalysts pre-
pared were shown to possess the uncommon cis-dichloro arrangement and were mostly inactive at room
temperature. By systematically modifying the size of the substituent groups at the chelating sulfur atom,
catalyst activity at different temperatures was significantly affected; more bulky substituents fomented
activity at lower temperatures. The catalysts were also shown to be stable in solution and retained their
catalytic activity even after being exposed to air for two weeks.
Received 7 January 2008
Accepted 26 March 2008
Available online 1 April 2008
Keywords:
Ruthenium
Ring closing metathesis
Latent catalysts
Sulfur chelation
Ó 2008 Elsevier B.V. All rights reserved.
During the last decade well defined olefin metathesis has
evolved from almost a chemical curiosity to one of the most widely
used carbon–carbon bond formation reactions in synthetic organic
chemistry [1]. One of the reasons for the tremendous success of
this method was the introduction of ruthenium based catalysts,
significantly more air and moisture stable than their predecessors
[2]. The judicious tuning of the ligand shell around the metal atom
spearheaded catalyst development, affording compounds with
acute stability and reactivity. Two of the most significant adjust-
ments to the environment of the ruthenium metal atom were the
introduction of strong r donating N-heterocyclic carbene (NHC) li-
gands [3] and the use of chelation to improve the stability of the
complex [4] (Fig. 1).
Most efforts in the field are geared towards the development of
olefin metathesis catalysts that react with a wider range of sub-
strates (electronically and sterically challenged alkenes), display
enhanced stability, and are also significantly more active. Still, pio-
neering examples of modified catalysts that afforded considerably
lower activities at room temperature were recently put forward by
Grela, Slugovc and co-workers [5]. The acquisition of catalysts with
very low activity and understanding the factors that cause this,
may possibly be the first step towards the generation of catalytic
switches. A reversible inhibition of catalyst activity might prove
beneficial not only in practical applications, but may possibly pro-
vide important insights on the catalyst mechanism. With this in
mind, we envisioned that the use of a sulfur chelating atom on
an analogue of Hoveyda–Grubbs catalyst 2 would strengthen the
ligation to ruthenium and lower the reaction rate. Thus, we
reported the synthesis, characterization and thermo-switchable
olefin metathesis activity of a latent sulfur chelated ruthenium cat-
alyst [6]. A different approach to influence reactivity of olefin
metathesis catalysts is by steric effects. For example, it has been
recently shown that the exchange of the isopropyl group by a
methyl group in catalyst 2 leads to lower activities, probably due
to a strengthening of the Ru–O chelation [7]. Herein we present
a novel series of modified sulfur chelated ruthenium catalysts
designed to alter initiation temperatures by systematic steric
acceleration effects.
Following the synthetic pathway presented in Scheme 1, sty-
rene ligands 5 were prepared. Compounds 5 were reacted with cat-
alyst 1 in the presence of CuCl to obtain catalysts 6 in moderate to
good yields [8].
All the sulfur chelated catalysts displayed the unusual cis-di-
chloro arrangement, even when the tert-butyl group was used as
the substituent, as evidenced by the asymmetry of the NMR spec-
tra (see Supplementary material). Furthermore, we were able to
corroborate this geometry by obtaining single crystal X-ray struc-
tures for all new catalysts (Fig. 2) [8]. This atypical geometry has
important effects on the catalyst behavior, for example, it has been
shown that catalysts in the cis-dichloro arrangement show lower
activity than their trans-dichloro counterparts [5c]. Grubbs et al.
have also suggested the cis-dichloro conformation as an intermedi-
ate in asymmetric olefin metathesis reactions [9]. Interestingly, in
* Corresponding authors. Tel.: +972 86461196; fax: +972 86461740 (N.G.
Lemcoff).
E-mail addresses: benasuly@bgu.ac.il (A. Ben-Asuly), lemcoff@bgu.ac.il (N.G.
Lemcoff).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.03.028

T. Kost et al. / Journal of Organometallic Chemistry 693 (2008) 2200–2203
2201
Fig. 1. Evolution of 1st generation Grubbs catalyst towards 2nd generation Grubbs catalyst 1 containing the NHC ligand and 2nd generation Hoveyda–Grubbs catalyst 2
additionally having a chelating isopropoxy group ortho to the benzylidene atom.
Table 1
Ruthenium–sulfur bond lengths for catalysts 6 and conversion to product 9
Catalyst
Ru–S bond length (Å)
Conversiona (%)
6a
6b
6c
6d
6e
2.364(1)c
2.347(2)
2.351(2)
2.340(1)
2.330(2)d
91
91
55b
25
17
a
0.1 M 8, 0.1 mol% catalyst 6. Reactions were carried out under air at 90 °C for
24 h.
b
c
See Ref. [8].
Average of four independent molecules.
Average of two independent molecules.
d
Scheme 1. Syntheses of catalysts 6a–e.
Fig. 2. Solid state structures of catalysts 6a, b, d, e. Ellipsoids drawn at 50% probability, solvent molecules and hydrogens omitted for clarity.

2202
T. Kost et al. / Journal of Organometallic Chemistry 693 (2008) 2200–2203
One of the salient features extracted from the crystal struc-
ture data is the dependence of the sulfur–ruthenium bond
length on the bulkiness of the sulfur substituent. As shown in
Table 1, there is a small, yet significant, elongation of the Ru–
S bond as the substituent on the sulfur atom becomes more
bulky. Given that according to the classical metathesis mecha-
nism the Ru–S bond must rupture for metathesis to occur,
we hypothesized that the more active catalysts would possess
longer Ru–S bonds.
Thermal activation behavior was tested by stepwise heating a
toluene solution of the catalysts in the presence of diethyl diallyl-
malonate 8. The benchmark ring closing metathesis (RCM) reaction
was monitored by GC–MS while quickly raising the temperature by
20 °C every 2 h until a temperature of 100 °C was reached. The
results are presented in Fig. 3.
As observed, the introduction of bulkier substituents indeed in-
creased the reactivity of the catalyst at lower temperatures. This
systematic steric acceleration effect is observed for all the alkyl
substituents. The phenyl substituted catalyst 6b shows a high
activity compared to the other catalysts, and we believe that stabi-
lizing electronic effects in the unbound state are in play here in
addition to the mainly steric effects that weakened the sulfur–
ruthenium bond.
Fig. 3. RCM reaction of 8 with catalysts 6a–e at increasing temperatures; each 2 h
block was kept at constant temperature. Reactions were run at 0.1 M concentration
with 1 mol% catalyst in toluene and conversion was monitored by GC–MS.
most cases observed to date, the cis-dichloro conformation is seen
when strong chelating ligands, such as pyridyl or quinoline, are
present [10].
Table 1 summarizes the ability of the catalysts to realize RCM
reactions at low loadings of catalyst. We observed that the most
Fig. 4. RCM activity for catalyst 6a at ambient conditions. 0.1 M 8, 1 mol% catalyst.
Fig. 5. RCM activity for catalysts 6b–e in air. 0.1 M 8, 1 mol% catalyst.

T. Kost et al. / Journal of Organometallic Chemistry 693 (2008) 2200–2203
2203
active catalysts also afforded higher turnover numbers, in line with
typical available Grubbs type olefin metathesis catalysts [11].
To further probe the catalytic properties and solution stability
of 6 we allowed the catalysts to stand with diethyl diallylmalonate
8 for at least two weeks at ambient conditions and monitored the
conversion to the cyclic product. Out of all the series, only catalyst
6a showed any significant catalytic activity at room temperature
(Fig. 4), albeit very mild. Notably, 6a was still active even after
20 days, reaching almost 50% conversion during this period.
In order to ascertain whether the dormant catalysts were still
active after two weeks in solution under air, 6b–e were heated first
for 2 h at 80 °C and then for 2 h at 100 °C (Fig. 5). To our satisfac-
tion, all the sulfur chelated catalysts showed noticeable RCM activ-
ity even after standing in toluene solution for two weeks under
ambient conditions that include, naturally, oxygen and humidity.
In conclusion, we present a series of cis-dichloro sulfur chelated
catalysts whose activity may be finely tuned by altering the steric
interactions on the labile ligand. The most reactive catalyst,
equipped with a tert-butyl substituent, induced an extremely slow
ring closing metathesis reaction at room temperature that, remark-
ably, was still active after more than 20 days. All catalysts showed
appreciable stability and performed ring closing metathesis reac-
tions even after prolonged exposure to air.
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2008.03.028.
References
[1] (a) A.H. Hoveyda, A.R. Zhugralin, Nature 450 (2007) 243;
(b) R.H. Grubbs, Tetrahedron 60 (2004) 7117;
(c) R.R. Schrock, Top. Organomet. Chem. 54 (1998) 1.
[2] T.M. Trnka, R.H. Grubbs, Acc. Chem. Res. 34 (2001) 18.
[3] M. Scholl, S. Ding, C.W. Lee, R.H. Grubbs, Org. Lett. 1 (1999) 953.
[4] (a) S.B. Garber, J.S. Kingsbury, B.L. Gray, A.H. Hoveyda, J. Am. Chem. Soc. 122
(2000) 8168;
(b) S. Gessler, S. Randl, S. Blechert, Tetrahedron Lett. 41 (2000) 9973.
[5] (a) R. Gawin, A. Makal, K. Wozniak, M. Mauduit, K. Grela, Angew. Chem., Int.
Ed. 46 (2007) 7206;
(b) X. Gstrein, D. Burtscher, A. Szadkowska, M. Barbasiewicz, F. Stelzer,
K. Grela, C. Slugovc, J. Poly. Sci.: Part A: Poly. Chem. 45 (2007) 3494;
(c) M. Barbasiewicz, A. Szadkowska, R. Bujok, K. Grela, Organometallics
25 (2006) 3599;
(d) C. Slugovc, B. Perner, F. Stelzer, K. Mereiter, Organometallics 23
(2004) 3622.
[6] A. Ben-Asuly, E. Tzur, C.E. Diesendruck, M. Sigalov, I. Goldberg, N.G. Lemcoff,
Organometallics 27 (2008) 811.
[7] (a) M. Barbasiewicz, M. Bieniek, A. Michrowska, A. Szadkowska, A. Makal,
K. Wozniak, K. Grela, Adv. Synth. Catal. 349 (2007) 193;
(b) For
a discussion of electronic effects on Ru–O chelation see also:L.
Gulajski, A. Michorwska, A. Bujok, K. Grela, J. Mol. Catal. A: Chem. 254
(2006) 118.
[8] The synthesis, crystal structure and catalytic activity of compound 6c have
been recently reported in Ref. [6].
[9] T.J. Seiders, D.W. Ward, R.H. Grubbs, Org. Lett. 3 (2001) 3225;
(a) For evidence on trans-conformation intermediates see:P.E. Romero, W.E.
Piers, J. Am. Chem. Soc. 127 (2005) 5032;
We are currently investigating the effects of modulating the
electronic density of the ligating atoms to modify the properties
of the catalysts and their geometry.
(b) J.M. Berlin, S.D. Goldberg, R.H. Grubbs, Angew. Chem., Int. Ed. 45 (2006)
7591.
Acknowledgement
[10] (a) We are currently actively investigating the effects of different chelating
ligand types on the cis- or trans-conformation preferences of ruthenium
benzylidene catalysts. For examples of cis-type catalysts see: X. Gstrein, D.
Burtscher, A. Szadkowska, M. Barbasiewicz, F. Stelzer, K. Grela, C.J. Slugovc,
Poly. Sci.: Part A: Poly. Chem. 45 (2007) 3494;
Partial funding of this work by the German-Israeli Foundation is
gratefully acknowledged.
(b) T. Ung, A. Hejl, R.H. Grubbs, Y. Schrodi, Organometallics 23 (2004) 5399;
Appendix A. Supplementary material
For
a theoretical study on cis–trans isomerization see:D. Benitez, W.A.
Goddard III, J. Am. Chem. Soc. 127 (2005) 12218.
[11] T. Ritter, A. Hejl, A.G. Wenzel, T.W. Funk, R.H. Grubbs, Organometallics 25
(2006) 5740.
Synthetic and characterization details for 3–6; full details for
RCM reaction of 8 and CIF files for 6 are available. Supplementary