Olefin cyclopropanation catalysed by half-sandwich ruthenium complexes

Article abstract of DOI:10.1016/S0040-4039(01)02292-4

Ruthenium complexes of the type [RuX(Cp′)(PPh₃)₂] (X=Cl and H; Cp′=Cp, Cp*, indenyl, and carboranyl) efficiently catalyse olefin cyclopropanation with diazoesters, and the cis/trans stereoselectivity of the resulting cyclopropanes strongly depends on the Cp′ ligand. With [RuCl(Cp*)(PAr₃)₂] complexes, cyclopropanation competes with the formal carbene insertion into C-H vinyl bonds of styrene, whereas ring-opening metathesis polymerisation takes place with norbornene, lending support to the formation of ruthenium-carbene and ruthenacyclobutanes as intermediates in these reactions.

Full text of DOI:10.1016/S0040-4039(01)02292-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 983–987
Olefin cyclopropanation catalysed by half-sandwich ruthenium
complexes
Oscar Tutusaus,a Se´bastien Delfosse,b Albert Demonceau,b,* Alfred F. Noels,b Rosario Nu´n˜ez,a
Clara Vin˜asa,* and Francesc Teixidora
aInstitut de Cie`ncia de Materials de Barcelona, CSIC, Campus de Bellaterra, Cerdanyola, 08193 Barcelona, Spain
bLaboratory of Macromolecular Chemistry and Organic Catalysis, University of Lie`ge, Sart-Tilman (B.6a), B-4000 Lie`ge,
Belgium
Received 23 October 2001; accepted 29 November 2001
Abstract—Ruthenium complexes of the type [RuX(Cp%)(PPh3)2] (X=Cl and H; Cp%=Cp, Cp*, indenyl, and carboranyl) efficiently
catalyse olefin cyclopropanation with diazoesters, and the cis/trans stereoselectivity of the resulting cyclopropanes strongly
depends on the Cp% ligand. With [RuCl(Cp*)(PAr3)2] complexes, cyclopropanation competes with the formal carbene insertion
into C–H vinyl bonds of styrene, whereas ring-opening metathesis polymerisation takes place with norbornene, lending support
to the formation of ruthenium–carbene and ruthenacyclobutanes as intermediates in these reactions. © 2002 Elsevier Science Ltd.
All rights reserved.
In recent decades there has been an exponential
increase in the use of transition metals in organic
synthesis. Among the different types of transition-
metal-based reagents described, carbene complexes are
among the most versatile.1 The applications of carbene
complexes include both their use as catalysts for a
number of important synthetic transformations2 and
their utilisation as stoichiometric reagents.3 In the
period between the discovery in the late 1950s that
copper catalysed the addition of diazo compounds to
olefins to yield cyclopropanes and the introduction of
inactivated olefins, and quite low turnover numbers. In
addition, the factors governing catalyst activity and
the mechanism by which ruthenium catalysts perform
olefin cyclopropanation are not known.8 Investigations
were undertaken to address both of these topics in the
following way: by varying the ligand sphere around
the ruthenium catalyst, we wished to determine how
the electronic and steric properties of the ligands affect
catalyst activity. Ruthenium complexes of the type
[RuX(Cp%)(PPh3)2] (X=Cl, H; Cp%=Cp, Cp*, indenyl,
and carboranyl) (1–4, Scheme 1) were chosen as
potential catalysts for two reasons: (1) upon Cp% lig-
and substitution, it is expected to modify the electronic
contributions in these systems. The higher electron
donating ability of Cp* compared to Cp is well-estab-
lished,9 and the capacity of carboranyl ligands
([C2B9H11]2−) to stabilise uncommon and high oxida-
tion states of the metals as well.10 (2) On the other
hand, Cp% substitution also results in changing the
steric properties of the ligands, which are expressed by
the cone angle. In this way, Cp* is obviously bulkier
than Cp and, most probably, than the carboranyl lig-
and [C2B9H11]2−,11a,b although the relative size of the
latter compared to Cp and Cp* is still a question
under debate.11c,d The indenyl ligand poses a more
complex problem since it is known to undergo a facile
metal ring slippage from h5- to h3-coordination, lead-
ing to the creation of a vacant coordination site on
the metal to host an entering ligand or substrate.12
chiral catalysts for asymmetric cyclopropanation,4
a
wide variety of useful transition-metal-based catalysts
has been discovered. Nowadays, rhodium carboxyl-
ates, discovered by Noels and Hubert in the early
1970s,5 play a prominent role in carbene chemistry,
and display some of the highest efficiency and versatil-
ity.
Ruthenium has been introduced recently as a much
cheaper alternative to rhodium,6 and quite interesting
results have been reported in the literature.7 Until
now, however, limitations associated with most ruthe-
nium complexes include failure to cyclopropanate
Keywords: cyclopropanation; diazo compounds; metathesis; olefins;
ruthenium and compounds.
* Corresponding authors. Fax: 32-(0)4 366 3497 (A.D.); fax: 34-(0)93/
580 57 29 (C.V.); e-mail: a.demonceau@ulg.ac.be; clara@icmab.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02292-4

984
O. Tutusaus et al. / Tetrahedron Letters 43 (2002) 983–987
Scheme 3.
In addition, it was observed that the product distribu-
tion does not change over the course of the reaction,
indicating that the products 5 and 6 are not formed by
ruthenium-assisted opening of the cyclopropanes. For
styrene, the combined cyclopropanation and CꢀH
insertion products were isolated in 97% yield, and the
product distribution was determined to be 57:22:18,
respectively. Noteworthy, formation of CꢀH insertion
products is distinctive of the [Ru–Cp*] fragment, and is
independent of the phosphine used: [RuCl(Cp*)] com-
plexes with isosteric para-substituted triarylphosphines
differing only by their electronic contributions (PPh3
(2a), P(p-C6H4-OCH3)3 (2b), and P(p-C6H4-CF3)3 (2c))
gave the same reactivity pattern.
Scheme 1.
Having established that complexes 1 and 4 show the
best catalytic performance for the cyclopropanation of
styrene, we then investigated the reaction of ethyl dia-
zoacetate with different styrene derivatives, 1-octene
and cyclooctene as well. The reactions were carried out
at 40°C, a temperature which, while being fairly moder-
ate, allows for olefin cyclopropanation to be accom-
plished efficiently (Fig. 1). The results of the
cycloaddition reactions are displayed in Table 2. In line
with the previous data obtained with styrene as starting
material, the reactions of ethyl diazoacetate with
styrenics afforded the corresponding cyclopropanes in
good yields, with predominantly cis stereoselectivity
In this paper, we report that ruthenium complexes of
the type [RuX(Cp%)(PAr3)2] (X=Cl, H; Cp%=Cp,13
Cp*,14 indenyl, and carboranyl15) efficiently catalyse the
cyclopropanation of styrenics with diazoesters.
To determine the relative activities and stereoselectivi-
ties of ruthenium catalysts 1–4, the cyclopropanation of
styrene with ethyl diazoacetate was measured under a
standard set of conditions (Scheme 2, R=Ph). The
results are summarised in Table 1. The half-sandwich
ruthenium complex, [RuCl(Cp)(PPh3)2] (1), proved to
be an effective catalyst for cyclopropanation of styrene.
Cyclopropane products were obtained in high yield and
with predominantly cis stereoselectivity. To examine the
influence of the Cp% ligand toward catalyst activity, the
half-sandwich ruthenium compounds 2–4 were then
employed. Compared to 1, complexes 2–4 led to the
reversed stereoselectivity, with the trans isomer as the
main product. On the other hand, ruthenacarboranes 4
gave rise to cyclopropanation yields similar to that
of 1. Noteworthy, with the related complexes,
[RuCl(Cp*)(PPh3)2] (2a) and [RuCl(Ind)(PPh3)2] (3), the
reaction of ethyl diazoacetate with styrene proceeded
smoothly. Catalysts 2a and 3 showed a lower catalytic
activity (around 60% cyclopropanation yield); however,
carbene insertion into the vinylic CꢀH bonds was also
observed to some extent. Upon monitoring by gas
chromatography the reaction of styrene and ethyl dia-
zoacetate catalysed by [RuCl(Cp*)(PPh3)2], it was
determined that in addition to forming the cyclo-
propanes, the CꢀH insertion products 5 and 6 (Scheme
3) appear initially as well.
Table 1. Ruthenium-catalysed cyclopropanation of styrene
by ethyl diazoacetatea
Cyclopropanation
Complex
Yield (%)b
cis/trans ratio
1
85
57
59
61
68
85
86
88
1.85
0.17
0.25
0.30
0.48
0.48
0.52
0.52
2a
2b
2c
3
4a
4b
4c
a Reaction conditions: complex, 0.005 mmol; styrene, 2 mL; ethyl
diazoacetate, 1 mmol diluted in 1 mL of styrene; addition time, 4 h;
40°C.
b Based on ethyl diazoacetate.
Scheme 2.

O. Tutusaus et al. / Tetrahedron Letters 43 (2002) 983–987
985
of dissociation of one phosphine.13,16 It is therefore
reasonable to assume that the initial stage of the cata-
lytic cyclopropanation of olefins implies the generation
of the carbene intermediate [RuCl(ꢁCHCO2Et)(Cp%)-
(PAr3)], by reaction of the diazo compound with the
16-electron complex [RuCl(Cp%)(PAr3)] formed by dis-
placement of one phosphine ligand.13 With this in
mind, the formation of the cyclopropanes could then
occur though the transfer of the carbene fragment onto
a non coordinated olefin. This hypothesis, however,
does not account for the formation of product 6.
According to the literature,8,13,17 the formation of 6
(and 5 as well) would result rather from the rearrange-
ment (likely via an h3-allylhydrido intermediate) of a
metallacyclobutane,
a key intermediate in olefin
metathesis which is in equilibrium with a metal–car-
bene–olefin complex. If we postulate the intermediacy
of ruthenacyclobutanes in the formation of compounds
5 and 6, metallacyclobutanes should be detectable by
their propensity to initiate the formation of polymers
from a suitable cyclic olefin. In order to test this
hypothesis, we tested complexes 1–4 in the ring-opening
metathesis polymerisation (ROMP) of norbornene
(Scheme 4). Moderate amounts of polynorbornenes (up
to 63% yields) were formed, essentially with catalysts 2
and 3 when activated by reaction with trimethylsilyldia-
zomethane, which is usually superior to diazoesters for
initiating metathesis.18 Most gratifyingly, we also noted
that the most efficient catalysts for olefin homologation
were also the most active ones for ROMP (Table 3),
therefore supporting the assumption of the intermedi-
acy of a ruthenacyclobutane in the homologation reac-
tion. A speculative catalytic cycle is presented on
Scheme 5.
Figure 1. Influence of the temperature on the decomposition
rate of ethyl diazoacetate in styrene in the presence of com-
plexes 1 (rt (ꢀ), and 40°C (ꢁ)), and 4a (rt (ꢂ), 40°C (ꢃ),
60°C (ꢄ) and 80°C (ꢅ). Reaction conditions same as in
Table 1.
with catalyst 1, and trans stereoselectivity with complex
4a. By contrast, non activated olefins (1-octene and
cyclooctene) were much less reactive, giving mainly
dimethyl maleate and only minor cyclopropanation
products. Apparently, electron-rich olefins show a
higher reactivity toward the intermediate carbene spe-
cies, and thus carbene dimerisation can be suppressed.
[RuCl(Cp%)(PAr3)2] are 18-electron complexes, and it is
generally agreed that the catalytic activity of this class
of ruthenium complexes depends on the relative facility
Scheme 4.
Table 2. Ruthenium-catalysed cyclopropanation of various olefins by ethyl diazoacetatea
Complex 1
Complex 4a
Cyclopropanation
Cyclopropanation
Olefin
Yield (%)b
cis/trans ratio
Yield (%)b
cis/trans ratio
Styrene
85
87
82
89
87
82
5
1.85
1.17
1.01
1.27
1.45
1.63
0.39
1.33c
85
86
81
84
79
84
39
15
0.48
0.49
0.59
0.68
0.62
1.13
0.60
0.60c
4-Methylstyrene
4-tert-Butylstyrene
4-Methoxystyrene
4-Chlorostyrene
a-Methylstyrene
1-Octene
Cyclooctene
16
a Same as in Table 1.
b Same as in Table 1.
c endo/exo ratio.

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O. Tutusaus et al. / Tetrahedron Letters 43 (2002) 983–987
Table 3. Ring-opening metathesis polymerisation of nor-
bornene catalysed by complexes 1–4a
phine ligands from the [RuCl(Cp%)(PAr3)2] complexes or
the release of only one phosphine and metal ring-slip-
page of indenyl in [RuCl(Ind)(PPh3)2]. The formation
of ruthenacyclobutanes also infers the generation of
RuIV species. Not surprisingly, the Cp* and indenyl
ligands are known to display electron-releasing proper-
ties that are much more pronounced than those of
Cp.19 Accordingly, the [RuCl(Cp%)] moiety (Cp%ꢁCp*
and Ind) should stabilise the RuIV–cyclobutane more
reliably than [RuClCp], favouring therefore a metathet-
ical reaction pathway.
b
b
c
Complex
Polymer yield (%)
Mn
Mw/Mn
|
c
1
1
26
19
63
55
7
–
–
–
–
–
–
–
–
7.7
10.6
11.5
10.7
0.38
0.54
0.44
0.36
0.54
0.66
–
2a
2b
2c
3
4a
4b
4c
35 500
33 500
39 000
39 500
1
2
0.36
The intermediacy of ruthenacyclobutanes in the
homologation reaction of olefins remains, however,
questionable, and observing olefin metathesis does not
infer that homologation takes place via the same
ruthenacyclobutane intermediate. Alternative mecha-
nisms could be proposed to account for olefin homolo-
gation. Nevertheless, it is well-known in organometallic
chemistry that ruthenacyclobutanes are very unstable
species, and only very few have been synthesised and
fully characterised.20 In olefin metathesis, a reaction in
which the intermediacy of a metallacyclobutane seems
to be no doubt, ruthenacyclobutanes have never been
detected despite numerous efforts world-wide.21 In
addition, theoretical studies, including molecular
dynamics simulations,22 revealed the formation of a
ruthenacyclobutane intermediate, but in a very high-
energy state. In the present case, since the activity of
the ruthenium catalysts for olefin homologation paral-
lels pretty well that for olefin metathesis, the involve-
ment of a common ruthenacyclobutane intermediate
for both reactions is quite plausible, though speculative.
However, due to the transient nature of ruthenacy-
clobutanes, a clear spectroscopic evidence for their
intermediacy in the homologation reaction seems to be
out of reach nowadays.
a Reaction conditions: 0.0075 mmol of catalyst and 0.5 g of norbor-
nene were dissolved under nitrogen in 30 mL of purified chloroben-
zene. The resulting solution was heated to 60°C over 20 min, and
0.1 mmol of trimethylsilyldiazomethane diluted in
1 mL of
chlorobenzene was then added to the reaction mixture via a syringe.
The reaction mixture was kept at 60°C for 5 h, then cooled to room
temperature, and precipitated in 700 mL of technical methanol.
b Determined by GPC, using polystyrene standards.
c Fraction of cis units, determined by 1H and 13C NMR.
The formation of ruthenacyclobutanes infers that the
carbene and the olefin are both coordinated to the
metal centre, and hence the presence of two cis vacan-
cies resulting either from the displacement of two phos-
In conclusion, we have shown that [RuCl(Cp)(PPh3)2]
(1) and their carboranyl derivatives (4) are highly
efficient catalyst precursors for promoting olefin cyclo-
propanation under mild conditions, with 1 exhibiting a
high cis stereoselectivity and 4 a significant preference
for the trans isomers. In addition to forming cyclo-
propanes, [RuCl(Cp*)(PAr3)2] (2) also catalyses the
insertion of carbenes into vinylic CꢀH bonds likely via
a ruthenacyclobutane intermediate. A detailed under-
standing of the reaction mechanism must await further
study, and improvements with this family of ruthe-
nium(II) complexes through modification of the stereo-
electronic parameters of the ligands are now under
investigation.
Acknowledgements
This work was financially supported by a grant-in-aid
(O.T.) joined to the project MAT98-0921 from CICYT
(Spain). Part of this work has also been carried out in
the framework of the COST D17/006/00 and TMR-
HPRN CT 2000-10 ‘Polycat’ programmes. The Belgian
team is also grateful to the ‘Fonds National de la
Scheme 5.

O. Tutusaus et al. / Tetrahedron Letters 43 (2002) 983–987
987
Recherche Scientifique’ (F.N.R.S.), Brussels, for the
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