Cp*Ru(II) and Cp*Ru(IV)-catalyzed reactions of CHX with vinyl C-H bonds: Competition between double bond homologation and olefin cyclopropanation by alkyl diazoacetate

Article abstract of DOI:10.1016/S0022-328X(98)00373-8

When diazoesters are used as carbene precursors, new Ru(II) and Ru(IV) complexes bearing various substitutents tethered ligands mediate the formal carbene insertion into C-H vinyl bonds of (substituted) styrenes to yield mostly E- and Z-styrylacetic esters (e.g. 4-phenylbut-3-enoates with styrene). This rarely observed reaction competes with the cyclopropanation of the double bond. The influence of steric and electronic factors on the two competitive reactions is reported. The observation that the most efficient C-H insertion catalysts also promote the ROMP of norbornene lend support to the formation of ruthenacyclobutanes as reaction intermediates.

Full text of DOI:10.1016/S0022-328X(98)00373-8

Journal of Organometallic Chemistry 558 (1998) 163–170
Cp*Ru(II) and Cp*Ru(IV)-catalyzed reactions of CHX with vinyl
C–H bonds: competition between double bond homologation and
olefin cyclopropanation by alkyl diazoacetate
a
a
a,
b
b
Fran c¸ ois Simal , Albert Demonceau , Alfred F. Noels *, D.R.T. Knowles , Shane O’Leary ,
b
c
Peter M. Maitlis , O. Gusev
a
CERM, Institut de Chimie-B6, Uni6ersit e´ de Li e` ge, B-4000, Sart Tilman, Belgium
Department of Chemistry, The Uni6ersity of Sheffield, Sheffield, S3 7HF, UK
b
c
A.N. Nesmeyano6 Institute of Organoelement Compounds. Academy of Sciences of Russia, Va6ilo6 St. 28, 117813, Moscow, Russia
Received 10 November 1997
Abstract
When diazoesters are used as carbene precursors, new Ru(II) and Ru(IV) complexes bearing various substitutents tethered
ligands mediate the formal carbene insertion into C–H vinyl bonds of (substituted) styrenes to yield mostly E- and Z-styrylacetic
esters (e.g. 4-phenylbut-3-enoates with styrene). This rarely observed reaction competes with the cyclopropanation of the double
bond. The influence of steric and electronic factors on the two competitive reactions is reported. The observation that the most
efficient C–H insertion catalysts also promote the ROMP of norbornene lend support to the formation of ruthenacyclobutanes
as reaction intermediates. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Ruthenium; Ruthenium-carbene; Olefin chelate; Diazoesters; Styrene; Insertion
1
. Introduction
carbene precursors [21–24], there is a dearth of infor-
2
mation on the insertion into sp ꢀC–H bonds. This
3
The understanding of how metal complexes mediate
may be because a vinyl C–H bond is stronger than sp
the formation of carbon–carbon bonds is a major
theme in organometallic chemistry. Much studied ex-
amples are the addition of metal–carbenes to olefins
and their insertion into carbon–hydrogen bonds. The
reaction products, depend on the metal, its oxidation
state and its ancillary ligands [1–3]. Ruthenium-based
carbene complexes have recently emerged as a new class
of versatile catalysts for olefin metathesis [4–11] and
olefin cyclopropanation [5,12–15,15–20]. Although in-
C–H bonds (for average bond dissociation energies, see
[25]) and that would favour either a (competitive) cy-
cloaddition onto the CꢀC double bond or an insertion
into an allylic C–H bond. In most of the examples
reported in the literature, vinyl C–H insertions are
intramolecular resulting from rearrangement of an in-
termediate cyclopropane [24,26–29]: we term these ‘ap-
parent vinyl C–H insertions’. So far as we are aware,
the most clear-cut examples of insertion into vinyl C–H
bonds of olefins ligated to a metal were described by
Werner and coworkers who recently observed the sub-
stitution of ethene C–Hs using various diazo com-
pounds [30–32]. Rhodia- and iridia-cyclobutanes were
proposed as intermediates in these reactions but no
unambiguous evidence for the involvement of the
metallacycles has yet been presented.
3
tramolecular carbenoid insertion reactions into sp C–
H bonds has proved to be a powerful method for the
construction of carbocycles, especially with dirhodiu-
m(II) catalysts and h-diazoesters or h-diazoketones as
*
Corresponding author.
0
022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00373-8

1
64
F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
Fig. 1. Effect of the catalyst on ethyl diazoacatate decomposition.
These molecules 4 and 5 formally result from an
insertion of the carbene into the different sp C–H
2
bonds of the olefin. The 4-phenylbut-3-enoates 4 are
obtained in a much larger amount than the 3-methyl-
cinnamates 5 (even with the best homologation cata-
lysts, the yield of 5 always remains below 3%)
indicating a favoured carbene ‘insertion’ into the termi-
nal C–H bonds and the possible importance of steric
factors on the course of the reaction. Diethyl maleate
and diethyl fumarate, together with some polymers and
oligomers make up the rest of the reaction products.
Blank experiments confirmed that the cyclopropanes
were stable to the reaction conditions and were not
isomerized to the homologated products, with either the
Ru(II)- or the Ru(IV)-complexes.
Scheme 1.
We now report on a new class of ruthenium(II) and
ruthenium(IV) complexes bearing various substituents
tethered ligands which mediate this rarely observed
formal carbene insertion into an olefin C–H vinyl bond
‘the homologation reaction’), a reaction which, as ex-
pected, competes with the cyclopropanation of the dou-
ble bond.
(
Table 1 summarizes the results obtained with com-
plexes 1a–h and 2a–h, respectively at 60 and 100°C.
The main trends showed by these results indicate
that:
2
. Results and discussion
The structures of the various 18 electron-Ru(II) and
Ru(IV) complexes tested as catalysts are shown in
Scheme 1 [33–36].
1. The main products are the cyclopropanes, both
from the Ru(II) and the Ru(IV) complexes, at both
60 and 100°C; the Ru(II) complexes generally give
more cyclopropanes than do the corresponding
Ru(IV) complexes.
When ethyl diazoacetate (the carbene precursor) is
added slowly to an excess of styrene containing the
catalyst, the unexpected compounds 4 to 5 are formed,
sometimes in considerable amount, in addition to the
expected cyclopropanes 3 (mixture of syn and anti-iso-
mers), see Schemes 2 and 1.
2. Substantial amounts of homologated products 4, are
also formed, in particular with Ru(IV) catalysts
2
b–f at 60°C. However the parent complexes, 1a
and 2a, form more 4 at 100°C.
3
. The syn/anti ratios of cyclopropanes are highest
when the Ru(II) complexes 1a–h are used at 60°C;
for the Ru(IV) catalysts they are somewhat higher
at 100°C than at 60°C and (for example, the 2b–f
series at 60°C) compare well with most of the ruthe-
nium-based cyclopropanation catalysts described to
date in the literature [5,12–15,15–20].
In order to get a better insight into the effect of steric
factors on the competitive homologation/cyclopropana-
tion reactions, the catalysts, 2c and 2f, were tested with
three different diazoacetates, Me-, Et- and t-Bu-dia-
Scheme 2.

F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
165
Table 1
Addition of ethyl diazoacetate to styrene catalyzed by ruthenium complexes 1 and 2 (a–h)
a
Complex
Temperature
0°C
6
100°C
Cyclopropanation
Yield, %b
Homologation (4)
Cyclopropanation
Yield, %b
Homologation (4)
syn/anti
Yield, %b
Z/E
syn/anti
Yield, %b
Z/E
1
2
1
2
1
2
1
2
2
2
1
1
a
a
b
b
c
c
d
d
e
f
70
69
62
57
75
60
73
57
63
56
57
71
0.39
0.39
0.51
0.21
0.53
0.19
0.57
0.18
0.14
0.15
0.51
0.45
7.5
16
1
30
t
1.0
1.0
2.4
1.7
—
1.75
—
1.35
1.65
1.15
—
72
57
0.22
0.20
19
38
1.0
1.0
72
96
84
94
71
66
73
90
84
0.34
0.41
0.41
0.31
0.22
0.18
0.31
0.27
0.27
18
1.5
10
2
23
25
17
2
1.3
2.1
32
t
1.65
1.75
1.4
1.55
1.13
1.45
1.16
33
29
37
B1
2
g
h
1.6
7
a
Reaction conditions: complex, 0.005 mmol; styrene, 20 mmol; under air; ethyl diazoacetate, 1 mmol diluted by styrene up to 1 ml; addition time,
4
h.
b
Determined by GLC analysis.
zoacetates (abbreviated hereafter as MeDA, EtDA and
t-BuDA, respectively) at 60°C. The results of the exper-
iments are summarized in Table 2.
The results of the experiments, summarized in Table
4, show that 6 is poor for homologation but an excel-
lent cyclopropanation catalyst (in fact the best of the
series), while 7 and its cationic analogue 8 do not show
any particular activity or selectivity when compared to
the other catalysts. However, the cationic complex 8
proved to be a quite efficient cyclopropanation catalyst
at 100°C.
The other Ru(IV) complexes 9 and 10, are similar to
the other Ru(IV) derivatives and mediate the formation
of homologated products. Their cyclopropanation ac-
tivity peaks at around 60–80°C and decreases signifi-
cantly at higher temperature. Further analysis however
indicates that there is an optimal temperature for the
homologation and for the cyclopropanation reactions
for all the complexes studied so far. This temperature
lies around 100°C for most of the complexes in Table 1
but is lower for some other complexes such as 2a, 7, 9
and 10. A partial explanation for this observation rests
on the different rates of diazoester decomposition as a
function of temperature with the different catalysts. As
an example, the rate of decomposition of EtDA cata-
lyzed by complexes 1d and 2d at 60 and 100°C is given
in the figure. The decomposition catalyzed by com-
plexes 7 (and also 9 and 10) is comparatively faster,
even at 25 or 60°C.
The different diazoesters do have a small influence on
the homologation reaction but little effect on the cyclo-
propanation. Using the less sterically demanding
MeDA favours the ‘insertion’ reaction whereas the
inverse trend is observed with the more bulky t-BuDA,
although the homologation reaction is then not com-
pletely suppressed. The Ru(IV) complex 2a without the
functionality, again, does not fit in with the general
trends.
Perhaps more surprising is the observation (Table 3)
that the electronic factors do not seem to affect the
outcome of the two reactions. Thus neither electron-do-
nating nor electron-withdrawing groups seem to have
any marked effect on the yields and stereoselectivities:
both 4-methyl styrene and 4-chloro styrene give similar
product distributions when reacted with EtDA in the
presence of a variety of (Ru(II) or Ru(IV) catalysts.
The preference for a homologation pathway with the
Ru(IV) catalysts is again observed.
These data do not show any strong influence of the
nature of the substituent/tether on the reactions stud-
ied: for example, neither the size nor its orientation
seems to play any major role. In order to investigate
further the influence of the tether, a new series of
complexes, including the cationic analogue of 1a
3. Mechanistic implications
(
Scheme 3) and complexes 7 and 8, where the sub-
stituent at CH2 is (at least initially) unambiguously
tethered to the metal centre, were then tested.
At this stage of the study, the reaction mechanism
remains speculative, although some rationalizations can

1
66
F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
Table 2
Addition of representative diazoacetates to styrene at 60°C
a
Complex
Diazoacetate
Cyclopropanation
Homologation (4)
Cyclopropanation/homologation ratio
Yield, %b
syn/anti ration
Yield, %b
Z/E ratio
1a
2a
2c
2f
N CHCO Me
66
70
66
0.54
0.39
0.38
6
7.5
3
1.43
1.00
0.98
11
9.5
22
2
2
N CHCO Et
2
2
N CHCO t-Bu
2
2
N CHCO Me
75
69
65
0.40
0.39
0.16
13
16
25
1.04
1.00
0.85
5.8
4.3
2.6
2
2
N CHCO Et
2
2
N CHCO t-Bu
2
2
N CHCO Me
57
60
60
0.20
0.19
0.06
37
32
29
1.65
1.75
1.85
1.55
1.85
2.05
2
2
N CHCO Et
2
2
N CHCO t-Bu
2
2
N CHCO Me
53
56
58
0.16
0.15
0.05
40
37
33
1.18
1.15
1.07
1.32
1.5
1.75
2
2
N CHCO Et
2
2
N CHCO t-Bu
2
2
a,b
Reaction conditions same as in Table 1.
be made based upon some quite simple assumptions on
the known chemistry of metal–carbene complexes. All
the catalysts employed here are initially 18 electron-
ruthenium(II) or (IV) complexes. In order to obtain the
metal–carbene—the key intermediate—the release of
one or more ligand(s) is necessary to create sites at
which the reaction can then proceed. This explains why
heating is needed to start the reaction and why the
different diazo compounds decompose at different rates
with the different ruthenium complexes.
The relative rates of decomposition of the diazo
compounds may be anticipated to be related to the
relative ease of the vacant site, in other words, to the
lability of the ligand. Thus, for example, the rutheniu-
m(II) complexes are expected to replace one CO ligand
by the diazo compound to form the key 18 electron-
ruthenium–carbene intermediate. This latter complex
could then react further with a non coordinated olefin
to yield a cyclopropane, in a manner similar to that
proposed for the dirhodium(II) catalysts [37,38]. The
situation for the ruthenium(IV) complexes is more com-
plex: one route by which they may be activated is by
loss of an allyl halide to yield a heavily unsaturated
Cp*RuCl, a 14 electron species which would allow the
coordination at the metal centre both of the carbene
species and of the olefin. That could give a ruthenacy-
clobutane. The chemistry of four-membered metallacy-
cles in general and that of ruthenacyclobutanes in
particular is quite varied and presently not well under-
stood [39]. Many reaction pathways are possible includ-
ing the reductive elimination of a cyclopropane as well
as decomposition to yield homologated products. How-
ever, examples of the latter reaction are relatively rare
drogen shift followed by migration of the hydride lig-
and to a terminal carbon atom of the allyl group
yielding the homologated molecule and reforming the
catalytically active species, as sketched in Scheme 4.
The preferential formation of 4 also suggests that
metallacyclobutane formation and its reaction(s) are
governed by steric and conformational factors. Accord-
ing to the proposed reaction pathway, preferential for-
mation of
4
implies preferential formation of
ruthenacyclobutane 11 relative to 12. This scheme also
rationalizes the relatively higher stereoselectivities in
cyclopropanation observed with ruthenium(IV) com-
plexes, the formation of metallacycles corresponding to
more sterically constrained transition states. The some-
what higher relative amount of homologation products
with ruthenium(II) complexes at 100°C might be due to
a more important participation of the ruthenacyclobu-
tane reaction pathway because of a (partial) disengage-
ment of the second CO ligand at this temperature. It is
also worth noting in this context that a significant
increase of homologated products is only obtained with
the less sterically hindered complex, the non-tethered
1
a.
In order to test the proposed mechanism and the
intermediacy of metallacycles, we have tested some of
our catalyst systems in the ring-opening-metathesis-
polymerization (ROMP) of bicyclo[2.2.1]hept-2-ene
(
norbornene). It is now accepted that metallacyclobu-
tanes are intermediates in olefin metathesis [4] and, if
we postulate their intermediacy in the homologation
reaction, the metallacycle should be detectable by its
propensity to initiate the formation of polymers from a
suitable cyclic alkene, e.g. norbornene. Thus, a compet-
itive reaction might lead to the formation of ROMP
products with this strained olefin, the relief of the olefin
ring-strain driving the reaction toward the formation of
polynorbornene according to Scheme 5.
[
39,40] and have not yet been reported in the chemistry
of ruthenium. In the present case, formation of isomers
and 5 can be readily explained by assuming a i-hy-
4

F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
167
Table 3
Addition of ethyl diazoacetate to 4-X-styrenes (X=H, Me, Cl) at 60°C
a
Complex
Olefin
Cyclopropanation
Yield, %b
Homologation (4)
syn/anti
yield, %b
Z/E
1a
2a
1d
2d
2f
Styrene
70
73
75
0.39
0.34
0.31
7.5
8
7
1.0
1.2
0.88
4
4
-methylstyrene
-chlorostyrene
Styrene
69
65
76
0.39
0.30
0.22
16
15
15
1.0
1.06
0.85
4
4
-methylstyrene
-chlorostyrene
Styrene
73
67
71
0.57
0.45
0.44
t
2
1
—
2.2
—
4
4
-methylstyrene
-chlorostyrene
Styrene
57
59
63
0.18
0.16
0.15
33
35
32
1.35
1.4
1.16
4
4
-methylstyrene
-chlorostyrene
Styrene
56
52
60
0.15
0.16
0.15
37
36
34
1.15
1.2
0.95
4
4
-methylstyrene
-chlorostyrene
a, b
Reaction conditions same as in Table 1.
Polymer formation is thus especially expected to be
observed with the ruthenium(IV)-based complexes.
Table 5 indicates that this is exactly what is observed
even at 60°C!). Moderate amounts of polynorbornenes
up to 54% conversion) are formed, essentially with
ruthenium(IV) catalysts when activated by reaction
with trimethysilyldiazomethane (TMSD, which is usu-
ally superior to diazoesters for initiating metathesis)
[41]. The polymers contain mostly cis-double bonds, as
often observed with Ru-based ROMP catalysts. The
relatively modest monomer conversions can be rational-
ized by the fact that the two competing reactions
(
(
(
homologation and cyclopropanation) are of course
chain termination steps in ROMP.
In the ruthenium(II) series, only complexes 7 and 8
give significant amounts of polymers. This can be ratio-
nalized by assuming a relatively easy loss of the CO
ligand in complex 7 and of both the olefin and the
nitrile ligand in 8. This hypothesis is also supported by
the observation that 7 is also the ruthenium(II) complex
which promotes the formation of higher amounts of
homologated products.
The at first sight unexpectedly small effects of the size
and nature of the CH2 substituent on the reaction
course may be due to the fact that the substituent is
normally bent back away from the metal. This tendency
is shown by a number of X-ray structure determina-
tions, which the substituent are always found bent
back. That the complexes with a tethered side chain
show similar effects suggests that the tether may be-
come unhooked early during the reaction.
4
. Conclusions
New ruthenium complexes have been shown for the
first time to catalyze the insertion of carbenes into
vinylic C–H bonds (homologation reaction). Metalla-
Scheme 3.

1
68
F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
Table 4
Addition of ethyl diazoacetate to styrene catalyzed by ruthenium complexes 6–10
a
Complex
Temperature
0°C
6
100°C
Cyclopropanation
Yield, %b
Homologation (4)
Cyclopropanation
Yield, %b
Homologation (4)
syn/anti
Yield, %b
Z/E
syn/anti
Yield, %b
Z/E
6
7
8
9
93
78
52
83
42
0.61
0.24
0.63
0.46
0.51
2.5
12
1.5
8
1.55
1.10
0.77
1.03
1.25
93
77
91
60
68
0.63
0.34
0.64
0.26
0.29
2.5
15.5
2.5
33
1.85
1.10
1.06
1.08
1.15
1
0
4
32
a, b
Reaction conditions same as in Table 1.
cyclobutanes are the putative intermediates responsible
for the homologation reaction, a reaction essentially
promoted by Ru(IV) complexes. Cyclopropanation of
the CꢀC double bond is competitive with homologation
and becomes the principal reaction with most of the
Ru(II) complexes, to the extent that some of them are
good cyclopropanation catalysts. Further studies are
now needed to assess the scope and limitations of the
catalyst systems and their potential in synthetic organic
chemistry.
5. Experimental section
5.1. General methods
The reactions were performed under nitrogen using
standard Schlenk or vacuum-line techniques. Solvents
and monomers were freshly distilled from standard
drying agents and kept under nitrogen. Ethyl diazoac-
etate and trimethylsylildiazomethane were reagent
grade and used without further purification. The ruthe-
Scheme 4.

F. Simal et al. / Journal of Organometallic Chemistry 558 (1998) 163–170
169
were based on the diazocompound and calculated with
dibutyl fumarate or diethyl phtalate as internal
standards.
5
.3. Polymerization reactions
Scheme 5.
The reaction were carried out in chlorobenzene (cata-
nium complexes were prepared according to literature
lyst 0.0075 mmol, diazo compound 0.1 mmol) and the
polymers (polynorbornenes) isolated and identified
along the lines reported in ref. [11] and [41].
[
33–36]. Products were identified by comparison of
their retention times with authentic samples on two
different GC capillary columns and by coupled GC-
MS. GC analysis were performed on a 30 m ×0.32
mm WCOT FFAP-CB column and a 30 m ×0.53
CP-Sil8-CB column, with FID and nitrogen as carrier
gas.
Acknowledgements
We thank the European Union for finantial support
(
Grant INTAS 94-393 and Human Capital and Mobil-
ity Contract ERBCHRX93 0147).
5.2. Cyclopropanation and homologation reactions
In a typical experiment, the catalyst (0.005 mmol)
and the olefin (20 mmol) were put into a round-bot-
tomed flask and heated to the reaction temperature
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1
solution of the diazoester in the olefin (1 mol l ) was
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[
[
[
[
1
2
1
2
2
1
2
2
6
7
7
8
8
9
1
a
a
c
c
c
d
d
d
N CHSi(CH )
B0.5
B0.5
B0.5
54
—
—
—
—
—
—
2
3 3
(
N CHSi(CH )
2
3 3
N CHSi(CH )
2
3 3
N CHSi(CH )
0.75 21 000
2
3 3
N CHCO C H
4
0.82
0.74
0.72 19 000
7000
2
2
2
5
N CHSi(CH )
B0.5
50
—
2
3 3
N CHSi(CH )
2
3 3
N CHCO C H
3
0.66
—
0.45 34 000
0.58 22 000
0.73 22 000
—
—
2
2 2
5
N CHSi(CH )
B0.5
47
23
2
3 3
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