Stable N-heterocyclic carbene (NHC)-palladium(0) complexes as active catalysts for olefin cyclopropanation reactions with ethyl diazoacetate

Article abstract of DOI:10.1002/chem.201102900

The Pd⁰ complexes [(NHC)PdL_n] (NHC=N-heterocyclic carbene ligand; L=styrene for n=2 or PR₃ for n=1) efficiently catalyse olefin cyclopropanation by using ethyl diazoacetate (EDA) as the carbene source with activities that improve on previously described catalytic systems based on this metal. Mechanistic studies have shown that all of these catalyst precursors deliver the same catalytic species in solution, that is, [(IPr)Pd(sty)], a 14e^- unsaturated intermediate that further reacts with EDA to afford [(IPr)Pd(=CHCO₂Et)(sty)], from which the cyclopropane is formed.

Full text of DOI:10.1002/chem.201102900

FULL PAPER
DOI: 10.1002/chem.201102900
Stable N-Heterocyclic Carbene (NHC)–Palladium(0) Complexes as Active
Catalysts for Olefin Cyclopropanation Reactions with Ethyl Diazoacetate
[
a]
[a]
[b]
[a]
Carmen Martꢀn, Francisco Molina, Eleuterio Alvarez, and Tomꢁs R. Belderrain*
0
Abstract: The Pd complexes [(NHC)PdL ] (NHC=N-heterocyclic carbene
n
ligand; L=styrene for n=2 or PR for n=1) efficiently catalyse olefin cyclopropa-
3
nation by using ethyl diazoacetate (EDA) as the carbene source with activities
that improve on previously described catalytic systems based on this metal. Mech-
Keywords: cyclopropanation · di-
azo compounds · homogeneous cat-
alysis · palladium
anistic studies have shown that all of these catalyst precursors deliver the same
ꢀ
catalytic species in solution, that is, [(IPr)Pd
A
H
U
G
E
N
N
(sty)], a 14e unsaturated intermediate
that further reacts with EDA to afford [(IPr)Pd
cyclopropane is formed.
ACHTUNGTRENN(NUG =CHCO Et) ACHNUTGTRENNNU(G sty)], from which the
2
Introduction
Cyclopropanes are widespread in nature and constitute the
[1]
building blocks of many biologically active compounds.
Among the various methods used for their preparation, the
transition-metal-catalysed cyclopropanation of olefins with
0
The already-described Pd -based catalysts for these trans-
[
2]
diazo reagents has attracted great interest [Eq. (1)]. Many
transition-metal complexes, especially those containing rho-
formations contain phosphanes or olefins as ancillary li-
[9d,11c]
gands.
On the other hand, Pꢀrez and co-workers have
[3]
[2e,4]
[5]
[5f,k,6]
[7]
dium, copper,
cobalt, iron,
or ruthenium, have
reported that group 11 metal complexes containing N-heter-
ocyclic carbene (NHC) ligands are active catalysts towards
carbene transfer from ethyl diazoacetate (N CHCO Et;
EDA) to several saturated or unsaturated substrates.
Although Pd –NHC compounds have been successfully em-
been employed to induce this type of transformation. Inter-
estingly, one of the most commonly employed metals in ho-
mogenous catalysis, palladium, has scarcely been described
as a catalyst for this process, in spite of the discovery of its
2
2
[
4q,12]
0
[8]
application for this purpose in the mid sixties. During the
last decade, a number of contributions relating to the use of
diazomethane as a carbene reagent with palladium-based
ployed as catalysts in several reactions, such as CꢀC and Cꢀ
N coupling reactions, oxidations, telomerisations and hydro-
[13]
genations,
to the best of our knowledge there have not
been any reports of the use of these compounds in olefin cy-
clopropanation. In addition, there is still debate about
the oxidation state of the palladium catalytic species in
these compounds.
[9]
catalysts have appeared, a topic recently reviewed by
[
10]
[14]
[15]
Wang and Zhang.
However, such catalysts seem to fail
[9a,b,11]
with alkyl diazoacetates as the reactant,
in spite of the
fact that they are stable and either commercially available
or easy-to-prepare reagents, in contrast to diazomethane,
which is known to be unstable.
On the basis of this, we decided to prepare a series of
novel (NHC)Pd complexes and investigate their potential
0
as catalysts for the olefin cyclopropanation reaction with
alkyl diazoacetate reagents. We have found that they are
quite active catalysts for the olefin cyclopropanation reac-
tion. A detailed mechanistic study, including kinetic and
thermodynamic data, has led to the proposal of a mecha-
nism for this transformation.
[
a] C. Martꢁn, F. Molina, Prof. T. R. Belderrain
Laboratorio de Catꢂlisis Homogꢀnea
Departamento de Quꢁmica y Ciencia de los Materiales
Unidad Asociada al CSIC
Centro de Investigaciꢃn en Quꢁmica Sostenible (CIQSO)
Campus de El Carmen s/n
Universidad de Huelva, 21007-Huelva (Spain)
Fax : (+) 34959219942
E-mail: trodri@dqcm.uhu.es
Results and Discussion
[
b] Dr. E. Alvarez
Instituto de Investigaciones Quꢁmicas
CSIC-Universidad de Sevilla
Avenida de Amꢀrico Vespucio 49
0
Synthesis and characterisation of complex [(IPr)Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(sty) ]
2
(
1): Although several methods have been employed for the
0
[16,17]
synthesis of mono-NHC–Pd –alkene complexes,
we
4
1092 Sevilla (Spain)
have prepared complex 1 in good yields by following a
method reported by Nolan and co-workers for the prepara-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102900.
Chem. Eur. J. 2011, 17, 14885 – 14895
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14885

0
tion of mono-NHC–Pd –PR complexes from [(NHC)Pd-
3
[18]
A( allyl)Cl].
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Thus, the reaction of [(IPr)PdACHNUTGTRENNUNG
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
KOtBu (1 equiv) in iPrOH in the presence of an excess of
styrene [sty; Eq. (2)] led to the formation of 1 in high yields
(
80%). This complex is stable in the solid state and can be
kept for an unlimited period of time under an inert atmos-
phere.
Complex 1 was characterised by NMR spectroscopy and
by X-ray crystallography. It shows fluxional behaviour, as in-
ferred from the line broadening observed in the variable
temperature (VT) NMR spectra (see the Supporting Infor-
mation). At room temperature the resonances correspond-
ing to the CH protons of iPr and the vinyl protons of the
olefin appear as three broad peaks at d=3.07, 3.19 and
1
3
.89 ppm in the H NMR spectrum (Figure 1I), which may
result from 1) fast (on the NMR timescale) exchange pro-
cesses with the free olefin, and/or 2) fast rotation of the
olefin around the palladium–olefin bond axis. At low tem-
peratures (ꢀ408C; Figure 1 II), the resonances of the me-
thine CH of the iPr groups split into two signals at d=2.83
1
Figure 1. Top: Olefinic resonances (labelled as a, b and c) in the H NMR
spectrum of 1 at low (II) and room temperature (I). Bottom: VT
H NMR spectra of 1 in the presence of six equivalents of styrene.
Sample conditions: [Pd]=14 mm and [sty]=84 mm in [D ]toluene.
8
(
heptet, J=6.5 Hz, 2H) and 3.43 ppm (heptet, J=6.5 Hz,
1
2H), whereas the olefinic protons appear as three signals at
2.94 (d, J=9.0 Hz, 2H), 3.07 (d, J=12.4 Hz, 2H) and
3.75 ppm (dd, J=9.3, J=12.4 Hz, 2H), at much higher field
than for free styrene (d=5.00, 5.59 and 6.55 ppm in C D ),
6
6
indicating strong back donation of electron density from the
in the presence of the olefin (Figure 2) enabled the determi-
°
palladium metal into the p* orbitals of the alkene. Accord-
nation of activation parameters: DH =22.4ꢁ1.6 Kcal/mol
13
1
°
ingly, in the C{ H} NMR spectrum of 1 at ꢀ408C, the
and DS =31ꢁ5 ue. This positive value for the activation
olefin carbon atoms resonate at d=74.7 (s, olefin CH) and
5
3.0 ppm (s, olefin CH ; d=135.5 and 112.0 ppm for the free
2
[19]
olefin).
Since we could not extract conclusions from the VT NMR
studies of the complex alone, we studied the effect of the
1
presence of excess olefin on the H NMR spectrum at room
temperature (Figure 1, bottom). Thus, the addition of free
olefin (6 equiv) afforded even broader peaks for the protons
of the coordinated olefin. Although this behaviour is expect-
ed for an associative olefin-exchange process at the metal
[20]
center, we could not discard the dissociative mechanism
since at 608C only one set of signals was observed for the
methine and the methyl protons of the iPr groups of the
NHC ligand. Furthermore, an Eyring plot obtained by use
Figure 2. Eyring plot of the temperature-dependent behaviour of the
olefin exchange reaction. Sample conditions: [Pd]=14 mm and [sty]=
1
of this VT H NMR study (monitoring the change in the sig-
nals corresponding to the methyl groups of the IPr ligand)
8
84 mm in [D ]toluene.
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Chem. Eur. J. 2011, 17, 14885 – 14895

Olefin Cyclopropanation
FULL PAPER
[16d]
entropy supports the proposal of a dissociative mechanism.
[(IMes)Pd
A
H
U
G
E
N
N
(DMF) ]),
and therefore the two styrene mole-
2
Such a pathway would involve the formation of the unsatu-
cules are coordinated to the metal in an almost planar fash-
ion.
ꢀ
rated 14e species [(IPr)Pd
A
H
U
G
E
N
N
(sty)] in a similar manner to that
proposed by Elsevier and co-workers in the (NHC)Pd-cata-
[
21]
lysed transfer hydrogenation of alkynes.
Catalytic cyclopropanation reaction of styrene by using
NHC–Pd complexes as catalysts: Once complex 1 was char-
0
The proposed structure of complex 1 has been confirmed
by single-crystal X-ray analysis (recrystallised by using tolu-
ene as the solvent). An ORTEP diagram of 1 is shown in
Figure 3. The asymmetric unit of the structure in the crystal
acterised, it was tested as the catalyst precursor in the sty-
rene cyclopropanation reaction by using EDA as the car-
0
bene source. A solution of the Pd complex (0.01 mmol) in
dichloromethane was charged with styrene (5 mmol) and
EDA (1 mmol) to give a [Pd]/
ACTHUNGTRENNUNG[ EDA]/ ACHTNUGTRENNUNG[ styrene] ratio of
1
:100:500. The reaction was monitored by GC, which
showed the gradual disappearance of EDA and appearance
of cyclopropanes. After 4 h, no diazo compound was detect-
ed in the reaction mixture, the analysis of the crude prod-
ucts (after volatles had been removed) showed the forma-
tion of a mixture of the cis- and trans-cyclopropanes
[
Eq. (3)] in 98% yield based on the remaining EDA. The re-
maining initial diazoacetate was converted into a mixture of
diethyl fumarate and maleate, a process also catalysed by
complex 1 [Eq. (4)].
2
Figure 3. ORTEP diagram of [(IPr)Pd ACHTNUGTRNEUNG( sty) ] (1). Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level. Selected bond lengths [ꢅ] and angles [8] for 1: PdꢀC1 2.101(7),
C15ꢀC16 1.397(8), PdꢀC15 2.147(6), PdꢀC16 2.189(6), C1-Pd-C15
9
7.89(17), C1-Pd-C16 135.23(15), N1-C1-N’1 104.6(6).
is formed of two structually equivalent, symmetrically inde-
pendent half molecules of the carbene complexes, with the
others halfs generated by a crystallographic twofold axis of
symmetry (see the Supporting Information). The PdꢀC1
After determining the catalytic capability of [(IPr)Pd-
AHCTUNGTNERNUG( sty) ] towards styrene cyclopropanation with EDA, we de-
2
cided to screen a series of related complexes of composition
[(NHC)PdL] in this test reaction (see the Experimental Sec-
tion for their preparation or in situ generation procedures).
bond length, 2.101(7) ꢅ, is somewhat longer than those
[16a–c]
found for other NHC–palladium olefin complexes,
but
Complexes 2–4 contain IPr, as well as a PR ligand; complex
3
0
is similar to that reported for [(IMes)Pd(DMF) ] (IMes=
A
H
U
G
R
N
U
G
5 corresponds to a biscarbene Pd complex, whereas com-
2
1
,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; DMF=di-
plex 6 contains an IMes ligand, along with PCy (Cy=cyclo-
3
[16d]
methyl fumarate).
nated styrenes, C15=C16 1.397(8) ꢅ, are longer than in free
styrene (1.346(20) ꢅ). This distance is similar to that re-
ported for the Rh complex [ (PNP)Rh
The C=C double bonds in the coordi-
hexane). The data in Table 1 show that complexes 1–4 and 6
display nearly identical catalytic behaviour in terms of activ-
ity and diastereoselectivity, with only the biscarbene com-
[22]
I
A
H
U
G
R
N
N
(sty)]X (PNP=2,6-
plex [IPr Pd] (5) being ineffective. Table 1, entries 1, 3 and 5
2
ꢀ
bis[(diphenylphosphino)methyl]pyridine; X=BF ) and
correspond to experiments carried out with a [Pd]/ ACHTUNGTRENNUNG[ EDA]/ ACHTUNGTRENNUNG[ -
4
II
II
[23]
longer than that for Pt and Pd complexes. This is again
consistent with significant metal to olefin p back donation.
The NHC plane is oriented at an angle of 59.87(26)8 to the
coordination plane of the styrenes (C15-Pd-C’15), in a simi-
styrene] ratio of 1:100:500, that is, 1 mol% of the Pd cata-
lyst, and provided nearly quantitative conversion into the
desired cyclopropanes. Interestingly, the cis:trans ratio was
also identical, providing what could be considered to be an
indication of the existence of a common catalytic species, at
least for catalysts 1–4. A second series of experiments car-
lar manner to the complex [(IMes)Pd
A
H
U
G
R
N
U
G
2
separation, 2.147(6) ꢅ, is shorter than the PdꢀC16 separa-
tion 2.189(6) ꢅ, as already reported for other styrene com-
ried out with 0.5 mol% of the catalyst ([Pd]/ ACHTUNGTREUNNNG[ EDA]/ ACHTUNGTRENNUNG[ styr-
[24]
plexes.
planes of the two styrenes (Pd-C15-C16 and Pd-C24-C25) is
1.23(35)8 (only slightly higher than in the case of
The dihedral angle between the coordination
ene]=1:200:1000; Table 1, entries 2 and 4 produced similar
conversions, yields and diastereoselectivities, although
longer reaction times were required.
1
Chem. Eur. J. 2011, 17, 14885 – 14895
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14887

T. R. Belderrain et al.
0
Table 1. Styrene cyclopropanation by using the NHC–Pd complexes 1–6
as the catalysts.
Table 2. Cyclopropanation of 1-hexene and cyclooctene by using EDA
[
a]
and [(IPr)Pd
A
H
U
G
R
N
N
(PPh
3
)] (3) as the catalyst precursor.
Time Conversion
h]
Olefin
Yield
[%]
cis/trans
[
b]
[c]
[
[%]
1
2
1-hexene
cyclooctene
24
24
98
63
95
58
40:60
13:87
[
a] [Pd]/
A
H
U
G
E
N
N
A
T
N
T
E
N
N
[olefin]=1:100:500. [b] Conversions were determined by
1
H NMR spectroscopy. [c] The percentage yield of the cyclopropane at
the end of the reaction determined by using 1,4-dimethoxybenzene as the
internal standard (diethyl fumarate and maleate accounted for the rest of
the EDA).
Catalyst
precursor
Time
[h]
Conversion
[%]
Yield
[%]
cis/trans
[c]
[d]
[
[
a]
b]
1
2
3
4
5
6
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IPr)Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(sty)
2
2
], 1
], 1
4
24
4
24
48
24
99
98
99
99
2
98
98
98
93
n.d.
90
38:62
37:63
37:63
37:63
n.d.
37:63
tablished the relative reactivity of these three olefins to be
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IPr)Pd(sty)
ACHTUNGTRENNUNG
3
.80:1.20:1.00, respectively. This trend is similar to that
[
a]
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IPr)PdL], 2–4
found by Noels and co-workers with Pd(OAc) as the cata-
AHCTUNGTRENNUNG
[
b]
2
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IPr)PdL], 2–4
[
a]
[e]
[e]
lyst (2.36:1.31:1.00), with the aforementioned differences in
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IPr)
2
Pd], 5
(PCy
0
[
a]
A
C
H
U
N
G
T
R
E
N
N
U
N
G
[(IMes)Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
)], 6
99
activity favouring the NHC–Pd -based system.
[
[
a] [Pd]/
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[EDA]/
ACHTUNGTRENNUNG[ olefin]=1:100:500. [b] [Pd]/ ACHTNUTRGENNUNG[ EDA]/ ACUTHNGTRENNUNG[ olefin]=1:200:1000.
1
c] Conversions were determined by H NMR spectroscopy. [d] The per-
Mechanistic studies
centage yield of the cyclopropane at the end of the reaction determined
by using 1,4-dimethoxybenzene as the internal standard (diethyl fumarate
and maleate accounted for the rest of the EDA). [e] n.d.=not deter-
mined.
Kinetics: Having demonstrated the unprecedented catalytic
activity of the aforementioned Pd complexes towards the
0
olefin cyclopropanation reaction with diazoacetates as the
carbene source, we focussed on the elucidation of the reac-
tion mechanism. To gain information about this, we first
monitored the evolution of nitrogen in the decomposition of
EDA in the abscense and presence of styrene with [(IPr)Pd-
As mentioned above, very few examples of the use of pal-
ladium-based catalysts for olefin cyclopropanation with diaz-
oacetates have been described most of which employ a Pd
II
precursor. Seminal work was described by Noels and co-
ACHTUNGTNERNUN(G sty) ] (1) as the catalyst precursor. Figure 4 shows that in
2
the absence of styrene the reaction does not reach comple-
[11c]
workers and utilised Pd
Styrene and EDA were employed in a 15:1 ratio to give
8% conversion into the cyclopropane. They also reported
the use of [Pd(PPh ) ] as the catalyst for the same reaction,
A
H
U
G
R
N
N
(OAc) as the catalyst precursor.
2
tion (1 mmol of EDA should provide 1 mmol of N ), in con-
2
9
trast with the experiment carried out in the presence of sty-
rene, for which all of the initial EDA is consumed. The use
of a phosphine-containing precatalyst, such as [(IPr)Pd-
ACHTUNGTRENNUNG
3
4
although only a 57% yield of the cyclopropane was ach-
0
ieved. In our case, Pd complexes 1–4 and 6 improved on
ACHTUNGTNERNUNG( PPh )] (3) provided additional information. In this case, an
3
this activity, with the added advantage of employing only a
initiation period of approximately 50 min was observed
prior to nitrogen evolution (Figure 5, line A). A series of ex-
5
:1 ratio of [styrene]/
cyclopropane (80% in 4 h) were obtained even if the reac-
tion was carried out by using only a 1:1 ratio of [styrene]/[E-
ACHTUNGTRENUNNG[ EDA]. Importantly, high yields of the
periments in which different amounts of PPh were added
3
A
H
U
G
R
N
U
led to the observation of an increase in the initiation period
(Figure 5, lines B and C) with eventual inhibition of the re-
DA]. This is a remarkable result since previous palladium-
based catalytic systems have required an excess of the
olefin. It seems that the replacement of phosphine ligands
with IPr is the key to this success. However, these systems
are less active than those reported for other transition
metals. For instance, the group of Pꢀrez has reported that
Ms
Ms
the turnover frequency (TOF) for the [Tp Cu] (Tp =hy-
ꢀ
1
drotris(3-mesitylpyrazolyl)borate) complex is 250 mmolh ,
whereas in the case of 1, the TOF is 24 mmolh .
ꢀ
1 [4 s]
Unusually, our system also extends to other usually less
reactive olefins, such as 1-hexene and cyclooctene. As
shown in Table 2, both the olefin in 1-hexene and cyclooc-
tene can be converted into the corresponding cyclopropane
in high (95%) or moderate (58%) yield, respectively. These
yields are much higher than those reported with Pd
A
H
U
G
R
N
U
G
Figure 4. Plot of nitrogen evolution in the reaction of EDA in the ab-
sence (A) and in the presence of styrene (B), catalysed by 1. Reaction
2
for the same olefins (30% for 1-hexene and 20% for cyclo-
conditions:
.95 mmol, solvent=CH
1]=0.02 mmol; [EDA]=0.95 mmol; [styrene]=5 mmol, solvent=
CH Cl (10 mL).
A) [catalyst]/
A
H
U
G
R
N
U
G
[1]=0.02 mmol;
[EDA]=
0
octene), providing evidence that the Pd -based catalysts re-
0
2
Cl
2
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
[11c]
ported herein are quite active.
Competition experiments
[
carried out with styrene, 1-hexene and cyclooctene have es-
2
2
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Chem. Eur. J. 2011, 17, 14885 – 14895

Olefin Cyclopropanation
FULL PAPER
curves until the addition of DEF, at which time the reaction
rate of the one containing added DEF dramatically de-
creased, providing evidence for the aforementioned inhibi-
tion pathway.
We have also carried out experiments to study the effect
that variation of the total concentration of Pd ([Pd]tot)
would have on the reaction rate of the cyclopropanation of
styrene by using 1 as the precatalyst. As shown in Figure 7,
kobs is linearly dependent on [Pd]tot.
Figure 5. Plots of nitrogen evolution in the reaction of EDA in the pres-
ence of styrene and different amounts of added PPh
action conditions: [3]/[EDA]/[styrene]=1:48:250; [catalyst]=0.02 mmol;
EDA]=0.96 mmol; [styrene]=5 mmol; solvent=CH Cl (10 mL).
added: A) none, B) 0.01 mmol, C) 0.02 mmol,
3
catalysed by 3. Re-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
[
2
2
Amount of PPh
D) 0.03 mmol.
3
action (Figure 5, line D). From the data in Figures 4 and 5,
we extracted the following conclusions: with the PPh -con-
3
taining precatalyst, the first step must consist of PPh disso-
3
ciation to generate the [(IPr)Pd] species, the equilibrium of
which is greatly affected by the addition of free phosphine.
The generation of the [(IPr)Pd] species, the common species
for 1 and 3 as precatalysts, initiates the catalytic cycle. In the
absence of styrene, this species catalyses the diazo coupling
reaction, which produces diethyl fumarate (DEF) and dieth-
yl maleate (DEM). These olefins would bind to the
Figure 7. Graph of kobs versus [Pd]tot by using 1 as the catalyst precursor
at room temperature.
The role of the olefin: a Hammett plot: Noels and co-work-
ers proposed that, in Pd-catalysed olefin cyclopropanation,
coordination of the olefin is crucial for the reaction to occur
in an intramolecular fashion (both the carbene and olefin
units are bound to the metal). This contrasts with copper- or
rhodium-based systems in which the reaction between the
metallocarbene and the olefin takes place intermolecularly,
that is, with the non-coordinated olefin attacking the metal-
locarbene carbon atom. Aiming to collect data to support
one route over the other, we performed competition experi-
ments with a series of para-substituted styrenes by using
complex 3 as the catalyst [Eq. (5)] and plotted the relative
rates against the Hammett constant s. As shown in Figure 8,
the data adequately fit a Hammett plot, although it has a
positive slope. This is in contrast with most of the catalytic
systems previously described that provide correlations with
negative slopes. The fact that the reactivity of an olefin to-
wards cyclopropanation increases if an electron-withdrawing
[
(IPr)Pd) unit to give [(IPr)Pd ACHUTNGERTNNGNU( DEF) ] or [(IPr)Pd ACTHUNGTRENNUNG( DEM) ],
2
2
blocking the active catalytic site. It is worth mentioning that
Cavell and co-workers have reported the complex
[16d]
[
(IMes)Pd The formation of [(IPr)Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(DMF) ].
ACHTUNGTNRENUNG( DEF) ]
2
2
or [(IPr)Pd ACHTUNGTRENNUNG( DEM) ] in the presence of a large excess of sty-
2
rene would be precluded, explaining the observation of
complete EDA consumption and subsequent cyclopropane
formation. To be sure that this proposal is correct, we ran
two identical cyclopropanation experiments and added, after
1
h graph suggests approximately 50 min, a certain amount
of diethyl fumarate to one of the reactions (Figure 6). The
two experiments showed nearly identical nitrogen evolution
Figure 6. Plot of nitrogen evolution in the reaction of EDA and styrene
catalysed by 3. Reaction conditions: [3]/
.02 mmol; [EDA]=0.95 mmol; [styrene]=5 mmol; solvent=CH
10 mL). Diethyl fumarate (5 mmol) was added to experiment B after
0 min.
ACTHUNGERTNNUNG[ EDA]/ ACHTGNUTRENNUNG
0
(
5
2
2
Figure 8. Hammett plot reflecting the electronic effect of the olefin in the
cyclopropanation reactions by using complex 3 as the catalyst.
Chem. Eur. J. 2011, 17, 14885 – 14895
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14889

T. R. Belderrain et al.
[
26]
1
group is present must be considered to be the result of pre-
vious coordination of the olefin to an electron rich metal
centre, that is, to the {(IPr)Pd} fragment.
and temperature. Accordingly, in the H NMR spectrum,
the resonance of the Ph PCH proton for the major isomer
(cis) could be observed at d=3.32 ppm (d, JHP =24 Hz). The
3
induction time observed with [(IPr)Pd ACHTUNGTERNNNU(G PR )] would then
3
correspond to the time required to consume the PR in the
3
reaction mixture, and from that point the catalytic reaction
would be indistinguishable from that carried out with 1 as
the catalyst.
The rate law for the styrene cyclopropanation with 1: Experi-
mental data collected to this point seem to imply that
We have already mentioned that the observation of simi-
lar activities and diastereoselectivities induced by complexes
[(IPr)Pd AHCTUNGRETNNUN(G olefin)] is the catalytic system in this reaction.
However, we have not yet distinguished between the two
posible synthetic routes that may explain the formation of
the cyclopropanes. The first pathway would involve attack
1
–4 in the styrene cyclopropanantion reaction with EDA
supports the proposal of a common intermediate. On the
basis of the data obtained from the previously discussed
of the diazocompound onto [(IPr)Pd ACHUTNGRNENUG( olefin)] B (Scheme 2,
ꢀ
competition experiments, this species should be the 14e ,
left) to give a transient olefin–palladacarbene that would
collapse into the products. However, it could also be possi-
ble that the unsaturated, olefin-free [(IPr)Pd] species (D)
could react with EDA to give the corresponding palladacar-
unsaturated [(IPr)Pd ACHUTNGRNENUG( olefin)] complex that could be formed
from 1 by olefin dissociation or from 2–4 by simultaneous
phosphine dissociation and olefin addition (Scheme 1).
bene [(IPr)Pd=C(H)CO Et] (E) that would then interact
2
with the olefin (Scheme 2, right). A detailed kinetic analysis
for both possible pathways has been carried out, leading to
the reaction rate laws given by Equations (6) and (9) (see
the Supporting Information for the derivation of both equa-
tions). The experimentally observed (Figure 7) linear de-
pendence of kobs on [Pd]tot (i.e., the initial amount of 1)
cannot be employed to distinguish between the two mecha-
nisms because, as shown in Equations (7) and (10), both
routes would show this behaviour. However, there is a sig-
nificant distinction regarding the dependence of the inverse
of kobs on [sty]. In one case, this dependence should be
linear [Eq. (8)], whereas, for the mechanism involving spe-
cies E as an intermediate, that is, with no olefin coordinated
to palladium, second-order behavior would be expected
[
Eq. (11)]. Consiquently, a series of kinetic experiments was
performed in which only the amount of styrene added to the
Scheme 1. Equilibria leading to the generation of the catalytic species
reaction was modified. Figure 9 shows a plot of 1/kobs versus
[
sty]. The linear correlation between the magnitude of these
leaves no doubt about the pathway responsible for this
ꢀ
These equilibria would control the relative amount of
transformation; it is the 14e unsaturated species B,
[(IPr)Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(olefin)] available to react with EDA in the first
[(IPr)Pd AHCTUNETGRNNUN(G sty)], that reacts with EDA to generate C prior to
step of the catalytic cycle. Whereas dissociation of styrene
cyclopropane formation. The intercept at the y-axis provides
from 1 would occur readily at room temperature, loss of the
the value of 1/(k [Pd] ) and hence the value for k is deter-
mined to be 1.77ꢆ10 min . The slope corresponds to the
1
tot
2
1
ꢀ1
PR ligand from [(IPr)Pd
A
H
U
G
R
N
N
(PR )] seems to be much slower, as
3
3
inferred from the observation of an initiation period
Figure 6). On the basis of the already described formation
of a phosphine ylide from the reaction of [Pd(PPh ) ] with
value of 1/k K [Pd] , from which the value of K was found
1
ꢀ
L
tot
L
2
(
to be 5ꢆ10 . This result explains the decrease in the reac-
tion rate with added olefin, since most of the palladium in
solution remains in the form of 1.
AHCTUNGTRENNUNG
3
4
[25]
the diazo ketone N CHCOtBu, we searched for this type
2
of compound in the reaction mixture when using 3 as the
catalyst precursor. We have observed that when three equi-
valents of EDA were added to a solution of 3 in C D
d½N ꢂ
d½EDAꢂ
½Pdꢂ
2
tot
6
6
¼ ꢀ
¼ k1
½EDAꢂ
ð6Þ
ð7Þ
½Styꢂ
dt
dt
(
0.6 mL), the intensity of the resonance corresponding to 3
1 þ
K
L
31
decreased in the P NMR spectrum and two new peaks ap-
peared at d=19.3 and 17.5 ppm (ratio 4:1) that correspond
to the two geometric isomers of the ylide (see the Support-
ing information), the ratio of which depends on the solvent
½
Pdꢂ
tot
½Styꢂ
KL
kobs
¼
1
þ
14890
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ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14885 – 14895

Olefin Cyclopropanation
FULL PAPER
2
Scheme 2. Possible catalytic cycles for the styrene cyclopropanation reaction with ethyl diazoacetate and [(IPr)Pd CAHTUNGTRENUNG( sty) ] as the catalyst: Left: [(IPr)Pd-
ACHTUNGTRENNUNG( sty)] as the species that reacts with EDA; Right: [(IPr)Pd] as the species that reacts with EDA.
[
(IPr)Pd
ACHTUNGTRENNUNG
the transient metallocarbene [(IPr)Pd
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
2
species (C), which collapses into the cyclopropanes (cis and
trans) and produces complex D on the way to forming B
and restarting the catalytic cycle.
At this stage, only the exact nature of the coupling of sty-
rene to the carbene unit CHCO Et remains somewhat un-
2
discovered. However, this has been proposed to occur by
facile and irreversible intramolecular [2+2] cycloaddition to
[27]
form a palladacyclobutane molecule
(Scheme 3). It is
worth mentioning that this would be the sole species in the
overall catalytic cycle in which the metal center would be in
a formal oxidation state of +2. The subsequent reductive
elimination would afford the cyclopropane and complex C.
Finally, although the mechanism proposed herein is clear-
Figure 9. Dependence of 1/kobs on the concentration of styrene. The value
of k
1
is obtained from the intercept and K
L
from the slope of the line.
1
kobs
1
½Styꢂ
¼
þ
ð8Þ
ly different from those reported for other transition metals,
k1½Pdꢂ
tot
k K ½Pdꢂ
1
L
tot
[2f,28]
such as copper (Scheme 4),
the diastereroselectivity is
similar to those observed for other complexes containing
NHC ligands. For instance, the N-heterocyclic carbene com-
plex [(IPr)CuCl] afforded a diastereoselectivity (cis/trans) of
d½N ꢂ
d½EDAꢂ
½Pdꢂ
2
¼ k0
tot
¼
ꢀ
½EDAꢂ
1
½Styꢂ
KL1
½Styꢂ2
KL2
ð9Þ
ð10Þ
ð11Þ
dt
dt
1
þ
þ
[4q]
3
2:68 in the cyclopropanation of styrene with EDA,
½
KL1
Pdꢂ
0
kobs ¼ k0
tot
which is almost indentical to those found for the NHC–Pd
1
½
Styꢂ
½Styꢂ
2
1
þ
þ
KL2
complexes. Theoretical calculations for copper C
2
-symmetric
[28b]
complexes
have revealed that the alkene substituent does
ꢀ
ꢁ
2
1
1
½Styꢂ ½Styꢂ
not interact significantly with the ligand in any exo pathway
(structure I in Scheme 3), meaning that the cis/trans selectiv-
ity is only a consequence of the relatively weak steric repul-
sion between the carboxylate and alkene substituents in the
transition state or intermediate, independent of the mecha-
nism (through metallacyclobutane or direct carbene transfer
to the olefin). However, other transition-metal systems are
¼
1 þ
þ
0
kobs k ½Pdꢂ
K
K
L2
1
tot
L1
Overall mechanism: On the basis of the collected experi-
mental data, an overall mechanistic proposal has been de-
veloped (Scheme 3) for the catalytic styrene cyclopropana-
tion by using complexes 1–4 as the catalyst precursors. First,
the catalyst precurors undergo either ligand dissociation (for
precursor 1) or transformation of a ligand into an ylide (for
precursors 2–4). In the latter case, the unsaturated species
Ms
highly diastereroselective. For example, the [Tp Cu] com-
plex affords a 98:2 cis/trans mixture of the cyclopropanes
[4s]
from styrene. In this case, the combination of C3v symme-
Ms
[(IPr)Pd] (D) would then be trapped by styrene to give the
try and the steric bulk of the ligand makes [Tp Cu] a
ꢀ
real catalytic species in this system, the 14e complex
highly diastereoselective catalyst.
Chem. Eur. J. 2011, 17, 14885 – 14895
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14891

T. R. Belderrain et al.
Conclusion
0
We have shown that several [(NHC)Pd ] complexes can act
as very active catalysts in the olefin cyclopropanation reac-
tion with ethyl diazoacetate as the carbene source, improv-
ing on the previously described catalytic systems based on
the use of this metal. On the basis of the kinetic data, as
well as other experimental data, we have proposed a mecha-
nism for this transformation in which, independent of the
catalyst precursor employed ([(IPr)Pd
ACHTUNGTNERNUN(G sty) ] or [(IPr)Pd-
2
AHCTUNGTRENNUNG
(PR )]), the same catalytic species is responsible of the de-
3
composition of EDA and subsequent carbene addition, that
is, the intermediate [(IPr)Pd(sty)] complex, which leads to
the formation of [(IPr)Pd(=CHCO Et)(sty)] after interac-
AHCTUNGTRENNUNG
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
2
tion with EDA and nitrogen extrusion. The cyclopropane
should then be formed by a [2+2] cycloaddition reaction, in-
volving a palladacyclobutane intermediate. The develop-
ment of this catalytic system for diazo compound decompo-
sition and carbene transfer, along with knowledge of the in-
tramolecular nature of the reaction (regarding the carbene–
olefin coupling), appears promising for the future design of
new catalysts to induce high levels of diastereo- or enantio-
selectivity by conveniently modifying the NHC ligand. Work
aimed at such catalyst improvement is currently underway
in our laboratory.
Experimental Section
General Methods: All reactions and manipulations were carried out
under an oxygen-free nitrogen atmosphere by using Schlenk techniques
or under a nitrogen atmosphere in an Mbraun glovebox. All substrates
were purchased from Aldrich. Solvents were dried and degassed before
[
18]
[29]
use. The Pd complexes 2–5 and the NHC carbene ligands were pre-
pared according to literature methods. NMR spectra were recorded on a
Varian Mercury 400 MHz spectrometer. GC data were recorded in a
Varian CP-3800 instrument. X-ray crystal structure analysis was per-
formed in the Unidad de Analyses Elemental of the Instituto de Investi-
gaciones de Quꢁmica, CSIC-Universidad de Sevilla and elemental analy-
ses in the Centro de Investigaciꢃn en Quꢁmica Sostenible, CIQSO-Uni-
versidad de Huelva.
Scheme 3. Top: Overall mechanism for the styrene cyclopropanation re-
action by using complexes 1–4 as the catalyst precursors and ethyl diazo-
acetate as the carbene source. Bottom: A plausible explanation for the
intramolecular formation of cyclopropane.
A
H
U
T
N
U
A
T
N
T
E
N
N
2
] (1): Styrene (0.8 mL, 6.98 mmol) was added to a solution
[18]
A
H
U
G
R
N
U
G
(1.05 g, 1.50 mmol) and KOtBu (0.185 g,
1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
1
ꢀ
1
1
n˜ =1505 cm (C=C); H NMR (400 MHz, ꢀ408C, [D
8
(
1
6
2.4 Hz, 2H), 2.94, (d, J=8.9 Hz, 2H), 2.76–2.89 (m, 2H), 1.40 (d, J=
.6 Hz, 6H), 1.16 (d, J=6.5 Hz, 6H), 1.07 (d, J=7.1 Hz, 6H), 1.05 ppm
1
3
(
2 2
d, J=6.8 Hz, 6H); C NMR (100 MHz, 408C, CD Cl ): d=146.8 (s; C
arom), 145.8 (s; C arom), 144.9 (s; C arom), 137.5 (s; C arom), 129.5 (s;
C arom), 127.5 (s; C arom), 124.2 (s; C arom), 124.1, (s; C arom), 124.0
Scheme 4. Proposed mechanism for the styrene cyclopropanation reac-
tion catalysed by copper systems.
(
brs; CH imid), 123.28 (s; CH arom), 74.66 (s; CH olefin), 53.01 (s; CH
olefin), 28.93 (s; CH(CH ), 28.32 (s; CH(CH ), 27.21 (s; CH(CH ),
5.28 (s; CH(CH ), 22.84 (s; CH(CH ), 22.31 ppm (s; CH(CH ); ele-
Pd: C 73.37, H 7.53, N 3.98;
2
A
H
U
G
R
N
U
G
3
)
2
A
H
U
G
R
N
U
G
3
)
2
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3 2
)
2
A
H
U
G
R
N
U
G
3
)
2
A
H
U
G
R
N
U
G
3
)
2
A
H
U
G
E
N
G
3 2
)
mental analysis calcd (%) for C43
found: C 72.75, H 7.52, N 3.97.
53 2
H N
In situ preparation of [(IMes)Pd
in situ by the following procedure: IMes
3
ACHTUNGTRNENUG( PCy )] (6): Compound 6 was generated
[28]
(30 mg, 0.01 mmol) was
14892
www.chemeurj.org
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14885 – 14895

Olefin Cyclopropanation
added to a solution of [Pd
FULL PAPER
[30]
A
H
U
G
R
N
U
G
3
)
2
]
(66 mg, 0.01 mmol) in CH
2
Cl
2
(EL-Press, Bronkhorst HI-TEC). The outlet of the pressure controller
was connected to a reaction flask (100 mL) that was also connected to a
Schlenk manifold to allow for manipulation of the reaction and degass-
(
10 mL). After 10 min of stirring, the solution was used directly in the cy-
clopropanation experiments. To characterise 6 by NMR spectroscopy this
complex was also prepared in situ by dissolving a stoichiometric amount
2
ing. The N pressure increase was measured after addition of EDA
1
of IMes and [Pd
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PCy
3
)
2
] in C
6
D
6
(0.7 mL). H NMR (400 MHz, C
6
D
6
):
(0.95 mmol) to a stirred solution of styrene and the palladium catalyst in
dichloromethane at room temperature. The apparatus was tested by car-
rying out the cyclopropanation reaction of styrene with EDA by using
d=6.79 (s, 4H), 6.21 (s, 2H), 2.30 (s, 12H), 2.16 (s, 6H), 2.0–1.0 ppm (m,
1
3
2
3
0H); C NMR (100 MHz, C
6 6
D ): d=198.2 (d, JCP =90.79 Hz; C car-
Br3
Br3
bene), 138.4 (s; C arom), 137.1 (s; C arom), 135.3 (s; CH arom), 128.7 (s;
[Tp Cu] as the catalyst. Styrene (5 mmol) and [Tp Cu] (0.0025 mmol)
were dissolved in dichlomethane (10 mL) and EDA (0.95 mmol) was
4
2
CH arom), 121.2 (d,
J
CP =4.3 Hz; C imid), 34.5 (d,
J
CP =12.1 Hz; CH
), 26.8 (s; CH ), 21.1 (d,
CP =9.0 Hz; CH
4
Br3
Cy), 31.9 (d, JCP =7.3 Hz; CH
2
Cy), 27.8 (s; CH
3
3
added. The [Tp Cu] complex catalysed the decomposition of 0.95 mmol
2
3
31
J
CP =16.1 Hz; CH
2
Cy), 18.7 ppm (d,
J
2
Cy);
P
2
of EDA (100 mL) and the N pressure increased by 0.25 bar.
(
100 MHz, C ): d=46.6 ppm.
6 6
D
Crystal data for 1: A single crystal of suitable size, crystallised from tolu-
ene, coated with dry perfluoropolyether was mounted on a glass fiber
and fixed in a cold nitrogen stream to the goniometer head. Reflections
were collected from a Bruker-Nonius X8Apex-II CCD diffractometer in
the range 2.74>2q>56.628. The data were reduced (SAINT) and cor-
Genaral catalytic cyclopropanation reaction: EDA (0.12 mL, 1 mmol)
was added to a solution of the palladium complex (0.01 mmol, 6 generat-
ed in situ) in CH Cl (10 mL) and the corresponding olefin (5 mmol).
2 2
The consumption of EDA was monitored by GC. When the reaction was
finished, the volatiles were removed under vacuum and the crude prod-
uct mixture was analysed by H NMR spectroscopy. All of the products
rected for Lorentz polarisation effects and absorption by the multiscan
1
[33,34]
method applied by SADABS.
The asymmetric unit of 1 is formed of
have previously been described and thus were identified by straightfor-
two independent half-complexes having twofold rotational symmetry and
also two solvating toluene molecules; one of the toluene molecules was
observed to be disordered and in two positions with an occupancy factor
fixed at 0.6 and 0.4. The structure was solved by direct methods (SIR-
[
31]
ward comparison with reported data. Conversions were determined by
1
H NMR spectroscopy by using 1,4-dimethoxybenzene as an internal
standard.
[
35]
2
2
002) and refined against all F data by full-matrix least-squares tech-
Reaction of 3 with EDA: ylide formation, Ph
equiv) was added to a solution of [(IPr)PdPPh
0.6 mL). After 24 h, the reaction was analysed by NMR spectroscopy at
3
P=CHCO
2
Et: EDA (7 mL,
[
35]
niques (SHELXL97).
57 68 2 52 2 7 8 w
C H N Pd [C43H N Pd, 2 AHTCNUGRTENNGNU( C H )]; M =887.53;
3
3
] (3 0.02 mmol) in C
6 6
D
orthorhombic; space group Pccn (no. 56); crystal size 0.43ꢆ0.42ꢆ
(
3
3
1
0.40 mm ; yellow prism; a=19.8389(14) ꢅ; b=22.3293(14) ꢅ; c=
room temperature and the ylide formation was observed. The P NMR
spectrum of the reaction mixture contained two peaks corresponding to
the geometric isomers of the ylide (d=19.3 and 17.4 ppm) and, accord-
3
2
1
0
2.5270(16) ꢅ; V=9979.2(12) ꢅ ; a=908; b=908; g=908; Z=8;
ꢀ
3
73(2) K; 1calcd =1.181 gcm ; l
A
H
U
G
R
N
U
G
ACHTUNGTRENNGNU( 000)=3760; m=
ꢀ1
=
ingly, a doublet corresponding to the coupling of the CH in Ph
3
P=
2
2
0.0695); 583 parameters; goodness of fit on F , S=1.072; R =0.0594 [I>
CHCO
2
Et can be observed at d=3.61 ppm ( JHP =24.0 Hz) in the
1
1
2s(I)]; wR =0.2017.
H NMR spectrum.
2
Reaction of 1 with DEF: detection of [(IPr)Pd
A
H
U
G
R
N
N
(DEF)
2
]: Diethyl fumarate
Cl
0.6 mL). The reaction was monitored by H NMR spectroscopy. At
(
(
10 mL, 3 equiv) was added to a solution of 1 (0.02 mmol) in CD
2
2
1
Acknowledgements
room temperature, fluxional behaviour was observed. At low tempera-
ture (ꢀ408C), selected resonances of the adduct [(IPr)Pd
A
H
U
G
R
N
N
(DEF)
2
] can be
3
We thank Prof. P. J. Pꢀrez (Univ. Huelva) for helpful and constructive
comments on these studies. We thank the Ministerio de Ciencia e Innova-
ciꢃn (grants CTQ2008–00042BQU and CTQ2011–24502) and the Junta
de Andalucꢁa (Proyecto P07-FQM-02794) for financial support. CM
thanks the MEC for a research fellowship.
assigned; two doublets at d=4.51 and 3.73 ppm (d, 2H,
J
HH =10.4 Hz)
were observed corresponding to the CH in the coordinated DEF, as-
[
16d]
signed by comparison with the related complex [(IMes)Pd
Competition reactions: EDA (60 mL, 0.5 mmol) was added in one portion
to a solution of [(IPr)Pd(PPh )] (3, 7.5 mg, 0.01 mmol), styrene (0.28 mL,
.5 mmol) and 1-hexene or cyclooctene (2.5 mmol) in dichloromethane
10 mL). The reaction mixture was monitored by CG until all of the
2
ACHTUNGTNRENUNG( DMF) ].
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
2
(
[
1] a) H. Pellissier, Tetrahedron 2008, 64, 7041–7095; b) A. de Meijere,
Small Ring Compounds in Organic Synthesis VI, Vol. 207: Topics in
Current Chemistry, Springer, Berlin, 2000.
[2] a) M. P. Doyle, A. McKervey, T. Ye, Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds: From Cyclopropanes to
Ylides, John Wiley & Sons, New York, 1998; b) M. P. Doyle, Chem.
Rev. 1986, 86, 919–939; c) H. Lebel, J.-F. Marcoux, C. Molinaro,
A. B. Charette, Chem. Rev. 2003, 103, 977–1050; d) H. M. L. Davies,
R. E. J. Beckwith, Chem. Rev. 2003, 103, 2861–2903; e) W. Kirmse,
Angew. Chem. 2003, 115, 1120–1125; Angew. Chem. Int. Ed. 2003,
EDA was consumed. The volatiles were removed under vacuum and the
1
crude reaction mixture was analysed by H NMR spectroscopy. The reso-
1
nances of the corresponding cyclopropanes were assigned in the H NMR
[
30]
spectra by comparison with the values reported in the literature. The
ratio of the cyclopropanes was obtained by integration (average of at
least two runs).
Cyclopropanation competition experiments of para-substituted styrenes
(
Hammett plot): Ethyl diazoacetate (0.5 mmol) was added to a solution
of 1 (0.01 mmol) and an equimolar mixture of styrene (2.5 mmol) and
the corresponding para-substituted styrene (2.5 mmol) in dichlorome-
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H NMR spectroscopy. The resonances of the corresponding cyclopro-
1
panes were assigned in the H NMR spectra by comparison with the
values reported in the literature. The ratios of the cyclopropanes ob-
[
[
32]
tained by integration (average of at least two runs): p-OMe/H=0.85; p-
3
2
3
Me/H=0.94; p-Cl/H=1.25; p-CF /H=1.50.
Olefin exchange reaction between [(IPr)Pd
(
A
H
U
G
R
N
U
G
2
]
(1) and styrene
2
(sty) ]
A
H
U
T
E
N
N
(
8
The solution was monitored by NMR spectroscopy at different tempera-
tures.
2
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Received: September 15, 2011
Published online: December 2, 2011
Chem. Eur. J. 2011, 17, 14885 – 14895
ꢄ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14895