Electronic tuning of a carbene center via remote chemical induction, and relevant effects in catalysis

Article abstract of DOI:10.1002/chem.201000870

The present report develops the idea that an N-heterocyclic carbene incorporating a remote anionic functionality-here, a malonate group-as a backbone component of its heterocy- clic framework, can be post-function- alized directly from its transition- metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the car- bene center over a rather broad range. Both the zwitterionic complex [Rh{malo-NHCj(cod)] and the cationic derivatives [Rh{malo-NHC^E}(cod)] ⁺ (where ma/o-NHC^E represents the ligand modified by a selected electro- phile E ) were used as pre-catalysts in two types of catalytic reactions, namely, the polymerization of phenyl- acetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catechol- borane substrate and the remote functionality of the N-heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.

Full text of DOI:10.1002/chem.201000870

DOI: 10.1002/chem.201000870
Electronic Tuning of a Carbene Center via Remote Chemical Induction, and
Relevant Effects in Catalysis
[
a]
Vincent Cꢀsar,* Noꢁl Lugan, and Guy Lavigne*
Abstract: The present report develops
the idea that an N-heterocyclic carbene
incorporating a remote anionic func-
tionality—here, a malonate group—as
a backbone component of its heterocy-
clic framework, can be “post-function-
alized” directly from its transition-
metal complexes, upon simple addition
of a variety of electrophiles interacting
directly with the malonate group in the
outer coordination sphere. From a pa-
lette of selected electrophilic reagents,
it was thus possible to modulate the
electronic donor properties of the car-
bene center over a rather broad range.
Both the zwitterionic complex [Rh-
A
T
N
T
E
U
G
A
T
N
R
N
U
G
the ligand donicity induced by the
added electrophile results in a concom-
itant reduction of catalytic activity. Ap-
parent deviations to such a trend in the
case of the hydroboration of styrene
were rationalized in terms of an inter-
action between the reactive catechol-
borane substrate and the remote func-
tionality of the N-heterocyclic carbene
leading to an in situ modification of
the nature of the active species. These
observations serve as a useful basis to
define the scope and limitations of the
present conceptual approach in cataly-
sis.
E
+
(where “malo-NHC ” represents the
ligand modified by a selected electro-
phile “E”) were used as pre-catalysts in
two types of catalytic reactions,
namely, the polymerization of phenyl-
acetylene and the hydroboration of sty-
rene. The results indicate that, in both
cases, the zwitterionic species is by far
the best catalyst, whereas a decrease in
Keywords: carbenes · heterocyclic
compounds
polymerization · rhodium
Introduction
apply them to the same concept, and we effectively see a
number of successful preliminary approaches pursuing such
a goal (see below).
The development of transition-metal complexes, the catalyt-
ic performance of which can be remotely controlled by an
external stimulus, represents a challenging conceptual ad-
vance in modern organometallic chemistry. To date, known
examples of such complexes include a few homogeneous
phosphine- and nitrogen-based catalysts incorporating either
In pioneering studies on the use of N-heterocyclic car-
benes as ancillary ligands, substantial efforts were first di-
rected toward the control and quantification of their steric
and electronic properties. The steric shielding of an NHC
[10]
was accurately quantified by Nolan et al. as the percent-
age of “occupied volume” (%Vbur), and the consequences of
its variation on the catalytic performances were examined in
[
1,2]
a redox-active ligand
or a “chemo-active” one possessing
either a basic binding site susceptible to be attacked by a
Lewis acid, or an acidic site susceptible to be deprotonat-
[3]
[11]
detail by Glorius and co-workers. On another hand, the
[
4]
ed.
Now if we consider N-heterocyclic carbenes,
which have blossomed into a class of powerful ligands for
electron-donating properties of NHCs were quantified by
measuring the infrared nCO stretching frequencies of two
[5,6]
(NHCs),
kinds of complexes, namely, [(NHC)Ni(CO) ], and
3
[7–9]
organometallic catalysis,
it would be highly beneficial to
[(NHC)MCl(CO) ] (M=Rh, Ir) which were independently
2
[12]
proposed as standard references. The corresponding inde-
pendent scales established from such complexes can be cor-
[
a] Dr. V. Cꢀsar, Dr. N. Lugan, Dr. G. Lavigne
CNRS, LCC (laboratoire de chimie de coordination)
[13,14]
related with the Tolman electronic parameter (TEP),
2
3
and
05 route de Narbonne
1077 Toulouse Cedex 4 (France)
which enables a direct comparison with the case of phos-
phine ligands. To date, attempts to modulate the electronic
donor properties of NHCs have followed three main strat-
egies (Scheme 1) including i) attachment of a pendant elec-
tron-donating or -withdrawing group at the periphery of the
Universitꢀ de Toulouse, UPS, INPT, 31077 Toulouse (France)
E-mail: vincent.cesar@lcc-toulouse.fr
guy.lavigne@lcc-toulouse.fr
11432
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Chem. Eur. J. 2010, 16, 11432 – 11442

FULL PAPER
[15]
nitrogen substituents (type A), or directly onto the back-
[12d,16]
bone atoms of the NHC (types B–F),
ii) incorporation
of a redox active group such as a ferrocene or a quinone
[17]
into the structure of the NHC, or iii) incorporation of an
easily deprotonable keto unit in close proximity to the car-
[18]
benic center. The latter two strategies allow a significant
modulation of the electronic properties of the carbene,
albeit only over two fixed values corresponding, for the
former, to the ligandꢂs reduced and oxidized states, respec-
tively, and for the latter, to the ligandꢂs protonated and de-
protonated forms.
represents a favorable case where the steric and electronic
properties are under control of totally independent parame-
ters. Indeed, whereas the shape of the metalꢂs cavity is con-
trolled only by the ligandꢂs steric parameters, determined by
the size of the ring and the nature of the nitrogen substitu-
ents, an interesting feature is that the remote malonate
group emerges in the outer coordination sphere, where it
becomes directly accessible to an external chemical stimulus,
possibly represented by external incoming electrophiles.
With these observations in mind, we anticipated that simple
addition of a range of electrophilic reagents exhibiting dif-
ferent electron-withdrawing properties might allow a rapid
and effective fine-tuning of the electronic properties of the
carbene center whithout change in the ligandꢂs steric proper-
ties. Beyond, we also intended to examine the effects of
such a modulation on the catalytic behavior of our complex.
The attractive possibility to incorporate the electrophile
after complexation appeared as a beneficial advantage in
view of a rapid catalyst screening, particularly relative to
most concurrent systems where the modulation has to be
carried out on the ligand precursor (Scheme 1, types A–
[15,16]
F).
Scheme 1. Selected examples of previously reported means to modulate
the electronic properties of an NHC ligand. The grey spheres indicate
the locations where the influencial substituents are introduced.
Results and Discussion
Synthesis of the pre-catalysts: The pyrimidinium betaꢄnes
(
(
1Me)·H and (1tBu)·H, incorporating two classical mesityl
2,4,6-trimethyphenyl) groups as the nitrogen substituents,
To date, the consequences of an electronic tuning of the
carbene center on the catalytic performances of relevant
were selected as the precursors of a series of target ligands
all exhibiting the same steric hindrance. In the first part of
the present investigation, we mainly considered the case
where a methyl group is installed as R substituent of the
malonate group. Later on, we found that in specific cases,
varying the nature of this group can affect the steric accessi-
bility of the oxygens.
[16a,d]
complexes have been examined only in few cases.
Inter-
estingly, very recently, Fꢃrstner provided experimental evi-
dence that a modulation of the p-acceptor properties of the
carbene center of an NHC is also prone to influence its cata-
[19]
lytic behavior.
In a preliminary communication of part of the present
work, we disclosed the modular synthesis of a new NHC
ligand family, a six-membered anionic N-heterocyclic car-
The starting compound of the present investigation, the
À
+
À
zwitterionic 14 e species [Rh
ACHTUNGTNERNUNG( cod) ACHTGNUTRENNUNG( 1 )], 2 , (see above)
R
R
was originally obtained in high yields by trapping the free
À
À
+
1
[20]
bene, “malo-NHC” (1 ) incorporating a malonate backbone
carbene (1R )·Li with = equivalent of [RhCl ACHTUNGTRENNUG( 1,5-cod)] .
2 2
synthetically accessible with different
R
groups (see
An X-ray structure analysis of 2Me revealed the existence of
a labile bonding interaction between the Cipso of one of the
mesityl arms and the Rh center. In solution, a dynamic pro-
cess, corresponding to the shuttling of the Rhodium center
between the two mesityl arms was found to occur even at
[20]
below).
À
Due to its anionic nature, 1R was found to react with nu-
merous transition-metal complexes to give unsaturated zwit-
À
terionic species, exemplified here by the 14 e prototype
À
1
[Rh
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(cod)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(1 )] (2 ). Such a zwitterionic metal/ligand system
À808C thus resulting in an averaging of the H and
R
R
Chem. Eur. J. 2010, 16, 11432 – 11442
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www.chemeurj.org
11433

G. Lavigne, V. Cꢀsar and N. Lugan
1
3
C NMR spectra of compounds 2R to a formal C2v symme-
try.
The addition of electrophiles E-OTf (E=Me,Tf, H) to the
zwitterionic complex 2 yielded the corresponding ionic
plex 2R in the presence of a halide source (Scheme 3, left
equation), a reaction which produced the new anionic com-
À
+
plex [RhCl(CO) ACUTHNGTRENNUNG( 1 )] ACHTUNGETRNNUN(G NEt ) (4 ).
2 R 4 R
Me
E
E
complexes [Rh
ACHTUNGTRENNUNG( cod) ACHTUGNETRNUNG( 1 )] ACHNUTGTRENNUN[G TfO] (3 ) in excellent yields
Me Me
(
>90%) (Scheme 2).
Scheme 3. Formation of standard chloro-carbonyl Rh complexes
RhCl(CO)
properties.
2
ACHTUNGERTNNUGN( NHC) to be used for the evaluation of the ligandꢂs donor
Scheme 2. Reaction of the zwitterionic complexes 2Me with various elec-
E
trophiles leading to the ionic complexes 3Me
.
The outcome of the conversion of 2 into 4 proved to be
R
R
highly dependent on the nature of the chloride salt since it
was observed that the counter-ion has a crucial role on the
All of them were obtained in spectroscopically and ana-
lytically pure form by evaporation of volatiles and a simple
solubility of the final ionic complex 4 . Our first attempts
R
1
washing with pentane. Analysis of the H NMR spectra of
based on chloride salts known for giving good results in the
solubilization of anionic metal complexes, such as bis(triphe-
E
complexes 3Me recorded at 258C, clearly revealed that the
[
22]
characteristic swing of the unsaturated Rh center between
nylphosphine)iminium chloride ([PPN]Cl)
or (5-azonia-
[23]
the mesityl groups, already observed for 2 , is still operative
spiro ACTHUNGRTNEN[UGN 4.4]nonane)chloride ([ASN]Cl) remained unsuccess-
R
here (as also the case for other cationic derivatives of close-
ful due to a poor solubility of the final products. Fortunately,
[21]
ly related NHCs). Again, this dynamic chemical exchange
could not be frozen by lowering the temperature down to
À808C, and is thus responsible for the observed averaging
the use of tetraethylammonium chloride ([NEt ]Cl) fur-
4
nished high yields of complexes 4 exhibiting high solubility
R
in common organic solvents (alcohols, THF, CH CN, chlori-
3
1
13
of H and C NMR spectra of the two isomeric forms. In
principle, the strong attachment of an electrophile to one of
the remote oxygens should reduce the symmetry of the
nated solvents).
For the neutral N-heterocyclic carbenes 1Me resulting
E
from the addition of electrophiles to the parent zwitterionic
ligand from C2v to C point group. This is effectively ob-
complex 2Me, the corresponding standard chloro-carbonyl
s
13
1
Me
E
served in the H and C NMR spectra of complexes 3Me
complexes 5Me were obtained by bubbling CO gas into a so-
Tf
E
and 3Me , where two sets of signals are indeed observed for
lution of in situ generated complexes 3Me in the presence of
H
the two mesityl arms. On the contrary, in the case of 3Me
,
a large excess of sodium chloride as the chloride source. For
H
the signals of the mesityl protons are averaged in the
5Me , the direct use of anhydrous HCl led to the transient
1
H
H NMR spectra at 258C, which can be rationalized in terms
formation of a neutral complex [RhCl
timately produced the complex [RhCl(CO)
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
(1 )] which ul-
Me
H
H
of a fast exchange of the proton between the two available
oxygen sites of the malonate backbone. This dynamic pro-
cess could not be frozen by lowering the temperature down
to À808C, indicating a very low activation energy barrier.
A
H
U
G
R
N
U
G
)
2
via displacement of the COD ligand by carbon monoxide.
Two representative complexes of the above series, namely,
Me
4Me and 5Me , were characterized by X-ray diffraction, and
their molecular structures are depicted in Figure 1.
À
Evaluation of the donor properties of the ligands 1R and
Both complexes possess distorted square-planar coordina-
tion geometries with the malo-NHC ligand being almost or-
thogonal to the mean coordination plane (N1-C3-Rh1-Cl1
E
1R : According to the literature, the donor properties of a
given NHC ligand can be accurately evaluated by measuring
the IR n(CO) stretching frequencies of complexes like
Me
torsion angle: 81.18 in 4Me, 85.48 in 5Me ). Very characteristi-
[
(NHC)Ni(CO) ], or [(NHC)MCl(CO) ] (M=Rh, Ir) taken
cally, examination of geometrical parameters within the mal-
onate backbone O3-C4-C5-C6-O4 of 4Me reveals that the
corresponding CÀO and CÀC bond lengths are all inter-
3
2
[12]
as standard references. For the present investigation, the
Rh scale was naturally selected. In view of evaluating the
donor properties of our anionic NHC 1R , it was first neces-
sary to prepare a standard anionic chloro-carbonyl Rh de-
rivative incorporating such a ligand. This was done simply
by bubbling CO into a solution of the zwitterionic Rh com-
À
mediate between single and double bond values, reflecting
the expected electron delocalization over the whole malo-
nate group. By contrast, examination of the same structural
I
Me
parameters in 5Me reveals that attachment of the electro-
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Chem. Eur. J. 2010, 16, 11432 – 11442

Electronic Tuning of a Carbene Center
FULL PAPER
Me
[25]
in compound 5Me is found to be 38.8%, which confirms
that the O-substitution has only little or no influence on the
[26]
1
steric properties of the malo-NHC ligand. In the H NMR
spectra of 4 , two sets of two singlets are observed for the
R
four aromatic CH protons and for the four ortho-methyl
groups of the mesityl rings, indicating a blocked or reduced
ligand rotation around the Rh-carbene bond (on the NMR
time scale). Such a symmetry break in the NMR spectra is
E
found in all complexes 5R and 4 . In addition, the two sides
R
Me
of the NHC are well differentiated in complexes 5
5Me by adjunction of the electrophile leading to a total loss
and
Me
Tf
of symmetry. Just like the cationic rhodium–COD complex
H
H
3Me , the complex 5Me experiences a fast exchange of the
hydrogen atom between the two oxygens of the malonate
group, a fluxional process which could not be frozen, even
at a temperature as low as À808C in CD Cl .
2
2
In order to gain further insight into the electronic proper-
ties of the newly formed NHCs and to compare them with
those of known derivatives, the IR spectra of the complexes
E
4R and 5R were recorded in CH Cl and the results are sum-
2
2
marized in Table 1. In the same Table, we also report the
values measured on two additional complexes incorporating
ligands which are of specific interest for comparative pur-
poses. One is 1,3-dimesityltetrahydropyrimidin-2-ylidene (6),
representing the closest comparable case of saturated six-
membered NHC ligand (IR nCO values recorded on the com-
plex [RhCl(6)(CO) ] in CH Cl ). The other, referred to as 7,
2
2
2
À
is formally derived from the anionic NHC 1Me by a C-meth-
ylation (IR nCO values recorded on the complex
RhCl(7)(CO) ] in CH Cl ).
2 2 2
[27]
[
Figure 1. Top: Molecular structure of the anionic unit of complex 4Me. Se-
lected bond lengths [ꢅ] and angles [8]: Rh1ÀC3 2.088(2), C3ÀN1
1
1
1
.338(3), C3ÀN2 1.335(3), N1ÀC4 1.448(3), N2ÀC6 1.437(3), O3ÀC4
2
Table 1. Comparison of nCO values for RhCl(CO) AHCTUNGTERNNUNG{ NHC} complexes.
.224(3), C4ÀC5 1.397(3), C5ÀC6 1.387(4), C6ÀO4 1.235(3); N1-C3-N2
16.1(2), N1-C3-Rh1-Cl1 81.1. Lower drawing: Molecular structure of
Me
complex
5
Me
.
Selected bond lengths [ꢅ] and angles [8]: Rh1ÀC3
2
1
1
.092(2), C3ÀN1 1.357(3), C3ÀN2 1.345(3), N1ÀC4 1.437(3), N2ÀC6
.405(3), O3ÀC4 1.217(3), C4ÀC5 1.438(4), C5ÀC6 1.348(3), C6ÀO4
.346(3); N1-C3-N2 116.04(19); N1-C3-Rh1-Cl1 85.4.
À1 [a]
av
À1
Entry
NHC ligand
n
CO [cm
]
n
CO [cm
]
[
b]
1
2
3
4
5
6
7
7
1
1
1
6
1
1
2086, 2005
2085, 2003
2082, 2000
2081, 1998
2076, 1988
2069, 1987
2068, 1986
2045
2044
2041
2039
2032
2028
2027
phile to the oxygen atom O4 restores the normal occurrence
of alternating single and double bonds along the same
chain. Besides, it is noteworthy that the NCN structural pa-
rameters are only slightly affected by the introduction of an
electrophile onto the backbone oxygen, as illustrated by the
small non significant changes of the C3ÀN1 and C3ÀN2
Tf
Tf
Me in 5Me
Me in 5Me
H
H
Me
Me
Me in 5Me
[
c]
MeÀ
tBuÀ
in 4Me
in 4tBu
[
2 2
a] IR spectra recorded in CH Cl . [b] 7: 1,3-Dimesityl-5,5-dimethyl-4,6-
bond lengths and of the N1-C3-N2 bond angle. The N1ÀC4
dioxotetrahydropyrimidin-2-ylidene. [c] 6: 1, 3-Dimesityltetrahydropyri-
midin-2-ylidene.
Me
(
4
: 1.448(3), 5
: 1.437(3) ꢅ) and N2ÀC6 (4 : 1.437(3),
Me
Me
Me
Me
5Me : 1.405(3) ꢅ) are indeed consistent with single bonds,
reflecting a lack of conjugation between the malonate and
the diaminocarbene moieties. Such a behavior was already
observed in the case of the substitution of quinoidal-type
zwitterions. The influence of the addition of the electro-
phile onto the malonate backbone can be quantified by cal-
culating the buried volume using the web application
SambVca” developed by Cavallo and co-workers.
anionic NHC in complex 4Me exhibits a buried volume of
9.6%, whereas the %Vbur of the methyl-substituted NHC
A comparison of these spectroscopic data leads to the fol-
lowing conclusions:
[24]
À
The anionic NHC ligands 1R (Table 1, entries 6–7) are
more electron-rich than all known six-membered NHCs re-
ported so far (nCO ranging from 2029 to 2038 cm ).
The nature of the apical R group attached to the central
carbon atom of the malonate group has little or no influence
on the electron-richness of the ligand (Table 1, entries 6–7).
av
À1 [28]
[10b]
“
The
3
Chem. Eur. J. 2010, 16, 11432 – 11442
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www.chemeurj.org
11435

G. Lavigne, V. Cꢀsar and N. Lugan
The attachment of an electrophile to one oxygen of the
malonate backbone of the NHC leads to a significant reduc-
tion of its electron-donor properties (entries 2–4), following
the order Me<H<Tf, where the highest electron-withdraw-
ing group, Tf, leads to the poorest electron-donor ligand
Figure 2 displays the kinetic curves obtained for the above
complexes under these standardized reaction conditions,
whereas the characteristics of the resulting poly(phenylacet-
ylene) (PPA) are summarized in Table 2.
within the present series of O-functionalized derivatives.
E
Noticeably, all the neutral ligands 1
resulting from the
Me
À
addition of an electrophile to 1Me appear to be poorer elec-
tron donors than the previously reported six-membered ring
NHCs.
The diamidocarbene 7, formally derived from the anionic
À
NHC 1Me by a C-methylation, appears as the poorest elec-
tron donor of the whole series, a point already discussed in
[27]
previous communications. Thus, it clearly appears that the
electronic properties of this class of ligands can be finely
À1
tuned over a range of nCO values covering about 18 cm .
In line with these observations, we were prompted to ex-
amine the consequences of such a modulation on the cata-
lytic performances of a series of comparable complexes in-
corporating these ligands. These included both the zwitter-
Figure 2. Kinetic profiles for the conversion of phenylacetylene as a func-
E
E
ionic complex 2 and the cationic adducts 3
generated by
tion of time using complexes 2 , 3
and 8.
R
Me
R
Me
addition of an electrophile associated with a non-coordinat-
ing anion. The catalytic assays were done on the two follow-
ing reactions.
Table 2. Characteristics of the poly(phenylacetylene) (PPA).
[
a]
À1 [b]
[b]
Entry
Catalyst
% cis
M
n
[gmol
]
w n
PDI (M /M )
Polymerization of phenylacetylene: The rhodium-catalyzed
polymerization of phenylacetylene was chosen as a first ref-
erence reaction for a comparative evaluation of the catalytic
activity of our new series of NHC ligands, firstly because the
reaction was known to be catalyzed by the complex [Rh-
1
2
3
4
5
6
2
2tBu
3
3
3
8
Me
78
81
39
40
40
39
28500
28600
23600
11700
12800
17900
2.18
2.33
2.57
4.65
6.11
3.39
Me
Tf
H
Me
Me
Me
+
[21b]
ACHTUNGTRENNUNG( cod)(6)] incorporating the closest comparable NHC, 6,
[
a] % cis corresponds to the percentage of the major cis-transoid struc-
and, secondly, because such a reaction represents a simple
elementary case where only one substrate, phenylacetylene,
and a molecular catalyst are involved.
1
ture of PPA and was measured by H NMR using the formula %-cis=
100ꢆ(6ꢆA5.84 ppm/Atotal).
[
21b]
[b] Measured by SEC.
In these assays, our objective was not to optimize the cat-
alytic system, but to work at the mildest possible conditions
in such a way to detect differences in the catalytic behavior
of our complexes. So, all catalytic runs were carried out in a
bath thermostated at 208C, on 1 mmol of phenylacetylene
to which 1 mol% of the rhodium catalyst was added
Very characteristically, it appears that modifications of
the malonate backbone leading to a modification of the li-
gandꢂs donicity are dramatically influencing the catalystꢂs ac-
tivity. The most significant aspects and trends are the follow-
ing:
(
Scheme 4).
The range of pre-catalysts considered in the present
The most active catalysts of the series are those bearing
the two anionic NHC ligands (complexes 2Me and 2tBu).
Given that they display about the same activity and that the
characteristics of the resulting PPA are the same, we can
conclude that, at least in the present case, the nature of the
substituent on the position 5 of the heterocyclic carbene has
no significant influence on the catalytic performances.
screening included the zwitterionic species 2, the cationic
E
complexes 3Me , and complex [Rh
A
H
U
G
E
N
N
ACHTUNGTRENNU(GN OTf) (8) gener-
ated in situ upon reaction of [RhClAHCUTNGTRENNUNG
Me
Although the activity of the O-methylated complex 3Me
is not negligible, only 64% of conversion was obtained after
2
4 h of reaction, whereas the selectivity in the cis-transoidal
form is only half the value recorded with catalysts 2Me and
2tBu, indicating that an electron-rich metal center is required
for the achievement and control of the reaction. Effectively,
all other complexes exhibiting lower electron donor proper-
ties gave very low conversions (10–20% after 24 h) with
very high polydispersities.
Scheme 4. Rhodium-catalyzed polymerization of phenylacetylene leading
to configurationally different poly(phenylacetylene) (PPA).
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Electronic Tuning of a Carbene Center
FULL PAPER
Clearly, the above tendency illustrates the requirement of
a strong electron-donor ligand to achieve the polymerization
of phenylacetylene. It is worth noting, however, that com-
ined here, the ratio between the linear anti-Markovnikov-
and branched Markovnikov-type products is highly depen-
dent on the activity of the catalyst, with the percentage of
branched product being maximized for the more efficient
complexes 2R (entries 1–2). However, as noted above, a
close look at the kinetic plots reveals several deviations
from the general trend, indicative of a more complex behav-
H
H
plex 3Me bearing the protonated ligand 1Me was shown to
be the least efficient catalyst, despite its relative electronic
richness. Here, our interpretation of such a discrepancy is
that the acidity of the ligand is inappropriate for the poly-
merization of phenylacetylene, a reaction which is known to
H
ior. Firstly, the acidified complex 3Me is not as active as one
[21b]
be quenched by addition of acids.
would have expected in account of the electron-donicity of
H
its supporting ligand 1Me , since it experiences a rapid de-ac-
Hydroboration of styrene: The rhodium-catalyzed hydrobo-
tivation. Secondly, although catalysts 2Me and 2tBu exhibit a
high activity in the early stages of the reaction, the activity
of 2tBu is seen to be drastically reduced after 3 min of reac-
tion. The latter observation suggests that the malonate back-
bone may exhibit a non-innocent behavior during the cataly-
sis. In order to detect the occurrence of an adventitious side
reaction between the NHC ligand backbone and the cate-
cholborane reagent (HBCat), we were led to carry out a sto-
ichiometric reaction between catecholborane and complexes
[29]
ration of styrene with pinacol or catecholborane has been
[30]
extensively studied with phosphine ligands,
knowledge, only once with NHC ligands.
but, to our
[16a]
The hydroboration was selected as a second test reaction
with the aim to examine the scope and limitations of the
present ligand types. Indeed, given that the borane reagent
contains both a hydride and a Lewis acidic site, the possibili-
ty of an interaction with the potentially reactive backbone
moiety of our NHC ligands could not be excluded. In order
to maximize such a potential interaction, catecholborane
was chosen as the hydride source since it is less sterically
hindered than pinacolborane and more reactive toward nu-
E
4R and 5Me which both possess a saturated metal center
being unlikely to react with such a substrate under the pres-
ent reaction conditions. An additional advantage of using
carbonyl derivatives in such an experiment was to allow a
facile detection of any change in the NHC ligand structure
by using IR nCO as a probe.
[31]
cleophiles. In a typical standard catalytic procedure, the
catalytic runs were conducted at 08C, using styrene as a sub-
strate with a catalyst loading of 1 mol%. The crude mixture
1
was analyzed by H NMR spectroscopy after 90 min to de-
termine the products yields and distribution (Table 3). The
conversion curves were also monitored by gas chromatogra-
phy in such a way to have a closer view to the reaction ki-
netics. A blank test (Table 3, entry 7) was carried out to
verify that no hydroboration occurs in the absence of cata-
lyst under the present reaction conditions.
The kinetic plots are particularly instructive, since, apart
from the exceptions discussed below, the respective activities
of the catalysts appear to be correlated with the electron-
E
donor properties of the ligand 1Me (Figure 3). Again, here,
the zwitterionic catalysts 2R are, by far, the most efficient
Me
ones. In an apparent trend following the order 2Me, 3Me
,
Tf
3Me , 8, a progressive reduction of the donor properties of
the carbene leads to a concomitant decrease of the observed
catalytic activity. It is also noteworthy that in all cases exam-
Figure 3. Kinetic curves of the catalytic hydroboration of styrene using
E
complexes 2
R
, 3Me and 8 as pre-catalysts.
The addition of an excess of
H
E
HBCat (5 equiv) to 5Me was
Table 3. Hydroboration of styrene with catecholborane (HBCat), using 2
R
, 3Me and 8 as catalyst precursors.
accompanied by the immediate
appearance (within less than
5
2
min) of a broad signal at d=
3.0 ppm in the B NMR of the
1
1
[
a]
[a]
Entry
[Rh] catalyst
Conversion [%]
b/lh Yield [%]
crude mixture, indicative of the
formation of a new compound
tentatively
[RhCl(CO)
(Scheme 5, left equation), and
resulting from a Brçnstedt acid-
base reaction between the
1
2
3
4
5
6
7
2
2
3
3
3
8
Me
91
63
50
48
25
31
0
44/37/10
31/24/6
13/22/13
3/39/6
0/22/1
0/20/10
–
tBu
formulated
as
)
Me
Me
Me
Me
BCat
BCat
Tf
H
2
A
H
U
G
R
N
U
G
Me
)] (5Me
none
1
H
[
a] Measured by H NMR, using mesitylene as internal standard.
acidic ligand 1Me and the hy-
Chem. Eur. J. 2010, 16, 11432 – 11442
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11437

G. Lavigne, V. Cꢀsar and N. Lugan
dride of catecholborane, followed by elimination of molecu-
cholborane is very clean and gives the anionic bimetallic
adduct [{5Me}2 ] , which most probably results from an in-
termolecular nucleophilic attack of the remote anionic
BCat
À
lar hydrogen and concomitant formation of the borate ester.
BCat
The formulation of the adduct 5Me
was confirmed by the
direct reaction of 2Me with B-chlorocatecholborane and sub-
sequent displacement of the COD ligand by CO (Scheme 5,
oxygen of a second molecular unit of 4 at the trigonal
boron center of the adduct 5Me . The inability of 4tBu to
Me
BCat
right equation). Thus, the above experiment provides con-
generate the analogous bimetallic borate-type adduct can be
reasonably ascribed to the steric bulk of the tert-butyl group
preventing further intermolecular nucleophilic attack. This
BCat
clusive evidence that ligand 1
can be readily generated
Me
H
in situ from 3Me under catalytic conditions. What we have
here is an uncommon case where a reactive substrate is seen
to interact with the ligand of the pre-catalyst, thereby modi-
fying the nature of the active species.
hypothesis was corroborated by a stoichiometric reaction
BCat
between isolated complexes 4 and 5R , in which a full
R
BCat
À
+
conversion to
A
H
U
G
E
U
G
] ACHTUNGTRENNUNG
4
methyl derivatives, wheras 4
tBu
BCat
and 5tBu
did not react togeth-
er. By comparing the IR nCO
spectra of the two types of ad-
Bcat
ducts observed here (5
:
tBu
av
À1
BCat
À
nCO =2045 cm , [{5Me}2 ] :
nCOav =2034 cm ), it is clear
that the lower electron-with-
drawing properties of the
anionic borate-type adduct will
lead to a carbene with en-
hanced electron-donor proper-
ties. The formation of two dif-
ferent types of boron substitut-
À1
H
Scheme 5. Parallel stoichiometric reactions between 5Me and catecholborane and between 2Me and B-chloroca-
techolborane.
The second critical point focusing our attention was the
observation of a significant difference in activity between
the two zwitterionic catalysts 2Me and 2tBu differing only in
the nature of the apical group on the central carbon of the
malonate group. We were thus led to check the respective
stoichiometric reactivities of their chlorocarbonyl counter-
parts 4Me and 4tBu toward catecholborane, which proved to
be different, as shown in Scheme 6.
ed adducts, each modifying differently the nucleophilicity of
the carbene, can therefore satisfactorily explain the ob-
served differences between the behavior of the pre-catalysts
2Me and 2tBu under catalytic conditions.
Conclusion
Bcat
[20]
Whereas the anionic complex 4tBu leads to adduct 5tBu
Whereas our previous report
drew attention on the re-
as the major product along with some decomposition
markable synthetic accessibility of a family of anionic N-het-
erocyclic carbenes incorporating a malonate group as back-
bone functionality, the present account not only demon-
strates the ability of these ligands to generate catalytically
active zwitterionic complexes, but also reveals new facets of
their reactivity. The most beneficial one in view of a rapid
catalyst screening rests on the possibility to achieve a “real-
time” tuning of the donor properties of their carbene center
in a very simple way, and even after their coordination to a
transition metal. Indeed, given that the nucleophilic oxygen
atom of the malonate group emerges in the outer coordina-
tion sphere of the complex, its interaction with a selected in-
coming electrophilic reagent will modify the electron distri-
bution through the heterocycle, thereby affecting the donor
properties of the carbene center in the inner coordination
sphere. From a palette of electrophilic reagents exhibiting
different electron-withdrawing abilities, we were thus able
(
Scheme 6, upper reaction), the reaction of 4Me with cate-
I
to generate a series of “modulated” cationic Rh complexes
whose catalytic activities were compared on the basis of two
test reactions. For the polymerization of phenylacetylene,
the experimental data revealed an apparent correlation be-
tween the activity of the complexes and the donicity of the
carbene center, thus placing the unmodified zwitterionic
Scheme 6. Divergent reactivities of 4Me and 4tBu toward catecholborane.
11438
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 11432 – 11442

Electronic Tuning of a Carbene Center
FULL PAPER
complex as the most efficient pre-catalyst. The specific case
of the hydroboration of styrene was useful to determine the
scope of the present conceptual approach and its possible
limitations when it comes to highly reactive substrates.
Indeed, stoichiometric reactions are seen to take place be-
tween catecholborane and the malonate group, leading to
the rapid formation of boron-substituted derivatives which
interfere in the catalytic system by modifying the nature of
the initial active species. Finally, whereas the two specific
examples reported here provide convincing evidence for an
apparent correlation between the nucleophilicity of the car-
bene and its catalytic performances, this may not be system-
atically the case, and it seems obvious that the present ap-
proach giving a simple access to weakly nucleophilic N-het-
erocyclic carbenes, will find its full justification in the future
as one will find specific transition metal-catalyzed reactions
requiring weakly nucleophilic ancillary ligands. By the way,
one of them was just discovered in a very recent report by
C
33
H
40
F
3
N
2
O
5
RhS·0.25CH
2
Cl
2
: C 52.69, H 5.39, N 3.70; found: C 52.65,
H 5.57, N 3.55.
4
(
h -1,5-Cyclooctadiene)(1,3-dimesityl-4-oxo-4H-5-methyl-6-(trifluorome-
thanesulfonato)pyrimidin-2-ylidene)rhodium(I)
nate (3Me ): To a solution of 2Me (58.7 mg, 0.102 mmol) in CH Cl (5 mL)
was added trifluoromethanesulfonic anhydride (18 mL, 0.11 mmol,
1.05 equiv) at 08C. After 15 min of reaction, the volatiles were removed
by evaporation under vacuum. The solid residue was washed with pen-
trifluoromethanesulfo-
Tf
2
2
tane (2 mL) and dried in vacuo leading to an orange powder (78 mg,
1
9
0%.). M.p. 98–1008C; H NMR (300 MHz, CDCl
3
): d=7.17 (s, 2H,
CHMes), 7.16 (s, 2H, CHMes), 4.62–4.58 (m, 2H, CHCOD), 3.17–3.14 (m,
H, CHCOD), 2.43 (s, 3H, CH3para), 2.42 (s, 3H, CH3para), 2.27 (s, 6H,
2
CH3ortho), 2.22 (s, 3H, CH3apical), 2.20 (s, 6H, CH3ortho), 1.80–1.74 (m, 2H,
CH2COD), 1.65–1.60 (m, 2H, CH2COD), 1.48–1.43 ppm (m, 4H, CH2COD);
1
3
C NMR (75 MHz, CDCl
3 2
): d=212.0 (br, N C), 159.5, 146.8 (C=O, C-
OTf), 141.6, 140.5, 136.8, 135.8 (CMes), 130.2, 129.9 (CHMes), 119.5 (q,
1
1
J
3 3
CF =319 Hz, CF ), 117.8 (q, JCF =322 Hz, CF ), 111.9 (Capical), 101.2 (br,
CHCOD), 69.1 (br, CHCOD), 32.0 (CH2COD), 26.7 (CH2COD), 21.1, 21.0
(CH3para), 18.8, 18.7 (CH3ortho), 10.7 ppm (CH3apical); IR (ATR): n˜ = 2923,
2
1
5
881, 1701, 1678, 1650, 1611, 1426, 1405, 1331, 1267, 1219, 1165, 1127,
À1
+
027, 966, 816, 766, 677 cm ; MS (ESI): m/z (%): 705 (46) [MÀTfO] ,
+
+
95 (37) [MÀOTfÀCOD] , 573 (100) [MÀ2OTf] .
[19]
Fꢃrstner, quoted above. Furthermore, weakly nucleophilic
carbenes related to those accessed here were recently shown
4
(
h -1,5-Cyclooctadiene)(1,3-dimesityl-4-oxo-4H-5-methyl-6-hydroxypyri-
H
midin-2-ylidene)rhodium(I) trifluoromethanesulfonate (3Me ): To a solu-
tion of 2Me (34.6 mg, 60.4 mmol) in CH Cl (2 mL) was added trifluorome-
[32]
[27b–c]
by Bertrand and Bielawski,
to be of high intrinsic in-
2
2
terest in their own right for metal free activation of industri-
ally important small molecules.
thanesulfonic acid (5.5 mL, 62 mmol, 1.05 equiv) at 08C. After 20 min, the
solution was concentrated to about 1 mL and the complex was precipitat-
ed by adding pentane (5 mL). The overlaying solution was evacuated and
the solid was dried in vacuo to give an orange powder (42 mg, 96%).
1
M.p. 103–1068C; H NMR (300 MHz, CDCl
3
): d=11.36 (br, 1H, OH),
7
6
4
.07 (s, 4H, CHMes), 3.53 (br, 2H, CHCOD), 3.37 (br, 2H, CHCOD), 2.34 (s,
H, CH3para), 2.31 (s, 12H, CH3para), 2.07 (s, 3H, CH3apical), 2.07–1.98 (m,
Experimental Section
1
3
H, CH2COD), 1.72–1.68 ppm (m, 4H, CH2COD); C NMR (75 MHz,
General methods: All manipulations were carried out under an inert at-
mosphere of dry nitrogen by using standard vacuum line and Schlenk
tube techniques. Glassware was dried at 1208C in an oven for at least
three hours before use. THF and diethyl ether were freshly distilled from
sodium/benzophenone, toluene from molten sodium, prior to use. Pen-
1
CDCl
3
): d=176.6 (d,
J
RhC =58 Hz, N
2
C), 158.4 (C=O, C-OH), 142.9,
1
1
35.9 (CMes), 130.3 (CHMes), 120.3 (CMes), 105.8 (d,
J
RhC =8 Hz, CHCOD),
1
9
9.0 (Capical), 75.2 (d,
J
RhC =19 Hz, CHCOD), 32.7 (CH2COD), 26.7 (CH
2
COD), 21.2 (CH3para), 18.4 (CH3 ortho), 8.7 ppm (CH3apical); IR (ATR): n˜ =
3
1
363, 2921, 2884, 1684, 1653, 1611, 1449, 1380, 1281, 1223, 1163, 1068,
+
2
tane and dichloromethane were dried over CaH and subsequently dis-
À1
025, 966, 854, 762 cm ; MS (ESI): m/z (%): 573 (100) [MÀTfO] , 363
tilled. NMR spectra were recorded on a Bruker ARX250, AV300 or
AV400 spectrometer, in the solvents indicated; chemical shifts are report-
ed in ppm (d) compared to TMS using the residual peak of deuterated
solvent as internal standard; coupling constants (J) in Hz. Infrared spec-
tra were obtained on a Perkin–Elmer Spectrum 100 FT-IR spectrometer.
Microanalyses were performed by the Laboratoire de Chimie de Coordi-
nation Microanalytical Service and MS spectra by the mass spectrometry
service of the Paul Sabatier University. Melting points were obtained
with a Stuart Scientific Melting Point apparatus SMP1 and were not cor-
rected. Complexes 2Me and 2tBu were synthesized according to our previ-
+
(
70) [MÀRh
A
H
U
G
R
N
N
(COD)OTf] ; HR-MS (ESI): m/z: calcd for C31
H
38
N
2
O
2
Rh:
5
73.1988, found: 573.1992.
Tetraethylammonium
oxo-6H(4H)-pyrimidin-2-ylidene-4(6)-olate)rhodium(I) (4Me): Complex
2
chloro(dicarbonyl)(1,3-dimesityl-5-methyl-6(4)-
Me (315 mg, 0.55 mmol) and NEt
4 2
Cl·xH O (200 mg, x~2, 1.21 mmol,
2
.2 equiv) were placed in a Schlenk tube and THF was syringed into
(
15 mL). CO gas was bubbled into the solution for 5 min during which
time solution became light orange. After 1 h, volatiles were evacuated in
vacuo and the residue was purified by flash chromatography (Al type
III, CH Cl /MeOH 95:5, R =0.38) to furnish the title compound as a
2
O
3
[
20]
ously reported procedure.
2
2
f
4
yellow compound (338 mg, 90%). Recrystallization from CH
2 2 2
Cl /Et O
(
h -1,5-Cyclooctadiene)(1,3-dimesityl-4-oxo-4H-5-methyl-6-methoxypyri-
Me
gave yellow crystals suitable for an X-ray diffraction experiment;
midin-2-ylidene)rhodium(I) trifluoromethanesulfonate (3Me ): To a solu-
tion of 2Me (49 mg, 85.6 mmol) in CH Cl (5 mL) was added methyl tri-
1
H NMR (250 MHz, CDCl
.03 (q, J=7.3 Hz, 8H, CH2ammonium), 2.27 (s, 6H, CH3ortho), 2.23 (s, 6H,
CH3ortho), 2.15 (s, 6H, CH3para), 1.87 (s, 3H, CH3apical), 1.09 ppm (t, J=
3
): d=6.87 (s, 2H, CHMes), 6.83 (s, 2H, CHMes),
2
2
3
fluoromethanesulfonate (10 mL, 90 mmol, 1.05 equiv) at room tempera-
ture. After 3 h of reaction, the volatiles were removed by evaporation
under vacuum. The solid residue was washed with pentane (2 mL) and
1
3
7
J
.3 Hz, 12H, CH3ammonium); C NMR (63 MHz, CDCl
RhC =41.9 Hz, N C), 186.2 (d, JRhC =53.2 Hz, Rh-CO), 183.4 (d, JRhC
77.8 Hz, Rh-CO), 161.8 (C4=O, C6-O), 138.4, 137.3, 136.9, 134.9 (CMes),
28.6, 128.1 (CHMes), 89.5 (Capical), 52.1 (CH2ammonium), 21.1, 19.6, 18.3
CH3Mes), 9.3 (CH3 apical), 7.4 ppm (CH3ammonium); IR (ATR): n˜ = 2986,
3
): d=198.1 (d,
2
=
dried in vacuo leading to an orange powder (62 mg, quant.). M.p. 106–
1
1
088C; H NMR (300 MHz, CDCl
CHMes), 4.26–4.24 (m, 2H, CHCOD), 3.80 (s, 3H, OCH
3
): d=7.13 (s, 2H, CHMes), 7.12 (s, 2H,
), 3.10–3.07 (m,
H, CHCOD), 2.42 (s, 3H, CH3para), 2.41 (s, 3H, CH3para), 2.26 (s, 6H,
1
(
3
2
2
1
920, 2859, 2059 (COsym), 1974 (COasym), 1663, 1609, 1480, 1458, 1390,
355, 1308, 1299, 1260, 1182, 1170, 1065, 1031, 1002, 919, 849, 783, 766,
CH3ortho), 2.17 (s, 6H, CH3ortho), 2.08 (s, 3H, CH3apical), 1.79–1.65 (m, 2H,
CH2COD), 1.65–1.54 (m, 2H, CH2COD), 1.47–1.41 ppm (m, 4H, CH2COD);
À1
À1
1
3
2 2 sym
728 cm ; IR (CH Cl ): n˜ = 2069 (CO ), 1987 cm (COasym); MS (ESI,
C NMR (75 MHz, CDCl
40.1, 140.1, 136.0, 129.8 (CMes), 129.8, 129.7 (CHMes), 105.6 (Capical), 100.4
br, CHCOD), 69.3 (br, CHCOD), 69.1 (br, CHCOD), 62.5 (OCH ), 32.1
CH2 COD), 26.7 (CH COD), 21.2, 21.1 (CH3para), 18.9, 18.8 (CH3ortho),
.4 ppm (CH3 apical); IR (ATR): n˜ =2917, 1662, 1613, 1476, 1450, 1384,
3
): d=161.3, 158.5 (C=O, C-OMe), 140.4, 140.4,
À
À
negative mode): m/z (%): 555 (69) [MÀH] , 527 (100) [MÀHÀCO] ,
1
(
(
9
1
(
2
6
55 (27); elemental analysis calcd (%) for C33
.61, N 6.12; found: C 57.52, H 6.93, N 5.97.
3 4
H45ClN O Rh: C 57.77, H
3
2
Tetraethylammonium chloro(dicarbonyl)(1,3-dimesityl-5-tert-butyl-6(4)-
oxo-6H(4H)-pyrimidin-2-ylidene-4(6)-olate)rhodium(I) (4tBu): Complex
2tBu (62 mg, 0.10 mmol) and NEt Cl·xH O (40 mg, x ~2, 0.24 mmol,
4 2
À1
359, 1302, 1272, 1064, 1034, 994, 959, 866, 853, 759 cm ; MS (ESI): m/z
+
%): 587 (100) [MÀTfO] ; elemental analysis calcd (%) for
Chem. Eur. J. 2010, 16, 11432 – 11442
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11439

G. Lavigne, V. Cꢀsar and N. Lugan
2
.4 equiv) were placed in a Schlenk tube. THF was syringed into (5 mL)
solution for 10 min. After 1 h, all volatiles were evacuated under vacuum
and the residue was washed with pentane (2ꢆ3 mL) to leave a yellow
solid (56.2 mg, 93%). M.p. 2108C (decomp); H NMR (300 MHz,
and CO gas was bubbled into the solution for 5 min during which time
solution color changed from orange to light yellow. After 1 hour, vola-
tiles were evacuated in vacuo and the residue was purified by flash chro-
1
3
CDCl ): d=6.95 (s, 2H, CHMes), 6.93 (s, 2H, CHMes), 5.25 (br, 1H, OH),
matography (Al
powder (66 mg, 91%). H NMR (300 MHz, CDCl
CHMes), 6.83 (s, 2H, CHMes), 2.97 (q, J=7.1 Hz, 8H, CH2ammonium), 2.27 (s,
H, CH3Mes), 2.26 (s, 6H, CH3Mes), 2.17 (s, 6H, CH3Mes), 1.37 (s, 9H,
2
O
3
type III, CH
2
Cl
2
/MeOH 95:5) to yield an orange
2.34 (s, 6H, CH3Mes), 2.17 (s, 6H, CH3Mes), 2.08 (s, 6H, CH3Mes), 1.92 ppm
1
13
3
): d=6.86 (s, 2H,
(s, 3H, CH3apical); C NMR (75 MHz, CDCl
3
): d=207.1 (d,
J
RhC
=
2
39.9 Hz, N C), 184.6 (d, JRhC =54.3 Hz, Rh-CO), 182.4 (d, JRhC =75.7 Hz,
Rh-CO), 159.0 (C4=O, C6-OH), 139.6, 136.6, 135.2, 133.8 (CMes), 129.9,
129.1 (CHMes), 97.1 (Capical), 21.2, 19.2, 18.1 (CH3Mes), 8.5 ppm (CH3apical);
6
1
3
CH3tBu), 1.08 ppm (t, J=7.1 Hz, 12H, CH3ammonium); C NMR (63 MHz,
CDCl ): d=196.7 (d, JRhC =41.5 Hz, N C), 186.4 (d, JRhC =53.0 Hz, Rh-
CO), 183.5 (d, JRhC =78.2 Hz, Rh-CO), 160.9 (C4=O, C6-O), 139.3, 137.5,
3
2
IR (ATR): n˜ = 2922, 2073 (COsym), 1988 (COasym), 1686, 1610, 1437,
À1
1376, 1309, 1276, 1206, 1165, 1061, 1033, 850, 771 cm ; IR (CH
2
Cl
2
): n˜ =
À1
+
1
3
36.6, 135.2 (CMes), 128.5, 128.0 (CHMes), 99.5 (Capical), 51.9 (CH2ammonium),
3.8 (C(CH ), 30.5 (C(CH ), 21.2, 19.6, 18.4 (CH3Mes), 7.4 ppm
2082 (COsym), 2000 cm (COasym); MS (ESI): m/z (%): 556 (100) [M] ,
+
+
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
)
3
A
H
U
G
E
N
N
3
)
3
534 (98), 521 (20) [MÀCl] , 515 (65), 493 (54) [MÀClÀCO] ; elemental
(
CH3ammonium); IR (ATR): n˜ = 2975, 2956, 2921, 2857, 2058, 1969, 1654,
2 4
analysis calcd (%) for C25H28ClN O Rh: C 53.73, H 5.05, N 5.01; found:
1
7
591, 1482, 1451, 1370, 1333, 1254, 1171, 1023, 999, 887, 856, 778,
C 52.95, H 4.68, N 5.03.
À1
À1
54 cm ; IR (CH
2
Cl
2
): n˜ = 2068 (COsym), 1986 cm (COasym); MS (ESI):
BCat
Complex 5Me
: Complex 2Me (55.6 mg, 97 mmol) and B-chlorocatechol-
+
+
m/z (%): 597 (77) [MÀNEt
4
À2H] , 569 (100) [MÀNEt
4
ÀHÀCO] ; ele-
borane (16.5 mg, 0.106 mmol, 1.1 equiv) were weighed in a Schlenk tube.
CH Cl (5 mL) was added and after 10 min, CO gas was bubbled into the
mental analysis calcd (%) for C36
3 4
H53ClN O Rh: C 59.22, H 7.32, N 5.75;
2
2
found: C 59.72, H 7.40, N 5.43.
solution for 20 min. The solution color changed from orange to pale
yellow. After 2 h, all volatiles were evacuated under vacuum and the resi-
due was washed twice with pentane (3 mL) to give, after drying, a yellow
Chloro(dicarbonyl)(1,3-dimesityl-4-oxo-4H-5-methyl-6-methoxypyrimi-
din-2-ylidene)rhodium(I) (5Me ): Complex 2Me (45.3 mg, 79.1 mmol) was
dissolved in CH
Me
1
2
Cl
2
(5 mL) and methyl triflate (9 mL, 83 mmol,
powder (59 mg, 90%). H NMR (400 MHz, CD
2
Cl
2
): d=6.99 (brs, 2H,
1
.05 equiv) was added at room temperature. After 3 h, an excess of dry
CHMes), 6.97 (brs, 2H, CHMes), 6.79 (br, 2H, CHCat), 6.22 (br, 2H, CHCat),
2.35 (s, 6H, CH3para), 2.17 (brs, 6H, CH3ortho), 2.10 (brs, 6H, CH3ortho),
NaCl was added and CO was bubbled into the solution for 10 min. The
light yellow reaction mixture was filtered through a plug of celite and
volatiles were removed in vacuo. The residue was washed with pentane
1
3
1.91 ppm (s, 3H, CH3apical); C NMR (100 MHz, CD
2 2
Cl ): d=206.8 (d,
J
RhC =43.8 Hz, N C), 185.0 (d, JRhC =54.1 Hz, Rh-CO), 182.5 (d, JRhC =
2
(
2ꢆ2 mL) and dried to give a pale yellow powder (41 mg, 91%).
74.8 Hz, Rh-CO), 158.9 (C4=O, C6-OBCat), 140.3, 139.9, 136.9, 135.6,
135.4, 135.1, 134.2, (CAr, CHAr), 129.7, 129.2 (CHMes), 97.3 (Capical), 20.9,
1
H NMR (300 MHz, CDCl
OCH ), 2.37 (s, 3H, CH3Mes), 2.34 (s, 3H, CH3Mes), 2.31 (s, 3H, CH3Mes),
.23 (s, 3H, CH3Mes), 2.20 (s, 3H, CH3Mes), 2.16 (s, 3H, CH3apical),
3
): d=6.99 (br, 4H, CHMes), 3.78 (s, 3H,
1
1
3
19.0, 18.3, 18.1, 18.0 (CH3Mes), 8.3 ppm (CH3apical); B NMR (128.4 MHz,
CD Cl ): d=23.06 ppm (br); IR (ATR): n˜ 2958, 2923, 2857, 2075
2
2
J
7
1
(
(
1
8
2
2
=
1
3
.14 ppm (s, 3H, CH3Mes); C NMR (75 MHz, CDCl
3
): d=208.8 (d,
(COsym), 1990 (COasym), 1689, 1609, 1466, 1377, 1335, 1309, 1274, 1204,
À1
À1
RhC =44.2 Hz, N C), 184.6 (d, JRhC =54.6 Hz, Rh-CO), 182.6 (d, JRhC
2
=
1165, 1031, 851, 771, 742 cm ; IR (CH
2
Cl
2
): n˜ = 2082 (COsym), 1999 cm
5.5 Hz, Rh-CO), 161.5, 158.7 (2C, C4=O, C6-OMe), 139.7, 139.2, 136.7,
36.3, 135.8, 135.6, 133.5, 133.2 (CMes), 130.0, 129.9, 129.0 (CHMes), 105.9
(COasym).
BCat
BCat
Complex [(5Me
)
2
]
A
H
U
G
E
N
N
(NEt
4
): To a solution of complex 5Me
(7.1 mg,
C
apical), 62.3 (O-CH
3
), 21.2, 19.8, 19.3, 18.5, 18.2 (CH3Mes), 9.7 ppm
1
0.5 mmol) in CH
2 2
Cl (1 mL) was added complex 4Me (7.2 mg, 10.5 mmol,
CH3apical); IR (ATR): n˜ = 2923, 2856, 2076 (COsym), 1985 (COasym), 1674,
1
.0 equiv) at room temperature. After 10 min, an aliquot was analyzed by
640, 1610, 1455, 1429, 1380, 1355, 1277, 1230, 1070, 997, 920, 909, 856,
IR spectroscopy which indicated a full conversion ( n˜ = 2075.0 (COsym),
À1
À1
49, 772, 727 cm ; IR (CH
2
Cl
2
): n˜ = 2081 (COsym), 1998 cm (COasym);
À1
1
993.6 cm (COasym). Evaporation of the solvent gave the title complex
+
+
MS (ESI): m/z (%): 548 (77) [MÀCO] , 507 (100) [MÀCOÀCl] ; ele-
mental analysis calcd (%) for C26 Rh, 0.1CH Cl : C 53.92, H
.24, N 4.82; found: C 53.78, H 5.61, N 4.90.
1
as an orange foam. H NMR (300 MHz, CD
2
Cl
2
): d=6.94 (s, 4H, CHMes),
H
30ClN
2
O
4
2
2
6
.91 (s, 4H, CHMes), 6.57 (s, 4H, CHCat), 3.07 (q, J=7.3 Hz, 8H,
5
CH2ammonium), 2.33 (s, 12H, CH3para), 2.14 (s, 12H, CH3ortho), 2.07 (s, 12H,
Chloro(dicarbonyl)(1,3-dimesityl-4-oxo-4H-5-methyl-6-(trifluoromethane-
sulfonato)pyrimidin-2-ylidene)rhodium(I) (5Me ): To a solution of 2Me
CH3ortho), 1.76 (s, 6H, CH3apical), 1.20 ppm (t, J=7.3 Hz, 12H,
Tf
13
CH3ammonium); C NMR (75 MHz, CD
2
Cl
2
): d=203.1 (d, JRhC =43.9 Hz,
(
40.0 mg, 69.8 mmol) in CH
2
Cl
2
(5 mL) was added trifluoromethanesul-
2
N C), 185.8 (d, JRhC =53.9 Hz, Rh-CO), 183.0 (d, JRhC =76.0 Hz, Rh-CO),
fonyl anhydride (12 mL, 73.3 mmol, 1.05 equiv) at room temperature.
After 30 min, a large excess of sodium chloride (>10 equiv) was added
to the solution as a solid and CO gas was bubbled into the solution for
160.6 (C4=O, C6-OB), 138.4, 137.0, 134.4 (CMes), 129.0, 128.6 (CHMes),
118.0, 108.3 (CHCat), 95.2 (Capical), 52.5 (CH2ammonium), 20.9 (CH3para), 19.1,
1
1
18.0 (CHortho), 8.6 (CH3apical), 7.3 ppm (CH3ammonium); B NMR (96.2 MHz,
CD Cl ): 14.16 ppm (s); IR (ATR): n˜ = 2983, 2922, 2859, 2066 (COsym),
2
0 min. After 1 hour of reaction, the solution was filtered through celite
2
2
to remove the remaining solids and evaporated. The residue was washed
1983 (COasym), 1662, 1610, 1482, 1458, 1426, 1395, 1376, 1309, 1276, 1264,
À1
with pentane (2ꢆ2 mL) and dried in vacuo to furnish a yellow solid
1233, 1205, 1170, 1057, 1032, 1001, 851, 770, 735 cm ; IR (CH
2
Cl
2
): n˜ =
1
À1
(
42 mg, 87%). H NMR (300 MHz, CDCl
3
): d=7.03 (s, 4H, CHMes), 7.02
2075 (COsym), 1994 cm (COasym).
(
s, 4H, CHMes), 2.37 (s, 3H, CH3Mes), 2.36 (s, 3H, CH3Mes), 2.31 (s, 3H,
BCat
Complex 5tBu
: Complex 2tBu (65 mg, 0.106 mmol) and B-chlorocate-
CH3apical), 2.30 (s, 3H, CH3Mes), 2.24 (s, 3H, CH3Mes), 2.23 (s, 3H, CH3Mes),
2
J
7
1
cholborane (18 mg, 0.116 mmol, 1.1 equiv) were weighed and placed in a
Schlenk tube and CH Cl (6 mL) was added. The solution became almost
1
3
.15 ppm (s, 3H, CH3Mes); C NMR (75 MHz, CDCl
3
): d=213.1 (d,
2
2
RhC =45.5 Hz, N C), 184.2 (d, JRhC =55.2 Hz, Rh-CO), 182.4 (d, JRhC =
2
immediately dark red. After 10 min, CO gas was bubbled into the solu-
tion for 20 min. The solution color changed to orange. The reaction was
left running overnight. The solution was filtered through a plug of celite
to remove a brown precipitate and the volatiles were evacuated under
vacuum. The residue was washed twice with pentane (3 mL) to give, after
4.9 Hz, Rh-CO), 159.5, 146.8 (2C, C4=O, C6-OTf), 141.0, 139.8, 137.1,
35.7, 135.5, 134.6, 134.2 (CMes), 130.4, 130.2, 129.7, 129.3 (CHMes), 113.1
(
(
C
apical), 21.1, 19.8, 19.1, 18.7, 18.2 (CH3Mes), 1.0 ppm (CH3apical); IR
ATR): n˜ = 2962, 2925, 2089 (COsym), 2001 (CO asym), 1718, 1658, 1418,
À1
1
(
7
358, 1262, 1224, 1123, 1087, 1028, 852, 820, 764, 735, 680 cm ; IR
1
drying, an orange-brown powder (55 mg, 72%). H NMR (300 MHz,
CD Cl ): d=7.01 (brs, 4H, CHMes), 6.80–6.76 (br, 4H, CHCat), 2.39 (s,
2 2
À1
CH
2
Cl
2
): n˜ = 2085 (COsym), 2003 cm (COasym); MS (ESI): m/z (%):
+
+
11 (34) [MÀCl+Na] , 683 (34), 653 [MÀCl] ; elemental analysis calcd
6
9
H, CH3ortho), 2.35 (s, 6H, CH3ortho), 2.15 (brs, 6H, CH3para), 1.32 ppm (s,
(
%) for C26
H
25ClF
3
N
2
O
6
RhS: C 45.33, H 3.66, N 4.07; found: C 45.76, H
13
H, CH
3
t
Bu); C NMR (75 MHz, CD
2
Cl
2
): d=232.6 (d, JRhC =46.2 Hz,
4
.16, N 3.80.
N
2
C), 185.3 (d, JRhC =51.1 Hz, Rh-CO), 182.2 (d, JRhC =74.4 Hz, Rh-CO),
165.8 (C4=O, C6-OBCat), 140.1, 140.0, 136.0, 134.8 (CAr), 130.2, 129.5
(CHMes), 121.0 (CHCat), 115.3 (CHCat), 92.3 (Capical), 40.4 (C(CH ), 28.7
), 20.8, 20.7, 18.7 ppm (CH3Mes); B NMR (96 MHz, CD Cl ):
d= 23.6 ppm (br); IR (ATR): n˜ 2966, 2922, 2079 (COsym), 1996
Chloro(dicarbonyl)(1,3-dimesityl-4(6)-hydroxy-5-methyl-6(4)-oxo-6H(4H)-
pyrimidin-2-ylidene)rhodium(I) (5Me ): To a solution of 2Me (62.3 mg,
0
0
H
A
H
U
G
R
N
N
3 3
)
1
1
.109 mmol) in CH
2
Cl
2
(4 mL), a solution of HCl (1m in Et
2
O, 120 mL,
(C
A
H
U
G
R
N
N
(CH
3
)
3
2
2
.120 mmol, 1.1 eq) was added and after 5 min, CO was bubbled into the
=
11440
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 11432 – 11442

Electronic Tuning of a Carbene Center
FULL PAPER
Me
(
1
(
COasym), 1752, 1728, 1666, 1606, 1511, 1468, 1410, 1374, 1297, 1274, 1252,
and C9 in the structure of 5Me were set in idealized position (R
3
CH, CÀ
À1
2
186, 1143, 1123, 1094, 1029, 851, 744, 654 cm ; IR (CH
COsym), 2005 cm (COasym).
2
Cl
2
): n˜ = 2086
H=0.96 ꢅ;
R
2
CH
2
,CÀH=0.97 ꢅ; RCH
3
,CÀH=0.98 ꢅ;
C
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(sp )ÀH=
À1
0.93 ꢅ; Uiso 1.2 or 1.5 times greater than the Ueq of the carbon atom to
which the hydrogen atom is attached), and their positions were refined as
“riding” atoms.
H
Stoichiometric reaction between 4
R
, 5Me and catecholborane: For the IR
or 5Me (10–30 mmol) was solubilized in CH Cl
1 mL) and the solution was cooled to 08C. Catecholborane (5 equiv)
was added and the IR spectra were recorded. For the NMR experiments,
H
monitoring, complex 4
R
2
2
Me
(
CCDC 772339 (4Me), 772340 (5Me ) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif
the reaction was carried out directly in a NMR tube using CD
solvent.
2 2
Cl as the
General procedure for the hydroboration of styrene: The rhodium cata-
lyst (0.005 mmol, 1 mol%) was weighed, placed in a 10 mL Schlenk tube
and solubilized in CH Cl (2 mL). Mesitylene (70 mL, 0.50 mmol, internal
2 2
standard) and styrene (58 mL, 0.50 mmol) were then syringed into the
Schlenk, and the solution was cooled to 08C using an ice/water bath.
Freshly distilled catecholborane (80 mL, 0.75 mmol, 1.5 eq) was added
within 5 s and the conversion was monitored by GC. For the GC samples,
Acknowledgements
Support from the CNRS and from the ANR (programme blanc ANR-08-
BLAN-0137-01) is gratefully acknowledged.
an aliquot of the reaction mixture was diluted with CH
through a small silica plug. For the determination of products distribu-
tion, the reaction was carried out in CD Cl (on 0.25 mmol scale of sty-
2 2
Cl and filtered
2
2
1
rene), and the yields of products were measured by H NMR spectrosco-
py after 90 min.
[1] For a review, see: A. M. Allgeier, C. A. Mirkin, Angew. Chem. 1998,
110, 936; Angew. Chem. Int. Ed. 1998, 37, 894.
[
2] Recent examples: a) M. R. Ringenberg, S. L. Kokatam, Z. M.
Heiden, T. B. Rauchfuss, J. Am. Chem. Soc. 2008, 130, 788;
b) C. K. A. Gregson, V. C. Gibson, N. J. Long, E. L. Marshall, P. J.
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3] J. D. Azoulay, R. S. Rojas, A. V. Serrano, H. Ohtaki, G. B. Galland,
G. Wu, G. C. Bazan, Angew. Chem. 2009, 121, 1109; Angew. Chem.
Int. Ed. 2009, 48, 1089.
General procedure for the polymerization of phenylacetylene: The rhodi-
um catalyst (0.01 mmol, 1 mol%) was solubilized in CH Cl (2.5 mL) and
2 2
the solution was placed in a temperature-controlled bath at 208C. Mesity-
lene (139 mL, 1.0 mmol, internal standard) and phenylacetylene (110 mL,
[
[
[
1
.0 mmol) were then successively syringed into the reaction mixture. The
conversion was measured by passing an aliquot of the solution through a
plug of silica gel using pentane as eluant and monitoring the decrease of
4] R. J. Lundgren, M. A. Rankin, R. McDonald, G. Schatte, M. Stra-
diotto, Angew. Chem. 2007, 119, 4816; Angew. Chem. Int. Ed. 2007,
the peak of phenylacetylene in the GC chromatogram. % cis values were
1
determined by H NMR spectroscopy using CD
2
Cl
2
as the deuterated sol-
4
6, 4732.
vent. The isolated polymer samples were dissolved in THF and the THF
solution was filtered (pore size=0.45 mm) before chromatographic analy-
sis. Size-exclusion chromatography (SEC) of poly(phenylacetylene) was
5] For monographs, see: a) N-Heterocyclic Carbenes in Transition
Metal Catalysis (Ed.: F. Glorius), Springer, Berlin, 2007; b) N-Heter-
ocyclic Carbenes in Synthesis (Ed.: S. P. Nolan), Wiley-VCH, Wein-
heim, 2006.
À1
carried out in filtered THF (flow rate: 1 mLmin ) on a 300ꢆ7.5 mm
PLgel 5 mm mixed-D column (Polymer Laboratories), equipped with a
multiangle light-scattering (miniDawn Tristar, Wyatt Technology Corpo-
ration) and a refractive index (R12000, Sopares) detectors. The column
was calibrated against linear polystyrene standards (Polymer Laborato-
ries).
[
6] For general reviews on NHCs, see: a) O. Schuster, L. Yang, H. G.
Raubenheimer, M. Albrecht, Chem. Rev. 2009, 109, 3445; b) F. E.
Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166; Angew. Chem.
Int. Ed. 2008, 47, 3122; c) D. Bourissou, O. Guerret, F. Gabbaꢄ, G.
Bertrand, Chem. Rev. 2000, 100, 39.
Me
X-ray diffraction studies: Crystals of 4Me and 5Me suitable for X-ray dif-
[
7] Reviews: a) S. Dꢇez-Gonzꢈlez, N. Marion, S. P. Nolan, Chem. Rev.
2 2 2
fraction were obtained through recrystallization from CH Cl /Et O (4Me)
2
2
009, 109, 3612; b) M. Poyatos, J. A. Mata, E. Peris, Chem. Rev.
009, 109, 3677; c) E. A. B. Kantchev, C. J. OꢂBrien, M. G. Organ,
Me
or EtOAc/pentane (5Me ) solutions. Data were collected on Oxford Dif-
Me
fraction Xclaibur (4Me) or Bruker D8 APEX II (5Me ) diffractometers.
Angew. Chem. 2007, 119, 2824; Angew. Chem. Int. Ed. 2007, 46,
768; d) V. Cꢀsar, S. Bellemin-Laponnaz, L. H. Gade, Chem. Soc.
All calculations were performed on a PC-compatible computer using the-
2
[
33]
WinGX system. The structures were solved by using the SIR92 pro-
Rev. 2004, 33, 619; e) C. M. Crudden, D. P. Allen, Coord. Chem.
Rev. 2004, 248, 2247; f) W. A. Herrmann, Angew. Chem. 2002, 114,
[
34]
gram, which revealed in each instance the position of most of the non-
hydrogen atoms. All remaining non-hydrogen atoms were located by the
usual combination of full matrix least-squares refinement and difference
1
342; Angew. Chem. Int. Ed. 2002, 41, 1290; g) O. Kꢃhl, Chem. Soc.
Rev. 2007, 36, 592; h) S. T. Liddle, I. S. Edworthy, P. L. Arnold,
Chem. Soc. Rev. 2007, 36, 1732.
8] NHCs in metathesis catalysts: a) T. Vorfalt, S. Leuthꢉußer, H.
Plenio, Angew. Chem. 2009, 121, 5293; Angew. Chem. Int. Ed. 2009,
[
35]
electron density syntheses by using the SHELXL97 program.
5
For
Me
Me , after completing the initial structure solution, it was found that
[
2
6% of the total cell volume was filled with disordered solvent, which
could not be modelled in terms of atomic sites. From this point on, resid-
ual peaks were removed and the solvent region was refined as a diffuse
4
8, 5191; b) C. Samojlowicz, M. Bieniek, K. Grela, Chem. Rev. 2009,
109, 3708 and references therein; c) G. C. Vougioukalakis, R.
Grubbs, Chem. Rev. 2010, 110, 1746.
[
36]
contribution without specific atom positions by using the PLATON
[
37]
module SQUEEZE, which subtracts electron density from the void re-
gions by appropriately modifying the diffraction intensities of the overall
structure. An electron count over the solvent region provides an estimate
for the number of solvent molecules removed from the cell. The number
of electrons thus located was 240 per unit cell. This residual electron den-
sity was assigned to 1.5 molecules of pentane per molecule of complex
[9] Recent examples of NHC-based catalysts for Suzuki–Miyaura cou-
pling reaction: a) M. G. Organ, S. Calimsiz, M. Sayah, K. H. Hoi,
A. J. Lough, Angew. Chem. 2009, 121, 2419; Angew. Chem. Int. Ed.
2009, 48, 2383; b) G. Altenhoff, R. Goddard, C. W. Lehmann, F.
Glorius, J. Am. Chem. Soc. 2004, 126, 15195; c) G. Altenhoff, R.
Goddard, C. W. Lehmann, F. Glorius, Angew. Chem. 2003, 115,
3818; Angew. Chem. Int. Ed. 2003, 42, 3690.
[10] a) For a general review, see: S. Diez-Gonzalez, S. P. Nolan, Coord.
Chem. Rev. 2007, 251, 874; b) for a leading reference on the calcula-
tion of the buried volume of monodentate NHCs, see: A. Poater, B.
Cosenza, A. Correa, S. Giudice, F. Ragone, V. Scarano, L. Cavallo,
Eur. J. Inorg. Chem. 2009, 1759.
(
6
240/4=60 electrons per molecule; 1.5 molecules of pentane would give
3 electrons), which were included in the formula, formula weight, calcu-
lated density, absorption coefficient, and F(000). Applying this procedure
AHCTUNGTRENNUNG
led to a dramatic improvement in all refinement parameters and a mini-
mization of residuals. Atomic scattering factors were taken from the
usual tabulations. Anomalous dispersion terms for Rh and Cl atoms were
included in Fc. All non-hydrogen atoms were allowed to vibrate aniso-
tropically. All the hydrogen atoms;except hydrogen atoms attached to C8
[11] For a comparison of the catalytic performance through a series of
electronically identical NHCs with varied steric constrain, see: a) S.
Chem. Eur. J. 2010, 16, 11432 – 11442
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11441

G. Lavigne, V. Cꢀsar and N. Lugan
Wꢃrtz, F. Glorius, Acc. Chem. Res. 2008, 41, 1523; b) see also
ref. [9b].
[23] a) T. A. Betley, J. C. Peters, Angew. Chem. 2003, 115, 2487; Angew.
Chem. Int. Ed. 2003, 42, 2385; b) C. C. Lu, J. C. Peters, J. Am.
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Chem. Soc. 2001, 123, 5100.
[24] P. Braunstein, O. Siri, J.-P. Taquet, M.-M. Rohmer, M. Bꢀnard, R.
Welter, J. Am. Chem. Soc. 2003, 125, 12246.
[25] The calculations used SambVca with the following preconized pa-
rameters: sphere radius: 3.5 ꢅ, distance from the center of the
sphere: 2.1 ꢅ, mesh spacing: 0.05 ꢅ, hydrogen atoms not included.
[26] It should be noted that the complex on which the % Vbur has a sig-
nificant influence since the anionic ligand displays % Vbur ranging
from 34.5 to 41.3%, respectively, in the zwitterionic iron and rhodi-
um complexes previously reported in our first communication.
[27] a) V. Cꢀsar, N. Lugan, G. Lavigne, Eur. J. Inorg. Chem. 2010, 361;
b) T. W. Hudnall, C. W. Bielawski, J. Am. Chem. Soc. 2009, 131,
16039; c) T. W. Hudnall, E. J. Moorhead, D. G. Gusev, C. W. Bielaw-
ski, J. Org. Chem. 2010, 75, 2763.
[
12] Recent examples: a) D. G. Gusev, Organometallics 2009, 28, 6458;
b) R. Tonner, G. Frenking, Organometallics 2009, 28, 3901; c) R. A.
Kelly III, H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens, J. Bord-
ner, I. Samardjiev, C. D. Hoff, L. Cavallo, S. P. Nolan, Organometal-
lics 2008, 27, 202; d) A. Fꢃrstner, M. Alcarazo, H. Krause, C. W.
Lehmann, J. Am. Chem. Soc. 2007, 129, 12676; e) W. A. Herrmann,
J. Schꢃtz, G. D. Frey, E. Herdtweck, Organometallics 2006, 25, 2437;
f) H. Tꢃrkmen, B. Cetinkaya, J. Organomet. Chem. 2006, 691, 3749;
g) R. Dorta, E. D. Stevens, N. M. Scott, C. Costabile, L. Cavallo,
C. D. Hoff, S. P. Nolan, J. Am. Chem. Soc. 2005, 127, 2485.
[
13] a) See reference [12c]; b) S. Wolf, H. Plenio, J. Organomet. Chem.
2
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Received: April 7, 2010
Revised: June 8, 2010
Published online: September 6, 2010
11442
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