Room-temperature synthesis of tetra-ortho-substituted biaryls by NHC-catalyzed Suzuki-Miyaura couplings

Article abstract of DOI:10.1002/chem.201102442

Into the groove: The introduction of a C₂-symmetric N-heterocyclic carbene ligand with appropriately substituted naphthyl side chains enables the efficient Suzuki-Miyaura coupling to form bulky tetra-ortho-substituted biaryls from aryl bromides and chlorides at room temperature. DFT calculations uncover the subtle steric phenomena at play that lead to the superior catalytic performance. Cyoct=cyclooctyl. Copyright

Full text of DOI:10.1002/chem.201102442

DOI: 10.1002/chem.201102442
Room-Temperature Synthesis of Tetra-ortho-Substituted Biaryls by NHC-
Catalyzed Suzuki–Miyaura Couplings**
Linglin Wu,[a] Emma Drinkel,[a] Fiona Gaggia,[a] Samanta Capolicchio,[a]
Anthony Linden,[a] Laura Falivene,[b] Luigi Cavallo,[b] and Reto Dorta*[a]
Transition-metal-catalyzed cross couplings have become
these important tetra-ortho-substituted biaryl structures by
way of the Suzuki–Miyaura coupling.[9]
some of the most powerful and widely used methods to con-
[1]
ꢀ
struct C C bonds. Among them, the Suzuki–Miyaura cou-
Recently, we have presented a new class of saturated
NHC ligands with naphthyl-derived side chains that showed
excellent reactivities in a variety of catalytic applications.[10]
In related studies,[11] we noticed that a ligand with a cyclooc-
tyl group in position 2 of the naphthalene moieties led to
significantly increased reactivity. On the basis of these ob-
servations, we now report the application of such NHC
ligand systems in the palladium-catalyzed Suzuki–Miyaura
couplings to give tetra-ortho substituted biaryls and present
conclusive evidence concerning the reasons leading to their
superior behavior in these reactions.
pling,[2] has emerged as a particularly attractive and practical
tool for synthetic organic chemistry.[3] Indeed, over the last
decade, several limitations of this methodology have been
successfully addressed by using bulky, electron-rich mono-
dentate phosphines or sterically demanding NHC ligands
(NHC=N-heterocyclic carbene).[4] One of the few challeng-
es remaining in the Suzuki–Miyaura coupling reaction in-
volves transformations with sterically demanding substrates
that lead to tetra-ortho-substituted products. Especially in
cases where aryl chlorides are used, the relatively poor nu-
cleophilicity of the arylboron reagents results in diminished
catalytic activities.[5] In 2004, Glorius and co-workers
showed for the first time that aryl chlorides can indeed be
coupled to aryl boronic acids to generate such tetra-ortho-
substituted biaryls at elevated temperature (1108C) by em-
ploying a very bulky, yet flexible derivative of their bioxazo-
line-derived NHC ligands in combination with a PdII metal
salt.[6] More recently in 2009 and following the same concept
of ꢀflexible steric bulkꢁ of the NHC ligand, Organ and co-
workers used the complex Pd-PEPPSI-IPent as the catalyst
for the Suzuki–Miyaura couplings to form bulky tetra-ortho-
substituted biaryls at milder conditions (658C).[7] Since then,
various other ligand systems have been shown to effect simi-
lar couplings involving aryl chlorides when appropriate heat-
ing is employed.[8] To date, systems that work at room tem-
perature have not been reported for the construction of
Reaction of NHC ligands with saturated[10c] and unsaturat-
ed N-heterocycles incorporating 2- or 2,7-cyclooctyl groups
on the naphthalene side chains with a PdACTHUNRTGNEUNG(cin)Cl dimer
(cin=cinnamyl) and appropriate workup gave the four com-
plexes depicted in Table 1 in good yield as single isomers
(anti-configured).[12]
To explore the effect of these new NHC ligands on biaryl
formation in difficult Suzuki–Miyaura couplings, we chose
the reaction between 2,4,6-trimethylphenyl chloride and 2,6-
dimethylphenyl boronic acid (Table 1). Under optimized re-
action conditions,[12] the four new catalyst systems were
benchmarked against the commercially available, SIPr/IPr-
modified congeners (Nolanꢁs catalysts) as well as Organꢁs
Pd-PEPPSI-IPent system, currently the most powerful pre-
catalyst for such transformations. At room temperature,
these reference systems resulted in low product yields
(Table 1, entries 1–5, GC yields).[13] In entries 4 and 5 in
Table 1, we used Organꢁs previously reported reaction con-
ditions,[7] which deteriorated the reaction outcome. Gratify-
ingly, all catalysts incorporating the new NHC structures
showed higher conversions and yields than the benchmark
systems. Among the four substructures tested, anti-C clearly
stands out as being particularly effective as it shows both
high conversions and yields at room temperature.
We then proceeded in evaluating the coupling of a variety
of hindered aryl bromides (Table 2) and aryl chlorides
(Table 3) employing precatalyst anti-C. As can be seen from
the data reported in Table 2, high isolated product yields
were normally obtained at room temperature within short
reaction times when employing aryl bromides. In entry 2 in
Table 2, where the coupling proceeded very slowly at room
temperature, slight heating (658C) was applied, leading to a
[a] L. Wu,+ E. Drinkel,+ F. Gaggia,+ S. Capolicchio,+
Priv.-Doz. Dr. A. Linden,+ Prof. Dr. R. Dorta+
Organisch-chemisches Institut, Universitꢂt Zꢃrich
Winterthurerstrasse 190, 8057 Zꢃrich (Switzerland)
E-mail: dorta@oci.uzh.ch
[b] L. Falivene, Prof. Dr. L. Cavallo
Dipartimento di Chimica
Universitꢄ di Salerno
Via Ponte don Melillo, 84084 Fisciano (Italy)
[+] New Address: School of Biomedical, Biochemical and Chemical
Sciences, University of Western Australia, 35 Stirling Highway,
Crawley, WA, 6009 (Australia)
[**] NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102442.
12886
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12886 – 12890

COMMUNICATION
Table 1. Screening of precatalysts in a representative example of room-
in the former case. Interestingly, the
present system also displayed excellent
temperature Suzuki–Miyaura coupling.[a] Cyoct=cyclooctyl.
chemoselectivity,
favoring
Suzuki–
Miyaura couplings over the Heck reac-
tion scenario (Table 3, entries 15 and
16). Precatalyst anti-C also promotes
less difficult Suzuki–Miyaura coupling
reactions, as evidenced from data gath-
ered in Table 3, entry 18. The last two
entries in Table 3 also highlight just how
different the reactivities are when
moving from a tri- to a tetra-ortho-sub-
stituted coupling product.
This last observation, together with
the uniquely high reactivity of anti-C,
prompted us to investigate the present
catalytic system further. As a first ap-
proach, we grew single crystals of anti-C
and of the unsaturated NHC–Pd deriva-
tive that lacks the 7-cyclooctyl groups
on the naphthalene units (anti-B) to
compare their structures in the solid
state (Figure 1).[14]
One notes that these anti-configured
NHCs lead to idealized C2-symmetry of
the ligand framework, in contrast to the
commonly used IPr/SIPr or Organꢁs
IPent ligands, which result in structures
with C2v symmetry. Between the NHC li-
gands in anti-B and anti-C, the main dif-
ference resides in the way the naphthyl
side chains are twisted with respect to
the central metal atom. The introduction
of additional steric crowding (7-cyclooc-
Entry
Pd complex
(cin)Cl
Conversion
[%]
Yield
[%]
1
2
3
4
5
6
7
8
9
G
G
39
40
33
20
7
45
60
95
65
35
33
29
A
ACHTUNGTRENNUNG
Pd-PEPPSI-IPent
Pd-PEPPSI-IPent
Pd-PEPPSI-IPent
anti-A
anti-B
anti-C
14[b]
<5[c]
39
51
90
61
anti-D
[a] Conditions: 258C, toluene (1 mL), 24 h, Ar–Cl (0.125 mmol), Ar–
(BOH)2 (0.250 mmol), KOtBu (0.312 mmol), 2 mol% Pd cat.; conversions
and yields determined by GC with internal standard. [b] KOtBu
(0.375 mmol), tBuOH (0.5 mL), 4 ꢆ molecular sieves. [c] KOH
(0.375 mmol), dioxane (0.5 mL).
dramatic increase in reactions rates. Indeed, entry 5 in
Table 2 shows that as little as 0.2 mol% of catalyst suffices
for the reaction to go to completion. More sterically de-
manding 2,6-diethylphenyl bromide could also be coupled
with 2,6-dimethylphenyl boronic acid at room temperature
in acceptable yield (Table 2, entry 10). Two representative
tetrasubstituted heterobiaryls were also successfully synthe-
sized in good yields (in Table 2, entries 11 and 12).
Table 3 shows examples for the synthesis of tetra-ortho-
substituted biaryls starting from the corresponding aryl
chlorides. Again, a variety of hindered biaryls were prepared
smoothly at room temperature. In two cases when sterically
demanding and electron-rich aryl chlorides were used, slight
heating was necessary for complete conversion (Table 3, en-
tries 2 and 3). In other cases, slight heating permitted the
use of much lower catalyst loadings (Table 3, entries 8, 12,
Figure 1. Overlay of the molecular structures of anti-B (gray) and anti-C
(yellow). Cinnamyl, chloride and hydrogens omitted for clarity.
tyl) leads to a structure in anti-C, in which the 2-cyclooctyl
groups point into the space occupied by the metal.[15]
and 13). Using the corresponding NHC/[PdACTHNURTGNENG(U dba)2] precata-
lyst system (dba=dibenzylideneacetone) instead of anti-C
completely suppresses the coupling reaction at room tem-
perature (Table 3, entries 4 vs. 5), meaning that the catalyti-
cally active 12-electron NHC–Pd0 complex is not generated
To further understand the difference between the catalyti-
cally preferred NHC ligand of anti-(2,7)-SICyoctNap and
the more established SIPr ligand, we calculated the buried
%VBur of the two NHC ligands.[16,17] The simple %VBur of
Chem. Eur. J. 2011, 17, 12886 – 12890
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12887

R. Dorta et al.
Table 2. Suzuki–Miyaura couplings generating tetra-ortho-substituted
products starting with aryl bromides catalyzed by anti-C.[a]
Table 3. Suzuki–Miyaura couplings generating tetra-ortho-substituted
products starting with aryl chlorides catalyzed by anti-C.[a]
Entry
1
Ar-Ar’
Pd [mol%]
2
Time [h]
8
Yield [%]
78
Entry
1
Ar–Ar’
Pd [mol%]
2
Time [h]
12
Yield [%]
96
2
3
4
5
2
1
0.5
0.2
72
2
3
80
2
1
8
80[b]
96[b]
92[b,c]
97[b,c]
12
3
2
2
72[b]
4
5
2
2
20
20
92
6
7
8
2
2
1
15
12
16
92
85
95
<10[c]
6
2
20
86
7
8
2
0.5
8
1.5
89
85[b,d]
9
2
2
10
18
91
67
9
2
16
82
10
11
12
13
1
1
0.5
0.2
10
1
1.5
7
90
91[b]
88[b,d]
80[b,d]
10
11
12
2
2
20
20
70
88
14
2
20
78
15
16
2
2
18
24
71
72
[a]Conditions: 258C, toluene (3.2 mL), Ar–Br (0.4 mmol), Ar–B(OH)2
(0.6 mmol), KOtBu
( 1.0 mmol); isolated products yields. [b]658C.
[c] Toluene (2 mL).
the NHC ligands in anti-C (41.5%) and (SIPr)PdACTHNUGRTNEUNG(cin)Cl
17
18
2
12
15
91
(34.9%) show that our ligand is bulkier than SIPr. That an
overall more bulky NHC ligand behaves better in catalysis
involving sterically more demanding substrates is not unusu-
al and the term ꢀflexible steric bulkꢁ has been introduced to
account for this phenomenon.[4b,6a,7] At least in a catalytic
coupling scheme that follows the classical path (i.e. no sec-
ondary reactivity/decomposition), this concept is neverthe-
less at odds with what one would expect. For this reason we
decided to perform a more detailed analysis by evaluating
the %VBur in the single quadrants around the Pd center and
plotted them as steric contour maps (Figure 2) for both
0.05
80[e]
[a]Conditions: 258C, Toluene (3.2 mL), Ar–Cl (0.4 mmol), Ar–B(OH)2
(0.8 mmol), KOtBu (1.0 mmol), anti-C; isolated products yields. [b]658C.
[c]2 mol% [PdACHTNUTRGENNG(U dba)2]/ACHTUNGTRNE(NUGN 2,7)-SICyoctNap. [d]2 mL toluene. [e]258C,
iPrOH (0.5 mL), Ar–Cl (0.5 mmol), Ar–B(OH)2 (1.1 equiv), 0.05 mol%
anti-C, KOtBu (1.3 equiv).
(SIPr)Pd
N
asymmetry in the way the ligand wraps around the metal.[18]
total %VBur into quadrant contributions quantifies any
Within this approach, the quadrant %VBur of SIPr is rather
12888
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12886 – 12890

Tetra-ortho-Substituted Biaryls
COMMUNICATION
Figure 3. Transition state of the reductive elimination for (SIPr)Pd(Ar)-
ACHUTNGREN(NUG Ar’) (left) and [anti-(2,7)-SICyoctNap]Pd(Ar)ACHTUTGNREN(NUGN Ar’) (right).
Figure 2. Steric maps of the NHC ligands in (SIPr)Pd
complexes.
ACHTUNGTREN(NUGN cin)Cl and anti-C
leasing of the bulky aryl groups from the metal by creating
a groove that is able to accommodate the ortho methyl
groups of the aryl moieties. Together with the overall very
bulky nature of this ligand, this translates into the clearly su-
perior catalytic performance that we see in these Suzuki–
Miyaura couplings.
constant (%VBur ca. 35%), while the quadrant %VBur values
of anti-(2,7)-SICyoctNap are largely different. Two quad-
rants heavily hindered (top left and bottom right, %VBur ca.
53%), two quadrants clearly more open (top right and
bottom left, %VBur ca. 30%) creating a groove that possibly
can host bulky substrates.
We therefore turned our attention to the catalytic step
where sterics play the crucial role, namely the transmetallat-
ed (NHC)Pd(Ar)ACHTUNGTRENNUNG(Ar’) [where Ar=2,4,6-trimethylphenyl
and Ar’=2,6-dimethylphenyl] intermediate and the follow-
ing reductive elimination. We first calculated the total
energy of the system when going from the starting naked
(NHC)Pd0 species (set at 0 kcalmolꢀ1), to the transmetallat-
In summary, we disclose the first catalyst system that is
able to efficiently perform the Suzuki–Miyaura coupling to
form bulky tetra-ortho-substituted biaryls from aryl bro-
mides and chlorides at room temperature. The central fea-
ture of the catalyst sees the introduction of a C2-symmetric
NHC ligand with appropriately substituted naphthyl side
chains. This simple modification to the well-known ligand
systems allows a dramatic increase in catalytic performance
in these difficult Suzuki–Miyaura coupling reactions, appa-
rently leaving the already high activity in less demanding
coupling reactions unaffected. Advanced DFT calculations
show how the ligandꢁs special steric properties, which are in-
herently associated with its symmetry, leave two of the four
quadrants relatively open. This enhances the reactivity of
the system with respect to the crucial, sterically demanding
ed (NHC)Pd(Ar)
ACHTUNGTRENNUNG(Ar’) intermediate according to Equa-
tion (1).[19]
ðNHCÞPd þ ArCl þ Ar0BðOHÞ2 þ KOtBu !
! ðNHCÞPdðArÞðAr0Þ þ KCl þ tBuOBðOHÞ2
ð1Þ
NHC–Pd(Ar)
ACHTUNGTREN(NUNG Ar’) intermediate and the following reductive
elimination step.
This approach eliminates the specific way the two aryl
moieties are loaded onto the metal. Loading of these two
aryls onto the (NHC)Pd0 species [Eq. (1)] is favored for
both (SIPr)Pd (by 23.8 kcalmolꢀ1) and [anti-(2,7)-SICyoct-
Nap]Pd (by 30.7 kcalmolꢀ1). This in turn means that the
groove created by the anti-(2,7)-SICyoctNap ligand is able
to more readily accommodate (by 6.9 kcalmolꢀ1) the bulky
aryl fragments than SIPr. An identical preference (6.9 kcal
molꢀ1) is also calculated at the level of the following transi-
tion state (overall 15 kcalmolꢀ1 higher in energy) leading to
the coupled Ar–Arꢁ product (see Figure S2 in the Support-
ing Information). Representations of these transition states
for both complexes show a distorted T-shaped geometry
(Figure 3). As already evidenced by the steric contour map
of Figure 2, the structures shown in Figure 3 perfectly illus-
trate how the open quadrants in anti-(2,7)-SICyoctNap are
able to host the ortho methyl groups of the aryl substituent
cis to the NHC ligand. Differently, in presence of the SIPr
ligand the same ortho methyl groups interact repulsively
with the NHC ligand. In other words, the special steric char-
acteristics of anti-(2,7)-SICyoctNap facilitate loading and re-
Acknowledgements
R. D. holds an Alfred Werner Assistant Professorship and thanks the
foundation for generous financial support. Support for this work was pro-
vided by SNF (grant to L. W.) and the Roche Research Foundation
(grant to E. D.).
Keywords: biaryls · cross-coupling · N-heterocyclic carbene
ligand · palladium
[1] Metal-Catalyzed Cross-Coupling Reactions, 2nd ed. (Eds.: A. de
Meijere, F. Diederich), Wiley-VCH, Weinheim, 2004.
[2] A. Suzuki, Angew. Chem. 2011, 123, 6854; Angew. Chem. Int. Ed.
2011, 50, 6854.
[3] For selected reviews, see: a) A. Suzuki, J. Organomet. Chem. 1999,
576, 147; b) N. Miyaura, Top. Curr. Chem. 2002, 219, 11; c) N.
Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457; d) J. Hassan, M.
Sevignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002, 102,
1359; e) S. Kotha, K. Lahiri, D. Kashinath, Tetrahedron 2002, 58,
9633; f) A. F. Littke, G. C. Fu, Angew. Chem. 2002, 114, 4350;
Chem. Eur. J. 2011, 17, 12886 – 12890
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12889

R. Dorta et al.
Angew. Chem. Int. Ed. 2002, 41, 4176; g) F. Bellina, A. Carpita, R.
Rossi, Synthesis 2004, 2419; h) U. Christmann, R. Vilar, Angew.
Chem. 2005, 117, 370; Angew. Chem. Int. Ed. 2005, 44, 366; i) F.
Alonso, I. P. Beletskaya, M. Yus, Tetrahedron 2008, 64, 3047;
j) Modern Arylation Methods (Ed.: L. Ackermann), Wiley-VCH,
Weinheim, 2009.
[9] For a single example of tetra-ortho-biaryl formation at room tem-
perature; C. Song, Y. Ma, Q. Chai, C. Ma, W. Jiang, M. B. Andrus,
Tetrahedron 2005, 61, 7438; unfortunately, trying to repeat the result
using the same reaction conditions and a very similar ligand, namely
(2,7)-SICyNap HBF4 (see Ref. [10c]) lead to no product formation.
[10] a) X. Luan, R. Mariz, M. Gatti, C. Costabile, A. Poater, L. Cavallo,
A. Linden, R. Dorta, J. Am. Chem. Soc. 2008, 130, 6848; b) L.
Vieille-Petit, X. Luan, R. Mariz, S. Blumentritt, A. Linden, R.
Dorta, Eur. J. Inorg. Chem. 2009, 1861. For the synthesis of more
members of the same ligand family, see: c) M. Gatti, L. Wu, E.
Drinkel, F. Gaggia, S. Blumentritt, A. Linden, R. Dorta, ARKIVOC
2011, 6, 176.
[4] For selected examples, see: a) J. Yin, M. P. Rainka, X. X. Zhang,
S. L. Buchwald, J. Am. Chem. Soc. 2002, 124, 1162; b) G. Altenhoff,
R. Goddard, C. W. Lehmann, F. Glorius, Angew. Chem. 2003, 115,
3818; Angew. Chem. Int. Ed. 2003, 42, 3690; c) O. Navarro, R. A.
Kelly, S. P. Nolan, J. Am. Chem. Soc. 2003, 125, 16194; d) S. D.
Walker, T. E. Barder, J. R. Martinelli, S. L. Buchwald, Angew.
Chem. 2004, 116, 1907; Angew. Chem. Int. Ed. 2004, 43, 1871;
e) T. E. Barder, S. D. Walker, J. R. Martinelli, S. L. Buchwald, J. Am.
Chem. Soc. 2005, 127, 4685; f) N. Marion, O. Navarro, J. Mei, E. D.
Stevens, N. M. Scott, S. P. Nolan, J. Am. Chem. Soc. 2006, 128, 4101;
g) O. Diebolt, P. Braunstein, S. P. Nolan, C. S. J. Cazin, Chem.
Commun. 2008, 3190; h) W. Tang, A. G. Capacci, X. Wei, W. Li, A.
White, N. D. Patel, J. Savoie, J. J. Gao, S. Rodriguez, B. Qu, N.
Haddad, B. Z. Lu, D. Krishnamurthy, N. K. Yee, C. H. Senanayake,
Angew. Chem. 2010, 122, 6015; Angew. Chem. Int. Ed. 2010, 49,
5879.
[5] For efficient tetra-ortho-substituted biaryl formation starting from
aryl bromides with appropriate heating, see references [4a, d, e, h].
For two examples of Stille couplings at elevated temperatures, see:
a) W. Su, S. Urgaonkar, P. A. McLaughlin, J. G. Verkade, J. Am.
Chem. Soc. 2004, 126, 16433. For two examples of Kumada cou-
plings at elevated temperatures, see: b) C. E. Hartmann, S. P. Nolan,
C. S. J. Cazin, Organometallics 2009, 28, 2915. For two examples of
Negishi couplings at elevated temperature, see: c) J. E. Milne, S. L.
Buchwald, J. Am. Chem. Soc. 2004, 126, 13028. For a series of exam-
ples of Negishi couplings that proceed at room temperature or
slightly above, underlining the higher reactivity compared to
Suzuki–Miyaura couplings, see: d) S. C¸ alimsiz, M. Sayah, D. Mallik,
M. G. Organ, Angew. Chem. 2010, 122, 2058; Angew. Chem. Int. Ed.
2010, 49, 2014.
[11] a) X. Luan, R. Mariz, C. Robert, M. Gatti, S. Blumentritt, A.
Linden, R. Dorta, Org. Lett. 2008, 10, 5569; b) X. Luan, L. Wu, E.
Drinkel, R. Mariz, M. Gatti, R. Dorta, Org. Lett. 2010, 12, 1912.
[12] See the Supporting Information for details.
[13] While Pd-PEPPSI-IPent is more effective at higher temperatures
(see Ref. [7], using higher temperature with (SIPr)Pd
(IPr)Pd(cin)Cl leads to extensive deboronation and only marginally
better yields.
ACHTUNGTRENNUNG(cin)Cl or
AHCTUNGTRENNUNG
[14] CCDC-832037 (anti-B), and CCDC-832038 (anti-C) contain the sup-
plementary crystallographic 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.
[15] VT NMR studies of the complexes could not yet establish whether
the respective ligand arrangements are maintained in solution. It is
of note however that different spatial arrangements of the cinnamyl
group seem likely and complicate the anaylsis (see 13C NMR spec-
tra).
[16] a) A. Poater, F. Ragone, R. Mariz, R. Dorta, L. Cavallo, Chem. Eur.
J. 2010, 16, 14348; b) A. Poater, B. Cosenza, A. Correa, S. Giudice,
F. Ragone, V. Scarano, L. Cavallo, Eur. J. Inorg. Chem. 2009, 1759;
c) A. C. Hillier, W. J. Sommer, B. S. Yong, J. L. Petersen, L. Cavallo,
S. P. Nolan, Organometallics 2003, 22, 4322; d) A. Poater, L. Cavallo,
Dalton Trans. 2009, 8885; e) P. Liu, J. Montgomery, K. N. Houk, J.
Am. Chem. Soc. 2011, 133, 6956.
[6] a) G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, J. Am.
Chem. Soc. 2004, 126, 15195; for a review on the use of NHC ligands
in LTM catalysis, see: b) S. Dꢇez-Gonzꢈlez, N. Marion, S. P. Nolan,
Chem. Rev. 2009, 109, 3612.
[7] M. G. Organ, S. C¸ alimsiz, M. Sayah, K. H. Hoi, A. J. Lough, Angew.
Chem. 2009, 121, 2419; Angew. Chem. Int. Ed. 2009, 48, 2383.
[8] a) T. Hoshi, T. Nakazawa, I. Saitoh, A. Mori, T. Suzuki, J. Sakai, H.
Hagiwara, Org. Lett. 2008, 10, 2063; b) L. Ackermann, H. K. Potu-
kuchi, A. Althammer, R. Born, P. Meyer, Org. Lett. 2010, 12, 1004;
c) A. Schmidt, A. Rahimi, Chem. Commun. 2010, 46, 2995; d) C. M.
So, W. K. Chow, P. Y. Choy, C. P. Lau, F. Y. Kwong, Chem. Eur. J.
2010, 16, 7996; e) D.-H. Lee, M.-J. Jin, Org. Lett. 2011, 13, 252.
[17] R. A. Kelly III, H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens, J.
Bordner, I. Samardjiev, C. D. Hoff, L. Cavallo, S. P. Nolan, Organo-
metallics 2008, 27, 202.
[18] a) I. C. Stewart, D. Benitez, D. J. OꢁLeary, E. Tkatchouk, M. W.
Day, W. A. Goddard, R. H. Grubbs, J. Am. Chem. Soc. 2009, 131,
1931; b) F. Ragone, A. Poater, L. Cavallo, J. Am. Chem. Soc. 2010,
132, 4249.
[19] See the Supporting Information for details on the DFT calculations.
Received: August 5, 2011
Published online: October 7, 2011
12890
www.chemeurj.org
ꢅ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12886 – 12890