Six-membered, chiral NHCs derived from camphor: Structure-reactivity relationship in asymmetric oxindole synthesis

Article abstract of DOI:10.1021/om201166b

A series of three chiral, expanded six-membered NHC-palladium(II) complexes was prepared with successively increased sterical demand, while retaining natural d-(+)-camphor as a chiral motif. The catalysts showed different reaction profiles in the asymmetr

Full text of DOI:10.1021/om201166b

Article
pubs.acs.org/Organometallics
Six-Membered, Chiral NHCs Derived from Camphor: Structure−
Reactivity Relationship in Asymmetric Oxindole Synthesis
Markus J. Spallek, Dominic Riedel, Frank Rominger, A. Stephen K. Hashmi, and Oliver Trapp*
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
S
* Supporting Information
ABSTRACT: A series of three chiral, expanded six-membered
NHC−palladium(II) complexes was prepared with successively
increased sterical demand, while retaining natural d-(+)-camphor
as a chiral motif. The catalysts showed different reaction profiles in
the asymmetric, intramolecular α-arylation of amides. The
molecular structure of two N-heterocyclic and one nitrogen
acyclic carbene palladium isonitrile complex was unequivocally
determined by X-ray crystallographic analysis. The results reported
herein account for a correlation of catalytic activity and
enantiodiscrimination in relation to the degree of chiral
substitution and steric congestion at the metal center. The
modular and convergent synthetic route of these air- and moisture-
stable palladium isonitrile complexes underlines the usefulness of this approach.
toward enantiomerically enriched 3,3-disubstituted oxindoles
with overall moderate success. Dorta et al.,26 Glorius et al.,27
INTRODUCTION
■
N-Heterocyclic carbenes (NHCs) have been known since the
late 1960s and have become an important class of ligands after
Arduengo et al. were able to isolate the first stable derivative in
1991.1 NHC ligands possess a strong σ-donor character and a
weak π-acceptor capability.2 The resulting metal complexes are
typically characterized by a high stability3 of the NHC−metal
bond, making them highly attractive for metal complex
formation. Due to their properties and reactivity, NHCs
revolutionized today's organometallic chemistry and homoge-
neous transition-metal catalysis.4−7 Particularly, chiral ligands
and their potential in asymmetric catalysis are receiving growing
attention.8−13 Various approaches toward the synthesis of
(chiral) NHC ligands were established.14−19 Besides the need
for a short access to chiral ligand frameworks and their
complexes, focus on modular synthetic pathways, preferably
high yielding and convergent, to investigate the impact of
ligand design on catalysis is emerging. Especially steric demand
is an important factor20 for effective chirality transfer as well as
for stabilization of low-coordinated metal complexes during
catalysis. The latter are needed for challenging transformations,
such as cross-coupling reactions of nonactivated aryl
chlorides.21−23 Furthermore, besides steric encumbrance of
the metal complex enhancing chirality transfer, retention of
certain flexibility24 within the catalyst seems to be a beneficial
factor as well.
Kundig et al.,28−30 and more recently Murakami et al.31
̈
managed this challenging type of α-amide arylation with high
enantiomeric excess and yields.32 While many of the ligands
employed are based on five-membered imidazole,25,28−30
imidazolines (or oxazoles),27,33 the use of expanded NHCs
and furthermore unsymmetrical substituted chiral NHC−metal
complexes have rarely been reported or investigated for this
type of transformation. Rather different properties are reported
for six-,15,34−39 seven-,15,40−43 and most recently eight-
membered44 derivatives including increased basicity (nucleo-
philicity),36,37,39 greater steric demand, and higher congestion
around the metal center (larger N−CNHC−N angle).37
We developed a set of three chiral palladium catalysts
featuring a six-membered N-heterocyclic hexahydropyrimidine
core. By modification of the flanking substituents and finally by
installing an additional group at the NHC backbone the steric
demand and the chiral information on the catalyst were
successively increased. Using these catalysts we want to
quantify the impact of substitution and steric hindrance on
catalyst reactivity and selectivity in the asymmetric oxindole
synthesis. This reaction, with rather low enantiomeric excess,
was chosen to be able to observe maximum effects of the
catalyst substitution. To get reliable insights, natural d-
(+)-camphor (respectively bornylamine) was chosen as the
chiral building block, and aryl bromides as well as aryl chlorides
with different substitution patterns were employed in the
asymmetric α-arylation of amides.
On the basis of the pioneering work of Hartwig et al.25 on
the intramolecular α-arylation of amides, we wish to contribute
to the growing field of ligands used for this transformation. The
reaction provides efficient and direct access to chiral 3,3-
disubstituted oxindoles, a common structural motif present in
many natural products. Currently there are various routes
Received: November 22, 2011
Published: January 30, 2012
© 2012 American Chemical Society
1127
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

Organometallics
Article
RESULTS AND DISCUSSION
Scheme 3. Palladium−Bis(isonitrile) Complex Formation
■
The synthesis of the six-membered, chiral NHC complexes was
achieved utilizing a straightforward synthetic protocol,
developed in the Hashmi group.45−49 The modular and quite
convergent pathway allowed us the unprecedented short
synthesis of three bornylamine-derived palladium catalysts
with varying types of backbone and wingtip substitution (cf.
Scheme 1).
Scheme 1. Chiral, Six-Membered NHC−Pd Complexes with
Enhanced Steric Demand
as their corresponding chloride salts (94% for 6a, 88% for 6b)
(cf. Scheme 4).
Scheme 4. Preparation of NHC-Backbone Synthons 6a and
6b
The cyclododecanone derivative of 6 was prepared in a
similar manner and obtained as colorless crystals after
recrystallization (79%, cf. Supporting Information). The
catalysts I−III were prepared by reaction with the appropriate
aminoalkyl chloride in the presence of excess triethylamine in
tetrahydrofuran. Nucleophilic attack of the amine nitrogen at
the isonitrile carbon atom followed by in situ intramolecular
cyclization of the resulting anion gave the bornyl-derived Pd−
isonitrile complexes I−III in 67% (I), 64% (II), and 41% (III)
yield (cf. Scheme 5). The reactions showed full conversions to
Bornylisonitrile 1 was prepared by neat reaction of ethyl
formate and 1R,2S-bornylamine in an autoclave at 200 °C for
12 h and an additional 5 h at 250 °C. This reaction furnished
pure bornylformamide 2 after precipitation, washings (n-
pentane), and recrystallization (acetone, petroleum ether) as
colorless crystals in 87% yield. Due to amide resonances, two
diastereomers were detected by NMR spectroscopic measure-
ments. Dehydration with phosphorus trichloride and excess
triethylamine in dichloromethane at −60 °C and then at room
temperature gave bornylisonitrile 3 in 83% yield as an off white
solid (cf. Scheme 2).
Scheme 5. Formation of Chiral Pd−Isonitrile Complexes I−
III by Intramolecular Cyclization
Scheme 2. Synthesis of Chiral Bornylisonitrile 3
The chiral palladium(II) isonitrile precursor 4 was obtained
in excellent yields (quantitative) by ligand exchange reaction of
Pd(MeCN)2Cl2 and bornylisonitrile 3 in toluene for 12 h at
room temperature (cf. Scheme 3). Aminoalkyl chlorides were
used as synthons for the installation of the NHC backbone.
Therefore, 1R,2S-bornylamine (1) was reacted with 1-bromo-3-
propanol in benzonitrile at elevated temperatures (95 °C) to
furnish 3-hydroxypropylbornylamine (5a) and 3-phenyl-3-
hydroxypropylbornylamine (5b) as colorless liquids after
purification. The chloride salts of 5a and 5b were prepared in
very good yields using thionyl chloride. To ease purification
and handling in further synthetic steps, 6a and 6b were isolated
the unsymmetrical, chiral isonitrile−NHC−palladium com-
plexes, as evidenced by IR-stretching frequencies and NMR
spectroscopic measurements of the isonitrile ligands. Formation
and cyclization of the backbone were validated by fading of the
carbonyl 13C-resonance of the isonitrile carbon at 168.0 ppm
and of the tertiary bornyl-bridgehead CH carbon at 63.7 ppm.
1128
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

Organometallics
Article
A shift of the corresponding characteristic isonitrile resonance
at 2233 cm−1 was observed as well.
Table 1. Selected Bond Lengths [Å] and Angles [deg] of
Camphor-Derived Pd−Isonitrile Complex I
Crystals suitable for X-ray diffraction studies were obtained
by vapor diffusion of diethyl ether into a saturated dichloro-
methane solution of either I or II. The molecular structures of
palladium complexes I and II were determined by crystal
structure analysis (cf. Figures 1 and 2). Two independent
Pd−C(1)
Pd−C(5)
Pd−Cl(1)
Pd−Cl(2)
C(5)−N(3)
2.027(1)
1.890(2)
2.364(6)
2.317(6)
1.140(2)
N(3)−C(6)
1.440(2)
170.0(2)
171.30(3)
89.60(9)
−79.00
Pd−C(5)−N(3)
C(5)−N(3)−C(6)
C(1)−Pd−C(6)
N(2)−C(1)−Pd−C(5)
Table 2. Selected Bond Lengths [Å] and Angles [deg] for
Camphor-Derived Pd−Isonitrile Complex II
Pd−C(1)
Pd−C(5)
Pd−Cl(1)
tPd−Cl(2)
C(5)−N(3)
2.014(4)
1.917(5)
2.331(1)
2.357(0)
1.160(6)
N(3)−C(6)
1.436(6)
178.0(5)
178.2(6)
88.45(17)
−73.54
Pd−C(5)−N(3)
C(5)−N(3)−C(6)
C(1)−Pd−C(6)
N(2)−C(1)−Pd−C(5)
Interestingly, an unprecedented dehydrochlorination side
reaction at the backbone of the palladium complex prior to the
cyclization step took place by reaction of 6b with bis-
(isobornylnitrile) 4. At room temperature this competitive
dehydrochlorination step to form the nitrogen acyclic carbene
(NAC) palladium complex 7 is favored. This clearly
demonstrates the high steric demand of the system. Higher
temperatures were proven to be beneficial for the cyclization,
thus suppressing dehydrochlorination to a certain extent.
Therefore preparation of III was conducted under reflux
conditions. Although, dehydrochlorination becomes less
favored, yields of the six-membered, phenyl-substituted NHC
palladium complex III did not exceed 41%. Furthermore, we
found that purification of the mixture by simple column
chromatography yields the pure cyclization product III (cf.
Scheme 6). The solid-state structure of NAC complex 7
Figure 1. Solid-state structure of the six-membered NHC−Pd−
isonitrile complex I.
molecules of I were obtained due to a conformational change of
the cyclododecane N-substituent. However, only the ORTEP
diagram of one isomer is depicted because of the small changes
observed between the two isomers. Note that the dihedral
torsion angle between N(2)−C(1)−Pd−C(5) of −79.0° (11°
deviation from the orthogonality) is indicative of steric
congestion around the quadratic planar metal center and
generally observed for NHC complexes with bulky substituents.
Selected bond lengths and angles of Pd−NHC complex I are
shown in Table 1. The solid-state structure of complex II,
bearing one chiral bornylamine moiety at the N-atoms each, is
depicted in Figure 2. The structure is in accordance with the
structural features observed for complex I. Due to the higher
steric demand and in particular the orientation of the exo-
methyl groups (C25 and C27) of the bornyl substituents, a
decrease of the dihedral torsion angle N(2)−C(1)−Pd−C(5)
to −73.5° is observed. However, packing effects in the crystal
structure cannot be excluded. In relation to the NHC axis the
isonitrile ligand points toward the sterically less crowded
camphor backbone (C24, C23, and C19). A significant change
of the Pd−carbene bond and Pd−isonitrile distances is not
observed, and the bornylisonitrile 3 is almost linearly
coordinated to the palladium center (Pd−C(5)−N(3) and
C(5)−N(3)−C(6), both 178°) (cf. Figure 2, Table 2).
Scheme 6. Dehydrochlorination to NAC−Palladium
Complex 7, Bearing an Unsaturated Styrenyl Backbone
Figure 2. Solid-state structure of six-membered NHC−Pd−isonitrile complex II: (a) side view and (b) top view.
1129
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

Organometallics
Article
revealed a distorted structure with the styrenyl unit being
displaced about −67.9° out of the C(1)−N(2)−C(2)−C(3)
plane. The isobornyl ligand is located in the opposite direction
with a deviation of −73.6° out of the N(3)−C(1)−N(2)−C(2)
plane. Note that due to rotational freedom, the chiral-bornyl N-
substituents changed orientation in the NAC complex
compared to the fused six-membered NHC Pd complexes I
and II (cf. Figure 3).
converted to their (R)-oxindole derivatives in very high yields
as well (entries 2−6, as determined by chiral HPLC). Benzyl-
substituted substrates were obtained with an enantiomeric
excess of 58% ee (bromide derivative, entry 3) and 63% ee
(chloride derivative, entry 4). The corresponding naphthyl
substrates showed higher enantioselectivities (68% ee and 72%
ee, entries 5 and 6). The results are noteworthy, since
temperature has a pronounced influence on the enantiose-
lectivity. The related five-membered NHC showed 70% ee,
however only 58% ee at 0 °C with catalyst loadings of 10 mol %
catalyst.25 We attribute the higher enantioselectivity of 72% ee
(entry 6) at higher temperatures to the NHC−metal angle
being enlarged within a six-membered NHC, thus increasing
steric congestion and chiral induction compared to the five-
membered derivative with the same chiral ligand pattern. A
slight increase of 5% in enantioselectivity was observed for the
chloro derivatives in each case (entries 3, 4 and 5, 6). These
data suggest that the halide is present on or in close proximity
to the catalytic active species during the enantiodiscriminating
step. Introduction of a cyclobutyl substituent at the N-terminus
of the substrate had no significant influence on enantiose-
lectivity compared to the N-methylated substrate (entries 2 and
3). Interestingly, catalyst III bearing the same chiral bornyl
ligand pattern but exhibiting a phenyl substituent at the NHC
backbone in close proximity to the N-substituent (and thus to
the chiral substituents) showed a different reactivity. Employing
the bromo-phenyl derivative (entry 7) high conversions were
observed as well, but enantioselectivity dropped almost
completely (8%). Besides small amounts of oxindole product
dehalogenation of the starting material was observed in 64%
yield (rac), as validated by NMR spectroscopic and chiral
HPLC-MS measurements. By changing the substrate to the
even more sterically demanding bromo-naphthyl derivative,
only the racemic dehalogenation products were observed.
Figure 3. Solid-state structure of six-membered NHC−Pd−isonitrile
complex 7.
After synthesis, characterization, and structural analysis of the
novel, chiral palladium−isonitrile complexes their catalytic
performance in the asymmetric α-amide arylation was
investigated. For a representative study overall seven different
substrates were synthesized according to the literature.25,50−53
Substrates bearing benzyl and naphthyl substituents and N-
methylation were chosen to evaluate the influence of the
substitution pattern on selectivity. The corresponding bromide
and chloride analogues and one substrate with an N-
cyclobutylated amide were prepared as well. Initially, solvent
screening (THF, diglyme, dioxane, DMSO) revealed dry
dimethoxyethane and sodium tert-butylate to be ideal, with
catalyst loadings of 2.5 mol % being sufficient for catalysis.
Unfortunately, related Pd−isonitrile complexes, which are
highly active catalysts in alcoholic media for Suzuki cross-
coupling of boronic acids,47 were not suitable. All reactions
were performed at 50 °C to furnish complete conversions
within a maximum of 18 h. With chiral catalyst I, featuring a
flexible cyclododecanyl substituent at one N-terminus, the
desired oxindole was obtained and isolated in quantitative
yields (cf. Table 3, entry 1). Enantioselective HPLC analysis of
CONCLUSION
■
In summary, we have presented the synthesis of three six-
membered (hexahydropyrimidine core), camphor-derived
NHC−Pd−isonitrile complexes and their application as
catalysts in the asymmetric α-amide arylation. The structure
of two Pd complexes and one nitrogen acyclic carbene Pd
complex was determined by X-ray crystallographic analysis.
Taking advantage of a convergent, modular synthetic pathway
the preparation of NHC ligands bearing different structural
motifs present at the N-termini was accomplished. By
successive increase of the chiral information and congestion
within the catalyst patternwhile retaining the chiral motif
(bornylamine) at the same timewe demonstrated their
influence on enantioselectivity. All catalysts showed good
conversions of bromo as well as chloro substrates with catalyst
loadings of 2.5 mol %. Whereas catalyst I showed no
enantiodiscrimination in the oxindole synthesis, catalyst II
proved to be more effective. Higher enantioselectivities
compared to the related, five-membered bornylamine-derived
ligand were observed at even higher temperatures. With the
sterically most demanding catalyst III a reaction profile leading
either to an arylation or dehalogenation product depending on
the employed substrate was observed. Furthermore, the
Table 3. Selected Bond Lengths [Å] and Angles [deg] for
NAC Pd−Isonitrile Complex 7
Pd−C(1)
Pd−C(5)
Pd−Cl(1)
Pd−Cl(2)
C(5)−N(3)
C(4)−C(5)
1.994(3)
1.937(3)
2.366(9)
2.319(3)
1.140(4)
N(3)−C(6)
Pd−C(5)−N(3)
C(5)−N(3)−C(6)
C(1)−Pd−C(6)
N(3)−C(1)−Pd−C(5)
C(1)−N(2)−C(2)−C(3)
1.461(4)
174.9(3)
178.2(6)
90.20(11)
−73.58
−67.85
the product showed a racemic mixture of oxindole enantiomers
and proved that no chiral induction takes place within a system
bearing only one chiral substituent. Therefore, catalysis was
continued using NHC−Pd−isonitrile complex II, featuring two
chiral bornyl substituents. With this catalyst all substrates were
calculated buried overlap volume54,55 of catalyst II (%Vbur
=
40.9) exceeds the value of 1,3-di(1-adamantyl)imidazol-2-
ylidene (IAd, %Vbur = 36.1)56 and represents the second
highest volume reported so far for chiral N,N-heterocyclic
carbene ligands (IBiox[(−)-menthyl], %Vbur = 47.7, Au
1130
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

Organometallics
Article
a
Table 4. Asymmetric Oxindole Synthesis Using NHC−Pd−Isonitrile Complexes I−III
b
c
entry
catalyst
X
R1
Me
Ar
A
B
yield [/%]
ee [A; %]
1
2
3
4
5
6
I
Br
Br
Br
Cl
Br
Cl
Br
Br
1-naphthyl
Ph
100
100
100
100
100
100
36
quant.
95
rac
55
58
63
68
72
8
II
II
II
II
II
III
III
c-C4H8
Me
Ph
90
Me
Ph
98
Me
1-naphthyl
1-naphthyl
Ph
92
Me
95
d
7
d
Me
64
89
8
Me
1-naphthyl
100
91
a
b
c
Reaction conditions: 0.3 mmol scale, NaOtBu (0.45 mmol), catalyst (2.5 mol %) in DME (5 mL) at 50 °C, 14−18 h. Isolated yield. Determined
d
by chiral HPLC (Chiralpak IA). Product configuration: R, determined by chiral HPLC (Chiralpak IB) of known compounds. Reaction at 80 °C.
Isolated yield of A and B.
complex).27 The results obtained show that higher hindrance of
the metal by substituents is still beneficial for enantioselectivity,
but at a certain level of congestion or restriction of flexibility a
further improvement of chiral induction within a given system
is limited.
(10) Rix, D.; Labat, S.; Toupet, L.; Crev
Inorg. Chem. 2009, 13, 1989−1999.
(11) Larsen, A. O.; Leu, W.; Oberhuber, C. N.; Campbell, J. E.;
Hoveyda, A. H. J. Am. Chem. Soc. 2004, 126, 11130−11131.
(12) Gillingham, D. G.; Kataoka, O.; Garber, S. B.; Hoveyda, A. H. J.
Am. Chem. Soc. 2004, 126, 12288−12290.
(13) Snead, D. R.; Seo, H.; Hong, S. Curr. Org. Chem. 2008, 12,
1370−1387.
(14) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R. Organometallics
2003, 22, 4384−4386.
(15) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.;
Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis,
I. A. Organometallics 2008, 27, 3279−3289.
́
isy, C.; Mauduit, M. Eur. J.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files (CIF) of compounds I, II,
and 7, as well as characterization data of all products including
HPLC data. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) Li, J.; Stewart, I. C.; Grubbs, R. H. Organometallics 2010, 29,
3765−3768.
(17) Flores-Figueroa, A.; Kaufhold, O.; Feldmann, K. O.; Hahn, F. E.
Dalton Trans. 2009, 9334−9342.
AUTHOR INFORMATION
■
Corresponding Author
(18) Bartolome, C.; Ramiro, Z.; Garcia-Cuadrado, D.; Gerez-Galan,
P.; Raducan, M.; Bour, M.; Echavarren, A. M.; Espinet, P.
Organometallics 2010, 29, 951−956.
*Phone: +49 6221-54-8470. Fax +49 6221-54-4904. E-mail:
trapp@oci.uni-heidelberg.de.
Notes
(19) Bartolome, C.; Garcia-Cuadrado, D.; Ramiro, Z.; Espinet, P.
Organometallics 2010, 29, 3589−3592.
The authors declare no competing financial interest.
(20) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523−1533.
̈
(21) Bellina, F.; Rossi, R. Chem. Rev. 2010, 110, 1082−1146.
ACKNOWLEDGMENTS
■
́
(22) Kantchev, E. A. B.; OBrien, C. J.; Organ, M. J. Angew. Chem., Int.
The authors thank the Deutsche Forschungsgemeinschaft
(DFG SFB 623 “Molecular Catalysts: Structure and Functional
Design”) for generous financial support. M.J.S. is grateful for a
doctoral fellowship (DFG GK 850 “Molecular Modeling”).
Ed. 2007, 46, 2768−2813.
(23) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366−
374.
(24) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am.
Chem. Soc. 2004, 126, 15195−15201.
REFERENCES
(25) Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66, 3402−3415.
(26) Luan, X.; Mariz, R.; Robert, C.; Gatti, M.; Blumentritt, S.;
Linden, A.; Dorta, R. Org. Lett. 2008, 10, 5569−5572.
■
(1) Arduengo, A. J. III; Harlow, R. L.; Kline, M. J. J. Am. Chem. Soc.
1991, 113, 2801.
(27) Wurtz, S.; Lohre, C.; Bergander, K.; Glorius, F. J. Am. Chem. Soc.
̈
(2) Díez-Gonzal
874−883.
(3) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247−
2273.
́
ez, S.; Nolan, S. P. Coord. Chem. Rev. 2007, 251,
2009, 131, 8344−8345.
(28) Jia, Y.-X.; Katayev, D.; Bernardinelli, G.; Seidel, T. M.; Kundig,
̈
E. P. Chem.Eur. J. 2010, 16, 6300−6309.
(29) Kundig, E. P.; Seidel, T. M.; Jia, Y.-X.; Bernardinelli, G. Angew.
̈
(4) Glorius, F. Topics in Organometallic Chemistry; Springer: Berlin,
2010; p 231.
(5) Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH:
Weinheim, 2006; p 319.
Chem., Int. Ed. 2007, 46, 8484−8487.
(30) Jia, Y.-X.; Hillgren, J. M.; Watson, E. L.; Marsden, S. P.; Kundig,
̈
E. P. Chem. Commun. 2008, 4040−4042.
(31) Liu, L.; Ishida, N.; Ashida, S.; Murakami, M. Org. Lett. 2011, 13,
1666−1669.
(32) Klein, J. E. M. N.; Taylor, R. J. K. Eur. J. Org. Chem. 2011, 34,
6821−6841.
(33) Glorius, F.; Altenhoff, G.; Goddard, R.; Lehmann, C. W. Chem.
Commun. 2002, 2704−2705.
(6) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290−1309.
(7) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47,
3122−3172.
́
(8) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. Rev.
2004, 33, 619−636.
(9) Douthwaite, R. E. Coord. Chem. Rev. 2007, 251, 702−717.
1131
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

Organometallics
Article
(34) Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki, B.; Dervisi, A.;
Fallis, I. A.; Cavell, K. J. Dalton Trans. 2009, 7099−7112.
(35) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R.
Chem.Eur. J. 2004, 10, 1256−1266.
(36) Siemeling, U.; Farber, C.; Leibold, M.; Bruhn, C.; Mucke, P.;
̈
̈
Winter, R.; Sarkar, B.; Hopffgarten, M.; Frenking, G. Eur. J. Inorg.
Chem. 2009, 31, 4607−4612.
(37) Bazinet, P.; Yap, G. P. A; Richeson, D. S. J. Am. Chem. Soc. 2003,
125, 13314−13315.
(38) Ces
11286−11287.
́
ar, V.; Lugan, N.; Lavigne, G. J. Am. Chem. Soc. 2008, 130,
́
(39) Bazinet, P.; Ong, T.-G.; OBrien, J. S.; Lavoie, N.; Bell, E.; Yap,
G. P. A.; Korobkov, I.; Richeson, D. S. Organometallics 2007, 26,
2885−2895.
(40) Newman, P. D.; Cavell, K. J.; Kariuki, B. M. Organometallics
2010, 29, 2724−2734.
(41) Scarborough, C. C.; Guzei, I. A.; Stahl, S. S. Dalton Trans. 2009,
2284−2286.
(42) Iglesias, M.; Beetstra, D. J.; Kariuki, B. M.; Cavell, K. J.; Dervisi,
A.; Fallis, I. A. Eur. J. Inorg. Chem. 2009, 13, 1913−1919.
(43) Scarborough, C. C.; Grady, M. J. W.; Guzei, I. A.; Gandhi, B. A.;
Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269−5272.
(44) Lu, W. Y.; Cavell, K. J.; Wixey, J. S.; Kariuki, B. Organometallics
2011, 30, 5649−5655.
(45) Hashmi, A. S. K.; Lothschutz, C. N-Heterocyclic Carbene
̈
Complexes, Their Preparation and Use; German Patent Application,
2010.
(46) Hashmi, A. S. K.; Lothschutz, C.; Bohling, C.; Hengst, T.;
̈
̈
Hubbert, C.; Rominger, F. Adv. Synth. Catal. 2010, 352, 3001−3012.
(47) Hashmi, A. S. K.; Lothschutz, C.; Bohling, C.; Rominger, F.
̈
̈
Organometallics 2011, 30, 2411−2417.
(48) Hashmi, A. S. K.; Lothschutz, C.; Graf, K.; Haffner, T.; Schuster,
A.; Rominger, F. Adv. Synth. Catal. 2011, 353, 1407−1412.
(49) Hashmi, A. S. K.; Hengst, T.; Lothschutz, C.; Rominger, F. Adv.
̈
̈
̈
Synth. Catal. 2010, 352, 1315−1337.
(50) Fananas, F. J.; Granados, A.; Sanz, R.; Ignacio, J. M.; Barluenga,
J. Chem.Eur. J. 2001, 7 (13), 2896−2970.
(51) Barluenga, J.; Fananas, F. J.; Sanz, R.; Fernandez, Y. Chem.
Eur. J. 2002, 8 (9), 2034−2046.
(52) Thompson, H. W.; Rashid, S. Y. J. Org. Chem. 2002, 67, 2813−
2825.
(53) Sorensen, U. S.; P.-Villar, E. Helv. Chim. Acta 2004, 87 (1), 82−
89.
(54) Poater, A.; Cosenza, B.; Correa, A.; Giudici, S.; Ragone, F.;
Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 27, 1759−1766.
́
(55) Clavier, H.; Correa, A.; Cavallo, L.; Escudero-Adan, E. C.;
Benet-Buchholz, J.; Slawin, A. M. Z.; Nolan, S. P. Eur. J. Inorg. Chem.
2009, 27, 1767−1773.
(56) Droge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940−
̈
6952.
1132
dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132