First chelated chiral N-heterocyclic bis-carbene complexes

Article abstract of DOI:10.1021/ol005694v

First chiral bidentate bis-carbene complex of Pd(II) prepared from a bis-imidazolium salt derived from (S)-2,2′-bromomethyl-1,1′-binaphthyl exists in both cis- and trans-square planar geometry. These and the corresponding trans-Ni(II) complexes are remarkably resistant to high temperature, air, water, and silica gel chromatography. The Pd complexes catalyze Heck reactions.

Full text of DOI:10.1021/ol005694v

ORGANIC
LETTERS
2000
Vol. 2, No. 8
1125-1128
First Chelated Chiral N-Heterocyclic
Bis-Carbene Complexes
Dean S. Clyne, Jian Jin, Evan Genest, Judith C. Gallucci, and T. V. RajanBabu*
Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
rajanbabu.1@osu.edu
Received February 19, 2000
ABSTRACT
First chiral bidentate bis-carbene complex of Pd(II) prepared from a bis-imidazolium salt derived from (S)-2,2′-bromomethyl-1,1′-binaphthyl
exists in both cis- and trans-square planar geometry. These and the corresponding trans-Ni(II) complexes are remarkably resistant to high
temperature, air, water, and silica gel chromatography. The Pd complexes catalyze Heck reactions.
The pioneering works of O¨ fele and Wanzlick, who prepared
the first authentic metal complexes of N-heterocyclic car-
benes,1 received little attention until the report of the
synthesis of a stable, free carbene by Arduengo and co-
workers in 1991.2 New protocols for the synthesis of these
carbenes and their metal complexes have spurred extensive
research into applications of these complexes in homoge-
neous catalysis.3 In contrast to the widely used phosphine
complexes, several complexes formed with these ligands
have been shown to be remarkably stable toward heat, air,
and moisture. The metal-carbon bonds in the carbene
complexes are much stronger than the metal-phosphorus
bonds of typical phosphine complexes, thereby eliminating
the problems associated with weak ligand-metal interactions
including deposition of free metal under catalytic condi-
tions.3,4 The N-heterocyclic carbene complexes have been
applied to a broad spectrum of catalytic reactions including
Heck, Suzuki, and Kumada couplings,5 olefin metathesis,6
and hydrosilylation7 processes. Herrmann and Fehlhammer
have subsequently reported methylene-bridged chelated car-
bene complexes.8 Carbene complexes with chiral ligands
have also been prepared by a number of groups.9
Conspicuously absent from the studies reported thus far
are chelated bis-carbene complexes of transition metals with
a chiral backbone. Known chiral carbene complexes are of
the type L*2M (M ) Rh,9a Ni,9c Pd;9d L* ) monocarbene
bearing a chiral group on the imidazoline-N) or a hybrid
(oxazoline/imidazoline-2-ylidene)M (M ) Rh).9f Lappert9a
(5) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G.
R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371. (b) Herrmann, W. A.;
Reisinger, C.-P.; Spiegler, M. J. Organomet. Chem. 1998, 557, 93. (c)
McGuinness, D. S.; Cavell, K. J.; Skelton, B. W.; White, A. H. Organo-
metallics 1999, 18, 1596. (d) Weskamp, T.; Bo¨hm, V. P. W.; Herrmann,
W. A. J. Organomet. Chem. 1999, 585, 348. (e) Huang, J.; Nolan, S. P. J.
Am. Chem. Soc. 1999, 121, 9889.
(6) (a) Ackermann, L.; Fu¨rstner, A.; Weskamp, T.; Kohl, F. J.; Herrmann,
W. A. Tetrahedron Lett. 1999, 40, 4787. (b) Huang, J.; Stevens, E. D.;
Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674. (c) Scholl,
M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999,
40, 2247. (d) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Herrmann,
W. A. Angew. Chem., Int. Ed. Engl. 1999, 38, 2416. (e) Frenzel, U.;
Weskamp, T.; Kohl, F. J.; Schattenmann, W. C.; Nuyken, O.; Herrmann,
W. A. J. Organomet. Chem. 1999, 586, 263.
(1) (a) O¨ fele, K. J. Organomet. Chem. 1968, 12, P42. (b) Wanzlick, H.-
W.; Scho¨nherr, H.-J. Angew. Chem., Int. Ed. Engl. 1968, 7, 141. (c) For
contributions of the Lappert group, see: Lappert, M. F. J. Organomet. Chem.
1988, 358, 185. (d) For an historical account, see: Regitz, M. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 725.
(2) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(3) For a review, see: Herrmann; W. A.; Ko¨cher, C. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2162.
(4) (a) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organo-
metallics 1999, 18, 5375 and references therein. (b) Ulman, M.; Grubbs,
R. H. J. Org. Chem. 1999, 64, 7202. (c) Coleman, A. W.; Hitchcock, P.
B.; Lappert, M. F.; Maskell, R. K.; Mu¨ller, J. H. J. Organomet. Chem.
1985, 296, 173.
(7) (a) Herrmann, W. A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2805. (b) Lappert, M. F.; Maskell,
R. K. J. Organomet. Chem. 1984, 264, 217.
(8) (a) Herrmann, W. A.; Schwarz, J.; Gardiner, M. G. Organometallics
1999, 18, 4082. (b) Herrmann, W. A.; Reisinger, C.-P.; Spiegler, M. J.
Organomet. Chem. 1998, 557, 93. (c) Fehlhammer, W. P.; Bliss, T.;
Kernbach, U.; Bru¨dgam, I. J. Organomet. Chem. 1995, 490, 149.
10.1021/ol005694v CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/25/2000

and Grubbs9g have described chiral 1,3-dialkyl-4,5-dihy-
droimidazol-2-ylidene complexes of Rh and Ru. Hydrosilyla-
tion9b and Heck reactions9d conducted with some of these
complexes have given only marginal asymmetric induction.
Since it is well-known that bidentate bisphosphine complexes
generally give much superior asymmetric induction in many
reactions vis-a´-vis the corresponding monodentate phosphine
complexes, we wondered whether a similar structural motif
could be constructed with chelating bis-carbenes.10 We have
recently completed a study of the synthesis, structure, and
reactivity of a new diimidazolin-2-ylidene ligand with a 2,2′-
binaphthyl backbone.11 In this paper we report the synthesis
and potential utility of novel chiral, chelated bis-carbene
complexes of palladium and nickel (1 and 2, eq 1) prepared
with this ligand.
organometallic carbene complexes (eq 1). (1) In situ forma-
tion of the ligand by reaction with a metal salt (MX) where
the X- counterion of the salt functions as the base required
for the carbene formation. (2) A two-step process where a
solution of the putative bis-carbene of the ligand was
generated under basic conditions and subsequently reacted
with the metal salt.
The reaction of Pd(OAc)2 with imidazolium salt 7 in
DMSO at 140 °C gave both the trans and the cis Pd
complexes 1a and 1b as yellow solids in 41% and 33% yields
after chromatography.14 The X-ray crystal structure analysis
confirmed the geometry of these Pd complexes (vide infra).
The absence of the signal for the imidazolium C-2 methine
protons in the 1H NMR spectra of the cis and trans
complexes indicated successful transformation of the salt into
the carbene. The NMR spectra indicated that the trans
complex was C2-symmetric, while the cis Pd complex 1b
had a more complex structure.13 A single downfield signal
in the 13C spectrum of 1a at 170.95 ppm for C-2 of the
imidazole moieties was present for the carbene carbons. The
(9) (a) Coleman, A. W.; Hitchcock, P. B.; Lappert, M. F.; Maskell, R.
K.; Mu¨ller, J. H. J. Organomet. Chem. 1983, 250, C9. (b) Herrmann, W.
A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2805. (c) Herrmann, W. A.; Goossen, L. J.; Artus, G. R. J.;
Ko¨cher, C. Organometallics 1997, 16, 2472. (d) Enders, D.; Gielen, H.;
Raabe, G.; Runsink, J.; Teles, J. H. Chem. Ber. 1996, 129, 1483. (e) Enders,
D.; Gielen, H.; Raabe, G.; Runsink, J.; Teles, J. H. Chem. Ber. 1997, 130,
1253. (f) Herrmann, W. A.; Goossen, L. J.; Spiegler, M. Organometallics
1998, 17, 2162. (g) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org.
Lett. 1999, 1, 953. (h) Other axial-chiral metal carbenes: Do¨tz, K. H.;
Tomuschat, P.; Nieger, M. Chem. Ber. 1997, 130, 1605.
The preparation of the ligand began with the reaction of
enantiomerically pure (S)-1,1′-bi-2-naphthol bis(trifluo-
romethanesulfonate) (3) with a solution of methylmagnesium
bromide under (dppp)NiCl2-catalyzed Kumada coupling
conditions to give dimethyl compound 4 (Scheme 1).12
(10) All chelated carbene complexes prepared previously have an N,N′-
methylene link between the imidazolines. Attempts to prepare other bridged
N,N′-complexes (C2, C3, C4, and o-xylylene) have not been successful. See
ref 8b.
(11) This work was presented by D.S.C. at the 218th National Meeting
of the American Chemical Society (New Orleans, August 1999), Paper
ORGN 167.
(12) (a) Kurz, L.; Lee, G.; Morgans, D. Jr.; Waldyke, M. J.; Ward, T.
Tetrahedron Lett. 1990, 31, 6324. (b) Uozumi, Y.; Tanahashi, A.; Lee, S.-
Y.; Hayashi, T. J. Org. Chem. 1993, 58, 1945. (c) Sengupta, S.; Leite, M.;
Raslan, D. S.; Quesnelle, C.; Snieckus, V. J. Org. Chem. 1992, 57, 4066.
(13) See Supporting Information for details.
Scheme 1. Synthesis of Diimidazolium Salts
(14) 1a and 1b: imidazolium salt 7 (358 mg, 0.512 mmol) and palladium
acetate (125 mg, 0.559 mmol) were stirred in DMSO (30 mL) at 140 °C
for 6 h. The solvent was removed by distillation under high vacuum, and
the residue was dissolved in CH2Cl2. Chromatography of the residue with
CH2Cl2 as eluant gave complex 1a (167 mg, 41%), and with ethyl acetate
gave complex 1b (135 mg, 33%) both as yellow solids. 1a: 1H NMR (300
MHz, CDCl3) δ 7.97 (d, J ) 7.8 Hz, 2H), 7.94 (d, J ) 7.5 Hz, 2H), 7.48
(dd, J ) 7.3, 7.5 Hz, 2H), 7.36 (dd, J ) 7.3, 8.7 Hz, 2H), 7.29 (d, J ) 8.7
Hz, 2H), 7.28 (d, J ) 7.8 Hz, 2H), 7.17 (d, J ) 1.8 Hz, 2H), 6.90 (d, J )
1.8 Hz, 2H), 5.70 (d, J ) 16.5 Hz, 2H), 4.70 (d, J ) 16.5 Hz, 2H), 3.81 (s,
6H); 13C NMR (75.469 MHz, CDCl3) δ 170.95 (2), 133.72 (2), 133.43 (2),
132.92 (2), 132.71 (2), 129.00 (2), 128.86 (2), 126.92 (2), 126.01 (2), 125.93
(2), 125.38 (2), 123.71 (2), 121.61 (2), 53.37 (2), 38.60 (2); MSFAB (SIMS-
(+)) m/z (relative intensity) 803 (C30H26N4PdI2, M+ + 1, 42), 802 (M+,
26), 675 (100); rev. phase HPLC (10% water in methanol, 1 mL/min, Zorbax
ODS, 300 nm) 3.87 min (100%). 1b: 1H NMR (300 MHz, CDCl3) δ 8.11
(d, J ) 8.7 Hz, 1H), 7.97 (d, J ) 8.4 Hz, 1H), 7.89 (d, J ) 7.5 Hz, 1H),
7.87 (d, J ) 8.4 Hz, 1H), 7.61 (d, J ) 8.7 Hz, 1H), 7.51 (dd, J ) 7.5, 7.5
Hz, 1H), 7.47 (dd, J ) 7.8, 8.4 Hz, 1H), 7.35 (d, J ) 1.8 Hz, 1H), 7.30-
7.35 (m, 2H), 7.22 (dd, J ) 7.8, 8.7 Hz, 1H), 7.10 (d, J ) 8.4 Hz, 1H),
6.98 (d, J ) 1.8 Hz, 1H), 6.80 (d, J ) 8.7 Hz, 1H), 6.41 (d, J ) 1.8 Hz,
1H), 5.98 (d, J ) 1.8 Hz, 1H), 5.80 (d, J ) 17.1 Hz, 1H), 5.71 (d, J ) 15.6
Hz, 1H), 4.87 (d, J ) 17.1 Hz, 1H), 4.69 (d, J ) 15.6 Hz, 1H), 3.89 (s,
3H), 3.73 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 164.21, 163.71, 134.03,
133.18, 133.08 (2), 133.02, 131.91, 131.44, 130.78, 129.91, 128.86, 128.77,
128.21, 127.47, 126.93, 126.65, 126.30, 126.14, 125.20, 124.28, 124.01,
123.34, 122.60, 121.27, 113.15, 55.31, 52.89, 39.58, 39.52; MSFAB (SIMS-
(+)) m/z (relative intensity) 803 (C30H26N4PdI2, M+ + 1, 6), 802 (M+,
12), 675 (100); rev. phase HPLC (10% water in methanol, 1 mL/min, Zorbax
ODS, 300 nm) 3.83 min (100%).
Photolysis in carbon tetrachloride with N-bromosuccinimide
gave dibromomethyl compound 5. Bis-alkylation with the
anion of imidazole followed by quaternization of the imi-
dazole ring at the 3-position with methyl iodide completed
the synthesis of the desired diimidazolium salt 7 in an overall
yield of greater than 65% for the four steps.13
A variety of conditions, which can be grouped into two
main categories,3 were examined for the preparation of the
1126
Org. Lett., Vol. 2, No. 8, 2000

Figure 1. ORTEP drawings of the molecular structure of trans Pd complex 1a and cis Pd complex 1b (thermal ellipsoids at the 50%
probability level). 1a: the atoms marked with an asterisk (*) are related to the unmarked atoms by a crystallographic 2-fold axis. Selected
bond lengths [Å] and angles [deg]: Pd-C1 2.016 (6), Pd-I 2.6011 (6), C1-Pd-C1* 175.6 (4), I-Pd-I* 174.64 (4), I-Pd-C1 90.1 (2).
1b, selected bond lengths [Å] and angles [deg]: Pd-C1 1.986 (6), Pd-C27 1.994 (7), Pd-I1 2.6890 (8), Pd-I2 2.6416 (7), C1-Pd-C27
89.7 (3), I1-Pd-I2 92.64 (2), I2-Pd-C1 86.5 (2), I1-Pd-C27 91.1 (2).
methylenes connecting the aromatic backbone to the het-
erocyclic rings were visible as an AX system of doublets at
5.70 and 4.70 ppm. The geometry of the 11-membered
chelated ring was such that these geminal protons occupied
quite different environments as indicated by the large ∆δ of
1.00 ppm. The 13C NMR spectrum for cis Pd complex 1b
contained two downfield signals at 161.23 and 159.28 ppm
of the ligands around the metal to be distorted square planar
(Figure 1).16 The Pd-C bond lengths were 2.016 Å for the
trans Pd complex and slightly shorter at 1.986 and 1.994 Å
for the cis Pd complex, indicating a more congested
environment at the metal center for complex 1b. The
C-Pd-C bond angles were determined to be 175.6° and
89.7°, respectively. It was hoped that the highly asymmetric
nature of the complexes would result in efficient chirality
transfer from the BINAP backbone in stereodifferentiating
reactions.
We next turned our attention to the preparation of Ni
complexes. The reaction of diimidazolium salt 7 with either
Ni(acac)2 or (Ph3P)2NiCl2 in hot DMSO gave either no
reaction or decomposition of starting materials with no trace
of the desired complex. Surprisingly, the reaction of Ni(acac)2
with the iodide salt in the higher boiling N-methylpyrroli-
dinone (NMP) at 200 °C was successful, giving after
chromatography trans Ni complex 2a as a red-colored solid
in 26% yield. The reaction of (Ph3P)2NiCl2 and a solution
of the preformed bis-carbene in THF also gave trans Ni
complex 2a in 45% yield after chromatography. In contrast
to the Pd complexes described above, only the trans Ni
complex was obtained, even under the harsh NMP reaction
conditions. Nickel tends to form trans complexes with
monocarbenes,17 and furthermore, no chelated bidentate
1
for the carbene carbons. In the H NMR spectrum, four
signals for the bridging benzylic methylene protons were
visible as doublets at 5.80, 5.71, 4.87, and 4.69 ppm. The
N-methyl groups appeared as a pair of singlets at 3.89 and
3.73 ppm.
The formation of the bis-carbene of the ligand was
attempted with a variety of base and solvent combinations.
It was found that reaction in THF at rt with potassium tert-
butoxide under dilute conditions gave the bis-carbene cleanly,
and in high yield.13 Reaction of a THF solution of the bis-
carbene of the ligand with Pd(OAc)2 gave exclusiVely the
trans palladium complex in 21% yield with spectral proper-
ties identical to those of the material prepared in hot DMSO.
Reaction of (Ph3P)2PdCl2 under similar conditions gave none
of the desired Pd complexes. Here, the larger ring size of
the chelate allowed for the trans configuration of the complex
in contrast to previous observations where only the cis
configuration of a methylene bridged bis-carbene Pd complex
was formed.8 Metal complexes with trans-spanning chelating
ligands are rare, and some have been shown to possess
unique catalytic properties.15
(16) Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication nos. CCDC-137339
(1a), CCDC-137340 (1b), and CCDC-137338 (2a). Copies of the data can
be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax (+44) 1223-336-033; e-mail deposit@
ccdc.cam.ac.uk). See Supporting Information for details of the crystal
structure analysis.
Single crystals of the Pd complexes were obtained by
diffusion of hexane into a solution of the complex in CH2-
Cl2. X-ray analysis of these crystals showed the geometry
(15) Sawamura, M.; Sudoh, M.; Ito, Y. J. Am. Chem. Soc. 1996, 118,
3309 and references therein.
(17) Herrmann, W. A.; Gerstberger, G.; Spiegler, M. Organometallics
1997, 16, 2209.
Org. Lett., Vol. 2, No. 8, 2000
1127

[carbene]NiX2 complexes have been reported even though
attempts to synthesize such complexes have been docu-
mented.8a
found to catalyze Heck coupling between ethyl acrylate and
bromo- or iodobenzene in 79% and 95% yields, respectively,
at 1-2 mol % of catalyst loading (eq 2).
The 13C NMR spectrum of Ni complex 2a contained 15
signals with one signal downfield at 176.41 ppm for the C-2
1
carbene of the imidazole moieties. The H NMR spectrum
in CDCl3 contained signals between 6.78 and 8.00 ppm for
the aromatic backbone and imidazole portions of the
complex. The bridging methylene protons were visible as
an AX system of doublets at 6.20 and 4.78 ppm. The
imidazole N-methyl groups appeared as a singlet at 4.07 ppm.
Single crystals of Ni complex 2a were obtained by diffusion
of hexane into a solution of the complex in CH2Cl2. X-ray
analysis of these crystals confirmed the trans configuration
of the ligand and further showed the geometry of the ligands
around the metal to be distorted square planar.18 The Ni-C
bond lengths were 1.889 and 1.901 Å, and the C1-Ni-
C27 bond angle was determined to be 177.7°. Taking into
account the shorter Ni-C bonds, the geometric parameters
are comparable to those of the trans Pd complex 1a.
As can be inferred from the method of preparation (polar
solvents and high temperature), analysis (reverse phase
HPLC with water/methanol solvent), and purification (silica
chromatography with no special attention to exclude air),13
these are remarkably stable complexes. Preliminary catalytic
studies support these conclusions. Complexes 1a and 1b were
In summary we have prepared the first catalytically
competent monomeric, chiral bis-carbene complexes of Pd
and Ni possessing a binaphthyl backbone. Further examina-
tion of this ligand motif and its structural variants as well as
the applications of these materials as enantioselective
catalysts are under investigation and will be reported in due
course.
Acknowledgment. This work was supported by the
National Science Foundation (CHE-9706766) and The Ohio
State University (postdoctoral fellowship, D.S.C.). We thank
Johnson Matthey for a gift of precious metals in support of
our research programs.
Supporting Information Available: Details of the prepa-
ration of the imidazolium salts and Ni complex 2a and
ORTEP diagram of 2a. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) See Supporting Information for details of the preparation and for
an ORTEP drawing of the complex with selected bond lengths and bond
angles.
OL005694V
1128
Org. Lett., Vol. 2, No. 8, 2000