Synthesis of N-heterocylic carbene-containing metal complexes from 2-(pentafluorophenyl)imidazolidines

Article abstract of DOI:10.1021/om060949f

N-Heterocyclic carbene (NHC)-containing metal complexes are prepared through a simple, base-free method involving the decomposition of 2-(pentafluorophenyl)imidazolidines under mild thermolytic conditions. Ruthenium, iridium, and rhodium complexes containing NHC ligands with different electronic and steric parameters are reported.

Full text of DOI:10.1021/om060949f

2
122
Organometallics 2007, 26, 2122-2124
Notes
Synthesis of N-Heterocylic Carbene-Containing Metal Complexes
from 2-(Pentafluorophenyl)imidazolidines
Angela P. Blum, Tobias Ritter, and Robert H. Grubbs*
The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, DiVision of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, California 91125
ReceiVed October 14, 2006
6
Summary: N-Heterocyclic carbene (NHC)-containing metal
complexes are prepared through a simple, base-free method
inVolVing the decomposition of 2-(pentafluorophenyl)imidazo-
lidines under mild thermolytic conditions. Ruthenium, iridium,
and rhodium complexes containing NHC ligands with different
electronic and steric parameters are reported.
advantage that they can be chemically manipulated. However,
the base that is often required to synthesize these adducts limits
the scope of complexes that can be prepared.
Waymouth, Hedrick, and co-workers recently reported the
base-free synthesis of a new set of air-stable NHC adducts that
uses a pentafluorophenyl substituent as a protecting group. These
adducts are formed by the condensation of diamines and
pentafluorobenzaldehyde and were shown to decompose with
N-Heterocyclic carbene (NHC)-derived complexes have been
1
used as powerful catalysts for effecting many transformations.
7
mild heat to the corresponding carbenes (eq 1).
Despite their popularity, few methods are reported for their
2
a-c
preparation. Current methods include the deprotonation
or
oxidative addition2 of imidazolium salts, thermolysis of
-(trichloromethyl)imidazolidine complexes, oxidative addition
of 2-chloroimidazolium salts, transmetalation of silver-NHC
complexes, and sodium reduction of imidazolidin-2-thiones.
d,e
2f
2
2g
2h
2i
Recently, Crabtree and co-workers reported that N,N′-dialkyl-
imidazolium-2-carboxylates can serve as precursors for the
preparation of NHC-containing metal complexes with the release
of CO2.3 Many of these methods utilize imidazolium salt
precursors, which are insoluble in many media and difficult to
further functionalize. “Protected” NHC adducts like 2-(alkoxy)-
The only byproduct of the reaction is pentafluorobenzene (bp
85 °C). Pentafluorophenyl adducts have been employed in three
instances in the literature to prepare NHC-containing transition-
metal complexes. Waymouth and Hedrick synthesized an NHC
7
complex from allylpalladium chloride, and Bedford et al.
8
a
reported phosphite-based palladacycle NHC compounds as
well as an iron-NHC complex that was formed in situ.8b These
reports illustrate the potential of utilizing pentafluorophenyl
adducts for synthesizing metal-NHC complexes, and it was
our goal to further explore the scope of compounds that can be
prepared from these adducts. Here, we report the preparation
of Ru-, Rh-, and Ir-NHC complexes by the thermolysis of
functionalized 2-(pentafluorophenyl)imidazolidines.
2f
imidazolidines, 2-(trichloromethyl)imidazolidines, 2-(trieth-
4
5
ylborane)imidazolidines and 2-(dimethylamino)imidazolidines
are used to prepare NHC-containing complexes and offer the
*
Corresponding author. E-mail: rhg@caltech.edu.
(1) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39-91. (b) Viciu, M. S.; Germaneau, R. F.; Nolan, S. P.
Org. Lett. 2002, 4, 4053-4056. (c) Trnka, T. M.; Grubbs, R. H. Acc. Chem.
Res. 2001, 34, 18-29. (d) Herrmann, W. A.; K o¨ cher, C. Angew. Chem.,
Int. Ed. 1997, 36, 2162-2187. (e) Herrmann, W. A. Angew. Chem., Int.
Ed. 2002, 41, 1290-1309. (f) Peris, E.; Crabtree, R. H. Coord. Chem. ReV.
A direct comparison between the strategies involving the
thermolysis of 1,3-bis(2,4,6-trimethylphenyl)-2-(pentafluorophe-
nyl)imidazolidine (1) and 1,3-bis(2,4,6-trimethylphenyl)-2-
2
004, 248, 2239-2246. (g) Perry, M. C.; Burgess, K. Tetrahedron:
Asymmetry 2003, 14, 951-961.
2) (a) Herrmann, W. A.; K o¨ cher, C.; Goossen, L. J.; Artus, G. R. J.
Chem.-Eur. J. 1996, 2, 1627-1636. (b) O¨ fele, K. J. Organomet. Chem.
968, 12, P42-P43. (c) M e´ ry, D.; Aranzaes, J. R.; Astruc, D. J. Am. Chem.
(trichloromethyl)imidazolidine (4) in the synthesis of a common
olefin metathesis catalyst, 3, is shown in Schemes 1 and 2. Both
methods afford 3 in comparable yield. The pentafluorophenyl
adduct route offers several advantages in that it requires no base
and fewer synthetic steps. In addition, 1 is stable at room
temperature for greater than 9 months, while 4 shows signs of
decomposition after a few weeks under the same conditions.
The progress of the thermolysis of pentafluorophenyl adducts
(
1
Soc. 2006, 128, 5602-5603. (d) McGuinness, D. S.; Cavell, K. J.; Skelton,
B. W.; White, A. H. Organometallics 1999, 18, 1596-1605. (e) Gr u¨ nde-
mann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R. H. J.
Chem. Soc., Dalton Trans. 2002, 2163-2167. (f) Trnka, T. M.; Morgan, J.
P.; Sanford, M. S.; Wilhelm, T. E.; Scholl, M.; Choi, T. -L.; Ding, S.; Day,
M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 2546-2558. (g)
F u¨ rstner, A.; Seidel, G.; Kremzow, D.; Lehmann, C. W. Organometallics
1
9
can be readily followed by F NMR by monitoring the
2
003, 22, 907-909. (h) Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005,
1
05, 3978-4008. (i) Hahn, F. E.; Paas, M.; Le Van, D.; L u¨ gger, T. Angew.
(6) Waltman, A. W.; Ritter, T.; Grubbs, R. H. Organometallics 2006,
25, 4238-4239.
Chem., Int. Ed. 2003, 42, 5243-5246.
(
3) Voutchkova, A. M.; Appelhans, L. N.; Chianese, A. R.; Crabtree, R.
H. J. Am. Chem. Soc. 2005, 127, 17624-17625.
4) Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura,
Y.; Kobayashi, K.; Ito, T. Chem. Commun. 2004, 19, 2160-2164.
5) Chamizo, J. A.; Morgado, J. Transition Met. Chem. 2000, 25, 161-
65.
(7) Nyce, G. W.; Csihony, S.; Waymouth, R. M.; Hedrick, J. L. Chem.
Eur. J. 2004, 10, 4073-4079.
(
(8) (a) Bedford, R. B.; Betham, M.; Blake, M. E.; Frost, R. M.; Horton,
P. N.; Hursthouse, M. B.; L o´ pez-Nicol a´ s, R.-M. Dalton Trans. 2005, 2774-
2779. (b) Bedford, R. B.; Betham, M.; Bruce, D. W.; Danopoulos, A. A.;
Frost, R. M.; Hird, M. J. Org. Chem. 2006, 71, 1104-1110.
(
1
1
0.1021/om060949f CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/06/2007

Notes
Scheme 1. Preparation of 3 from the Thermolysis of
Organometallics, Vol. 26, No. 8, 2007 2123
Table 2. Comparison of Percent Conversion to
Pentafluorobenzene in Different Solvents in the Thermolysis
of 1 after 1 h at 45 °C
1
,3-Bis(2,4,6-trimethylphenyl)-2-(pentafluorophenyl)imida-
zolidine, 1
solvent
% convn
C6D6
toluene-d8
THF-d8
8
8
29
CD2CI2
32
acetone-d6
CD3CN
90
>95
groups, respectively) to reach the same conversion (Table 1).
A study of the thermolysis of 1 at 45 °C in varying solvents
(
eq 1, R ) Mes) showed a dramatic range of rates of conversion
to pentafluorobenzene as determined by percent conversions
1
9
after 1 h quantified by F NMR spectroscopy (Table 2). More
polar solvents such as acetone and acetonitrile were shown to
yield higher conversions than less polar solvents such as
benzene. Although studies with acetone and acetonitrile showed
faster conversions to pentafluorobenzene, the apparent instability
of some of the metal precursors prevented the preparation of
NHC-containing organometallic complexes in these solvents.
Studies in tetrahydrofuran and methylene chloride were also
challenging because most of the reactions we attempted, which
primarily involved ruthenium alkylidene species, required
temperatures that exceeded the boiling points of these solvents.
Despite prolonged reaction times, we found toluene to give the
highest yields in our applications.
Table 1. Comparison of the Thermolysis of
2
-(Pentafluorophenyl)imidazolidines to >95% in Benzene or
Toluene
Ruthenium-, rhodium-, and iridium-NHC complexes were
easily prepared from pentafluorophenyl adducts in generally high
yield under mild thermolytic conditions, as shown in Table 3.
We investigated substrates with different electronic and steric
properties, further illustrating the effectiveness of this strategy.
The preparation of a complex containing an unsymmetrically
substituted NHC ligand (1-mesityl-3-methylimidazol-1-ylidene)
is also reported (entry 6). The synthesis of the product of entry
7
presented a particular challenge. Other methods that were
attempted to prepare the complex were unsuccessful due to the
apparent instability of the product. The synthesis of this
complex, although in low yield, highlights the utility of this
method. It is likely that the ortho methoxy groups of this adduct
do not present the steric environment needed to convert to
pentafluorobenzene and the carbene at lower temperatures.
Indeed, a lack of hindered rotation around the C-Caryl(C6F5) bond
appearance of pentafluorobenzene. This fact enabled us to
observe that the electron-rich adduct 1,3-bis(4-methoxy-2,6-
dimethyl)-2-(pentafluorophenyl)imidazolidine (5) decomposes
to pentafluorobenzene and the N-heterocyclic carbene at a faster
rate (>95% conversion in 2 h at 60 °C) than 1,3-bis(2,4,6-
trimethylphenyl)-2-(pentafluorophenyl)imidazolidine (1) (>95%
conversion in 4 h at 60 °C), as shown in Table 1. In addition,
adducts lacking steric bulk in the ortho position of their phenyl
rings do not form the carbene as readily. For example, the
thermolysis of 1,3-bis[3,5-di(tert-butyl)phenyl)]-2-(pentafluo-
rophenyl)imidazolidine (6), an adduct with steric bulk only in
the meta position of its phenyl groups, required 14 days and
is evident by the presence of only two resonances in the 19
F
NMR spectrum of this adduct, in contrast to the five resonances
seen for 1.
A strategy for preparing NHC-containing organometallic
complexes from the thermolysis of 2-(pentafluorophenyl)-
imidazolidines was presented. It should be noted that this method
is not meant to replace traditional strategies for preparing metal
complexes containing NHC ligands. Instead it is meant to be
viewed as a simple, base-free alternative that may offer access
to more complicated metal-NHC complexes by eliminating
possible side reactions. The reaction scope was shown to include
ligands with different electronic and steric properties. This
107 °C to reach >95% conversion to pentafluorobenzene, while
all adducts with substituents (methyl or isopropyl groups) in
the ortho position required less heat (60 °C) and shorter reaction
times (4 or 5 h for the adducts containing methyl or isopropyl
Scheme 2. Preparation of 3 from the Thermolysis of 1,3-Bis(2,4,6-trimethylphenyl)-2-(trichloromethyl)imidazolidine, 4

2
124 Organometallics, Vol. 26, No. 8, 2007
Table 3. N-Heterocyclic Carbene Complexes Prepared from 2-(Pentafluorophenyl)imidazolidines
Notes
method requires no base and fewer synthetic steps than many
strategies currently in use, making possible the preparation of
complexes with functionality that may be incompatible with
other methods.
Michael W. Day and Lawrence M. Henling are thanked for
crystallographic work.
Supporting Information Available: Crystallographic data for
4
chloro(η -1,5-cyclooctadiene)(1,3-dimesitylimidazole-2-ylidene)-
rhodium(I), experimental procedures, and NMR data for all new
compounds are available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. T.R. thanks the German Academic
Exchange Service (DAAD) for a postdoctoral fellowship. The
authors thank Irina Gorodetskaya, Dr. Anna G. Wenzel, and
Dr. Georgios C. Vougioukalakis for starting materials. Dr.
OM060949F