A facile preparation of imidazolinium chlorides

Article abstract of DOI:10.1021/ol800628a

A process for the preparation of symmetric and unsymmetric imidazolinium chlorides that involves reaction of a formamidine with dichloroethane and a base (a) is described. This method makes it possible to obtain numerous imidazolinium chlorides under solvent-free reaction conditions and in excellent yields with purification by simple filtration. Alternatively, symmetric imidazolinium chlorides can be prepared directly in moderate yields from substituted anilines by utilizing half of the formamidine Intermediate as sacrificial base (b).

Full text of DOI:10.1021/ol800628a

ORGANIC
LETTERS
2008
Vol. 10, No. 10
2075-2077
A Facile Preparation of Imidazolinium
Chlorides
Kevin M. Kuhn and Robert H. Grubbs*
The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of
Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125
rhg@caltech.edu
Received March 18, 2008
ABSTRACT
A process for the preparation of symmetric and unsymmetric imidazolinium chlorides that involves reaction of a formamidine with dichloroethane
and a base (a) is described. This method makes it possible to obtain numerous imidazolinium chlorides under solvent-free reaction conditions
and in excellent yields with purification by simple filtration. Alternatively, symmetric imidazolinium chlorides can be prepared directly in
moderate yields from substituted anilines by utilizing half of the formamidine intermediate as sacrificial base (b).
Since the first isolation of a stable N-heterocyclic carbene
(NHC) by Arduengo,1 their use as ligands in organometallic
complexes has become routine. NHCs, as neutral, two-
electron donors with little π-accepting character, have
replaced phosphines in a variety of applications.2 Particularly,
the use of NHCs as ligands in ruthenium-based olefin
metathesis has allowed for great gains in both activity and
stability.3 There is also increasing interest in the use of NHCs
as nucleophilic reagents and organocatalysts, with wide
application in reactions such as the benzoin condensation,
among others.4
NHCs are often prepared in situ via the deprotonation of
their corresponding imidazol(in)ium salts (eq 1).5 Therefore,
facile and high-yielding methods for the synthesis of imi-
dazol(in)ium salts are of great interest.
The synthesis of unsaturated imidazolium salts, previously
optimized by Arduengo et al., involves a one-pot procedure
from glyoxal, substituted aniline, formaldehyde, and acid
starting materials.6 Saturated imidazolinium salts, however,
are prepared from the reaction of triethyl orthoformate with
the corresponding diamine.7 This approach suffers several
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. A J. Am. Chem. Soc.
1991, 113, 361–363.
(2) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39–91. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002,
41, 1290–1309. (c) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004,
248, 2239–2246. (d) Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004,
248, 2247–2273. (e) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. ReV.
2007, 251, 874–883.
¨
(5) (a) Ofele, K. J. Organomet. Chem. 1968, 12, P42-P43. (b)
Herrmann, W. A.; Ko¨cher, C.; Goossen, L. J.; Artus, G. R. J. Chem.sEur.
J. 1996, 2, 1627–1636. (c) Me´ry, D.; Aranzaes, J. R.; Astruc, D. J. Am.
Chem. Soc. 2006, 128, 5602–5603.
(3) (a) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H.
Tetrahedron Lett. 1999, 40, 2247–2250. (b) Scholl, M.; Ding, S.; Lee, C. W.;
Grubbs, R. H. Org. Lett. 1999, 1, 953–956. (c) Huang, J. K.; Stevens, E. D.;
Nolan, S. P.; Peterson, J. L. J. Am. Chem. Soc. 1999, 121, 2674–2678.
(4) For a review of recent work, see: Marion, N.; Diez-Gonzalez, S.;
Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2988–3000.
(6) Arduengo, A. J., III. Preparation of 1,3-Disubstituted Imidazolium
Salts. U.S. Patent No. 5077414, 1991.
(7) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999,
55, 14523–14534.
10.1021/ol800628a CCC: $40.75
Published on Web 04/16/2008
2008 American Chemical Society

drawbacks: the preparation of the diamine generally includes
either a palladium C-N coupling or a condensation and
reduction sequence (Scheme 1);8 moreover, purification of
Our preliminary efforts focused on the preparation of 1,3-
dimesitylimidazolinium chloride from N,N′-bis(mesityl)for-
mamidine. The formamidine can act as both substrate and
sacrificial base in the reaction. After optimization, the
reaction led to nearly quantitative, reproducible yields of pure
1,3-dimesitylimidazolinium chloride (Scheme 2) N,N′-bis-
Scheme 1. Common Syntheses of Imidazolinium Salts
Scheme 2
a Yield in parentheses based on a 50% theoretical yield with half of
the substrate considered as a sacrificial base.
the unstable diamine sometimes requires careful chroma-
tography. Unsymmetric imidazolinium salts are especially
challenging synthetic targets due to the introduction of the
differing substituents.9
Recently, Bertrand et al. developed an alternative ret-
rosynthetic disconnection and prepared a range of five-, six-,
and seven-membered imidazolinium salts from the addition
of “di-electrophiles” to lithiated formamidines.10 For ex-
ample, 1,3-dimesitylimidazolinium lithium sulfate was pre-
pared in high yield with 1,3,2-dioxathiolane-2,2-dioxide as
the dielectrophile (eq 2).
(mesityl)formamidine hydrochloride could also be isolated
and easily reverted to the formamidine for future use by
solvation in pyridine and precipitation into water.11
Numerous bases were screened to find an effective
replacement for the sacrifical formamidine. Only diisopro-
pylethylamine (DIPA) was shown to perform well in the
reaction. Bases such as pyridine and triethylamine were too
nucleophilic and reacted preferentially with dichloroethane.
Strong bases such as sodium hydride deprotonated the final
product.
The two methods were both successful in preparing a series
of other imidazolinium chlorides starting from a variety of
anilines (Table 1). In all cases, reactions were completed
neat in 10-20 equiv of dichloroethane and a slight excess
of base. Products were easily purified by removal of excess
dichloroethane, trituration in acetone or hot toluene, and
filtration.
Notably, two challenging unsymmetrical imidazolinium
chlorides were prepared in good yields (entries 5 and 6).
Our synthesis of 1-(2,6-difluorophenyl)-3-(mesityl)imidazo-
linium chloride (entry 5), prepared here in two steps and a
65% overall yield, is a marked improvement over its previous
four-step synthesis.9a This method should allow for the
properties and applications of unsymmetrical NHCs to be
further explored.
In our ongoing efforts, we have found several substrate
limitations. As steric bulk at the N-aryl ortho positions is
increased, the steric hindrance decreases reaction rate, and
longer reaction times are necessary. This is exemplified by
the reaction of N,N′-bis(2-tert-butylphenyl)formamidine,
which only reached 60% conversion after 7 days (entry 4A).
Highly electron-withdrawing N-aryl subsituents also hinder
thereaction;thereactionofN,N′-bis(2,6-trifluoromethylphenyl)
formamidine was unsuccessful. Finally, reaction of a dialkyl
formamidine, N,N′-bis(cyclohexyl)formamidine, gave only
poor yields of the desired product, most likely due to the
increased basicity of dialkyl formamidines.
Following Bertrand’s report, we reasoned that imidazo-
linium chlorides could be more easily prepared directly from
the reaction of formamidines with dichloroethane (DCE) in
the presence of a base. Formamidines are ideal precursors
for the preparation of imidazolinium chlorides because they
are generally prepared in a one-step solvent-free reaction
from materials already utilized in imidazolinium salt syn-
thesis, namely anilines and triethylorthoformate.
Herein, we report this new synthetic strategy for the
preparation of imidazolinium chlorides under solvent-free
reaction conditions and in excellent yields with purification
by simple filtration. This strategy also allows for the
preparation of symmetric imidazolinium chlorides in a one-
step, three-component procedure directly from substituted
anilines.
(8) (a) For recent examples, see: Ritter, T.; Day, M. W.; Grubbs, R. H.
J. Am. Chem. Soc. 2006, 128, 11788–11789. (b) Stylianides, N.; Danopoulos,
A. A.; Pugh, D.; Hancock, F.; Zanotti-Gerosa, A. Organometallics 2007,
26, 5627–5635. (c) Courchay, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud,
A.; Wagener, K. B. Organometallics 2006, 25, 6074–6086. (d) Beletskaya,
I. P.; Bessmertnykh, A. G.; Averin, A. D.; Denat, F.; Guilard, R. Eur. J.
Org. Chem. 2005, 261–280.
(9) (a) Vougioukalakis, G. C.; Grubbs, R. H. Organometallics 2007,
26, 2469–2472. (b) Winkelmann, O.; Linder, D.; Lacuor, J.; Na¨ther, C.;
Lu¨ning, U. Eur. J. Org. Chem. 2007, 22, 3687–3697.
Further efforts focused on a one-step, three-component
synthesis of commonly utilized symmetric 1,3-diarylimidi-
(10) (a) Jazzar, R.; Liang, H.; Donnadieu, B.; Bertrand, G. J. Organomet.
Chem. 2006, 691, 3201–3205. (b) Jazzar, R.; Bourg, J.-B.; Dewhurst, R. D.;
Donnadieu, B.; Bertrand, G. J. Org. Chem. 2007, 72, 3492–3499.
(11) Dains, F. B.; Malleis, O. O.; Meyer, J. T. J. Am. Chem. Soc. 1913,
35, 970–976.
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Org. Lett., Vol. 10, No. 10, 2008

Table 1. Preparation of 1,3 Diarylimidazolinium Chlorides from
Formamidines
Table 2. Preparation of 1,3 Diarylimidizolinium Chlorides from
Substituted Anilines in One-Step
a Isolated yield of the desired imidazolinium chloride. b Yield in
parentheses based on a 50% theoretical yield with half of the substrate
considered as a sacrificial base.
chloride has been prepared on a 20 g scale without loss in
yield or ease of purification.
In conclusion, we have devised a new synthetic strategy
for the preparation of symmetric and unsymmetric imida-
zolinium chlorides from formamidines in excellent yields.
Because the formamidine precursors and imidazolinium
products are both formed in solvent-free conditions and
purified by simple trituration and filtration, this approach is
more straightforward as well as more atom-economical than
the previously available methods. We have also demonstrated
that symmetric imidazolinium chlorides can be prepared
directly in moderate yields from substituted anilines. We
believe these experimentally convenient procedures will find
wide application as N-heterocyclic carbenes become even
more common as ligands and organocatalysts.
a Isolated yield of the desired imidazolinium chloride. b Yield in
parentheses based on a 50% theoretical yield with half of the substrate
considered as a sacrificial base. c A suitable non-nucleophilic base could
not be found to replace the formamidine as base. d While neither reaction
had reached 100% conversion, the reactions were stopped after 7 days.
zolinium chlorides from substituted anilines (Table 2). The
formamidine, as base and intermediate, is formed in situ,
and the cyclization then proceeds as normal. Regrettably,
replacement of the sacrificial formamidine with diisopropyl-
ethylamine hindered the initial reaction and resulted in very
limited product formation. While yields are lower than the
two-step procedure, reaction optimization, and recycling of
the formamidine hydrochloride could be successful on the
large scale. In our studies, 1,3-dimesitylimidazolinium
Acknowledgment. We gratefully acknowledge financial
support from NIH Grant No. GM31332 to R.H.G.
Supporting Information Available: Full experimental
details for all new procedures and characterization data for
formamidines (entries 1-6, Table 1) and imidazolinium
chlorides (entries 1-6, Table 1). This material is available
free of charge via the Internet at http://pubs.acs.org.
OL800628A
Org. Lett., Vol. 10, No. 10, 2008
2077