Persistent carbenes containing acetylenes: 4,5-dialkynylimidazol-2- ylidene

Article abstract of DOI:10.1039/b001622p

A nucleophilic imidazole carbene featuring acetylene substituents in the 4- and 5-positions is prepared in three steps from dialkynyl diimines and is characterised by low temperature NMR spectroscopy and trapping reactions with MeI, CS₂ and HgC

Full text of DOI:10.1039/b001622p

Persistent carbenes containing acetylenes: 4,5-dialkynylimidazol-2-ylidene
Rüdiger Faust*a and Bernd Göbeltbc
a Christopher-Ingold-Laboratories, Department of Chemistry, University College London, 20 Gordon Street, London,
UK WC1H 0AJ. E-mail: r.faust@ucl.ac.uk
b Pharmazeutisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 364, D-69120
Heidelberg, Germany
c
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA
Received (in Liverpool, UK) 23rd February 2000, Accepted 20th April 2000
Published on the Web 15th May 2000
A nucleophilic imidazole carbene featuring acetylene sub-
stituents in the 4- and 5-positions is prepared in three steps
from dialkynyl diimines and is characterised by low
temperature NMR spectroscopy and trapping reactions with
MeI, CS2 and HgCl2.
The remarkable renaissance of nucleophilic carbenes, triggered
by the isolation of the first crystalline imidazol-2-ylidene by
Arduengo et al.,1 is witnessed by recent syntheses of a variety
of derivatives of this class of compounds2 and has culminated in
the development of imidazol-2-ylidenes as versatile ligands in
transition metal complexes for use in catalysis applications.3 In
the course of these investigations it emerged that the variation of
the 4,5-imidazole substituents can have striking effects on the
stability of the corresponding carbenes. For example, the notion
that a CC double bond between these positions is a crucial
prerequisite for the generation of isolable carbenes could not be
maintained after the preparation of a stable imidazolidine
Scheme 1
carbene.4 Furthermore,
a
4,5-dichloroimidazol-2-ylidene
proved to be the first heterocyclic carbene stable to the air,5 and
an analogous 4,5-bistrifluoromethyl derivative could even be
subjected to bulb-to-bulb distillations.6
effected by quenching diimine dianions with CS2.9 Imidazole-
2-thiones in turn are precursors to Arduengo-type carbenes
which can be obtained either directly by alkali metal reduction
of the thiones10 or via deprotonation of an intermediate
imidazolium salt derived from them.1 Hence, stirring 3 together
with sodium in dry diethyl ether at room temperature furnished
an initially deep blue solution which slowly turned orange-red
to indicate the formation of the dianion of 3. Treatment of this
solution with CS2 led to the imidazole-2-thione 4 in 70% yield.
Attempts to isolate an acetylenic carbene directly from 4 by a
potassium-mediated reductive desulfurisation were unsuccess-
ful and led only to the decomposition of the starting material.
We therefore prepared the dialkynyl imidazolium salt 5 by
oxidative desulfurisation of 4 with dilute nitric acid in THF
followed by anion exchange with sodium tetrafluoroborate.
Gratifyingly, deprotonation of 5 could be effected with BuLi in
THF at 278 °C and resulted in a yellow-coloured solution of
4,5-dialkynyl imidazol-2-ylidene 6. Interestingly, carbene 6
could only be generated from 5 using BuLi or LDA in THF.
With potassium tert-butanolate, sodium or potassium hydride in
either THF or DMSO on the other hand, only decomposition
products were observed even at temperatures as low as
2100 °C.
We were intrigued by the challenge to introduce seemingly
carbene-incompatible alkynyl groups into the 4,5-positions of
imidazol-2-ylidenes as in 1 (Fig. 1), and are driven by the
prospect to use the acetylene substituents as delocalising
linkages able to connect several carbene sites within one
molecular framework (2). Monomolecular multi-carbene arrays
like 2 are deemed to be of particular interest with respect to
metal coordination and to the exploitation of synergistic metal
cooperativity. We report here the synthesis of a 4,5-dialkynyl
imidazol-2-ylidene as the first representative of this class of
compounds.
The acetylenic carbene 6 was clearly identified in the low
temperature 13C NMR spectrum (90.56 MHz, THF with
external D2O-lock, 278 °C) featuring a diagnostic signal for the
ylidene carbon at 214.3 ppm.11 However, and in contrast to
many non-acetylenic imidazol-2-ylidenes, we were unable to
isolate 6 in neat or in crystalline form. THF solutions of 6 could
be kept at 278 °C for several hours without change, but
allowing the solutions to gradually warm to 240 °C led to
brown, unidentified decomposition products. Apparently, a
4,5-diacetylene substitution appears to have a detrimental effect
on the thermal stability of the imidazol-2-ylidene. While this
finding is somewhat unexpected in light of the remarkable
persistence of 4,5-dichloroimidazol-2-ylidene5 and the elec-
Fig. 1
We have recently reported two complementary pathways to
dialkynyldiimines such as 3 (Scheme 1) from oxalyl chloride
and bisimidoyl chlorides, respectively.7,8 These building blocks
were deemed to be convenient starting materials for the rapid
assembly of the 4,5-diacetylenyl imidazole core, since ring
closure to the corresponding imidazole-2-thiones can be
DOI: 10.1039/b001622p
Chem. Commun., 2000, 919–920
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on crystals of 9 confirmed the presence of a bridging mercury
atom between two imidazole ring structures. Unfortunately,
severe disorder in the crystals caused by the anions and the
floppy triisopropylsilyl groups prevented a refinement of the
observed data below an R-value of 15%.
This work has demonstrated the feasibility of introducing
alkynyl groups into the structural framework of nucleophilic
carbenes and has therefore opened the way for the exploration
of the metal coordination behaviour of these species and for the
assembly of acetylene-linked, multinuclear metal-carbene com-
plexes. Developments along those lines are currently under-
way.
We acknowledge the generous support from Professor Dr R.
Neidlein, Pharmazeutisch-Chemisches Institut of the University
of Heidelberg, Germany and from the Fonds der chemischen
Industrie, Germany.
Notes and references
1 A. J. Arduengo, III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361.
2 D. Bourisson, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev.,
2000, 100, 39.
3 T. Weskamp, W. C. Schattenmann, M. Spiegler and W. A. Herrmann,
Angew. Chem., Int. Ed., 1998, 37, 2490.
Scheme 2
4 A. J. Arduengo, III, J. R. Goehrlich and W. J. Marshall, J. Am. Chem.
Soc., 1995, 117, 11 027.
5 A. J. Arduengo, III, F. Davidson, H. V. R. Dias, J. R. Goerlich, D.
Khasnis, W. J. Marshall and T. K. Prakasha, J. Am. Chem. Soc., 1997,
119, 12 742.
6 A. J. Arduengo, III, Acc. Chem. Res., 1999, 32, 913.
7 R. Faust and B. Göbelt, Tetrahedron Lett., 1997, 38, 8017.
8 R. Faust, B. Göbelt and C. Weber, Synlett, 1998, 64.
9 M. Zettlitzer, H. tom Dieck, E. T. K. Haupt and L. Stamp, Chem. Ber.,
1986, 119, 1868.
10 N. Kuhn and T. Kratz, Synthesis, 1993, 561.
11 A coordination of the carbene to lithium ions can not be ruled out. See:
R. W. Alder, M. E. Blake, C. Bortolotti, C. Bufali, C. P. Butts, E.
Linehan, J. M. Oliva, A. G. Orpen and M. J. Quayle, Chem. Commun.,
1999, 241.
tron-withdrawing properties of alkynyl substituents, it should
be noted that nucleophilic carbenes are known to react with
activated acetylenes12 and that 6 therefore may be susceptible to
intermolecular cycloaddition reactions.
The reactivity of 6 even at low temperature prompted us to
further establish its identity by trapping reactions with various
electrophiles (Scheme 2). Hence, treatment of a solution of 6 in
THF with methyl iodide furnished, after anion exchange, the
corresponding 2-methylimidazolium ion 7 in 58% yield.
Similarly, the zwitterionic imidazolium dithiocarboxylate 8
could be obtained from the reaction of 6 with CS2. Lastly, a
further indirect proof of the intermediacy of carbene 6 was
assembled by performing one of Wanzlick’s original experi-
ments,13,14 namely the reaction of 6 with mercury dichloride.
The reaction furnished, after anion exchange, the diimidazol-
2-yl-mercurium salt 9 as a colourless solid. All adducts of 6 are
fully characterised by spectroscopic data and elemental analy-
ses. In addition, a qualitative X-ray diffraction study performed
12 D. Enders, K. Breuer, J. Runsink and J. H. Teles, Liebigs Ann. Chem.,
1996, 2019.
13 H.-J. Schönherr and H.-W. Wanzlick, Liebigs Ann. Chem., 1970, 731,
176.
14 For some recent developments pertaining to Wanzlick’s original work
see: Y. Liu and D. M. Lemal, Tetrahedron Lett., 2000, 41, 599.
920
Chem. Commun., 2000, 919–920