N-heterocyclic carbenes can coexist with alkenes and C-H acidic sites

Article abstract of DOI:10.1039/b107238b

The isolation of compounds 3 and 7a,b proves that a singlet carbene center can coexist withalkenes or C-H acidic sites in proximity without spontaneous annihilation.

Full text of DOI:10.1039/b107238b

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N-Heterocyclic carbenes can coexist with alkenes and C–H acidic sites
Alois Fürstner,* Helga Krause, Lutz Ackermann and Christian W. Lehmann
Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim/Ruhr, Germany.
E-mail: fuerstner@mpi-muelheim.mpg.de; Fax: +49-208-306-2994
Received (in Cambridge, UK) 10th August 2001, Accepted 21st September 2001
First published as an Advance Article on the web 18th October 2001
The isolation of compounds 3 and 7a,b proves that a singlet
carbene center can coexist with alkenes or C–H acidic sites in
proximity without spontaneous annihilation.
no significant electron density. To the best of our knowledge,
this is the first example of an isolated carbene that coexists with
an olefin in its vicinity.
With 3 in hand, it was possible to improve the synthesis of the
targeted ruthenium complex 5 (Scheme 1). Thus, reaction of
(PCy3)2Cl2RuNCHPh with isolated 3 in toluene affords complex
4 in 86% yield which cyclizes upon heating to form the desired
product 5. The use of toluene instead of n-pentane in the latter
reaction is beneficial for solubility reasons and constitutes a
noteworthy improvement over our original procedure.7
Among the nucleophilic carbenes that can be isolated in pure
form,1 N-heterocyclic carbenes (NHC’s) of the general structure
12 are rapidly gaining importance as electron-donating ligands
for transition metal centers in different oxidation states.3
Applications of NHC-complexes to catalysis have recently met
with considerable success, olefin metathesis4 and cross cou-
pling chemistry5 being the most convincing cases.6 Usually,
NHC ligands bearing simple alkyl or aryl substituents R on their
N-atoms are employed, although functionalized ones may be of
even greater preparative appeal. A pertinent example is the
ruthenium complex 5 which serves as a ‘designer’ catalyst for
olefin metathesis capable of regenerating itself once the
substrate is depleted.7 The reported synthesis of 5 involves the
in situ generation of carbene 3 as a transient species which is
immediately trapped by admixed (PCy3)2Cl2RuNCHPh8 to form
product 4; though easy to carry out, this in situ protocol turned
out to be rather low yielding (41%). Heating 4 in an inert solvent
then effects the intramolecular metathesis of the tethered olefin
to form the metallacyclic complex 5.
To improve the synthesis of this promising catalyst, we were
pursuing the isolation of compound 3 containing an alkene next
to the carbene center. Although cyclopropanation of olefins is a
prototype carbene reaction with low activation barrier,9–11 the
preparation of 3 is surprisingly simple. This compound is
obtained in essentially quantitative yield as a crystalline solid by
treatment of the imidazolium salt 2 with KOt-Bu in THF at 0 °C
followed by a standard work-up.12 The spectral data and the
molecular structure in the solid state (Fig. 1) show the presence
of a carbene center in proximity to the intact alkene group.†
Difference maps clearly reveal the location of all hydrogen
atoms; the volume element adjacent to the carbene C-atom has
Scheme 1 Reagents and conditions: [a] KOt-Bu, THF, 0 °C, 98%; [b]
(PCy3)2Cl2RuNCHPh, toluene, 0 °C, 86%; [c] n-pentane (reflux) or toluene
(65 °C), 41–77%; Mes = 2,4,6-trimethylphenyl ( = mesityl).
Next, we investigated if other functional groups that are
generally believed to be incompatible with free carbenes can
similarly be incorporated into NHC’s. The insertion of carbenes
into acidic C–H bonds is known to be a facile process;9
therefore the preparation of compounds 7a,b bearing an ester or
a nitrile group seemed challenging, in particular since Ardu-
engo13 has previously reported that closely related dihydro-
imidazol-2-ylidenes rapidly insert into the acidic C–H bonds of
MeCN, MeSO2Ph, HCCl3 or HC·CH.14
Gratifyingly, however, carbenes 7a,b are obtained in ex-
cellent yields by deprotonation of the corresponding imidazol-
ium salts 6a,b (Scheme 2),† although they were found to
deteriorate within hours even at 0 °C. Reaction of (PCy3)2-
Cl2RuNCHPh8 with these functionalized carbenes in toluene at
ambient temperature provides the novel ruthenium complexes 8
(74%) and 9 (38%).§ The X-ray structure of 8 (Fig. 2) shows the
characteristic p–p stacking between the benzylidene ligand and
the mesityl group which was previously recognized as a highly
Fig. 1 Molecular structure of 3.‡ Anisotropic displacement parameters are
shown at 50% probability level. There are two independent molecules in the
asymmetric unit. The major difference is found in the conformation of the
alkyl chain. The torsion angle C6–C7–C8–C9 is 67.6(5)° compared to
177.1(4)° in the second molecule. The difference of the dihedral angles
formed between the mesityl and the NHC is 15.8°.
Scheme 2 Reagents and conditions: [a] KOt-Bu, THF, 210 °C, 95% (7a),
89% (7b); Mes = 2,4,6-trimethylphenyl ( = mesityl).
2240
Chem. Commun., 2001, 2240–2241
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107238b

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3H), 2.30 (br, 3H), 2.01 (s, 6H), 1.19 (t, 3H, J = 7.1 Hz); 13C NMR (d8-
THF) d 218.2, 138.4, 136.0, 129.5, 128.9, 121.1, 118.3, 59.8, 21.5, 21.1,
17.7, 15.2. 7b: 1H NMR (d8-THF) d 7.11 (d, 1H, J = 1.6 Hz), 6.91 (s, 2H),
6.85 (d, 1H, J = 1.6 Hz), 4.11 (t, 2H, J = 6.9 Hz), 2.35 (t, 2H, J = 6.9 Hz),
2.29 (s, 3H), 1.98 (s, 6H), 1.95–1.80 (m, 2H), 1.75–1.40 (m, 4H).
‡ Crystal data. 3: C17H22N2, M = 254.37 g mol21, colorless, crystal
dimensions 0.26 3 0.22 3 0.11 mm, monoclinic P21/n (no. 14), at 100 K a
= 9.5288(4), b = 29.1967(15), c = 11.0965(6) Å , b = 94.419(2)°, V =
3078.0(3) Å3, Z = 8, r = 1.098 Mg m23, m = 0.065 mm21, l = 0.71073
Å, 10536 reflections measured, 3885 unique (Rint = 0.148), final R =
0.078, wR(F2) = 0.209, CCDC 168097. 8: C42H61Cl2N2O2PRu, M =
828.87 g mol21, red–brown, crystal dimensions 0.18 3 0.12 3 0.02 mm,
monoclinic P21/c (no. 14), at 100 K a = 17.4060(2), b = 10.2810(1), c =
23.6677(3) Å, b = 96.673(1)°, V = 4206.67(8) Å3, Z = 4, r = 1.309 Mg
m23, m = 0.573 mm21, l = 0.71073 Å, 44732 reflections measured, 15979
unique (Rint = 0.075) final R = 0.049, wR(F2) = 0.113. CCDC 168098.
See http://www.rsc.org/suppdata/cc/b1/b107238b/ for crystallographic files
in CIF or other electronic format.
§ Data for ruthenium complexes. 8: 1H NMR (CD2Cl2) d 19.16 (s, 1H), 7.82
(br, 2H), 7.53 (m, 1H), 7.42 (m, 1H), 7.12 (t, 2H, J = 7.6 Hz), 6.87 (d, 1H,
J = 2.1 Hz), 6.30 (m, 2H), 5.96 (q, 1H, J = 7.4 Hz), 4.40–4.20 (m, 2H),
2.40–2.20 (m, 3H), 2.34 (s, 3H), 2.00–1.50 (m, 15H), 1.99 (d, 3H, J = 7.3
Hz), 1.93 (s, 6H), 1.45–1.00 (m, 15H), 1.33 (t, 3H, J = 7.2 Hz); 31P NMR
(CD2Cl2) d 35.6; C42H61Cl2N2O2PRu (915.86) calcd. C, 60.86; H, 7.42; N,
3.38; found C, 60.75; H, 7.38; N, 3.32%; 9: 1H NMR (C6D6) d 19.75 (s, 1H),
8.17 (br, 2H), 7.60–6.80 (m, 5H), 6.20 (s, 1H), 6.17 (d, 1H, J = 1.8 Hz),
4.54 (t, 2H, J = 7.2 Hz), 3.26 (m, 2H), 2.70–2.50 (m, 3H), 2.30–1.10 (m,
36H), 2.18 (s, 3H), 1.83 (s, 6H); 31P NMR (C6D6) d 34.2; C43H62Cl2N3PRu
(823.93) calcd. C, 62.68; H, 7.58; N, 5.10; found C, 62.70; H 7.49; N,
4.97%.
Fig. 2 Molecular structure of complex 8.‡ Anisotropic displacement
parameters are shown at 50% probability level. Selected bond lengths [Å]
and angles [°]: Ru1–C9 1.839(2), Ru1–C1 2.066(2), Ru1–Cl2 2.3944(5),
Ru1–Cl1 2.4056(5), Ru1–P1 2.4156(6), Cl2–Ru1–Cl1 162.299(19), C1–
Ru1–P1 162.02(6).
Table 1 Ring closing metathesis reactions (RCM) using complex 8 as the
catalyst; E = COOEta
Substrate
Product
Yield
91%
1 Review: D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem.
Rev., 2000, 100, 39.
2 A. J. Arduengo, Acc. Chem. Res., 1999, 32, 913.
3 W. A. Herrmann and C. Köcher, Angew. Chem., Int. Ed. Engl., 1997, 36,
2162; D. Enders and H. Gielen, J. Organomet. Chem., 2001, 617–618,
70.
4 Reviews: T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18;
A. Fürstner, Angew. Chem., Int. Ed., 2000, 39, 3012.
69%
5 C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org. Chem., 1999,
64, 3804; A. Fürstner and A. Leitner, Synlett, 2001, 290; S. Lee and J. F.
Hartwig, J. Org. Chem., 2001, 66, 3402; G. A. Grasa and S. P. Nolan,
Org. Lett., 2001, 3, 119; W. A. Herrmann, M. Elison, J. Fischer, C.
Köcher and G. R. J. Artus, Angew. Chem., Int. Ed., 1995, 34, 2371; J.
Huang and S. P. Nolan, J. Am. Chem. Soc., 1999, 121, 9889 and
literature cited therein.
78%b
6 See also: A. Fürstner and H. Krause, Adv. Synth. Catal., 2001, 343, 343;
H. M. Lee, T. Jiang, E. D. Stevens and S. P. Nolan, Organometallics,
2001, 20, 1255; A. C. Chen, L. Ren, A. Decken and C. M. Crudden,
Organometallics, 2000, 19, 3459; J. Louie and R. H. Grubbs, Chem.
Commun., 2000, 1479.
a All reactions were performed in toluene at 80 °C using 4–7 mol% of 8. b In
CH2Cl2 at 40 °C.
7 A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann, R.
Mynott, F. Stelzer and O. R. Thiel, Chem. Eur. J, 2001, 7, 3236.
8 P. Schwab, R. H. Grubbs and J. W. Ziller, J. Am. Chem. Soc., 1996, 118,
100.
conserved structural motif in unsymmetrical NHC-complexes
of this type.7 Moreover, the preliminary data displayed in Table
1 illustrate that complex 8 exhibits promising catalytic activity
in prototype metathesis reactions.
We are grateful to the DFG (Leibniz award to A. F.) and the
Fonds der Chemischen Industrie for generous financial sup-
port.
9 W. Kirmse, Carbene Chemistry, Academic Press, New York, 1971.
10 NHC’s rapidly react with electron deficient alkenes, cf. D. Enders, K.
Breuer, J. Runsink and J. H. Teles, Liebigs Ann., 1996, 2019.
11 For a NHC’s with adjacent alkyne groups prepared by a different
method see: R. Faust and B. Göbelt, Chem. Commun., 2000, 919.
12 A. J. Arduengo, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R. Goerlich,
W. J. Marshall and M. Unverzagt, Tetrahedron, 1999, 55, 14 523.
13 A. J. Arduengo, J. C. Calabrese, F. Davidson, H. V. Rasika Dias, J. R.
Goerlich, R. Karfczyk, W. J. Warshall, M. Tamm and R. Schmutzler,
Helv. Chim. Acta, 1999, 82, 2348.
14 Note, however, that carbonyl-containing NHC’s have previously been
invoked as transient species in the preparation of metal complexes, cf.
D. S. McGuinness and K. J. Cavell, Organometallics, 2000, 19, 741; H.
Glas, E. Herdtweck, M. Spiegler, A.-K. Pleier and W. R. Thiel,
J. Organomet. Chem., 2001, 626, 100.
Notes and references
† Data for functionalized carbenes. 3: 1H NMR (d8-THF) d 7.09 (d, 1H, J
= 1.5 Hz), 6.90 (s, 2H), 6.83 (d, 1H, J = 1.5 Hz), 5.84 (ddt, 1H, J = 17.0,
10.3, 6.6 Hz), 5.03 (ddt, 1H, J = 17.1, 1.9, 1.6 Hz), 4.94 (ddt, 1H, J = 10.1,
2.1, 1.2 Hz), 4.08 (t, 2H, J = 6.9 Hz), 2.29 (s, 3H), 2.09 (m, 2H), 1.97 (s,
6H), 1.92 (m, 2H); 13C NMR (d8-THF) d 217.1, 139.9, 138.9, 137.5, 135.8,
1
129.5, 121.1, 119.4, 115.2, 50.8, 31.8, 31.6, 21.0, 18.0. 7a: H NMR (d8-
THF) d 7.25–7.00 (m, 3H), 6.95 (m, 2H), 4.08 (q, 2H, J = 7.1 Hz), 2.30 (s,
Chem. Commun., 2001, 2240–2241
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