N,N′-Bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene: A stable nucleophilic carbene derived from benzimidazole

Article abstract of DOI:10.1002/(SICI)1521-3765(19990604)5:6<1931::AID-CHEM1931>3.0.CO;2-M

N,N′-Dialkylated benzimidazol-2-thiones 6 can be prepared in a three-step procedure from o-phenylene-diamine. Thione 6a was reduced with sodium/potassium under desulfurization leading to carbene 4. Compound 4 is the first stable free carbene derived from benzimidazole. It exhibits the topology of an unsaturated carbene including a short C4-C5 bond. Its ¹³C NMR spectrum [δ(C2)=231.47] and selected structural parameters [angle N1-C2-N3 103.5(1)° and 104.2(1)°], however, correspond to those of saturated carbenes of the imidazolidin-2-ylidene type 3. The observation that the unsaturated carbene 4 behaves like a saturated imidazolidin-2-ylidene is corroborated by the tendency of the sterically less bulky N,N′-dialkylated thione 6b to yield the olefin 8 after reduction with Na/K. Carbene 4 can be coordinated to a W(CO)₅ fragment yielding the complex 9, which contains a nonplanar carbene ring.

Full text of DOI:10.1002/(SICI)1521-3765(19990604)5:6<1931::AID-CHEM1931>3.0.CO;2-M

FULL PAPER
N,N'-Bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene:
A Stable Nucleophilic Carbene Derived from Benzimidazole
F. Ekkehardt Hahn,*[a] Lars Wittenbecher,[a] Roland Boese,[b] and Dieter Bläser[b]
Dedicated to Professor Wolf-Peter Fehlhammer on the occasion of his 60th birthday
Abstract: N,N'-Dialkylated benzimid-
azol-2-thiones 6 can be prepared in a
three-step procedure from o-phenylene-
diamine. Thione 6a was reduced with
sodium/potassium under desulfurization
leading to carbene 4. Compound 4 is the
first stable free carbene derived from
benzimidazole. It exhibits the topology
of an unsaturated carbene including a
short C4 C5 bond. Its 13C NMR spec-
trum [d(C2) ˆ 231.47] and selected
structural parameters [angle N1-C2-N3
103.5(1)8 and 104.2(1)8], however, cor-
respond to those of saturated carbenes
of the imidazolidin-2-ylidene type 3. The
observation that the unsaturated car-
bene 4 behaves like a saturated imida-
zolidin-2-ylidene is corroborated by the
tendency of the sterically less bulky
N,N'-dialkylated thione 6b to yield the
olefin 8 after reduction with Na/K.
Carbene 4 can be coordinated to a
W(CO)5 fragment yielding the complex
9, which contains a nonplanar carbene
ring.
Keywords: carbenes ´ carbene com-
plexes ´ NMR spectroscopy ´ tung-
sten
Introduction
Stable, N-heterocyclic carbenes, which were first proposed by
Wanzlick about 40 years ago,[1] have been intensely studied[2]
since the isolation and characterization of 1,3-bis(adamant-
yl)imidazolin-2-ylidene in 1991.[3] In the meantime, stable
imidazolin-2-ylidenes (1),[4] triazol-5-ylidene (2),[5] imidazoli-
din-2-ylidenes (3),[6] (Figure 1) and other N,S-[7] and acyclic
N,N'-stabilized carbenes[8] have been reported. Stable car-
benes derived from annelated N-heterocyclic five-membered
rings are hitherto unknown despite the successful stabilization
of N,N'-dialkylated benzimidazolin-2-ylidenes[9] and benzox-
azolin-2-ylidenes[10] in metal complexes. We report here the
preparation and molecular structure of 1,3-bis(2,2-dimethyl-
propyl)benzimidazolin-2-ylidene (4) (Figure 1), which is the
first stable free carbene derived from benzimidazole,[11]
together with the crystal structure of its tungsten complex
[W(CO)5(4)] 9.
Figure 1. Stable N-heterocyclic carbenes.
Results and Discussion
The synthesis of carbenes of type 4 containing an annelated
N-heterocyclic five-membered ring by methods described by
Arduengo et al.[4a,b] or Kuhn et al.[4c] is hampered by two
factors; i) the deprotonation of sterically less bulky substi-
tuted N,N'-dialkylated benzimidazolium ions with strong
bases like nBuLi, tBuLi, or NaH in THF, in contrast to the
corresponding imidazolium ions, does not yield stable car-
benes but instead C2 C2 connected olefins,[12] and ii) the
preparation of bulky (tBu, neopentyl) substituted N,N'-
dialkylated benzimidazolium salts starting from benzimid-
[a] Prof. Dr. F. E. Hahn, Dipl.-Chem. L. Wittenbecher
Anorganisch-chemisches Institut der Universität
Wilhelm-Klemm-Strasse 8, D-48149 Münster (Germany)
Fax: (‡49)251-83-33108
E-mail: FEHahn@nwz.uni-muenster.de
[b] Prof. Dr. R. Boese, Dr. D. Bläser
Institut für Anorganische Chemie, Universität-Gesamthochschule-
Essen, Universitätsstrasse 5 ± 7, D-45117 Essen (Germany)
E-mail: boese@structchem.uni-essen.de
Chem. Eur. J. 1999, 5, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0506-1931 $ 17.50+.50/0
1931

F. E. Hahn et al.
FULL PAPER
Table 1. Selected spectroscopic and geometric parameters for N-hetero-
cyclic carbenes.[a]
azole, which cannot dimerize after C2 deprotonation, proved
unsuccessful to date.
Carbene
C2
13C NMR [d]
N1-C2-N3
Angle
Ref.
For the synthesis of stable carbenes with a benzimidazole
frame, a new synthetic strategy was developed which is
outlined in Scheme 1. For the preparation of bulky substituted
o-phenylenediamines of type 5 we used a procedure originally
developed by Lappert et al. for the synthesis of N-heterocyclic
stannylenes.[13] The dialkylated o-phenylenediamines N,N'-
bis(2,2-dimethylpropyl)-1,2-diaminobenzene (5a) and N,N'-
dimethyl-1,2-diaminobenzene (5b) were subsequently treated
with thiophosgene to yield the thiones 6a and 6b, respecti-
vely.[4c]
The reduction of the thione 6a with Na/K in THF (reaction
time 20 days) leads, in contrast to the same reaction with non
annelated imidazol-2-thiones,[4c] not to a carbene, but instead
to the benzimidazoline 7. Compound 7 was identified by NMR
spectroscopy and by comparison with an authentic sample
1a
1b
1c
1d
1e
1 f
1g
1h
2
211.4
215.2
215.8
219.7
216.3
213.7
211.1
205.9
214.6
244.5
238.2
231.5
198.9
102.2(2)
±
3
4a
4a
4a
4a
4a, 4b
4c
4c
101.2(1)
101.4(2)
101.7(1)
101.5(1)
±
±
100.6(2)
104.7(3)
106.44(9)
103.5(1), 104.3(1)
103(1), 103(1)
5
6a
6b
3a
3b
4
this study
this study
9
[a] The numbering scheme for carbenes is illustrated in Figure 1.
carbene carbon atom in car-
benes of type 3 relative to type
1 was attributed to the lack of
aromatic stabilization in the
saturated N-heterocyclic sys-
tems.[6] Carbene 4 exhibits an
unsaturated
N-heterocyclic
five-membered ring containing
formally 6p electrons, yet its
13C NMR spectrum indicates
that it behaves like a saturated
derivative of the imidazolidin-
2-ylidene type 3 which lacks
aromatic stabilization.
Further evidence for an un-
usual electronic situation in 4 is
furnished by the X-ray crystal
structure analysis of crystals of
4. The asymmetric unit contains
two, within experimental error,
identical, enantiomeric, C2-chi-
ral molecules of 4 (Figure 2).
The geometric parameters of
Scheme 1. Synthesis of 4, 7, and 8.
the five-membered ring in 4 are
typical for imidazolin-2-yli-
obtained by reaction of 5a with bis(dimethylamino)methane.
We assume, that during the relatively long reaction time of 6a
with Na/K in THF reduction of THF occurs, leading to
hydrogenation of the intermediately formed carbene.
Reduction of 6a with Na/K in the more inert solvent
toluene leads to carbene 4 in 60% yield. The reaction, which
takes about 20 days, can be monitored by 13C NMR spectro-
scopy: the resonance for the thione carbon atom (d ˆ 175.24)
vanishes and the low-field shifted resonance for the carbene
carbon atom appears at d ˆ 231.47.
denes of type 1. One notheworthy exception is the enlarged
N1-C2-N3 angle (103.5(1)8 and 104.3(1)8) in 4 (Table 1),
which falls in the range normally observed for saturated
N-heterocyclic carbenes of type 3. The 13C NMR spectra and
the N1-C2-N3 angle of 4 indicate that the unsaturated carbene
4 actually behaves like a saturated N-heterocyclic carbene of
type 3. The main difference between 4 and saturated carbenes
of type 3 is the length of the C C bond in the five-membered
ring. Saturated carbenes possess a C C single bond [d(C C)
1.505(6) Š for 3a, 1.512(2) Š for 3b], while 4 exhibits the
topology of an unsaturated five-membered ring with a C C
double bond. The long C C single bond in carbenes of type 3
has been used to explain the widening of the N1-C2-N3 angle
in saturated carbenes relative to their unsaturated analog-
ues.[6a] Since 4 exhibits a C C double bond in the five-
membered ring and still shows an enlarged N1-C2-N3 angle
Surprisingly, the resonance for the carbene carbon atom in
4 does not fall in the range normally observed for unsaturated
N-heterocyclic carbenes of the imidazolin-2-ylidene type 1
(dC(2) ꢀ 215), but is observed instead in the range typical for
saturated N-heterocyclic carbenes of the imidazolidin-2-
ylidene type 3 (dC(2) ꢀ 240, Table 1). The deshielding of the
1932
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0506-1932 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 6

N-Heterocyclic Carbenes
1931±1935
of our proposal that unsaturated carbenes derived from
benzimidazole exhibit the topology of an unsaturated N-het-
erocyclic carbene but show spectroscopic and structural
properties and the reactivity of carbenes with a saturated
N-heterocyclic ring.
Not only carbene 4 itself, but also its metal complexes show
remarkable properties. Reaction of 4 with [W(CO)5(thf)]
leads to the air-stable complex [W(CO)5(4)] 9. The X-ray
structure analysis of crystals of 9 (Figure 3) shows that the
ligand 4 has to undergo a conformational change with the loss
Figure 2. ORTEP plot of the two crystallographically independent mole-
cules of 4 in the asymmetric unit (molecule A top, molecule B bottom).
Selected bond lengths [Š] and angles [8] for molecule A [molecule B]:
N1 C2 1.361(2) [1.360(2)], N1 C5 1.397(2) [1.394(2)], N1 C10 1.458(2)
[1.458(2)], N3 C2 1.374(2) [1.368(2)], N3 C4 1.388(2) [1.397(2)], N3 C15
1.456(2) [1.457(2)], C4 C5 1.395(2) [1.386(2)]; C2-N1-C5, 112.80(12)
[111.99(14)], C2-N1-C10 122.32(14) [123.1(2)], C5-N1-C10 124.88(13)
[124.89(14)], C2-N3-C4 112.36(13) [111.83(14)], C2-N3-C15 121.61(14)
[122.4(2)], C4-N3-C15 125.94(14) [125.8(2)], N1-C2-N3 103.49(13)
[104.26(14)].
Figure 3. ORTEP plot of the two crystallographically independent mole-
cules of 9 in the asymmetric unit (molecule A top, molecule B bottom).
Selected bond length [Š] and angles [8] for molecule A [molecule B]:
W C2 2.27(1) [2.29(1)], N1 C2 1.42(2) [1.38(2)], N1 C5 1.37(2) [1.40(2)],
N3 C2 1.33(2) [1.36(2)], N3 C4 1.39(2) [1.39(2)], C4 C5 1.35(2) [1.36(2)];
N1-C2-N3 103(1) [1.03(1)].
relative to unsaturated N-heterocyclic carbenes, this explan-
ation now appears less satisfactory.
The assumption that stable, unsaturated carbenes derived
from benzimidazole exhibit properties comparable to those of
saturated imidazolin-2-ylidenes of type 3 is also corroborated
by the reactivity of the N,N'-dimethyl-substituted derivative
of 4 (Scheme 1). Wanzlick[1] noted more than 30 years ago that
saturated carbenes of type 3 dimerize rapidly to the corre-
sponding olefins if the substituents at the nitrogen atoms
allow this geometrically, an observation which was only
recently confirmed.[6b] This dimerization is not observed for
unsaturated carbenes of type 1. It can, however, be enforced
by geometric constraints.[14] No tendency for dimerization was
observed for 4, which we attribute to the steric bulk at the
nitrogen atoms. However, reduction of the N,N'-dimethyl-
substituted thione 6b with Na/K leads not to a carbene, but
instead to the olefin 8. The same product was obtained by
Lappert et al. upon deprotonation of N,N'-dimethylbenzimi-
dazolium iodide with NaH.[12a] In both cases, the rapid
dimerization of the most likely intermediately formed car-
bene is typical for saturated N-heterocyclic carbenes of the
imidazolidin-2-ylidene type 3. We take this as a confirmation
of the C2-chirality for coordination. In coordinated 4 both
neopentyl groups are positioned on the same side of the
N-heterocycle. This minimizes the steric interaction with the
CO groups at the tungsten atom. However, two of the C2-W-
C
cis angles (C20, C21) are still larger than 908, while the other
two C2-W-Ccis angles (C22, C23) are smaller than 908. The
carbene carbon atom (C2) does not reside in the plane of the
N-heterocyclic ring but is shifted from that plane by
0.207(11) Š (molecule A) or 0.190(11) Š (molecule B)
towards the side of the ring, which is not occupied by the
neopentyl groups. This geometric situation allows 4 to
coordinate tightly to the W(CO)5 fragment inspite of the
steric bulk at the nitrogen atoms. This is also confirmed by the
W C2 distances of 2.27(1) Š (molecule A) and 2.29(1) Š
(molecule B), which are in the typical range of W(CO)5
complexes with N-heterocyclic carbenes.[15]
Chem. Eur. J. 1999, 5, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0506-1933 $ 17.50+.50/0
1933

F. E. Hahn et al.
FULL PAPER
Tetramethyltetraazafulvalene (8): Sodium (0.17 g, 7.4 mmol) and potassium
(0.58 g, 14.9 mmol) were melted under vacuum. The resulting alloy was
cooled to 08C and treated with a solution of 6b (0.43 g, 2.41 mmol) in dry
toluene (40 mL). The mixture was stirred for 20 days at room temperature.
The cloudy solution was then filtered and the solvent was removed under
reduced pressure yielding pure 8 (0.29 g, 83%) as a colorless oil, which
crystallized over a period of two days. 1H NMR (250 MHz, C6D6): d ˆ 6.86
(m, 4H, Ar-H), 6.47 (m, 4H, Ar-H), 2.73 (s, 12H, CH3); 13C NMR
(62.9 MHz,C6D6): d ˆ 143.61 (Ar-C), 124.42 (N-C-N) 121.04, 108.59 (Ar-C),
Experimental Section
General remarks: 1H and 13C NMR spectra were recorded on a Bruker AC
200 or Bruker AM 250 spectrometer. Elemental analyses were obtained
with a Vario EL Elemental Analyzer at the Freie Universität Berlin. Mass
spectra (EI, 70 eV) were taken on a Finnigan MAT 112 spectrometer. Air-
sensitive compounds were prepared and handled under argon by Schlenk
techniques. All solvents used were rigorously dried (typically over Na or
Na/K and benzophenone) and freshly distilled prior to use. X-ray data sets
were collected on a Siemens SMART CCD area detector system at
293(2) K (in a sealed capillary for air-sensitive 4) or on a CAD-4 counter
diffractometer at 298(2) K (for air-stable 9).The programs used were
Siemens SHELXL-97 UNIX Version, Release 97-1 for 4 and MolEN for 9.
Molecular structure plots were generated with ORTEP. Further details on
the crystal structure investigations may be obtained from the Fachinfor-
mationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany
(fax: (‡49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting
the depository numbers CSD-408863 and CSD-59646.
‡
35.98 (CH3); MS (EI) m/z (%): 292 (10) [M ], 147 (100).
N,N'-Bis(2,2-dimethylpropyl)-benzimidazolin-2-ylidene-pentacarbonyl-
tungsten (9): A solution of [W(CO)6] (1.41 g, 4.0 mmol) in THF (100 mL)
was irridiated (mercury-vapor lamp) for 5 h at room temperature. Then a
solution of 4 (0.42 g, 1.63 mmol) was added dropwise. After the mixture had
been stirred at room temperature overnight, the solvent was removed and
the residue was purified by column chromatography (SiO2, hexane/
dichlormethane 6:1, v:v). Yield 0.81 g (85%, relative to 4) of pale yellow
crystals. 1H NMR (200 MHz, CDCl3): d ˆ 7.40 (m, 2H, Ar-H), 7.14 (m, 2H,
Ar-H), 4.51 (s, 4H, CH2), 0.98 (s, 18H, CH3); 13C NMR (62.9 MHz, CDCl3):
d ˆ 200.47 (trans-CO), 198.25 (cis-CO), 196.86 (N-C-N), 135.45, 122.30,
112.65 (Ar-C), 60.30 (CH2), 34.72 (CMe3) 29.08 (CH3).
N,N-Bis(2,2-dimethylpropyl)-1,2-diaminobenzene (5a): The compound
was prepared according to the method described by Lappert et al.[13]
Colorless oil. 1H NMR (250 MHz, CDCl3): d ˆ 6.80 (m, 2H, Ar-H), 6.65
(m, 2H, Ar-H), 3.40 (s, 2H, NH), 2.81 (s, 4H, CH2), 1.05 (s, 18H, CH3);
13C NMR (62.9 MHz, CDCl3): d ˆ 138.33, 119.15, 112.09 (Ar-C), 56.41
(CH2), 31.44 (CMe3), 27.81 (CH3).
X-ray structure analysis of N,N-Bis(2,2-dimethylpropyl)-benzimidazolin-2-
ylidene (4): Suitable crystals of 4 were obtained by stripping the solvent
from a solution of the compound in toluene. The oily product crystallizes
upon standing for one week. The crystals melt at 268C. The data crystal was
mounted in a glass capillary. Compound 4 crystallizes with two independent
molecules in the asymmetric unit. Formula C17H26N2, Mrˆ 258.40,
0.84  0.53  0.45 mm3, a ˆ 10.1057(3), b ˆ 10.6191(4), c ˆ 16.7234(6) Š,
N,N'-Dimethyl-1,2-diaminobenzene (5b): The compound was prepared as
reported in ref. [16]. Colorless crystals (m.p. 22 ± 24 oC). 1H NMR
(250 MHz, CDCl3): d ˆ 6.83 (m, 2H, Ar-H), 6.59 (m, 2H, Ar-H), 3.20 (s,
2H, NH), 2.63 (s, 6H, CH3); 13C NMR (62.9 MHz, CDCl3): d ˆ 137.94,
118.59, 109.97 (Ar-C), 30.61 (CH3).
a ˆ 77.50(1), b ˆ 73.72(1), g ˆ 73.452(1)o, Vˆ 1633.0(1) Š3, 1calcd
ˆ
1
1.051 gcm 3, m(MoKa) ˆ 0.061 mm , Z ˆ 4, triclinic, space group P1 (no.
2), MoKa radiation ˆ 0.71073 Š, Tˆ 293(2) K, w scans, 12060 refelctions
collected, 2V range 4.04 ± 51.46o, 5238 independent and 3864 observed
reflections [I > 2(I)], 343 refined parameters (refinement on F2), R ˆ
0.0534, wR2 ˆ 0.1472, max. (min.) residual electron density 0.215
Å
N,N'-Bis(2,2-dimethylpropyl)-benzimidazolin-2-thione (6a): A stirred sol-
ution of 5a (1.2 g, 4.83 mmol) in THF (60 mL) was treated with triethyl-
amine (5.4 mL, 38.7 mmol). The solution was cooled to
158C and
thiophosgene (0.55 mL, 7.21 mmol) was added dropwise under formation
of a yellow precipitate. The solution was allowed to warm to room
temperature and stirred for another 14 h. Subsequently the solvent was
removed and the dark orange residue was extracted with diethyl ether (8 Â
10 mL). The solvent was removed and the oily product was purified by
column chromatography (SiO2, hexane/ethyl acetate, 10:1, v:v). Yield
1.23 g (89%) of a colorless oil. 1H NMR (250 MHz, [D8] THF): d ˆ 7.44 (m,
2H, Ar-H), 7.26 (m, 2H, Ar-H), 4.40 (s, 4H, CH2), 1.22 (s, 18H, CH3);
13C NMR (62.9 MHz, [D8] THF): d ˆ 175.24 (C ˆ S), 134.37, 122.77, 111.05
(Ar-C), 55.43 (CH2), 36.24 (CMe3), 29.60 (CH3).
(
0.179) eŠ 3, hydrogen atoms on calculated positions.
X-ray structure analysis of 9: Suitable crystals of 9 were grown by
evaporation of a CH2Cl2/hexane (5:1, v:v) solution. The crystals are air-
stable. Compound 9 crystallizes with two independent molecules in the
asymmetric unit. Formula C22H26N2O5W, Mr ˆ 582.31, 0.4  0.2  0.2 mm3,
a ˆ 7.030(2), b ˆ 39.552(14), c ˆ 17.160(9) Š, b ˆ 101.50(3)o, Vˆ 4676(6) Š3,
1calcd ˆ 1.654 gcm 3, m(MoKa) ˆ 5.078 mm 1, Z ˆ 8, monoclinic, space group
Cc (no. 9), MoKa radiation ˆ 0.71073 Š, Tˆ 298(2) K, w scans, 6495 unique
reflections collected, 2V range 2.0 ± 46.0o, 4922 observed reflections [F2 >
3(F2)], 539 refined parameters (refinement on F), R ˆ 0.0371, wR2 ˆ 0.0410,
max. (min.) residual electron density 1.82 ( 0.39) eŠ 3, hydrogen atoms
on calculated positions.
N,N'-Dimethylbenzimidazolin-2-thione (6b): Compound 6b was synthe-
sized from 5b (0.87 g, 6.4 mmol), triethylamine (7.12 mL, 51.1 mmol), and
thiophosgene (0.73 mL, 9.6 mmol) as described for 6a. Column chroma-
tography (SiO2, hexane/ethyl acetate, 8:1, v:v) yielded 0.93 g (82%) of 6b
as white powder. 1H NMR (250 MHz, [D8] THF): d ˆ 7.15 (m, 4H, Ar-H),
3.68 (s, 6H, CH3); 13C NMR (62.9 MHz, [D8] THF): d ˆ 171.64 (C ˆ S),
133.32 123.15, 109.36 (Ar-C), 31.17 (CH3).
N,N'-Bis(2,2-dimethylpropyl)-benzimidazoline (7): Compound 7 was ini-
tially obtained by Na/K reduction of 6a in THF over a reaction time of
20 days. An authentic sample of 7 was synthesized by heating a mixture of
5a (0.62 g, 2.5 mmol) and bis(dimethylamino)methane (1.0 g, 9.8 mmol) in
diethylether (40 mL) in the presence of a trace (0.03 g) of p-toluenesulfonic
acid under reflux for 6 h. After standard workup 0.19 g (88%) of a yellow
solid was obtained. 1H NMR (250 MHz, C6D6): d ˆ 6.81 (m, 2H, Ar-H),
6.38 (m, 2H, Ar-H), 4.54 (s, 2H, N-CH2-N), 2.53 (s, 4H, CH2), 0.86 (s, 18H,
CH3); 13C NMR (62.9 MHz, C6D6): d ˆ 143.67, 118.92, 105.47 (Ar-C), 83.19
(N-C-N), 64.19 (CH2), 33.66 (CMe3), 27.91 (CH3).
Acknowledgment
This work was financially supported by the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie. We thank Mrs. T. Pape,
Freie Universität Berlin, for technical assistance during the synthetic work.
[1] H.-W. Wanzlick, Angew. Chem. 1962, 7, 129; Angew. Chem. Int. Ed.
Engl. 1962, 1, 75.
N,N'-Bis(2,2-dimethylpropyl)-benzimidazolin-2-ylidene
(4):
Sodium
[2] a) M. Regitz, Angew. Chem. 1996, 108, 791; Angew. Chem. Int. Ed.
Engl. 1996, 35, 725; b) W. A. Herrmann, C. Köcher, Angew. Chem.
1997, 109, 2257; Angew. Chem. Int. Ed. Engl. 1997, 36, 2162.
[3] A. J. Arduengo, III, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991,
113, 361.
[4] 1a: ref. [3]; a) 1b ± 1 f: A. J. Arduengo, III, H. V. Rasika Dias, R. L.
Harlow, M. Kline, J. Am. Chem. Soc. 1992, 114, 5530. b) The X-ray and
neutron diffraction analysis of perdeuterated 1 f was reported by A. J.
Arduengo, III, H. V. Rasika Dias, D. A. Dixon, R. L. Harlow, W. T.
Klooster, T. F. Koetzle, J. Am.Chem. Soc. 1994, 116, 6812; c) 1g ± 1h:
N. Kuhn, T. Kratz, Synthesis 1993, 561.
(0.08 g, 3.5 mmol) and potassium (0.28 g, 7.2 mmol) were melted together
under vacuum. The resulting alloy was cooled to 08C and treated with a
solution of 6a (0.31 g, 1.07 mmol) in dry toluene (30 mL). The mixture was
stirred for 20 days at room temperature. The cloudy solution was then
filtered and the solvent was removed under reduced pressure. Analytically
pure 4 was obtained as a colorless oil, which crystallized over a period of
one week upon standing. Yield 0.21 g (76%), m.p. 268C. 1H NMR
(250 MHz, [D8]THF): d ˆ 6.95 (m, 4H, Ar-H), 3.98 (s, 4H, CH2), 1.01 (s,
18H, CH3); 13C NMR (62.9 MHz, [D8]THF): d ˆ 231.47 (N-C-N), 136.39,
120.77, 110.82 (Ar-C), 58.74 (CH2), 33.82 (CMe3), 28.49 (CH3).
1934
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0506-1934 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 6

N-Heterocyclic Carbenes
1931±1935
[5] D. Enders, K. Breuer, G. Raabe, J. Runsik, J. H. Teles, J.-P. Melder, K.
Ebel, S. Brode, Angew. Chem. 1995, 107, 1119; Angew. Chem. Int. Ed.
Engl. 1995, 34, 1021.
[6] a) A. J. Arduengo, III, J. R. Goerlich, W. J. Marshall, J.
Am. Chem. Soc. 1995, 117, 11027; b) M. K. Denk, A. Thadani, K.
Hatano, A. J. Lough, Angew. Chem. 1997, 109, 2719; Angew. Chem.
Int. Ed. Engl. 1997, 36, 2067.
[7] a) A. J. Arduengo, III, J. R. Goerlich, W. J. Marshall, Liebigs Ann
1997, 365; b) G. Boche, C. Hilf, K. Harms, M. Marsch, J. C. W.
Lohrenz, Angew. Chem. 1995, 107, 509; Angew. Chem. Int. Ed. Engl.
1995, 34, 487.
[8] R. W. Alder, P. R. Allen, M. Murray, A. G. Orpen, Angew. Chem.
1996, 108, 1211; Angew. Chem. Int. Ed. Engl. 1996, 35, 1121.
[9] a) K. M. Lee, C. K. Lee, I. J. B. Lin, Angew. Chem. 1997, 109, 1936;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1850; b) K. Öfele, W. A.
Herrmann, D. Mihalios, M. Elison, E. Herdtweck, T. Priermeier, P.
Kiprof, J. Organomet. Chem. 1995, 498, 1.
metallics, 1997, 16, 763; h) U. Kernbach, T. Lügger, F. E. Hahn, W. P.
Fehlhammer, J. Organomet. Chem. 1997, 541, 51; i) C. Lambert, I.
Lopez-Solera, P. R. Raithby, Organometallics, 1996, 15, 452; j) J.
Barluenga, F. Aznar, B. Weyershausen, S. Garcia-Granda, E. Martin,
Chem. Commun. 1996, 2455.
[11] During the preparation of this manuscript we became aware, that Prof.
M. Lappert, University of Sussex at Brighton had prepared 4 in the
same manner as we did and stopped his efforts to crystallize the
carbene, when he became aware of our work. See also Organometallic
Chemistry in the South Pacific, 24 ± 28.1.1999, Book of Abstracts, L21
[12] a) E. Cetikkaya, P. B. Hitchcock, H. Kücükbay, M. F. Lappert, S. Juaid,
J. Organomet. Chem. 1994, 481, 89; b) Z. Shi, R. P. Thummel, J. Org.
Chem. 1995, 60, 5935.
[13] H. Braunschweig, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, Z.
Anorg. Allg. Chem. 1995, 621, 1922.
[14] T. A. Taton, P. Chen, Angew. Chem. 1996, 108, 1098; Angew. Chem.
Int. Ed. Engl. 1996, 35, 1011.
[10] a) F. E. Hahn, M. Tamm, J. Chem. Soc. Chem. Commun. 1993, 482;
b) F. E. Hahn, M. Tamm, J. Organomet. Chem. 1993, 456, C11; c) F. E.
Hahn, M. Tamm, T. Lügger, Angew. Chem. 1994, 106, 1419; Angew.
Chem. Int. Ed. Engl. 1994, 33, 1356; d) F. E. Hahn, M. Tamm, J. Chem.
Soc. Chem. Commun. 1995, 569; e) F. E. Hahn, M. Tamm, Organo-
metallics 1995, 14, 2597; f) M. Tamm, T. Lügger, F. E. Hahn,
Organometallics, 1996, 15, 1251; g) F. E. Hahn, L. Imhof, Organo-
[15] M. Tamm, A. Grzegorzewski, F. E. Hahn, J. Organomet Chem. 1995,
501, 309.
[16] F. A. Neugebauer, B. Funk, H. A. Staab, Tetrahedron Lett. 1994, 35,
4755.
Received: March 1, 1999 [F1639]
Chem. Eur. J. 1999, 5, No. 6
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0506-1935 $ 17.50+.50/0
1935