Stable carbenes. Synthesis and properties of benzimidazol-2-ylidenes

Article abstract of DOI:10.1134/S1070428006120116

A new stable crystalline carbene, 1,3-bis(1-adamantyl)benzimidazol-2- ylidene, was synthesized by decomposition of 1,3-bis(1-adamantyl)-2,3-dihydro- 1H-benzimidazol-2-ylacetonitrile on heating under reduced pressure. Heteroaromatic 1,3-R₂-disub

Full text of DOI:10.1134/S1070428006120116

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2006, Vol. 42, No. 12, pp. 1822–1833. © Pleiades Publishing, Inc., 2006.
Original Russian Text © N.I. Korotkikh, G. F. Raenko, T.M. Pekhtereva, O.P. Shvaika, A.H. Cowley, J.N. Jones, 2006, published in Zhurnal Organicheskoi
Khimii, 2006, Vol. 42, No. 12, pp. 1833–1843.
Stable Carbenes. Synthesis and Properties
of Benzimidazol-2-ylidenes
N. I. Korotkikha, G. F. Raenkoa, T. M. Pekhterevaa, O. P. Shvaikaa,
A. H. Cowleyb, and J. N. Jonesb
a Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine,
ul. R. Lyuksemburg 70, Donetsk, 83114 Ukraine
b Texas University, Austin, USA
Received November 2, 2005
Abstract—A new stable crystalline carbene, 1,3-bis(1-adamantyl)benzimidazol-2-ylidene, was synthesized by
decomposition of 1,3-bis(1-adamantyl)-2,3-dihydro-1H-benzimidazol-2-ylacetonitrile on heating under reduced
pressure. Heteroaromatic 1,3-R2-disubstituted benzimidazol-2-ylidenes, both stable (R = 1-Ad) and generated
in situ (R = Me, Bzl), as well as 1,3,4,5-tetraphenylimidazol-2-ylidene (generated in situ), reacted with aceto-
nitrile to give the corresponding insertion products, 1,3-disubstituted 2-cyanomethyl-2,3-dihidro-1H-(benz)-
imidazoles. The geometric parameters of 1,3-bis(1-adamantyl)benzimidazol-2-ylidene, determined by X-ray
analysis, suggest its lower aromaticity as compared to imidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes. The
structures of 2-cyanomethyl-2,3-dihydro-1H-benzimidazoles, 2-cyanomethyl-1,3,4,5-tetraphenyl-2,3-dihydro-
1H-imidazole, and 1-(1-adamantyl)-5-cyanomethyl-3,4-diphenyl-4,5-dihydro-1H-1,2,4-triazole are character-
ized by partial conjugation in the heteroring; some compounds exhibit luminescence under UV irradiation.
1,3-Bis(1-adamantyl)benzimidazol-2-ylidene reacted with molecular sulfur in benzene to give 1,3-bis-
(1-adamantyl)-2,3-dihydro-1H-benzimidazole-2-thione, but it failed to react with selenium under analogous
conditions.
DOI: 10.1134/S1070428006120116
posed an improved synthesis of imidazol-2-ylidenes
via reduction of imidazole-2(3H)-thiones with potas-
sium in boiling THF. Important steps in this line were
also syntheses of saturated heterocyclic carbenes, acy-
clic carbenes, and some multidentate systems [6].
A long and important period including experiments
with carbenes generated in situ [1, 2] has come to the
end when the first stable nucleophilic carbene, 1,3-bis-
(1-adamantyl)imidazol-2-ylidene, has been isolated
[3]. It was synthesized by deprotonation of 1,3-bis(1-
adamantyl)imidazolium chloride with sodium or potas-
sium hydride in the presence of t-BuOK or dimsyl
anion [4]. Following analogous approaches, several
research teams later synthesized new stable heteroaro-
matic carbenes of the triazole [5] and some other series
[6]. The procedures were based on deprotonation of
the corresponding azolium salts by the action of such
bases as lithium diisopropylamide [7], butyllithium
[8], and potassium hexamethyldisilylamide [9]. Her-
mann et al. [10] succeeded in effecting deprotonation
of imidazolium salts with sodium hydride at a rela-
tively low temperature using the system liquid ammo-
nia–tetrahydrofuran as solvent. Stable 1,3,4-triphenyl-
4,5-dihydro-1H-1,2,4-triazol-5-ylidene was obtained
by decomposition of intermediate 5-methoxy-4,5-di-
hydro-1H-1,2,4-triazole [5]. Kuhn and Kratz [11] pro-
We previously reported [12, 13] that C–H-inser-
tions of heteroaromatic benzimidazol-2-ylidenes gen-
erated in situ with acetonitrile lead to the formation of
the corresponding 2-cyanomethyl derivatives. A short
time later, Arduengo et al. [14] described analogous
reactions of nonaromatic 1,3-dimesityl-4,5-dihydro-
imidazol-2-ylidene with acetylene, dimethyl sulfone,
and acetonitrile [14]. On the other hand, 1,3-dimesityl-
and 1,3-diadamantyl-substituted imidazol-2-ylidenes
failed to react with the same compounds. We presumed
that the above dihydrodiazole derivatives may be inter-
mediate products in the thermal synthesis of stable
heteroaromatic carbenes. The first results obtained in
this line were briefly reported in [13]. We also found
that deprotonation of 1,2,4-triazolium salts with
sodium hydride in acetonitrile can be used for direct
1822

STABLE CARBENES.
1823
synthesis of 1,2,4-triazol-5-ylidenes [15]. Under more
severe conditions (100°C), 1,2,4-triazol-5-ylidenes did
react with acetonitrile to produce 5-cyanomethyl-4,5-
dihydro-1H-1,2,4-triazoles.
nitric acid according to [17], followed by anion ex-
change reaction.
We have found that benzimidazolium salts Ia–Ic
and imidazolium salt II having no substituent at the
2-position react with sodium hydride in acetonitrile at
room temperature to give products of carbene insertion
into the C–H bond of acetonitrile, 2-cyanomethyldi-
hydroazoles IIIa–IIIc and IV. Compounds IIIa–IIIc
and IV were isolated as crystalline substances and
were characterized by the data of elemental analysis
With a view to estimate the possibility for prepara-
tion of stable carbenes of the benzimidazole and
related imidazole series, as well of dihydroazole struc-
tures based thereon, we examined reactions of a num-
ber of benzimidazolium and imidazolium salts with
sodium hydride in acetonitrile. In the present publica-
tion we report the following main results: (1) a new
synthesis of a stable carbene, 1,3-bis(1-adamantyl)-
benzimidazol-2-ylidene; (2) detailed synthesis of
1,3-disubstituted 2-cyanomethyl-2,3-dihydro-1H-benz-
imidazoles and 2-cyanomethyl-1,3,4,5-tetraphenyl-2,3-
dihydro-1H-imidazole; (3) X-ray diffraction data for
1,3-bis(1-adamantyl)benzimidazol-2-ylidene, carbene
precursors [the corresponding benzimidazolium salt,
1,3-disubstituted 2-cyanomethyl-2,3-dihydro-1H-benz-
imidazoles, 2-cyanomethyl-1,3,4,5-tetraphenyl-2,3-di-
hydro-1H-imidazole, 1-(1-adamantyl)-2-cyanomethyl-
3,4-diphenyl-4,5-dihydro-1H-1,2,4-triazole], and
1,3-bis(1-adamantyl)- and 1,3-dibenzylimidazole-2-
thiones; and (4) reactions of 1,3-bis(1-adamantyl)benz-
imidazol-2-ylidene with acetonitrile and sulfur.
1
and IR and H and 13C NMR spectroscopy (see
1
Experimental). In the H NMR spectra of IIIa, IIIb,
and IV, signals from protons in the CH–CH2 fragment
appeared as doublets at δ 2.67–3.23 ppm (CH2) and
3
triplets at δ 4.27–5.23 ppm (CH, J = 2.8–6.0 Hz).
Their 13C NMR spectra contained signals from aromat-
ic carbon atoms (δC 106.5–148.6 ppm) and carbon
atoms of the cyano (δC 116.3–122.4 ppm), CHN
(δC 86.3–86.7 ppm), and CH2 groups (δC 27.8–
34.9 ppm). Compounds IIIa, IIIb, and IV showed in
the IR spectra (Nujol) absorption bands corresponding
to aromatic C=C (1595–1600, 1480–1495 cm–1) and
C–H bonds (3045–3070 cm–1) and C≡N group (2235–
2240 cm–1). The purity of IIIa, IIIb, and IV was
checked by TLC and 1H NMR.
As carbene precursors we used the corresponding
azolium salts. Benzimidazolium perchlorates Ia–Ic
were synthesized in two steps. Initially, the corre-
sponding benzimidazolium halides were prepared by
alkylation of azoles with alkyl halides (methyl iodide,
benzyl chloride, and 1-adamantyl bromide) in metha-
nol (Ia) or in acetic acid in the presence of sodium
acetate (Ib, Ic) or by reaction of 1-(1-adamantyl)-1H-
benzimidazole with 1-adamantyl bromide in dichloro-
benzene (Ic). The synthesis of adamantyl-substituted
imidazoles, benzimidazoles, and benzimidazolium
salts was described in [16]. The second step was anion
exchange reaction performed by treatment of azolium
halides with an aqueous solution of sodium perchlo-
rate. Crystallization of benzimidazolium perchlorate Ic
from dichloroethane gave crystals suitable for X-ray
analysis. Salt II was obtained by oxidation of 1,3,4,5-
tetraphenyl-2,3-dihydro-1H-imidazole-2-thione with
As shown above, carbene insertion into the C–H
bond of acetonitrile occurs even in the reaction with
sterically hindered 1,3-(1-adamantyl)benzimidazolium
salt Ic where the C2–H bond is shielded by bulky
adamantane fragments. However, the product whose
composition corresponded to 2-cyanomethyl derivative
IIIc resembled ionic compounds in its chromatograph-
ic behavior. Like ionic species, it is poorly soluble in
organic solvents (even in such a strongly polar solvent
as DMSO). Compound IIIc underwent decomposition
on heating in benzene or DMSO. The spectral param-
eters of IIIc somewhat differed from those of com-
1
pounds IIIa, IIIb, and IV. Although the H NMR
spectrum of benzimidazole IIIc in DMSO-d6 contained
a triplet at δ 5.27 ppm (CH), as in the spectra of IIIb
and IV, the CH2CN signal appeared in a stronger field
(δ 1.98 ppm) as a doublet (J = 5.8 Hz). Considerable
shielding of the CH2 protons may be caused by strong
R
Ph
R
Ph
Ad-1
Ad-1
Ph
Ph
Ph
Ph
N
N
N
N
N
N
N
N
X–
X–
N
N
N
N
N
N
CN
CN
CN
Ph
Ph
R
Ph
R
Ph
IV
Ph
R
Ia–Ic
II
IIIa–IIIc
V
VI
I, III, R = Me (a), Bzl (b), 1-Ad (c); VI, R = p-BrC6H4; X = ClO4.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

KOROTKIKH et al.
1824
Table 1. Selected bond lengths (pm) and bond and torsion
angles (deg) in the molecules of 1,3-dibenzyl-2,3-dihydro-
1H-benzimidazol-2-ylacetonitrile (IIIb), 1,3-bis(1-ada-
mantyl)-2,3-dihydro-1H-benzimidazol-2-ylacetonitrile
(IIIc), and 1,3,4,5-tetraphenyl-2,3-dihydro-1H-imidazol-
2-ylacetonitrile (IV)
IIIb (Δδ = 0.4 ppm) and in a stronger field relative to
those of salt Ic (Δδ = 0.8–1.2 ppm). An analogous pat-
tern was observed in the spectra of some dihydroazoles
having no substituent in the 2-position, as compared to
the corresponding azolium salts [18, 19]. The C2 signal
in the 13C NMR spectrum of Ic is displaced upfield
relative to analogous signals of IIIa, IIIb, and IV
(ΔδC = 15.1–15.5 ppm), while the chemical shifts of
the CN and CH2 carbon nuclei are similar to those in
IIIa and IV. These data indicate anisotropic shielding
of the C2 atom and CH2 protons in molecule IIIc; i.e.,
dihydroazole IIIc is a sterically hindered compound.
We thus presumed that compound IIIc exists in a po-
larized form with extended C2–C6 bond.
Bond or angle
IIIba
IVb
IIIc
C2–N1
145.9(2)
145.9(2)
139.4(2)
139.5(2)
140.4(2)
153.9(2)
c146.3(2)c
103.7(1)
109.0(1)
108.6(1)
110.8(1)
005.2(2)
057.5(2)
147.6(3)
146.2(3)
143.9(3)
139.6(3)
141.1(3)
154.8(3)
150.7(3)
106.0(2)
102.4(2)
110.4(2)
111.5(2)
017.0(2)
065.0(2)
146.6(3)
147.9(3)
145.3(3)
142.0(3)
136.4(3)
153.5(3)
143.0(3)
105.0(2)
104.5(2)
108.9(2)
110.9(2)
018.8(3)
064.9(3)
C2–N3
C5–N1
C4–N3
C4–C5
C2–C6
N1–C10
N1C2N3
C2N1C5
N1C5C4
C2C6C7
C2N1C5C4
N1C2C6C7
To verify the above assumption, the structure of
compounds IIIb, IIIc, and IV was studied by the
X-ray diffraction method (Table 1; Figs. 1, 2). Single
crystals of IIIc were obtained by crystallization from
benzene-d6, and single crystals of IIIb and IV, by
crystallization from acetonitrile. Compound IV crystal-
lized as solvate (C29H23N3)2 ·4CH3CN. We have found
no published X-ray diffraction data for 2-cyanomethyl
derivatives of dihydroazoles.
a
b
c
Other bonds: N3–C16 145.8(2), C9–C10 152.0(2), C16–C20
151.5(2) pm.
Other angles: C2N1C10C11 54.3(3), C4N3C20C21 158.2(3),
N3C4C30C31 –50.1(3), N1C5C40C41 178.8(2)°.
N1–C9 bond.
The structures of molecules IIIc and IV are asym-
metric with respect to the plane passing through the C2
atom and mid C4–C5 bond. The C2–C6 bond declines
toward one of the nitrogen atoms; therefore, the
C5–N1/C4–N3 and C2–N1/C2–N3 bond lengths are
slightly different in pairs. The dihydroimidazole frag-
ment N3C4C5N1 is almost planar, but the C2 atom
deviates from that plane by an angle of 17.0–18.8°.
The benzene ring attached to C5 in structure IV is
almost coplanar to the N3C4C5N1 fragment, while the
other benzene rings with the N3C4C5N1 plane form
dihedral angles of 22 to 54°. The C2–C6 bond in IIIc is
slightly longer (d = 154.8 pm), and the N1C2N3 bond
angle is larger (106.0°), than the corresponding param-
eters of molecules IIIb and IV (153.9, 153.5 pm and
103.7, 105.0°, respectively).
steric effect of the adamantyl substituents, especially
of the nearest C–H bonds. The aromatic protons in IIIc
give only one multiplet at δ 6.82 ppm, i.e., in a weaker
field relative to the corresponding signals of IIIa and
C11
C30
C10
N1
C31
N8
C5
C4
C2
C7
C32
Unlike structures IIIc and IV, the molecule of com-
pound IIIb is symmetric with respect to the plane pas-
sing through the C2 atom and mid C4–C5 bond (Fig. 3).
The heteroring in IIIb is characterized by the least
deviation from planarity (the C2N1C5C4 dihedral angle
is 5.2°) among the examined cyanomethyl derivatives.
All endocyclic bonds C5–N1, C4–N3, C2–N1, and C2–N3
are shorter than in molecules IIIc and IV. Specific
attention should be given to shortening of the N1–C9
(146.3 pm) and N3–C16 bonds relative to standard C–N
bond (N1–C10 150.7 pm), which may be due to elec-
N3
C6
C33
C20
C21
Fig. 1. Structure of the molecule of 1,3-bis(1-adamantyl)-
2,3-dihydro-1H-benzimidazol-2-ylacetonitrile (IIIc) accord-
ing to the X-ray diffraction data.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

STABLE CARBENES.
1825
tron-withdrawing effect of the benzyl group. It should
also be noted that the C2–N1(3) bonds in molecules
IІIa–IIIc are slightly shorter than standard single C–N
bond.
C21
C20
C6
N3
C2
C7
N8
We compared the X-ray diffraction data for dihy-
drobenzimidazoles IIIb and IIIc and dihydroimidazole
IV with those for dihydrotriazole V (which was syn-
thesized previously [15]); the structure of V was also
compared with the structure of related carbene VI
(Table 2; Figs. 3, 4). Crystals of compounds V and VI
suitable for X-ray analysis were obtained by crystal-
lization from acetonitrile and toluene, respectively. The
distances C5–N1 and C5–N4 in molecule V were similar
to those in IIIb, IIIc, and IV. The C5–C6 bond length
(153.3 pm) and N1C5N4 angle turned out to be the
lowest among the examined cyanomethyl derivatives.
All single bonds in carbene VI (C–N and N–N) are
shorter by 2–5 pm, and the double C3=N2 bond is
longer by 2 pm, than the corresponding bonds in
molecule V, indicating higher aromaticity of the car-
bene structure. The N4–C30 bond in V is shorter than
the standard C–N bond; this means that the aromatic
ring on N4 is conjugated with the heteroring to the
greatest extent.
C4
C30
N1
C10
C11
C5
C31
C40
C41
Fig. 2. Structure of the molecule of 1,3,4,5-tetraphenyl-2,3-
dihydro-1H-imidazol-2-ylacetonitrile (IV) according to the
X-ray diffraction data.
N8
C7
C21
C20
C6
N3
C4
C16
C2
N1
Thus we obtained first information on the structure
of insertion products formed by stable heteroaromatic
benzimidazol-2-ylidenes and imidazol-2-ylidene. The
insertion product of 1,2,4-triazol-5-ylidene was de-
scribed previously [15]. C–H-Insertion reactions of the
saturated 1,3-diphenyl-4,5-dihydroimidazol-2-ylidene
dimer with cyclopentanone, nitromethane, and malo-
nonitrile were reported in [20, 21]. However, the
products were not identified unambiguously, and it
remained unclear whether the observed reactions in-
volved monomeric or dimeric carbene species. Apart
from publications [12–14], the problem concerning the
reactivity of heteroaromatic carbenes toward acetoni-
trile was discussed by some other authors; but the
corresponding insertion products were not isolated [6].
An unsuccessful attempt to react imidazol-2-ylidenes
with acetonitrile was also reported in [10]. According
to [5], attempts to isolate C–H-insertion products of
triphenyl-substituted 1,2,4-triazol-5-ylidene were un-
successful, for such reactions require elevated tem-
peratures (>150°C) which promote decomposition of
the carbene. Unsaturated heteroaromatic 1,3-bis(1-ada-
mantyl)imidazol-2-ylidene also failed to react with
acetonitrile, but its solvate with CH3CN was formed
instead [14].
C33
C11
C10
C5
C30
C32
C9
C31
Fig. 3. Structure of the molecule of 1,3-dibenzyl-2,3-dihy-
dro-1H-benzimidazol-2-ylacetonitrile (IIIb) according to the
X-ray diffraction data.
C11
C10
C31
C5
C30
N1
N4
N2
C6
C21
C20
C3
C7
N8
Fig. 4. Structure of the molecule of 1-(1-adamantyl)-3,4-di-
phenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylacetonitrile (V) ac-
cording to the X-ray diffraction data.
As postulated in [22], carbenes are capable of being
inserted into X–H bonds according to electrophilic and
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

KOROTKIKH et al.
1826
Table 2. Selected bond lengths (pm) and N1C5N4 angles
(deg) in the molecules of 1-(1-adamantyl)-3,4-diphenyl-4,5-
dihydro-1H-1,2,4-triazol-5-ylacetonitrile (V) and 1-(1-ada-
mantyl)-4-(4-bromophenyl)-3-phenyl-4,5-dihydro-1H-1,2,4-
triazol-5-ylidene (VI)
analogous conditions to give azirines [23], while reac-
tions of electrophilic N-fluoropyridylidene with nitriles
and cyanates lead to the formation of pyrido[1,2-a]-
[1,3,5]triazin-4-ones [24].
Compounds IIIa–IIIc and especially IV exhibit
luminescence under UV irradiation. Taking into ac-
count the lack of conjugated substituents in structures
IIIa–IIIc, this fact is surprising; it resembles lumines-
cence of N-phenyl-substituted nonaromatic 4,5-dihy-
dropyrazoles. Neither benzimidazole nor its quaternary
salts Ia–Ic show even weak luminescence in the
visible region. A probable reason is the presence of
specific conjugation system (NCCN) in dihydroazole
molecules III and IV.
Bond or angle
V
VI
C5–N1
C5–N4
C3–N4
C3=N2
N1–N2
C3–C20
N4–C30
C5–C6
N1C5N4
148.5(4)
147.1(4)
140.5(4)
128.2(4)
144.4(4)
147.6(5)
141.9(4)
153.3(5)
103.9(3)
134.6(3)
138.6(3)
138.4(3)
130.1(3)
139.4(3)
147.3(3)
143.5(3)
–
2-Cyanomethyl derivatives IIIa–IIIc and IV are
relatively stable on storage; they remained almost un-
changed at room temperature under argon for several
months. Compound IIIc was found to undergo decom-
position on heating to 180°C under reduced pressure to
give pure carbene VIIa (colorless powder) in high
yield (method a). Analogous results were obtained by
heating in aromatic solvents (benzene, toluene), but the
process took a longer time. Although additional polar-
ization and extension of the C2–C6 bond in molecule
IIIc are weak (Δd = 13 pm), these factors are likely to
favor dissociation of that bond on heating. Sterically
unhindered dihydroazoles IIIa, IIIb, and IV undergo
thermal decomposition in a complex manner; as a re-
sult, unidentified tarry products are formed.
100.3(2)
nucleophilic mechanisms (or, most probably, following
an intermediate mechanism). Taking into account that
benzimidazol-2-ylidenes are clearly nucleophilic, their
reaction with acetonitrile should follow nucleophilic or
mixed path, as shown in Scheme 1. An alternative
version includes initial generation of NCCH2– anion
and its subsequent combination with azolium ion. To
elucidate details of the reaction mechanism, we ex-
amined the reaction of 2-deuterio-1,3-dibenzylbenz-
imidazolium chloride (Id) with acetonitrile in the
presence of sodium hydride. Salt Id was prepared by
heating the corresponding 1H-containing salt Ib in D2O
until the 2-H signal (δ 10.78 ppm) disappeared from
Thus we were the first to isolate an individual
stable carbene of the benzimidazole series (compound
VIIa, mp 198–202°C). The product can be recrystal-
lized from toluene and can be stored for several
months at room temperature under argon without
appreciable decomposition. Carbene VIIa can also be
obtained by treatment of salt Ic in toluene with potas-
1
the H NMR spectrum. The transformation into 2-cy-
anomethyl derivative IIIb was accompanied by loss of
deuterium, which indicated carbenoid mechanism of
the process (Scheme 1).
For comparison, it should be noted that acyclic
phosphanylsilylcarbenes react with acetonitrile under
Scheme 1.
Bzl
Bzl
Bzl
Bzl
N
N
N
N
H
NaH
MeCN
H
D
CH2CN
N
N
N
N
CH2
CN
Bzl
Bzl
Bzl
Bzl
IIIb
Id
Bzl
–
N
D
CH2CN (CH3CN + NaH)
Id
CH2CN
N
Bzl
IIIb
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

STABLE CARBENES.
1827
sium tert-butoxide at 80°C (the initial salt is poorly
soluble in toluene). It is seen that the synthetic ap-
proaches to 1,2,4-triazol-5-ylidenes [15] are also effec-
tive in the synthesis of carbene VIIa.
C2
C20
N3
C4
C10
N1
All signals from aromatic protons and protons in
C21
1
the adamantyl substituents in the H NMR spectrum
C11
C5
of compound V in benzene-d6 are displaced upfield
relative to the corresponding signals of salt Ic (Δδ =
0.4–0.6 ppm), while no 2-H signal (typical of Ic) is
C33
C32
C30
13
present. The C NMR spectrum contains a signal at
C31
δC 223.0 ppm, which belongs to the carbene carbon
atom, signals from carbon atoms in the adamantyl
groups (δC 29.5–57.5), and aromatic carbon signals
(δC 113.7–134.8 ppm). The IR spectrum of dihydro-
azole V in Nujol resembles those reported for 1,2,4-tri-
azol-5-ylidenes [15]: the most typical are absorption
bands due to stretching vibrations of C–H and C=C
bonds at 3035 and 1600, 1500 cm–1, respectively.
Fig. 5. Structure of the molecule of 1,3-bis(1-adamantyl)-
benzimidazolium perchlorate (Ic) according to the X-ray dif-
fraction data.
C2
C20
3 C10
N1
N
C21
The reaction reverse to the decomposition IIIc →
VIIa, i.e., insertion VIIa→IIIc, is fairly fast: the con-
version of carbene VIIa in acetonitrile is complete in
several minutes, and in toluene, in 15–30 min at room
temperature.
C11
C5
C4
C30
C33
C31
C32
Thus the insertion into the C–H bond of acetonitrile
effectively occurs with strongly basic heteroaromatic
carbenes (aryl-substituted imidazol-2-ylidenes and
benzimidazol-2-ylidenes having aliphatic substituents);
analogous reactions with 1,2,4-triazol-5-ylidenes are
much slower [15]. We also synthesized a new hetero-
aromatic carbene, benzimidazol-2-ylidene VIIa, which
is stable on prolonged storage.
Fig. 6. Structure of the molecule of 1,3-bis(1-adamantyl)-
2,3-dihydro-1H-benzimidazol-2-ylidene (VIIa) according to
the X-ray diffraction data.
Crystals of carbene VIIa for X-ray analysis were
grown from a solution in anhydrous toluene. For com-
parison, we also performed X-ray analysis of perchlo-
rate Ic; single crystals of the latter were obtained by
crystallization from dichloroethane. The structural
parameters of compounds VIIa and Ic are given in
Table 3, and their molecular structures are shown in
Figs. 5 and 6. It is seen that the bonds at the carbene
center in VIIa (C2–N1(3) ~137 pm) are appreciably
longer than in azolium salt Ic (Δ d = 3.8 pm) (Fig. 4)
and appreciably shorter than the ordinary N1(3)–C10(20)
bond (149–150 pm). These data suggest electron dens-
ity delocalization in the carbene molecule. The corre-
sponding bonds in salt Ic are also delocalized, but their
lengths are closer to those of typical double bonds
(d = 132.2, 133.6 pm). The benzimidazole fragment in
molecule VIIa is almost planar due to conjugation.
For comparison, Table 3 also contains crystallographic
data for Hahn’s carbene VIIb [25].
Almost simultaneously with us [13], Hahn et al.
[25] synthesized another carbene of the benzimidazole
series, 1,3-dineopentylbenzimidazol-2-ylidene (VIIb),
by reduction of the corresponding thione with sodium–
potassium alloy at room temperature (reaction time
20 days). However, compound VIIb is a mixture of
stereoisomers, and it has a low melting point, 26°C.
The synthesis of VIIb is much more difficult than the
synthesis of high-melting carbene VIIa; in addition,
the latter is a single isomer and is more convenient to
handle with.
R
R
N
N
S
N
N
R
R
The structural parameters of molecules VIIa and
VIIb are fairly similar, but the C2–N1 and C5–N1 bonds
in VIIa are slightly longer (Δ d = 0.8–1.4 pm), while
VIIa, VIIb
VIIIa, VIIIb
VII, R = 1-Ad (a), t-BuCH2 (b); VIII, R = 1-Ad (a), Bzl (b).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

KOROTKIKH et al.
1828
Table 3. Selected bonds lengths (pm) and bond angles (deg) in the molecules of compounds Ic and VII,a 1,3-bis(1-ada-
mantyl)-2,3-dihydro-1H-imidazol-2-ylidene (IX) [3],b carbene VIIb [25], and thiones VIIIa and VIIIb
VIIb
Bond or angle
Ic
VIIa
VIIIa
VIIIb
IX
A
B
C2–N1
C5–N1
C5–C4
N1–C10
N1C2N3
C2N1C5
N1C5C4
133.6(4)
139.9(4)
141.5(4)
148.9(4)
112.2(3)
107.6(2)
106.9(2)
137.4(4)
140.2(4)
139.8(4)
149.3(4)
103.8(2)
112.2(2)
105.8(2)
136.1(2).0
139.4(2).0
139.5(2).0
145.8(2).0
103.5(1.3)
112.8(1.2)
–
136.0(2).0
139.4(2).0
138.6(2).0
145.8(2).0
104.3(1.4)
112.0(1.4)
–
138.9(2)00
140.4(2)00
140.3(3)00
152.1(2)00
108.14(15)
108.27(15)
107.80(15)
136.7(2)00
139.0(2)00
139.8(3)00
146.5(2)00
106.37(19)
110.30(14)
106.51(9)0
136.5(4)
138.1(4)
133.5(5)
144.1(4)
101.4(2)
112.8(3)
106.5(3)
a
Other bond lengths, pm: Ic: C2–N3 132.2(4), C4–N3 139.6(4), N3–C20 150.0(4); VII: C2–N3 137.2(4), C4–N3 139.9(4), N3–C20 149.4(3);
VIII: C2–S 168.05(19) (a), 168.0(2) (b).
b
Data for 1,3-bis(1-adamantyl)-2,3-dihydro-1H-imidazol-2-ylidene.
the N1–C10 bond is appreciably longer (Δ d = 3.5 pm).
The difference in the C5–N1 and N1–C10 bond lengths
increases in going to 1,3-bis(1-adamantyl)imidazol-2-
ylidene (IX). The observed variations originate from
steric effects of the bulky adamantyl groups and fused
benzene ring, which make carbene VIIa less aromatic
than VIIb and IX [3]. We determined the bond angles
in the benzimidazol-2-ylidene fragment with a higher
accuracy as compared to [25] (Table 3).
matic delocalization; this parameter indicates higher
aromaticity of 1,2,4-triazol-5-ylidenes.
As noted above, nucleophilic carbene VIIa is much
more reactive than triazolylidenes toward acetonitrile.
We examined reactions of VIIa with sulfur and sele-
nium in benzene. The reaction with sulfur was very
fast (1–2 min at room temperature), and it resulted in
the formation of thione VIIIa and intensely red–brown
compound X. The composition of the latter was
determined as (C27H34N2)2S7 ·4C6H6, i.e., it is a solvate
with benzene. Presumably, it has a structure like
Crb+–S–(S+=S–)3=Crb, where Crb is the carbene VIIa
fragment as azolium ion (Crb+) or dihydroazole (=Crb).
We succeeded in isolating compound X due to its poor
solubility in benzene; the product separated from the
reaction mixture during the process. Compound X is
very sensitive to atmospheric oxygen and moisture: on
dissolution in chloroform, it is converted into colorless
thione VIIIa (16%) and pale yellow salt Ic with
HS3O4– as counterion (60%). Anion exchange reaction
of the latter with sodium perchlorate gave perchlorate
Ic described above. We failed to record IR or NMR
spectra of X and determine its crystalline structure
because of its fast transformation in polar solvents.
Judging by the elemental composition, color, and
chemical behavior, product X can be regarded as
a hypervalent sulfur compound. The yield of thione
VIIIa strongly depends on the reactant ratio: it
changes from 49% in the reaction with excess carbene
to 90% with excess sulfur. Obviously, the reason is
the formation of compound X. Our failure to isolate
a product like X in the reactions of 1,2,4-triazol-5-
ylidenes with sulfur [15] may be explained by its fast
All bonds in carbene VIIa are shorter than the cor-
responding bonds in IIIc due to aromatic character of
the carbene ring. The C4=C5 bond in VIIa is also ap-
preciably shorter, indicating lesser bond leveling than
in VI; the C3=N2 bond in the latter is longer than in
dihydroazole V. These findings may be interpreted in
terms of higher aromaticity of 1,2,4-triazol-5-ylidenes
compared to benzimidazol-2-ylidenes. On the other
hand, our experimental data and the results of theoret-
ical calculations [26] suggest similar aromaticities of
benzimidazol-2-ylidenes and 1,2,4-triazol-5-ylidenes
in keeping with the delocalization criterion; the latter
are only slightly more aromatic.
The endocyclic N1C2N3 angle in molecule VIIa
(103.8°) is considerably smaller than in salt Ic (Δω =
8.4°) due to repulsion between sp2-electrons at the car-
bene carbon atom and electrons at the endocyclic C–N
bonds. The N1C2N3 angle is intermediate between
imidazol-2-ylidenes (101–102°) [3, 4] and dihydro-
imidazol-2-ylidene VI (104.7–106.4°) [18], but it
appreciably exceeds the corresponding bond angle in
1,2,4-triazol-5-ylidenes (100.3°) [5, 15]. Presumably,
the bond angle at the carbene carbon atom reflects
stabilization of nucleophilic cyclic carbenes via aro-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

STABLE CARBENES.
1829
decomposition in solution to the corresponding 1,2,4-
triazole-5-thione.
C5
N1
C4
N3
Crystals of thione VIIIa for X-ray analysis were
obtained from a solution in DMF. With a view to
compare its structure with that of a related sterically
unhindered thione, we synthesized 1,3-dibenzylbenz-
imidazole-2-thione by heating perchlorate Ib with
sulfur in acetonitrile in the presence of triethylamine.
Single crystals of thione VIIIb were grown from a so-
lution in propan-2-ol. The NMR spectra of thiones
VIIIa and VIIIb were essentially similar (see Experi-
mental). The most characteristic signals from C2 in the
13C NMR spectra of VIIIa and VIIIb were located at
δC ~170.8 ppm.
C10
C20
C2
S
Fig. 7. Structure of the molecule of 1,3-bis(1-adamantyl)-
2,3-dihydro-1H-benzimidazole-2-thione (VIIIa) according
to the X-ray diffraction data.
S
According to the X-ray diffraction data, adamantyl-
substituted thione VIIIa is characterized by longer
endocyclic C–N bonds as compared to VIIIb, presum-
ably due to electron-donor effect of the 1-adamantyl
groups. The same factor is likely to be responsible for
the shorter exocyclic N–CBzl bonds in VIIIb relative to
the corresponding bonds in molecule VIIIa. The C–S
distances in the molecules of both compounds are
equal. This fact indicates the absence of appreciable
steric shielding of the sulfur atom. The N1C2N3 angle
in molecule VIIIa is larger than in VIIIb; a probable
reason is steric repulsion between hydrogen atoms in
the fused benzene ring and adamantyl substituent in
molecule VIIIa.
Unlike the reaction with sulfur, selenium failed to
react with carbene VIIa under analogous conditions.
Presumably, the reaction is hampered by steric hindr-
ances to the formation of intermediate hypervalent
selenium compound from cyclic molecular selenium
(but not atomic selenium whose radius is only slightly
larger than that of sulfur: S 1.04, Se 1.17 Å). As far as
we know, this is the first example of strong steric
hindrances to carbene reactions with chalcogens.
C2
C20
C10
N3
C4
N1
C5
Fig. 8. Structure of the molecule of 1,3-dibenzyl-2,3-di-
hydro-1H-benzimidazole-2-thione (VIIIb) according to the
X-ray diffraction data.
X-Ray analysis was performed on a Nonius-Kappa
CCD diffractometer [λ 0.71069–0.71073 Å; tempera-
ture 153(2) K] with no correction for absorption. The
results of measurements were treated by the full-matrix
least-squares procedure with respect to F2. The prin-
cipal crystallographic parameters of the examined com-
pounds are given in Table 4. The calculations were
performed according to manuals [27] (data acquisition
and cell refinement), [28] (data reduction), [29] (struc-
ture resolution), and [30] (structure refinement and
molecular graphics). The complete sets of the crystal-
lographic data are available from the authors.
EXPERIMENTAL
2,3-Dihydro-1H-(benz)imidazol-2-ylacetonitriles
IIIa–IIIc and IV (general procedure). Sodium hydride
(separated from mineral oil), 0.026 g (1.1 mmol), was
added in one portion under argon to a solution of
1 mmol of azolium salt Ia–Ic or II in anhydrous aceto-
nitrile (5–50 ml). The mixture was stirred for 0.5–1 h
(IIIa, IIIb, IV), the progress of the reaction being
monitored by the volume of evolved hydrogen. The
mixture was then passed under argon through a thin
layer of degassed neutral silica gel, and the solvent was
removed under reduced pressure. The oily residue was
ground with hexane, and the precipitate was filtered
All experiments with benzimidazol-2-yilidene VIIa
and with carbenes generated in situ were carried out
under argon. The solvents were dried by standard
procedures. The NMR spectra were recorded on
a Varian Gemini-200 spectrometer at 200 (1H) and
50.3 MHz (13C) using tetramethylsilane as internal
reference. The IR spectra were measured in Nujol on
a Specord 75IR spectrometer. Thin-layer chromatog-
raphy was performed on silica gel using chloroform or
chloroform–methanol (10:1) as eluent; development
with iodine vapor.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

KOROTKIKH et al.
1830
Table 4. Principal crystallographic parameters and conditions of X-ray diffraction experiments for compounds Ic, IIIb, IIIc,
IV, V, VII, VIIIa, and VIIIb
Parameter
Crystal system Orthorhombic Monoclinic Triclinic
Ic
IIIb
IIIc
IV
V
VIIa
VIIIa
VIIIb
Triclinic Orthorhombic Monoclinic Monoclinic Monoclinic
Space group
a, Å
Pbca
P21/n
08.120
18.031
12.404
90.000
94.290
90.000
P-1
P-1
P212121
P21/c
C2/c
C2/c
18.047(4)
12.858(3)
26.796(5)
90.000(5)
90.000(5)
90.000(5)
09.968(2) 13.428(3)
10.847(2) 14.379(3)
11.484(2) 15.519(3)
097.02(3) 093.55(3)
107.80(3) 114.05(3)
102.99(3) 091.79(3)
10.892(5)
11.031(5)
36.066(5)
90.000(5)
90.000(5)
90.000(5)
12.233(5) 21.1300(3) 18.6940(2)
11.768(5) 10.4150(2) 09.7410(2)
14.737(5) 21.8010(3) 09.9250(2)
90.000(5) 90.000(5) 90.000(10)
93.716(5) 90.000(5) 111.0930(10)
90.000(5) 90.000(5) 00.000(10)
b, Å
c, Å
α, deg
β, deg
γ, deg
Crystal habit, 0.2×0.2×0.2 0.4×0.3×0.3 0.2×0.2×0.2 0.2×0.2×0.2 0.3×0.3×0.3 0.3×0.2×0.2 0.4×0.3×0.2 0.3×0.3×0.2
mm
Total number
of reflections
23980
7681
8768
18905
12314
9506
4847
1917
Number of
independent
reflections
08631
4135
5173
12304
08509
5712
4847
1917
Completeness,
%
91.1
99.2
98.9
98.9
77.3
88.7
99.3
99.0
θ, deg
30.06
1.005
30.47
1.169
30.47
0.754
27.44
1.014
30.06
1.267
30.47
0.977
30.06
1.061
30.06
1.159
Goodness of
fit (by F2)
R1 [I > 2σ(I)]
0.0766
0.2327
0.0609
0.1181
0.0542
0.1428
0.0776
0.1582
0.0978
0.1314
0.0837
0.1850
0.0495
0.1043
0.0476
0.0861
wR2 [I > 2σ(I)]
R1 (all reflec-
0.1379
0.0749
0.0985
0.1588
0.1266
0.1775
0.0803
0.0590
tions)
wR2 (all reflec-
0.2603
0.1226
0.1820
0.1952
0.1405
0.2266
0.1216
0.0932
tions)
32
33
off, dried, and recrystallized. In the synthesis of com-
pound IIIc, the reaction time was 4 h (25°C); the mix-
ture was held for 0.5 h at 0°C, and the precipitate was
filtered off and dried.
116.3 (C7), 106.5 (C31, C ), 119.8 (C30, C ); 141.6
(C4, C5). Found, %: C 70.5; H 6.9; N 22.7. C11H13N3.
Calculated, %: C 70.6; H 7.0; N 22.4.
1,3-Dibenzyl-2,3-dihydro-1H-benzimidazol-2-yl-
acetonitrile (IIIb). Yield 72%, Rf 0.87, mp 134–136°C
(from MeCN). IR spectrum, ν, cm–1: 3045 w (C–H),
2240 w (C≡N), 1600 m, 1495 m (C=Carom), 1300 m,
1280 m, 1220 m, 1140 m, 1120 m, 1070 m, 1010 m,
730 s. 1H NMR spectrum (CDCl3), δ, ppm: 2.67 d (2H,
1,3-Dimethyl-2,3-dihydro-1H-benzimidazol-2-yl-
acetonitrile (IIIa). Yield 89%, Rf 0.79, mp 104–105°C
(from MeCN). IR spectrum, ν, cm–1: 3070 w (C–H),
2235 m (C≡N), 1595 m, 1495 m (C=Carom), 1300 m,
1270 m, 1215 m, 1165 m, 1110 m, 1055 m, 1010 m,
730 s. 1H NMR spectrum (CDCl3), δ, ppm: 2.63 s (3H,
CH3N), 2.83 d (2H, CH2, 3J = 3.0 Hz), 4.27 t (1H, 2-H,
3
2-CH2, J = 2.8 Hz), 4.27 s (2H, CH2N), 5.00 t (1H,
3
2-H, J = 2.8 Hz), 6.40 m (4H, Harom), 7.27 m (10H,
3J = 3.0 Hz), 6.47 m (4H, Harom). C NMR spectrum
H
arom). 13C NMR spectrum (CDCl3), δC, ppm: 24.8
13
(CDCl3), δC, ppm: 23.2 (C6), 34.9 (C10, C20), 86.7 (C2),
(C6), 53.3 (C10, C20), 83.0 (C2), 116.3 (C7), 106.9 (C31,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

STABLE CARBENES.
1831
C 32), 119.8 (C30, C 33), 137.2 (C11, C21), 141.0 (C4, C5).
Found, %: C 81.2; H 6.3; N 12.4. C23H21N3. Calculat-
ed, %: C 81.4; H 6.2; N 12.4.
1.60 m (12H), 2.04 m (6H), and 2.55 m (12H) (1-Ad);
3
4
7.07 d.d (2H), 7.66 d.d (2H, J = 6.1, J = 3.2 Hz)
(Harom). 13C NMR spectrum (C6D6), δC, ppm: 29.5 (C12,
C14, C16, C22, C24, C26), 36.0 (C13, C15, C19, C23, C25,
C29), 42.5 (C11, C17, C18, C21, C27, C28) (1-Ad); 57.5
(C10, C20) 113.7 (C31, C32), 119.2 (C30, C33), 134.6 (C4,
C5, Carom), 223.0 (C2). Found, %: C 84.0; H 8.9; N 7.1.
C27H34N2. Calculated, %: C 83.9; H 8.9; N 7.3.
1,3-Bis(1-adamantyl)-2,3-dihydro-1H-benzimid-
azol-2-ylacetonitrile (IIIc). a. Yield 79%, mp 187–
189°C (from toluene–acetonitrile, 3:1). IR spectrum,
ν, cm–1: 3060 w (CH), 2235 m (C≡N), 1590 m
(C=Carom), 1300 m, 1280 m, 1240 m, 1190 m, 1170 m,
1
b. Potassium tert-butoxide, 0.56 g (5 mmol), was
added in one portion under argon to a suspension of
2.43 g (5 mmol) of benzimidazolium perchlorate Ic in
15 ml of anhydrous toluene. The mixture was stirred
for 0.5 h, heated to 80°C, and cooled to room tempera-
ture, the precipitate of KClO4 was filtered off, and the
filtrate was evaporated under reduced pressure. Yield
1.71 g (82%), crystalline substance, mp 198–202°C
(from toluene). The product was identical to a sample
prepared as described above in a.
1080 m, 1035 m, 1025 m, 980 m, 730 s. H NMR
spectrum (C6D6), δ, ppm: 1.42 m (12H), 1.82 m (12H),
3
1.93 m (6H) (1-Ad); 1.98 d (2H, 2-CH2, J = 5.8 Hz),
3
5.27 t (1H, 2-H, J = 5.8 Hz), 6.83 m (4H, Harom).
13C NMR spectrum (C6D6), δC, ppm: 26.8 (C6), 30.1
(C12, C14, C16, C22, C24, C26), 36.6 (C13, C15, C19, C23,
C25, C29), 41.3 (C11, C17, C18, C21, C27, C28) (1-Ad);
55.7 (C10, C20, 1-Ad), 71.2 (C2), 118.0 (C7), 116.2 (C31,
C32), 120.3 (C30, C33), 139.0 (C4, C5). Found, %:
C 81.9; H 8.7; N 9.8. C29H37N3. Calculated, %: C 81.5;
H 8.7; N 9.8.
1,3-Bis(1-adamantyl)-2,3-dihydro-1H-benzimid-
azole-2-thione (VIIIa). A solution of 0.42 g
(1.1 mmol) of carbene VII in 1 ml of anhydrous ben-
zene was added dropwise under stirring to a solution of
0.049 g (1.56 mmol) of sulfur in 5 ml of anhydrous
benzene. The precipitate, 0.02 g (5%) of by-product X,
was filtered off, and the filtrate was evaporated under
reduced pressure. Yield 0.41 g (90%), mp 275–276°C
(from DMF). IR spectrum, ν, cm–1: 3050 w (C–Harom),
1700 m (N–C=S), 1600 m, 1555 m, 1500 m (C=Carom).
1H NMR spectrum, δ, ppm: in CDCl3: 1.72 m (12H),
2.25 m (6H), and 2.42 m (12H) (1-Ad); 7.38 m and
7.82 m (2H each, Harom); in pyridine-d5: 1.55 m (6H),
1.74 m (6H), 2.08 m (6H), and 3.13 m (12H) (1-Ad);
b. Anhydrous acetonitrile, 2 ml, was added drop-
wise to a solution of 0.08 g (0.21 mmol) of carbene
VIIa in 1 ml of anhydrous benzene. The mixture was
stirred for 0.5 h, and the precipitate was filtered off,
dried, and recrystallized as described above. Yield
0.08 g (90%). Analogous results were obtained in pure
acetonitrile where the product is soluble very poorly.
In this case, the reaction was complete in several
minutes at room temperature, and the mixture was
additionally stirred for 0.5 h.
1,3,4,5-Tetraphenyl-2,3-dihydro-1H-imidazol-
2-ylacetonitrile (IV). Yield 55%, Rf 0.97, mp 143–
144°C (from MeCN). IR spectrum, ν, cm–1: 3050 w
(C–H), 2235 w (C≡N), 1590 m, 1480 m (C=Carom),
1300 m, 1255 m, 1115 m, 1040 m, 1020 m, 760 s.
1H NMR spectrum (CD3CN), δ, ppm: 3.23 d (2H,
13
7.05 m and 7.87 m (2H each, Harom). C NMR spec-
trum (pyridine-d5), δC, ppm: 31.1 (C12, C14, C16, C22,
C24, C26), 36.4 (C13, C15, C19, C23, C25, C29), 40.9 (C11,
C17, C18, C21, C27, C28), 67.2 (C10, C20), 114.3 (C31,
C32), 121.5 (C30, C33), 134.0 (C4, C5), 170.8 (C2).
Found, %: C 77.5; H 8.5; N 6.9; S 7.9. C27H34N2S.
Calculated, %: C 77.4; H 8.2; N 6.7; S 7.7.
3
3
2-CH2, J = 6.0 Hz), 5.23 t (1H, 2-H, J = 6.0 Hz),
13
7.12 m and 7.60 m (10H each, Harom). C NMR spec-
trum (CD3CN), δC, ppm: 27.8 (C6); 86.3 (C2); 122.4
(C7); 124.4 (C4, C5); 129.1, 130.0, 130.4 (Carom); 137.5
(C20, C30); 148.6 (C10, C40). Found, %: C 84.1; H 5.8;
N 10.2. C29H23N3. Calculated, %: C 84.2; H 5.6; N 10.2.
Compound X. mp 150°C (decomp.). Found, %:
C 71.8; H 7.0; N 4.3; S 17.3. C54H68N4S7·4C6H6. Cal-
culated, %: C 71.5; H 7.1; N 4.3; S 17.1.
1,3-Bis(1-adamantyl)-2,3-dihydro-1H-benzimid-
azol-2-ylidene (VIIa). a. Compound IIIc was heated
under reduced pressure, gradually raising the tempera-
ture from 100 to 180°C, and was then kept for 1–1.5 h
at 180°C. The product was an almost colorless crystal-
line substance. Yield 91%, mp 198–202°C (from tolu-
ene). IR spectrum, ν, cm–1: 3035 w (C–Harom); 1600 w,
1,3-Dibenzyl-2,3-dihydro-1H-benzimidazole-2-
thione (VIIIb) was synthesized by heating equimolar
amounts of perchlorate Ib and triethylamine and
a small excess of sulfur (20%) in MeCN. mp 184°C
1
(from i-PrOH). H NMR spectrum (CDCl3), δ, ppm:
5.65 s (4H, CH2N), 7.09 m (4H, Harom), 7.33 m (10H,
Harom). 13C NMR spectrum (CDCl3), δC, ppm: 48.36
(CH2N), 109.42 (C6, C7), 122.81 (C5, C8), 127.26 (Cp),
1
1500 w (C=Carom). H NMR spectrum (C6D6), δ, ppm:
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 42 No. 12 2006

KOROTKIKH et al.
1832
127.56 (Co), 128.53 (Cm), 131.78 (C4a, C7a), 135.40
(Ci), 170.81 (C2).
Khimiya, 1990; Shvaika, O.P., Korotkikh, N.I., and
Aslanov, A.F., Khim. Geterotsikl. Soedin., 1992, no. 9,
p. 1155; Regitz, M., Angew. Chem., Int. Ed. Engl., 1996,
vol. 35, p. 725; Herrmann, W.A. and Köcher, C., Angew.
Chem., Int. Ed. Engl., 1997, vol. 36, p. 2162; Ardu-
engo, A.J., III, Acc. Chem. Res., 1999, vol. 32, p. 913;
Bourissou, D., Guerret, O., Gabbai, F.P., and Ber-
trand, G., Chem. Rev., 2000, vol. 100, p. 39; Car-
malt, C.J. and Cowley, A.H., Adv. Inorg. Chem., 2000,
vol. 50, p. 1.
Oxidative decomposition of compound X. Com-
pound X, 0.1 g (0.076 mmol), was dissolved in 1 ml
of chloroform, and the solution was stirred for 20 min
on exposure to air. The mixture was diluted with 2 ml
of hexane, and the pale precipitate was filtered off
and dried. Yield of salt Ic (X = HS3O4) 0.05 g (60%).
1H NMR spectrum (DMSO-d6), δ, ppm: 1.84 m (12H),
2.37 m (6H), 2.54 m (12H) (1-Ad); 7.56 d.d (2H,
7. Alder, R.W., Allen, P.R., Marray, M., and Orpen, A.G.,
Angew. Chem., Int. Ed. Engl., 1996, vol. 35, p. 1121.
8. Denk, M., Rodezno, J.M., Gupta, S., and Lough, A.J.,
J. Organomet. Chem., 2001, vols. 617–618, p. 242.
9. Danopoulos, A.A., Winston, S., Gelbrich, T., Hurst-
house, M.B., and Tooze, R.P., Chem. Commun., 2002,
p. 482.
10. Herrmann, W.A., Elison, M., Fischer, J., Köcher, C., and
Artus, G.R.J., Chem. Eur. J., 1996, vol. 2, p. 722;
Herrmann, W.A., Köcher, C., Goossen, L.J., and
Artus, G.R.J., Chem. Eur. J., 1996, vol. 2, p. 1627.
11. Kuhn, N. and Kratz, T., Synthesis, 1993, p. 561.
12. Korotkіkh, M.I., Raenko, G.F., Pekhtereva, T.M., and
Shvaika, O.P., Dopov. Nats. Akad. Nauk Ukraini, 1998,
vol. 6, p. 149.
13. Korotkikh, N.I., Rayenko, G.F., and Shvaika, O.P.,
Abstracts of Papers, 17th Congr. on Heterocyclic
Chemistry, Vienna, 1999, PO-383; Korotkikh, N.I.,
Rayenko, G.F., and Shvaika, O.P., Rep. Ukr. Acad. Sci.,
2000, no. 2, p. 135.
3
4
H
arom), 8.01 d.d (2H, Harom, J = 6.5, J = 3.2 Hz),
9.10 s (1H, 2-H). Found, %: C 59.3; H 6.5; N 5.3;
S 17.6. C27H36N2S3O4. Calculated, %: C 59.1; H 6.6;
N 5.1; S 17.5.
Evaporation of the filtrate gave 0.01 g (16%) of
thione VIIIa which was identical to a sample
described above in the IR spectrum and melting point.
By treatment of salt Ic (X = HS3O4) with excess
aqueous sodium perchlorate we obtained initial salt
Ic (X = ClO4).
This study was performed under financial support
by the Ukrainian State Foundation for Basic Research,
by the Ministry of Education and Science of Ukraine
(project no. 03.07/00), and by the Robert A. Welch
Foundation (USA). The authors are also thankful to
Dr. K.Yu. Chotii (Institute of Physical Organic and
Coal Chemistry, National Academy of Sciences of
Ukraine) for recording the IR spectra.
14. Arduengo, A.J., Calabrese, J.C., Davidson, F.,
Dias, H.V.R., Goerlich, J.R., Krafczyk, R., Mar-
shall, W.J., Tamm, M., and Schmutzler, R., Helv. Chim.
Acta, 1999, vol. 82, p. 2348.
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