Structural and theoretical investigation of 2-iminoimidazolines - Carbene analogues of iminophosphoranes

Article abstract of DOI:10.1039/b615418b

The preparation of 2-iminoimidazolines 3a-3f has been accomplished by the Staudinger reaction of the carbenes 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1b), 1,3- diisopropylimidazolin-2-ylidene (1c), 1,3

Full text of DOI:10.1039/b615418b

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Structural and theoretical investigation of 2-iminoimidazolines – carbene
analogues of iminophosphoranes†
Matthias Tamm,*a Dejan Petrovic,a So¨ren Randoll,a Stephan Beer,a Thomas Bannenberg,a
Peter G. Jonesa and Jo¨rg Grunenbergb
Received 24th October 2006, Accepted 1st December 2006
First published as an Advance Article on the web 4th January 2007
DOI: 10.1039/b615418b
The preparation of 2-iminoimidazolines 3a–3f has been accomplished by the Staudinger reaction of the
carbenes 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene
(1b), 1,3-diisopropylimidazolin-2-ylidene (1c), 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (1d),
1,3-bis(2,6-diisopropylphenylimidazolin-2-ylidene (1e) and 1,3,4,5-tetramethylimidazolin-2-ylidene (1f)
with trimethylsilyl azide (Me3SiN3) followed by desilylation of the resulting 2-trimethylsilylimino-
imidazolines 2a–2e. The X-ray crystal structures of 2d and 2e have been established, revealing
C1–N1–Si1 angles that are more obtuse than the corresponding P–N–Si angles observed in related
trimethylsilyl iminophosphoranes. Together with 2c, the disilylated side product 1,3-diisopropyl-2-
(trimethylsilylimino)-4-trimethylsilylimidazoline (4) has been isolated and structurally characterized.
Cleavage of the N–Si bonds in 2a–2f and formation of 3a–3f is easily achieved by stirring in methanol.
The molecular structures of the 2-iminoimidazolines 2a–2c are reported, indicating that the structural
parameters are best described by non-ylidic resonance structures and that electron delocalization within
the imidazole heterocycle does not play a crucial role in these imine systems. Compound 2a forms a
head-to-head dimer in the solid state via weak intermolecular N–H · · · N contacts, which have
additionally been characterized by means of compliance constants. To further analyze the electronic
structure of these imines in comparison to related guanidine ligands, the proton affinities (PAs) of the
model compounds 2-imino-1,3-dimethylimidazoline (5), 2-imino-1,3-dimethylimidazolidine (6) and
tetramethylguanidine (7) have been calculated by means of density functional theory. Finally, the
charge distribution in 5–7 and the relative contribution of relevant resonance structures have been
determined using natural bond orbitals (NBO) and natural resonance theory (NRT).
Introduction
Ligands derived from the 1H-imidazole heterocycle currently play
a major role in organotransition metal and coordination chem-
istry. In particular, N-heterocyclic carbenes of the imidazolin-
2-ylidene type1 are nowadays ubiquitous and indispensable to
the development of diverse research areas such as homogeneous
catalysis,2 materials science3 and medicinal chemistry.4 The stabil-
Scheme 1 Mesomeric structures of imidazole-based ligands.
ity of these carbenes can be attributed inter alia to the capability
of the imidazolium ring to effectively stabilize a positive charge
of negative charge at X affords compounds with considerably
leading to strongly basic and highly nucleophilic ligands. This
enhanced basicity and nucleophilicity. Accordingly, the heavier
behaviour can be transferred to an exocyclic moiety X at the
2-chalcogenoimidazolines (X = S, Se, Te) have been regarded as
2-position of the N-heterocycle, so that for species such as 2-
neutral analogues of thiolate, selenolate and tellurolate ligands,
methylen-, 2-imino- and 2-oxo-imidazolines (X = CH2, NH,
O) a strong contribution from the ylidic mesomeric structure
B (Scheme 1) must be considered.5,6 The resulting build-up
respectively7 and, as a further example, the extensive use of the
tripodal ligand hydrotris(methimazolyl)borate is based on the
excellent electron-donating ability of the exocyclic sulfur atoms.8,9
We have exploited this concept in the synthesis of various novel
2-trimethylsilyliminoimidazolines (X = NSiMe3) and have shown
that these are suitable precursors for the synthesis of transition
metal complexes incorporating ancillary imidazolin-2-iminato
ligands (X = N−).10 Since these ligands can act as 2r,4p-electron
donors in a similar fashion to that described for related phos-
phoraneimides, R3PN−,11 they can be regarded as monodentate
analogues of cyclopentadienides, C5R5−. With this contribution,
we now wish to give a full account of the synthesis and structural
aInstitut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t
Carolo-Wilhelmina zu Braunschweig, Hagenring 30, D-38106 Braunschweig,
Germany. E-mail: m.tamm@tu-bs.de
bInstitut fu¨r Organische Chemie, Technische Universita¨t Carolo-Wilhelmina
zu Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany. E-mail:
joerg.grunenberg@tu-bs.de
† Electronic supplementary information (ESI) available: Cartesian co-
ordinates of the atomic positions for all calculated structures and the
most important resonance structures obtained for 5–7 by NRT (natural
resonance theory) analysis. See DOI: 10.1039/b615418b
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characterization of several 2-trimethylsilyliminoimidazolines (X =
NSiMe3) and their resulting 2-iminoimidazolines (X = NH), which
are useful building blocks for the preparation of poly(imidazoline-
2-imine) ligands currently under investigation in our laboratory.12
In addition, we report a comparative theoretical study of the
electronic structure of these imines and some related guanidine
systems.
the case of 2b, silylation leads to a marked low-field shift of the
septet CH resonance from 3.95–4.61 ppm. The resonances of the
trimethylsilyl protons of the compounds 2a–2c and 2f appear in the
range of 0.47–0.53 ppm, whereas in the case of the aryl-substituted
imines 2d and 2e the corresponding signals appear at −0.10 and
−0.16 ppm, respectively. In the 13C NMR spectra of all silylated
imines 2, the resonances of the former carbene carbon atoms are
found between 139 and 154 ppm, which is a shift of approximately
80 ppm up-field from the related resonances in the free carbenes 1.
According to their 1H and 13C NMR spectra, the silylated imines 2
exhibit pseudo-C2v symmetry in solution, implying either that the
N1–C1–Si axis is linear or that rotation around the N1–C1 axis
is fast on the NMR time scale. In fact, the◦C1–N1–Si angle in 2a
was found to be close to linearity [169.3(2) ].10a
Results and discussion
Preparation and characterization of N-silylated
2-iminoimidazolines
Silylated iminophosphoranes have been widely used for complex-
ation reactions with various metal halides or oxides to yield
phosphoraneiminato complexes by the elimination of trialkylsi-
lyl halides or hexaalkyldisiloxanes, respectively.11 Therefore, the
syntheses of analogous N-silylated 2-iminoimidazolines 2 are
of great interest in view of the possibility of generating novel
transition metal complexes via the cleavage of the N–Si bond. We
have previously reported a general high-yield synthesis of the 2-
(trimethylsilylimino)imidazolines 2;10 the reaction between readily
available stable carbenes 1 and trimethylsilyl azide (TMS-N3)
in boiling toluene furnishes the N-silylated 2-iminoimidazolines
2 in a similar way to that described for the preparation of
silylated phosphoraneimines (Scheme 2).11d,13 It is reasonable to
assume that this conversion also follows the mechanism of the
Staudinger reaction,14 which would involve nucleophilic attack of
the carbene on the terminal azide nitrogen atom and intermediate
triazene formation followed by N2 dissociation.15 This mechanism
is supported by the observation that stable triazenes can be isolated
by treatment of imidazolin-2-ylidenes with alkyl and aryl azides.16
To study the effect of different substituents on the nitrogen
atoms, we have established the molecular structures of 2d (Fig. 1)
and 2e (Fig. 2) by means of X-ray diffraction analysis, revealing
significantly smaller angles of 147.2(1)◦ in 2d and 155.4(1) and
157.8(1)◦ for the two independent molecules in 2e (Table 1).
[A least-squares fit of the central parts of the molecules (five-
membered ring plus NSi and ipso C atoms) gave an r.m.s. deviation
˚
of 0.05 A; the interplanar angles of the aryl groups to the central
ring of 78.5 and 78.5◦ for molecule 1, and 81.6 and 89.0◦ for
molecule 2 show, however, that the molecular conformations
are somewhat different.] Although electronic factors might also
account for these differences,10a,b the observed trend follows the
increased steric requirements of the N-substituents [tBu (2a)
>Dipp (2e) >Mes (2d)]. This implies that the potential energy
surface is quite shallow with respect to the C–N–Si angle, as has
been previously demonstrated for phosphoraneiminato transition
metal complexes.17 On average, these angles are more obtuse
than the corresponding P–N–Si angles observed in trimethylsi-
lyl iminophosphoranes such as Ph3PNSiMe3 [140.2(2)◦] and
Scheme 2 Preparation of 2-iminoimidazolines 3.
Compounds 2a (R = tBu, Rꢀ = H), 2d (R = Mes, R1 = H), 2e
(R = Dipp, Rꢀ = H) and 2f (R = Me, Rꢀ = Me) are obtained as
colourless solids. In contrast, the N-silylated 2-iminoimidazolines
2b (R = iPr, Rꢀ = Me) and 2c (R = iPr, Rꢀ = H) are isolated as
brownish oils, which can be purified by bulb-to-bulb distillation at
180 ◦C/9 mbar affording colourless liquids. The conversion of the
carbenes bearing 4,5-hydrogen atoms (1a, 1c, 1d, 1e) can easily be
followed by 1H NMR spectroscopy, as pronounced high-field shifts
of about −0.70 ppm are observed for the resonances of the NCH
hydrogen atoms upon formation of the corresponding imines. In
Fig. 1 ORTEP drawing of 2d with thermal displacement parameters
drawn at 50% probability.
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◦
˚
Table 1 Selected bond distances (A) and angles ( ) for 2d, 2e, 3a–3c and 4
2d
2e
3a
3b
3c
4
C1–N1
C1–N2
C1–N3
N1–Si
1.267(2)
1.394(2)
1.391(2)
1.687(1)
1.264(2)/1.265(2)
1.400(2)/1.397(2)
1.399(2)/1.397(2)
1.677(2)/1.677(2)
1.295(2)
1.390(2)
1.395(2)
1.294(3)
1.386(3)
1.389(3)
1.298(1)
1.384(1)
1.383(1)
1.274(4)
1.386(3)
1.392(3)
1.665(2)
C1–N1–Si
N1–C1–N2
N1–C1–N3
N2–C1–N3
147.2(1)
125.0(1)
131.6(1)
103.4(1)
155.4(1)/157.8(1)
131.6(2)/131.4(2)
125.6(2)/125.6(2)
102.9(2)/103.0(2)
150.9(2)
126.5(2)
129.7(2)
103.9(2)
123.5(1)
131.2(1)
105.3(1)
124.1(2)
130.9(2)
105.1(2)
127.1(1)
127.9(1)
105.0(1)
Scheme 3 Formation of the imines 2c (65%) and 4 (15%).
Fig. 2 ORTEP drawing of 2e (one of two independent molecules) with
thermal displacement parameters drawn at 50% probability.
Fig. 3 ORTEP drawing of 4 with thermal displacement parameters drawn
at 50% probability.
Cy3PNSiMe3 [149.8(2)◦] (Cy = cyclo-C6H11).18,19 Whereas the
˚
exocyclic C1–N1 bond distances found in 2d [1.267(2) A] and
(vide supra). Finally, it should be emphasized that the activation
and silylation of the 4-position in 2c is in line with previous recent
reports on the ‘abnormal’ binding of N-heterocyclic carbenes in
transition metal complexes by activation of the HC CH imidazole
fragment.20
˚
˚
2e [1.263(3), 1.265(3) A] are the same as in 2a [1.275(3) A] within
experimental error, slightly longer N–Si distances are observed
˚
˚
˚
[1.687(1) A (2d) and 1.677(2), 1.677(2) A (2e) versus 1.655(3) A
(2a)]. For comparison, the N–Si bonds in phosphoraneimines
=
˚
˚
fall in the same range [1.686(2) A (Ph3PNSiMe3), 1.656(4) A
(Cy3PNSiMe3)]. A more detailed evaluation and discussion of
these structural aspects will be given below in connection with
the structures of the desilylated 2-iminoimidazolines 3.
Preparation and characterization of 2-iminoimidazolines
The N-silylated imines 2 are useful compounds, not only be-
cause of their application in the synthesis of transition metal
complexes,5b,10,21 but also because of the possibility of producing 2-
iminoimidazolines 3 by desilylation and cleavage of the N–Si bond.
These compounds and their corresponding bases, the imidazolin-
2-imides, can serve not only as valuable ligands, but also
as building blocks for the preparation of poly(imidazoline-2-
imine) ligands.5b,12,22 Prior to this work, the 2-imino-1,3-dimethyl-
imidazoline reported by Kuhn et al. was the only well documented
2-iminomidazoline derivative, and its synthesis was achieved
by a multi-step protocol from 2-aminoimidazole.23 In con-
trast, our novel procedure for the preparation of 2-trimethyl-
silyliminoimidazolines 2 (vide supra) now allows convenient access
to 2-iminoimidazolines 3 with a variable substitution pattern by
It should be noted that the conversion of 1c into 2c
was accompanied by the formation of noticeable amounts
(15%) of the compound 1,3-diisopropyl-2-(trimethylsilylimino)-4-
trimethylsilylimidazoline (4) (Scheme 3). Imine 2c can be separated
by bulb-to-bulb distillation, and sublimation of the residue affords
the disilylated 4 as a crystalline solid, which was subjected to a
single-crystal X-ray structure analysis. The molecular structure
is presented in Fig. 3, unequivocally proving that the second
trimethylsilyl substituent has been incorporated into the molecule
in the 4-position of the N-heterocycle. The additional silyl
substituent does not alter the metric parameters significantly, and
◦
˚
the C1–N1–Si1 angle [150.9(2) ] and the C1–N1 [1.274(4) A]
˚
and N1–Si1 distances [1.665(2) A] fall in the expected ranges
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desilylation in methanol at ambient temperatures (Scheme 2).
The reaction is complete within two hours, and the formation
of the 2-iminoimidazolines 3 can be easily followed by 1H NMR
spectroscopy with a singlet resonance for the imine proton (NH)
emerging in the range between 4.20 and 4.77 ppm. In addition,
the desilylation is accompanied by a marked low-field shift of the
NCN 13C NMR resonance, which can be observed in the range
from 159.5 to 153.0 ppm, by about 10 ppm.
Sublimation of compounds 3 affords crystalline solids, which
proved suitable for X-ray crystal structure determination in the
cases of 3a (Fig. 4), 3b (Fig. 5) and 3c (Fig. 6). Most noticeably,
the C1–N1 bond distance in all imines 3 is increased with respect to
the corresponding distance in the silyl derivatives 2 [1.295(2) (3a),
˚
˚
1.294(3) (3b) and 1.298(1) A (3c) versus 1.275(3) (2a), 1.267(2) A
˚
(2d) and 1.263(3)/1.265(3) A (2e)], indicating that the imines 3
Fig. 6 ORTEP drawing of 3c with thermal displacement parameters
drawn at 50% probability; the position of the N1–H hydrogen atom is
disordered.
might exhibit a slightly stronger ylidic behaviour than their silyl
congeners 2. In contrast, all other structural parameters remain
almost completely unaffected by the variation of the substitution
pattern. Consequently, the previously structurally characterized
2-imino-1,3-dimethylimidazoline also exhibits similar distances
is also supported by the fact that the imidazole heterocycles
in all compounds 2 and 3 show consistent differences from the
structures of imidazolium ions, which have N–C–N angles in the
23
˚
and angles, e.g. d(C1–N1) = 1.296(2) A. All C1–N1 distances
observed so far clearly fall in the range expected for a C(sp2)–
range 108.3–109.7◦ and C1–N2(3) bonds ranging from 1.315 to
◦
2
24
˚
˚
N(sp ) double bond (1.28 A), suggesting that the solid-state
structures of the imines 2 and 3 are in good agreement with
the non-ylidic mesomeric structure A shown in Scheme 1. This
1.335 A.25 For 2d and 2e, the N2–C1–N3 angles of 103.4(1) and of
102.9(2) and 103.0(2)◦, respectively, are close to the corresponding
angles observed for free imidazolin-2-ylidenes (101.2–102.2◦),1,25,26
whereas the angles in 3a–3c of 105.3(1), 105.1(2) and 105.0(1)◦
adopt an intermediate position between the values found in
imidazolium ions and imidazolin-2-ylidenes. Finally, the internal
˚
C1–N2 and C1–N3 distances of about 1.39 A (Table 1) in both
the imines 2 and 3 are significantly longer than the corresponding
bonds in imidazolium ions (vide supra) and also in imidazolin-2-
˚
ylidenes (1.363–1.373 A), indicating that from a structural point of
view electron delocalization within the heterocycle does not play
a crucial role in these imine systems.
Investigation of the crystal packing in 3a–3c does not reveal
the formation of strong intermolecular N–H · · · N hydrogen
bonds, which one might have anticipated from the basicity of
the exocyclic nitrogen atoms (vide infra). Only 3a exhibits N1–
˚
H1 · · · N1 contacts below 2.75 A, which qualify for hydrogen
Fig. 4 DIAMOND drawing of a hydrogen-bonded dimer of 3a showing
weak inter- and intra-molecular non-covalent contacts with thermal
displacement parameters drawn at 50% probability.
bonding according to the van der Waals cut-off criterion [rvdW(N) =
27
˚
˚
1.55 A, rvdW(H) = 1.20 A]. Two molecules 3a adopt a head-
to-head arrangement with the two imine groups forming a four-
membered NHNH parallelogram (Fig. 4). The position of the
exocyclic imine hydrogen atom could be refined with individual
isotropic displacement parameters producing N–H distances of
˚
0.87(2) and 2.57(2) A and N1–H1 · · · N1 and H1–N1 · · · H1 angles
of 125(1) and 55(1)◦, respectively. These geometric parameters,
specifically the comparatively long non-covalent N1 · · · H1 contact
and the small N1–H1 · · · N1 angle, indicate the weakness of
the hydrogen bonds.28 In addition, intramolecular C–H · · · N1
contacts can be identified with the tert-butyl groups acting as
weak hydrogen donors towards the imine nitrogen atom.29,30 As a
consequence, the methyl groups adopt a staggered conformation
with respect to the N1–H1 bond, and the shortest N1 · · · H
distances to each of the four methyl groups below and above the
˚
N1–H1 moiety are 2.41 and 2.50, and 2.58 and 2.79 A, respectively.
The two shortest contacts are those to the tert-butyl groupattached
to N2, and the imine group is clearly bending over towards this
alkyl substituent, since a significantly smaller N2–C1–N1 angle of
Fig. 5 ORTEP drawing of 3b with thermal displacement parameters
drawn at 50% probability.
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123.5(1)◦ is observed in comparison with the adjacent N3–C1–N1
angle of 131.2(1)◦.
To investigate the relative strength of these intra- and inter-
molecular hydrogen bonds, we performed DFT (density functional
theory) calculations on a dimeric unit of 3a using an augmented
triple zeta basis set. Further details are given in the following
section. The fully optimized gas phase structure of the dimer is
similar to the solid-state structure, and the head-to-head hydrogen
bond arrangement is retained, while the non-covalent N1 · · · H1
˚
distance of 2.644 A is now slightly elongated. Because geometrical
parameters are not always ideal interaction strength descriptors,29
we additionally computed generalized compliance constants as
a second order property.31 The results indeed point to a very
weak non-covalent N1 · · · H1 interaction, which is reflected by the
−1
˚
high value of 24.40 A mdyn . In comparison, strong N–H · · · N
interactions such as in the adenine–thymine or guanine–cytosine
Scheme 4 Calculated proton affinities (PAs) of guanidine model systems
5–7.
base pairs are indicated by short contacts with d(N · · · H) <
−1
˚
˚
2 A and compliance constants of 4.50 and 2.28 A mdyn ,
respectively.32 The intramolecular C–H · · · N1 contacts mentioned
above are also reproduced by our gas phase calculation with
(246.0 kcal mol−1) and 6 (244.6 kcal mol−1). This trend is in
agreement with recent calculations on 6 and 7.35,36
Since proton affinities should not be regarded as the one and
only descriptor for ligand Lewis basicity, we have additionally
computed the NBO (natural bond orbital) charges for the exocyclic
nitrogen atoms. In contrast to the trend in the PAs, a pronounced
difference is found between 5 (−0.822), its saturated analogue 6
(−0.781) and acyclic 7 (−0.755). In order to explain these results
further in terms of resonance structures, all possible contributions
to the total electronic wave function have been determined by
means of natural resonance theory (NRT).37,38 Evaluation of the
relative contribution of all ylidic resonance structures of type B for
compounds 5–7 reveals (Scheme 5) an increase of the zwitterionic
nature for the guanidine 7 (to 14.1%), the saturated ligand 6
(to 16.7%) and the 2-iminoimidazoline system 5 (to 20.6%). A
complete list of the most important NRT structures is given in the
ESI.† This trend is consistent with the stronger capability of the
imidazole ring in 5 to stabilize a positive charge in comparison
with the NCN moieties in 6 and 7. Accordingly, 2-iminoidazolines
should be particularly well suited for the complexation of Lewis-
acidic transition metal complex fragments and for the stabilization
of electron deficient transition metal complexes.
˚
distances ranging from 2.42 to 2.72 A. The associated compliance
−1
˚
constants amount to 8.71–9.31 mdyn A , indicating that the
intramolecular non-covalent interactions are significantly stronger
than the intermolecular ones. Accordingly, we suggest that these
C–H · · · N contacts are responsible for the reduced tendency of
the notably polarized imine moiety to aggregate via strong N–
H · · · N hydrogen bonds and that the negative charge on the
exocyclic nitrogen atom (vide infra) is consequently tempered by
delocalization. The other structures discussed here have not been
theoretically analysed in such detail. The structures as determined
do not display significant N–H · · · N contacts, and the intra- and
inter-molecular C–H · · · N contacts are in general appreciably
˚
longer than in 3a (e.g. intramolecular Cmethyl–H · · · N of 2.53 A
˚
in 3b, or intermolecular C3–H · · · N of 2.66 A in 3c).
Comparative calculations of ligand properties
Since 2-iminoimidazolines represent interesting ligands in their
own right and can also serve as valuable building blocks for the
preparation of poly(imidazoline-2-imine) ligands,5b,12 22 we became
interested in further investigation of their electronic structure
in comparison with related guanidine-type ligands.33 Thus, we
have performed extensive and systematic DFT calculations on the
model systems 5–7 (Scheme 4). All computations were performed
using the hybrid density functional method B3LYP implemented
in the Gaussian03 program34 in combination with a triple zeta
basis set augmented with polarization functions on hydrogen and
all heavy atoms [6-311++G(d,p)]. Since the ligand basicity can be
regarded as a key feature in terms of their reactivity towards tran-
sition metals, we calculated the proton affinities (PAs) of 2-imino-
1,3-dimethylimidazoline (5), 2-imino-1,3-dimethylimidazolidine
(6) and tetramethylguanidine (7). The computed values are given
in Scheme 4. In contrast to earlier calculations employing a
reduced basis set [6-31+G(d,p)], we could not reproduce the
exceptionally high proton affinity of 253.4 kcal mol−1 reported
for 5.5b Our calculations reveal only slight differences in the
proton affinities obtained for 5–7 with the highest value observed
for tetramethylguanidine (7) (246.6 kcal mol−1) followed by 5
Scheme 5 Relative contributions from the ylidic mesomeric structures B
and NBO charges of the exocyclic nitrogen atom for the guanidine model
systems 5–7.
Conclusions
With this contribution, we have presented a convenient and flexible
route for the preparation of various 2-iminoimidazolines. Our
results suggest that the basic and nucleophilic properties of the
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imidazolin-2-ylidenes are transferred to the exocyclic nitrogen
atom. As a result, these imines are strong bases that are likely
to exhibit even stronger nucleophilic properties than related
guanidine-type systems and might therefore serve as valuable
ligands in their own right, and also as important building blocks
for the design and preparation of novel multidentate poly(2-
iminoimidazoline) ligands. In addition, applications in organic
synthesis such as the aza-Wittig reaction can be envisaged.14c,39
153.4 (NCN), 113.3 (NCMe), 44.8 (CHMe), 20.4 (CHCH3), 9.6
(NCCH3) ppm.
Compound 3c. Yield: 91%. Found: C, 64.59; H, 10.71; N,
1
25.13%. Calc. for C9H17N3: C, 64.63; H, 10.24; N, 25.12%; H
NMR (270 MHz, C6D6, 25 ◦C): d 5.81 (s, 2 H, NCH), 4.25 (s,
1 H, NH), 4.13 (br., 2 H, CHMe), 0.96 (d, 3JH,H = 6.7 Hz, 12 H,
CH3) ppm. 13C NMR (67.93 MHz, C6D6): d 153.0 (NCN), 106.4
(NCMe), 44.2 (CHMe), 21.1 (CHCH3) ppm.
Compound 3d. Yield: 95%. 1H NMR (400 MHz, C6D6, 25 ◦C):
d 6.76 (s, 4 H, m-H), 5.71 (s, 2 H, NCH), 4.28 (s, 1 H, NH), 2.23 (s,
12 H, o-CH3), 2.11 (s, 6 H, p-CH3) ppm. 13C NMR (100.52 MHz,
C6D6): d 151.7 (NCN), 137.7 (ipso-C), 137.2 (p-CMe), 134.3 (o-
CMe), 129.2 (m-CH), 112.0 (CH), 20.8 (p-CCH3), 17.9 (o-CCH3)
ppm.
Experimental
All operations were performed in an atmosphere of dry argon by
using Schlenk and vacuum techniques. All solvents were purified
by standard methods and distilled prior to use. 1H and 13C NMR
spectra were recorded on JEOL-GX 400 (400 MHz), JEOL-
GX 270 (270 MHz), Bruker AC 200, Bruker DPX 200, Bruker
AV 300 and Bruker DPX 400 devices. The chemical shifts are
given in ppm relative to TMS. The spin coupling patterns are
indicated as s (singlet), d (doublet), m (multiplet), sept (septet)
and br (broad, for unresolved signals). Elemental analysis (C, H,
N) succeeded by combustion and gas chromatographical analysis
with a Vario EL III CHNS, Carlo Erba Mod. 1106 and a Vario
Micro Cube. Trimethylsilyl azide was received from Aldrich and
Compound 3e. Yield: 97%. Found: C, 80.27; H, 9.02; N,
1
10.26%. Calc. for C27H37N3: C, 80.35; H, 9.24; N, 10.41%; H
NMR (400 MHz, C6D6, 25 ◦C): d 7.22 (m, 4 H, m-H), 7.14 (s,
2 H, p-H), 5.87 (s, 2 H, NCH), 4.21 (br., 1 H, NH), 3.22 (sept.,
4 H, CHMe), 1.35 (d, 12 H, CH3), 1.22 (d, 12 H, CH3) ppm. 13
C
NMR (100.52 MHz, C6D6): d 159.5 (NCN), 154.6 (ipso-C), 148.6
(o-C), 129.6 (p-CH), 124.3 (m-CH), 113.6 (CH), 29.0 (CHMe),
24.1 (CHCH3), 24.0 (CHCH3) ppm.
˚
dried over molecular sieve (4 A). The imidazolin-2-ylidenes 1a–
1f 40–42 and the 2-(trimethylsilylimino)imidazolines 2a–2e 10 were
Compound 3f. Yield: 97%. Found: C, 60.63; H, 9.53; N,
29.83%. Calc. for C7H13N3: C, 60.40; H, 9.41; N, 30.19%; 1H NMR
(200.13 MHz, C6D6, 25 ◦C): d 4.20 (s, 1 H, NH), 2.75 (s, 6 H,
NCH3), 1.48 (s, 6 H, CCH3) ppm. 13C NMR (50.32 MHz, C6D6):
d 155.4 (NCN), 113.0 (NCMe), 44.8 (CHMe), 27.7 (CHCH3), 8.4
(NCCH3) ppm.
prepared according to literature procedures.
1,3,4,5-Tetramethyl-2-(trimethylsilylimino)imidazoline (2f)
A
solution of 1,3,4,5-tetramethylimidazolin-2-ylidene (1f)
(1.242 g, 10 mmol) in toluene (20 mL) was treated dropwise
with trimethylsilyl azide (14 mmol) at ambient temperature, and
the resulting reaction mixture was subsequently heated in boiling
toluene for 24 h. Filtration and evaporation of the solvent afforded
the desired product as a yellowish solid, which can be further
purified by sublimation (yield: 1.416 g, 67%). 1H NMR (200 MHz,
C6D6, 25 ◦C): d 2.80 (s, 6 H, NCH3), 1.45 (s, 6 H, CCH3), 0.53 (s,
12 H, SiCH3) ppm. 13C NMR (50.32 MHz, C6D6): d 152.9 (NCN),
113.1 (NCMe), 28.4 (CHMe), 8.6 (NCCH3), 5.1 (SiCH3) ppm.
1,3-Diisopropyl-2-(trimethylsilylimino)-4-trimethylsilylimidazoline
(4)
A
solution of the 1,3-diisopropyl-4,5-dimethylimidazolin-2-
ylidene (1c) (1.522 g, 10 mmol) in toluene (20 mL) was treated
dropwise with trimethylsilyl azide (20 mmol) at ambient tempera-
ture, and the resulting reaction mixture was subsequently heated in
boiling toluene for 24 h. Filtration and evaporation of the solvent
afforded the mixture of the mono- and di-silylated product in the
ratio 5 : 1 according to the integration of the 1H NMR peaks. The
monosilylated product was distilled off by bulb-to-bulb distillation
at 180 ◦C/9 mbar, and the disilylated product was sublimed from
the residue to give a white solid (yield: 0.498 g, 16%). Found:
C, 57.83; H, 10.26; N, 13.85%. Calc. for C15H33N3Si2: C, 57.82; H,
10.67; N, 13.48%; 1H NMR (400 MHz, C6D6, 25 ◦C): d 6.19 (s, 1 H,
NCH), 4.27 (sept., 1 H, CHMe), 3.94 (sept., 1 H, CHMe), 1.54 (d,
6 H, CCH3), 0.95 (d, 6 H, CCH3), 0.48 (s, 9 H, SiCH3) and 0.14 (s,
9 H, SiCH3) ppm. 13C NMR (100.52 MHz, C6D6): 143.8 (NCN),
121.8 (NCH), 115.2 (NCSi), 50.5 (CHMe), 43.9 (CHMe), 22.0
(CHCH3), 19.9 (CHCH3), 4.4 (NSiCH3), −0.6 (CSiCH3) ppm.
General procedure for the preparation of 2-iminoimidazolines 3
2-(Trimethylsilylimino)imidazolines 2 (1 equiv.) were treated with
an excess of CH3OH (15 equiv.) at ambient temperature for 2 h.
The solvent was then removed in vacuo and the product extracted
with n-hexane. Filtration and evaporation of n-hexane afforded
the imines as colourless solids (3a, 3c, 3d and 3e) or as brownish
oils (3b and 3f), respectively, of which the latter can be purified by
bulb-to-bulb distillation at 70 ◦C/0.1 mbar.
Compound 3a. Yield: 96%. Found: C, 67.39; H, 11.01; N,
1
21.27%. Calc. for C11H21N3: C, 67.65; H, 10.84; N, 21.51%; H
NMR (270 MHz, C6D6, 25 ◦C): d 5.94 (s, 2 H, NCH), 1.39 (s,
18 H, CCH3) ppm. 13C NMR (100.52 MHz, C6D6): d 153.1 (NCN),
107.0 (NCH), 53.8 (NCMe), 27.7 (CCH3) ppm.
Single-crystal X-ray crystal structure determinations
Compounds 2d, 3a, 3b. Data were recorded at −100 ◦C on
an area detector (NONIUS, MACH3, j-CCD) at the window
of a rotating anode (NONIUS, FR591) using MoKa radiation
Compound 3b. Yield: 97%. Found: C, 67.29; H, 11.15; N,
1
21.45%. Calc. for C11H21N3: C, 67.65; H, 10.84; N, 21.51%; H
◦
˚
NMR (270 MHz, C6D6, 25 C): d 4.31 (s, 1 H, NH), 4.17 (sept.,
(k = 0.71073 A). Absorption corrections were applied during the
3JH,H = 6.8 Hz, 2 H, CHMe), 1.63 (s, 6 H, CH3), 1.37 (d, 3JH,H
=
scaling procedure. Structures were refined on F2 using the program
7.2 Hz, 12 H, CH3) ppm. 13C NMR (100.52 MHz, C6D6): d
528 | Org. Biomol. Chem., 2007, 5, 523–530
system SHELXL-97 (G. M. Sheldrick, University of Go¨ttingen,
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Germany). All other calculations were performed with the
STRUX-V system, including the programs PLATON (A. L. Spek,
University of Utrecht, Netherlands), SIR92 (C. Giacovazzo,
University of Bari, Italy). All hydrogen atom positions were found
in difference maps. The hydrogen positions were refined with
individual isotropic displacement parameters. Exceptions/special
features: for 3a, hydrogen atoms were included using rigid methyl
groups allowed to rotate but not tip, or a riding model; for 3b, in the
absence of significant anomalous scattering, no Friedel opposite
reflections were registered.
Crystal data for 4: C15H33N3Si2, monoclinic, space group P21/n,
◦
˚
a = 11.259(2), b = 9.8467(18), c = 18.752(3) A, b = 103.035(6) ,
Z = 4. A colourless prism 0.5 × 0.15 × 0.1 mm was used to record
a total of 17 512 reflections to 2hmax 53.7◦, of which 4145 were
unique. The structure was refined to wR2 = 0.158 (all reflections),
R1 = 0.063 [I > 2r(I)] for 191 parameters; S = 1.13, max.
−3
˚
Dq 0.39 e A .
CCDC reference numbers 618031 (2e), 618034 (3c), 618035 (4).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b615418b.
¯
Crystal data for 2d: C24H33N3Si, triclinic, space group P1, a =
˚
8.4729(1), b = 8.9346(1), c = 15.8498(3) A, a = 88.5376(6), b =
Acknowledgements
80.8585(6), c = 83.8057(6)◦, Z = 2. A yellow irregular fragment
0.40 × 0.35 × 0.35 mm was used to record a total of 8419 reflections
to 2hmax 50.7◦, of which4282were unique. The structure was refined
to wR2 = 0.104 (all reflections), R1 = 0.0374 [I > 2r(I)] for 385
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG Ta 189-6/1). D. P. thanks the Bavarian Science
Foundation (Bayerische Forschungsstiftung) for a scholarship
(2003–2006).
−3
˚
parameters; S = 1.03, max. Dq 0.24 e A .
Crystal data for 3a: C11H21N3, monoclinic, space group P21/◦c,
˚
a = 9.8042(1), b = 11.0624(2), c = 11.6724(2) A, b = 112.3293(7) ,
References
Z = 4. A colourless fragment 0.3 × 0.15 × 0.13 mm was used to
record a total of 4140 reflections to 2hmax 50.7◦, of which 2137 were
unique. The structure was refined to wR2 = 0.0981 (all reflections),
1 M. Regitz, Angew. Chem., 1996, 108, 791–794; M. Regitz, Angew.
Chem., Int. Ed. Engl., 1996, 35, 725–728; W. A. Herrmann and C.
Ko¨cher, Angew. Chem., 1997, 109, 2256–2282; W. A. Herrmann and
C. Ko¨cher, Angew. Chem., Int. Ed. Engl., 1997, 36, 2162–2187; A. J.
Arduengo, III and R. Krafczyk, Chem. Unserer Zeit, 1998, 32, 6–14;
Anthony J. Arduengo, III, Acc. Chem. Res., 1999, 32, 913–921; D.
Bourissou, O. Guerret, F. P. Gabba¨ı and G. Bertrand, Chem. Rev.,
2000, 100, 39–91; C. Wentrup, Science, 2001, 292, 1846–1847; V. Nair,
S. Bindu and V. Sreekumar, Angew. Chem., 2004, 116, 5240–5245; V.
Nair, S. Bindu and V. Sreekumar, Angew. Chem., Int. Ed., 2004, 43,
5130–5135; F. E. Hahn, Angew. Chem., 2006, 118, 1374–1378; F. E.
Hahn, Angew. Chem., Int. Ed., 2006, 45, 1348–1352.
2 L. Jafarpour and S. P. Nolan, Adv. Organomet. Chem., 2000, 46, 181–
222; T. Weskamp, V. P. W. Bo¨hm and W. A. Herrmann, J. Organomet.
Chem., 2000, 600, 12–22; W. A. Herrmann, T. Weskamp and V. P. W.
Bo¨hm, Adv. Organomet. Chem., 2001, 48, 1–69; L. Jafarpour and S. P.
Nolan, J. Organomet. Chem., 2001, 617–618, 17–27; D. Enders and H.
Gielen, J. Organomet. Chem., 2001, 617–618, 70–80; W. A. Herrmann,
Angew. Chem., 2002, 114, 1342–1363; W. A. Herrmann, Angew. Chem.,
Int. Ed., 2002, 41, 1290–1309; A. C. Hillier, G. A. Grasa, M. S. Viciu,
H. M. Lee, C. Yang and S. P. Nolan, J. Organomet. Chem., 2002, 653,
69–82; D. Enders and T. Balensiefer, Acc. Chem. Res., 2004, 37, 534–
541; V. Cesar, S. Bellemin-Laponnaz and L. H. Gade, Chem. Soc. Rev.,
2004, 33, 619–636; E. Peris and R. H. Crabtree, Coord. Chem. Rev.,
2004, 248, 2239–2246; V. Nair, R. S. Menon and V. Sreekumar, Pure
Appl. Chem., 2005, 77, 1191–1198.
3 A. J. Boydston, K. A. Williams and C. W. Bielawski, J. Am. Chem. Soc.,
2005, 127, 12496–12497; M. Egen, K. Kahle, M. Bold, T. Gessner, C.
Lennartz, S. Nord, H.-W. Schmidt, M. Thelakkat, M. Ba¨te, C. Neuber,
W. Kowalsky, C. Schildknecht and H.-H. Johannes, (BASF AG), WO
2006056418, 2006; M. Bold, C. Lennartz, M. Prinz, H.-W. Schmidt,
M. Thelakkat, M. Ba¨te, C. Neuber, W. Kowalsky, C. Schildknecht and
H.-H. Johannes, (BASF AG), WO 2005019373, 2005.
4 J. C. Garrison and W. C. Youngs, Chem. Rev., 2005, 105, 3978–4008.
5 (a) N. Kuhn, M. Go¨hner, G. Frenking and Y. Chen, in Unusual
Structures and Physical Properties in Organometallic Chemistry, ed.
M. Gielen, R. Willem and B. Wrackmeyer, John Wiley & Sons, The
Atrium, West Sussex (UK), 2002, pp. 337–386; (b) N. Kuhn, M. Go¨hner,
M. Grathwohl, J. Wiehoff, G. Frenking and Y. Chen, Z. Anorg. Allg.
Chem., 2003, 629, 793–802.
R1 = 0.038 [I > 2r(I)] for 137 parameters; S = 1.04, max.
−3
˚
Dq 0.16 e A .
Crystal data for 3b: C11H21N3, monoclinic, space group Cc, a =
◦
˚
7.5606(2), b = 16.9628(4), c = 9.1745(2) A, b = 92.2557(8) , Z = 4.
A colourless fragment 0.36 × 0.18 × 0.15 mm was used to record a
total of 1080 reflections to 2hmax 50.8◦, of which 1080 were unique.
The structure was refined to wR2 = 0.0673 (all reflections), R1 =
−3
˚
0.0296 [I > 2r(I)] for 211 parameters; S = 1.10, max. Dq 0.09 e A .
CCDC reference numbers 618030 (2d), 618032 (3a), 618033 (3b).
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b615418b.
Compounds 2e, 3c, 4. Data were recorded at −140 ◦C on a
Bruker SMART 1000 CCD diffractometer (MoKa radiation, k =
˚
0.71073 A). No absorption corrections were applied. Structures
were refined on F2 using the program system SHELXL-97
(G. M. Sheldrick, University of Go¨ttingen, Germany). Hydrogen
atoms were included using rigid methyl groups allowed to rotate
but not tip, or a riding model. Exceptions/special features: for
3c, the hydrogen atom of the NH group was disordered over two
positions, which were refined with half occupancy and an N–H
distance restraint.
¯
Crystal data for 2e: C30H45N3Si, triclinic, space group P1, a =
˚
9.6170(8), b = 12.4620(10), c = 25.214(2) A, a = 88.322(2),
b = 84.261(2), c = 79.785(2)◦, Z = 4. A colourless irregular
fragment 0.40 × 0.35 × 0.3 mm was used to record a total of
20 896 reflections to 2hmax 56.6◦, of which 14 394 were unique. The
structure was refined to wR2 = 0.127 (all reflections), R1 = 0.055
−3
˚
6 C. J. Carmalt and A. H. Cowley, Adv. Inorg. Chem., 2000, 50, 1–
32; N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005, 249, 829–
857.
7 N. Kuhn, K. Kotowski, T. Kratz and G. Henkel, Phosphorus, Sulfur
Silicon Relat. Elem., 1998, 136–138, 517–520.
8 M. Garner, J. Reglinski, I. Cassidy, M. D. Spicer and A. R. Kennedy,
Chem. Commun., 1996, 1975–1976.
9 Selected recent publications: V. K. Landry, J. G. Melnick, D. Buccella,
K. Pang, J. Ulichny and G. Parkin, Inorg. Chem., 2006, 45, 2588–2597;
J. G. Melnick, G. Zhu, D. Buccella and G. Parkin, J. Inorg. Biochem.,
[I > 2r(I)] for 635 parameters; S = 0.90, max. Dq 0.42 e A .
Crystal data for 3c: C9H17N3, monoclinic, space group P21/◦c,
˚
a = 10.3194(11), b = 8.4799(9), c = 12.1497(13) A, b = 106.293(2) ,
Z = 4. A colourless tablet 0.3 × 0.3 × 0.1 mm was used to record a
total of 11 493 reflections to 2hmax 61◦, of which 3121 were unique.
The structure was refined to wR2 = 0.128 (all reflections), R1 =
0.050 [I > 2r(I)] for 120 parameters and 1 restraint; S = 1.03,
−3
˚
max. Dq 0.31 e A .
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The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 523–530 | 529
©

View Article Online
2006, 100, 1147–1154; M. Rombach, J. Seebacher, M. Ji, G. Zhang, G.
He, M. M. Ibrahim, B. Benkmil and H. Vahrenkamp, Inorg. Chem.,
2006, 45, 4571–4575; C. A. Dodds, M. Garner, J. Reglinski and M. D.
Spicer, Inorg. Chem., 2006, 45, 2733–2741; A. Cetin and C. J. Ziegler,
Dalton Trans., 2006, 1006–1008; S. L. Kuan, W. K. Leong, L. Y. Goh
and R. D. Webster, J. Organomet. Chem., 2006, 691, 907–915; C. A.
Dodds, J. Reglinski and M. D. Spicer, Chem.–Eur. J., 2006, 12, 931–939;
I. R. Crossley, A. F. Hill and A. C. Willis, Organometallics, 2006, 25,
289–299.
27 A. Bondi, J. Phys. Chem., 1964, 68, 441–451; R. S. Rowland and R.
Taylor, J. Phys. Chem., 1996, 100, 7384–7391.
28 G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford University
Press, Oxford, 1997.
29 T. Steiner, Angew. Chem., 2002, 114, 50–80; T. Steiner, Angew. Chem.,
Int. Ed., 2002, 41, 48–76; G. R. Desiraju and T. Steiner, The Weak
Hydrogen Bond in Structural Chemistry and Biology, Oxford University
Press, Oxford, 1999.
30 G. R. Desiraju, Chem. Commun., 2005, 2995–3001.
10 (a) M. Tamm, S. Randoll, T. Bannenberg and E. Herdtweck, Chem.
Commun., 2004, 876–877; (b) M. Tamm, S. Beer and E. Herdtweck,
Z. Naturforsch., B: Chem. Sci., 2004, 59, 1497–1504; (c) M. Tamm, S.
Randoll, E. Herdtweck, N. Kleigrewe, G. Kehr, G. Erker and B. Rieger,
Dalton Trans., 2006, 459–467.
11 (a) D. W. Stephan, Organometallics, 2005, 24, 2548–2560; (b) K.
Dehnicke and A. Greiner, Angew. Chem., 2003, 115, 1378–1392; K.
Dehnicke and A. Greiner, Angew. Chem., Int. Ed., 2003, 42, 1340–
1354; (c) K. Dehnicke, M. Krieger and W. Massa, Coord. Chem. Rev.,
1999, 182, 19–65; (d) K. Dehnicke and F. Weller, Coord. Chem. Rev.,
1997, 158, 103–168; (e) K. Dehnicke and J. Stra¨hle, Polyhedron, 1989,
6, 707–726.
31 Compliance constants, whether recorded experimentally or computed
theoretically, measure the displacement of a single internal coordinate
due to an external force. For details, see: J. Grunenberg and N.
Goldberg, J. Am. Chem. Soc., 2000, 122, 6045–6047; G. Frantzius,
R. Streubel, K. Brandhorst and J. Grunenberg, Organometallics, 2006,
25, 118–121; L. H. Jones and B. I. Swanson, Acc. Chem. Res., 1976, 9,
128–134.
32 J. Grunenberg, J. Am. Chem. Soc., 2004, 126, 16310–16311.
33 C. Wu¨rtele, E. Gaoutchenova, K. Harms, M. C. Holthausen, J.
Sundermeyer and S. Schindler, Angew. Chem., 2006, 118, 3951–
3954; C. Wu¨rtele, E. Gaoutchenova, K. Harms, M. C. Holthausen,
J. Sundermeyer and S. Schindler, Angew. Chem., Int. Ed., 2006, 45,
3867–3869; S. Herres-Pawlis, U. Flo¨rke and G. Henkel, Eur. J. Inorg.
Chem., 2005, 3815–3824; M. Schatz, V. Raab, S. P. Foxon, G. Brehm,
S. Schneider, M. Reiher, M. C. Holthausen, J. Sundermeyer and S.
Schindler, Angew. Chem., 2004, 116, 4460–4464; M. Schatz, V. Raab,
S. P. Foxon, G. Brehm, S. Schneider, M. Reiher, M. C. Holthausen, J.
Sundermeyer and S. Schindler, Angew. Chem., Int. Ed., 2004, 43, 4360–
4363; S. Herres, A. J. Heuwing, U. Flo¨rke, J. Schneider and G. Henkel,
Inorg. Chim. Acta, 2005, 358, 1089–1095; H. Wittmann, V. Raab, A.
Schorm, J. Plackmeyer and J. Sundermeyer, Eur. J. Inorg. Chem., 2001,
1937–1948; V. Raab, J. Kipke, O. Burghaus and J. Sundermeyer, Inorg.
Chem., 2001, 40, 6964–6971; S. H. Oakley, M. P. Coles and P. B.
Hitchcock, Inorg. Chem., 2004, 43, 5168–5172; S. Pohl, M. Harmjanz, J.
Schneider, W. Saak and G. Henkel, J. Chem. Soc., Dalton Trans., 2000,
3473–3479; H. Wittmann, A. Schorm and J. Sundermeyer, Z. Anorg.
Allg. Chem., 2000, 626, 1583–1590.
34 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03, Revision
C.02, Gaussian, Inc., Wallingford, CT, 2004.
12 M. Tamm, D. Petrovic, S. Beer, S. Randoll, S. Filimon and T. Glo¨ge,
unpublished results.
13 A similar reaction resulting directly in the formation of 1,3-dimesityl-
2-iminoimidazoline in low yield has been reported previously, see: J. M.
Hopkins, M. Bowdridge, K. N. Robertson, T. S. Cameron, H. A.
Jenkins and J. A. C. Clyburne, J. Org. Chem., 2001, 66, 5713–5716;
in a similar fashion, the reaction of stable N-heterocyclic silylenes with
covalent azidosilanes leads to the intermediate formation silaimines,
which undergo further nucleophilic addition or cycloaddition reactions,
see: M. Denk, R. K. Hayashi and R. West, J. Am. Chem. Soc., 1994,
116, 10813–10814; N. J. Hill, D. F. Moser, I. A. Guzei and R. West,
Organometallics, 2005, 24, 3346–3349.
14 (a) H. Staudinger and J. Meyer, Helv. Chim. Acta, 1919, 2, 635–
646; (b) Y. G. Gololobov, I. N. Zhmurova and L. F. Kasukhin,
Tetrahedron, 1981, 37, 437–472; (c) S. Bra¨se, C. Gil, K. Knepper and V.
Zimmermann, Angew. Chem., 2005, 117, 5320–5374; S. Bra¨se, C. Gil,
K. Knepper and V. Zimmermann, Angew. Chem., Int. Ed., 2005, 44,
5188–5240.
15 W. Q. Tian and Y. A. Wang, J. Org. Chem., 2004, 69, 4299–4308.
16 D. M. Khramov and C. W. Bielawski, Chem. Commun., 2005, 4958–
4960.
17 A. Diefenbach and F. M. Bickelhaupt, Z. Anorg. Allg. Chem., 1999,
625, 892–900.
18 F. Weller, H.-C. Kang, W. Massa, T. Ru¨benstahl, F. Kunkel and K.
Dehnicke, Z. Naturforsch., B: Chem. Sci., 1995, 50, 1050–1054.
19 A. Mu¨ller, M. Mo¨hlen, B. Neumu¨ller, N. Faza, W. Massa and K.
Dehnicke, Z. Anorg. Allg. Chem., 1999, 625, 1748–1751.
20 D. Bacciu, K. J. Cavell, I. A. Fallis and L.-l. Ooi, Angew. Chem., 2005,
117, 5416–5418; D. Bacciu, K. J. Cavell, I. A. Fallis and L.-l. Ooi,
Angew. Chem., Int. Ed., 2005, 44, 5282–5284; H. Lebel, M. K. Janes,
A. B. Charette and S. P. Nolan, J. Am. Chem. Soc., 2004, 126, 5046–
5047; A. R. Chianese, A. Kovacevic, B. M. Zeglis, J. W. Faller and R. H.
Crabtree, Organometallics, 2004, 23, 2461–2468; S. Gru¨ndemann, A.
Kovacevic, M. Albrecht, J. W. Faller and R. H. Crabtree, J. Am. Chem.
Soc., 2002, 124, 10473–10481.
35 A. A. Kolomeitsev, I. A. Koppel, T. Rodima, J. Barten, E. Lork, G.-V.
Ro¨schenthaler, I. Kaljurand, A. Ku¨tt, I. Koppel, V. Ma¨ements and I.
Leito, J. Am. Chem. Soc., 2005, 127, 17656–17666.
21 N. Kuhn, R. Fawzi, M. Steimann and J. Wiethoff, Z. Anorg. Allg.
Chem., 1997, 623, 769–774; N. Kuhn, R. Fawzi, M. Steimann and J.
Wiethoff, Z. Anorg. Allg. Chem., 1997, 623, 554–560.
22 N. Kuhn, M. Grathwohl, M. Steimann and G. Henkel, Z. Naturforsch.,
B: Chem. Sci., 1998, 53, 997–1003; N. Kuhn, M. Grathwohl, C.
Nachtigal and M. Steimann, Z. Naturforsch., B: Chem. Sci., 2001,
56, 704–710.
23 N. Kuhn, R. Fawzi, M. Steimann, J. Wiethoff, D. Bla¨ser and R. Boese,
Z. Naturforsch., B: Chem. Sci., 1995, 50, 1779–1784 and refs cited
therein.
36 I. A. Koppel, R. Schwesinger, T. Breuer, P. Burk, K. Herodes, I. Koppel,
I. Leito and M. Mishima, J. Phys. Chem. A, 2001, 105, 9575–9586.
37 E. D. Glendening and F. Weinhold, J. Comput. Chem., 1998, 19,
593.
38 The NRT analysis was done using the NBO 5.0 G program extension
for Gaussian; E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E.
Carpenter, J. A. Bohmann, C. M. Morales and F. Weinhold, Theoretical
Chemistry Institute, University of Wisconsin, Madison, WI, 2001.
39 S. Shah and J. D. Protasiewicz, Coord. Chem. Rev., 2000, 210, 181–
201; F. P. Coss´ıo, C. Alonso, B. Lecea, M. Ayerbe, G. Rubiales and F.
Palacios, J. Org. Chem., 2006, 71, 2839–2847.
24 J. March, Advanced Organic Chemistry, John Wiley & Sons, Inc., New
York, 4th edn, 1992, p. 21.
40 A. J. Arduengo, H. Bock, H. Chen, M. Denk, D. A. Dixon, J. C. Green,
W. A. Herrmann, N. L. Jones, M. Wagner and R. West, J. Am. Chem.
Soc., 1994, 116, 6641–6649.
25 A. J. Arduengo, III, H. V. Rasika Dias, R. L. Harlow and M. Kline,
J. Am. Chem. Soc., 1992, 114, 5530–5534.
26 A. J. Arduengo, III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361–363; A. J. Arduengo, III, H. V. Rasika Dias, D. A.
Dixon, R. L. Harlow, W. T. Klooster and T. F. Koetzle, J. Am. Chem.
Soc., 1994, 116, 6812–6822.
41 A. J. Arduengo, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R.
Goerlich, W. J. Marshall and M. Unverzagt, Tetrahedron, 1999, 55,
14523–14534.
42 N. Kuhn and T. Kratz, Synthesis, 1993, 561–562.
530 | Org. Biomol. Chem., 2007, 5, 523–530
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