A new class of organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5- dimethylimidazol-2-ylidene amines: Synthesis, structure, pKBH+ measurements, and properties

Article abstract of DOI:10.1002/chem.201102249

A series of stable organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5- dimethylimidazol-2-ylidene amines, were efficiently synthesized from N,N′-dialkylthioureas and 3-hydroxy-2-butanone and their basicities were measured in acetonitrile. The derivativ

Full text of DOI:10.1002/chem.201102249

FULL PAPER
DOI: 10.1002/chem.201102249
A New Class of Organosuperbases, ACTHNUTRGNENNUG -Alkyl- and N-Aryl-1,3-dialkyl-4,5-
dimethylimidazol-2-ylidene Amines: Synthesis, Structure, pKBH+
Measurements, and Properties
Roman A. Kunetskiy,*[a] Svetlana M. Polyakova,[a] Jirˇꢀ Vavrˇꢀk,[a] Ivana Cꢀsarˇovꢁ,[b]
Jaan Saame,[c] Eva Roos Nerut,[c] Ivar Koppel,[c] Ilmar A. Koppel,[c] Agnes Kꢂtt,[c]
Ivo Leito,*[c] and Ilya M. Lyapkalo†*[a]
Abstract: A series of stable organosu-
perbases, N-alkyl- and N-aryl-1,3-di-
ACHTUNGTRENNUNGalkyl-4,5-dimethylimidazol-2-ylidene
amines, were efficiently synthesized
from N,N’-dialkylthioureas and 3-hy-
droxy-2-butanone and their basicities
were measured in acetonitrile. The de-
rivatives with tert-alkyl groups on the
imino nitrogen were found to be more
discussed. In acetonitrile and in the gas
phase, the basicity of the alkylimino
derivatives depends on the size of the
substituent at the imino group, which
influences the degree of aromatization
of the imidazole ring, as measured by
13C NMR chemical shifts or by the cal-
culated DNICS(1) aromaticity parame-
ters, as well as on solvation effects. If a
wider range of imino-substituents, in-
cluding electron-acceptor substituents,
is treated in the analysis then the influ-
ence of aromatization is less predomi-
nant and the gas-phase basicity be-
comes more dependent on the field-in-
ductive effect, polarizability, and reso-
nance effects of the substituent.
Keywords: amines · aromaticity ·
basicity · imidazolylidene amines ·
steric hindrance · substituent ef-
fects · superbases
basic than the tBuP1ACTHNUGRTNEUNG(pyrr) phospha-
zene base in acetonitrile. The origin of
the high basicity of these compounds is
Introduction
Uncharged organosuperbases,[1] such as amidines, guani-
dines, phosphazenes, and the Verkade phosphatrane bases
(Figure 1), are an important class of reagent in modern or-
ganic chemistry. The synthesis of organosuperbases,[1c,2] their
basicity in solution[2,3,4] and in the gas phase,[3b,5,6] different
modifications,[1c,3a,7,8] and potential applications[1a–h] have
been extensively studied. Their success is based on the ease
Figure 1. The common organosuperbases with their basicity in acetonitri-
le[1j,4c] and in the gas phase.[3a,5]
ˇ
[a] R. A. Kunetskiy, Dr. S. M. Polyakova, J. Vavrꢀk, Dr. I. M. Lyapkalo
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic
Flemingovo nam. 2, 16610 Prague 6 (Czech Republic)
Fax : (+420)220-183-578
of their molecular modification, good solubility in most or-
ganic solvents, and recycling possibilities. In addition, bases
such as DBU, TMG or the Schwesinger P1 phosphazenes
are often applied as strong metal-free Brønsted bases in
stoichiometric amounts to substrates.[1c] In spite of the afore-
mentioned favorable properties, limitations, such as their nu-
cleophilicity, scope of applicability, functional-group com-
patibility, high molecular weight, high price, toxicity, stabili-
ty to air, and hydrolysis, remain. That is the reason that the
design, synthesis, and application of new classes of organo-
superbases are of general interest in modern organic chemis-
try. In particular, the search for simple and accessible mole-
cules that display high basicity (comparable to or higher
than the P1-phosphazene bases) and exploit other principles
governing basicity is certainly required.
E-mail: roman.kunetskiy@gmail.com
ˇ
[b] Dr. I. Cꢀsarovꢁ
Department of Inorganic Chemistry, Charles University
Hlavova 2030, 12840 Prague 2 (Czech Republic)
[c] J. Saame, E. R. Nerut, Dr. I. Koppel, Prof. I. A. Koppel, Dr. A. Kꢂtt,
Prof. I. Leito
Institute of Chemistry, University of Tartu
Ravila 14a, 50411 Tartu (Estonia)
E-mail: ivo.leito@ut.ee
[†] Deceased, September 10, 2010.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102249. It contains synthesis
and characterization data for compounds 1a–1j, 1aH+–1jH+,
1aMe+, 6a, 7, and 8, X-ray crystallographic data for salts 1a·HBF4,
1h·HBF4, 1g·HBF4, 1i·HOTs, 1j·HBF4, and base 1h, a detailed de-
scription of measurement of the DpKBH+ values in MeCN, computa-
tional data, and NMR spectra.
Recently, the outstanding thermal and hydrolytic stability
of bis(imidazol-2-ylidene)ammonium cations (BIMAs,
Figure 2) in alkaline media was discovered.[9] Their proper-
Chem. Eur. J. 2012, 18, 3621 – 3630
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3621

on the basicity of the bases is reported. To support the re-
sults a comprehensive study of the role of the field, reso-
nance, polarizability, and steric substituent effects[13,14] of a
wider range of alkyl and electronegative substituents, R
(e.g., F, Cl, Me, triflate (OTf), CH2X, etc.), for the subseries
of bases 1, in which Alk=iPr, is included in the computa-
tional study.
Figure 2. The principle behind the design of novel superbases 1.
ties were tentatively attributed to efficient resonance charge
delocalization and stabilization of the positive charge over
the entire cation. The bulky lipophilic N-alkyl substituents
also contribute by sterically protecting the cation. This sug-
gests that the N,N’-dialkyl-4,5-dimethylimidazol-2-ylidene
amine motif 1 serves exceptionally well as the basis for the
design of new uncharged strong bases (Figure 2). An exami-
nation of the literature data revealed that only one relevant
Experimental Section
Synthesis: A representative synthetic procedure is provided for 1a. The
synthesis of compounds 1b–1j is described in the Supporting Informa-
tion.
Synthesis of 2-chloro-1,3-diisopropyl-4,5-dimethylimidazolium tetrafluoro-
borate (5a) from 1,3-diisopropyl-4,5-dimethyl-1H-imidazole-2(3H)-thione
(4a),
a one-pot potassium-free protocol: Freshly distilled diglyme
(150 mL) was added to a flask with Na (5.80 g, 250 mmol) under an inert
atmosphere. The flask was heated with a heat gun until the sodium
melted and stirring was continued at this temperature for 10 min. After
cooling to room temperature, imidazole-2-thione 4a[9] (10.6 g, 50.0 mmol)
was added and the reaction mixture was stirred at 1108C for 24 h. After
again cooling to room temperature, stirring was stopped, and the inor-
ganic residue was allowed to precipitate over 12 h. The supernatant was
carefully transferred through a cannula into a second flask (traces of fine
white particles are not a problem) and cooled to ꢀ408C. Hexachloro-
ethane (13.0 g, 55.0 mmol) was added with vigorous stirring, and the reac-
tion mixture was warmed to room temperature over 2 h. After stirring
overnight, toluene (150 mL) was added, and the resulting precipitate was
collected on a glass filter over Celite and washed twice with toluene
(50 mL). The product/Celite mixture was taken up in CHCl3. The result-
ing suspension was filtered from Celite and the organic layer was trans-
ferred with chloroform (200 mL) directly into a separating funnel. A di-
luted aqueous solution of NaBF4 (27.5 g, 250 mmol) was added, the two-
phase mixture was shaken vigorously, and the layers were separated. The
aqueous layer was extracted once with a small portion of chloroform.
The combined chloroform fractions were dried (MgSO4), filtered, and
evaporated under reduced pressure, yielding product 5a (9.68 g, 64%) as
a colorless crystalline solid after recrystallization from EtOAc/MeOH
(20:1). The analytical data were in agreement with those previously re-
ported.[15a]
base,
1-methyl-1,5,6,7-tetrahydroimidazoACTHNGUTERN[UNG 1,2-a]pyrimidine
(ImPyr, Figure 1) has been described so far, for which a reli-
able experimental pKBH+ of 24.55 in acetonitrile (MeCN)
has been determined.[1i,10] This base was also investigated
computationally.[11]
Herein, we report a new family of lipophilic organosuper-
bases incorporating a fully substituted pseudo-imidazole
core: N-[1,3-dialkyl-4,5-dimethyl-1H-imidazol-2(3H)-ylide-
ne]alkyl- or -arylamines 1 (Figure 2). Preliminary experi-
ments indicated that the base 1a (Alk=iPr and R=tBu)
has a pKBH+ in acetonitrile close to 30. At the same time its
cyclic counterpart ImPyr has a pKBH+ value of only 24.55.
The dramatic change in solution basicity as a result of small
structural changes prompted a detailed analysis of the struc-
ture of these compounds. This leads to the hypothesis that a
synergistic effect of aromatization and some release of steric
strain upon protonation influence their basicity (Figure 3).
Herein, the validity of this hypothesis is demonstrated based
on an experimental and computational study.
Synthesis of 1a·HBF4: KF (3.49 g, 60.0 mmol) was heated in a reaction
flask with a heat gun under vacuum for a few minutes to remove all
traces of moisture. After cooling, salt 5a (3.03 g, 10.0 mmol) and a mag-
netic stirring bar were added, followed by MeCN (30 mL) and tBuNH2
(3.15 mL, 30.0 mmol). The reaction mixture was stirred at ambient tem-
perature for 72 h. Once the 1H NMR spectrum of the reaction mixture in-
dicated full conversion of the starting material had occurred, chloroform
(30 mL) was added. After stirring for 5 min, the suspension was filtered
through a Schott glass filter directly into a separating funnel, and the
solid residue was washed a few times with small portions of chloroform.
A diluted aqueous solution of NaBF4 (5.50 g, 50.0 mmol) was added to
the solution in chloroform, followed by vigorous shaking. The layers
were separated. The aqueous layer was extracted once with a small por-
tion of chloroform. The combined chloroform fractions were dried
(MgSO4), filtered, and evaporated under reduced pressure, yielding salt
1a·HBF4 (3.05 g, 90%) as colorless crystals after recrystallization. M.p.
149–1508C (EtOAc); 1H NMR (400 MHz, CDCl3, SiMe4): d=1.28 (s,
9H; CMe3), 1.54 (d, 3JH,H =7.1 Hz, 12H; 2ꢄCHMe2), 2.34 (s, 6H; 2ꢄ
Me), 4.51 (s, 1H; NH), 5.01 ppm (sept, 3JH,H =7.1 Hz, 2H; 2ꢄCH);
13C NMR (101 MHz, CDCl3): d=10.2 (Me), 21.3 (CHMe2), 30.7 (CMe3),
50.3 (CHMe2), 55.6 (CMe3), 124.3 (C=C), 141.6 ppm (CNH); IR (neat):
v˜ =1013 (vs), 1053 (vs), 1085 (vs), 1192 (m), 1212 (m), 1373 (m), 1391
(m), 1409 (m), 1472 (w), 1519 (m), 1629 (w), 2982 (w) 3342 cmꢀ1 (w); MS
(ESI+): m/z (%): 252 (100) [M+]; HRMS (ESI+): m/z calcd for
C15H30N3+: 252.2434 [M+]; found: 252.2434; elemental analysis calcd (%)
Figure 3. Hypothesis for the role of aromaticity in causing the strong ba-
sicity of superbases 1.
An efficient and general method for the synthesis of imi-
dazol-2-ylidene amines 1 (hereafter named IMAMs) is de-
scribed, which allows the variation of the steric and elec-
tronic properties of the substituents Alk and R in the sub-
strates. Their pKBH+ values in acetonitrile were determined
and a computational study of gas-phase basicities (GB), as
well as the nucleus-independent chemical shift (NICS)
values,[12] for evaluation of the possible role of aromatization
3622
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3621 – 3630

Organosuperbases
FULL PAPER
for C15H30BF4N3 (339.22): C 53.11, H 8.91, N 12.39; found: C 53.19, H
9.00, N 12.20.
Crystallographic data for 1h: C18H27N3O; monoclinic; space group P21/c;
a=12.4823(4), b=10.8254(4), c=12.9650(5) ꢅ; b=98.714 (2)8; Z=4; col-
orless crystal 0.78ꢄ0.43ꢄ0.32 mm; reflections/parameters 3981/206;
Liberation of base 1a: Salt 1a·HBF4 (13.6 g, 40.0 mmol) was dissolved in
MeOH (40 mL) and hexane (200 mL) was added, followed by aqueous
KOH (50%; 40 mL) with vigorous stirring. The mixture was stirred at
ambient temperature for several minutes. The hexane layer was carefully
separated through a cannula into a flask filled with argon. Extraction of
the mixture with hexane (200 mL) was repeated twice. The combined
hexane fractions were dried over BaO, filtered, evaporated, and dried in
a high vacuum at 508C to give pure base 1a (7.64 g, 76%) as a colorless
oily substance. 1H NMR (400 MHz, C6D6): d=1.23 (d, 3JH,H =7.1 Hz,
12H; 2ꢄCHMe2), 1.57 (s, 9H; CMe3), 1.72 (s, 6H; 2ꢄMe), 4.54 ppm
(sept, 3JH,H =7.1 Hz, 2H; 2ꢄCH); 13C NMR (101 MHz, C6D6): d=11.1
(Me), 21.0 (CHMe2), 33.7 (CMe3), 47.1 (CHMe2), 51.7 (CMe3), 116.1 (C=
C), 146.2 ppm (NCN); IR (hexane): v˜ =1632 (vs), 1679 (m), 1685 cmꢀ1
(m); MS (CI+): m/z (%): 154 (15) [M+HꢀC3H6ꢀC4H8]+, 196 (10)
[M+HꢀC4H8]+, 208 (24) [MꢀHꢀC3H6]+, 209 (52) [MꢀC3H6]+·, 210 (75)
[M+HꢀC3H6]+, 236 (40) [MꢀCH3]+, 250 (20) [MꢀH]+, 251 (48) [M]+,
252 (100) [M+H]+; HRMS (CI+): m/z calcd for C15H30N3+: 252.2440
[M+H]+; found: 252.2445.
[F2>2s(F2)]=0.044; wR(F2)=0.117 (all data); S=1.03; D1max
RACHTUNGETRNUNNG =
0.47 eꢅꢀ3
.
Crystallographic data for 1h·HBF4: C18H28N3O·BF4; monoclinic; space
group C2/c; a=33.575(3), b=8.6587(8), c=15.1732(12) ꢅ; b=111.928
(3)8; Z=8; colorless crystal 0.56ꢄ0.28ꢄ0.27 mm; reflections/parameters
4702/268;
RACTHNGUTERNNUG
[F2>2s(F2)]=0.055; wR(F2)=0.152 (all data); S=1.03;
D1max =0.43 eꢅꢀ3
.
Results
Synthesis: The synthesis of monocyclic IMAM bases 1a–1h
commenced with a condensation reaction of commercially
or synthetically available N,N’-dialkylthioureas 3a or 3b
with 3-hydroxybutan-2-one 2 (Scheme 1). The corresponding
1,3-dialkyl-4,5-dimethyl-1H-imidazole-2(3H)-thiones 4a and
4b were obtained in good yields by heating at reflux in amyl
alcohol, by following a literature procedure.[15]
Computational methods: All calculations were carried out with the Gaus-
sian 09 program package[16] by using density functional theory (DFT)
hybrid functional B3LYP and a 6–311+G** basis set. For optimized
structures, vibrational frequencies were calculated to confirm that the sta-
tionary point corresponds to a true minimum and to calculate zero-point
vibrational energies (ZPVE), enthalpies, and Gibbs free energies.
All NICS values [ppm] were calculated at the B3LYP/6–311+G** level
of theory for all neutral and protonated bases. More negative NICS
values indicate an increase in the aromatic character, whereas positive
values show that the system is antiaromatic. Values close to zero appear
if the system has neither aromatic nor antiaromatic character.
NICS(0)[12] values are calculated at the mass center of the ring, however,
it has been noted that in the case of smaller rings some of the shielding
effect comes from s electrons and therefore NICS(1) values are pre-
ferred.[12] NICS(1) values are calculated 1 ꢅ above and/or below the
mass center of the ring, corresponding to NICS(+1) and NICS(ꢀ1), re-
spectively. In this study NICS(1) corresponds to the average of the
values, since not all of the substances studied are symmetrical and there-
fore, in some cases, there are slight differences in the NICS(+1) and
NICS(ꢀ1) values. The term DNICS(1) denotes the difference in NICS(1)
values between the protonated and neutral forms of a substance.
The GB of a base B refers to the reaction given in Equation (1):
K
B þ Hþ
BHþ
H
ð1Þ
G
Scheme 1. Synthesis of bases 1a–1h: a) amyl alcohol, reflux, 24 h; b) Na,
diglyme, 1108C, 24 h, then decantation; c) C2Cl6, ꢀ408C to RT, 2 h;
d) R-NH2, KF, MeCN, RT, 2–3 d, then NaBF4; e) aqueous KOH (50%),
MeOH, RT, 15 min, hexane extraction; f) KF, MeCN, RT, 3 d; g) R-NH2,
MeCN, 608C, 2–3 d (*Isolated yields are given for the two steps starting
from 5 for bases 1a–1 f and from 6 for bases 1g and 1h).
The GB of a base B at a temperature T is defined to be the negative
value of the Gibbs free energy [Eq. (2)] for the reaction given in Equa-
tion (1):
GB ꢁ ꢀDG ¼ RTlnK
ð2Þ
pKBH+ measurements: The pKBH+ values of the protonated forms of the
bases were measured according to the method described in refs. [4a and
c]. The uncertainties in the pKBH+ values are estimated to be ꢂ0.15
pKBH+ units. See the Supporting Information for more details.
Although a two-step procedure for the synthesis of 5
from 4 via the intermediate imidazol-2-ylidenes (N-hetero-
cyclic carbenes (NHC))[20] is documented,[9,15,21] a significant-
ly modified and simplified one-pot protocol was developed,
enabling the synthesis of 5a from 4a in multigram quanti-
ties. Heating of 4a with sodium in diglyme at 1108C, fol-
lowed by quenching of the intermediate NHC with hexa-
chloroethane provided 5a in an overall yield of 64% by
using neither potassium nor reflux conditions.
X-Ray crystallography: Data were collected on Bruker APEX-II CCD
(1h, 1h·HBF4) and Nonius Kappa CCD diffractometers at a temperature
of 150 K. Structures were solved by direct methods (SIR92)[17] and re-
fined by full-matrix least-squares techniques based on F2
(SHELXL97).[18] The hydrogen atoms on carbon were fixed into ideal-
ized positions (riding model) and assigned temperature factors of either
Hiso(H)=1.2 Ueq(pivot atom) or Hiso(H)=1.5 Ueq(pivot atom) for the
ꢀ
methyl moiety. The hydrogen atom of N H moieties was found on differ-
The IMAM salts 1aH+–1 fH+ were synthesized in good
yields by KF-mediated coupling of 2-chloroimidazolium
salts 5a or literature-known compound 5b[9] with the corre-
ence Fourier maps and refined isotropically. The crystallographic data for
compounds 1a·HBF4, 1h·HBF4, 1h, 1g·HBF4, 1i·HOTs (Ts=tosyl), and
1j·HBF4 are included in the Supporting Information.[19]
Chem. Eur. J. 2012, 18, 3621 – 3630
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3623

R. A. Kunetskiy, I. Leito, I. M. Lyapkalo et al.
sponding primary amines in acetonitrile. The presence of
KF was mandatory for the synthesis of the IMAM salts with
no conversion of 5a being observed with an excess of tert-
butylamine in [D3]-acetonitrile after 1 day of heating in the
absence of KF. It was firmly proven that KF played a dual
role in this synthesis. On the one hand, it served as an acti-
vating agent, forming the intermediate 2-fluoroimidazolium
salt 6, which is a significantly better electrophile than
2-chloroimidazolium salt 5. On the other hand, it acted as a
strong base, scavenging protons and thus driving the process
to completion.
afforded bicyclic salts 1iH+ and 1jH+. The IMAM bases 1i
and 1j were liberated, as described above for 1a–1h, in high
yields.
An attempt to perform a Staudinger-type synthesis of
base 1a from an N-tert-butyl triazene precursor[9] was unsuc-
cessful. Its thermolysis under various conditions led invaria-
bly to extrusion of isobutene and nitrogen, yielding 1k
(Figure 2, Alk=iPr, R=H). Alternatively, base 1k was ob-
tained through the corresponding N-trimethyl silyl (TMS)-
derivative in 60% yield, by following a literature proce-
dure.[25]
The IMAM bases 1a–1h were liberated in high yields
from the corresponding salts by using a 1:1 v/v mixture of
aqueous KOH (50%) and methanol, which is similar to the
conditions Schwesinger et al.[22] applied for the liberation of
P1 bases from their salts.
Unfortunately, para-anisidine underwent a fast double
substitution reaction with 5a, affording the dicationic salt 7
(Scheme 2). Consequently, to obtain the desired product
IMAM bases are stable liquids (1a–1 f and 1i) or crystal-
line solids (1g, 1h, 1j, and 1k) that can be stored for several
months as stock solutions in hexane at 58C over BaO. They
are readily miscible with common aprotic organic solvents,
including hydrocarbons. The most basic representatives
were found to be sensitive to atmospheric moisture and
CO2, since they rapidly convert to the protonated form
(probably as the bicarbonate salt). Despite its highly hydro-
phobic substitution pattern, base 1a is soluble in D2O, in
which it exists in the fully ionized form 1aD+·ODꢀ, as
shown in the 13C NMR spectrum. It is quite stable towards
hydrolysis, and no changes were observed after heating at
1008C in D2O for one hour.
The nucleophilicities of bases 1a and 1 f were compared
to that of the known compound tBuP
competition experiments. Equimolar mixtures of 1a/tBuP1-
(pyrr) or 1 f/tBuP1A(pyrr) and methyl iodide in [D3]-MeCN
1ACHTUNGTREN(NGNU pyrr) by use of NMR
A
CHTUNGTRENNUNG
1
were monitored by H NMR spectroscopy at room tempera-
Scheme 2. Formation of dication 7. a) 5a, KF, MeCN, 608C, 3 d.
ture. The nucleophilicity of 1a and 1 f proved to be approxi-
mately 60 and 5 times higher than that of tBuP1ACTHUNRTGNE(GNU pyrr), re-
spectively.
1hH+, the intermediate 2-fluoroimidazolium salt 6a was iso-
lated first, followed by direct coupling with an excess of
para-anisidine in the absence of KF (Scheme 1, 5!6!1H+
pathway). A similar indirect coupling approach was also
used for the synthesis of 1gH+ because a considerably
better yield was achieved.
The synthesis of the bicyclic IMAM bases 1i and 1j was
accomplished from 2. Its reaction with butylamine,[23] fol-
lowed by condensation of the intermediate a-aminoketone
with cyanamide,[24] gave an aminoimidazolium salt, which
was converted into the free base 8 by deprotonation with
ammonia (Scheme 3). Alkylation of 8 with 1,3-propylene di-
Selected solid-state structures: The solid-state structures of
the free base 1h (Figure 4) and the salts 1h·HBF4
(Figure 5), 1a·HBF4, 1g·BF4, 1i·OTs, and 1j·HBF4 (for the
last four structures, see the Supporting Information) were
characterized by X-ray crystal-structure analysis.[19] The
average N1(3)-C2-N4-C14 torsion angle (34.88) for base 1h
ꢀ
indicates a deviation of the C2 N4 double bond from a
planar geometry. A slight distortion of the exo-nitrogen
atom (N4) out of the imidazole ring plane (6.88) was also
observed for base 1h. On protonation of base 1h, the
ꢀ
ꢀ
N1(3) C2 and N1(3) C5(4) bonds were shortened by 0.010–
ꢀ
tosylate or 2,2-dimethyl-1,3-propylene bis
N
0.036 ꢅ, whereas the C4 C5 bond was elongated by
0.013 ꢅ, thus averaging of the bond lengths in the planar
imidazolium ring in cation 1hH+ is observed. The C2 N4
ꢀ
bond was elongated by 0.064 ꢅ. The average N1(3)-C2-N4-
C14 torsion angle became closer to orthogonal, at 70.08, for
salt 1hH+.
Basicity of imidazol-2-ylidene amines 1a–1k: The alkyl-sub-
stituted imidazol-2-ylidene amines (bases 1a–1g) are strong
bases in acetonitrile, the strongest of which are those con-
taining a sterically demanding alkyl substituent on the imine
nitrogen atom (Table 1). However, increasing the steric
demand beyond a tert-butyl substituent does not significant-
Scheme 3. Synthesis of the bicyclic bases 1i and 1j. a) BuNH2, cyclohex-
ane, reflux, 1 h, then aqueous HCl; b) NCNH2, H2O, 90 8C, 24 h, then
NH3 (1m); c) (TsOCH2)2CH2, DMF or (TfOCH2)2CMe2, MeCN, RT, 3 d;
d) aqueous KOH (50%), MeOH, RT, 15 min, hexane extraction.
3624
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3621 – 3630

Organosuperbases
FULL PAPER
vs. 1b). As the steric demand of the imino substituent de-
creases, the basicity generally becomes lower (1a>1c>
1e>1k). Surprisingly, the N-methyl derivative 1e is slightly
more basic than the N-neopentyl imidazole 1d and one of
the most hindered derivatives 1g is slightly less basic than
1a. In the gas phase, the same base pairs have almost equal
basicity. These differences imply the importance of solvation
on the basicity in MeCN. The solvation effect gives addition-
al stability to the protonated forms of the bases and the sol-
vent molecules interact preferentially with the protonated
basicity center. The bulkier the imino substituent, the more
hindered the solvation of this center. Nevertheless, the in-
trinsically higher basicity of the compounds with bulkier
imino substituents outweighs the solvation effects.
Compound 1h with an aromatic imino substituent dis-
played by far the weakest basicity among bases 1. Bicyclic
compounds 1i and 1j display a significantly lower basicity
than the monocyclic bases, but still a higher one than 1k
and ImPyr (Figure 1). The difference in basicity between 1i
and ImPyr provides an estimation of the contribution of two
additional donor methyl groups to the bicyclic system to be
calculated, giving a value of approximately one pKBH+ unit.
Figure 4. X-Ray crystal structure of base 1h. The displacement ellipsoids
ꢀ
are drawn at the 30% probability level. Selected bonds lengths [ꢅ]: N4
ꢀ
ꢀ
ꢀ
ꢀ
C2 1.308(2), N1 C2 1.370(2), N3 C2 1.382(2), N1 C5 1.396(2), N3 C4
Basicity of 1a–1k in solution and the effect of aromaticity:
The contribution of aromatic resonance structure 1’
(Figure 2) to the structure of bases 1a–1k affects their
chemical shifts. Aromaticity is reflected by a net leveling of
the differences in the chemical shifts at C2 and C4,5 of the
imidazole ring. Thus, the 13C NMR chemical-shift difference
ꢀ
1.414(2), C4 C5 1.341(2); average torsion angle N1(3)-C2-N4-C14: 34.88.
D=d(C2)ꢀdACHTUGNTREN(NUNG C4,5) can be used to assess the level of aroma-
ticity in the imidazole rings in bases 1a–1k, as well as in
their salts (see Table S7 in the Supporting Information). The
smaller the value of D, the larger the contribution of the
aromaticity to the structure. The D values of the free bases 1
increase in the order: 1a (R=tBu, 30.1 ppm)<1c (R=iPr,
32.6 ppm)<1e (R=Me, 35.9 ppm)<1k (R=H, 40.1 ppm).
The same order is found for the salts 1H+: 1aH+ (R=tBu,
17.3 ppm)<1cH+ (R=iPr, 19.5 ppm)<1eH+ (R=Me,
21.6 ppm)<1kH+ (R=H, 23.4 ppm). The steric hindrance
caused by the substituent on the Nexo atom increases in the
opposite order. This suggests that as the steric bulk of the
substituent at Nexo increases and the steric repulsion with
the iPr substituents on N1 or N3 increases, the double-bond
character of the imine is reduced and the contribution of
the aromatic imidazolium amide resonance structure 1’ in-
creases.
Reasonable linear correlations were found when the
chemical shift differences, D, for bases 1a–1k and the corre-
sponding salts 1aH+–1kH+ were plotted against the mea-
sured pKBH+ values (Figure 6). The correlation holds even
better for the average D values of bases 1 and their proton-
ated forms (middle line). Thus, the more bulky the Nexo sub-
stituent, the stronger the contribution of the aromatic reso-
nance structure 1’. This rationale is also supported by the
lower basicity of 1i and 1j than that of the monocyclic com-
pounds. The additional rigid six-membered ring prevents
C=N rotation upon protonation, thus thwarting the synergis-
Figure 5. X-Ray crystal structure of the salt 1h·HBF4. The displacement
ellipsoids are drawn at the 30% probability level. The distorted counter-
ꢀ
ꢀ
ion BF4 is omitted for clarity. Selected bonds lengths [ꢅ]: N4 C2
ꢀ
ꢀ
ꢀ
ꢀ
1.372(2), N1 C2 1.341(2), N3 C2 1.346(2), N1 C5 1.386(2), N3 C4
1.391(2), C4 C5 1.354(3); average torsion angle N1(3)-C2-N4-C14: 70.08.
ꢀ
ly change the basicity in acetonitrile (1a vs. 1 f and 1g).
Some variability of the substituents in the 1 and 3 positions
is possible without diminishing the basicity significantly (1a
Chem. Eur. J. 2012, 18, 3621 – 3630
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3625

R. A. Kunetskiy, I. Leito, I. M. Lyapkalo et al.
Table 1. Summary of basicity measurements and calculations for compounds 1.
dene amines (see Tables S5 and
S6 in the Supporting Informa-
tion) was carried out to further
dissect the contributing factors
to basicity. The DNICS(1)
values characterize the increase
in aromatization of the imida-
zole ring upon protonation (see
Table S6 in the Supporting In-
formation). The influence of
the substituents on the GB can
be quantitatively described by
two essentially similar linear
Base
Structure
Basicity
pKBH+
MeCN
DNICS(1)
Change in
dihedral angle
on protonation
GBcalc
1a
1b
30.21
29.90
257.8
ꢀ4.23
ꢀ4.01
24.6!84.5 (exp: 87.7)[a]
22.6!73.9
256.2
1c
1d
1e
1 f
1g
1h
1i
27.98
26.29
27.2
254.7
253.1
253.4
259.2
257.6
249.4
250.2
251.0
248.0
ꢀ3.88
ꢀ3.44
ꢀ2.90
ꢀ3.89
ꢀ4.38
ꢀ2.98
ꢀ2.18
ꢀ2.31
ꢀ2.52
13.1!80.5
free-energy
relationship
(LFER) approaches, which take
into account the field-inductive
(s* or sF constants),[13,14,26–29]
polarizability (sa constants or
15.8!30.1
molecular
refractivity
(MRD)[26,27]), resonance (sR0 ,
sꢀR, or sþR constants)[13,14,29], and
steric effects (expressed by the
Charton u constant)[30] of the
11.5!72.0
29.97
29.47
23.26
25.54
25.33
24.46
30.6!89.4
substituents.
Equation (3)
serves as the basis for elucidat-
ing the relationship between
the molecular structure and ba-
sicity of members of compound
class 1 (Alk=iPr).
25.7!89.9 (exp: 88.5)[a]
28.3 (exp: 34.8)[a]!57.5 (exp: 70.0)[a]
4.2!3.4 (exp: 7.9)[a]
2.8!10.5 (exp: 12.7)[a]
0.8!20.1
GB ¼ a0 þ a1sF þ a2sa
ð3Þ
þa3sꢀR þ a4u
The GB values for the N-
alkyl derivatives are within the
range
253.1–259.2 kcalmolꢀ1,
the highest in a large series of
compounds. Attachment of
other, especially electronega-
tive, substituents to the imine
nitrogen atom leads to a de-
crease in the GB to the level of
222.2–249.4 kcalmolꢀ1.
1j
1k
[a] Value obtained from the crystal structure.
Examination of the correla-
tion statistics (Table 2) shows
tic effect of aromatization and the steric-strain release.
Compounds 1i and 1j were consequently found to have ba-
sicities almost five orders of magnitude lower than that of
the monocyclic base 1a. This correlation is only valid if the
substituents on Nexo have a similar electronic nature. There-
fore, the correlation does not hold for compound 1h.
that the first three terms in Equation (3)—the field-induc-
tive (a1sF), polarizability (a2sa), and resonance (a3sꢀR) com-
ponents—are all statistically significant (at a probability of
0.99), whereas the steric term a4u is insignificant. The CN
substituent deviates significantly from Equation (3) and thus
was excluded from the general correlation. This is valid be-
cause independent calculations at the DFT 6–311+G**
level showed that protonation of the neutral base is expect-
ed to take place on the nitrogen atom of the cyano group
and not, as in all other members of the series, at the nitro-
gen atom of the imino group (Figure 3). The sensitivity coef-
Basicity of imidazol-2-ylidene amines in the gas phase and
the effect of aromaticity: A computational study into the
gas-phase basicities and DNICS(1) values of compounds
1a–1k (Table 1) and representative other imidazole-2-yli-
3626
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3621 – 3630

Organosuperbases
FULL PAPER
Table 2. The statistical least-squares treatment of GB data for the derivatives of 1.
[c]
Scales
a0
a1
a2
a3
a4
R[a]
s[b]
n/n0
sF, sa, sꢀR, u
sF, sa, sꢀR
249.1ꢂ1.4
249.1ꢂ1.4
ꢀ32.8ꢂ1.9
ꢀ32.8ꢂ1.9
ꢀ9.99ꢂ2.00
ꢀ9.99ꢂ2.00
ꢀ18.0ꢂ5.0
ꢀ18.0ꢂ5.0
1.21ꢂ1.29
0.981
0.981
1.95
1.95
20/21[d]
20/21[d]
–
[a] R=correlation coefficient. [b] s=standard deviation. [c] n0 reflects the total number of data points involved in the correlation; n=the number of
points remaining after exclusion of significantly deviating points. [d] The CN derivative was excluded.
constants (see Table 2) for R=Alk and H is observed. The
results of the correlation analysis thus demonstrate that if a
wide range of electronically different substituents R is con-
sidered, their field-inductive effect dominates the gas-phase
basicity most strongly. The influence of resonance and polar-
izability is less pronounced but still significant. In contrast,
the steric effects for this type of proton transfer are negligi-
ble.
For the same selection of compounds the DNICS(1)
values were calculated (Figure 7). For some subfamilies of
compounds with fixed sF values of zero or close to zero, for
Figure 6. Dependence of the pKBH+ values (in MeCN) of the bases on
the 13C NMR chemical shift differences of bases 1a–1k and the corre-
2
^
sponding salts. Compound 1h is not shown. : y=ꢀ0.7087x+42.008; R =
2
2
~
&
0.8762. : y=ꢀ0.6732x+45.852; R =0.8961. :y=ꢀ0.576x+47.125; R =
0.8212.
ficients a1, a2, and a3 all have negative signs. This means that
for all substituents with sF ꢃ0, the field-inductive effect de-
creases the GB relative to R=H by up to a large amount
(e.g., for SO2CF3) since 0.84ꢄ(ꢀ32.8)=ꢀ27.6 kcalmolꢀ1,
which corresponds to more than 20 pKBH+ units. The nega-
tive sign of a2 =ꢀ9.99 for the polarizability effect (with sa ꢄ
0) shows that the GB of bases 1 could be increased relative
to R=H by up to as much as (ꢀ0.82)ꢄ(ꢀ9.99)=8.2 kcal
molꢀ1 or by approximately 6 powers of ten, as compared
with the parent compound 1k. Likewise, the negative re-
gression coefficient a3 for a given selection of R substituents
means that for compounds with positive values of the sRꢀ
resonance constant, for example, for R=NO2 (sꢀR =0.18ꢄ
(ꢀ18.0)=ꢀ3.2 kcalmolꢀ1), some decrease in the GB value
of the parent base 1, by up to 2.3 powers of ten, is expected,
whereas for those with negative values (e.g., F with sꢀR =
ꢀ0.25) a similarly moderate increase in the basicity is ex-
pected (by up to 2.4 pKBH+ units). No statistically significant
value for regression coefficient a4 was found for the a4u
term in Equation (3). A similar conclusion was also drawn
by Taft and Topsom, who found no evidence for the influ-
ence of steric effects on protonation (in particular, steric
“relief” upon protonation of the neutral base) in the case of
a series of neutral amines (RNH2, RNMe2, etc.) by using the
Taft steric constant Es.[13] This analysis shows that by far the
greatest effect on the basicity of neutral bases can be
reached by increasing its field-inductive effect component
(up to 20 powers of ten), whereas the influence of the two
other components could add “only” a few powers of ten.
There is no possibility to check the applicability of Equa-
tion (3) for only the alkyl substituents (R=Alk). Instead, a
rough linear correlation between pKBH+ and the Taft s*
Figure 7. Correlation between GB and DNICS(1) values of bases 1 (See
Table S6 in the Supporting Information; y=ꢀ3.607x+241.21; R2 =0.660).
^
The imino substituents are denoted as follows: for alkyl and aryl sub-
^
stituents and for the remaining substituents.
example, for R=Alkyl, H (sF =0), or Ph (sF =0.1), a linear
correlation between the GB and DNICS(1) values was
found. Among the remaining substituents, due to the notice-
able variation in their sF values, a scattered “Milky-Way-
like” picture with much worse statistical characteristics and
no linear relationship between GB and DNICS(1) values
emerges. It follows from this NICS analysis and from a cor-
relation analysis in terms of both Equation (3) and the slope
of the linear region in Figure 7 for the alkyl substituents and
those with sF values close to zero that there is only a
modest share of the “aromatization” in these compounds
(3.6 kcalmolꢀ1 of GB for every DNICS(1) unit).
The calculated GB and experimental pKBH+ values for
1a–1k roughly correlate, although slight variations were
found that most likely relate to solvation differences for the
bases in MeCN. A linear correlation was found between the
measured pKBH+ values in MeCN and the DNICS(1) values
for 1a–1k (Figure 8), which offers support for the suggested
physical meaning of the correlation shown in Figure 6.
Chem. Eur. J. 2012, 18, 3621 – 3630
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3627

R. A. Kunetskiy, I. Leito, I. M. Lyapkalo et al.
strain on protonation, for which the effective bond order of
the C=N double bond is reduced and the substituent in the
2 position can rotate to minimize steric interactions. In con-
trast to bases 1a–1k, an increase in steric hindrance beyond
an N-methyl group leads to a decrease in the basicity of P1
phosphazene bases in acetonitrile.[4c,22]
A recent publication by Mayr et al.[32] concluded that both
aromatic and saturated NHCs display similar nucleophilici-
ties (and similar basicities). The same was shown earlier for
the effect of ligation of NHCs to metals.[20,33] Although
cyclic guanidines[34] and imidazoline-2-imines[34c,35] have been
described as superbases, no experimental pKBH+ values are
available. To determine the contribution of aromaticity to
the basicity of 1, the GB of structurally analogous com-
pounds 9 and 10 and of saturated analogues 11, 12, and 13
was calculated. The unsubstituted derivative 11 displays
only a 1.6 kcalmolꢀ1 lower basicity than the unsaturated
compound 9. Comparing the GB of saturated compound 12
to that of compound 1a, the latter is 10.8 kcalmolꢀ1 more
basic. The GB difference decreases by 4 kcalmolꢀ1 for tetra-
methyl-substituted compound 13 in comparison with 1a. In
the gas phase, the number and size of the alkyl fragments
near the basicity center significantly affect the basicity. In
addition, two methyl groups in positions 4 and 5 in 13 can
have an additional steric influence on the isopropyl groups
that is not present in 1a. Thus, structure 12 can be consid-
ered to be the closest analogue of 1a and a significant part
of the GB difference can be traced to the contribution of
the aromaticity in 1a.
The computational analysis of the contributing factors to
the GB of a larger series of imidazole-2-imines demon-
strates that if a wide range of electronically different sub-
stituents at the 2 position is considered, their field-inductive
effect dominates the gas-phase basicity (affecting it by up to
20 powers of ten). The influence of resonance and the polar-
izability effects of substituents in the 2 position are less pro-
nounced, but still significant. In contrast, steric effects are
negligible for this type of proton transfer. The GB values of
all of these compounds are, however, lower than those of
1a–1k.
Figure 8. Correlation between the DNICS(1) values and the measured
MeCN pKBH+ values (y+ꢀ2.358x+19.67; R2 =0.811). Compound 1h has
been excluded from linear fitting (see the text for an explanation).
Discussion
The high basicity of compounds 1a–1k deserves comment.
This is the first class of practically useful bases with a basici-
ty markedly increased by forced aromatization of a substruc-
ture.[31] This is based on the synergistic effect of three crucial
structural features on the unsaturated imidazole ring—a
bulky imino substituent, bulky substituents in the 1 and 3
positions, and the methyl groups in the 4 and 5 positions.
The alkyl substituents on the imidazole ring increase the
electron density and stabilize the positive charge in the de-
veloping imidazolium cation. The extent to which the
methyl groups in the 4 and 5 positions increase the calculat-
ed GB of compound 10 amounts to approximately 5 kcal
molꢀ1 (Figure 9). Enlarging the substituents on the 1 and 3
positions from methyl in 10 to isopropyl in 1e increases the
GB by approximately 4 kcalmolꢀ1. This increase in crowding
places the bulky substituents in the 1 and 3 positions into an
orientation in which steric interactions with the imine sub-
stituent are maximized, leading to another 4.4 kcalmolꢀ1
gain in GB for the most hindered derivatives, such as 1a.
Conclusion
A series of stable organosuperbases, N-alkyl- and N-aryl-
1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines, have
been designed. Their synthesis was accomplished in 4–
5 steps, starting from N,N’-dialkylthioureas, with overall
yields of 27–44%. This unified synthetic strategy is based on
a KF-mediated coupling of 2-chloroimidazolium salts with
primary amines, allowing the synthesis of a large number of
imidazol-2-ylidene amines with widely varying substitution
patterns. The structures of the 2-aminoimidazolium cations
1aH+–1jH+ and the neutral bases 1a–1j were elucidated in
depth. The origin of the high basicity of these compounds is
discussed. It is demonstrated that the increase in the basicity
of the compounds in the gas phase and in solution is influ-
Figure 9. Calculated GB [kcalmolꢀ1] for representative imidazoli(di)ne-2-
imines to delineate substituent and resonance contributions (*The GB of
12 is the average for the cis/trans isomers).
In acetonitrile, an effective weakening of the C=N double
bond in bases 1a–1k with increasing bulkiness of the imine
substituent and a significantly more pronounced contribu-
tion of the imidazolium amide resonance structure 1’ is ex-
pected to contribute to the increase in basicity. The correla-
tions between 13C NMR chemical shift/NICS values and so-
lution basicity/GB support this hypothesis. Another factor
contributing to the observed basicity is the release of steric
3628
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3621 – 3630

Organosuperbases
FULL PAPER
[8] a) N. Peran, Z. B. Maksic, Chem. Commun. 2011, 47, 1327–1329;
b) V. Raab, I. Kipke, R. M. Gschwind, J. Sundermeyer, Chem. Eur.
J. 2002, 8, 1682–1693.
enced by the degree of aromatization of the imidazolium
ring as measured by the 13C NMR chemical shifts in solu-
tion, as well as by the calculated DNICS(1) values in the gas
phase. If a wider range of imino substituents, including elec-
tron-acceptor substituents, is studied the influence of the
field-inductive effect, polarizability, and resonance effects of
the substituent becomes more significant than aromatiza-
tion.
ˇ
ˇ
[9] R. A. Kunetskiy, I. Cꢀsarovꢁ, D. Saman, I. M. Lyapkalo, Chem. Eur.
J. 2009, 15, 9477–9485.
[10] pKBH+ denotes the pKa value of the conjugate acid of a base B.
ˇ
´
´
[11] B. Kovacevic, Z. B. Maksic, Org. Lett. 2001, 3, 1523–1526.
[12] P. von R. Schleyer, Ch. Maerker, A. Dransfeld, H. Jiao, N. J. R.
van E. Hommes, J. Am. Chem. Soc. 1996, 118, 6317–6318.
[13] a) R. W. Taft, R. D. Topsom, Prog. Phys. Org. Chem. 1987, 16, 1–83;
b) R. W. Taft, Prog. Phys. Org. Chem. 1983, 14, 247–350.
[14] R. W. Taft, I. A. Koppel, R. D. Topsom, F. Anvia, J. Am. Chem. Soc.
1990, 112, 2047–2052.
[15] a) N. Kuhn, T. Kratz, Synthesis 1993, 561–562; b) for an improved
protocol, see Ref. [9].
Acknowledgements
[16] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
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, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢉ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT, 2009.
[17] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C.
Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994, 27, 435–
436.
[18] G. M. Sheldrick, Acta Crystallogr. 2008, A64, 112–122.
[19] CCDC-769494 (1a·HBF4), 830319 (1g·HBF4), 830320 (1h), 830321
(1h·HBF4), 830322 (1i·HOTs), 830323 (1j·HBF4) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[20] F. E. Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166–3216;
Angew. Chem. Int. Ed. 2008, 47, 3122–3172.
[21] a) N. Kuhn, A. Abu-Rayyan, M. GçÅhner, M. Steimann, Z. Anorg.
Allg. Chem. 2002, 628, 1721–1723; b) N. Kuhn, J. Fahl, R. Fawzi, C.
Maichle-Moessmer, M. Steimann, Z. Naturforsch. 1998, 53b, 720–
726.
[22] R. Schwesinger, J. Willaredt, H. Schlemper, M. Keller, D. Schmitt,
H. Fritz, Chem. Ber. 1994, 127, 2435–2454.
[23] J. A. S. Laks, J. R. Ross, S. M. Bayomi, J. W. Sowell Sr., Synthesis
1985, 291–293.
[24] G. C. Lancini, E. Lazzari, J. Heterocycl. Chem. 1966, 3, 152–154.
[25] M. Tamm, D. Petrovic, S. Randoll, S. Beer, T. Bannenberg, P. G.
Jones, J. Grunenberg, Org. Biomol. Chem. 2007, 5, 523–530.
[26] I. A. Koppel, M. M. Karelson, Org. React. 1975, 11, 985–1010.
[27] I. A. Koppel, U. H. Mçlder, Org. React. 1983, 20, 3–44.
[28] I. A. Koppel, U. H. Mçlder, R. J. Pikver, Org. React. 1981, 18, 380–
410.
[29] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165–195.
[30] M. Charton, Top. Curr. Chem. 1983, 114, 57–91.
[31] a) Cyclopropenimine systems have been proposed as strong organic
Continuous support of this work by the IOCB under grant No. Z4 055
0506 and by the Ministry of Education (grant No. MSM0021620857 for
I.C.) is gratefully acknowledged. This work was also supported by the Es-
tonian Science Foundation (grant Nos. 7374 and 8162), by the UT Centre
of Excellence “High-Technology Materials for Sustainable Development”
Project (SLOKT117T), and by the target financing projects
SF0180061s08 and SF0180089s08 from the Ministry of Education and Sci-
ence of Estonia. The authors are grateful to Dr. Ullrich Jahn (IOCB,
Prague) for helpful discussions and for support in the preparation of the
manuscript. The authors are thankful to Pavel Fiedler (IOCB, Prague)
for recording the IR spectra of the IMAM bases.
[1] a) D. Leow, C.-H. Tan, Synlett 2010, 11, 1589–1605; b) H. Bruns, M.
Patil, J. Carreras, A. Vazquez, W. Thiel, R. Goddard, M. Alcarazo,
Angew. Chem. 2010, 122, 3762–3766; Angew. Chem. Int. Ed. 2010,
49, 3680–3683; c) Superbases for Organic Synthesis: Guanidines,
Amidines, Phosphazines and Related Organocatalysts (Ed.: T. Ishika-
wa), Wiley, New York, 2009; d) V. R. Chintareddy, K. Wadhwa, J. G.
Verkade, J. Org. Chem. 2009, 74, 8118–8132; e) U. Wild, O. Hꢂbner,
A. Maronna, M. Enders, E. Kaifer, H. Wadepohl, H.-J. Himmel,
Eur. J. Inorg. Chem. 2008, 4440–4447; f) J. Barluenga, P. Moriel, F.
Aznar, C. Valdꢆs, Org. Lett. 2007, 9, 275–278; g) W. Su, S. Urgaon-
kar, P. A. McLaughlin, J. G. Verkade, J. Am. Chem. Soc. 2004, 126,
16433–16439; h) M. Sauthier, J. Forniꢆs-Cꢁmer, L. Toupet, R. Rꢆau,
Organometallics 2000, 19, 553–562; i) R. Schwesinger, Nachr. Chem.
Tech. Lab. 1990, 38, 1214–1226; j) P. B. Kisanga, J. G. Verkade, R.
Schwesinger, J. Org. Chem. 2000, 65, 5431–5432.
[2] R. Schwesinger, H. Schlemper, C. Hasenfratz, J. Willaredt, T. Dam-
bacher, T. Breuer, C. Ottaway, M. Fletschinger, J. Boele, H. Fritz, D.
Putzas, H. W. Rotter, F. G. Bordwell, A. V. Satish, G. Z. Ji, E.-M.
Peters, K. Peters, H. G. von Schnering, L. Walz, Liebigs Ann. 1996,
1055–1081.
[3] a) A. A. Kolomeitsev, I. A. Koppel, T. Rodima, J. Barten, E. Lork,
G.-V. Rçschenthaler, I. Kaljurand, A. Kꢂtt, I. Koppel, V. Mꢇemets,
I. Leito, J. Am. Chem. Soc. 2005, 127, 17656–17666; b) I. A. Koppel,
R. Schwesinger, T. Breuer, P. Burk, K. Herodes, I. Koppel, I. Leito,
M. Mishima, J. Phys. Chem. A 2001, 105, 9575–9586.
[4] a) A. Kꢂtt, V. Movchun, T. Rodima, T. Dansauer, E. B. Rusanov, I.
Leito, I. Kaljurand, J. Koppel, V. Pihl, I. Koppel, G. Ovsjannikov, L.
Toom, M. Mishima, M. Medebielle, E. Lork, G.-V. Rçschenthaler,
I. A. Koppel, A. A. Kolomeitsev, J. Org. Chem. 2008, 73, 2607–
2620; b) L. Soovꢇli, I. Kaljurand, A. Kꢂtt, I. Leito, Anal. Chim. Acta
2006, 566, 290–303; c) I. Kaljurand, A. Kꢂtt, L. Soovꢇli, T. Rodima,
V. Mꢇemets, I. Leito, I. A. Koppel, J. Org. Chem. 2005, 70, 1019–
1028.
[5] I. Kaljurand, I. A. Koppel, A. Kꢂtt, E.-I. Rꢈꢈm, T. Rodima, I.
Koppel, M. Mishima, I. Leito, J. Phys. Chem. A 2007, 111, 1245–
1250.
[6] E.-I. Rꢈꢈm, A. Kꢂtt, I. Kaljurand, I. Koppel, I. Leito, I. A. Koppel,
M. Mishima, K. Goto, Y. Miyahara, Chem. Eur. J. 2007, 13, 7631–
7643.
ˇ
´
´
bases: B. Kovacevic, Z. B. Maksic, R. Vianello, J. Chem. Soc. Perkin
Trans. 2 2001, 886–891; b) for application as ligands, see ref. [1b].
[32] B. Maji, M. Breugst, H. Mayr, Angew. Chem. Int. Ed. 2011, 50,
6915–6919.
[33] a) Topics in Organometallic Chemistry, Vol. 21, N-Heterocyclic Car-
benes in Transition Metal Catalysis (Ed.: F. Glorius), Springer,
Berlin, 2007; b) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P.
Nolan), Wiley-VCH, Weinheim, 2006.
[7] R. W. Alder, Chem. Rev. 1989, 89, 1215–1223, and references there-
in.
Chem. Eur. J. 2012, 18, 3621 – 3630
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3629

R. A. Kunetskiy, I. Leito, I. M. Lyapkalo et al.
[34] a) J. R. Bell, H. Luo, S. Dai, Tetrahedron Lett. 2011, 52, 3723–3725;
b) T. Ishikawa, Chem. Pharm. Bull. 2010, 58, 1555–1564; c) the cal-
culated MeCN pKBH+ for mono-/1,8-bis(dimethylethyleneguanidi-
no)-substituted naphthalenes is one/three powers of ten lower than
for their unsaturated analogues: V. Raab, K. Harms, J. Sundermeyer,
Ref. [23]; b) N. Kuhn, M. Gçhner, M. Grathwohl, J. Wiethoff, G
Frenking, Y. Chen, Z. Anorg. Allg. Chem. 2003, 629, 793–802.
´
´
B. Kovacevic, Z. B. Maksic, J. Org. Chem. 2003, 68, 8790–8797.
[35] a) The calculated proton affinity PAcalc of N,N’-dimethyl-2-iminoimi-
dazoline is 1.4 kcalmolꢀ1 higher than its guanidine analogue, see
Received: July 21, 2011
Revised: December 10, 2011
Published online: February 14, 2012
3630
www.chemeurj.org
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3621 – 3630