2,3-Dihydroimidazol-2-ylidene

Article abstract of DOI:10.1002/(SICI)1099-0690(199808)1998:8<1517::AID-EJOC1517>3.0.CO;2-H

Irradiation of imidazole-2-carboxylic acid (3) - matrix-isolated in argon at 10 K - with a wavelength of 254 nm leads to decarboxylation and the formation of a complex between 2,3-dihydroimidazol-2-ylidene and carbon dioxide (1·CO₂). Upon irradiation of 2,3-dihydroimidazol-2-ylidene (1) with λ = 254, 193, and 185 nm no imidazole can be detected. On the other hand flash vacuum pyrolysis of imidazole-2-carboxylic acid (3) produces imidazole and carbon dioxide. 2,3-Dihydroimidazol-2-ylidene (1) - the possible intermediate - cannot be trapped under these conditions.

Full text of DOI:10.1002/(SICI)1099-0690(199808)1998:8<1517::AID-EJOC1517>3.0.CO;2-H

FULL PAPER
2,3-Dihydroimidazol-2-ylideneȠ
Günther Maier* and Jörg Endres
Institut für Organische Chemie der Justus-Liebig-Universität Gießen,
Heinrich-Buff-Ring 58, D-35392 Gießen, Germany
Fax (internat.): ϩ 49(0)641/9934309
Received March 3, 1998
Keywords: Nucleophilic carbenes / Heterocycles / Decarboxylation / Photochemistry / Matrix isolation
Irradiation of imidazole-2-carboxylic acid (3)
isolated in argon at 10 K – with a wavelength of 254 nm leads
to decarboxylation and the formation of a complex between
–
matrix-
λ = 254, 193, and 185 nm no imidazole can be detected. On
the other hand flash vacuum pyrolysis of imidazole-2-
carboxylic acid (3) produces imidazole and carbon dioxide.
2,3-dihydroimidazol-2-ylidene and carbon dioxide (1·CO2). 2,3-Dihydroimidazol-2-ylidene (1) – the possible interme-
Upon irradiation of 2,3-dihydroimidazol-2-ylidene (1) with diate – cannot be trapped under these conditions.
In 1991 Arduengo and co-workers[1] reported the iso- thermodynamic stability of 2,3-dihydroimidazol-2-ylidene
lation and characterization of the first stable carbene, 1,3- (1) over 2,3-dihydrothiazol-2-ylidene, which has recently
di(1-adamantyl)-2,3-dihydroimidazol-2-ylidene. Since that been prepared both in the gas phase[7] and in cryogenic ma-
time many substituted nucleophilic carbenes have been pre- trices,[8] is revealed by the calculated heats of hydrogenation
pared.[2] The remarkable stability of 2,3-dihydroimidazol-2- of the carbene carbon atom (the calculated heat of hydro-
ylidenes cannot be rationalized only by steric protection of genation for 2,3-dihydroimidazol-2-ylidene is Ϫ12.0 kcalи
the carbenic center, because 1,3-dimethyl-2,3-dihydroimida- molϪ1 and for 2,3-dihydrothiazol-2-ylidene Ϫ26.6 kcalи
zol-2-ylidene is also a stable molecule. From ab initio stud- molϪ1). It follows that 2,3-dihydroimidazol-2-ylidene (1) is
ies it can be pointed out that cyclic electron delocalization thermodynamically more stable than 2,3-dihydrothiazol-2-
is responsible for the extraordinary stability of 2,3-dihydro- ylidene.
imidazol-2-ylidenes (1).[3] Recently, Schwarz and co-work-
ers[4] were able to demonstrate that 2,3-dihydroimidazol-2- zole-2-carboxylic acid is an easy way to generate 2,3-di-
As we have recently shown the decarboxylation of thia-
ylidene (1) is a stable molecule in the gas phase.
hydrothiazol-2-ylidene.[8] Therefore we have chosen imida-
zole-2-carboxylic acid (3) as a precursor for our matrix iso-
lation studies of 1. In principle four rotamers of imidazole-
2-carboxylic acid (3) are conceivable. The experimental
spectrum of matrix-isolated imidazole-2-carboxylic acid (3)
(Figure 1) is in very good agreement with the calculated
spectrum of the most stable rotamer 3a.
Scheme 1. Calculated reaction energies for the intramolecular 1,2-
hydrogen shift of 2,3-dihydroimidazol-2-ylidene
The calculated infrared spectra of 3b؊3d do not agree
as well with the experimental infrared spectrum of matrix-
isolated imidazole-2-carboxylic acid (3). Due to strong hy-
drogen bonds in solid imidazole-2-carboxylic acid (3) it is
not possible to evaporate 3 without decarboxylation. There-
fore in all our experimental spectra the signals of imidazole
(2) and carbon dioxide are visible.
The calculated activation energy for the decarboxylation
of 3a is 20.8 kcalиmolϪ1 (Scheme 2).
Photofragmentation of Imidazole-2-carboxylic Acid (3)
Calculations
Shortly after the beginning of the irradiation of 3a with
Although 2,3-dihydroimidazol-2-ylidene (1) is 26.3 kcalи a wavelength of λ ϭ 254 nm the intensities of the signals of
˜
molϪ1 less stable than imidazole (2), the calculated energy 3a decreased, while a new signal arose at ν ϭ 1738.1 cmϪ1
,
barrier for the intramolecular hydrogen shift is very high which possibly is caused by rotamer 3b. After 20 min of
(41.5 kcalиmolϪ1) (Scheme 1).[5] Because of the interaction irradiation with λ ϭ 254 nm, the signals of imidazole-2-
of the nitrogen lone pairs with the empty p-orbital of the carboxylic acid (3) had completely disappeared. The new
carbenic center, 1 should be a singlet carbene. The calcu- signals do not decrease upon further irradiation with λ ϭ
lated singlet/triplet gap is 81.6 kcalиmolϪ1. The higher 254 nm. In the region above 700 cmϪ1 they correspond very
Eur. J. Org. Chem. 1998, 1517Ϫ1520
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
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1517

G. Maier, J. Endres
FULL PAPER
Figure 1. Comparison of a section of the experimental spectrum of
matrix-isolated imidazole-2-carboxylic acid (3) (argon, 10 K) with
the calculated infrared spectra of rotamers 3a؊3d; relative energies
of the four rotamers are given in kcalиmolϪ1
Scheme 2. Calculated reaction energies for the decarboxylation of
3a; bond lenghts are given in pm
with λ ϭ 254 nm again produced, beside other signals, a
˜
band at ν ϭ 623.7 cmϪ1. The absence of an isotopic shift
and our experience with thiazole-2-carboxylic acid, which
yields a complex between carbon dioxide and 2,3-dihydro-
thiazol-2-ylidene upon irradiation,[8] led us to the con-
clusion that irradiation of imidazole-2-carboxylic acid (3)
also results in the formation of a complex between 2,3-di-
hydroimidazol-2-ylidene (1) and carbon dioxide.
Quantum-mechanical calculations predict a complex be-
tween 2,3-dihydroimidazol-2-ylidene (1) and carbon diox-
ide, with a stabilization energy of 4.3 kcalиmolϪ1 relative
to its fragments. The comparison between the calculated
well with the calculated infrared spectrum of 2,3-dihydro- infrared spectra of 1·CO2 and [D2]-1·CO2 reveals that there
imidazol-2-ylidene (2). Below 700 cmϪ1 one additional sig- are only four signals showing no or a very small isotopic
˜
nal, compared to the calculated infrared spectrum, at ν ϭ shift (Tables 1 and 2). They can all be assigned to vibrations
623.7 cmϪ1 is observed. In order to decide whether this sig- of the carbon dioxide ligand. Due to the unsymmetrical ar-
nal belongs to 2,3-dihydroimidazol-2-ylidene (1) we also rangement of the carbon dioxide ligand the bending vi-
studied the photochemical decarboxylation of deuterated bration, which is degenerated in free carbon dioxide, splits
imidazole-2-carboxylic acid ([D2]-3). Irradiation of [D2]-3a into two distinct signals (calculated: 667.2 and 611.0 cmϪ1
)
1518
Eur. J. Org. Chem. 1998, 1517Ϫ1520

2,3-Dihydroimidazol-2-ylidene
FULL PAPER
Figure 2. Comparison of the experimental difference spectrum (sec-
tion from 1700 to 400 cmϪ1) of the irradiation of imidazole-2-car-
boxylic acid (3) with λ ϭ 254 nm (the bands with negative values
diminish, while those with positive values are formed upon irradi-
ation) with the calculated infrared spectrum of 1·CO2; bands due to
vibrations of the carbon dioxide ligand are marked with an asterisk
and the symmetrical stretching vibration becomes infrared-
active (calculated: 1365.8 cmϪ1). The signals, which arise
during the photolysis of imidazole-2-carboxylic acid (3), are
in acceptable agreement with the calculated infrared spec-
trum of 1·CO2. In the presence of the carbon dioxide the
high C2v symmetry of 2,3-dihydroimidazol-2-ylidene (1) is
disturbed, so the previously infrared- inactive ring bending
vibrations of 2,3-dihydroimidazol-2-ylidene (1) below 700
cmϪ1 should now be detectable. Due to very low calculated
intensities of these bending vibrations (Table 1) it is not
possible to detect any of them in the experimental difference
spectrum (Figure 2).
Table 1. Comparison of the calculated infrared spectrum for the
carbon dioxide complex (1·CO2) with the experimental (argon, 10
K) infrared spectrum after irradiation of imidazole-2-carboxylic
acid (3) with λ ϭ 254 nm; the five vibrations below 400 cmϪ1 are
not listed; vibrations due to carbon dioxide are marked with an
asterisk
˜
˜
No. Sym.
νexp [cmϪ1] (Iexp) νcalcd [cmϪ1] (Icalcd
)
Figure 3. Comparison of the experimental difference spectrum (sec-
tion from 1700 to 400 cmϪ1) of the irradiation of deuterated imida-
zole-2-carboxylic acid ([D2]-3) with λ ϭ 254 nm (the bands with
negative values diminish, while those with positive values are for-
med upon irradiation) with the calculated infrared spectrum of
[D2]-1·CO2; bands due to vibrations of the carbon dioxide ligand
are marked with an asterisk
ν1
ν2
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
νNH
νNH
3510.7 (0.31)
3493.4 (0.37)
-
3667.9 (0.31)
3649.4 (0.46)
3281.7 (0.01)
3260.3 (0.00)
2421.4* (2.99)
1611.9 (0.04)
1438.7 (0.27)
1404.2 (0.03)
1384.3 (0.02)
1365.8* (0.03)
1211.7 (0.00)
1176.6 (0.01)
1121.5 (0.10)
1090.2 (0.06)
1044.6 (0.15)
932.7 (0.02)
926.4 (0.04)
824.5 (0.00)
756.8 (1.00)[a]
706.5 (0.07)
685.2 (0.02)
667.2* (0.10)
620.6 (0.00)
611.0* (0.76)
592.7 (0.16)
ν3
νCH
ν4
νCH
Ϫ
ν5
νas, CO
2332.1* (2.76)
1581.0 (0.02)
1410.7 (0.12)
1376.9 (0.06)
1352.5 (0.02)
Ϫ
ν6
νCϭC
ν7
δNH (i.p.)
δNH (i.p.)
δCHN
ν8
ν9
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν22
ν23
ν24
ν25
ν26
ν27
ν18
ν28
νs, CO
δNCN
Ϫ
Ϫ
δCNC
δNH (i.p.)
δCH (i.p.)
δCNC
1096.3 (0.06)
1072.8 (0.03)
1028.9 (0.14)
913.7 (0.02)
906.9 (0.03)
Ϫ
δring
δring
AЈЈ δCH (o.o.p.)
AЈЈ δNH (o.o.p.)
AЈЈ δs, CH (o.o.p.)
AЈЈ δNH (o.o.p.)
AЈЈ δΩCO (o.o.p.)
AЈЈ δring (o.o.p.)
732.1 (1.00)
Ϫ
Ϫ
665.8* (0.06)
Ϫ
AЈ
δOCO (i.p.)
623.7* (0.09)
579.4 (0.14)
AЈЈ δring (o.o.p.)
[a]
Calculated absolute intensity: 185.9 kmиmolϪ1
.
mentation products or 2,3-dihydroimidazol-2-ylidene (1) di-
rectly produces 5؊8.
The calculated infrared spectrum of [D2]-1·CO2 is in very
In the case of 2,3-dihydrothiazol-2-ylidene annealing of
good agreement with the experimental difference spectrum the matrix at 60 K leads to the formation of thiazole.[8]
of the photolysis of deuterated imidazole-2-carboxylic acid Annealing of a matrix which contains 2,3-dihydroimidazol-
([D2]-3) (Figure 3, Table 2).
2-ylidene (1) at 60 K leads to the disappearance of the sig-
Isomerization of 2,3-dihydroimidazol-2-ylidene (1) to nals of 2,3-dihydroimidazol-2-ylidene (1). Afterwards only
imidazole could not be detected during the irradiation with signals of imidazole (2) are present in the infrared spectrum.
λ ϭ 254 nm. Irradiation with a shorter wavelength (λ ϭ But due to the decarboxylation of imidazole-2-carboxylic
193 or 185 nm) results in the formation of acetylene (7), acid (3) and formation of 2 during the evaporation process
acetonitrile (4), methyl isocyanide (5), its ylidic tautomer it is not possible to decide whether additional imidazole (2)
6,[9] and hydrocyanic acid (8). To decide whether these is produced through an intramolecular hydrogen transfer of
products arise from 2,3-dihydroimidazol-2-ylidene (1) or 2,3-dihydroimidazole (1) during the annealing process.
from its isomer imidazole (2), which is also present in the
Pyrolysis of imidazole-2-carboxylic acid (1), performed at
matrix (due to the decarboxylation during the evaporation only 400°C, leads to nearly complete decarboxylation. The
process), we irradiated imidazole under the same conditions experimental spectrum of the trapped products showed only
and found again the above-mentioned products. It is not the signals of imidazole (2) and imidazole-2-carboxylic acid
clear whether 2,3-dihydroimidazol-2-ylidene (1) first iso- (3). The signals of 2,3-dihydroimidazol-2-ylidene (1) could
merizes into imidazole (2) and 2 leads to the observed frag- not be found.
Eur. J. Org. Chem. 1998, 1517Ϫ1520
1519

G. Maier, J. Endres
FULL PAPER
Table 2. Comparison of the calculated infrared spectrum for the
carbon dioxide complex ([D2]-1·CO2) with the experimental (argon,
10 K) infrared spectrum after irradiation of deuterated imidazole-
2-carboxylic acid ([D2]-3) with λ ϭ 254 nm; the five vibrations be-
low 400 cmϪ1 are not listed; vibrations due to carbon dioxide are
marked with an asterisk
Sources: Excimer laser LPX 105 MC from Lambda Physics, mer-
cury high-pressure lamp HBO 200 from Osram, and mercury low-
pressure spiral lamp from Gräntzel. Ϫ Imidazole-2-carboxylic Acid
(3) was prepared according to the literature (see ref.[10]). Ϫ [D2]-
Imidazole-2-carboxylic Acid ([D2]-3) was prepared by recrystalliza-
tion of imidazole-2-carboxylic acid (3) in D2O.
˜
˜
No. Sym.
νexp [cmϪ1] (Iexp) νcalcd [cmϪ1] (Icalcd
)
Ƞ
Dedicated to Professor Gernot Boche on the occasion of his
ν1
ν2
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
νCH
νCH
Ϫ
Ϫ
3281.8 (0.02)
3260.4 (0.00)
2694.6 (0.28)
2679.6 (0.82)
2421.2* (7.24)
1585.8 (0.00)
1380.4 (0.01)
1365.8* (0.06)
1341.5 (0.20)
1272.4 (0.02)
1145.9 (0.01)
1114.6 (0.09)
1089.3 (0.07)
946.5 (0.08)
929.3 (0.02)
893.0 (0.23)
865.2 (0.45)
823.3 (0.00)
714.8 (1.00)[a]
668.2* (0.37)
626.6 (0.00)
623.2 (0.00)
610.9* (1.85)
532.9 (0.28)
494.9 (0.66)
60th birthday.
[1]
A. J. Arduengo III, R. L. Harlow, M. Kline J. Am. Chem. Soc.
ν3
νND
Ϫ
2616.6 (1.61)
2332.1* (11.31)
Ϫ
1991, 113, 361Ϫ363.
[2]
ν4
νND
For a recent review see: W. A. Herrmann, C. Köcher Angew.
ν5
νas, CO
Chem. 1997, 109, 2257Ϫ2282; Angew. Chem. Int. Ed. Engl.
1997, 36, 2162Ϫ2187.
ν6
νCϭC
[3]
ν7
δCH (i.p.)
νs, CO
Ϫ
Ϫ
C. Heinemann, W. Thiel. Chem. Phys. Lett. 1994, 217, 11Ϫ16.
ν8
Ϫ C. Heinemann, T. Müller, Y. Apeloig, H. Schwarz J. Am.
Chem. Soc. 1996, 118, 2023Ϫ2036. Ϫ C. Boehme, G. Frenking
J. Am. Chem. Soc. 1996, 118, 2037Ϫ2046.
ν9
δNCN
νCN
1316.7 (0.44)
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν22
ν23
ν24
ν25
ν26
ν18
ν27
ν28
Ϫ
Ϫ
[4]
δs, CH (i.p.)
G. A. McGibbon, C. Heinemann, D. J. Lavorato, H. Schwarz
AЈ δas, CH (i.p.)
1088.0 (0.09)
1073.2 (0.06)
934.0 (0.14)
Ϫ
871.0 (0.41)
844.5 (0.74)
Ϫ
709.2 (1.00)
666.1* (0.58)
Ϫ
Angew. Chem. 1997, 109, 1572Ϫ1575; Angew. Chem. Int. Ed.
Engl. 1997, 36, 1478Ϫ1481.
AЈ
AЈ
AЈ
δCH
δCNC
δring
[5]
All calculations were carried out by using the program package
Gaussian 94 (Revision D.3)[6] and the hybrid HF/DFT method
B3LYP with a standard 6-311G(d,p) basis set. Zero point vi-
brational energies are included in the calculated energies.
AЈ δas, ND (i.p.)
AЈ δs, ND (i.p.)
[6]
AЈЈ δas, CH (o.o.p.)
AЈЈ δs, CH (o.o.p.)
AЈЈ δOCO (o.o.p.)
AЈЈ δring (o.o.p.)
AЈЈ δring (o.o.p.)
M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G.
Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Peters-
son, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V.
G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B.
Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y.
Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R.
Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees,
J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, J. A.
Pople, Gaussian 94, Revision D.3, Gaussian, Inc., Pittsburgh
PA, 1995.
Ϫ
AЈ
δOCO (i.p.)
623.7* (1.11)
510.3 (0.43)
485.8 (1.24)
A” δND (o.o.p.)
A” δND (o.o.p.)
[a]
Calculated absolute intensity: 76.9 kmиmolϪ1
.
[7]
Graham A. McGibbon, Jan Hrusak, David J. Lavarto, Helmut
Schwarz, Johan K. Terlouw, Chem. Eur. J. 1997, 3, 232Ϫ236
[8]
G. Maier, J. Endres, H. P. Reisenauer Angew. Chem. 1997, 109,
1788Ϫ1790; Angew. Chem. Int. Ed. Engl. 1997, 36, 1709Ϫ1712.
[9]
Experimental Section
Cryostat for Matrix Isolation: Displex closed-cycle refrigeration
system CSA 202 from Air Products. Ϫ Spectrometers: IR: FT-IR
spectrometer IFS 55 from Bruker, resolution 0.5 cmϪ1 Ϫ Light
G. Maier, C. Schmidt, H. P. Reisenauer, E. Endlein, D. Becker,
J. Eckwert, B. A. Hess, L. J. Schaad Chem. Ber. 1993, 126,
2337Ϫ2352.
[10]
N. J. Curtis, R. S. Brown J. Org. Chem. 1980, 45, 4038Ϫ4040.
[98082]
1520
Eur. J. Org. Chem. 1998, 1517Ϫ1520