(1-Pyridinio)perfluoro-phenacyl-ide: A new stable pyridinium ylide in the enol form

Article abstract of DOI:10.1107/S0108270108006318

The title compound, C13H6F5NO, exists in the enol form and adopts the E configuration about the enol double bond. It is the first example of an enol-type pyridinium ylide. The enol structure was unambiguously determined on the basis of the significantly longer C - O bond and shorter C - C bond. Intra-molecular C - H...O and C - H...F hydrogen bonds are responsible for promotion of the enol form and for the stability of this compound.

Full text of DOI:10.1107/S0108270108006318

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
Several pyridinium ylides have previously been structurally
characterized, all with an electron-withdrawing group at C6
contributing to the stabilization of the anionic charge. The
bond lengths around the anionic moieties of these compounds
ISSN 0108-2701
(1-Pyridinio)perfluorophenacylide: a
new stable pyridinium ylide in the enol
form
Shinji Yamada* and Emiko Ohta
Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku,
Tokyo 112-8610, Japan
Correspondence e-mail: yamada.shinji@ocha.ac.jp
are compared with those of (I) in Table 1. The C6—C7
distances for the reported pyridinium ylides are in the range
˚
1.411–1.443 (5) A, and the C7—O1 distances occur between
Received 29 February 2008
Accepted 6 March 2008
Online 15 March 2008
˚
1.211 (3) and 1.242 A. The C6—C7 bond length of
˚
1.3648 (19) A in (I) is the shortest reported for a pyridinium
ylide and is also much shorter than the general Csp2—Csp2
bond length (Allen et al., 1987). It is, however, very close to
that of the Csp2 Csp2 bond of a classic enol tautomer (Allen
et al., 1987) and is also close to the reported value of
The title compound, C13H6F5NO, exists in the enol form and
adopts the E configuration about the enol double bond. It is
the first example of an enol-type pyridinium ylide. The enol
structure was unambiguously determined on the basis of the
significantly longer C—O bond and shorter C—C bond.
Intramolecular C—Hꢀ ꢀ ꢀO and C—Hꢀ ꢀ ꢀF hydrogen bonds are
responsible for promotion of the enol form and for the
stability of this compound.
˚
1.360 (13) A for the C C double bond in lithium 3,3-dime-
thyl-1-buten-2-olate [Cambridge Structural Database (CSD,
Version 5.28; Allen, 2002) refcode DETRAV (Laube et al.,
˚
1985)]. In contrast, the C7—O1 distance of 1.257 (2) A is the
longest among those reported and is intermediate between a
C
O double bond and a C—O single bond. The relatively
shorter N1—C6 distance of (I) is also in agreement with the
sp2 character of atom C6. These observed bond lengths
strongly suggest that this pyridinium ylide exists in the enol
form. It is important to note that when the perfluorophenyl
ring was replaced with a phenyl ring, the ylide became
unstable and could not be isolated. Therefore, the perfluoro-
phenyl ring determines the stability of the pyridinium ylide.
The crystal structure of (I) is built up by two intramolecular
C—Hꢀ ꢀ ꢀO and C—Hꢀ ꢀ ꢀF hydrogen bonds and two inter-
molecular C—Hꢀ ꢀ ꢀO and C—Hꢀ ꢀ ꢀF hydrogen bonds (Fig. 2
and Table 2) which are the result of the partial negative
Comment
Pyridinium ylides are an important family of synthetic inter-
mediates for a variety of organic compounds, such as indol-
idines (Tsuge et al., 1985; Kanemasa et al., 1989; Zhang et al.,
2000; Wu & Chen, 2003; Kakehi, 2005; Xia et al., 2006) and
cyclopropanes (Shestopalov et al., 1989; Vo et al., 1997; Kojima
et al., 2000; Yamada et al., 2007). Among the various types of
pyridinium ylides, pyridinium acylmethylides are often
employed for organic synthesis because they can be readily
prepared from the corresponding pyridinium salts with a base.
These pyridinium ylides are generally unstable and are
therefore used in reactions without isolation. However, when
an electron-withdrawing group is attached to the C atom next
to the pyridinium ring, the stabilization of the ylides increases
significantly.
In the course of our research on the reactivity of pyridinium
ylides (Yamada et al., 2007), we found that the title compound,
(I), is significantly stable without having an electron-with-
drawing group on the C atom next to the pyridinium ring.
Moreover, the X-ray structure clarifies that the equilibrium in
the keto–enol tautomerism is shifted significantly to the enol
form.
The pyridinium ring (Fig. 1) is almost coplanar with the enol
plane, and the pyridinium and perfluorophenyl rings adopt the
E configuration. The N1—C6—C7—O1 and N1—C6—C7—
C8 torsion angles are 0.2 (2) and ꢁ178.72 (13)ꢂ, respectively,
suggesting double-bond character for the C6—C7 bond.
Figure 1
The molecular structure of the title pyridinium ylide, (I), showing the
atom-numbering scheme. Displacement ellipsoids are drawn at the 30%
probability level and H atoms are shown as small spheres of arbitrary
radii.
o230 # 2008 International Union of Crystallography
doi:10.1107/S0108270108006318
Acta Cryst. (2008). C64, o230–o232

organic compounds
Table 1
Comparison of the bond lengths (A) in various pyridinium ylides.
˚
Comparative data from the Cambridge Structural Database (CSD, Version
5.28; Allen, 2002).
CSD Refcode
N1—C6
C6—C7
C7—O1
Reference
(I)
1.4110 (17)
1.400 (2)
1.4105
1.444 (5)
1.422 (4)
1.404 (3)
1.408 (2)
1.457 (3)
1.4193
1.3648 (19)
1.421 (2)
1.4113
1.443 (5)
1.412 (5)
1.426 (4)
1.416 (3)
1.428 (4)
1.4285
1.2572 (17)
1.231 (2)
1.2273
1.229 (5)
1.241 (4)
1.219 (3)
1.230 (2)
1.211 (3)
1.2415
a
b
c
d
e
f
g
h
i
AHENAC
BCAIMP
FALPAJ
FEFLEI
FOBNEQ
IZARAC
LAHLAI
PYINDB
VAMQEG
YARYUM
1.4400 (11)
1.407 (12)
1.4105 (12)
1.427 (2)
1.2269 (12)
1.221 (2)
j
k
Notes: (a) this work; (b) Kolev et al. (2002); (c) Friedman et al. (1978); (d) Banks et al.
(1986); (e) Uc¸ar et al. (2005); (f) Kolev, Yancheva et al. (2005); (g) Kolev et al. (2004); (h)
Bansal et al. (2004); (i) Kaminskii et al. (1976); (j) Kuhn et al. (2003); (k) Kolev, Wortmann
et al. (2005).
Figure 2
Part of the crystal structure of compound (I), viewed along the b axis.
Table 2
Hydrogen-bond geometry (A, ).
1
Hydrogen bonds are shown as dashed lines. [Symmetry codes: (i) x ꢁ ,
ꢂ
˚
2
3
2
1
2
3
2
ꢁy + , z; (ii) x + , ꢁy + , z.]
D—Hꢀ ꢀ ꢀA
D—H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D—Hꢀ ꢀ ꢀA
C6—H6ꢀ ꢀ ꢀF1
C1—H1ꢀ ꢀ ꢀO1
C5—H5ꢀ ꢀ ꢀO1i
C6—H6ꢀ ꢀ ꢀF5i
0.94 (2)
0.97 (2)
0.97 (2)
0.94 (2)
2.72 (2)
2.10 (2)
2.39 (2)
2.48 (2)
2.994 (2)
2.842 (2)
3.240 (2)
3.396 (2)
97.7 (11)
131.7 (15)
147.3 (17)
164.9 (14)
charges on oxygen and fluorine. The intramolecular C5—
H5ꢀ ꢀ ꢀO1 hydrogen bond results in a small C1—N1—C6—C7
torsion angle [7.7 (2)ꢂ]. The C5ꢀ ꢀ ꢀO1 distance is much shorter
than those of the related pyridinium ylides, suggesting a
stronger interaction for this hydrogen bond. On the other
hand, the C6—C7—C8—C9 torsion angle of ꢁ56.04 (19)ꢂ is
the result of the attractive C6—H6ꢀ ꢀ ꢀF1 interaction and
electrostatic repulsion between the negatively charged O and
F atoms. These hydrogen bonds promote the formation of the
enol form.
1
3
Symmetry code: (i) x ꢁ ; ꢁy þ ; z.
2
2
decomposition point 377 K). IR (KBr, ꢀ, cmꢁ1): 1634 (s), 1570 (m),
1484 (m), 1372 (s); 1H NMR (400 MHz, CDCl3): ꢁ 9.50 (d, J = 6.4 Hz,
2H), 7.58 (t, J = 7.2 Hz, 1H), 7.53 (m, 2H), 6.17 (s, 1H); MS m/z: 287
(M+, 97%), 285 (100), 120 (48), 65 (51).
Molecules of (I) in the crystal structure are linked through
two weak C—Hꢀ ꢀ ꢀF and C—Hꢀ ꢀ ꢀO hydrogen bonds, forming
chains running along the a axis (Fig. 2). Atoms C5 and C6 in
the molecule at (x, y, z) act as hydrogen-bond donors via
Crystal data
3
˚
C13H6F5NO
Mr = 287.19
V = 1131.24 (7) A
Z = 4
Cu Kꢃ radiation
ꢄ = 1.47 mmꢁ1
T = 296.1 K
Monoclinic, P21=a
1
˚
a = 11.5658 (4) A
atoms H5 and H6 to atoms C5 and F5 in the molecule at (x ꢁ ,
2
˚
b = 8.0116 (3) A
3
2
ꢁy + , z). There are no other significant intermolecular
˚
c = 12.2308 (4) A
0.40 ꢃ 0.25 ꢃ 0.10 mm
interactions.
ꢂ = 93.453 (2)ꢂ
In summary, the significantly longer C—O and shorter C—
C bond lengths around the anionic moiety demonstrate that
ylide (I) exists in the enol form. The perfluorophenyl group is
responsible for the stability and promotion of the enol
tautomer through intramolecular hydrogen bonds, which will
provide insight into the control of keto–enol equilibrium in
related systems.
Data collection
Rigaku R-AXIS RAPID
diffractometer
Absorption correction: empirical
(using intensity measurements)
(ABSCOR; Higashi, 1995)
Tmin = 0.592, Tmax = 0.867
12696 measured reflections
2056 independent reflections
1825 reflections with I > 2ꢅ(I)
Rint = 0.047
Refinement
Experimental
R[F2 > 2ꢅ(F2)] = 0.034
wR(F2) = 0.104
S = 1.08
207 parameters
All H-atom parameters refined
Triethylamine (75 ml, 0.53 mmol) was added to a solution of 1-[2-oxo-
2-(perfluorophenyl)ethyl]pyridinium bromide (129 mg, 0.35 mmol) in
CH2Cl2 (4 ml). The mixture was stirred at room temperature for 2 h.
The product was extracted three times with CH2Cl2. The combined
organic layer was dried over anhydrous MgSO4 and evaporated to
give (I) (95.5 mg). Recrystallization of (I) from CH2Cl2 gave yellow
crystals suitable for X-ray crystallographic analysis (yield 96%;
ꢁ3
˚
Áꢆmax = 0.18 e A
ꢁ3
˚
2056 reflections
Áꢆmin = ꢁ0.16 e A
All H atoms were located in difference Fourier maps and refined
isotropically without restraints; the C—H distances are in the range
˚
0.926 (19)–0.98 (2) A.
ꢄ
Acta Cryst. (2008). C64, o230–o232
Yamada and Ohta C13H6F5NO o231

organic compounds
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Promotion of Science.
¨
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o232 Yamada and Ohta C13H6F5NO
Acta Cryst. (2008). C64, o230–o232