2-Chloroallyl cation. Structure, FT-IR spectra, and matrix isolation

Full text of DOI:10.1021/jo971481g

J . Org. Chem. 1999, 64, 4931-4934
4931
Notes
2-Ch lor oa llyl Ca tion . Str u ctu r e, F T-IR
Sp ectr a , a n d Ma tr ix Isola tion
Davor Ki]emet,† Zlatko Mihalic´,*,† Igor Novak,‡ and
Hrvoj Vancˇik*,†
Department of Chemistry, Faculty of Science and
Mathematics, University of Zagreb, Strossmayerov trg 14,
10000 Zagreb, Croatia, and Department of Chemistry,
National University of Singapore, Singapore 119260
Received August 11, 1997
F igu r e 1. MP2(fc)/6-311G(d,p) optimized geometries and
relative energies (including ZPVE correction) of cations 1a and
1b and transition structure for their interconversion.
Although a series of chloro-substituted allyl cations
were already characterized by NMR and IR spectra,1 the
problem of the structure of the 2-chloroallyl cation (1)
still remained an open question.2 It is known that allyl
cations are sensitive to the electron donating capabilities
of their substituents. Chlorine bound to the terminal
C-atom of the allyl group has a stabilizing effect that can
be ascribed to the back-donation of n-electrons. This effect
is manifested in the lowering of the rotational barrier
observed for the cis-trans isomerization of 1-chloroallyl
cations.1a Moreover, the extent of the stabilization by a
terminal chlorine is almost the same as by a terminal
methyl group.
However, an electron donating substituent bound to
the central carbon atom of the allyl group has no
stabilizing effect by π-conjugation. It was found3 that the
solvolysis rate of 2-chloro- and 2-bromoallyl esters is
much lower than that of the parent allyl tosylate. This
inter alia may be a consequence of the π-orbital non-
interaction between the halogen n-electrons and the allyl
cation LUMO. The orbital that describes halogen lone
pair electrons lies in the bisecting nodal plane of the
LUMO of the allyl cation. From such a point of view, it
may be expected that the preferred effect of chlorine in
1 is bridging rather than a back-donation of n-electrons,
and the Cs structure 1a seemed to be more favorable.
such a structure.2 The more symmetric C2v structure 1b
has not been taken into account in an explanation of
these spectral data. However, in spite of the lack of
π-interaction between the allyl group and the chlorine
atom, the structure 1b could be considered to be at least
as stable as isomer 1a . Stability of 1b may originate from
a perfect allyl group π-electron delocalization. Addition-
ally, the destabilizing sterical and strain effects are
absent in this structure.
In this work, we represent a reinvestigation of the
structure of 1 by ab initio calculations on the MP2 and
DFT level of theory. The results of the calculations are
supplemented with the experimental approach to the
isolation and FT-IR characterizations of the ion.
Resu lts a n d Discu ssion
Resu lts of Ca lcu la tion s. The energies of cations 1a
and 1b and the transition structure for their intercon-
version were calculated at the MP2(fc)/6-311G(d,p) level
of theory (Figure 1). The C2v structure 1b is 7.5 kcal/mol
(ZPVE-corrected value) more stable than the chlorine-
bridged isomer 1a . The activation energy for the isomer-
ization of 1a to 1b is calculated to be 14.7 kcal/mol. At
the DFT level (B3LYP/6-311G(d,p)), the stability relation
between 1a and 1b is not much different than at the MP2
level. Isomer 1b (C2v) is 10.1 kcal/mol (ZPVE-corrected
value) more stable than 1a . Theoretically, both of the
carbocations are isolable. Noninteraction of the chlorine
n-electrons in 1b could be demonstrated by unchanged
values of the C-Cl bond distance. It is less shortened
(1.703 Å, compared to 1.640 Å in 1-chloroallyl cation,
1.747 Å in 2-chloropropene, or 1.735 Å in 2,3-dichloro-
propene (2), all at MP2(fc)/6-311G(d,p)) than could be
expected for the structures with more significant chlorine
interaction. A decrease in the C-Cl bond length by
0.0674a and 0.0584b Å in going from a neutral chloride to
an R-chlorocarbocation has been observed by X-ray dif-
This structure could also be considered as a kind of
allene chloronium ion. Previously published 1H NMR
spectra of 1 in a SbF5/SO2 solution were explained by
† University of Zagreb.
‡ National University of Singapore.
(1) (a) Vancˇik, H.; Mihalic´, Z.; Ki]emet, D. Croat. Chem. Acta 1996,
69, 1511. (b) West, R.; Kwitowski, P. T. J . Am. Chem. Soc. 1966, 88,
5280. Deno, N. C.; Holland, G. W. J .; Schulze, T. J . Org. Chem. 1967,
32, 1496. Deno, N. C. In Carbonium Ions; Olah, G. A., Schleyer, P. v.
R., Eds.; Wiley-Interscience: New York, 1970; Vol. 2, pp 783-806.
(2) Bollinger, J . M.; Brinich, J . M.; Olah, G. A. J . Am. Chem. Soc.
1970, 92, 4025.
(4) (a) Laube, T.; Bannwart, E.; Hollenstein, S. J . Am. Chem. Soc.
1993, 115, 1731. (b) Chen, G. S.; Glaser, R.; Barnes, C. L. J . Chem.
Soc., Chem. Commun. 1993, 1530. (c) Review: Weinhold, F.; Carpenter,
J . L. In The Structure of Small Molecules and Ions; Naaman, R., Vager,
Z., Eds.; Plenum: New York, 1988; p 227. (d) Bader, R. F. W. Atoms
in Molecules-A Quantum Theory; Clarendon Press: Oxford, 1990.
(3) Bentley, T. W.; Norman, S. J .; Kemmer, R.; Christl, M. Liebigs
Ann. 1995, 599.
10.1021/jo971481g CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/06/1999

4932 J . Org. Chem., Vol. 64, No. 13, 1999
Notes
calculated at 1850 cm-1 (1757 cm-1 scaled by 0.9506) for
1a and at 1619 cm-1 (1537 cm-1 scaled) for the structure
1b. The former frequency is typical for the allene chlo-
ronium ion and the second for the allyl cation, respec-
tively.
Exp er im en ta l Ap p r oa ch . Preparation of 1 in solid
SbF5 failed. The expected characteristic signals for an
asymmetric stretching mode of the CCC+ group (in the
1500-1600 cm-1 region) or of the CdC+ group (in the
1800-1900 cm-1 region) did not appear in the spectra.
Instead, broad absorptions, typical for polymeric materi-
als, were recorded. This failure encouraged us to test
other Lewis acids, such as SbCl5, as a superacid matrix
material for the preparation of these sensitive cations.
The precursor of 1, 2,3-dichloropropene (2), was co-
deposited5 with an excess of SbCl5 (500:1) on the CsI
window cooled to 77 K. The spectrum changed, by
warming the matrix to 120 K and the dominant peaks
at 1574 and 1424 cm-1 started to grow. The spectra are
shown in Figure 2c,d. All the new peaks were compared
with the spectra calculated for the structures 1a and 1b
on the MP2(fc)/6-311G(d,p) level of theory. A good agree-
ment with experimental frequencies and intensities was
found only for the spectrum calculated for structure 1b.
Comparison between experimental and calculated fre-
quencies for the four peaks that can be readily identified,
two C-H stretching modes (3023 and 2967 cm-1) and two
stretching modes of the CCC+ group (1574 and 1424
cm-1), gives a correlation coefficient of 0.9997 and a
scaling factor of 0.94, in agreement with the published
value.6 The characteristic CCC+ asymmetric stretching
vibration of 1b (1574 cm-1) is almost the same as in the
parent allyl cation (1577 cm-1).7 This supports the
theoretical prediction that the nodal position of the
chlorine has no direct effect on the π-electron density in
the ground-state allyl cation. For structure 1a , which
may be perceived as an allene chloronium ion, calculation
predicts the high-frequency signal of CdC stretching at
1850 cm-1 (Figure 2, part a), but in our experiments, no
signal appeared in this spectral region (see Figure 2, part
d).
Alternative assignment of the spectrum in part b of
Figure 2 to the complex 2‚SbCl5 rather than to the
carbocation 1b can be excluded because a change in the
CdC frequency in going from the precursor 2 to such a
complex is calculated to be only 9 cm-1! Namely, calcu-
lated CdC stretching frequencies at the MP2(fc)/6-31G-
(d) level of theory are 1714 cm-1 in 2 and 1705 cm-1 in
2‚SbF5 (Figure 3). On the other hand, a typical decrease
in the CdC stretching frequency in going from an alkene
to the allyl cation is nearly 70 cm-1, as it has already
been found in a series of simple acyclic allyl cations. In
the case of 1b, we have measured a 65 cm-1 frequency
downshift (from 1639 cm-1 in 2,3-dichloropropene to 1574
cm-1 in 1b).
To support the assignment of the spectrum of 1b, we
have also prepared its cyclic chloroallyl cation analogue
5. The ion was independently prepared in the SbF5
matrix from two different precursors 38a and 48b under
similar experimental conditions used in preparation of
1b (Figure 4). The asymmetric CCC+ stretching signal
F igu r e 2. IR spectra calculated at MP2(fc)/6-311G(d,p) level
of theory (in parentheses scaled6 values) of (a) Cs cation 1a
and (b) C2v cation 1b, (c) experimental FT-IR spectrum of 2,3-
dichloropropene in an SbCl5 matrix at 77 K, and (d) reaction
product after the matrix was warmed to 120 K.
fraction measurements. The same conclusion also follows
from results of the NPA4c and AIM4d analyses of MP2
densities at the same level of theory. In going from the
trans-1-chloroallyl cation to the 2-chloroallyl cation 1b,
the charge on chlorine decreases from 0.333 to 0.156
(NPA) or from 0.127 to -0.001 (AIM), respectively. Also,
the C-Cl bond order, calculated by the AIM method,
changes from 1.445 in the trans-1-chloroallyl cation to
1.183 in the 2-chloroallyl cation 1b.
The less stable structure 1a possess characteristics of
the allene chloronium ion. Its CdC bond is shorter than
in nonbridged allyl cations. The Cl-atom takes a central
position between carbons above the C-C bond with
almost equal C-Cl bond distances (see Figure 1).
Starting from these geometries, the frequencies and
intensities of IR-active vibrations are calculated for both
1a and 1b. The corresponding simulated spectra are
represented in Figure 2 (spectra a and b). Characteristic
signals assigned to the CdC stretching vibrations are
(6) Scott, A. P.; Radom, L. J . Phys. Chem. 1996, 100, 16502.
(7) Buzek, P.; Schleyer, P.v. R.; Vancˇik, H.; Mihalic´, Z.; Gauss, J .
Angew. Chem., Int. Ed. Engl. 1994, 33, 448.
(5) Vancˇik, H.; Sunko, D. E. J . Am. Chem. Soc. 1989, 111, 3742.
Vancˇik, H. Pure Appl. Chem. 1995, 67, 761.
(8) (a) Makosza, M.; Wawrzyniewicz, M. Tetrahedron Lett. 1969,
4659. (b) Bergman, E. J . Org. Chem. 1963, 28, 2210.

Notes
J . Org. Chem., Vol. 64, No. 13, 1999 4933
F igu r e 4. FT-IR spectra of 2-chlorocyclohexenyl cation (5) in
an SbF5 matrix from the precursors (a) 2,3-dichlorocyclohexene
and (b) 6,6-dichlorobicyclo[3.1.0]hexene.
typical absorption at 1574 cm-1 assigned to the allyl
CCC+ asymmetric stretching mode. Another isomer, 1a ,
has not been observed because no signal of the expected
allene chloronium ion CdC stretching vibration appeared
in 1800-1900 cm-1 spectral region.
F igu r e 3. IR spectra (nonscaled) calculated at MP2(fc)/6-31G-
(d) level of theory of (a) 2,3-dichloropropene (2) and (b) complex
of 2,3-dichloropropene with SbF5 (2‚SbF5). Bond distances
given in parentheses correspond to 2,3-dichloropropene.
The calculated decrease in the CdC stretching fre-
quencies (9 cm-1) expected by the formation of the
complex of the precursor with a Lewis acid is small in
comparison with the frequency downshift (65 cm-1
)
measured for the ionization of 2. Consequently, such a
complex has not been observed.
Exp er im en ta l Section
Gen er a l. All reagents and solvents were pure analytical
grade materials purchased from commercial sources and were
used without further purification. Infrared spectra were recorded
on Perkin-Elmer 1725x FT-IR spectrometer with 2 cm-1 resolu-
tion. The 13C and 1H NMR spectra were recorded on a Varian
Gemini 300 spectrometer operating at 75 and 300 MHz, respec-
tively, with CDCl3 as solvent.
6,6-Dich lor obicyclo[3.1.0]h exa n e.10 Chloroform (9 mL),
cyclopentene (9.2 mL; 0.1045 mol), 50% sodium hydroxide (20
mL), and triethylbenzylammonium chloride (0.2 g) were placed
in a 100 mL flask fitted with a stirrer. The mixture was
vigorously stirred for 1 h. Then, the cooled reaction mixture was
added in water (50 mL). The layers were separated, and the
water layer was extracted with dichloromethane (25 mL). The
combined organic layers were washed with water (25 mL), dried
over sodium sulfate, and evaporated. Distillation at 20 mmHg
gave 6.70 g of product, bp 70-72 °C in 42% yield. 13C NMR (δ/
ppm): 24.88; 27.46; 37.92; 68.01.
was found in the same spectral region, i.e. at 1523 cm-1
.
Again, this frequency is very close to that measured for
the unsubstituted cyclohexenyl cation (1521 cm-1),9
implying that the central position of the chlorine atom
does not significantly change π-electron distribution in
the allyl group.
Con clu sion s
Among the two possible isomers of a 2-chloroallyl
cation, it was found that the C2v structure 1b is 7.5 kcal/
mol (MP2) or 10.1 kcal/mol (DFT) more stable than the
Cs isomer 1a .
Our FT-IR experiments in the cryogenic solid SbCl5
matrix have shown that the ion 1b is a major product
under these conditions. It can be characterized by a
2,3-Dich lor ocycloh exen e.11 6,6-dichlorobicyclo[3.1.0]hexane
(2.5 g; 0.016 mol) was heated for 3 h under nitrogen at 210 °C.
Distillation gave 2.5 g of 2,3-dichlorocyclohexene, bp 81 °C, 15
mmHg, in 100% yield. IR (cm-1): 1641 (CdC str.). 13C NMR (δ/
ppm): 15.84; 25.89; 32.84; 59.00; 129.80; 131.28.
(9) Unpublished result from this laboratory.
(10) Makosza, M.; Wawrzyniewicz, W. Tetrahedron Lett. 1969, 4659.
(11) Bergman, E. J . Org. Chem. 1963, 28, 2210.

4934 J . Org. Chem., Vol. 64, No. 13, 1999
Notes
with Gamess12 and Gaussian 9413 programs at MP2(fc)
and B3LYP level of theory. Split-valence basis set
6-311G(d,p) was used for cations and the precursor
molecule, and the structure and spectra of the 2‚SbF5
complex was calculated using the 6-31G(d) basis set for
2 and 3-21G(d) for the SbF5 moiety. The existence of a
true minima or saddle point was, in each case, verified
by the absence or presence of imaginary vibrational
frequencies. Natural population analysis (NPA)4c and
atoms in molecules theory (AIM)4d calculations, both
based on MP2(fc)/6-311G(d,p) densities, were performed
by Gaussian 94.
Ma tr ix Exp er im en t. All the matrices were prepared on a
CsI window cooled by a closed cycle cryostat ROK 10-300 Leybold
Heraeus connected to vacuum line, equipped with an oil diffusion
pump. During the deposition, the temperature was held at 70
K and the vacuum at 10-5 mmHg. The flow of the sample was
regulated by a Teflon valve. The deposition times were 6 min in
all experiments. The matrix material (SbF5 or SbCl5) to sample
ratio was estimated to be 500:1.
Meth od s of Ca lcu la tion s
Full geometry optimizations and harmonic vibrational
frequency calculations were, for all molecules, performed
(12) GAMESS 97: Schmidt, M. W.; Baldridge, K. K.; Boatz, J . A.;
Elbert, S. T.; Gordon, M. S.; J ensen, J . H.; Koseki. S.; Matsunaga, N.;
Nguyen, K. A.; Su, S. J .; Windus, T. L.; Dupuis, M.; Montgomery, J .
A. J . Comput. Chem. 1993, 14, 1347.
(13) Gaussian 94 Revision D.1: Frisch, M. J .; Trucks, G. W.;
Schlegel, H. B.; Gill, P. M. W.; J ohnson, B. G.; Robb, M. A.; Cheeseman,
J . R.; Keith, T.; Petersson, G. A.; Montgomery, J . A.; Raghavachari,
K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.;
Cioslowski, J .; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.;
Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.;
Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.;
Defrees, D. J .; Baker, J .; Stewart, J . P.; Head-Gordon, M.; Gonzalez,
C.; Pople, J . A.; Gaussian, Inc., Pittsburgh, PA, 1995.
Ack n ow led gm en t. This work was supported by the
Ministry of Science of Croatia (Grant No. 119402) and
a research grant RP940601 from the National Univer-
sity of Singapore.
J O971481G