C-H Hyperconjugation in α-chlorocarbocations

Article abstract of DOI:10.1039/a805772i

The lowering of C_β-H stretching frequencies in carbocations 1a-d and 2a-c induced by hyperconjugation was tested as a possible probe for estimating the electron donating ability of α-substituents. Conclusions are based on the results of high level quantum chemical calculations confirmed with experimental FT-IR spectra. Because the decrease in the C_β-H stretching frequency is comparable in 1b and in 1c, and in 2b and 2c respectively, it follows that α-substitution by a methyl group or by chlorine stabilizes a carbocation with almost the same effectiveness.

Full text of DOI:10.1039/a805772i

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C–H Hyperconjugation in á-chlorocarbocations
Milan Mesic´,a Igor Novak,b Dionis E. Sunkoa and Hrvoj Vancˇik*a
a Department of Chemistry, Faculty of Science, University of Zagreb, Strossmayerov trg 14,
10000 Zagreb, Croatia
b Department of Chemistry, National University of Singapore, Singapore 119260, Singapore
Received (in Cambridge) 23rd July 1998, Accepted 17th September 1998
The lowering of Cβ–H stretching frequencies in carbocations 1a–d and 2a–c induced by hyperconjugation was tested
as a possible probe for estimating the electron donating ability of α-substituents. Conclusions are based on the results
of high level quantum chemical calculations confirmed with experimental FT-IR spectra. Because the decrease in the
Cβ–H stretching frequency is comparable in 1b and in 1c, and in 2b and 2c respectively, it follows that α-substitution
by a methyl group or by chlorine stabilizes a carbocation with almost the same effectiveness.
calculations and NMR6a–c and IR6d–g spectroscopic data. Of
the two effects exhibited by the electronegative chlorine atom,
Introduction
The stability of carbocations depends on the type of atoms and
i.e. the destabilizing inductive and stabilizing resonance effect,
atomic groups attached to the positive carbon atom and on
the latter is more pronounced. The electron back-donation by
steric restrictions imposed by sp3–sp2 rehybridization. The sim-
the chlorine atom is isoelectronic with hyperconjugation
plest carbocation, CH3ϩ is the least stable and in the gas phase1
(Scheme 1). A similar intramolecular electron density transfer
it has only recently been experimentally accessible. The order
in the two cases is manifested by comparable 13C NMR chem-
of stability of simple alkyl cations is 3Њ > 2Њ > 1Њ and is practic-
ical shifts in corresponding α-CH3 and α-Cl substituted carbo-
ally independent of the mode of their generation in the gas
cations.7 Moreover, the barrier of the cis–trans isomerization of
phase or in solution. Major factors governing the stability of
methyl or chloro substituted allyl cations has been calculated to
carbocations are hyperconjugation, steric and field effects. Res-
onance effects are important in alkenyl and aryl substituted
cations. In molecular orbital terms, hyperconjugation is
described as the interaction between the vacant p-type orbital
be almost equal.6f
The objective of the research described in this paper was to
further investigate the relationship between hyperconjugation
and the α-chlorine effect in simple carbocations. The hyper-
on the cationic center and the pseudo π-orbital of the adjacent
conjugative effect of a neighboring C–H bond can be easily
C–H or C–C bond. The energy of this interaction depends on
recognized in the IR spectrum of carbocations generated in
the square of the Fock matrix element between the donor and
cryogenic SbF5 matrices. We have shown that the C–H stretch-
acceptor orbitals which is in turn a function of the dihedral
ing frequency in the methylcyclopentyl cation is 2775 cmϪ1 i.e.
angle and the type of donor σ-(pseudo-π) orbital.2 A better
about 200 cmϪ1 lower than the average C–H frequency in satur-
overlap between the hyperconjugating σ orbital with the empty
ated hydrocarbons (2950 cmϪ1).8a In contrast, in the tert-butyl
2
3
p orbital leads to a shortening of the Csp –Csp bond and a
lengthening of the properly oriented adjacent CH (CC) bond.
On the other hand charge delocalization reduces the electron
donation of the hyperconjugating orbital which affects the
change in the Csp –Csp bond length in the opposite direction.
Recent crystallographic studies by Laube et al.3 on cumyl
hexafluoroantimonate provide strong support for the effect
of phenyl delocalization on Cϩ–C bond lengths and hyper-
conjugative interactions.
Electronegative heteroatoms attached directly to the cationic
carbon increase the stability of the ion by the so called α-effect
and explain, inter alia, the higher solvolytic reactivity of α–halo
ethers.4 Thus, the methoxymethyl cation has been isolated as a
solid salt, CH3OCH2ϩ SbF6Ϫ. This effect is ascribed to the con-
tribution of a second resonance structure as in Scheme 1a.
cation9 with three adjacent methyl groups the C–H frequency is
lowered to 2845 cmϪ1. Taking cation 1 as a model (R = H, CH3,
Cl) the hyperconjugative shift of a Cβ–H bond will depend on
electron donating properties of the substituent R. With a better
2
3
R
2a: R = H
2b: R = CH3
2c: R = Cl
1a: R = H
1b: R = CH3
1c: R = Cl
R
+
+
H3C
H3C
CH3
Cl
Cl
1d
+
CH3
+
R
H
H
Cβ
H+
H
H
+
X
X+
+
C
C
H2
H2
donor, the electron demand from the C–H bond is smaller and
the change in the Cβ–H stretching frequency is less pronounced
(vide supra). Accordingly, the frequency of the Cβ–H bond may
be used as a measure of stabilization of a carbocation by sub-
stituent R. In order to test this hypothesis the C–H stretching
frequencies of two series of carbocations (1a–d and 2a–c) were
calculated by density functional theory and compared with
experimentally observed frequencies of the respective cations
generated in cryogenic SbF5 matrices (Table 1).
a) X = O, Cl
b)
Scheme 1
Crystal structure analysis of α-chlorosubstituted carbo-
cations by Laube et al.5 suggests the presence of a partial C᎐Cl
᎐
double bond character, a conclusion supported by ab initio
J. Chem. Soc., Perkin Trans. 2, 1998, 2371–2374
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Table 1 Cβ–H stretching vibrations affected by hyperconjugation
νCβ–H/cmϪ1
Calc.
Scaleda
Experimental
H
+
1a
H3C
2888
2776
2730
CH3
CH3
1b
2974
2965
2981
2820
2859
2851
2865
2711
2845
2845
—
+
H3C
CH3
CH3
Cl
Cl
+
1c
H3C
Cl
1d
+
H3C
H
—
+
2a
CH3
+
2902
2934
2789
2820
2765
2780
2b
2c
Cl
+
a Scaling by a factor of 0.9614, see ref. 10c.
Results and discussion
Structures and IR active vibrational frequencies of cations 1a–
1d, and 2a–2c were calculated by the density functional method
at B3-LYP/6-31G(d) level.10 Full geometry optimizations were
performed for all cations and the obtained structures were
genuine minima on potential energy surfaces as demonstrated
by the absence of imaginary vibrational frequencies. The
selected experimentally observed Cβ–H stretching vibrations
affected by hyperconjugation (Table 1) appear as signals of
lowest frequencies in the C–H stretching region. The decrease in
these frequencies is almost the same in methyl as in chloro sub-
stituted cations. However, both differ by about 100 cmϪ1 from
the corresponding values in parent 2-propyl9 and cyclopentyl
cations.8b
Changes in calculated bond lengths (Fig. 1) also support the
assumption that chlorine and methyl groups have similar stabil-
izing effects. The Cβ–H bonds suitably positioned for hypercon-
jugation in 1b and 1c are almost equally elongated relative to
1a (1.1077 and 1.1081 Å, respectively). Introduction of two
chlorine atoms, as in 1d leads merely to a discrete lengthening
of the Cβ–H bond to 1.0926. However, in 1a these bonds are
significantly longer (1.1175 Å). Similarly, the Cβ–H bond in the
chlorine derivative 2c (1.1111 Å) is only marginally shorter than
in the methyl substituted cation 2b (1.1134 and 1.1152 Å,
respectively). The hyperconjugatively shortened Cϩ–Cβ bonds
in all substituted cations are approximately 1.47 Å long, i.e. 0.03
Å longer than in the parent cation 1a. By using the Reindl–
Clark–Schleyer relationship11 it is possible to calculate the
hyperconjugation energy (Ehyp) from the known Cϩ–Cβ bond
length. Applying this relationship, the estimated Ehyp is 21 kcal
molϪ1 for the substituted cations, compared to 24 kcal molϪ1 for
the parent ion 1a. The difference in the Ehyp values (3 kcal
molϪ1) corresponds to a difference of 100 cmϪ1 (vide supra) in
the hyperconjugative red shift of Cβ–H stretching frequencies.
Fig. 1 Geometries of cations 1a–1d and 2a–2c calculated by B3-LYP/
6-31G(d). Bond lengths in Å; bond angles in degrees. Structures reflect
connections only. C–H bond distances lengthened by hyperconjugation
are underlined.
A decrease in the Cβ–H frequency of approximately 33 cmϪ1
corresponds to a hyperconjugation energy of 1 kcal molϪ1
.
Comparison with the experimental spectra
In most experiments the ionization started in the temperature
range between 110 and 180 K. Since under the applied experi-
mental conditions it was not possible to get better spectra of
the cations, all the experimental data were obtained repetitively
in a series of experiments. By our experience, the solid SbF5
matrix material does not possess the best optical quality and the
transparency of the matrix is highly sensitive to changes in
temperature. This is reflected in a remarkable change of the
spectral baseline, especially in the 2500–3500 cmϪ1 region. It is
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Fig. 3 FT-IR spectra of the 1-chloro-1-cyclopentyl cation prepared at
200 K a) from 1,2-dichlorocyclopentane, b) from 1,1-dichlorocyclo-
pentane. Impurities are labeled with *.
Fig. 2 FT-IR spectra of 2,2-dichloropropane-SbF5 matrix a) at 77 K
and b) at 240 K.
known that even in the argon matrix, there are difficulties in
recording the C–H stretching region because of the low inten-
sity of the signals. The structures of the ions obtained under
such experimental conditions are confirmed by comparison
with the calculated frequencies. After scaling the C–H stretch-
ing frequencies by a recommended factor of 0.9614,10c a satis-
factory agreement between the calculated and experimental
spectra was achieved (Table 1). The frequencies below 2000
cmϪ1 agree well without scaling.
different precursors. Similarly, the IR spectrum of the parent
allyl cation12d was recorded starting from three different pre-
cursors. These are examples of isolations of carbocations which
are known as being very unstable but, in spite of this, no
precursor-SbF5 complexes have been observed. The cation was
identified by the appearance of a characteristic strong Cϩ–Cl
wagging mode band at 1075 cmϪ1. The stretching vibration of
the hyperconjugatively weakened Cβ–H bond was observed at
2780 cmϪ1
.
Further warming of 2c in the matrix (above 200 K) resulted
in its decomposition, which was indicated by the appearance of
a new band at 1495 cmϪ1. This indicates that 2c eliminates HCl
and rearranges to the stable cyclopentenyl cation 3. The 1495
2-Propyl cations
When 2,2-dichloropropane was codeposited with an excess of
SbF5 on the CsI window at 77 K, only the starting material was
identified in the IR spectrum. The spectrum of α-chloro substi-
tuted 2-propyl cation 1c started to appear by warming the mat-
rix to 240 K. The characteristic signal at 2846 cmϪ1 (Fig. 2)
could be assigned to the Cβ–H stretching vibration of the bond
which interacts hyperconjugatively with the positive carbon
atom. 1d was prepared from 1,1,1-trichloroethane under similar
conditions. By comparing the spectral data for 2-propyl cations
1a–1c, it can be seen that the hyperconjugative lowering of the
Cβ–H stretching frequency is almost equal in 1b (2845 cmϪ1) as
in 1c (2846 cmϪ1), but differs from that in 1a (2730 cmϪ1).9 This
result corroborates our hypothesis that both substituents, CH3
and Cl, are almost equally strong electron donors.
Cl
+
+
2c
3
cmϪ1 band could tentatively be assigned to νas of the allyl group
since all reported allyl cations afford a signal between 1500 and
1580 cmϪ1 6f,13
The same rearrangement of 2c was previously
.
observed by NMR in superacid medium.14
The Cβ–H stretching frequency in the chloro ion 2c (2780
cmϪ1) is almost the same as in the methyl substituted ion 2b
(2765 cmϪ1). This implies, as in the previous case, that the extent
of the electron donation of CH3 and Cl substituents in this
cation is also of comparable magnitude.
The remarkable effect of n-electron back-donation on
increasing the C–Cl stretching frequency was not observed in
1c, but in 2c an increase in frequency of the C–Cl wagging
vibration (at 1075 cmϪ1) was observed instead. In our previous
work,6g we have shown that changes in C–Cl vibrations caused
by n-electron back-donation differ for different α-chloro-
carbocations. The calculated C–Cl bond lengths in 1c (1.667 Å)
and in 2c (1.661 Å) are 0.04 Å shorter than a typical C(sp3)–Cl
bond distance (1.7 Å). This shortening of C–Cl bonds caused
Cyclopentyl cations
The chloro substituted cyclopentyl cation 2c was prepared
independently from two precursors, i.e. 1,1-dichlorocyclo-
pentane and 1,2-dichlorocyclopentane. In both experiments,
identical spectra assigned to ion 2c were obtained (Fig. 3).
Assignation of the spectrum should, whenever possible,
additionally be confirmed by a preparation of the same cation
from different precursors under similar experimental condi-
tions. In that sense, we have recorded the spectrum of the chlo-
rocyclopentyl cation starting from two different dichlorides.
Also, the 7-norbornyl cation12c has been isolated from three
J. Chem. Soc., Perkin Trans. 2, 1998, 2371–2374
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2 D. E. Sunko, I. Szele and W. J. Hehre, J. Am. Chem. Soc., 1977, 99,
5000 and references therein.
3 T. Laube, G. A. Olah and R. Bau, J. Am. Chem. Soc., 1997, 119,
3087.
by the α-chloro effect is smaller than the value of 0.067 Å
reported by Laube et al.5 from X-ray single crystal
measurements.
4 For an early reference on the α-effect see: A. Streitwieser, Jr.,
Solvolytic Displacement Reactions, McGraw Hill, New York, 1962,
pp. 25, 102. See also, J. March, Advanced Organic Chemistry, 4th
edn., Wiley Interscience, New York, 1992, p. 351.
5 T. Laube, E. Bannwart and S. Hollenstein, J. Am. Chem. Soc., 1993,
115, 1731.
Conclusion
The lowering of Cβ–H stretching frequencies in carbocations
induced by hyperconjugation was tested as a possible probe for
estimating the electron donating ability of α-chloro substitu-
ents. It was found that both a methyl group and chlorine as α-
substituents stabilize a carbocation with almost the same
effectiveness. From the spectra of various α-chloro carboca-
tions it seems that the C–Cl stretching frequency does not
6 (a) G. A. Olah and Y. K. Mo, Halogenated Carbocations, in
Carbonium Ions, Vol. V, Wiley & Sons, New York, 1976; (b) G. A.
Olah, L. Heiliger and G. K. S. Prakash, J. Am. Chem. Soc., 1989,
111, 8020; (c) G. A. Olah and M. R. Bruce, J. Am. Chem. Soc., 1979,
101, 4765; (d) H. Vancˇik, K. Percacˇ and D. E. Sunko, J. Am. Chem.
Soc., 1990, 112, 7418; (e) H. Vancˇik, K. Percacˇ and D. E. Sunko,
always occur at frequencies as high as 1000 cmϪ1
.
J. Chem. Soc., Chem. Commun., 1991, 807; ( f ) H. Vancˇik,
_
Z. Mihalic´ and D. Kidemet, Croat. Chem. Acta, 1996, 69, 1511;
_
(g) H. Vancˇik, I. Novak and D. Kidemet, J. Phys. Chem. A, 1997,
Experimental
101, 1523.
7 Ref. 6(a), p. 2188.
The precursors for cations 1c and 1d, i.e. 2,2-dichloropropane
and 1,1,1-trichloroethane, are commercially available (Aldrich).
Cation 2c was prepared separately from two precursors, i.e.
trans-1,2-dichlorocyclopentane and 1,1-dichlorocyclopentane.
The former was prepared from cyclopentene by a previously
described procedure.15 1,1-Dichlorocyclopentane was obtained
by the reaction of cyclopentene with SbCl5 in tetrachloro-
ethane.16
8 (a) H. Vancˇik and D. E. Sunko, J. Am. Chem. Soc., 1989, 111,
3742; (b) P. Buzek, Strukturbestimmung von Carbokationen, PhD
Dissertation, Friedrich-Alexander-Universität, Erlangen-Nürnberg,
1993.
9 G. A. Olah, E.-B. Baker, J. C. Evans, W. S. Tolgyesi, J. S. McIntire
and I. J. Bastien, J. Am. Chem. Soc., 1964, 86, 1360.
10 (a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098; (b) C. Lee, W. Yang
and R. G. Parr, Phys. Rev. B, 1988, 37, 785; (c) A. P. Scott and
L. Radom, J. Phys. Chem., 1996, 100, 16502.
11 B. Reindl, T. Clark and P. v. R. Schleyer, J. Comput. Chem., 1996, 17,
1406.
SbF5-matrix experiments
12 (a) N. C. Craig, R. K.-Y. Lai, L. G. Matus, J. H. Miller and
S. Palfrey, J. Am. Chem. Soc., 1980, 102, 38; (b) J. M. Jarret, N. Sin,
T. Ramsey and M. Saunders, J. Phys. Org. Chem., 1989, 2, 51; (c)
M. Mesic´, D. E. Sunko and H. Vancˇik, J. Chem. Soc., Perkin
Trans. 2, 1994, 1135; (d) P. Buzek, P. v. R. Schleyer, H. Vancˇik,
Z. Mihalic´ and J. Gauss, Angew. Chem., Int. Ed. Engl., 1994, 33,
448.
13 (a) H. Vancˇik, V. Gabelica, D. E. Sunko, P. Buzek and P. v. R.
Schleyer, J. Phys. Org. Chem., 1993, 6, 427; (b) P. Buzek, P. v. R.
Schleyer, H. Vancˇik, Z. Mihalic´ and J. Gauss, Angew. Chem., Int.
Ed. Engl., 1994, 33, 448.
The samples were deposited in a liquid nitrogen cooled cryostat
and/or closed cycle helium cryostat Leybold-Heraeus ROK 10-
300 and the CsI cold window was held at 77 K under 10Ϫ5 Torr
vacuum. The matrix:sample ratio was about 500:1. All ions
were prepared by the published procedure8a,17 from the corres-
ponding chloride (methyl substituted) and dichloride (chloro
substituted) precursors by codeposition with excess SbF5
(1:500) on a CsI window cooled to 10 or 77 K respectively.
Deposition times were 10–12 min. Spectra were recorded on
a Perkin-Elmer 1725x FT-IR spectrometer with 2 cmϪ1
resolution.
14 G. A. Olah, G. Liang and Y. K. Mo, J. Am. Chem. Soc., 1972, 94,
3544.
15 B. Carrol, D. G. Kubler, H. W. Dawis and A. M. Whaley, J. Am.
Chem. Soc., 1951, 73, 5382.
16 S. Uemura, A. Onoe and M. Okano, Bull. Chem. Soc. Jpn., 1974, 47,
692.
Acknowledgements
This work has been supported by the Ministry of Science of
Croatia (Grant No. 119402) and a research grant RP940601
from the National University of Singapore.
17 H. Vancˇik, Pure Appl. Chem., 1995, 67, 761.
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