Stereoelectronic structure of α-bromoalkenyl trifluoromethyl ketones

Article abstract of DOI:10.1134/S1070428009100017

The results of quantum-chemical calculations at the B3LYP/6-311G level of theory showed that (Z)-α-bromo-β-arylalkenyl trifluoromethyl ketones are more stable than the corresponding E isomers by 4-5 kcal/mol. Relatively large positive charge on the olefin

Full text of DOI:10.1134/S1070428009100017

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 10, pp. 1431–1436. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.N. Chipanina, T.N. Aksamentova, A.Yu. Rulev, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 10,
pp. 1447–1452.
Stereoelectronic Structure
of α-Bromoalkenyl Trifluoromethyl Ketones
N. N. Chipanina, T. N. Aksamentova, and A. Yu. Rulev
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: nina_chipanina@irioch.irk.ru
Received December 14, 2008
Abstract—The results of quantum-chemical calculations at the B3LYP/6-311G** level of theory showed that
(Z)-α-bromo-β-arylalkenyl trifluoromethyl ketones are more stable than the corresponding E isomers by
4–5 kcal/mol. Relatively large positive charge on the olefinic β-carbon atom and strong polarization of the C=C
bond in both Z-s-cis and Z-s-trans conformers makes bromoalkenyl trifluoromethyl ketones the most potent
Michael acceptors among α,β-unsaturated carbonyl compounds. The calculated data are very consistent with
the experimental IR spectra.
DOI: 10.1134/S1070428009100017
Unsaturated carbonyl compounds play an important
role in synthetic organic chemistry and attract interest
as model structures for theoretical studies. α-Halo
derivatives of α,β-unsaturated carbonyl compounds
exhibit a high reactivity toward nucleophilic reagents
and are convenient models for studying transmission
of electronic effects through the C=C(Hlg)C=O con-
jugated system [1, 2]. It is known that introduction of
a fluorine atom or fluorine-containing group into
organic molecules considerably changes their physical
and chemical properties. It is not accidental that the
chemistry of unsaturated trifluoromethyl ketones has
developed extensively during the past decades, and
these compounds have been widely used in the syn-
thesis of acyclic and carbo- and heterocyclic com-
pounds [3–6]. Polyfunctionality and accessibility of
haloalkenyl trifluoromethyl ketones makes them un-
doubtedly advantageous as intermediate products for
the preparation of various biologically active sub-
stances and analogs of natural compounds. For ex-
ample, we recently reported on unexpected one-pot
transformation of 4-aryl-3-bromo-1,1,1-trifluoro-
methylbut-3-en-2-ones into indenols by the action of
secondary amines [7, 8].
the stereoelectronic structure of α-haloalkenyl tri-
fluoromethyl ketones in comparison with their non-
fluorinated analogs on the basis of the results of DFT
quantum-chemical calculations (B3LYP/6-311G**)
and IR spectral data.
α-Bromo enones IIIa–IIId were synthesized ac-
cording to a classical procedure implying bromination
of initial enones Ia–Id and subsequent dehydro-
bromination of dibromo derivatives IIa–IId thus ob-
tained [7, 8] (Scheme 1). α-Haloalkenyl trifluoro-
methyl ketones are typical captodative alkenes, which
can exist as several stereoisomers A–D arising from
cis–trans isomerism about the C=C bond and rotation
of the trifluoroacetyl group about the C–C bond. Ac-
1
13
cording to the H and C NMR data, α-bromo enones
Scheme 1.
O
Br
O
Br2, CHCl3, 10°C
Ar
CF3
Ar
CF3
Br
IIa–IId
Ia–Id
O
Et3N, Et2O, 10°C
With a view to understand specific reactivity of
alkenyl trifluoromethyl ketones having a halogen atom
in the α-position, it is necessary to thoroughly analyze
their structure and the nature of electronic interactions
in their molecules. In the present work we examined
Ar
CF3
Br
IIIa–IIId
Ar = Ph (a), 4-MeOC6H4 (b), 2,5-(MeO)2C6H3 (c), 2-thienyl (d).
1431

CHIPANINA et al.
Quantum-chemical calculations performed for
1432
O
Ar
O
isolated molecules showed that the Z isomers of IIIa
(s-trans or s-cis; heavy atoms constituting the π-elec-
tron system lie in one plane) are more stable than the
corresponding E isomers by 4–5 kcal/mol (Table 2).
The reason is that the C=C double bond in the
E-isomers is forced out from both aromatic ring plane
(by ~30°) and carbonyl group plane (by 53 and 43° for
the s-trans and s-cis conformers, respectively). The
difference in the energies of formation of the s-trans
and s-cis conformers of E-IIIa does not exceed
0.5 kcal/mol, whereas the s-trans-Z structure of IIIa is
more stable than s-cis-Z by 1.48 kcal/mol. According
to the MP2/6-311G** calculations, the s-trans con-
former is also more stable, but the energy difference is
∆E = 1.82 kcal/mol. Taking into account small dif-
ference in the dipole moments of the s-trans and s-cis
conformers (μ = 5.09 and 4.80 D, respectively), we
cannot assume stabilizing effect of solvent polarity.
Stabilization of the less polar s-cis conformer of Z-V
(μ = 3.08 D) in the solid state may be determined
by specificity of its crystal packing. While studying
α-bromo-β-aminoalkenyl trifluoromethyl ketones [11]
we already observed stabilization of the corresponding
s-cis conformers despite lower energy of s-trans con-
formers (by 1.4–1.8 kcal/mol). Almost similar energy
differences between the conformers were obtained by
calculations at the B3LYP/6-31G**, PBE/QZ3P, and
riMP2/cc-pVTZm levels of theory.
Ar
CF3
CF3
Br
Br
s-cis, Z
s-cis, E
A
B
CF3
Ar
CF3
Ar
O
O
Br
Br
s-trans, Z
s-trans, E
C
D
IIIa–IIId have Z configuration with respect to the
C=C bond with s-cis orientation with respect to the
single C–C bond in the C=C–C=O fragment [8].
The IR spectra of enones IIIa–IIId recorded from
thin film (IIIa) or KBr pellets (IIIb–IIId) contain
strong absorption bands with approximately equal
intensities due to stretching vibrations of the C=O and
C=C bonds, indicating s-cis orientation of these
groups [9, 10]. In the spectrum of IIIa, the ν(C=O) and
ν(C=C) bands appear at 1714 and 1593 cm–1, respec-
tively, for the neat substance, and they shift to 1716
and 1595 cm–1 and to 1721 and 1596 cm–1 in going to
CDCl3 and CCl4, respectively. However, the intensity
ratio of these bands remains unchanged, which may be
regarded as an additional support to s-cis conformation
of the Z isomer of IIIa in inert medium. In the IR
spectra of both liquid and crystalline bromo enone
Z-IV and bromo enal Z-V having no electron-with-
drawing CF3 group in their molecules, the ν(C=O)
band is displaced toward lower frequencies by
~30 cm–1, as compared to enone IIIa, while the
ν(C=C) band has a higher frequency but by no more
than 10 cm–1 (Table 1). As in the spectra of IIIa, the
intensity ratio corresponds to the s-cis conformer. On
the other hand, the intensity of the carbonyl absorption
band in the IR spectra of solutions of IV and V in
CDCl3 (1685 and 1697 cm–1, respectively) is higher
than the intensity of the ν(C=C) band (~1600 cm–1).
The difference in the absorption intensities increases in
going to weakly polar carbon tetrachloride. The
ν(C=O) bands of IV and V in this solvent (1692 and
1705 cm–1, respectively) are very strong, whereas the
ν(C=C) bands have medium intensity. This means that,
unlike α-bromoalkenyl trifluoromethyl ketones Z-III,
compounds Z-IV and Z-V in inert medium exist as
equilibrium mixtures of s-trans and s-cis conformers.
Increase of solvent polarity shifts the equilibrium
toward s-trans conformer.
Nucleophilic 1,4-addition reactions are typical
transformations of α,β-unsaturated carbonyl com-
pounds. Therefore, in order to evaluate general reac-
tivity of α-bromoalkenyl trifluoromethyl ketones we
examined the effects of their conformation and nature
of substituent in the benzene ring on the polarizability
of the C=C bond in the conjugated system C=C–C=O.
For this purpose, we defined the polarity of the double
Cα=Cβ bond (∆q) as the difference in the charges on
the Cα and Cβ atoms. The latter were calculated for the
Z isomers of a series of enones with s-trans and s-cis
orientations of the C=C and C=O bonds and different
substituents in the benzene ring, as well as for the
s-trans and s-cis conformers of a series of α-substi-
tuted enones and enals (Table 1).
The results showed that the s-trans conformers of
unsaturated trifluoromethyl ketones IIIa–IIIe and VI
are energetically more favorable than their s-cis struc-
tures by 1.3–1.8 kcal/mol (Table 1). The s-trans con-
formers of acrolein (VII) and its homologs both in the
gas phase and in inert solvents are also more stable [9].
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STEREOELECTRONIC STRUCTURE OF α-BROMOALKENYL TRIFLUOROMETHYL
1433
Table 1. Calculated (B3LYP/6-311G**) parameters of s-trans and s-cis conformers of substituted enones Ia, IV, VI, and
X–XII, enals V and VII–IX, and Z isomers of α-bromoalkenyl trifluoromethyl ketones IIIa–IIIe
C=C–C=O (s-trans)
∆qc
C=C–C=O (s-cis)
∆q
Comp.
no.
qb
q
R
X
H
Y
∆Ea
ν(C=C)d ν(C=O)d ∆E
ν(C=C) ν(C=O)
Cβ
C=C C=O
Cβ
C=C C=O
Ia
Ph
CF3 1.27 –0.020 0.136 0.377 1672
1767 0.00 –0.001 0.181 0.379 1658
1782 1.48 –0.058 0.328 0.410 1602
1786
1771
IIIa Ph
Br CF3 0.00 –0.036 0.321 0.432 1630
e(1593)e e(1714)e
IIIb 4-MeOC6H4 Br CF3 0.00 –0.041 0.332 0.439 1626
1775 1.39 –0.061 0.337 0.416 1599 1761 f
f(1585)f (1692)f
IIIc 2,5-(MeO)2- Br CF3 0.00 –0.068 0.344 0.433 1627
1779 1.80 –0.098 0.365 0.408 1600 1767 f
f(1570)f (1695)f
C6H3
IIId 2-Thienyl
Br CF3 0.00 –0.081 0.373 0.435 1632
1776 1.28 –0.099 0.377 0.414 1606 1762 f
f(1580)f (1700)f
IIIe 4-O2NC6H4 Br CF3 0.00 –0.037 0.319 0.422 1630
1792 1.33 –0.058 0.324 0.400 1614
1781
IV
V
Ph
Ph
Br CH3 1.81 –0.016 0.297 0.545 1650
Br H 0.31 –0.023 0.287 0.513 1650
Br CF3 0.00 –0.075 0.196 0.406 1652
1765 0.00 –0.055 0.341 0.552 1626 1757 e
e(1603)e (1685)e
1788 0.00 –0.059 0.367 0.528 1628 1774 f
f(1602)f (1681)f
VI
H
H
1809 1.81 –0.061 0.197 0.397 1633
1800
1790
VII
H
H
0.00 –0.168 0.001 0.472 1680 e 1786 1.67 –0.129 0.083 0.472 1666
(1618)e a(1704)e
Br H 0.00 –0.107 0.151 0.495 1656 1805 0.13 –0.072 0.211 0.510 1648
VIII H
1793
1798
IX
H
Cl H 0.01 –0.090 0.168 0.515 1663 e 1806 0.00 –0.054 0.233 0.531 1657
(1610)e a(1730)e
X
H
H
H
Me 0.41 –0.169 0.012 0.490 1683 e 1761 0.00 –0.127 0.074 0.506 1670 1780 e
(1618)e a(1685)e e(1618)e (1706)e
CF3 0.83 –0.139 0.021 0.364 1680 1792 0.00 –0.115 0.070 0.376 1672
XI
H
H
1809
XII
Br Me 1.16 –0.107 0.149 0.521 1656
1783 0.00 –0.071 0.195 0.536 1643 1776 e
e(1609)e (1697)e
a
b
c
d
e
f
Difference in the total energies of formation, kcal/mol.
Charge on atom, e.
Difference in the charges on atoms, e.
Calculated stretching vibration frequency, cm–1.
Experimental frequency (neat),
Experimental frequency (KBr).
The difference in the energies of formation of the
above conformers sharply decreases (∆E = 0.4–
0.01 kcal/mol) in going to α-haloacroleins VIII and
IX, bromo enone V, and unsubstituted enone X. The
s-cis conformer turned out to be more stable for com-
pounds Ia, IV, XI, and XII (∆E ≤ 1.81 kcal/mol).
formers. The polarities of the C=O group in different
conformers of Ia and VI–XII differ insignificantly,
and molecules having no bromine atom possess con-
siderably less polar C=O bond.
The presence of phenyl and trifluoromethyl groups
in bromo enones III makes the C=C bond more polar,
and the difference in the polarities of that bond in the
s-trans and s-cis conformers is small. In addition, the
Cβ atom bears a positive charge which is considerably
larger in the s-cis conformers (Table 1). Unsubstituted
trifluoromethyl ketone Ia is characterized by lower
Both conformers of Ia and VI–XII are charac-
terized by a negative charge on Cβ and low polarity of
the double C=C bond. The negative charge on Cβ in the
s-cis conformers is smaller, while the C=C bond is
more polar than in the corresponding s-trans con-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 10 2009

CHIPANINA et al.
1434
Table 2. Charge distribution (q, e) in different stereoisomers of 3-bromo-1,1,1-trifluoro-4-phenylbut-3-en-2-one (IIIa), cal-
culated by the B3LYP/6-311G** method
O–0.270
0.140
Ph
O
CF3
0.184
Ph
CF3
0.092
–0.255
0.058
0.036
0.148
0.054
–0.213
0.043
Ph
–0.270 CF3
CF3
Ph
–0.285 O
O
–0.230
–0.248
–0.234
Br0.027
Br
Br
Br
0.034
0.024
0.038
A
B
C
D
Parameter
A (Z, s-cis)
B (E, s-cis)
C (Z, s-trans)
D (E, s-trans)
∆Ea
μb
1.48
4.80
5.02
5.09
0.00
5.09
5.57
4.10
∆qc
00.328
00.273
00.321
00.267
a
b
c
Difference in the total energies of formation, kcal/mol.
Dipole moment, D.
Polarity of the C=C bond, e.
positive charge on Cβ and less polar C=C bond. Intro-
duction of an electron-withdrawing substituent (e.g.,
nitro group) into the benzene ring (IIIe) weakly affects
the above parameters, whereas electron-donating
groups (e.g., MeO in IIIb) favor increased positive
charge on Cβ and higher polarity of the C=C bond. The
strongest effect was observed for compounds IIIc and
IIId having a 2,5-dimethoxyphenyl or thienyl group,
respectively, on the β-carbon atom, especially for their
s-cis conformers. The polarity of the carbonyl group
is weakly sensitive to the substituent nature and con-
formation. Compounds IV and V having no trifluoro-
methyl group occupy an intermediate place. The posi-
tive charge on Cβ and polarization of the C=C bond in
the s-cis conformers of IV and V approach the corre-
sponding values for α-bromoalkenyl trifluoromethyl
ketones IIIa–IIIe but considerably exceed those found
for the s-trans conformers of IV and V. Thus, unlike
nonfluorinated analogs, bromoalkenyl trifluoromethyl
ketones IIIa–IIIe are characterized by fairly large
positive charge on the β-carbon atom at the double
bond and stronger polarization of that bond. This
means that bromoalkenyl trifluoromethyl ketones III
should behave as the most reactive Michael acceptors
among α,β-unsaturated carbonyl compounds.
higher than those found for their s-trans conformers,
which may be due to weaker conjugation in the s-cis
structure. These data are very consistent with the IR
spectra of enones [9]. However, introduction of a halo-
gen atom into the α-position leads to a considerably
different pattern. The ν(C=O) and ν(C=C) frequencies
calculated for the s-trans conformers of Z-IIIa and
E-IIIa are higher than for their s-cis conformers. The
same applies to other α-halo-substituted unsaturated
carbonyl compounds: the difference in the ν(C=O) fre-
quencies is 10–20 cm–1. The lower ν(C=C) frequency
for the s-cis conformers of all the examined α-halo
derivatives corresponds to increased polarization of the
C=C bond therein.
The calculated ν(C=O) frequencies of the s-trans
conformers of α-halo-substituted acroleins VIII and
IX are higher, while the ν(C=C) frequencies are lower
by ~20 cm–1, than those of more stable s-trans con-
former of acrolein (VII). These data are consistent
with variation of the corresponding frequencies in the
IR spectra of α-chloroacrolein (1730 and 1610 cm–1)
[12] as compared to acrolein (1704 and 1618 cm–1)
[9], which exist in solution preferentially as s-trans
conformers. α-Bromovinyl methyl ketone (XII) and
bromine-free analog X displayed similar variations of
the calculated ν(C=O) and ν(C=C) frequencies of the
s-trans conformers and reduction of ν(C=C) of the
s-cis conformers, the ν(C=O) frequency remaining
almost unchanged. This is reflected in the ν(C=O)
values in the IR spectra of enone X which exists as
s-trans and s-cis conformers (1685 and 1706 cm–1)
[9] and bromo enone XII in the s-cis conformation
(1697 cm–1). In addition, the ν(C=C) frequency of
We also calculated normal vibration frequencies of
the compounds under study in s-trans and s-cis con-
formations with a view to compare the calculated
values with the IR spectral data and electron density
distribution in their molecules (Table 1). The calculat-
ed ν(C=O) frequencies for the s-cis conformers of
unsubstituted acrolein (VII), methyl vinyl ketone (X),
trifluoromethyl vinyl ketone (XI), and enone Ia are
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STEREOELECTRONIC STRUCTURE OF α-BROMOALKENYL TRIFLUOROMETHYL
1435
compound XII decreases to 1609 cm–1 relative to
ν(C=C) of enone X (1618 cm–1). Thus introduction of
a bromine atom into the α-position of α,β-unsaturated
carbonyl compounds reduces the negative charge on
the β-carbon atom, increases polarization of the C=C
bond, and decreases is stretching vibration frequency.
of this series). The corresponding differences in the ex-
perimental IR spectra range from 110 to 120 and from
70 to 90 cm–1, respectively.
EXPERIMENTAL
Enones IIIa–IIId were synthesized according to
the procedure described in [8]. Enone IV was prepared
by successive bromination–dehydrobromination of
benzylideneacetone as described in [13]. α-Bromo-
cinnamaldehyde (V) was synthesized according to
[14]. The IR spectra were recorded from neat sub-
stances (films), KBr pellets, and solutions in CCl4 and
CDCl3 on a Varian 3100 FT-IR spectrometer. Quan-
tum-chemical calculations, including calculations of
vibration frequencies, were performed in terms of the
density functional theory (B3LYP/ 6-311G**) with
complete geometry optimization using Gaussian 03
software package [15]. The total energies of formation
were determined with correction for zero-point vibra-
tion energy.
Replacement of the methyl group in enone IV by
trifluoromethyl (compound IIIa) or introduction of
a halogen atom (compounds VI–XII) results in lower
calculated ν(C=C) frequencies and increased ν(C=O)
values. In the IR spectrum of enone IIIa the corre-
sponding absorption maxima are displaced to 1593 and
1714 cm–1 relative to analogous bands in the spectrum
of IV (1603 and 1685 cm–1).
According to the calculations, stretching vibrations
of the C=C bond in bromo enones III are strongly
overlapped by vibrations of the aromatic ring, and
introduction of an electron-withdrawing substituent
into the benzene ring weakly affects the calculated
ν(C=C) frequency, although this leads to considerable
polarization of the double bond in the s-cis con-
formers. Nevertheless, increased polarization of the
C=C bond is accompanied by reduction of the ν(C=C)
frequency in the IR spectra of enones IIIb and IIIc by
10–20 cm–1 as compared to enone IIIa having no
substituent in the benzene ring. Considerable increase
in the positive charge on the β-carbon atom and in the
polarity of the C=C bond in bromo enone IIId
corresponds to both reduction in the calculated ν(C=C)
frequency and low-frequency shift of the ν(C=C) band
in the IR spectrum. The calculated ν(C=O) frequency
decreases by 10 and 4 cm–1 upon introduction of one
(IIIb) or two (IIIc) methoxy groups, respectively. In
the IR spectra of these compounds, the low-frequency
shift of the ν(C=C) band reaches ~20 cm–1. Insofar as
the polarity of the C=O bond remains almost un-
changed, reduction of the ν(C=O) frequency is largely
determined by variation of geometric parameters of
that bond, i.e., its length and CCO bond angle, rather
than by electronic effect of substituents, which is
typical of carbonyl-containing compounds [10]. The
same applies to the reduced ν(C=O) frequency for s-cis
conformers of bromo enones with simultaneous de-
crease in the C=O bond polarization.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 08-03-00067-a).
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