Chiroptical properties of organic radical cations. The electronic and vibrational circular dichroism spectra of α-tocopherol derivatives and sterically hindered chiral hydroquinone ethers

Article abstract of DOI:10.1021/jp0463520

Qualitatively and quantitatively reliable electronic and vibrational circular dichroism (ECD and VCD) spectra of chiral organic radical cations were obtained for the first time with α-tocopherol derivatives and sterically hindered chiral hydroquinone ethers. The isolation and spectral measurements of chiral radical cation salts were made possible by using nitrosonium or antimony derivatives as electron-transfer oxidants, which can cleanly oxidize the substrate donors without giving any byproducts in the sample solution. Such reliable ECD spectra enabled us to fully examine the chiroptical properties of organic radical cations and also compare them with those of the corresponding neutral compounds. The observed VCD spectra of neutral and radical cationic species of chiral hydroquinone ether were nicely simulated by density functional theory (DFT) calculations, from which the relative contribution of each radical cation conformer in solution was evaluated. Thus, the combined synthetic, spectroscopic, and theoretical protocol, composed of chiral modification, clean oxidation to form stable radical cations, ECD/VCD spectral analyses, and DFT calculations, was demonstrated to be a powerful, indispensable tool for elucidating a comprehensive picture of radical cationic species in solution.

Full text of DOI:10.1021/jp0463520

9540
J. Phys. Chem. A 2004, 108, 9540-9549
Chiroptical Properties of Organic Radical Cations. The Electronic and Vibrational Circular
Dichroism Spectra of r-Tocopherol Derivatives and Sterically Hindered Chiral
Hydroquinone Ethers
,†
‡
,†,§
Tadashi Mori,* Hiroshi Izumi, and Yoshihisa Inoue*
Department of Molecular Chemistry, Graduate School of Engineering, Osaka UniVersity,
-1 Yamada-oka, Suita 565-0871, Japan, National Institute of AdVanced Industrial Science and Technology
AIST), AIST Tsukuba West, 16-1, Onogawa, Tsukuba, Ibaraki 305-8569, Japan, and Entropy Control Project,
2
(
ICORP, JST, 4-6-3 Kamishinden, Toyonaka 560-0085, Japan
ReceiVed: August 12, 2004
Qualitatively and quantitatively reliable electronic and vibrational circular dichroism (ECD and VCD) spectra
of chiral organic radical cations were obtained for the first time with R-tocopherol derivatives and sterically
hindered chiral hydroquinone ethers. The isolation and spectral measurements of chiral radical cation salts
were made possible by using nitrosonium or antimony derivatives as electron-transfer oxidants, which can
cleanly oxidize the substrate donors without giving any byproducts in the sample solution. Such reliable
ECD spectra enabled us to fully examine the chiroptical properties of organic radical cations and also compare
them with those of the corresponding neutral compounds. The observed VCD spectra of neutral and radical
cationic species of chiral hydroquinone ether were nicely simulated by density functional theory (DFT)
calculations, from which the relative contribution of each radical cation conformer in solution was evaluated.
Thus, the combined synthetic, spectroscopic, and theoretical protocol, composed of chiral modification, clean
oxidation to form stable radical cations, ECD/VCD spectral analyses, and DFT calculations, was demonstrated
to be a powerful, indispensable tool for elucidating a comprehensive picture of radical cationic species in
solution.
Introduction
photoreactions has been low, and the mechanism of chiral
information transfer upon ET is still unclear.
1
3
Chiral photochemistry has attracted considerable attention in
1
,2
Electronic circular dichroism (ECD) spectroscopy is an
essential and powerful tool for obtaining the chiroptical and
stereochemical information of natural and synthetic optically
active compounds, and in recent years, a great deal of chiroptical
recent years. Enantiodifferentiating photosensitization is one
of the most successful strategies for obtaining moderate-to-high
enantioselectivities by manipulating environmental factors such
3
as temperature, solvent, pressure, and concentration. Asym-
14
metric photoisomerizations performed within modified zeolite
property data have been accumulated. Nevertheless, practically
no systematic endeavor has hitherto been devoted to the ECD
spectral study of chiral organic radical cations, apart from the
preliminary study on a tocopherol derivative described here.15
There are several reports on ECD spectra of transient redox
species generated electrochemically and measured in situ.16
However, this spectroelectrochemical technique has not been
applied to the detection of radical ions and may have some
limitations in fully examining the chiroptical properties of radical
ions, because the spectral overlap with the original neutral
species cannot be avoided, particularly at the shorter wave-
lengths, and the contribution of possible chiral contaminants
cannot rigorously be excluded. In the present study, we report
the unequivocal ECD spectra of stable organic radical cations,
which are compared with those of the corresponding neutral
species. We further performed vibrational circular dichroism
4
supercages and asymmetric photocyclization/additions of achiral
5
substrates in chiral crystals have also been studied extensively.
In contrast, asymmetric (photo)reactions via radical ionic
species, produced through an electron-transfer (ET) process,
have been investigated much less, although a couple of recent
investigations have shown that asymmetric ET reactions do
occur but give low stereoselectivity. This is rather surprising,
because radical cations are widely recognized as important
6
7
intermediates in physical and organic chemistry, in particular,
8
9
in redox, electrophilic aromatic substitution, and addition-
10
11
elimination reactions. Furthermore, radical cations and
charge-transfer complexes12 are known to play crucial roles in
biological systems and are indeed characterized for a variety
of biologically active compounds. Photoinduced ET reactions,
by exciplex or excited charge-transfer complexes, are considered
to be important from both synthetic and mechanistic points of
view, but the stereoselectivity obtained in asymmetric ET
(VCD) spectral measurements and the relevant ab initio calcula-
tions at the Hartree-Fock (HF) and DFT levels in order to
elucidate the relative contribution of the possible conformers
in the solution phase. These studies provide us with the
indispensable chiroptical and stereochemical information about
usually much less stable organic radical cations and further
contribute to the assignment of radical cationic species observed
*
To whom correspondence should be addressed. Fax: 81-6-6879-7923.
Phone: +81-6-6879-7920. E-mail: tmori@chem.eng.osaka-u.ac.jp (T.M.),
inoue@chem.eng.osaka-u.ac.jp (Y.I.).
†
Osaka University.
National Institute of Advanced Industrial Science and Technology
‡
(
AIST).
§
17
ICORP.
by the recently developed time-resolved CD spectral technique
1
0.1021/jp0463520 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/09/2004

Chiroptical Properties of Organic Radical Cations
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9541
CHART 1: Radical Cations of Derivatives of
and, also, to the mechanistic understanding of asymmetric
electron-transfer reactions.
r-Tocopherol
Our own approach to authentic ECD and VCD spectra of
organic radical cations, free from artifacts, is to synthesize
candidate donor compounds that are expected to generate stable
radical cations upon ET oxidation in solution. For that purpose,
we chose trimethyloxonium hexachloroantimotate and nitroso-
nium salts as single-electron-transfer (SET) oxidants and
annulated hydroquinone ethers as readily oxidizable substrates
to obtain stable and isolable chiral radical cations and reveal
their chiroptical properties. Furthermore, these stable radical
cations are of great interest from the viewpoint of designing
chiral magnetic materials, because a novel magneto-optical
phenomenon was predicted theoretically but has yet been not
fully supported experimentally, and only a limited number of
chiral-molecule-based magnetic materials have been synthe-
through the release of a proton.29 Optical rotation of R-toco-
pherol is strongly solvent dependent (e.g., the specific rotation
2
5
of (2R,4′R,8′R)-R-tocopherol, [R]365, varies from +0.32° in
24
ethanol to -3.0° in benzene. There is only one ECD spectrum
assigned to a radical cation of R-tocopherol acetate (Chart 1).1
In the study, optically active R-tocopherol acetate, (2R,4′R,8′R)-
5,25
1
a, was partially oxidized in trifluoroacetic acid to its radical
•+
1
8
cation (1a ), and the ECD measurement of the resulting solution
sized. Hence, the first unambiguous ECD and VCD spectra
and relevant chiroptical properties of organic radical cations
reported here are not only scientifically interesting but also
important for the development of such magneto-optical materi-
als.
-
1
-1
gave a negative CE of ∆ꢀext ) -65 M cm at 465 nm.
However, this is an extraordinarily large ∆ꢀ value compared to
those reported for neutral tocopherol and relevant optically active
-
2
-1
-1 26
alkylbenzenes: ∆ꢀ ≈ 10
M
cm . Our own attempts to
oxidize R-tocopherol acetate 1a with a slight-to-large excess
amount (up to 10 equiv) of nitrosonium tetrafluoroborate or
triethyloxonium hexachloroantimonate in dichloromethane (eqs
Results and Discussion
2
7
p-Dimethoxybenzenes are known to give the corresponding
radical cations upon mild oxidation, which are relatively stable
and well-characterized.19 Hence, we first introduced a chiral
auxiliary to anisole by the Mitsunobu coupling reaction to yield
1 and 2) were unsuccessful, for which the relatively high
+
-
•+
-
ArH + NO BF f ArH (BF ) + NO (eq 1)
4
4
+
-
•+
-
2
ArH + 3Et O SbCl f 2ArH (SbCl ) +
(1S,2S,5R)-p-neomenthyloxyanisole and (1S,2S,4S)-p-isobornyl-
3
6
6
oxyanisoles, which were then subjected to SET oxidation with
[EtCl, Et O, SbCl ] (eq 2)
2
3
+
-
triethyloxonium hexachloroantimonate (Et3O SbCl6 ) in dichlo-
+
-
romethane. An addition of Et3O SbCl6 to the solution of chiral
anisole under an argon atmosphere produced a new absorption
band at wavelengths around 450 nm, which is assignable to the
d0-d2 transition of the relevant radical cation on the basis of
the reported spectrum of the parent p-dimethoxybenzene radical
cation.19 We preliminary reported the CD spectra of these
solutions and discussed the Cotton effect (CE) of the chiral
organic radical cations semiquantitatively.20 It was revealed that
both the neutral and oxidized forms of chiral alkoxyanisole
derivatives exhibit CE of the same sign with analogous
oxidation potential of the acetate may be responsible, at least
in part. Strong acids were often used in various oxidation
reactions, but recent studies have revealed the formation of
concomitants, such as reduced species and undesirable byprod-
2
8
ucts derived therefrom. If they are chiral, as is often the case,
the obtained CD spectrum of the radical cationic species of
interest will not be reliable or reproducible. In the oxidation of
vitamin E and its derivatives, the radical cations are stable only
for a brief period of time as reactive intermediates and readily
deprotonate to give the corresponding neutral radical species.29
Furthermore, R-tocopherol possesses a fused dihydropyran
coplanar to the aromatic ring, which prevents the full relaxation
of the produced radical cationic species and eventually acceler-
ates the possible secondary (oxidation) reaction(s). We further
examined other oxidants (e.g., antimony pentachloride or
trifluoromethanesulfonic acid in dichloromethane) under various
conditions but failed to produce a radical cation that is stable
enough to obtain a fully consistent CD spectrum (∼30 min).
Hence, we performed a trace experiment under the reported
conditions (trifluoroacetic acid/dichloromethane ) 2:1) and
measured the CD spectrum of the oxidized species of 1a
immediately after the treatment, which is shown in Figure 1
1
ellipticities and anisotropy (g) factors in the Lb band regions.
However, because of the subsequent sluggish reactions, such
2
8
as chlorination and protonation of the aromatic ring, it was
difficult to analyze these first CD data more quantitatively, and
hence, the in-depth examinations of the structure and conforma-
tion of the radical cation in solution from the obtained CD
spectra have not been done. Nevertheless, the use of the SET
oxidant was obviously advantageous over the conventional
technique of using strong acids, in particular, for the preparation
and quantitative spectral measurement of radical cations.
1. Electronic CD Spectra of R-Tocopherol Derivatives and
their Radical Cations. We first chose tocopherol and the related
chromanols as substrate donors for the SET oxidation, not only
because this class of compounds is known to be biologically
active and behave as a natural antioxidant,21 but also because
the first and only chiral organic radical cation for which the
CD spectrum was claimed to have been measured was prepared
from vitamin E acetate.15 Tocopherols have attracted much
attention, especially in the fields of peroxyl radical chemistry
and of oxidations in vitro22 and in organic solvents. More
recently, the EPR and ENDOR spectra of radical cations of
vitamin E-related compounds measured in various solvents
revealed that these radical cations are very short-lived and are
spontaneously converted to the corresponding neutral radicals
15
(middle right, dashed line). Contrary to the previous report,
the solution exhibited an oppositely signed (positive) CE.30 Thus,
the observation of EPR signals does not immediately guarantee
that the detected radical cation of interest is the major species
responsible for the CD signals, particularly in such a complex
mixture resulting from strong acid oxidation. We would rather
suggest that protonated 1a, which is likely to be abundantly
formed under the highly acidic condition, gives a slightly red-
shifted absorption and CD signal. Therefore, chiral vitamin E
analogue 1b was newly synthesized. This compound is expected
to be readily oxidized to the corresponding radical cation under
a much milder condition using either nitrosonium tetrafluo-
23

9542 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Mori et al.
Figure 1. Electronic absorption (UV-vis) and ECD spectra and anisotropy (g) factors of neutral (left panels) and radical cationic (right panels)
•
+
•+
R-tocopherol derivatives (dashed lines for 1a and 1a ; solid lines for 1b and 1b ) in dichloromethane at 25 °C under an argon atmosphere. The
-
4
-3
UV-vis and ECD spectra were recorded at 1.0 × 10 M, except for the UV-vis spectra of the radical cations which were recorded at 1.0 × 10
+
-
M. The radical cations of 1a and 1b were obtained by treating the neutral donors with CF
3
CO
2
H and NO BF
4
, respectively. The vertical scale
(∆ꢀ) of the ECD spectra of the radical cationic species is tentative, as we assume a quantitative formation of the radical cation.
roborate or triethyloxonium hexachloroantimonate (eqs 1 and
), because its achiral analogue, prepared recently, has an
oxidation potential as low as 1.05 V (versus SCE, as compared
with E ) ca. 1.5 V for NO ) in dichloromethane. In such
an oxidation strategy, we can greatly reduce the amount of
contaminants and even eliminate the byproducts (such as NO,
EtCl, and SbCl3) by evaporation if necessary and exclude the
possible contamination by the arenium ion (protonated arene).
As expected, we could obtain the radical cation in ca. 70% yield
safely conclude that the previous report of a CD signal of the
radical cation of vitamin E acetate was erroneous. Although
the amount of the radical cation formed upon oxidation of 1a
was small, the analogous g factor observed indicates that the
2
+
31
32
•
+
CE, and the structure, of 1a is practically the same as that of
b•+. It is a good sign that the use of SET oxidants greatly
1
reduces the contamination by artifacts in the production and
subsequent CD measurement of radical cations from the
qualitative and semiquantitative points of view. However, even
(
estimated from the absorbance of the solution) within 30 min
•+
in the 1b case, its instability did not allow us to fully determine
+
-
by mixing 1b with Et3O SbCl6 in dichloromethane at 0 °C.
The absorption maximum of methyl ether 1b is slightly red-
shifted from that of acetate 1a in dichloromethane (probably
due to the lower oxidation potential of 1b (vide infra)), yet keeps
the same spectral shape. As can be seen from Figure 1 (middle
traces), the CD spectra of neutral (2R,4′R,8′R)-R-tocopherol
acetate 1a and methyl ether 1b show quite similar profiles,
the reliable ∆ꢀ value.
2
. Electronic Circular Dichroism Spectra of 9,10-Bis[(R)-
1
-methylpropyloxy]-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-octahy-
•+
droanthrancene (2b) and its Radical Cation (2b ). Ab Initio
Calculations of Annulated Hydroquinone Ethers. Of several
stable organic radical cations
2
7,28,34
that were structurally well-
1
characterized by X-ray crystallography, those derived from
sterically hindered hydroquinone ethers (2 and 3), possessing
two 1,3-cyclopentanediyls or 1,4-cyclohexanediyls, are suitable
for CD spectral examination. This is because (i) these radical
cations can be prepared under relatively mild conditions and
are totally stable not only in the solid state but also in
dichloromethane solution at ambient temperatures for more than
a month under inert conditions, (ii) the electrochemical and
UV-vis spectral properties were known for dimethyl analogues
and should not greatly be affected by chiral modification, and
exhibiting the absorptions attributable to the Lb band at ∼280
nm with a fairly strong negative CE with comparable intensities
33
of ∆ꢀ ≈ -0.14. This CD spectrum of neutral 1a is in good
15
agreement with the one previously reported.
•
+
The CD spectrum of 1b obtained by oxidation with
+
-
NO BF4 is shown in Figure 1, along with that of the neutral
•
+
donor 1b. Radical cation 1b exhibited a positive CE around
80 nm for the d0-d2 transition. Judging from the benzene
4
26
chirality and sector rules, as well as the conformational
resemblances of 1a with 1b and 1a with 1b , we can now
•
+
•+

Chiroptical Properties of Organic Radical Cations
iii) chiral ether moiety(ies) can be readily introduced to the
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9543
(
CHART 2: Stable Radical Cations of Sterically
Hindered Hydroquinone Ethers
aromatic ring by conventional coupling reactions.
It is known that the two methoxy groups, perpendicular to
the aromatic plane, are in either syn or anti conformation, and
34
the two are equilibrated with each other in the neutral donors.
Thus, the observed CD should be a sum of the signals from the
two conformers. The relative contributions of the syn and anti
conformers in 2 and 3 were crudely estimated by ab initio
calculations, using the Boltzmann populations obtained from
the calculated Gibbs free energies of the structure-optimized
conformers (See Table S1 in Supporting Information). The
optimization was performed with the economical HF/3-21G*
basis set for each conformer employed, which was followed
by high-level energy calculations using DFT methods at the
B3LYP/6-311+G(2d,p) level. The calculated energies were
further corrected for the zero-point energy obtained in frequency
CHART 3: Conformations of Neutral and Radical
Cationic Chiral Hydroquinone Ethers 2b and 2b•
+
3
5
calculations at HF/3-21G* level with a scaling factor of 0.94.
For the known dimethyl derivatives (2a and 3a), the calculated
free energies were comparable to the syn and anti isomers,
predicting a roughly 1:1 syn/anti ratio. However, the X-ray
crystallographic studies found only the anti conformers in
crystalline 2a and 3a,34 which can be accounted for by the
significant role played by the packing forces. In contrast, the
corresponding radical cations favor the anti conformation over
the syn conformation to a great extent.
We further moved on to the chiral hydroquinone derivatives
36
(2b,c and 3b,c) with (R)- and/or (S)-1-methylpropyl auxiliaries.
Interestingly, ab initio calculations yielded distinctly different
energy profiles and structure preferences for 2b and 3b, despite
the same homochiral modification. Thus, bis(cyclopentanediyl)-
annulated 2b prefers the anti form, while the bis(cyclohex-
anediyl)-annulated homologue 3b favors the syn form. In the
oxidized radical cation form, however, both 2b and 3b
exclusively prefer the anti conformation. Merely for comparison
purposes, meso donors 2c and 3c, possessing antipodal (R)- and
in Supporting Information), and the calibration with added
ferrocene gave the oxidation potential of 2b: E1/2 ) 1.19 V
versus SCE. This is slightly higher than that for the dimethyl
analogue 2a, but small enough to be oxidized by convenient
SET oxidants such as NO or Et3O SbCl6 (eqs 1 and 2). This
slight shift of E1/2 may be attributed to steric effects, because
the larger substituents in 2b may hinder the alkoxy group from
taking the favorable conformation coplanar to the aromatic ring.
+
+
-
(S)-1-methylpropyl groups, were also subjected to calculations
giving a similar tendency. However, we will not further analyze
or discuss the results obtained with these achiral donors, because
the stereoselective synthesis of the meso donors is rather
complicated, and no ECD or VCD signals are expected to occur.
Chemical Oxidations of 2b and CD Spectra of Neutral 2b
•
+
and Radical Cation 2b . To avoid ambiguity and obtain the
absolute ∆ꢀ value of a radical cation, we employed chiral
•
+
•+
derivatives of stable radical cations 2 and 3 , whose parent
•
+
•+
compounds (2a and 3a ) were known to be isolable as pure
salts (Chart 2). We will report the use of this isolated radical
cation for the first chiroptical measurements of the organic
radical cation (vide infra).
Among the species examined, 2b is conformationally the most
intriguing in both the neutral and radical cationic states, showing
populations to the energetically less favorable syn conformer
in 10% and 12%, respectively. Higher-level calculations at the
B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) levels afforded the
relative contribution of the syn in 21% for neutral 2b and 4%
The electronic CD and UV-vis spectra of the neutral donor
2b in dichloromethane are shown in Figure 2, along with the
anisotropy (g) factor. The observed CD extrema are in good
•
+
for 2b . Such a critical balance between the conformers is not
a drawback but an advantage for precisely analyzing how the
conformational changes affect the ECD and VCD spectral
behavior in solution (vide infra), and hence, we decided to
exploit this substrate for further experimental studies.
1
1
agreement with the absorption maxima for both Lb and La
bands (279 and 243 nm), affording negative CEs of ∆ꢀ ≈ -0.13
(Table 1). The ∆ꢀ values are significantly smaller than those
1
reported for related chiral aromatic ethers. For example, the Lb
bands of p-neomenthyloxyanisole and p-isobornyloxyanisole
display much larger ∆ꢀ’s of +1.2 and +1.8, respectively.20 The
unusually small ∆ꢀ value for 2b may be attributed at least in
part to the mixed conformers with opposite CD signs. This idea
is supported by our preliminary ab initio calculations at the HF
level and indeed proved by VCD measurements combined with
DFT calculations (vide infra). The observed g factors are on
Electrochemical Oxidation of 9,10-Bis[(R)-1-methylpropyl-
oxy]-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-octahydroanthrancene (2b).
Donor 2b was prepared by the Mitsunobu coupling of 9,10-
dihydroxy-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-octahydroanthran-
3
4
cene with (S)-(+)-2-butanol. The desired (R,R)-2b was
37
obtained in >99% optical purity. The dimethyl analogue 2a
is known to be electrochemically oxidized at a redox potential
-
4
of 1.11 V versus SCE.34 Hence, the chiral analogue 2b (1 mM)
the order of 10 , which is typical for chiral benzenes with
2
6
1
simple substitutents. A negative CE for the Lb transition is
was subjected to electrochemical oxidation with platinum
electrodes in anhydrous dichloromethane containing 0.2 M tetra-
n-butylammonium hexafluorophosphate as a supporting elec-
trolyte. A reversible cyclic voltammogram (CV) and a linear
sweep oxidation wave (OSWV) were obtained (See Figure S1
reasonable in view of the empirical benzene chirality rule.26
Chiral donor 2b was readily oxidized by either 1.5 equiv of
triethyloxonium hexachloroantimonate or 1 equiv of nitrosonium
tetrafluoroborate in dichloromethane to give a clear orange

9544 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Mori et al.
-
3
Figure 2. Electronic absorption (UV-vis) and circular dichroism (CD) spectra and anisotropy (g) factors of (a) neutral 2b (10 M) and (b) radical
•
+
-4
•+
-
-
4
cation 2b (10 M) in dichloromethane at 25 °C under argon atmosphere. The counteranion of 2b was either SbCl
6
(dashed line) or BF
(solid
line).
TABLE 1: UV-Vis and Circular Dichroism Spectra of
atmosphere to eliminate the precipitates. Anhydrous ether was
added to the solution to give the salt as a precipitate, which
was filtered and washed with dry ether under argon. Method 2:
Chiral Radical Cations and Their Parent Compoundsa
UV-vis
max/nm (log ꢀ)
CD
b
ext/nm (∆ꢀ)b
compd configuration oxidant
λ
λ
+
-
2
b and NO BF4 (1 equiv) were mixed in dichloromethane,
+
1
a
R,(R,R)
286 (3.34)
281 (-0.142)
234 (+0.271)
496 (∼+0.01)
285 (-0.137)
237 (+0.177)
470 (∼+0.01)
279 (-0.129)
243 (-0.125)
529 (-0.081)
488 (-0.082)
315 (-0.274)
and the suspension was stirred at 0 °C. As the NO salt
gradually dissolved, the color of the solution became darker.
The resulting solution was treated as in Method 1 to yield the
BF4 salt of 2b . These two salts were more than 98% pure,
as determined by the iodometric titration. These salts were
redissolved in dichloromethane, and the solutions were subjected
to the UV-vis and ECD spectral measurements under argon.
<
230
a•+
b
R,(R,R)
R,(R,R)
CF
CO
H
492 (>2.4)
289 (3.38)
230
c
c
1
1
3
2
-
•+
<
+
-
1
2
b•+
b
R,(R,R)
R,R
NO BF
4
487 (>3.0)
279 (2.74)
232 (3.89)
+
-
4
2
b•+
R,R
NO BF
520 (3.40)
d
4
3
85 (3.23)
The two radical cation salt solutions showed exactly the same
UV-vis spectra in the d0-d2 transition region around 520 nm
but revealed significant differences at shorter wavelengths, most
probably because of the overriding absorption by the counter-
d
15 (3.48)
a
Measured in dichloromethane at 25 °C under argon atmosphere.
b
c
Molar extinction coefficient ꢀ and molar circular dichroism ∆ꢀ.
•+
•+
Quantitative formation of 1a or 1b assumed; hence, the ∆ꢀ value
•+
anion. The CD spectra of 2b with different counteranions were
d
could be larger. Shoulder.
measured, and the results are shown in Figure 2b (traces A and
B). The two spectra are almost superimposable in the d0-d2
transition region but deviate appreciably at shorter wavelengths.
solution at -20 °C to room temperature under argon.38 This
solution was stable for a long period of time even at ambient
temperature, if protected from air. The new broad absorption
band centered at 520 nm is assigned to the d0-d2 transition of
-
Because SbCl6 is not transparent at shorter wavelengths, the
•
+
∆
ꢀ values of 2b were determined from the CD spectrum of
-
•+
the BF4 salt as -0.081, -0.082, and -0.274 at 529, 488, and
315 nm, respectively. Both neutral and radical cationic (R,R)-
2b species exhibit the negative CE for the two major bands. It
is noted that the d0-d2 transition of radical cation 2b• gives
an unexpectedly small anisotropy (g) factor of -2 × 10 at
ca. 500 nm, which is roughly 10 times smaller than those of
the allowed transitions of neutral 2b. This is simply due to the
radical cation 2b from the spectral comparison with its methyl
analogue (2a ) and the p-dimethoxybenzene radical cation.19
•+ 34
Two different salts of 2b•+ were isolated by the following
+
procedures. Method 1: A solution of 2b was mixed with 1.5
-
5
+
-
equiv of Et3O SbCl6 under argon in dichloromethane, and the
resulting mixture was stirred for 1 h at 0 °C. After the
evaporation of the solvent in vacuo, the residue was redissolved
in anhydrous dichloromethane and filtered under an inert
•+
higher transition probability (ꢀ) of the d0-d2 transition of 2b .

Chiroptical Properties of Organic Radical Cations
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9545
+
-
CHART 4: Conformers of (R)-2-Butanol with Different
Dihedral Angles around the C-C-C*-C and
H-C*-O-H Bond
ers around the H-C*-O-H bond (i.e., t, g , and g ) were
also taken into account to give information about more detailed
conformer structures and their distributions. It should be noted,
however, that, although the VCD-DFT approach is a versatile
and reliable tool for analyzing the structure of chiral molecules49
and has indeed been successful in determining the solution-
5
0
phase conformation of small molecules, it is not always
suitable for larger molecules with many more degrees of
freedom. Hence, we first examined the validity of such an
approach to the relatively large molecule 2b and then expanded
•
+
the method to the radical cationic species 2b .
A dichloromethane-d2 solution of 2b (0.22 M), placed in a
BaF2 cell (2 cm φ), was subjected to VCD measurement. The
path length of the cell was separately determined as 72 µm.
-
1
The VCD signals with a 4 cm resolution were accumulated
for 3 h to afford a reasonable spectrum, which was corrected
for the solvent signals. Figure 3a illustrates the VCD and
simultaneously obtained IR spectra of neutral 2b.
3
. Vibrational Circular Dichroism Spectra of 9,10-Bis-
[
(R)-1-methylpropyloxy]-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-oc-
•
+
tahydroanthrancene (2b) and its Radical Cation (2b ).
Neutral Donor. In the foregoing experiments, we measured the
first reliable electronic CD spectra of organic radical cations,
although this alone does not immediately provide us with enough
information to elucidate the structure and conformation of the
relevant radical cations; we therefore further measured the VCD
spectra of the relevant species in solution, which were compared
with the calculated spectra.
It is not very easy in general to know the population and
structural details of such conformers that are equilibrated with
each other in solution by using conventional spectroscopic
techniques such as UV-vis, electron paramagnetic resonance
Ab initio calculations of neutral donor 2b were performed at
two different levels (Table S2 in Supporting Information). First,
the geometrical optimization of each conformer was done by
HF calculation, which is faster and more economical than the
DFT and post-SCF calculations. This was followed by frequency
calculations at the same level and a single-point energy
calculation at the B3LYP/6-311G+(2d,p) level. Theoretical
absorption and VCD spectra were simulated with Lorentzian
-
1
band shapes of 6 cm full-width at half-mean, and the
frequencies were scaled by a factor of 0.97 or 0.91 for the
B3LYP/6-31G(d) or HF/3-21G* calculations, respectively. The
hydroquinone ether 2b may take 12 different conformations,
because there is syn-anti isomerism of two alkoxyls with
(
EPR), or NMR measurements. For example, the conformational
dynamics of R-tocopherol in solution have recently been studied
+
1
3
39
respect to the aromatic plane and also anti (T), gauche plus (G ),
in detail by C NMR spectroscopy. However, this elegant
methodology cannot be applied to the radical cation species.
EPR spectroscopy is a very sensitive technique to detect radical
cations, but it is usually used for the detection of transient
species in a solid matrix and is not suitable for studying the
conformer distribution in solution, particularly when several
-
and gauche minus (G ) rotamers around the C-C-C*-C bond
for each of the two 1-methylproyl groups. These isomers are
+
+
distinguished by the following abbreviations: anti-G G , anti-
+
+
-
-
-
-
+
+
G T, anti-G G , anti-TT, anti-TG , anti-G G , syn-G G , syn-
+
+
-
-
-
-
G T, syn-G G , syn-TT, syn-TG , and syn-G G . Similar
optimizations were done by the DFT method, which is relatively
fast and affords satisfactory results as precise as post-SCF
calculations. Thus, almost all of the IR peaks calculated for each
conformer can be assigned to the observed signals (see Sup-
porting Information).
conformers are present. The combined use of DFT calculations
and VCD spectroscopy40 has been recognized in recent years
as a powerful tool for determining the absolute configuration,
as well as the predominant conformations of ground state (or
neutral) chiral molecules in the solution phase. DFT theory
4
1
The VCD spectra calculated for all possible conformers are
shown in Figure 4 (right), along with the Boltzmann populations
calculated from their Gibbs free energies. Each conformer
displays a significantly different simulated VCD spectrum, and
no single conformer can correctly reproduce the observed VCD
profile. This is also true for the IR signals (see Figure S2 in
Supporting Information). The observed VCD and IR spectra
are best simulated in frequency and intensity by using the DFT
method: Compare spectrum a with b in Figure 4 (see Figure
S2 for the IR counterparts). The HF calculations gave less
satisfactory results: Compare spectrum a with d in Figure 4.
As expected, even if the contribution from the minor conformers
can provide vibrational frequencies and intensities, the accuracy
of which is comparable to post-self-consistent field (post-SCF)
calculations taking electron correlation into account. Standard
software such as Gaussian 9842 now includes the calculation
of VCD intensities as well. This allows us to use VCD for the
reliable determination of the absolute configuration and con-
formational analysis in solution for simple chiral molecules such
4
3
44
as 2-butanol and R-deuterioethanol. Recently, such tech-
4
5
niques have been further applied to a variety of systems, but
only for the rather simple stable compounds.
The absolute configuration of 2-butanol is of particular
importance in stereochemistry, because a large number of chiral
compounds are related to this simple chirogenic group. Thus,
the conformational preference of 2-butanol is also a crucial
recent topic, which has been studied carefully by using VT-
(of <10% population) is neglected, the simulated VCD spectrum
c (and e) show satisfactory matches. It is concluded, therefore,
that the DFT calculation can generate a reasonable VCD
spectrum of a relatively large neutral molecule such as 2b by
taking into account the conformer population that is estimated
from the calculated Gibbs free energy of each conformer.
polarimetry,46 NMR spectroscopy, Raman spectra combined
47
48
with MM3 calculations and VCD spectroscopy combined with
DFT calculations.43 These experiments indicate that the T
conformer, possessing the anti C-C-C*-C bond, is lowest in
Radical Cation. Because radical cation 2b•+ is totally stable
under inert atmosphere (vide supra), highly reliable and
reproducible VCD and IR spectra could be measured in solution.
Although the measurements were run under inert conditions,
+
energy, but the other two conformers (i.e., gauche plus (G )
-
+
-
and minus (G )) are also populated in the ratio T:G :G ) ca.
:4:1 (Chart 4). In the VCD-DFT study,43 additional conform-
5

9546 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Mori et al.
Figure 3. VCD and IR spectra of (a) neutral 2b (0.22 M) and (b) radical cation 2b•+ (0.22 M) in dichloromethane-d
at ambient temperature.
2
Figure 4. Experimental and calculated VCD spectra of 2b. Left: (a) observed; (b) DFT simulation using all 12 conformers; (c) DFT simulation
using 5 major conformers; (d) HF simulation using all conformers; (e) HF simulation using 3 conformers. Right: a breakdown of the DFT-
simulated spectrum b into each conformer with relative contribution. Y-axis of the calculated spectra is shifted for clarity.
using purge gas (N2) in a closed BaF2 cell, a short exposure to
the air (upon transfer of the sample to the cell) and continuous
IR irradiation (during the VCD measurement) did not ap-
preciably damage the sample. However, prolonged exposures
to air or IR may lead to decomposition of the radical cation to
give the corresponding benzoquinone derivative51 and other
unidentified byproducts during the spectrum accumulation (see
Figure S4 in Supporting Information for detail). Hence, we chose
the SbCl6 , rather than the BF4 , salt for greater stability and
used the VCD data obtained in the initial 3-h accumulation. If
the sample preparation was done very carefully (fully air-
protected), the VCD spectrum obtained did not change for up
to 5-6 h under the conditions employed. The VCD and IR
•
+
spectra of 2b thus obtained are shown in Figure 3b.
Calculations of the IR and VCD frequencies and intensities
were also performed for all 12 conformations and optimized
independently for the relevant conformations of radical cation
•+
2b at both HF and DFT levels as for the neutral species
(Figures 5 and 6). As anticipated, the HF-level calculations led
to significant deviations from the experimental IR and VCD
spectra, while the DFT calculations gave satisfactory results.
Again, the DFT-calculated frequencies and intensities for each
conformation do not immediately agree with the experimental
values, especially for the VCD spectrum. However, the Boltz-
-
-

Chiroptical Properties of Organic Radical Cations
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9547
•
+
Figure 5. Experimental and calculated IR (left) and VCD (right) spectra of 2b . (a) Observed; (b) DFT simulation using all 12 conformers; (c)
DFT simulation using 4 selected conformers; (d) HF simulation using all conformers; (e) HF simulation using 4 conformers. Y-axis of the calculated
spectra is shifted for clarity.
•
+
Figure 6. Calculated IR (left) and VCD (right) spectra of 2b . The DFT-calculated IR and VCD profiles of each conformer and the relative
contribution estimated from the Gibbs free energies are shown. Y-axis of the calculated spectra is shifted for clarity.
mann statistics and the Gibbs free energies as determined by
the high-level energy calculations with a relatively large basis
set (6-31G(d)) allow the population determination of the 12
conformers (See Table S2 in Supporting Information), and if
the relative distribution of these conformers is taken into
account, the calculated IR and VCD spectra of the radical cation
not match to any of the calculated peaks of 2b•+ but are found
in the observed spectrum of neutral 2b, indicating possible
contamination by the neutral species, especially at the later
stages of the data accumulation. Thus, the slight differences
between the simulated and observed VCD and IR intensities
•+
may partly originate from a small amount of 2b formed during
sample preparation or spectral measurement.
•+
2b
are in excellent agreement the experimental ones, as shown
in Figure 5b.
The population-weighted theoretical spectra of neutral 2b and
•+
Although the iodometric titration of the radical cation sample
indicated a purity of >95%, the observed IR and VCD spectra
of the solution of 2b appear to contain some neutral species.
For instance, the IR peaks at 1468 and 1203 cm , both of which
are seen in the spectrum of 2b solution (Figure 3, right), do
radical cation 2b are in good agreement with the experimental
spectra obtained in dichloromethane-d2 under inert conditions.
It is noted that the DFT calculations nicely simulate the VCD
spectra of not only the neutral but also the radical cationic
species by using the calculated energies and populations of all
•
+
-1
•
+

9548 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Mori et al.
1
2 possible conformations. The anti-T,T is the most stable
Y. Org. Mol. Photochem. 1999, 3, 71. (d) Everitt, S. R. L.; Inoue, Y. In
Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze,
K. S., Eds.; Marcel Dekker: New York, 1999; Vol. 3, p 71. (e) Inoue, Y.;
Ramamurthy, V. Chiral Photochemistry; Marcel Dekker: New York, 2004.
(2) (a) Cauble, D. F.; Lynch, V.; Krische, M. J. J. Org. Chem. 2003,
68, 15. (b) Nakamura, A.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 966. (c)
Wada, T.; Nishijima, M.; Fujisawa, T.; Sugahara, N.; Mori, T.; Nakamura,
A.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 7492. (d) Asaoka, S.; Wada,
T.; Inoue, Y. J. Am. Chem. Soc. 2003, 125, 3008. (e) Inoue, Y.; Jiang, P.;
Tsukada, E.; Wada, T.; Shimizu, H.; Tai, A.; Ishikawa, M. J. Am. Chem.
Soc. 2002, 124, 6942. (f) Inoue, Y.; Sugahara, N.; Wada, T. Pure Appl.
Chem. 2001, 73, 475. (g) Inoue, Y.; Wada, T.; Sugahara, N.; Yamamoto,
K.; Kimura, K.; Tong, L.-H.; Gao, X.-M.; Hou, Z.-J.; Liu, Y. J. Org. Chem.
conformation for neutral 2b, as was the case with (R)-2-
butanol,4 while the anti-G ,G and anti-T,T are almost
3
+
+
comparable in energy and equally populated for radical cation
2
prediction with the experimental verification, provides us with
a versatile and powerful tool for conveniently and reliably
analyzing a complex conformer mixture and even an experi-
mentally difficult-to-access unstable species.
•
+
b . This chiroptical approach, combining the theoretical
Conclusion
2000, 65, 8041. (h) Bach, T.; Bergmann, H.; Grosch, B.; Harms, K. J. Am.
Chem. Soc. 2002, 124, 7982. (i) Bach, T.; Bergmann, H. J. Am. Chem.
Soc. 2000, 122, 11525.
The only electronic CD spectrum reported for a radical cation
of R-tocopherol acetate has turned out to be incorrect and is
thought to arise from an artifact, because our own examinations
under the reported conditions gave the oppositely signed
(3) (a) Inoue, Y.; Wada, T.; Asaoka, S.; Sato, H.; Pete, J.-P. Chem.
Commun. 2000, 251. (b) Inoue, Y.; Ikeda, H.; Kaneda, M.; Sumimura, T.;
Everitt, S. R. L.; Wada, T. J. Am. Chem. Soc. 2000, 122, 406. (c) Inoue,
Y.; Matsushima, E.; Wada, T. J. Am. Chem. Soc. 1998, 120, 10687. (d)
Inoue, Y.; Yokoyama, T.; Yamasaki, N.; Tai, A. Nature (London) 1989,
341, 225.
(positive) Cotton effect for the d0-d2 transition with a much
lower ∆ꢀ value. This was made possible by employing a vitamin
E analogue with a low oxidation potential (E1/2) and SET
(4) (a) For a recent review, see Ramamurthy, V. J. Photochem.
Photobiol., C 2000, 1, 145. (b) Cheung, E.; Chong, K. C. W.; Jayaraman,
S.; Ramamurthy, V.; Scheffer, J. R.; Trotter, J. Org. Lett. 2000, 2, 2801.
(c) Shailaja, J.; Ponchot, K. J.; Ramamurthy, V. Org. Lett. 2000, 2, 937.
(d) Joy, A.; Scheffer, J. R.; Ramamurthy, V. Org. Lett. 2000, 2, 119. (e)
Lakshminarasimhan, P.; Sunoj, R. B.; Chandrasekhar, J.; Ramamurthy, V.
J. Am. Chem. Soc. 2000, 122, 4815. (f) Joy, A.; Uppili, S.; Netherton, M.
R.; Scheffer, J. R.; Ramamurthy, V. J. J. Am. Chem. Soc. 2000, 122, 728.
(g) Also, see Wada, T.; Shikimi, M.; Inoue, Y.; Lem, G.; Turro, N. J. Chem.
Commun. 2001, 1864.
+
oxidants such as NO and antimony derivatives, which elimi-
nates the possible contamination by side products such as the
arenium ion. Thus, the first reliable ECD spectra of radical
cations were obtained by quantitatively oxidizing chirally
modified, sterically hindered hydroquinone ethers with triethyl-
oxonium hexachloroantimonate or nitrosonium tetrafluoroborate.
These first CD spectral observations revealed that the CE and
the g factor of the radical cations of vitamin E derivatives and
chiral hydroquinone ether are relatively small and comparable
to those of the relevant neutral species.
(5) (a) Chong, K. C. W.; Sivaguru, J.; Shichi, T.; Yoshimi, Y.;
Ramamurthy, V.; Scheffer, J. R. J. Am. Chem. Soc. 2002, 124, 2858. (b)
Scheffer, J. R.; Wang, K. Synthesis 2001, 1253. (c) Scheffer, J. R. Can. J.
Chem. 2001, 79, 349. (d) Cheung, E.; Kang, T.; Netherton, M. R.; Scheffer,
J. R.; Trotter, J. J. Am. Chem. Soc. 2000, 122, 11753. (e) Cheung, E.;
Netherton, M. R.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1999, 121,
Vibrational CD spectroscopy, associated with ab initio
calculations, can be used as a standard protocol for analyzing
2919. (f) Borecka-Bednarz, B.; Bree, A. V.; Patrick, B. O.; Scheffer, J. R.;
(
and even predicting) the configuration, conformation and
Trotter, J. Can. J. Chem. 1998, 76, 1616. (g) Leibovitch, M.; Olovsson,
G.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1998, 120, 12755. (h)
Hirotsu, K.; Okada, K.; Mizutani, H.; Koshima, H.; Matsuura, T. Mol. Cryst.
Liq. Cryst. Sci. Technol., Sect. A 1996, 277, 99. (i) Koshima, H.; Maeda,
A.; Masuda, N.; Tatsuura, T.; Hirotsu, K.; Okada, K.; Mizutani, H.; Ito,
Y.; Fu, T. Y.; Scheffer, J. R.; Trotter, J. Tetrahedron: Asymmetry 1994, 5,
•
+
conformer population of chiral radical cations such as 2b .
Low-level optimizations at the HF/3-21G* level do not appear
to be useful, while DFT calculations provide satisfactory
agreements of the VCD and IR spectra with the experimental
ones both for neutral and radical cationic species. Despite the
limited number of successful examples presented here, it is clear
that the use of chiral probes in ECD and VCD spectral studies
combined with the (rather economical) theoretical DFT calcula-
tion at 6-31G(d)-level optimization allows us to elucidate the
structural details of a variety of chemically and biologically
important radical cations in solution, which is complimentary
or even superior to the X-ray crystallographic structure obtained
in the solid state.
1415.
(6) For example, see the following: (a) Maekawa, K.; Igarashi, T.;
Kubo, K.; Sakurai, T. Tetrahedron 2001, 57, 5515. (b) Hamada, T.; Ohtsuka,
H.; Sakaki, S. Chem. Lett. 2000, 364.
(
7) For reviews, see the following: (a) Symons, M. C. R. Chem. Soc.
ReV. 1984, 13, 393. (b) Garcia, H.; Roth, H. D. Chem. ReV. 2002, 102,
947. (c) Gruetzmacher, H.-F. Curr. Org. Chem. 2003, 7, 1565. (d) Harmata,
3
M. Chemtracts 2003, 16, 429-434. (e) Mizuno, K.; Hayamizu, T.; Maeda,
H. Pure Appl. Chem. 2003, 75, 1049. (f) Yoon, U. C.; Su, Z.; Mariano, P.
S. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd
ed.; Horspool, W., Lenci, F., Eds.; CRC Press: Boca Raton, 2004; pp 101,
1
-101, and 20.
8) Eberson, L. Electron-transfer reactions in organic chemistry;
Springer-Verlag: Berlin, 1987.
9) Taylor, R. Electrophilic aromatic substitution; John Wiley &
Sons: Chichester, 1990.
10) de la Mare, P. B. D.; Bolton, R. Electrophilic additions to
(
Acknowledgment. Financial support (to T. M.) of this work
by a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology of Japan
(
(
(
grants 12740346 and 16750034), Handai FRC, and the 21st
unsaturated systems, 2nd ed.; Elsevier Scientific Publishing Company: New
Century COE for Integrated EcoChemistry are gratefully
acknowledged. We thank Professor Qing-Xiang Guo at the
University of Science and Technology of China for preliminary
ab initio calculations and Dr. Guy A. Hembury for assistance
in the preparation of this manuscript.
York, 1982.
(
11) (a) Yim, M. B.; Kang, S.-O.; Chock, P. B. Ann. N.Y. Acad. Sci.
2
000, 899, 168. (b) Ozaki, S. i.; Matsui, T.; Roach, M. P.; Watanabe, Y.
Coord. Chem. ReV. 2000, 198, 39. (c) Edge, R.; Truscott, T. G. AdV.
Photosynth. 1999, 8, 223. (d) Morales, F. J.; Babbel, M.-B. J. Agric. Food
Chem. 2002, 50, 4657. (e) Charurin, P.; Ames, J. M.; del Castillo, M. D. J.
Agric. Food Chem. 2002, 50, 3751. (f) Fritz, T. A.; Liu, L.; Finer-Moore,
J. S.; Stroud, R. M. Biochemistry 2002, 41, 7021. (g) Bhaskar, B.; Bonagura,
C. A.; Li, H.; Poulos, T. L. Biochemistry 2002, 41, 2684. (h) Watanabe, Y.
ReV.Heteroat. Chem. 2000, 22, 135.
(12) (a) Gutman, F.; Johnson, C.; Keyzer, H.; Molnar, J. Charge-Transfer
Complexes in Biological Systems; Marcel Dekker: New York, 1998. (b)
Semenov, A. Y.; Mamedov, M. D.; Chamorovsky, S. K. FEBS Lett. 2003,
Supporting Information Available: General experimental
details, full tables of ab initio calculations, electrochemical
analysis of 2b, observed and calculated IR spectra of 2b, time
dependence of the VCD spectra of the radical cation, and VCD
and IR spectra of relevant reference compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
553, 223. (c) Chateauneuf, G. M.; Brown, R. E.; Brown, B. J. Int. J.
Quantum Chem. 2001, 85, 685. (d) Prokhorova, L. I.; Revina, A. A. Radiats.
Biol. Radioekol. 2001, 41, 24. (e) Mrozek, A.; Karolak-Wojciechowska,
J.; Bsiri, N.; Barbe, J. Acta Pol. Pharm. 2000, 57, 345. (f) Helton, M. E.;
Pacheco, A.; McMaster, J.; Enemark, J. H.; Kirk, M. L. J. Inorg. Biochem.
2000, 80, 227. (g) Wagner, M. A.; Trickey, P.; Chen, Z.-w.; Mathews, F.
S.; Jorns, M. S. Biochemistry 2000, 39, 8813. (h) Worthington, S. E.; Krauss,
References and Notes
(
1) For reviews, see the following: (a) Rau, H. Chem. ReV. 1983, 83,
5
35. (b) Inoue, Y. Chem. ReV. 1992, 92, 741. (c) Everitt, S. R. L.; Inoue,

Chiroptical Properties of Organic Radical Cations
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9549
+
+
M. Comput. Chem. 2000, 24, 275. (i) Yu, S.-W.; Kim, Y.-R.; Kang, S.-O.
Biochem. J. 1999, 341, 755. (j) Brown, B. J.; Deng, Z.; Karplus, P. A.;
Massey, V. J. Biol. Chem. 1998, 273, 32753. (k) Yamamoto, K.; Sawanishi,
H.; Miyamoto, K.-I. Biol. Pharm. Bull. 1998, 21, 356. (l) Font, M.; Monge,
A.; Ruiz, I.; Heras, B. Drug Des. DiscoVery 1997, 14, 259. (m) Lecomte,
S.; Baron, M. H. Biospectroscopy 1997, 3, 31. (n) Rehn, C.; Pindur, U.
Monatsh. Chem. 1996, 127, 645.
(36) All calculations in Table 2 were performed for the (G , G )
conformations with respect to the 2-methypropyl groups. This is indeed
not complete, however, because other conformations such as (T, T) may
be energetically much more favorable in some cases. For instance, the anti
+
+
•+
(G , G ) conformer is most stable for 2b and 2b at B3LYP/6-311+G-
(2d, p)//HF/3-21G* level calculations, while the most stable conformer of
•+
2b is the anti (T, T) conformation at the B3LYP/6-311+G(2d, p)//B3LYP/
6-31G(d) level. Accordingly, in Table 2, we can only roughly estimate the
relative energetical preference of anti and syn forms. This phenomenon is
further examined in Table 3 for the compound 2b.
(37) We have prepared similar derivatives using achiral 2-butanol. From
GC analysis, the meso form and dl pairs were obtained in ca. 1:1 and
appeared separately in the GC chromatogram. In use of (S)-2-butanol, such
epimers were not observed, and a single stereoisomer was detected by GC.
This indicates that the stereoretention was not occurring during the
Mitsunobu coupling reaction.
(13) Recently, we have demonstrated that the exciplex derived from
direct irradiation of donor and excited charge-transfer complex affords
different photoreactivity as well as diastereoselectivity. Saito, H.; Mori, T.;
Wada, T.; Inoue, Y. J. Am. Chem. Soc. 2004, 126, 1900.
(
14) (a) Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism,
Principles and Applications, 2nd ed.; John Wiley & Sons: New York, 2000.
b) Lightner, D. A.; Gurst, J. E. Organic Conformational Analysis and
(
Stereochemistry from Circular Dichroism Spectroscopy; John Wiley &
Sons: New York, 2000.
(
(
15) Sur, S.; Colpa, J. P. Chem. Phys. Lett. 1986, 127, 577.
16) (a) Redl, F. X.; Lutz, M.; Daub, J. Chem.sEur. J. 2001, 7,
(38) For stoichiometry of oxidation steps in eqs 1 and 2, see refs 27
and 28.
5
350. (b) Beer, G.; Niederalt, C.; Grimme, S.; Daub, J. Angew. Chem., Int.
(39) Witowski, S.; Wawer, I. J. Chem. Soc., Perkin Trans. 2 2002,
433.
Ed. 2000, 39, 3252. (c) Westermeier, C.; Gallmeier, H.-C.; Komma, M.;
Daub, J. Chem. Commun. 1999, 2427.
(40) (a) Cheeseman, J. R.; Ashvar, C. S.; Frisch, M. J.; Devlin, F. J.;
Stephens, P. J. Chem. Phys. Lett. 1996, 252, 211. (b) Bak, K. L.; Jorgensen,
P.; Helgaker, T.; Ruud, K.; Jensen, H. J. A. J. Chem. Phys. 1994, 100,
6620. (c) Bak, K. L.; Jorgensen, P.; Trygve, H.; Kenneth, R.; Jensen, H. J.
A. J. Chem. Phys. 1993, 98, 8873. (d) Stephens, P. J. Chem. Phys. Lett.
1991, 180, 472. (e) Jalkanen, K. J.; Kawiecki, R. W.; Stephens, P. J.; Amos,
R. D. J. Phys. Chem. 1990, 94, 7040. (f) Stephens, P. J.; Jalkanen, K. J.;
Amos, R. D.; Lazzeretti, P.; Zanasi, R. J. Phys. Chem. 1990, 94, 1811. (g)
Stephens, P. J.; Jalkanen, K. J.; Lazzeretti, P.; Zanasi, R. Chem. Phys. Lett.
1989, 156, 509. (h) Amos, R. D.; Jalkanen, K. J.; Stephens, P. J. J. Phys.
Chem. 1988, 92, 5571. (i) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.;
Handy, N. C. J. Phys. Chem. 1988, 92, 1781. (j) Amos, R. D.; Handy, N.
C.; Jalkanen, K. J.; Stephens, P. J. Chem. Phys. Lett. 1987, 133, 21. (k)
Stephens, P. J. J. Phys. Chem. 1987, 91, 1712. (l) Stephens, P. J. J. Phys.
Chem. 1985, 89, 748.
(17) (a) Kliger, D. S.; Lewis, J. W. In Circular Dichroism, Principles
and Applications, 2nd ed.; Berova, N., Nakanishi, K., Woody, R. Eds.; John
Wiley & Sons: New York, 2000; p 2431. (b) Dartigalongue, T.; Hache, F.
J. Opt. Soc. Am. B 2003, 20, 1780.
(
18) Wagniere, G. H. Chem. Phys. 1999, 245, 165.
(19) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier:
Amsterdam, 1988; p 257.
20) Mori, T.; Shinkuma, J.; Sato, M.; Saito, H.; Wada, T.; Inoue, Y.
Enantiomer 2002, 7, 115.
(
(
21) Machlin, L. J. Vitamin E; Dekker: New York, 1980.
(22) (a) Ahrens, B.; Davidson, M. G.; Forsyth, V. T.; Mahon, M. F.;
Johnson, A. L.; Mason, S. A.; Price, R. D.; Raithby, P. R. J. Am. Chem.
Soc. 2001, 123, 9164. (b) Culbertson, S. M.; Vinqvist, M. R.; Barclay, L.
R. C.; Porter, N. A. J. Am. Chem. Soc. 2001, 123, 8951.
(
23) (a) Amorati, R.; Ferroni, F.; Lucarini, M.; Pedulli, G. F.; Valgimigli,
(41) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (b) Becke, A. D.
J. Chem. Phys. 1993, 98, 5648.
L. J. Org. Chem. 2002, 67, 9295. (b) Sch a¨ del, U.; Gruner, M.; Habicher,
W. D. Tetrahedron 2002, 58, 5081. (c) Sch a¨ del, U.; Habicher, W. D.
Synthesis 1998, 293.
(42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.11; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(43) Wang, F.; Polavarapu, P. L. J. Phys. Chem. A 2000, 104,
10683.
(44) (a) Shaw, R. A.; Wieser, H.; Dutler, R.; Rauk, A. J. Am. Chem.
Soc. 1990, 112, 5401. (b) Dothe, H.; Lowe, M. A.; Alper, J. S. J. Phys.
Chem. 1989, 93, 6632.
(45) (a) Izumi, H.; Yamagami, S.; Futamura, S.; Nafie, L. A.; Dukor,
R. K. J. Am. Chem. Soc. 2004, 126, 194. (b) Freedman, T. B.; Cao, X.;
Rajca, A.; Wang, H.; Nafie, L. A. J. Phys. Chem. A 2003, 107, 7692. (c)
Tomankova, Z.; Setnicka, V.; Urbanova, M.; Matejka, P.; Kral, V.; Volka,
K.; Bour, P. J. Org. Chem. 2004, 69, 26. (d) Kuppens, T.; Langenaeker,
W.; Tollenaere, J. P.; Bultinck, P. J. Phys. Chem. A. 2003, 107, 542. (e)
Devlin, F. J.; Stephens, P. J.; Cheeseman, J. R.; Frisch, M. J. J. Am. Chem.
Soc. 1996, 118, 6327.
(24) (a) Baxter, J. G.; Robeson, C. D.; Taylor, J. D.; Lehman, R. W. J.
Am. Chem. Soc. 1943, 65, 918. (b) Sebrell, W. H.; Harris, R. S. The Vitamins;
Academic Press: New York, 1972; p 166.
(25) There are other related reports on ECD of optically active organic
radical anions and neutral radicals; however, the precise data such as molar
ellipticities were not reported due to similar difficulties. (a) Sur, S.; Colpa,
J. P.; Wan, J. K. S. Chem. Phys. 1986, 108, 133. (b) Ito, O.; Hatano, M. J.
Am. Chem. Soc. 1974, 96, 4375. (c) Ito, O.; Tajiri, A.; Hatano, M. Chem.
Phys. Lett. 1973, 19, 125. (d) Irurre, J.; Santamaria, J.; Gonzalez-Rego, M.
C. Chirality 1995, 7, 154.
(
26) Smith, H. E. Chem. ReV. 1998, 98, 1709.
(27) Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org.
Chem. 1998, 63, 5847.
28) Rathore, R.; Zhu, C.; Lindeman, S. V.; Kochi, J. K. J. Chem. Soc.,
Perkin Trans. 2 2000, 1837.
29) Lehtovuori, P.; Joela, H. Phys. Chem. Chem. Phys. 2002, 4,
928.
30) Judging from the highly reliable ECD spectrum of the quantitatively
(
(
1
(
•
+
prepared methyl ester radical cation (1b ), we determined that the sign of
•
+
CE of 1a reported previously was erroneous (vide infra). Although we
do not have an immediate answer to this discrepancy, protonated and/or
sulfonated 1a formed upon treatment with strong acid would play some
role.
(
31) Gwaltney, S. R.; Rosokha, S. V.; Head-Gordon, M.; Kochi, J. K.
J. Am. Chem. Soc. 2003, 125, 3273.
32) Mori, T.; Takamoto, M.; Wada, T.; Inoue, Y. HelV. Chim. Acta
001, 84, 2693.
(46) Bernstein, H. J.; Pederson, E. E. J. Chem. Phys. 1949, 17,
885.
(47) Abe, K.; Ito, K.; Sueyawa, H.; Hirota, M.; Nishino, M. Bull. Chem.
Soc. Jpn. 1986, 59, 3125.
(48) Hagemann, H.; Mareda, J.; Chiancone, C.; Bill, H. J. Mol. Struct.
1997, 410, 357.
(49) Stephens, P. J.; Devlin, F. J.; Aamouche, A. In Chirality: Physical
Chemistry; Hicks, J. M., Ed.; ACS Symposium Series 810; American
Chemical Society: Washington, DC, 2002; p 18.
(
2
(
33) Note that, in ref 15, the ∆ꢀ value of 1a was reported to be -20,
which is not in agreement with our value. However this report may be
erroneous, because the g value (∆ꢀ/ꢀ) reported in the same literature almost
coincided with ours.
(
(
34) Rathore, R.; Kochi, J. K. J. Org. Chem. 1995, 60, 4399.
35) (a) Scott, A.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (b)
Wong, M. W. Chem. Phys. Lett. 1996, 256, 391. (c) Pople, J. A.; Scott, A.
P.; Wong, M. W.; Radom, L. Isr. J. Chem. 1993, 33, 345. (d) Pople, J. A.;
Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.;
Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J. Quantum Chem., Quantum
Chem. Symp. 1981, 15, 269.
(50) (a) Gigante, D. M. P.; Long, F.; Bodack, L. A.; Evans, J. M.;
Kallmerten, J.; Nafie, L. A.; Freedman, T. B. J. Phys. Chem. A 1999, 103,
1523. (b) Nafie, L. A. Enantiomer 1998, 3, 283.
(51) Rathore, R.; Bosch, E.; Kochi, J. K. J. Chem. Soc., Perkin Trans.
2 1994, 1157.