Optical stability of axially chiral push-pull-substituted buta-1,3dienes: Effect of a single methyl group on the C60 surface

Article abstract of DOI:10.1002/anie.200906853

A new spin on C₆₀: Axially chiral, pushpull buta-1,3-diene chromophores conjugated to vicinally methylated fullerenes (see picture) have high barriers of rotation about their chiral axes. These high barriers allowed their optical resolution and the determination of the absolute configuration by comparison of experimental and calculated circular dichroism spectra. The methyl group is rigidly fixed upon the surface of the carbon sphere and is responsible for the high rotational barriers. (Figure Presented).

Full text of DOI:10.1002/anie.200906853

Communications
DOI: 10.1002/anie.200906853
Hindered Rotation
Optical Stability of Axially Chiral Push–Pull-Substituted Buta-1,3-
dienes: Effect of a Single Methyl Group on the C60 Surface**
Michio Yamada, Pablo Rivera-Fuentes, W. Bernd Schweizer, and Franꢀois Diederich*
Axial chirality stems from the restricted rotation around a
single bond, and the most abundant class of compounds
featuring this type of chirality are biaryl derivatives. Biaryl
structural motifs are encountered in a large number of
optically active natural products,[1] and also provide the basis
for some of the most versatile classes of enantiomerically pure
ligands for asymmetric catalysis, such as 1,1’-binaphthalene
derivatives.[2] A chiral axis is also found in substituted
allenes,[3] alkylidenecycloalkanes, spiranes,[4] and buta-1,3-
dienes. Only a few examples of sterically congested, axially
chiral buta-1,3-dienes have been reported, and a summary of
them, resulting in particular from early work by Kꢀbrich,
Mannschreck, and co-workers,[5] is included in Figure 1SI in
the Supporting Information.
Over the past few years, we have prepared a large number
of nonplanar push–pull-substituted buta-1,3-dienes by [2+2]
cycloaddition of either tetracyanoethene (TCNE) or 7,7,8,8-
tetracyano-p-quinodimethane (TCNQ) with electron-donor-
substitued alkynes, and subsequent cycloreversion.[6–8] In
several of these systems, the buta-1,3-diene moieties are
highly sterically congested and NMR spectroscopy indicated
the presence of different conformers undergoing slow
erene surface is shown to raise the barrier for rotation about
their chiral axis to such an extent that separation, isolation,
and chiroptical characterization of the enantiomers of non-
planar buta-1,3-diene-based charge-transfer (CT) chromo-
phores are possible for the first time.[10]
The synthesis of conjugates 1–6 (Scheme 1) begins with a
protocol for fullerene alkynylation introduced in 1994 inde-
pendently by Komatsu et al.[11] and our group.[12] Addition of
lithiated N,N-dimethylanilino(DMA)-substituted acetylene
to C60, and then either protonation (AcOH) or methylation
(MeI), afforded intermediates 7 and 8; the latter was
characterized by X-ray analysis (see Figure 2SI in the
Supporting Information). Subsequent addition of TCNE
and TCNQ, respectively, yielded the fullerene–CT chromo-
phore conjugates 1–4 having the nonplanar push–pull buta-
1,3-dienes directly attached to the carbon sphere. Conjugates
5 and 6, having an acetylenic spacer separating the CT
chromophore and the fullerene, were obtained by addition of
ꢀ ꢁ ꢀ
Me3Si C C Li and subsequent deprotection to give 9, which
then underwent oxidative hetero-Hay-coupling to yield 10 for
subsequent cycloaddition/cycloreversion. All fullerene–CT
chromophore conjugates are deeply colored solids that are
stable at ambient temperature in air. Due to the nonplanarity
of the push–pull butadiene moiety, they are highly soluble in
common organic solvents such as CH2Cl2. The [6,6]-addition
pattern (addition to the double bond shared by two six-
membered rings) on the fullerene was confirmed by the
characteristic UV/Vis absorption maxima at lmax = 431–434
and 702 nm in the precursors 7, 8, and 10.[13] The UV/Vis
spectra of 1–6 show intense broad CT bands with maxima
between 450 and 700 nm (see Figures 18–23SI in the Support-
ing Information). Clear hypochromic shifts were observed
after addition of CF3COOH, and the original CT bands were
recovered upon addition of a base (Et3N or K2CO3).
exchange, resulting from hindered rotation about their central
[7]
ꢀ
C C single bond. However, efforts to separate the axially
chiral enantiomers were unsuccessful.
The stable and highly soluble nonplanar push–pull
chromophores obtained by the TCNE and TCNQ additions
are potent electron acceptors,[6] and this motivated us to
conjugate them to C60 to improve the solubility of the carbon
sphere and enhance its electron uptake capacity.[9]
Herein, we report the synthesis and X-ray structure of
conjugates between C60 and push–pull chromophores
obtained from TCNE and TCNQ by a cycloaddition/cyclo-
reversion sequence. The focus lies on the remarkable
stereochemical properties of the resulting axially chiral
buta-1,3-dienes: a single methyl group attached to the full-
The structures of the conjugates 2, 4, and 6 were addi-
tionally confirmed by X-ray structure analysis (Figure 1). The
molecular structures confirmed not only the attachment of
the two groups to the [6,6] junction, but also provided
evidence for substantial bond length alternation in the
DMA ring, indicative of efficient push–pull conjugation in
the ground state (see Figure 3SI in the Supporting Informa-
tion). The push–pull chromophores in 2, 4, and 6 are highly
distorted from planarity, mainly by rotation around the chiral
axis of the buta-1,3-diene moieties.
[*] Dr. M. Yamada, P. Rivera-Fuentes, Dr. W. B. Schweizer,
Prof. Dr. F. Diederich
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Hꢁnggerberg, HCI, 8093 Zꢀrich (Switzerland)
Fax: (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
Homepage: www.diederich.chem.ethz.ch
[**] This work was supported by the Swiss National Science Foundation
and the ETH Research Council. We thank the C4 Competence Center
for Computational Chemistry at ETH Zꢀrich for the allocation of
computational resources. M.Y. acknowledges the receipt of a JSPS
Research Fellowship for Young Scientists.
The NMR spectra of the conjugates recorded in C2D2Cl4
show large differences, depending on the nature of the second
group on the fullerene surface (H or Me) and on the absence
or presence of an acetylenic spacer (see the Supporting
ꢀ
Information). Two rotational processes around C C single
bonds need to be considered: rotation about the bond
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906853.
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Chemie
only for 1, 3, 5, and 6. Nota-
bly, conjugates 1, 3, 5, and 6
retain their Cs symmetry
even at 193 K in a CD2Cl2
solution. In contrast, conju-
gates 2 and 4, having a Me
group on the fullerene sur-
face and lacking an acety-
lenic spacer, are C1 symmet-
ric according to the 13C NMR
spectra. The decrease in the
molecular symmetry in 2 and
4 reflects hindered rotation
around the central buta-1,3-
ꢀ
diene C C single bond, ren-
dering
axially
the
chiral
The
compounds
and
Me
C1 symmetric.
group, rigidly attached to
the fullerene surface, is the
origin of the axial chirality.
When replaced by a H (as in
1 and 3), the rotation around
the butadiene central bond
becomes fast on the NMR
time scale. This is also the
case, when the distance
between Me group and buta-
diene is increased by acety-
lene insertion (as in 5 and 6).
The axially chiral enantio-
meric pairs (P)-2/(M)-2 and
(P)-4/(M)-4 are shown in
Scheme 1. a) 1. {[4-(N,N-dimethylamino)phenyl]ethynyl}lithium, THF, 208C; 2. AcOH or MeI, 208C or 408C,
25% (7), 31% (8); b) 1. [(trimethylsilyl)ethynyl]lithium, THF, 208C; 2. MeI, 408C; 3. K2CO3, THF/MeOH
(2:1), 208C, 18% (9); c) 4-ethynyl-N,N-dimethylaniline, CuCl, O2, TMEDA, PhCl, 208C, 61% (10); d) TCNE,
1,2-dichloroethane, 208C, 39% (1), 80% (2); e) TCNQ, 1,2-dichloroethane or PhCl, 808C, or 1328C, 62% (3),
94% (4); f) TCNE, PhCl, 208C, 97% (5); g) TCNQ, PhCl, 808C, 41% (6). TMEDA=N,N,N’,N’-tetra-
methylethylenediamine.
connecting the CT chromophore to the C60 core and rotation
about the central bond of the buta-1,3-diene moiety.
Although the conjugates 1–6 have inherently achiral C60
addition patterns, the 13C NMR spectra reveal Cs symmetry
Figure 2 below.
ꢀ
Atropisomerism about the C C single bond connecting
the butadiene moiety to the fullerene core is observed by
1H NMR analysis of 1–3 at 298 K. The two atropisomers of Cs-
Figure 1. ORTEP drawings of a) 2, b) 4, and c) 6 with thermal ellipsoids shown at the 50% probability level for 173–223 K. Solvate molecules are
omitted for clarity. Selected dihedral angles [8]: a) C78-C63-C62-C73 74.1, C78-C63-C64-C65 46.0; b) C64-C63-C62-C84 79.8, C64-C63-C73-C78 54.0;
c) C72-C62-C83-C85 60.1, C72-C62-C63-C64 26.6.
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symmetric 1 are observed in a 1.0:0.65 ratio (DG = 0.3 kcal
preferred atropisomers of 2 and 4 closely resembles the one
observed by X-ray crystallography.
molꢀ1). Increased broadening of the peaks is observed upon
heating the sample to 368.2 K (see Figure 24SI in the
Supporting Information). Since the coalescence temperature
is higher than 368.2 K, it is estimated that the free enthalpy of
The remaining question was whether the barrier for
rotation around the chiral axis in 2 and 4 would be sufficiently
large to allow separation of the C1-symmetrical conjugates
into their enantiomers. This could indeed be accomplished
with an enantiomeric ratio (e.r.) of greater than 99:1 by HPLC
methods using the chiral stationary phase (S,S)-WHELK-O1
(see Figure 28SI in the Supporting Information). Large
optical rotations at l = 589 nm were measured: + 770 for
(+)-2 and + 5800 for (+)-4. The CD spectra of (+)-2/(ꢀ)-2
and (+)-4/(ꢀ)-4, respectively, are mirror images along the
abscissa and show Cotton effects associated with the CT
absorptions (Figure 2). This observation gives conclusive
evidence that (ꢂ )-2 and (ꢂ )-4 are axially chiral butadienes.
The CD curves of (+)-2/(ꢀ)-2 change drastically upon
acidification with TFA and were fully recovered after
neutralization with Et3N (see Figure 29SI in the Supporting
Information). In contrast, acidification of a CH2Cl2 solution of
(+)-4/(ꢀ)-4 with TFA and subsequent neutralization with
Et3N caused a large decrease in intensity of the CD features
(see Figure 30SI in the Supporting Information). Apparently
the barrier for racemization of (+)-4/(ꢀ)-4 is lower than for
(+)-2/(ꢀ)-2.
¼
activation, DG , is larger than 17.2 kcalmolꢀ1 based on the
1
Eyring equation. The H NMR spectrum of C1-symmetric 2
also exhibits two sets of signals assignable to atropisomers in a
ratio of 1.0:0.19, yielding DG = 1.0 kcalmolꢀ1. Again, peak
¼
broadening is observed upon heating and DG is estimated
larger than 17.6 kcalmolꢀ1. A similar atropisomerism (ratio
1.0:0.48; DG = 0.4 kcalmolꢀ1) was observed for Cs-symmetric
3. Here, the coalescence temperature (TC) was reached [TC =
¼
(345 ꢂ 5) K] and DG calculated as (16.1 ꢂ 0.3) kcalmolꢀ1. In
contrast, the 1H NMR spectrum of 4 only shows one C1-
symmetric atropisomer; the equilibrium is shifted because of
the steric repulsion between the methyl group on C60 and the
cyclohexa-2,5-diene-1,4-diylidene moiety. The energy differ-
ence between the two atropisomers of 4 was estimated to be
4.2 kcalmolꢀ1 by density functional theory (DFT) calculations
at the B3LYP/6-31G* level of theory (see the Supporting
Information). Notably, the calculated structures of the
A
kinetic study
was carried out to
determine the activa-
tion parameters for
racemization.
compounds
Both
(+)-2/
(ꢀ)-2 and (+)-4/(ꢀ)-4
racemized readily
upon moderate heat-
ing, with the former
being the more opti-
cally stable conjugate.
The racemization pro-
cesses of the (+)- and
(ꢀ)-enantiomers were
monitored by CD
spectroscopy of 1,2-
dichloroethane solu-
tion, and the first-
order rate constants
were obtained by
linear
line-fitting
plots of the molar cir-
cular dichloism De as a
function of time at
different temperatures
(see
Figures 31SI–
41SI in the Supporting
Information). From
the Arrhenius plot
and the Eyring equa-
tion, the activation
parameters
obtained
As already deduced
were
(Table 1).
Figure 2. a) Top: CD spectra of enantiopure (+)-2 (black line) and (ꢀ)-2 (gray line). Bottom: UV/Vis spectrum.
b) Top: CD spectra of enantiopure (+)-4 (black line) and (ꢀ)-4 (gray line). Bottom: UV/Vis spectrum. All spectra
recorded for compounds in CH2Cl2 solutions.
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Chemie
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¼
¼
¼
298K
[kcalmolꢀ1
DH
DS
DG
Ea
t1/2 (313K)
[h]
[kcalmolꢀ1
]
[calmolꢀ1 Kꢀ1
]
]
[kcalmolꢀ1
]
2
4
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9.7ꢂ6.7
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t1/2 = 5.9 h) is approximately 1.5 kcalmolꢀ1 larger than that for
(+)-4/(ꢀ)-4 (t1/2 = 1.2 h). This difference can be attributed to
the fact that the cyclohexa-2,5-diene-1,4-diylidene moiety is
more flexible, and its distortion by bending out of plane is
energetically less disfavored, as compared to the dicyanovinyl
moiety.
The absolute configuration of the axially chiral butadienes
was determined through comparison of the experimental CD
spectra with spectra obtained in theoretical calculations
(time-dependent DFT (TD/DFT) using the the B3LYP[14]
functional and the 6-31G* basis set). As shown in Figure 46SI
in the Supporting Information, the basic patterns of the CD
curves of (ꢀ)-2 and (ꢀ)-4 (sign, magnitude, and position of
Cotton effects) are in agreement with the TD/DFT calcu-
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absolute configurations of both (ꢀ)-2 and (ꢀ)-4 were
determined as M. In addition, the TD/DFT calculations are
in agreement with the absorption spectra of 2 and 4 (see
Figures 42SI and 43SI in the Supporting Information).
In summary, we have prepared a new family of axially
chiral buta-1,3-dienes involving conjugation of nonplanar
push–pull chromophores to C60, separated their enantiomers,
and determined their absolute configuration. This investiga-
tion adds a new aspect to fullerene chirality, in which the
surface of the carbon sphere plays a key role.[10,15] It reveals a
particularly dramatic difference between a Me and a H—both
rigidly fixed on the fullerene surface—in sterically affecting
the rotation around a neighboring single bond. This study
clearly demonstrates that the integrity of C60 can serve as
powerful steric force to control molecular configurations and
opens up a new avenue of chiral buta-1,3-diene chemistry.
Received: December 4, 2009
Published online: April 1, 2010
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Keywords: axial chirality · buta-1,3-dienes ·
charge-transfer chromophores · circular dichroism · fullerenes
.
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