New functional hexahelicenes - Synthesis, chiroptical properties, X-ray crystal structures, and comparative data bank analysis of hexahelicenes

Article abstract of DOI:10.1002/ejoc.200300084

Seven new hexahelicenes (3-7, 9, 10) containing different functional group substituents have been synthesized and, in three cases (4, 8, 10), optically resolved. Optical rotations were measured and CD spectra are reported. X-ray crystal structures of the

Full text of DOI:10.1002/ejoc.200300084

FULL PAPER
New Functional Hexahelicenes ؊ Synthesis, Chiroptical Properties, X-ray
Crystal Structures, and Comparative Data Bank Analysis of Hexahelicenes
[a]
[a]
[b]
[a]
´ ´
Claudia Wachsmann, Edwin Weber,* Matyas Czugler, and Wilhelm Seichter
Keywords: Chiroptical properties / Circular dichroism / Helicenes / Pi interactions / Supramolecular chemistry
Seven new hexahelicenes (3−7, 9, 10) containing different
functional group substituents have been synthesized and, in
three cases (4, 8, 10), optically resolved. Optical rotations
were measured and CD spectra are reported. X-ray crystal
yielding stacks of molecules (1, 3, 4, 8). Statistical analysis of
the molecular dimensions in these and related crystal struc-
tures was carried out in order to identify potential parameters
relevant to macroscopic phenomena, including optical rota-
structures of the helicenes 1, 3, 4, 8, and 9 have been deter- tion.
mined, of which 4 represents a 1:1 clathrate with acetone.
These show a concerted interplay of C−H···O, C−H···π, and
π···π supramolecular interactions in the packings, mostly
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
(CD) of helicenes, using two new functional helicenes (1,
2). Here we report on the synthesis of a further eight func-
Since the pioneering paper on hexahelicene in 1956,[1] tional hexahelicenes 3؊10 (Scheme 1), of which seven are
both the carbo-[2] and the heterohelicenes,[3] in which a reg- new compounds, including successful optical resolution in
ular cylindrical helix is formed through an all-ortho annel- three (4, 8, 10) and X-ray crystal structures in five cases (1,
ation of aromatic or heteroaromatic rings, have continued 3, 4, 8, 9). The helicene 8 is a known compound,[6] but was
to be an attractive class of compounds.[4] This arises from obtained here by a different approach. These new structures
their specific structural features, involving particular stereo- encouraged a statistical analysis of the molecular dimen-
chemical and chiroptical properties.[5] In exploitation of sions in these and related crystal structures in order to
this, helicene backbones have been used to design chiral identify potential parameters that might be relevant to the
crown compounds,[6] molecular recognizing clefts,[7] and optical properties.
clathrates.[8] The first chiral phosphane ligands based on the
helicene moiety have recently been synthesized,[9] and have
proved successful in enantioselective transition metal ca-
talysis.[10] The helicene structural element has also attracted
some attention in the topical field of crystal engineering,[11]
including helicene alignment in the honeycomb sheets of
trimesic acid.[12] Nevertheless, helicenes (carbohelicenes in
particular) bearing functional groups suitable to act as
starting points for further structural modification are rare,
due to the synthetic efforts required, including separation
of the enantiomers.
In a previous work[13] we demonstrated the use of the
time-dependent density functional theory (TDDFT) for
investigation of molecular electronic circular dichroism
[a]
Institut für Organische Chemie, Technische Universität
Bergakademie Freiberg,
Scheme 1. Hexahelicenes studied
Leipziger Strasse 29, 09596 Freiberg/Sachsen, Germany
Fax: (internat.) ϩ49-(0)3731-393170
E-mail: edwin.weber@chemie.tu-freiberg.de
Institute of Chemistry, Chemical Research Center,
Hungarian Academy of Sciences,
[b]
Synthesis
P. O. Box 17, 1525 Budapest, Hungary
The syntheses of the substituted hexahelicenes 1؊4 were
accomplished in reasonable yields (14Ϫ63%) by use of es-
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2003, 2863Ϫ2876
DOI: 10.1002/ejoc.200300084
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
FULL PAPER
tablished procedures for hexahelicene formation.[9,10,14] The
preparation started from naphthalene-2,7-bis(triphenylpho-
sphonium) dibromide[9,10,15] and the appropriate benzal-
dehyde by double Wittig reaction[16] to give the correspond-
ing distilbenes 11aϪd (Scheme 2), which were subjected a
double photocyclodehydrogenation[17] in diluted solutions
(10Ϫ3 mol·LϪ1, or suspensions in cases of 11b and 11c) in
toluene in the presence of iodine as oxidizing agent. The
relatively moderate yields of 30 and 14% for 2 and 3, respec-
tively, are perhaps a consequence of the unfavorable solu-
bility properties of 11b and 11c in toluene, necessitating
their use as suspensions. The more lipophilic bis(isobutyl
ester) 4 was a favorable substitute for 3, however, and so is
used below. As usual,[9,10] the double Wittig reaction pro-
duced distilbenes 11 (aϪd) as mixtures of (E) and (Z) iso-
mers. This was of no concern, though, since irradiation
gives rise to rapid interconversion of the Z and E iso-
mers.[18] Nevertheless, we were able to isolate pure (Z,Z)
isomers on recrystallization of 11b and 11d either from
methanol or from toluene.
Table 1. Specific [α] and molecular [M] rotations of the helicenes
in chloroform
20
Helicene
[α]
M
c [g/100 mLϪ1
]
589
1
3440 Ϯ 400
3271.7 Ϯ 7
3660 Ϯ 100
2672.8 Ϯ 3
1851.1 Ϯ 100
3640 Ϯ 10[a]
13027
12709
19348
10383
13864
11950
0.003
0.06
0.03
0.1
0.03
0.098
2
4
8
10
Hexahelicene[1]
[a]
24
[α]D
.
mate) column used for reasons of stability has been re-
ported as being compatible only with solvents such as meth-
anol, 2-propanol, or n-hexane.[24] However, by systematic
variation of the solvent composition, we were able to per-
form optical base-line separation of the helicenes 1, 2, 4, 8,
and 10, of which 1 and 2 have been reported elsewhere,
together with their chiroptical data.[13]
The optical rotations, including those for 1 and 2, are
summarized in Table 1. The molar rotations (M) vary be-
tween 10383, measured for the dialcohol 8, and 19348, for
the bis(isobutyl ester) 4, indicating influences of the sub-
stituents. For comparison, the unsubstituted hexahelicene
has a molar rotation of 11950,[1] which is exceeded by the
new helicenes, except in the case of 8.
The CD spectra of these helicenes were also measured
(Figure 1) and indicated that complete optical resolution
had occurred, which makes assignment of the absolute con-
figurations possible.[13,25,26] Figure 1 (top) shows the dichro-
gram of the bis(hydroxymethyl)-substituted helicene 8,
which is characterized by two intensive bands (around λ ϭ
250 and 330 nm) of opposite sign, demonstrating no signifi-
cant change relative to the CD of unsubstituted hexahel-
icene.[27] Similar behavior is seen for 10 [Figure 1 (middle)],
which is rather unexpected because of the presence of the
two bulky bis(4-methylphenyl)hydroxymethyl substituents.
Obviously, these substituents do not involve conformational
changes effective on dichroism.
On the other hand, the dichrogram of the helicene diester
4 [Figure 1 (bottom)] shows a characteristic change from
those of unsubstituted hexahelicene or 8 and 10. This
change involves a splitting of the original single band
around λ ϭ 250 nm into two bands near λ ϭ 240 and
260 nm, which are also found for the dicyano- and the di-
methoxy-substituted helicenes 1 and 2.[13] It appears prob-
able that electronic influences of π and lone-pair electrons
of the particular functional groups, being conjugated to the
helicene framework, are producing an effect here while the
non-conjugated substituents in 8 and 10 are ineffective.
Scheme 2. Distilbene starting compounds
The hexahelicenes 5؊10 were synthesized from 1, 2, and
4 by modification of their functional groups. More pre-
cisely, the diamine 5 was obtained from dinitrile 1 by stand-
ard LiAlH4 reduction,[19] and the diphenol 6 by ether cleav-
age of 2 with boron tribromide.[20] From diester 4, basic
hydrolysis[21] yielded dicarboxylic acid 7, LiAlH4 re-
duction[19] the dialcohol 8, and addition of the respective
aryllithium agents[22] produced the bis-tertiary alcohols 9
and 10.
Optical Resolution
Although optical resolution of helical compounds, in-
cluding helicenes, by HPLC on derivatives of cellulose as
chiral stationary phases has proved successful,[10,23] the
limitations of solvents for non-destruction elution of the
column is a problem.[24] It also turned out to be a problem
X-ray Structural Studies
Crystallographic data for 1, 3, 4·acetone (1:1), 8, and 9
here, because of the unfavorable solubility characteristics of are given in Table 2. Structural features, including those for
these helicenes. The cellulose tris(3,5-dimethylphenylcarba- hexahelicene, are summarized in Table 3.
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New Functional Hexahelicenes
FULL PAPER
the same helicity, running along the crystallographic c axis
[Figure 3 (a)]. Adjacent molecules within the stacks are
interlocked through their aromatic units, which assume face
˚
to face orientations with an average distance of 3.75 A be-
tween ring centers. This distance indicates the presence of
well-defined intermolecular π-π-interactions.[29] The func-
tional groups of the helicene form weak CϪH···N hydrogen
bonds[30] [Figure 3 (b)], which are slightly different in
˚
their bond lengths [H(5Ј)···N(151)
C(5Ј)ϪH(5Ј)···N(152) ϭ 152°; H(13ЈЈ)···N(22) ϭ 2.8 A,
C(13ЈЈ)ϪH(13ЈЈ)···N(22) ϭ 151°].
ϭ
2.6 A,
˚
The structural similarity to the dinitrile helicene 1 persists
in the crystal packing of the diester helicene 3. This com-
pound did not undergo spontaneous resolution, though its
space group is the noncentrosymmetric Pna21. The inner
pitch elevations (d1, C1···C16 distance) of the two indepen-
˚
dent molecules of the asymmetric unit are 3.10 and 3.06 A,
while the torsion angles 1 and 2 are again moderately elev-
ated, at values of 18.8 and 16.0°, and of 13.1 and 11.7°,
respectively, for the two independent molecules. The pack-
ing illustrates that the helicene molecules also aggregate in
stacks, but that the neighboring stacks have opposite chiral-
ities (Figure 4). Consecutive molecules within a stack are
rotated around their helical axis, so that strong π···π-inter-
actions[29] develop between the terminal aryl groups (d ϭ
˚
3.66Ϫ3.67 A) of the two independent molecules of the
asymmetric unit. This molecular arrangement within a
stack produces different binding behavior. Only a pro-
portion of the carbonyl oxygen atoms can form weak inter-
molecular CϪH···O[30] interactions to methyl and aryl hy-
drogen atoms.
Recrystallization of the diisobutyl ester 4 from acetone
yielded a clathrate with acetone of host-guest stoichiometry
¯
1:1. These crystals are racemic (space group P1). The crys-
tal structure of helicene 4 (Figure 5) shows that the racemic
pair of molecules form a centrosymmetric dimer in the cell.
A weak CϪH···O hydrogen bond[30] [C(9)ϪH(9)···O(22),
H(9)···O(22) ϭ 2.59 A], together with a CϪH···π[31] interac-
˚
tion between the inner aryl groups, of which the shortest
˚
distance is 3.38 A, link the two host molecules into this
dimeric-like arrangement. The crystal packing shows a
channel structure along the a axis, which is partially occu-
pied by guest molecules (Figure 6). Parts of this lattice voids
are occupied by the protruding ester groups of the helicene
host molecules, so that an alternating arrangement of guests
and substituents occurs. This could be the reason why only
pairwise stacking develops here, in contrast to the diester 3.
Another reason could be the steric congestion caused by
Figure 1. Experimental CD spectra of helicene 8 (top), 10 (middle),
and 4 (bottom) in n-hexane
Crystallization of the dinitrile helicene derivative 1 (Fig- the substituents on the helicene. The inner pitch elevation
˚
ure 2) from benzene gave orange-red crystals. The hexa- (d1, C1···C16 distance) is 3.11 A. The torsion angles 1 and
gonal space group P6122 indicates that spontaneous resolu- 2 over the penultimate bonds are small, at 13.7° in both
tion had occurred.[28] The inner pitch elevation (d1, cases.
˚
C1···C16 distance) is 3.08 A. The torsion angles at the inner
Diol 8 has the functional groups with the smallest steric
helical rim are also a convenient measure of the helicity. requirements. Figure 7 reveals that each hydroxy group of
Those over the penultimate bonds and the helicene is part of an eight-membered cyclic hydrogen
[C(1)ϪC(16e)ϪC(16d)ϪC(16c) and
C(16c)ϪC(16b)Ϫ bond system[32] with almost ideal hydrogen bond geometry
1
2
C(16a)ϪC(16)] are 15.7 and 15.1°. The helicene molecules [O(18)ϪH(18)··· O(20)
.
[Ϫ0 5ϩx, 0.5Ϫy, 0.5ϩz], H(18)··· O(20) ϭ
˚
1 pack in the crystal in stacks of columns, trivially with 1.74 A, O(18)ϪH(18)···O(20) ϭ 168°; O(20)ϪH(20)···
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
Table 2. Crystal data and details of the structure determination for the compounds studied
FULL PAPER
Compound
1
3
4
8
9
Formula
C28H14N2
378.41
C30H20O4
444.46
C36H32O4·C3H6O
586.69
C28H20O2
388.44
C52H36O2
692.8
Molecular mass
Crystal system
Space group
hexagonal
P6122 (No.178)
9.736(1)
9.736(1)
70.731(3)
90
90
120
5806.3(9)
12
1.299
orthorhombic
Pna21 (No. 33)
20.040(1)
16.835(1)
13.140(1)
90
90
90
4433.1(5)
8
1.332
triclinic
monoclinic
P21/n (No. 14)
13.437(3)
11.197(3)
14.643(3)
90
115.51(2)
90
1988.3(9)
4
monoclinic
P21/a (No. 14)
12.540(3)
22.271(3)
12.901(3)
90
94.51(3)
90
3591.8(13)
4
1.281
0.59 (Cu-Kα)
0.20 ϫ 0.20 ϫ 0.25
298
1.5418 (Cu-Kα)
8191/6888, 0.029
4436
¯
P1 (No. 2)
˚
a (A)
11.119(4)
11.426(3)
13.779(4)
94.44(2)
100.90(2)
104.73(2)
1647.9(9)
2
˚
b (A)
˚
c (A)
α [°]
β [°]
γ [°]
3
˚
V (A )
Z
D(calcd.) [g·cmϪ3
]
1.182
0.08 (Mo-Kα)
1.298
µ [mmϪ1
]
0.59 (Cu-Kα)
0.71 (Cu-Kα)
0.63 (Cu-Kα)
0.30 ϫ 0.35 ϫ 0.35
298
1.5418 (Cu-Kα)
4260/4086, 0.015
3417
Crystal size [mm]
T [K]
Radiation [A]
Total/uniq. data, R(int) 15515/3989, 0.077
0.20 ϫ 0.20 ϫ 0.40 0.20 ϫ 0.45 ϫ 0.50 0.20 ϫ 0.3 ϫ 0.5
298
1.5418 (Cu-Kα)
296
298
˚
1.5418 (Cu-Kα)
6423/5990, 0.035
5143
0.71073 (Mo-Kα)
5294/4612, 0.080
2338
Observed data
[IϾ 2.0 σ(I)]
Nref, Npar
3053
3682, 274
5985, 614
4612, 379
4086, 274
6887, 495
R, wR2, S
0.0411, 0.1186, 1.06 0.0384, 0.1053, 1.05 0.0842, 0.2686, 0.93 0.0436, 0.1333, 1.05 0.0528, 0.1416, 1.12
Ϫ0.12/0.14 Ϫ0.13/0.15 Ϫ0.22/0.70 Ϫ0.17/0.16 Ϫ0.18/0.18
Min./max. resd. dens.
Ϫ3
˚
[e·A
]
Table 3. Structural features (selected conformations and non-bond-
ing distances of terminal ring atoms) of the helicenes studied, in-
cluding hexahelicene
[c]
Torsion angle, [°][a,b]
Distance [A]
˚
Compound
ϕ1
ϕ2
ϕ3
ϕ4
d1
d2
d3
1
3
15.7
18.8
13.1
13.7
13.4
13.0
15.1
16.0
11.7
13.7
15.1
17.6
30.0
28.1
28.8
27.8
26.4
22.9
25.8
30.0
24.9
25.0
27.8
26.4
27.6
29.7
15.2
3.08 4.21 5.25
3.10 4.20 5.00
3.06 4.31 5.00
3.11 4.36 5.44
2.94 3.85 4.89
3.13 4.32 5.44
3.22 4.58 5.63
4
8
9
Hexahelicene 11.3
[a]
Standard deviations are typically 0.3Ϫ0.5 ° for the torsion
˚
angles (ϕ) and less than 0.01 A for the interatomic distances (d).
Figure 2. Molecular structure and numbering scheme of helicene 1
[b] ϕ1 ϭ C(1)ϪC(16e)ϪC(16d)ϪC(16c); ϕ2
ϭ C(16)ϪC(16a)Ϫ
C(16b)ϪC(16c); ϕ3
ϭ
C(16e)ϪC(16d)ϪC(16b); ϕ4
ϭ
[c]
vious structures, molecules of opposite configuration are ar-
ranged in stacks in which their aryl units assume an edge
C(16a)ϪC(16b)ϪC(16c)ϪC(16d).
C(2)···C(15); d3 ϭ C(3)···C(14).
d1 ϭ C(1)···C(16); d2 ϭ
to face relationship[33] with CϪH pointing into the center
1.87 A, of an aromatic ring at an intermolecular distance of 3.0 A.
˚
˚
O(18)[0.5Ϫx,
,
H(20)··· O(18)
ϭ
Ϫ0.5ϩy, 0.5Ϫz]
O(20)ϪH(20)···O(18) ϭ 176°]. Accordingly, nets of hydro-
In the crowded diol 9, the hydroxy groups are shielded
gen bonds link molecules intimately together. The rigid hel- from their external environment (Figure 8). One of the
icene skeleton and the H-bridge requirements cause both functional groups is visibly turned inward and has its OϪH
ϪCH2OH groups to adopt anticlinal conformations vector directed towards one of the compound’s own ben-
(CϪCϪOϪH torsion angles are Ϫ78.0 and Ϫ88.2°). The zene rings. Analysis of the XϪH ring center of gravity dis-
infinite two-dimensional network is characterized by base tances reveal several weak XϪH···π interactions,[30] two of
vectors {1 0 1}, 2: {0 1 0} and in plane: {1 0 Ϫ1} so that which are appreciably shorter than the sum of the CϪH
˚
directed four-membered (OϪH···OϪH···) rings are formed. van der Waals radii (ഠ 2.9Ϫ3.0 A). The strongest of these
The inner pitch elevation (d1, C1···C16 distance) is the is the above OϪH···π interaction,[34] which directs the OϪH
˚
shortest here, with the value of 2.94 A. The values of the bond such that it points into the center of the terminal aryl
˚
torsion angles 1 and 2 of the inner helix bonds are again ring of the helicene framework at a distance of 2.71 A
˚
relatively small, at 13.4 and 15.1°. In contrast to the pre- [O(15)ϪH(15) Cg(1), d ϭ 2.71 A, 171°). Another short
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New Functional Hexahelicenes
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Figure 3. Packing structure of helicene 1: top: as seen from the crystallographic c direction; bottom: a side view indicating CϪH···N and
π···π interactions (broken lines)
˚
contact distance of 2.68 A is from one of the CϪH atoms packing structure of molecules characteristic of helicenes in
of the ring to a neighboring phenyl ring center of another general. Instead they are interlocked such that the phenyl
molecule [C(4)ϪH(4) Cg(7)[1Ϫx, Ϫy, 1 Ϫ z], d ϭ 2.68 A, 172°]. groups may form CϪH···π interactions.[31]
˚
Both contacts maintain near linear angles around these hy-
drogen atoms. The high steric demand of the diphenylmeth-
anol units results in there being no conventional hydrogen
bonds, the shortest intermolecular distance being 2.67 A
Data Bank Analysis of Hexahelicene Structures
˚
The question is how one can best compare these rather
[O(22)ϪH(22)···O(15)[1Ϫx, y, 1Ϫz], 170.1°]. Helicene 9 has no rigid and mostly symmetric molecules, which, at least in a
˚
molecular symmetry within 0.80 A tolerance. The inner naive approach, should have C2 symmetry. The answers
˚
pitch elevation (d1, C1···C16 distance) is 3.13 A. The ter- would perhaps relate solid-state structural variability to
minal inner helix torsion angles 1 and 2 show somewhat their chiroptical properties. Relationships between molecu-
unequal but relatively small opening at 13.0 and 17.6°. Ob- lar structures could address chirality-related issues such as
viously the bulky units of helicene 9 prevent the column-like the occurrence of spontaneous resolution and so on.
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
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Figure 4. Packing structure of helicene 3 with a CϪH···O contact shown in dotted line; chirality of opposite stacks are indicated
Figure 5. Packing excerpt of helicene 4·acetone (1:1) with CϪH···O contacts (broken lines) and π···π interactions (dotted lines) in the
core of the helicenes
We chose to analyze structural data from our six indepen- helicenes were retrieved from the CSD (Release 5.23, April
dent hexahelicene models from five crystals against those 2002), merged with our own with the aid of PREQUEST,[36]
from the Cambridge Structural Database[35] (CSD). Hexa- and analyzed by use of the statistical package VISTA.[37]
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New Functional Hexahelicenes
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Figure 6. Packing structure of helicene 4·acetone (1:1); shaded areas are for alternating guest and terminal methyl group sites
Figure 7. Packing structure of helicene 8 showing some CϪH···O intermolecular contact dimensions and CϪH···π approach
An initial three-dimensional connectivity search from the ing more than one hexahelicene molecule in their asymmet-
main database gave 24 independent structure hits[38] con- ric units (such as compound 3) or that are polymorphic
taining hexahelicene moieties with no steric constraints increase the number of independent observations of hexa-
such as covalent bonds between either the C1ϪC16 or the helicene moieties up to 36 altogether, including those from
C2ϪC15 atomic positions (cf. Figure 2). Structures contain- this study. No structural fragment was excluded from the
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
FULL PAPER
Figure 8. Centrosymmetric pair of helicene molecules 9 indicating
significant CϪH···O intermolecular contact dimensions and
CϪH···π contact distances
further analyses. The following results are listed in an order
as one looks at these structures from their innermost bay
region out towards the flanks of the molecule.
The Outer C؊C Bond Lengths Opposite the Bay Region
of Hexahelicene: In an attempt to describe the shape of
hexahelicenes in general, a good starting point seems to be
to look at different aspects of their ability to preserve their
C2 symmetry more or less perfectly. This is first analyzed
by selecting two chemical bonds that should reflect C2 sym-
metry violations at bond length levels. A C2 axis would dis-
sect an ideal C2 symmetry molecule at the outer aromatic
CϪC bonds (junction of C8ϪC8a and C9ϪC8a bonds, cf.
Figure 2). As bond lengths usually deviate from ideal aro-
matic values here, it is equally interesting to see how far this
deviation goes. Figure 9 (a) shows this distribution, indicat-
ing that only four out of the 36 data points fall on the ide-
ally symmetric diagonal line. However, the majority of val-
ues lie within one sample standard deviation (1SD), so these
could be considered practically symmetric for many of the
cases, also including our five crystal structures. An apparent
Figure 9. (a) Scattergram of the C8ϪC8a and C9ϪC8a bonds in
the database of thirty hexahelicene crystal structures. Numbered
black dots relate to the crystal structures of the respective com-
pounds from this paper in this and in the following diagrams
throughout; diagonal represents ideally symmetric bonding; dotted
circle indicates approximate standard deviation range from the
mean value of the sample, indicated by an asterisk; (b) polar histo-
gram of the π3 angle (°) distribution for 36 observations, with the
mean value of 40.2°; (c) scattergram of the π1 vs. π2 angles (°);
diagonal line indicates positioning of ideally symmetric π1 ϭ π2
cases (not a regression line); the asterisk * denotes placement of the
mean value; broken circle indicates one sample standard deviation
ranges (1SD) from the mean; arrows indicate two prominent out-
liers from the mean (POZFIT01 and QEYWAS in CSD)
2
elongation of these bonds against an aromatic Csp2ϪCsp
standard is also quite obvious, with the mean value of 1.420
˚
A of the sample. However, there are a few outliers with
slightly eccentric behavior that is also obvious in the follow-
ing charts. Among these, one finds asymmetrically substi-
tuted hexahelicenes at positions 1 and 2 (MHXHEL10,
BHXHEL20 and FHXHLA). Environmental effects oper-
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New Functional Hexahelicenes
FULL PAPER
ate in another branch of these outliers, too. Signs of steric
congestion, either intramolecular (QEYWAS, NOQTIW)
or intermolecular (due to heteromolecular association) in
nature (ABUPAO; ABUNIU, ABUNUG), are probably
manifested in these deviations.
The Hexahelicene Opening Angle (π3) (for specification
see ref.[2]): This is perhaps the most characteristic general
molecular shape descriptor (cf. Figure 2), so its distribution
merits mention. The polar histogram in Figure 9 (b) shows
a slightly bimodal, or strongly distorted Gaussian distri-
bution with a relative large spread of about 32°, unexpected
in a rigid molecule. The relative small number of data in
the sample does not allow for a lengthy discussion of this
histogram though. We do note though, that those samples
at the minor peak are all from crystal structures with chiral
space groups (P1 and P212121).
Comparison of the Flank Angles (π1 and π2) (for specifica-
tion see ref.[2]): The angles of these least-squares planes
measure the tilt of the planes of the last ten atoms of each
flank against a central ten-atom plane (i.e. how three, par-
tially overlapping naphthalene moieties kink pairwise
towards each other). It should therefore again be expected
that a perfectly symmetric hexahelicene must have these
angles equal: along the diagonal of Figure 9 (c). As this
case is again somewhat similar to that in Figure 9 (a), here
we consider those that lie within 1SD value of the mean as
being practically symmetric within error limits. Here again
are two notable outliers: those possessing either non-centro-
symmetric or chiral space groups. Compound 8 from this
study also lies far from the diagonal, albeit 8 has a trivial
centrosymmetric space group P21/n. The relatively large dis-
tortion is thus caused by a strong asymmetric hydrogen
bond involvement in a centrosymmetric space group.
The Hexahelicene Opening Angle (π3) Against the Non-
bonded Distance (d1), π3 vs. d2 and π3 vs. d3 (for specifica-
tion of d1؊d3 see Table 3): The purpose of these three con-
secutive diagrams [Figure 10 (aϪc)] is to illustrate how dis-
tortions from the hexahelicene center propagate towards the
flanks of these molecules. This is best shown by the π3 an-
gle [cf. Figure 9 (b)]. Figure 10 (a) shows that no correlation
exists between π3 and d1, a minor surprise since a rigid
helicene model would imply that these two parameters cor-
relate well. A substantial number of structures lie outside
the 1SD limiting circle. Thus, opposing atoms C1 and C16
(Figure 2) suffer from substantial strain, forcing them away
from the planes made up by neighboring atoms. A mean
value of this C1···C16 distance also shows this clearly. It is
˚
˚
˚
of only 3.14 A, about 0.4 A shorter than the van der Waals
radii sum. As we proceed from d1 towards the outer elev-
ation distances d2 and d3, we see this correlation developing
nicely [Figure 10 (bϪc)]. The elevation distance between the
atom positions C2···C15 (d2) shows a fair correlation (corr.
coefficient: 90.9%). Compound 8 is still deviating, as are
Figure 10. (a) Scattergram of the π3 angles (°) vs. d1 distances (A);
˚
an asterisk * denotes placement of the mean value (40.2°, 3.139 A);
broken circle indicates one sample standard deviation ranges (1SD)
from the mean; (b) scattergram of the π3 angles (°) vs. d2 distances
˚
(A); an asterisk * denotes placement of the mean value (40.2°,
˚
4.165 A); regression line is computed for all 36 data; (c) scattergram
˚
of the π3 angles (°) vs. d3 distances (A); an asterisk * denotes place-
˚
ment of the mean value (40.2°, 5.032 A); regression line is com-
some of the noncentrosymmetric crystal structures. There puted for all 36 data
still is a tendency for the scatter of the distances to increase
as the π3 angle reduces. An obvious interpretation of this eff. ϭ 99.1%]. On looking through all these structures, it
is steric congestion. Correlation between the π3 angle and becomes clear that this pattern is fairly independent of the
the d3 distance is almost perfect [Figure 10 (c), corr. co- substituent positions. Substituents, albeit differing slightly
Eur. J. Org. Chem. 2003, 2863Ϫ2876
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
FULL PAPER
in their relative frequencies and also in whether they are this, it does have one interesting feature: the unusually high
mono- or disubstituted, occur at all three positions. M value of compound 4. The association and solvation
A further analysis was also made for non-bonding π···π properties of 4 may be markedly different from those of the
type interactions. Overlap would be expected to appear pre- other molecules; the inclusion of acetone in its crystal might
eminently on the more aromatic outer flank regions be- be an indication of this. It is known that some helicenes
tween symmetry-related phenanthrene rings. Analysis of exhibit solvent-, concentration-, and temperature-depen-
outer ring center distances shows that appreciable overlap dent molar rotation and CD spectra.[40] These dependencies
of this type occurs only in six cases, three of which, as al- are primarily due to variable self-assembling capabilities of
luded to earlier, are from the compounds reported in this the molecules.
paper. Compounds 1, 3, and 4 have π···π ring center dis-
˚
˚
tances under 4.0 A of this kind (3.74 and 3.75 A for 1, 3.66
˚
˚
and 3.67 A for 3, and 3.98 A for 4). The other three struc-
Conclusions
tures (FHXHLA, FHXHLB, and GOJDAK) have all their
respective ring center distances longer than 4 A.
˚
Seven new hexahelicenes (3؊7, 9, 10) containing different
Distribution of Chiral, Centric, and Acentric Space Groups functional group substituents in positions 2 and 15 of the
in the Sample: Crystallographic shorthand is used to desig- helicene framework have been synthesized by a synthetic
nate centrosymmetric space groups (centric) and also non- route involving a double Wittig reaction, a photocycliz-
centrosymmetric ones (acentric). Caution is again advised, ation, and subsequent modification of functional groups
since the sample distribution is unequal and especially (Scheme 1). A corresponding helicene 8, a known com-
undersized for acentric crystals. There are 15 centric, 11 chi- pound but previously obtained by a different approach,[6]
ral, and only three acentric space group occurrences in the has also been synthesized by the described reaction se-
sample, including the five structures reported in this work. quence, while the known 2,15-disubstituted hexahelicene
Apparently the occurrence of more than one molecule in derivatives 1 and 2 were prepared by literature meth-
the asymmetric unit (i.e. ZЈ Ͼ 1) seems to be frequent in ods.[13,14] Optical resolution with a cellulose tris(3,5-dimeth-
acentric crystals (two out of three) while it is perhaps less ylphenylcarbamate) column[24] was successfully performed
frequent both in centric crystals (two out of 15) and in chi- in three cases (4, 8, 10) and the molar rotations of these
ral ones (one out of 11). However, this disparity may also compounds have been determined. These vary between
be due to the relatively small number of cases, and caution 10383 and 19348, indicating influences of the substituents
is advised even for hexahelicene itself when classifying it in relative to the unsubstituted hexahelicene, most distinctly
the chiral class.[39] There seems to be no clear tendency be- exhibited in 4, the bulky bis(isobutyl ester). The CD spectra
tween substitution site and the frequency of these molecules agree with this.
crystallizing either in chiral or in non-centrosymmetric
space groups.
The crystal structures show a beautiful concerted inter-
play of weaker/stronger CϪH···N, CϪH···O, and CH···π in-
Principal Component (PC) Analysis of the d1, d2, and d3 teractions, π···π overlapping, and dispersion. An outstand-
Distances (as specified in Table 3) vs. Molar Rotation (M): ing feature of some of the molecules of this study Ϫ 1, 3,
Since there is some information available on the molar ro- 8, and 9 Ϫ is their capability for association with like mol-
tation of hexahelicene and the compounds in this study, it ecules, while only 4, in its inclusion compound with ace-
is tempting to plot these quantities against some possibly tone, shows heteromolecular association in the crystal. This
meaningful structural parameter. The principal components is in contrast to a recent comparative and exhaustive
(PCs) were derived from the d1, d2, and d3 distance values. study[12] on the association properties of polyaromatic sys-
Beforehand, we derived mean molecular dimensions (angles tems, hexahelicenes among others, which describes hetero-
and distances) of the two independent molecules of 3 so as molecular association between differing polyaromatic mol-
to have a single point for this parameter against one M ecules in considerable detail. As pointed out in the data-
data. PCs were first generated from a set of 35 sample bank analysis section, 1 and 3 rely especially upon π···π
points using all structures. The major component PC1 ex- interactions to build their crystal lattices. This interaction
plains 65.0% of the variation of the three generated compo- is only marginally present in the crystal structure of 4, as
nents. The minor component PC2 accounts for the rest of well as in three other helicene derivative crystals from the
the variance (34.8%). PC1 shows high (Ͼ97%) correlation literature.[38]
with both d1 and d2, indicating that these two distances
Statistical analysis of the molecular dimensions in these
explain variance in this small sample equally well (Fig- and related crystal structures gave hints as to what struc-
ure 11, Supporting Information, see also the footnote on tural parameters could be relevant to other macroscopic
the first page of this article).
physical phenomena such as optical rotation. Decreased
Next, only sample points with known molar rotation val- aromaticity in the central molecular portion is obvious,
˚
2
2
ues were kept (i.e., five structures from this study and hexa- with Csp ϪCsp bonds as long as 1.420 A mean value. Over-
helicene, Table 1). Since the number of data sets in the all mean values of the experimentally measured C1···C16
sample is reduced here to six, caution is due. A scatter plot (d1), C2···C15 (d2), and C3···C14 (d3) distances are 3.14(2),
˚
of the major PC against the M values (Figure 12, Support- 4.16(5), and 5.03(8) A, respectively; the mean π3 opening
ing Information) shows no usable regression. In spite of angle is 40.2°. Consistently with intuition, the internal bay
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New Functional Hexahelicenes
FULL PAPER
126.91, 127.18, 127.77, 128.55, 129.61, 130.07, 130.22, 131.48,
133.58, 135.36, 158.75 ppm. C28H24O2 (EI-MS): calcd. 392.1776;
found 392.2.
regions of these molecules bear considerable strain. Obser-
vations relating the distortion of theses structures and mo-
lar rotation cannot be simply deduced, but apparently mo-
lecular association and solvation may play an important
role here, also in the solid state. The statistical data also
shed light on the difference ranges between some molecules
in their solid-state fine structure.
Ester 11c: Methyl 4-formylbenzoate was used. Workup was as given
for 11a. Column chromatography (Al2O3, eluent: benzene, Rf ϭ
0.64) yielded 87% of a colorless solid [(E,Z) isomers of 11c]. M.p.
around 195 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 3.90 (s, 6 H,
COOCH3), 6.69Ϫ6.84 (m, 4 H, CϭCϪH), 7.20Ϫ7.95 (m, 14 H,
ArϪH) ppm. 13C NMR (75.5 MHz, CDCl3): δ ϭ 51.98, 107.41,
121.00, 127.01, 127.48, 128.27, 128.95, 129.57, 129.69, 132.04,
133.46, 134.69, 142.05, 166.86 ppm. C30H24O4 (FAB-MS): calcd.
448.1675; found 448.1674.
Experimental Section
General: Melting points: Kofler melting point microscope (uncor-
1
rected). IR: PerkinϪElmer FT-IR 1600. H NMR (internal stand-
Ester 11d: 2-Methylpropyl 4-formylbenzoate and the diphos-
phonium salt in dry THF (40 mL) were used. Sodium 2-methylpro-
poxide (1  in 2-methyl-1-propanol, 20 mL, 20 mmol) was added
at room temp. under Ar, and the mixture was stirred for 2 days.
After quenching with water, the mixture was extracted with diethyl
ether, washed with water, and dried (Na2SO4). Evaporation of the
solvent under vacuum and column chromatography (SiO2, eluent:
dichloromethane, Rf ϭ 0.84) yielded 91% of a pale yellow solid
[(E,Z) isomers of 11d; recrystallization from toluene yielded the
pure (Z,Z) isomer]. M.p. 189Ϫ190 °C. 1H NMR (300 MHz,
ard TMS): Bruker WH 250, WM 300, and WM 400. 13C NMR
(internal standard TMS): Bruker WM 250, WM 300, and WM 400.
MS: A.E.I MS 50 and Kratos FAB-MS Concept 1H. Elemental
analyses: Heraeus CHN rapid analyzer. Column chromatography:
Silica gel (63Ϫ100 µm, Merck) and Al2O3 (150 µm, neutral, Ald-
rich) Ϫ Enantiomeric separation: HPLC (Gilson, λ ϭ 254 nm),
cellulose-tris(3,5-dimethylphenylcarbamate) (CDMPC), 50
ϫ
0.46 cm (MallinkrodtϪBaker). Optical rotation: PerkinϪElmer 241
Polarimeter. Circular dichroism: Jasco J-720. Organic solvents were
purified by standard procedures. For optical separation, eluents of
HPLC quality (Merck) were used, for CD measurement n-hexane
of Uvasol quality (Merck).
3
CDCl3): δ ϭ 1.05 (d, JH,H ϭ 8.93 Hz, 12 H, CH3), 2.01 (m, 2 H,
3
3
CH), 4.01 (d, JH,H ϭ 9.02, 4 H, CH2), 7.25 (d, JH,H ϭ 11.8 Hz,
2 H, CϭCϪH), 7.4 (d, 3JH,H ϭ 11.8 Hz, 2 H, CϭCϪH), 7.60Ϫ8.10
(m, 14 H, ArϪH) ppm. 13C NMR (75.5 MHz, CDCl3): δ ϭ 19.24,
27.74, 70.59, 125.48, 126.37, 127.41, 127.61, 127.74, 128.40, 128.62,
128.77, 130.19, 131.70, 133.52, 134.71, 166.12 ppm. C36H36O4
(FAB-MS): calcd. 532.2614; found 532.2613.
Naphthalene-2,7-bis(triphenylphosphonium) dibromide was pre-
pared according to the ref.[9,10,15] procedure. The aromatic alde-
hydes were purchased from Fluka.
General Procedure. Synthesis of Distilbenes 11a؊d: Lithium me-
thoxide (1  in methanol, 20 mL, 20 mmol) was added over 0.5 h
under Ar to a refluxing solution of naphthalene-2,7-bis(triphenyl-
phosphonium) dibromide (6.71 g, 8 mmol) and the corresponding
aromatic aldehyde (16 mmol) in dry methanol (25 mL). Heating at
reflux was continued for 3 h. The mixture was cooled to room
temp. and stirred for 12 h. Compound 11d was an exception in that
dry THF (40 mL) and sodium 2-methylpropoxide were used and
room temp. was employed rather than heating at reflux. Details for
workup and purification, as well as data for the individual com-
pounds, are given below.
General Procedure. Synthesis of Helicenes 1؊4: A solution (or a
suspension in the cases of 11b and 11c) of the respective distilbene
11 (aϪd) (1 mmol) and iodine (38.1 mg, 0.15 mmol) in toluene
(1 L) was irradiated for 3.5Ϫ4 h in a photoreactor fitted with a
water-cooled immersion well and a high-pressure Hg lamp. Evapor-
ation of the solvent and column chromatography (SiO2) yielded the
pure racemic compounds. Enantiomeric separation was effected by
HPLC on a cellulose tris(3,5-dimethylphenylcarbamate) column at
25 °C. Experimental details and specific data for the individual
compounds are given below.
Hexahelicene-2,15-dicarbonitrile (1):[13] 2,7-Bis(4-cyanostyryl)naph-
thalene (11a) was used to give 62% of a light yellow solid; Rf ϭ
0.65 (eluent: cyclohexane/ethyl acetate, 1.5:1). M.p. Ͼ 300 °C
(dec.).
2,7-Bis(4-cyanostyryl)naphthalene (11a): 4-Cyanobenzaldehyde was
used. The reaction mixture was hydrolyzed with water and concen-
trated under vacuum to half its volume. The yellow precipitate was
collected and purified by column chromatography (SiO2, eluent:
dichloromethane, Rf ϭ 0.60) to yield 86% of pale yellow solid
2,15-Dimethoxyhexahelicene (2):[13,14c] 2,7-Bis(4-methoxystyryl)-
naphthalene (11b) was used to give 30% of a lemon solid; Rf ϭ
0.68 (eluent: toluene). M.p. 206 °C.
1
[(E,Z) isomers of 11a]. M.p. around 155 °C. H NMR (300 MHz,
CDCl3): δ ϭ 6.63Ϫ6.72 (m, 4 H, CϭCϪH), 7.15Ϫ7.89 (m, 14 H,
ArϪH) ppm. 13C NMR (75.5 MHz, CDCl3): δ ϭ 110.73, 123.87,
126.93, 127.57, 127.71, 128.26, 128.93, 129.60, 132.06, 132.23,
132.54, 133.0, 141.76 ppm. C28H18N2 (EI-MS): calcd. 382.1469;
found 382.2.
Dimethyl Hexahelicene-2,15-dicarboxylate (3): 2,7-Bis[4-(methoxy-
carbonyl)styryl]naphthalene (11c) was used to give 14% of a color-
less solid; Rf ϭ 0.20 (eluent: dichloromethane). M.p. 285 °C. IR
2,7-Bis(4-methoxystyryl)naphthalene (11b):[14c] 4-Methoxybenzal- (KBr): ν˜ ϭ 3032.5 cmϪ1 (CϪH, Ar), 2939 (CϪH, OCH3), 1712.5
dehyde was used. Workup and purification (Rf ϭ 0.85) as given for (CϭO), 1432 (Ar), 1244 (CϪO). 1H NMR (400 MHz), CDCl3):
3
11a yielded 86% of a colorless solid [(E,Z) isomers of 11b; recrys-
δ ϭ 3.58 (s, 6 H, CH3), 7.74 (d, JH,H ϭ 8.22 Hz, 2 H, ArϪH),
3
3
tallization from methanol yielded the pure (Z,Z) isomer]. M.p. 7.75 (d, JH,H ϭ 8.22 Hz, 2 H, ArϪH), 7.82 (d, JH,H ϭ 8.41 Hz, 2
152Ϫ154 °C. 1H NMR (250 MHz, CDCl3): δ ϭ 3.79 (s, 6 H, H, ArϪH), 7.96 (d, 3JH,H ϭ 8.41 Hz, 2 H, ArϪH), 8.02 (d, 3JH,H ϭ
3
OCH3), 6.59 (d, 3JH,H ϭ 12.32 Hz, 2 H, CϭCϪH), 6.68 (d, 3JH,H ϭ
8.22 Hz, 2 H, ArϪH), 8.05 (s, 2 H, ArϪH), 8.06 (d, JH,H
ϭ
3
12.32 Hz, 2 H, CϭCϪH), 6.75 (d, JH,H ϭ 8.4 Hz, 4 H, ArϪH), 8.22 Hz, 2 H, ArϪH) ppm. 13C NMR (100.6 MHz, CDCl3): δ ϭ
3
3
7.25 (d, JH,H ϭ 8.4 Hz, 4 H, ArϪH), 7.35 (d, JH,H ϭ 8.6 Hz, 2
51.82, 124.16, 125.49, 125.58, 126.07, 127.55, 127.89, 127.94,
128.58, 128.81, 129.30, 131.00, 132.05, 133.82, 135.18, 166.86 ppm.
3
H, ArϪH), 7.57 (d, JH,H ϭ 8.6 Hz, 2 H, ArϪH), 7.65 (s, 2 H,
ArϪH) ppm. 13C NMR, (62.9 MHz, CDCl3): δ ϭ 55.15, 113.57, C30H20O4 (EI-MS): calcd. 444.1361; found 444.2.
Eur. J. Org. Chem. 2003, 2863Ϫ2876
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C. Wachsmann, E. Weber, M. Czugler, W. Seichter
FULL PAPER
Bis(2-methylpropyl) Hexahelicene-2,15-dicarboxylate (4): 2,7-Bis[4-
ArϪH), 7.98 (d, 3JH,H ϭ 8.33, 2 H, ArϪH), 8.11 (d, 3JH,H ϭ 8.64,
(2-methylpropyloxycarbonyl)styryl]naphthalene (11d) was used to 2 H, ArϪH), 8.15 (s, 2 H, ArϪH), 8.22Ϫ8.28 (m, 6 H, ArϪH)
give 63% of a light yellow solid; Rf ϭ 0.75 (eluent: dichlorometh-
ppm. 13C NMR (100.6 MHz, [D6]DMSO: δ ϭ 122.74, 125.10,
126.54, 127.41, 127.68, 127.75, 127.78, 128.02, 128.12, 128.66,
ane). M.p. 162Ϫ164 °C (from n-hexane). IR (KBr): ν˜ ϭ 3051 cmϪ1
(CϪH, Ar), 2961 (CϪH, aliph.), 1714 (CϭO), 1616 (Ar), 1241 (C- 129.38, 131.37, 133.18, 134.36, 166.68 ppm. C28H16O4 (EI-MS):
1
3
O). H NMR (250 MHz, CDCl3): δ ϭ 0.81 (d, JH,H ϭ 6.72 Hz, 6
calcd. 416.1049; found 416. C28H16O4: calcd. C 80.76, H 3.87;
found C 80.52, H 3.82.
3
H, CH3), 0.85 (d, JH ϭ 6.72 Hz, CH3); 1.82 (m, 2 H, CH), 3.78
(dd, 2JH,H ϭ 17.68, 3JH,H ϭ 10.76 Hz, 2 H, CH2), 3.80 (dd, 2JH,H ϭ
1,1Ј-(Hexahelicene-2,15-diyl)dimethanol (8): LiAlH4 (49.3 mg,
1.3 mmol) was added under Ar to a solution of 4 (528.6 mg,
1 mmol) in dry THF (150 mL). The suspension was stirred at room
temp. for 12 h, then quenched with cooling with half-concd. hydro-
chloric acid and extracted into diethyl ether. The organic layer was
washed with water, then with 20 % aqueous solution of sodium
carbonate and dried (Na2SO4). Evaporation of the solvent under
vacuum, recrystallization from ethanol, and column chromatogra-
phy (SiO2, eluent: dichloromethane/acetone, 5:1, Rf ϭ 0.40) yielded
a pale yellow solid (365.2 mg, 94%). M.p. 234 °C (ref.[6] M.p.
3
17.68, JH,H ϭ 10.68 Hz, 2 H, CH2), 7.72Ϫ8.35 (m, 14 H, ArϪH)
ppm. 13C NMR (62.9 MHz, CDCl3): δ ϭ 19.25, 27.68, 70.58,
123.75, 125.46, 126.23, 127.42, 127.60, 127.76, 128.31, 128.65,
128.68, 129.58, 130.16, 131.64, 133.46, 134.66, 166.15 ppm.
C36H32O4 (FAB-MS): calcd. 528.2301; found 528. Enantiomeric
separation (eluent: n-hexane/2-propanol, 9:1, 2 mL·minϪ1, 28 bar):
(M)-(Ϫ)-4: tr ϭ 15.5 min, kЈ ϭ 1.0; (P)-(ϩ)-4: tr ϭ 31.0 min, kЈ ϭ
3.0; α ϭ 3.0, Rs ϭ 0.26. [α]2D0 ϭ 3660 Ϯ 100 (c ϭ 0.03, CHCl3).
Hexahelicene-2,15-dimethanamine (5): LiAlH4 (90 mg, 2.34 mmol)
was added to a solution of 1 (100 mg, 0.26 mmol) in dry THF 232Ϫ235 °C). IR (KBr): ν˜ ϭ 3423 cmϪ1 (OH), 3043.5 (CϪH, Ar),
(15 mL). The suspension was stirred at room temp. for 12 h, and
then quenched with water and extracted into dichloromethane. The
amine hydrochloride was precipitated by addition of diluted hydro-
chloric acid and separated by centrifugation. The solid was sus-
pended in THF and stirred with aqueous NaOH (20 %). The or-
ganic layer was separated, washed with water, and dried (Na2SO4).
Evaporation of the solvent under vacuum yielded a light yellow
solid (45.5 mg, 45%). M.p. 178 °C. H NMR (400 MHz, CDCl3): 125.31, 127.37, 127.46, 127.47, 127.70, 127.76, 127.86, 128.06,
δ ϭ 1.62 (s, 4 H, NH2), 3.16 (d, JH,H ϭ 14.07 Hz, 2 H,. CH2), 130.01, 131.58, 132.15, 133.64, 133.83 ppm. C28H20O2 (EI-MS):
2923 (CϪH, aliph.), 1616 (Ar), 102.5 (CϪO). 1H NMR (400 MHz,
CDCl3): δ ϭ 3.95 (d, 2JH,H ϭ 12.42, 2 H, CH2O), 4.12 (d, 2JH,H ϭ
3
12.42 Hz, 2 H, CH2OH), 7.24 (d, JH,H ϭ 8.26, 2 H, ArϪH), 7.50
3
(s, 2 H, ArϪH), 7.85 (d, JH,H ϭ 8.26, 2 H, ArϪH), 7.92 (d,
3
3JH,H ϭ 8.57, 2 H, ArϪH) 7.96 (d, JH,H ϭ 8.57, 2 H, ArϪH),
3
3
8.00 (d, JH,H ϭ 8.23, 2 H, ArϪH), 8.05 (d, JH,H ϭ 8.23, 2 H,
ArϪH) ppm. 13C NMR (100.6 MHz, CDCl3): δ ϭ 65.73, 124.42,
1
2
2
3
3.31 (d, JH,H ϭ 14.07, 2 H, CH2), 7.17 (d, JH,H ϭ 7.99, 2 H,
calcd. 388.1463; found 388.0. Enantiomeric separation (eluent: n-
ArϪH), 7.48 (s, 2 H, ArϪH), 7.79 (d, JH,H ϭ 7.99, 2 H, ArϪH), hexane/2-propanol, 9:1, 0.4 mL·minϪ1, 49 bar): (M)-(Ϫ)-10: tr ϭ
3
7.89 (d, 3JH,H ϭ 8.72, 2 H, ArϪH), 7.92 (d, 3JH,H ϭ 8.72 Hz, 2 H,
32.5 min, kЈ ϭ 9.83; (P)-(ϩ)-10: tr ϭ 51.5 min, kЈ ϭ 16.17; α ϭ
ArϪH), 7.98 (d, 3JH,H ϭ 8.08, 2 H, ArϪH), 8.03 (d, JH,H ϭ 8.08, 1.64. [α]2D0 ϭ 2672.8 Ϯ 3.4 (c ϭ 0.1, CHCl3).
2 H, ArϪH) ppm. 13C NMR (100.6 MHz, CDCl3): δ ϭ 30.67,
3
1,1,1Ј,1Ј-Tetraphenyl-1,1Ј-(hexahelicene-2,15-diyl)dimethanol
(9):
124.43, 125.77, 125.89, 126.35, 126.80, 127.64, 127.71, 127.92,
Phenyllithium (1.6
 in cyclohexane/diethyl ether, 1.3 mL,
128.01, 130.23, 131.01, 132.01, 133.57, 136.12 ppm. C28H22N2 (HR
MS): calcd. 386.1783; found 386.1777. C28H22N2: calcd. C 87.01,
H 5.74, N 7.25; found C 86.80, H 5.90, N 7.22.
2.1 mmol) was added at Ϫ15 °C under Ar to a suspension of 4
(158.6 mg, 0.3 mmol) in dry THF (8 mL). The mixture was stirred
for 1 h at the same temperature, then for 12 h at room temp. and
was then heated at reflux for 4 h. After quenching with ice/water,
the aqueous phase was extracted with diethyl ether, and the extract
was washed with water and dried (NaSO4). Evaporation of the sol-
vent under vacuum and purification by column chromatography
(SiO2, eluent: dichloromethane, Rf ϭ 0.39) yielded a pale yellow
solid (160 mg, 77%). M.p. 223 °C. IR (KBr): ν˜ ϭ 3448 cmϪ1 (OH),
3058 (CϪH, Ar), 1609.5 (Ar), 1015.5 (CϪO). 1H NMR (400 MHz,
CDCl3): δ ϭ 1.72 (s, 2 H, OH), 6.75Ϫ7.20 (m, 20 H, ArϪH),
7.78Ϫ7.94 (m, 14 H, ArϪH) ppm. 13C NMR (100.6 MHz, CDCl3):
Hexahelicene-2,15-diol (6): Boron tribromide (1  in hexane,
2.1 mL, 2.1 mmol) was added at room temp. over 30 min to a
stirred solution of 2 (100 mg, 26 mmol) in dry dichloromethane
(10 mL). Stirring was continued at the same temp. for 12 h. The
mixture was quenched with ice/water. The precipitate was separated
and washed with water. Recrystallization from dichloromethane
yielded a light yellow solid (58.1 mg, 62%), sensitive to light. M.p.
295 °C. IR (KBr): ν˜ ϭ 3215 cmϪ1 (OH), 2891, 1479.5 (Ar), 1192.5
1
3
(CϪO). H NMR (400 MHz, [D4]MeOH): δ ϭ 6.82 (dd, JH,H
ϭ
4
4
8.63, JH,H ϭ 2.35 Hz, 2 H, ArϪH), 7.00 (d, JH,H ϭ 2.35 Hz, 2 δ ϭ 81.88, 124.13, 126.76, 126.91, 127.12, 127.17, 127.19, 127.26,
H, ArϪH), 7.71 (d, 3JH,H ϭ 8.63 Hz, 2 H, ArϪH), 7.75 (d, 3JH,H ϭ
127.34, 127.60, 127.61, 127.90, 127.94, 127.99, 128.23, 128.43,
3
8.52 Hz, 2 H, ArϪH), 7.86 (d, JH,H ϭ 8.52 Hz, 2 H, ArϪH), 7.95 129.84, 131.22, 131.69, 133.40, 143.94, 146.11, 146.89 ppm.
3
3
(d, JH,H ϭ 8.21 Hz, 2 H, ArϪH), 7.98 (d, JH,H ϭ 8.21 Hz, 2 H,
ArϪH) ppm. 13C NMR (100.6 MHz, [D4]MeOH: δ ϭ 111.69,
117.65, 124.10, 125.67, 127.74, 127.88, 128.07, 128.14, 128.98,
130.15, 133.19, 133.27, 133.99, 156.00 ppm. C26H16O2 (HR MS):
calcd. 360.1150; found 360.1152. C26H16O2: calcd. C 86.65, H 4.47;
found C 86.64, H 4.36.
C52H36O (EI-MS): calcd. 692.2715; found 692. C52H36O: calcd. C
90.14, H 5.24; found C 89.76, H 5.10.
1,1,1Ј,1Ј-Tetrakis(4-methylphenyl)-1,1Ј-(hexahelicene-2,15-diyl)-
dimethanol (10): A solution of 4-methylphenyllithium was prepared
by addition, under Ar, of p-bromotoluene (359.2 mg, 2.1 mmol) in
dry diethyl ether (4 mL) to lithium granules (29 mg, 4.2 mmol) in
dry diethyl ether (4 mL), heating at reflux for 1 h, and cooling
Hexahelicene-2,15-dicarboxylic Acid (7):
A suspension of 4
(528.7 mg, 1 mmol) in 10 % aqueous NaOH (50 mL) and methanol down. This solution was added over 30 min at Ϫ15 °C under Ar
(10 mL) was heated at reflux for 2 days. After filtration, the filtrate
was acidified carefully with diluted hydrochloric acid. The precipi-
tate that had formed was separated and washed with water to yield
a light yellow solid (62.5 mg, 15%). M.p. 290 °C. IR (KBr): ν˜ ϭ
3440 cmϪ1 (OH), 3049 (CϪH, Ar), 1689.5 (CϭO), 1619.5 (Ar). 1H
to a solution of 4 (158.6 mg, 0.3 mmol) in dry diethyl ether (8 mL).
The mixture was stirred at room temp. for 1 h, heated at reflux for
7 h, cooled down, and quenched with ice water. The aqueous phase
was extracted with diethyl ether, and the extract was washed with
water and dried (Na2SO4). Evaporation of the solvent under vac-
NMR (400 MHz, [D6]DMSO): δ ϭ 7.69 (d, 3JH,H ϭ 8.33 Hz, 2 H, uum and purification by column chromatography (SiO2, eluent: di-
2874  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org
Eur. J. Org. Chem. 2003, 2863Ϫ2876

New Functional Hexahelicenes
FULL PAPER
chloromethane, Rf ϭ 0.76) yielded a light yellow solid (141.6 mg, measurements. Financial support by the Deutsche Forschungs-
63%). M.p. 129 °C. IR (KBr): ν˜ ϭ 3472.5 cmϪ1 (OH), 3065 (CϪH,
gemeinschaft (DFG) and the Fonds der Chemischen Industrie is
Ar), 2966.5 (CϪH, aliph.), 1630.5 (Ar), 1232 (CϪO). 1H NMR gratefully acknowledged. MC thanks NATO for a scholarship
(250 MHz, CDCl3): δ ϭ 1.68 (s, 2 H, OH), 2.18 (s, 6 H, CH3), 2.31
administered by the Ministry of Education (OM KFHAT, Grant
No. 2090/NATO/02). This work is part of the Graduate School
3
(s, 6 H, CH3), 6.64 (d, JH,H ϭ 8.66 Hz, 4 H, ArϪH), 6.67 (d,
3
3JH,H ϭ 8.66 Hz, 4 H, ArϪH), 6.78 (d, JH,H ϭ 8.07 Hz, 4 H, Program (GRK, 208) of the TU Bergakademie Freiberg supported
3
3
ArϪH), 6.90 (d, JH,H ϭ 8.07 Hz, 4 H, ArϪH), 7.13 (d, JH,H
ϭ
by the DFG.
8.42 Hz, 2 H, ArϪH), 7.14 (d, 3JH,H ϭ 8.42 Hz, 2 H, ArϪH), 7.79
3
(d, JH,H ϭ 8.59 Hz, 2 H, ArϪH), 7.84 (s, 2 H, ArϪH), 7.86 (d,
3JH,H ϭ 8.59 Hz, 2 H, ArϪH), 7.88 (d, JH,H ϭ 8.25 Hz, 2 H,
3
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3
[2]
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www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2875

C. Wachsmann, E. Weber, M. Czugler, W. Seichter
FULL PAPER
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with their literature references are deposited and available upon
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2876
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 2863Ϫ2876