λ5-Phospha[7]helicenes: Synthesis, properties, and columnar aggregation with one-way chirality

Article abstract of DOI:10.1002/anie.201106157

One-way street: A new family of λ^5-phospha[7]helicenes which form one-dimensional columnar stacks in the solid state were synthesized (see scheme). Neighboring stacks have opposite dipole directions and, in the case of the racemic phosphole sul

Full text of DOI:10.1002/anie.201106157

Angewandte
Chemie
DOI: 10.1002/anie.201106157
Heterocyclic Chemistry
l5-Phospha[7]helicenes: Synthesis, Properties, and Columnar
Aggregation with One-Way Chirality**
Koji Nakano, Hiromi Oyama, Yoshio Nishimura, Satoshi Nakasako, and Kyoko Nozaki*
Helical extension of p-conjugated systems is of great interest
for the production of novel molecules and materials with
unusual properties and applications. A typical example is
helical polyacetylenes, in which continuous p conjugation is
achieved through covalent bonds.[1,2] Another example is the
assembly of short helical fragments, such as helicenes.[3,4] For
instance, helicenebisquinone derivatives were reported to
form one-dimensional columnar aggregates with the aid of
long alkyl side chains.[5] Unique optical properties, such as
high nonlinear optical susceptibility[5c,f] and circularly polar-
ized luminescence, were reported for these aggregates.[5e,6]
Single crystals would also be suitable for creating such
architectures if the p-conjugated helical molecules are
appropriately stacked. Nevertheless, our analysis of the
Cambridge Crystallographic Database revealed that struc-
tures with one-dimensional columnar helicene packing that is
driven by p–p stacking interactions are rather uncommon,[7]
and in the majority of structures with helicene packing, the
packing is noncolumnar and mostly driven by CH-p inter-
actions. We hypothesized that the use of dipole–dipole
interactions in addition to inherent p–p stacking interactions
might produce a one-dimensional columnar arrangement of
helicenes. Dipole–dipole interactions have been used as part
of systems that contain cooperative interactions for the
formation of one-dimensional molecular arrangements.[8]
Theoretical studies on the corannulene dimer have also
demonstrated that dipole–dipole interactions should be a
significant part of the binding energy of bowl-shaped
molecules.[9] Previously, we have reported the synthesis of
oxa- and aza[7]helicenes 3 and 4 by using palladium-catalyzed
reactions.[10] These helicenes do not possess dipole-moment
vectors that are parallel to their helical axes, and no one-
dimentional columnar stack was formed. Herein, we report
the preparation of l5-phospha[7]helicenes 1 and 2 as a new
family of helicenes.[11,12] The phosphole oxide and phosphole
sulfide moieties give these helicenes a dipole-moment vector
that is parallel to their helical axes, and thus the columnar
stacking of 1 and 2 was achieved. More importantly, a
racemate of phosphole sulfide 2 crystallized with a unique
packing motif: the columns with one dipole direction consist
of a single enantiomer and the columns with the opposite
dipole direction consist of the other enantiomer (Figure 1).
l5-Phospha[7]helicenes 1 and 2 were synthesized as shown
in Scheme 1. A racemic 4,4’-biphenanthryl-3,3’-diyl bis(tri-
fluoromethanesulfonate, rac-6), from which we previously
synthesized aza[7]helicene,[10] was selected as the starting
compound. The palladium-catalyzed cross-coupling of rac-6
with ethyl phenylphosphinate gave a 46% yield of the
monophosphrous compound rac-7 as a mixture of diastereo-
mers (axial chirality and P-centered chirality).[13] l3-Phos-
pha[7]helicene rac-9, which has a phosphole moiety, was
[13c,14]
obtained by the reduction of rac-7 with LiAlH4
and a
subsequent palladium-catalyzed intramolecular P-arylation.
[*] Dr. K. Nakano, H. Oyama, Y. Nishimura, S. Nakasako,
Prof. Dr. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyoku, Tokyo 113-8656 (Japan)
E-mail: nozaki@chembio.t.u-tokyo.ac.jp
Dr. K. Nakano
PRESTO Science and Technology Agency
4-1-8 Honcho Kawaguchi, Saitama (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (“Emergence of Highly Elaborated p-Space and Its
Function” 21108508) from the Ministry of Education, Culture,
Sports, Science and Technology (Japan). We thank Dr. Hiroyasu
Sato (Rigaku Co.) for assistance with the X-ray analysis of (P)-1.
Figure 1. Structures of heterohelicenes and a representation of the
columnar packing in a single crystal of rac-2. Columns with a given
dipole moment consist of a single enantiomer (P or M isomer) and
columns with the opposite dipole moment consist of the other
enantiomer.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106157.
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Scheme 1. Synthesis of l5-phospha[7]helicenes 1 and 2. DPPB=1,4-
bis(diphenylphosphino)butane; Tf=trifluoromethanesulfonyl.
Figure 2. a) UV/Vis absorption (UV) and photoluminescence spectra
(PL) of (P)-1 (solid line, 2.4ꢀ10À6 m) and (P)-2 (dashed line,
7.5ꢀ10À6 m) in CHCl3 solution. b) CD spectra of 1 (1.6ꢀ10À5 m for
both enantiomers, CHCl3 solution).
As the phosphorus center of rac-9 is susceptible to oxidation
under ambient conditions, rac-9 was directly oxidized without
purification to give racemic l5-phospha[7]helicene rac-1,
which has a phosphole oxide moiety, in a 34% overall yield
from rac-7. l5-Phospha[7]helicene rac-2, which has a phosp-
hole sulfide moiety, was synthesized from rac-1 in 97% yield
by using Lawessonꢀs reagent. Racemic 1 was separated into
enantiopure (P)-1 and (M)-1 by HPLC on a chiral stationary
phase. The absolute configuration of enantiopure 1 in each
fraction was determined by X-ray crystallographic analysis
(see below). In contrast, rac-2 could not be separated into
enantiopure forms by HPLC. Therefore, both (P)-2 and (M)-2
were synthesized from enantiopure (P)-1 and (M)-1, respec-
tively.[15]
À5.87 eV, respectively (see the Supporting Information). l5-
Phospha[7]helicenes 1 and 2 have similar broad luminescent
spectra with maxima (lem) at 462 nm and 460 nm, respectively,
in solution. Thus, larger Stokes shifts were observed for both
l5-phospha[7]helicenes (2390 cmÀ1 for 1 and 2800 cmÀ1 for
rac-2) than for 3 and 4 (650 cmÀ1 for 3 and 580 cmÀ1 for 4).
Such large Stokes shift are common for phosphole-containing
compounds and suggest a large rearrangement of the p-
conjugated framework upon photoexcitation.[17] The quantum
yield of 1 in solution is 7.8%, whereas the quantum yield in
the solid state is much lower (0.1%). In contrast, 2 has a low
quantum yield of 0.1% both in solution and in the solid state.
Based on DFT calculations for 1 and 2, the HOMOs are
mainly located on the two phenanthrene moieties and
chalcogen atoms, and contain a nodal plane at the phosphorus
atom (see the Supporting Information). In contrast, the
LUMOs are largely located on the phosphole/chalcogenide
moieties, where (thio)phosphoryl groups work as an electron-
withdrawing group through s*–p* hyperconjugation. Such
electronic perturbation of the phosphole/chalcogenide moi-
eties may cause an intramolecular charge transfer to induce a
large Stokes shift.
The photophysical properties of l5-phospha[7]helicenes 1
and 2 were evaluated by UV/Vis absorption and photo-
luminescence spectroscopy (Figure 2), cyclic voltammetry,
and theoretical calculations. The photophysical data are
summarized in Table S5 in the Supporting Information.
Both l5-phospha[7]helicenes 1 and 2 have almost identical
spectra. The longest absorption maxima (labs) at 416 nm of 1
and 2 (Figure 2a) are significantly red-shifted relative to that
of 5-phenyldibenzophosphole-5-oxide (332 nm), 5-phenyldi-
benzophosphole-5-sulfide (330 nm),[12f] and phenanthrene
(293 nm), which indicates effective p conjugation over the
helical frameworks. The absorption edges of 1 and 2 were
both at 432 nm, which are red-shifted relative to the related
hetero[7]helicenes 3–5.[10,16] The HOMO–LUMO energy gaps
of 2.87 eV that were estimated from the absorption edges
agree with those calculated by DFT calculations at the
B3LYP/6-31G(d) level (see the Supporting Information).
Cyclic voltammetry experiments show two irreversible oxi-
dation waves for 1 and 2, and the HOMO energy levels
(EHOMO) of 1 and 2 were estimated to be À5.90 eV and
The optical properties of 1 and 2 that are imparted by their
chiral structures were characterized. Similar to other known
(P)-heterohelicenes, both (P)-1 and (P)-2 are dextrorotatory.
The specific rotations [a]D of (P)-1 and (P)-2 are + 3014 (c =
0.10, CHCl3) and + 3198 (c = 0.10, CHCl3), respectively,
which are much larger than those of 3 and 4 (see below).
The circular dichroism (CD) spectra of (P)-1 and (P)-2 had a
small negative dichroic signal at around 410 nm, a intense
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positive signal around 340 nm, and a relatively intense
negative signal around 250 nm (Figure 2b). This trend is
similar to that of the known (P)-heterohelicenes.[10,16b,18] In
contrast, the dichroic signs of (P)-1 and (P)-2 at around
300 nm are opposite to that of the known (P)-heterohelicenes.
The structures of 1 and 2 were confirmed by single-crystal
X-ray analysis.[19] Single crystals that were suitable for X-ray
analysis were obtained from rac-1, enantiopure (P)-1, enan-
tiopure (M)-1, rac-2, and enantiopure (M)-2. It is noteworthy
that whereas a single crystal of rac-1 contained only one
enantiomer by spontaneous resolution, a racemic crystal that
contained both enantiomers was obtained with rac-2. The
absolute configuration of enantiopure 1 with the longer
HPLC retention time was confirmed to be the P configuration
by the refinement of the Flack parameter with data collected
by using Cu Ka radiation (see the Supporting Information).
Accordingly, the absolute configuration of enantiopure 1 and
2 was determined. Helicenes 1 and 2 have more distorted
structures than the other heterohelicenes: the sums of the five
À
dihedral angles that are derived from the seven C C bonds
[C(17)-C(17a)-C(17b)-C(17c),
C(17a)-C(17b)-C(17c)-C-
(17d), C(17b)-C(17c)-C(17d)-C(17e), C(17c)-C(17d)-C(17e)-
C(17f), and C(17d)-C(17e)-C(17f)-C(1)] are 95.28 for 1
(based on the crystallographic data of enantiopure (M)-1,
Figure 3) and 99.68 for 2 (based on the crystallographic data
of rac-2). These angles are larger than those of hetero[7]-
helicenes 3–5 (79–888). This can be attributed to the larger
angles between the two double bonds of phosphole oxide
(508) and phosphole sulfide (508) relative to furan (328),
pyrrole (358), and thiophene (458): the larger angle causes a
larger overlap of the two terminal benzene rings in the l5-
phospha[7]helicenes and, therefore, a stronger steric repul-
sion (see the Supporting Information). These larger distor-
tions in 1 and 2 explain the higher tolerance of 1 and 2 towards
racemization. The enantiopurity of 1 and 2 did not decrease
after heating the compounds to 1708C for 68 h in CH2Cl2,
whereas the enantiopurity of 3 and 4 did decrease after
heating (3: from > 90% ee to 40% ee at 1508C for 20 min in
mesitylene; 4: from 99% ee to 40% ee at 1508C for 68 h in
mesitylene). The larger specific rotations of enantiopure 1
and 2 might also be attributed to their more-distorted
structures.
The packing structures in the single crystals of rac-2 and
enantiopure (M)-2 are shown in Figure 3b–3e.[20] Columnar
alignments of the p systems were detected for the racemate
and for the single enantiomer of 2, as well as the single
enantiomer of 1. In these structures, each column consists of
helicenes with a staggered packing pattern (Figure 3b,c). The
intermolecular, face-to-face, p–p interaction distances of
(M)-1, rac-2, and (M)-2 are 3.44 ꢁ, 3.35 ꢁ, and 3.46 ꢁ,
respectively. The dipole-moment vectors that are perpendic-
ular to the helical axis cancel each other through the two
interacting molecules, whereas the vectors that are parallel to
the axis are aligned in one column. Neighboring columns have
opposite dipole-moment vectors that cancel each other and
the overall dipole moment. Most notable is the packing of rac-
2. Each column contains a single enantiomer of either (P)-2 or
(M)-2. As shown in Figure 3d, all of the columns that contain
(P)-2 have dipole moments from the bottom to the top and all
Figure 3. X-ray structures of l5-phospha[7]helicenes: a) ORTEP draw-
ings of (M)-1 packing in the single-crystal structure of rac-2 (the single
crystal was obtained from enantiopure (M)-1; 50% thermal ellipsoids;
all hydrogen atoms are omitted for clarity). b) Side and c) top views of
(P)-2 and (M)-2 in the racemic crystal structure of rac-2. d) Columnar
arrangement of (P)-2 and (M)-2 in the single-crystal structure of rac-2.
e) Columnar arrangement of (M)-2 in the single-crystal structure of
(M)-2.
of the columns that contain (M)-2 have dipole moments in the
opposite direction, from the top to the bottom. Thus, the
crystal is chirally anisotropic. In other words, if chirality can
be sensed to differentiate (P)-2 from (M)-2 (or (M)-2 from
(P)-2), the moment is a one-way street from bottom to top (or
top to bottom, Figure 1).[21]
In conclusion, we have developed a new synthetic
approach to l5-phospha[7]helicenes and revealed their
unique packing structures. One-dimentional columnar pack-
ing of the l5-phospha[7]helicenes, which achieves the ideal p–
p stacking of the helicenes, was successfully created with the
aid of dipole moments. A racemate of phosphole sulfide 2
exhibited a unique packing structure that creates “one-way
chirality”.
Experimental Section
A mixture of LiAlH4 (55 mg, 1.46 mmol) in THF (7.3 mL) was placed
in a 20 mL Schlenk tube under argon and cooled to À788C in a dry
ice/acetone bath. Me3SiCl (126 mL, 1.46 mmol) was added to the
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white suspension of LiAlH4 and the resulting mixture was warmed to
room temperature and stirred for 1 h. The resulting colorless solution
was then cooled to À428C in a dry ice/acetonitrile bath. rac-7 (210 mg,
0.31 mmol) was dissolved in tetrahydrofuran (6.0 mL) and this
solution was added slowly to the Me3SiCl/LiAlH4 mixture. The
reaction mixture was warmed to room temperature and stirred for
16 h, then cooled to 08C and quenched with degassed ethyl acetate
(0.5 mL) and degassed water (0.5 mL). After stirring the mixture at
room temperature for 15 min, it was filtered through a pad of basic
alumina, under argon. The filtrate was concentrated under reduced
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pressure to yield
a diastereomeric mixture of rac-8 (187 mg).
31P NMR (500 MHz, [D8]THF): d = À44.9, À45.1 ppm].
A mixture of rac-8 (187 mg, 0.31 mmol), Pd(OAc)2 (6.9 mg,
0.031 mmol), 1,4-bis(diphenylphosphino)butane (DPPB, 13 mg,
0.031 mmol), iPr2NEt (186 mL, 1.07 mmol) in DMSO (3.0 mL) was
placed in a 100 mL Schlenk tube and degassed by three freeze-pump-
thaw cycles. After stirring under argon at 1108C for 79 h, the reaction
mixture was cooled to ambient temperature and diluted with CH2Cl2
(10 mL), then aqueous H2O2 (30–35%) was added. The resulting
mixture was stirred at room temperature for 12 h. Water (20 mL) was
added, and the resulting mixture was extracted with CH2Cl2 (10 mL ꢂ
5). The combined organic layers were dried over anhydrous Na2SO4,
filtered, and concentrated under reduced pressure. The crude residue
was purified by column chromatography on silica gel with gradient
elution from CH2Cl2 to CH2Cl2/ethyl acetate (4:1) to give rac-1 as a
yellow solid (49 mg, 34% from rac-7, Rf = 0.30(ethyl acetate)).
Received: August 31, 2011
Revised: October 30, 2011
Published online: December 1, 2011
Keywords: chirality · helical structures · helicenes ·
.
molecular arrangement · phosphorus heterocycles
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(M)-1 to (P)-2 and (M)-2, respectively, was assumed to proceed
without any loss of enantipurity, as no decrease in the
enantiopurity of (P)-1 was observed after heating it to 1708C
for 64 h in 1,2-dichlorobenzene.
[16] a) A. Dore, D. Fabbri, S. Gladiali, G. Valle, Tetrahedron:
Asymmetry 1995, 6, 779; b) G. Gottarelli, G. Proni, G. P.
Spada, D. Fabbri, S. Gladiali, C. Rosini, J. Org. Chem. 1996,
61, 2013.
[17] J. Yin, R. F. Chen, S. L. Zhang, Q. D. Ling, W. Huang, J. Phys.
Chem. A 2010, 114, 3655.
[18] a) M. B. Groen, H. Wynberg, J. Am. Chem. Soc. 1971, 93, 2968;
b) I. Pischel, S. Grimme, S. Kotila, M. Nieger, F. Vogtle,
Tetrahedron: Asymmetry 1996, 7, 109.
[19] Crystal data for (P)-1: C34H22O1.5P; Mr = 485.52; a = 10.113(4),
b = 12.110(6), c = 19.665(9) ꢁ; a = b = g = 908; V= 2408(2) ꢁ3;
T= 93(2) K; orthorhombic; space group P212121; Z = 4; m =
1.231 mmÀ1; 1calcd = 1.339 gcmÀ3; 35543 reflections measured;
4404 unique reflections (Rint = 0.1014); final R1 = 0.0826; wR2 =
0.2177 [I > 2.0s(I)]. (M)-1: C34H21OP; Mr = 476.48; a =
10.050(6), b = 12.175(7), c = 19.520(11) ꢁ; a = b = g = 908; V=
2389(2) ꢁ3; T= 103(2) K; orthorhombic; space group P212121;
Z = 4; m = 0.142 mmÀ1; 1calcd = 1.325 gcmÀ3; 15536 reflections
measured; 4187 unique reflections (Rint = 0.0319), final R1 =
0.0630; wR2 = 0.1397 [I > 2.0s(I)]. rac-2: C34H21PS; Mr =
492.54; a = 26.353(10), b = 12.458(4), c = 19.163(8) ꢁ; a = g =
908; b = 130.0975(11)8; V= 4813(3) ꢁ3; T= 103(2) K; mono-
clinic; space group C2/c; Z = 8; m = 0.224 mmÀ1
; 1calcd =
1.360 gcmÀ3; 15314 reflections measured; 4187 unique reflec-
tions (Rint = 0.0566); final R1 = 0.0681; wR2 = 0.1217 [I > 2.0s(I)].
(M)-2: C34H21PS; Mr = 492.54; a = 10.055(3), b = 12.389(4), c =
20.027(6) ꢁ; a = b = g = 908; V= 2494.6(13) ꢁ3; T= 93(2) K;
orthorhombic; space group P212121; Z = 4; m = 0.216 mmÀ1
;
1calcd = 1.311 gcmÀ3; 16935 reflections measured; 4376 unique
reflections (Rint = 0.0591); final R1 = 0.1024; wR2 = 0.2833 [I >
2.0s(I)]. CCDC 851214 [(P)-1], 841807 [(M)-1], 841809 [rac-2],
and 841808 [(M)-2] contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif. The crystal data of the single
crystal from rac-1 is not described since the enantiopure crystal
was obtained by spontaneous resolution, and the crystallo-
graphic data was the same as that for the single crystal of
enantiopure (M)-1.
[20] Note that none of the previous O-, S-, or N-containing
[7]helicenes form such columnar aggregates.
[21] E. W. Meijer, E. E. Havinga, Synth. Met. 1993, 57, 4010.
Angew. Chem. Int. Ed. 2012, 51, 695 –699
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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