1H-phosphindoles as structural units in the synthesis of chiral helicenes

Article abstract of DOI:10.1002/anie.201202024

Building helicenes: A photochemical cyclization approach affords helicenes in which the fused ring sequence ends with a phosphole unit (see scheme). The stereogenic phosphorus centers of the substrates control the screw sense of helical chirality. The terminal phosphole units undergo photochemical [2+2] annulations to give dimeric helical structures. Copyright

Full text of DOI:10.1002/anie.201202024

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DOI: 10.1002/anie.201202024
Phosphorus Heterocycles
1H-Phosphindoles as Structural Units in the Synthesis of Chiral
Helicenes**
Keihann Yavari, Souad Moussa, Bꢀchir Ben Hassine, Pascal Retailleau, Arnaud Voituriez, and
Angela Marinetti*
Helical arrangements and the associated chirality have well-
known implications in a variety of areas ranging from natural
biological systems to materials science. In particular ortho-
fused polyaromatic systems with p-conjugated helical struc-
tures display unique physicochemical properties which have
stimulated countless studies[1] oriented towards self-assembly
and chiral recognition,[2] chiroptical devices,[3] molecular
Figure 1. Targeted helical structures: phosphorus heterocycles at the
external edge of ortho-fused aromatic sequences.
machines,[4] material chemistry,[5] organometallic and organo-
catalysis,[6,7] as well as asymmetric synthesis.[8,9]
In the general context of the growing interest in helical
derivatives, helicenes displaying phosphorus functions repre-
sent promising scaffolds. Trivalent phosphorus functions and
the related metal complexes are easily designed to tune the
electronic and physicochemical properties of helicenes,[10] as
well as to offer a wide range of potential uses in organome-
tallic chemistry and catalysis. It is therefore rather surprising
to note that the field today is still in its infancy, first of all in
terms of number of compounds as well as structural diversity.
So far, most phosphorus derivatives having helical chirality
display polyaromatic (or heteroaromatic) helical scaffolds
with pendant phosphorus functions (phosphites,[2a,6b,11] triva-
lent phosphines and phosphine oxides,[6a,12] phospholes,[10a,b]
etc.). Very little is known about their properties and
behaviors.
The aim of this work is to expand the range of hetero-
helicenes to chiral helicenes where phosphorus is embedded
within the helical framework itself, at the external edge of the
fused ring sequence (Figure 1). Helical phosphines of this
class would not only display peculiar electronic features
resulting from extended p conjugation, but also take full
advantage of the dissymmetric steric environment generated
by the helical chirality at the external edge. Helicenes of this
class are unknown to date, whereas phosphahelicenes,
wherein phosphorus is embedded in the internal section of
the helical framework, have been reported only recently by
Tanaka and co-workers and Nozaki and co-workers.[13]
We intend to disclose here: a) the first diastereo- and
enantioselective access to [6]- and [7]helicenes in which the
fused ring sequence ends with a phosphole unit; b) the
structural and chiroptical properties of enantiomerically pure
derivatives of this class; c) the enantiospecific self-assembly
of columnar arrangements in the solid state; and d) the
photochemical [2+2] annulations of phosphole-fused [6]hel-
icenes into dimeric helical structures.
When targeting helicenes where phosphorus is embedded
within the helical framework itself, we envisioned 1H-
phosphindole oxides as suitable building blocks and have
adapted the classical helicene synthesis, based on the photo-
chemical cyclization/dehydrogenation of diarylolefins, to
these substrates.[14] Our starting materials are the olefins 5
which combine a 1H-phosphindole unit and a benzo[c]phe-
nanthrene fragment. They have been prepared according to
a known procedure,[15] by using two consecutive palladium-
promoted Suzuki couplings. First the alkene 1 is coupled with
2-bromobenzo[c]phenanthrene (2)[16] and the resulting prod-
uct 3 is then coupled with either the 1H-phosphindol-5-yl
tosylate 4a or 4b[17] (Scheme 1).
[*] K. Yavari, Dr. P. Retailleau, Dr. A. Voituriez, Dr. A. Marinetti
Institut de Chimie des Substances Naturelles, CNRS UPR2301
1, av. de la Terrasse, 91198 Gif-sur-Yvette (France)
E-mail: angela.marinetti@icsn.cnrs-gif.fr
Dr. S. Moussa, Prof. B. Ben Hassine
Laboratoire de Synthꢀse Organique Asymꢁtrique et Catalyse
Homogꢀne, Facultꢁ des Sciences, 5019 Monastir (Tunisia)
[**] We thank the Institut de Chimie des Substances Naturelles and the
COST Actions CM0802 “ PhoSciNet ” for supporting this research.
Scheme 1. Synthesis of the olefinic substrates 5 by palladium-pro-
moted Suzuki couplings. Men*=(1R,2S,5R)-2-isopropyl-5-methylcyclo-
hexyl, SPhos=2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl,
Tf =trifluoromethanesulfonyl, THF=tetrahydrofuran.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202024.
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Of the two 1H-phosphindole oxides used in this work, 4b
displays a phenyl substituent on phosphorus, and 4a displays
a chiral l-menthyl group. The P-menthyl-substituted phos-
phindole oxide (RP)-4a was obtained in diastereomerically
pure form by chromatographic separation of a mixture of
epimers with opposite configurations at the stereogenic
phosphorus center. The RP configuration of 4a has been
assigned by X-ray crystal studies.
initially formed [6]helicene (RP,P)-6a. According to this
hypothesis, compounds 7a,a’ could be conveniently prepared
by photolysis of the pure helicene (RP,P)-6a, as shown in
Scheme 3 (25 min, 75% total yield, 1:1 ratio).
The olefins 5 have been converted into helical derivatives
by the oxidative photocyclization method illustrated in
Scheme 2. A priori, photocyclization of 5 might afford four
different isomers of the polyaromatic structures depending on
Scheme 3. Photochemical dimerization of the helical phosphindole
oxide (RP,P)-6a.
The solid-state structure of compound 7a was unambig-
uously established by the X-ray crystal structure shown in
Figure 2. X-ray data demonstrate a head-to-head dimeriza-
tion of (RP,P)-6a through a [2+2] cyclization of the olefinic
functions. The two homochiral helical units, which have
P configurations, are connected by a cyclobutane moiety with
a (R,S,S,R) configuration, while the stereogenic phosphorus
center displays an R configuration ([a]D =+ 1505, (c = 1,
CHCl3)).
Scheme 2. Diastereoselective synthesis of a phosphindole-based [6]hel-
icene.
the carbon atoms that are involved in the ring closure, that is,
either carbon atoms b or b’ for the phosphindole fragment,
and either a or a’ for the benzo[c]phenanthrene unit. More-
over, starting from (RP)-5a, the resulting helical structures
might be produced as mixtures of two diastereomers with
opposite relative configurations of the stereogenic phospho-
rus center and the helical scaffold. Gratifyingly, the photo-
cyclization of olefins 5 proved to be regio- and stereochem-
ically controlled in such a way that only one or two, out of six
possible isomers, were typically obtained. For instance, UV
irradiation of a solution of the l-menthyl-substituted sub-
strate (RP)-5a in cyclohexane for 1 hour afforded (RP,P)-6a as
the major product [27% yield upon isolation; 31P NMR: d =
61 ppm; [a]D =+ 1860, (c = 1, CHCl3)].[18]
À
The phosphine oxide (RP,P)-6a results from C C bond
formation between the carbon atoms a and b of the starting
olefin. In the 1H NMR spectrum the phosphole moiety shows
peaks at d = 6.27 ppm (dd, JH-P = 24.5 Hz, JH-H = 8.5 Hz, H-9)
and d = 7.32 ppm (dd, JH-P = 35.5 Hz, JH-H = 8.5 Hz, H-8).
These signals are not significantly shifted with respect to the
starting material. The terminal phenyl ring shows peaks at d =
6.77 ppm (t, JH-H = 7.0 Hz) and d = 7.18 ppm (t, JH-H = 7.5 Hz),
which are significantly shifted to high-field values with respect
Figure 2. X-ray crystal structure of the dimeric phosphahelicene (RP,P)-
7a. For reasons of clarity the pendant alkyl groups are in gray. Thermal
ellipsoids are shown at 30% probability.
The second compound 7a’ ([a]D =+ 1150, (c = 1, CHCl3))
is assumed to be the epimer of the head-to-head dimer, with
(S,R,R,S) configuration of the cyclobutane ring. This assign-
ment is based on NMR data and mass spectrometry since
crystals suitable for X-ray studies could not be obtained.
The dimerization reaction in Scheme 3 demonstrates that
the presence of a phosphole unit at the end of helical
sequences opens the way to unprecedented extended helical
structures. To the best of our knowledge, [2+2] dimerizations
of analogous heterohelicenes have never been mentioned in
the literature, although they are easily anticipated to take
to the starting material [(RP)-5a: d = 7.47 ppm (t, JH-H
7.5 Hz) and d = 7.57 ppm (t, JH-H = 7.2 Hz)].
=
NMR analysis of the crude reaction mixture of the
photocyclization experiment in Scheme 2, showed the pres-
ence of two minor products, 7a,a’, whose amounts increased
after prolonged UV irradiation of the mixture. Based on the
well-known behavior of phospholes, benzothiophenes, and
analogous heterocycles under photochemical conditions,[19]
we postulated that such minor products might result from
an intermolecular photochemical [2+2] cyclization of the
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place from a variety of five-membered heterocycles under
photochemical conditions.
cene-type structure 8b. Similar to the packing of the rac-
helicene described by Nozaki and co-workers,[13b] crystals of
rac-8b display a columnar arrangement, aligned with the
helical axis, in which each column is made of a single
enantiomer (see the Supporting Information). Both phospha-
helicenes 6b and 8b have been obtained in enantiomerically
pure form by SFC on columns having a chiral stationary phase
(see the Supporting Information).
An intriguing feature of the photochemical cyclization/
dehydrogenation reactions is that they always afford the
corresponding helical compounds with relative RP*,P*-helical
configurations as the unique (or largely predominant) diaste-
reomer. Unexpectedly, in these diastereisomers the phospho-
rus substituents are oriented toward the polyaromatic scaf-
fold, that is, in the most hindered space region. This sense of
stereochemical control is not well understood thus far. It
seems to have, however, a kinetic origin, as suggested by the
experiments shown in Scheme 5.
In the crude reaction mixture of the photochemical
cyclization in Scheme 2, compounds (RP,P)-6a, 7a, and 7a’,
and the residual starting material represent a total of 75% of
the molar amount. A nonhelical side product is also observed
(15%). This mass balance demonstrates that the photocycli-
zation reaction takes place with a high level of diastereose-
lectivity with the screw sense of the helical chirality being
efficiently controlled by the chiral substrate. We will see
hereafter that this stereochemical control does not relate to
the presence of the chiral menthyl group on phosphorus, since
the same control operates in the case of P-phenyl-substituted
phosphindoles. It is the R-configured stereogenic phosphorus
center of (RP)-5a that induces a P helical configuration in the
final products.
In additional experiments, the racemic P-phenylphosphin-
dole oxide 5b was subjected to the photochemical cyclization
reaction under standard reaction conditions. This substrate
undergoes the expected cyclization at a lower rate compared
to that of the P-menthyl-substituted derivative (RP)-5a. As
a consequence, the photochemical [2+2] dimerization starts
well before total consumption of the substrate and the
reaction affords mixtures of products, with product ratios
depending upon the reaction time. From these mixtures we
have isolated the three compounds shown in Scheme 4. The
X-ray crystal structures for the three compounds are given in
the Supporting Information.
Scheme 5. Reduction of the phosphindole oxide (RP*,P*)-6b and sub-
sequent oxidation leading to an epimeric mixture. Thermal ellipsoids
are shown at 30% probability.
Phosphindole oxide (RP,P)-6b (d 31P NMR = 42 ppm) was
reduced with a HSiCl3/Et3N mixture to give the correspond-
ing trivalent phosphine as a mixture of two isomers, 9b,b’
(d 31P NMR = 0.2 and À2.4 ppm), in a 1:0.8 ratio. These
epimeric phosphines are assumed to equilibrate slowly,
because of the comparatively low pyramidal inversion
barriers of benzophospholes with respect to other phos-
phines.[20] The subsequent oxidation of the mixture with H2O2
produced the two diastereomers of the phosphahelicene
oxide with opposite phosphorus configurations, ((RP,P)-6b
and (SP,P)-6b’; d 31P NMR = 42 and 40 ppm, respectively) in
a 1:1 ratio. X-ray diffraction studies afforded direct evidence
for the molecular structure of the (SP*,P*)-configured oxide
6b’ (Scheme 5).[21]
Scheme 4. Helical phosphindole derivatives obtained from rac-5b
under oxidative photocyclization (I2, cyclohexane, propylene oxide, hn
150 Watt). X-ray crystal structure of (RP,P)-6b. Thermal ellipsoids are
shown at 30% probability.
Compounds 6b and 7b are the analogues of the P-
menthyl-substituted compounds (RP,P)-6a and 7a, respec-
tively. The new compound 8b is a phospha[7]helicene
À
obtained through C C bond formation between the carbon
This experiment demonstrates that both epimers of the
phosphine oxide can be formed and seems to be equally
favored. Thus, the preferential formation of (RP*,P*)-6b
under photochemical conditions does not correlate with
atoms a and bꢀ of the olefinic precursor (Scheme 2). The
helical pitch of 8b (3.33 ꢁ) is slightly larger than that of 6b
(3.12 ꢁ), thus suggesting an increased strain of the [7]heli-
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Chem. 2010, 122, 103 – 106; Angew. Chem. Int. Ed. 2010, 49, 99 –
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a possible stabilization of the final product. An alternative
rational to the observed diastereoselectivity might be a pref-
erential helical folding of the substrate, driven by intra-
molecular p stacking or steric repulsions, which would be
locked then in the photocyclization process.[22]
In summary, we have used 1H-phosphindole as a new
platform to access polyaromatic helicene scaffolds terminat-
ing with a heterocyclic moiety. We have shown that, in the
photochemical synthesis of these new systems, helicity can be
controlled by taking advantage of the stereogenic phosphorus
atom. Moreover, the reactive olefinic function of the phos-
phole unit allows unprecedented dimeric helicene derivatives
to be accessed. Further studies will be oriented toward uses of
these new phosphahelicenes in organometallic chemistry and
catalysis.
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Typical photocyclization procedure: Iodine (0.4 mmol, 157 mg),
propylene oxide (20 mmol, 1.4 mL), and cyclohexane (350 mL)
were added to (Z)-5-(5-(benzo[c]-phenanthren-2-yl)oct-4-en-4-yl)-1-
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