Stereospecific synthesis of chiral P-containing polyaromatics based on 7-membered P-rings

Article abstract of DOI:10.1039/d1cc02293h

We present the stereospecific synthesis of helicenoid-based phosphepines (7-membered P-rings) as well as chiral P-containing polycyclic aromatic hydrocarbons. In these systems, an axial to central chirality transfer takes place from the BINAP moiety to th

Full text of DOI:10.1039/d1cc02293h

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Stereospecific synthesis of chiral P-containing
polyaromatics based on 7-membered P-rings†
Cite this: Chem. Commun., 2021,
57, 7256
abc
Reka Mokrai,
´
Elsa Caytan,
Anabella Mocanu,a Matthew P. Duffy,a Thomas Vives,a
Vincent Dorcet,a Thierry Roisnel,
Laszlo Nyulaszi,
a
a
bc
Received 29th April 2021,
Accepted 21st June 2021
´ ´ ´
Zoltan Benko, *b Pierre-Antoine Bouit *a and Muriel Hissler
*
a
+
´
DOI: 10.1039/d1cc02293h
rsc.li/chemcomm
We present the stereospecific synthesis of helicenoid-based phos- or OFETs. The 7-membered P-ring is a potential source of
phepines (7-membered P-rings) as well as chiral P-containing chirality that has not been exploited so far in the field of
polycyclic aromatic hydrocarbons. In these systems, an axial to molecular materials.7 In the present article, we describe the
central chirality transfer takes place from the BINAP moiety to the stereospecific synthesis of helicenoid-based phosphepine as
P-atom. The impact of the molecular design on the structure, the well as a chiral P-containing PAH through an axial to central
(chir)optical (including circularly polarized luminescence) and chirality transfer from readily available 2,20-bis(diphenyl-
redox properties are investigated.
phosphino)-1,10-binaphthalene (BINAP). The impact of the
molecular design on the structure, the (chir)optical and redox
Polycyclic aromatic hydrocarbons (PAHs) with fused-benzene properties are investigated using a joint experimental/theore-
rings have been widely investigated due to their potential tical approach, highlighting the potential of these chiral deri-
applications in optoelectronic devices.1 The properties of PAHs vatives for further applications in optoelectronics or catalysis.
are determined by their molecular structure which is mainly
defined by their fusion pattern: ortho- and peri-fused systems originally for
For the synthesis, we adapted Widhalm’s strategy, developed
racemic phosphepine-based helicenoid
a
possess a polyaromatic framework which can be planar structure ((RP,M)-1/(SP,P)-1, abbreviated rac-1 for simplicity,
(PAH A)1 or helicoidal (PAH B) (Fig. 1).2 The insertion of non- Scheme 1).8–11 Using enantiopure BINAP-oxides as starting
benzenoids rings in the framework is another way to tune the materials, we observed that the reaction is fully stereospecific
properties since a deviation of the planarity (for example C, (see ESI† for chiral supercritical fluid chromatography (SFC))
Fig. 1) modifies the p-systems leading to potentially chiral and, thus, afford enantiopure compounds (Rp,M)-1 in 54% yield
compounds.3 In most of the cases, stereospecific access to such and (Sp,P)-1 in 63% yield. Enantiomers of 1 were fully
compounds is difficult. In the meantime, chiral p-conjugated
systems appeared as promising building blocks in organic
electronics, nanotechnology, sensing or catalysis.4 Another
approach to tune the properties of p-systems is to introduce
heterocycles within the C-framework.5 Recently, the phosphe-
pine ring (7-membered unsaturated P-ring, D, Fig. 1) also
gained attention for its potential applications in opto-
electronics.6 Its distorted framework allows the preparation of
twisted p-systems that were successfully introduced into OLEDs
a Univ Rennes, CNRS, ISCR–UMR 6226, Rennes F-35000, Hungary.
E-mail: muriel.hissler@univ-rennes1.fr
b Department of Inorganic and Analytical Chemistry,
´
´
Budapest University of Technology and Economics, Szt. Gellert ter 4,
Budapest H-1111, Hungary. E-mail: zbenko@mail.bme.hu
c
´
´
MTA-BME Computation Driven Chemistry Research Group. Szt. Gellert ter 4,
Budapest H-1111, Hungary
Fig. 1 Up: Example of fully benzenoid planar PAH (A) and chiral (B); chiral
PAH featuring 7-membered ring (C). Down: PAH featuring 7-membered
P-ring (D) and structures (E) studied in this article.
† Electronic supplementary information (ESI) available. CCDC 1921440, 1921441,
1921442, 1921443, 1921466, 1921447 and 1921449. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d1cc02293h
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Fig. 2 X-Ray crystallographic structure of (RP, M)-1 (left) and (RP, M)-2
(right).
(Fig. S9 and S10, ESI†). This confirms the conformational
stability of the compounds even in their trivalent form and,
thus paves the way toward the stereospecific synthesis of a great
diversity of chiral coordination complexes.
Scheme 1 Synthesis of 1–3.
The molecular structures of derivatives 1–3 were all con-
characterized by multinuclear NMR spectroscopy and MS firmed by single-crystal X-ray diffraction (Fig. 2). The study will
analysis and additionally by single-crystal X-ray diffraction. focus on 1 and 2 as the structure of complex 3 is similar to 1. In
Based on X-ray analysis (vide infra), the use of (R)-BINAP leads all of these compounds, the s4,l5-P atom is in a tetrahedral
to the formation of (RP, M)-1 in which the P center has a R coordination environment with usual valence angles and C–P
configuration and the helicenoid backbone forms a left-handed bond lengths. As reported by Widhalm7 rac-1 crystallizes as a
helix and thus adopts a M configuration. The (S)-BINAP leads to conglomerate and the analyzed crystal revealed the presence of
the formation of (SP, P)-1 in 63% yield. In these systems, an only the (RP, M)-enantiomer. The X-ray structures of (RP, M)-1
axial to central chirality transfer takes place as the BINAP and (SP, P)-1 confirm the helicenoid arrangement of the rings
moiety controls the stereochemistry of the P-atom.12
with a ‘‘helicity’’ angle of respectively 68.21 and 68.11 (angle
The binaphthyl backbone of rac-1 was further fused in between the two rings at each extremity of the helix, repre-
‘‘Scholl conditions’’ to afford the perylene fused phosphepine sented here in blue and red, Fig. 2). The seven-membered rings
((RP,M)-2/(SP,P)-2, abbreviated rac-2) in 63% yield (Scheme 1). present the expected boat-like conformation with the phenyl
The reaction carried out with the enantiopure starting materi- substituents on the P-atom in axial position. Intramolecular
als affords (RP,M)-2 (61%) and (SP,P)-2 (50%) with retention of p-stacking between this exocyclic P-phenyl and one of the
the configuration at the P and the helicenoid part (see X-ray). naphthyl fragments (d = 3.55 Å for (RP, M)-1 and d = 3.56 Å
Again, the enantiopurity was checked by chiral SFC.
for (SP, P)-1) takes place (Fig. S11, ESI†). The structure of rac-2
The conformational stability of these chiral compounds was consists of a racemic mixture of the two enantiomers. This
confirmed as no sign of epimerization was observed upon derivative is a contorted PAH featuring a negative curvature
heating the compounds at 140 1C (for 2.5 hours) during the with angles between the phenyl ring situated at the 2–3 (in
synthesis. For both 1 and 2 scaffolds, we have calculated the green, Fig. 2) and 6–7 position (in orange, Fig. 2) of 111.21 and
activation barriers for the inversion of configuration of the PAH angle between the phenyl at the 4–5 position (in purple) and the
backbone and the resulting products at the B3LYP-D3/cc-pVDZ mean phosphepine plane of 149.31 in the opposite direction.
level. The inversion barrier for the helical backbone in the Again, the seven-membered ring displays a boat-like conforma-
case of (SP,P)-1 (leading to (SP,M)-1) is 46.2 kcal molꢀ1
,
tion with the P-phenyl substituents in axial position. In this
which is larger than the racemization barrier of [5]helicene case, no intramolecular p-stacking is observed. This structural
(Ea = 23.5 kcal molꢀ1) or [6]helicene (Ea = 35.0 kcal molꢀ1
(Fig. S29–S31 and Table S4, ESI†).13
)
part highlights the fact that molecular engineering of polyaro-
matic phosphepines allows preparing derivatives with a great
This confirms that insertion of heptagons allows obtaining structural variety such as helicenoid derivatives and negatively
conformationally stable [5]helicene without functionalization curved PAH.
in the fjord position (unlike carbo[5]helicenes).3c,14 The for-
The spectroscopic properties of all compounds were inves-
mation of the new C–C bond decreases the inversion barrier for tigated in diluted CH2Cl2 solutions (c = 5 ꢁ 10ꢀ6 mol Lꢀ1, Fig. 3
(RP,M)-2 to 32.5 kcal molꢀ1
.
and Table S2, ESI†) at room temperature. rac-1, (SP, P)-1 and
To illustrate the versatility of the synthetic approach and (RP, M)-1 display almost identical UV/Vis spectra consisting of a
offer post-functionalization possibilities, phosphepine oxide strong absorption band at 260 nm (e = 3.6 ꢁ 104 Mꢀ1 cmꢀ1) and
rac-1, (RP, M)-1 and (SP, P)-1 were reduced to the corresponding several bands of medium and low intensity (4 ꢁ 103 Mꢀ1 cmꢀ1
s3, l3-analogues. These intermediates were directly complexed o e o 14 ꢁ 103 Mꢀ1 cmꢀ1) from 282 to 334 nm (Fig. 3). TD-DFT
to AuI to afford rac-3, (RP, M)-3 and (SP, P)-3 by reacting with calculations employing different functionals (B3LYP, oB97XD,
Me2SAuCl (Scheme 1). The enantiopurity and the retention M06-2X, cam-B3LYP) and ADC(2) calculations predict similar
of the configuration (both the helicenoid fragment and the excitation energies as observed in the experimental spectrum.
P-atom) were confirmed by chiral SFC and X-ray analysis The best overall fit was found at the TD-B3LYP/6-31G* level,
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Fig. 4 ECD spectra of (RP, M)-1 (red), (SP, P)-1 (red, dotted) - (RP,M)-2,
(green) (SP,P)-2 (green, dotted) in diluted DCM at 10ꢀ5 M.
cases, the lowest energy ECD bands are attributed to HOMO–
LUMO transitions. (RP, M)-1 (resp. (SP, P))-1 display also CPL
with glum B 2 ꢁ 10ꢀ3 at 385 nm (Fig. S27, ESI†). These values
are rather classical for organic CPL emitters16 but higher than
previously reported organophosphorus based-organic CPL
Fig. 3 UV-Vis absorption and normalized emission spectra of (SP, P)-1
(dotted) and (RP, M)-1 (solid line) in CH2Cl2 (c = 5 ꢁ 10ꢀ6 M). HOMO and emitter.17 This first result thus paves the way toward the
LUMO of 1 (up) UV-Vis absorption and normalized emission spectra of
introduction of optimized helical phosphepine into chiroptical
devices.
(SP, P)-2 (dotted) and (RP, M)-2 (solid line) in CH2Cl2 (c = 5 ꢁ 10ꢀ6 M).
HOMO and LUMO of 2 (down) (calculations at the B3LYP/6-31G*//B3LYP/
Rac-2, (SP, P)-2 and (RP, M)-2, having a smaller ‘‘helicity’’
6-31+G* level).
angle (vide supra) and thus a larger p-conjugated platform than
their helicenoid precursor, exhibit markedly different optical
which shows two strong absorption bands at 263 and 256 nm and redox properties. The UV-vis absorption spectra of (RP,M)-2,
(oscillator strength: f = 0.137 and f = 0.111, respectively) as well (SP,P)-2 and rac-2 display a structured absorption band in the
as four closely spaced transitions with lower oscillator strength visible (lmax = 441 nm, Fig. 3), characteristic of their perylene-
in the 331 and 272 nm region (for details see ESI†). At this level based structure (Fig. S17, ESI†). The TD-DFT calculations show
of theory, the S1 state (at 331 nm) corresponds to a HOMO– that this absorption is attributed to an intense p–p* HOMO–
LUMO transition (Fig. 3) involving the p system with a modest LUMO transition at 442 nm (f = 0.238) of the perylene backbone
oscillator strength (f = 0.086). The absorption bands are asso- without any significant contribution of the P-atom (Table S8,
ciated with several, near lying p–p* electronic transitions of the ESI†). The red-shift is easily rationalized by the extension of the
helicenoid backbone without contribution of the P-atom indi- conjugation between compounds 1 and 2. Compounds 2 dis-
cating that these properties are predominantly determined by play strong luminescence in solution with the same gradual
the helical p-electron system of such compounds, in accor- red-shift (lem (rac-1) = 371 nm and lem (rac-2, (RP,M)-2,
dance to the non aromatic NICS values of the seven membered (SP,P)-2) = 461 nm). It is noteworthy that 2 displays excellent
ring as shown in Fig. S33 of the ESI.†
fluorescence quantum yield in solution (f = 79%). Interest-
The emission properties of rac-1, (SP, P)-1 and (RP, M)-1 were, ingly, despite its polyaromatic platform, 2 also luminesce in the
then, examined in CH2Cl2 at a concentration of 10ꢀ5 M. At RT, solid-state without noticeable impact of the chirality (Fig. S19,
all compounds behave as blue fluorescent emitters, with emis- ESI†). (RP,M)-2 displays a strong negative structured ECD band
sion wavelengths at 371 nm and a quantum yield of 18% in (De = ꢀ50.4 Mꢀ1 cmꢀ1 at 313 nm) followed by another negative
solution. The insertion of a phosphepine in the helicenoid structured band in the visible (De = ꢀ8.31 Mꢀ1 cmꢀ1 at 440 nm)
structure induced a blue-shift of the emission maximum and (Fig. 4). The spectra were nicely fitted by DFT calculations.
increased emission performance compared to [5]helicene Again, the lowest energy ECD bands are attributed to HOMO–
(lem = 424 nm, f = 4%).15 All compounds also luminesce as LUMO transitions. This red-shift compared to 1 is again
powders (Fig. S18, ESI†). To get more insights into the impact attributed to a larger p-platform. In addition, the CPL of
of chirality on the properties of these systems, electronic RP,M-2 (resp. SP,P-2) was too weak to be accurately
circular dichroism (ECD) spectra have been recorded in diluted measured (glum o 10ꢀ4). The weaker chiroptical properties of
DCM solutions. Firstly, (RP, M)-1 and (SP, P)-1 displayed ECD 2 (gabs = 5 ꢁ 10ꢀ4 at 442 nm; glum o 10ꢀ4) compared to
with the expected mirror-image relationship (Fig. 4). For exam- helicenoids 1 (gabs = 2 ꢁ 10ꢀ3 at 330 nm; glum = 2 ꢁ 10ꢀ3 at
ple, (RP, M)-1 displays a strong negative ECD band (De = ꢀ99.0 385 nm) are in accordance with the fact that these p–p*
at 252 nm) and a positive band (De = 6.4 Mꢀ1 cmꢀ1 at 330 nm). transitions are localized mainly on the nearly planar perylene
The spectra were nicely fitted by DFT calculations. In both backbone (Fig. 3) and are rather far from the source of chirality.
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However, these data nicely illustrate that the presence of the Notes and references
7-membered P-ring endows the perylene scaffold with appeal-
ing chiroptical properties.
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In the present article, we describe the stereospecific synth-
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complexes 3, and chiral P-containing PAH 2. In these systems,
an axial to central chirality transfer takes place from the BINAP
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the structure, the (chir)optical and redox properties are inves-
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This work is supported by the MESR, the CNRS, the Region
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¨
´
Progressio Alapıtvany, Tempus Kozalapıtvany, Janos Bolyai
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´
Research Fellowship, UNKP 20-5-BME-317, TET_16-1-2016-
0128, NRDI Fund (TKP2020 IES,Grant No. BME-IE-NAT), PICS
SmartPAH (08062)-MTA NKM-44/2019, China-French AIL in
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Conflicts of interest
There are no conflicts to declare.
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