Confining Nitrogen Inversion to Yield Enantiopure Quinolino[3,2,1-k]Phenothiazine Derivatives

Article abstract of DOI:10.1002/adfm.201803140

The first examples of optically pure 9H-quinolino[3,2,1-k]phenothiazine (QPTZ)-based molecules are reported. The inversion of the nitrogen atom, locked in the QPTZ fragment, is confined with a high-energy barrier that has allowed the isolation at room temperature of each stereoisomer. Considering the growing necessity in molecular electronics to introduce chiral characteristics within highly efficient molecular fragments, a molecular strategy to generate enantiopure derivatives constructed on the very promising electron rich core QPTZ is provided. As this work aims to report the foundations of the QPTZ chirality, the present findings may open avenues towards the use of this fragment in its optically pure form.

Full text of DOI:10.1002/adfm.201803140

FULL PAPER
Nitrogen Inversion
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Confining Nitrogen Inversion to Yield Enantiopure
Quinolino[3,2,1-k]Phenothiazine Derivatives
Cassandre Quinton, Lambert Sicard, Nicolas Vanthuyne, Olivier Jeannin,
and Cyril Poriel*
molecular orbital level and of the spin–
orbit coupling) has been demonstrated.
The first examples of optically pure 9H-quinolino[3,2,1-k]phenothiazine
QPTZ)-based molecules are reported. The inversion of the nitrogen atom,
[5]
(
The QPTZ fragment incorporated in
spiroconfigured materials has thus led
to highly efficient blue phosphorescent
OLEDs, displaying better performance
than those obtained with the widely used
locked in the QPTZ fragment, is confined with a high-energy barrier that has
allowed the isolation at room temperature of each stereoisomer. Considering
the growing necessity in molecular electronics to introduce chiral charac-
teristics within highly efficient molecular fragments, a molecular strategy to
generate enantiopure derivatives constructed on the very promising electron
rich core QPTZ is provided. As this work aims to report the foundations of
the QPTZ chirality, the present findings may open avenues towards the use of
this fragment in its optically pure form.
[5]
phenylacridine. In the present work, we
wish to focus on the dynamic behavior of
the nitrogen atom in QPTZ-based mole-
cules. Indeed, N-pyramidal amines with
three different substituents attached to
the nitrogen atom can be optically active
if there is a thermodynamically predomi-
nant conformer in the conformational
equilibrium, but the resulting conformers often cannot be iso-
1
. Introduction
[
7]
lated as the nitrogen inversion barrier is too low. In fact, for
configurational stability to be observed, the nitrogen inversion
The incorporation of chirality in highly efficient molecular
fragments is one of the chief future directions for all three
types of organic electronics devices, namely organic light-
−
1
energy barrier should be higher than 100 kJ mol at room tem-
[1]
[8]
perature. This value can be reached by inserting the nitrogen
emitting diodes (OLEDs), field effect transistors,[3] and solar
cells.[ The incorporation of a chiral compound in device can
enable the detection and emission of chiral light or a enhance
charge transport.[ However and despite recent promising
studies, only a few examples have been reported to date. This
is partly due to the limited number of chiral fragments dis-
playing a high efficiency when incorporated in devices. Two
years ago, a new electron rich core, 9H-quinolino[3,2,1-k]phe-
nothiazine (QPTZ), has appeared in literature displaying its
[2]
atom into a ring, adding strain in the planar transition state.
[9]
4]
Interestingly, in the rigid QPTZ fragment investigated herein,
the confinement of the nitrogen atom within a dissymmetric
bridged structure increases the energy of this inversion bar-
rier which therefore enables the isolation of the resulting iso-
mers. Moreover, the sulfur atom seems to contribute to the
structural and chiral characteristics described herein. Indeed,
the sulfur/carbon linkage rigidifies the system compared to the
phenylacridine fragment but retains enough flexibility, unlike
1]
[5]
[10]
potential in the field of phosphorescent OLEDs. This frag-
ment consists of a phenylacridine core[ to which a sulfur
bridge was added between the top and side phenyl rings. In
this previous work, the crucial role played by this sulfur atom
in the electronic properties (i.e., raising of the highest occupied
indoloacridine, to allow the inversion of the nitrogen atom
6]
when heated. Note that a structurally related analogue pos-
sessing an oxygen atom instead of the sulfur atom has been
[
11]
reported with nevertheless no mention of nitrogen inversion.
Thus, this work presents our investigations on the nitrogen
inversion of the QPTZ fragment and the first examples of opti-
cally pure QPTZ-based molecules (1–3, Figure 1) described in
literature. Constraining the nitrogen atom in a pyramidal con-
figuration at room temperature appears hence as an efficient
strategy to generate enantiopure compounds. Furthermore, the
spiro carbon of spirofluorene-based compounds also allows the
Dr. C. Quinton, L. Sicard, Dr. O. Jeannin, Dr. C. Poriel
Univ Rennes
CNRS
ISCR-6226, F-35000 Rennes, France
E-mail: cyril.poriel@univ-rennes1.fr
[
12]
introduction of chirality in such systems.
Dr. N. Vanthuyne
Aix Marseille Univ
CNRS
Centrale Marseille
iSm2, Marseille F-13397, France
Three molecular models based on the fluorene-spiro-QPTZ
F-s-QPTZ) architecture have been chosen for this study: the
(
unsubstituted F-s-QPTZ (2) and two substituted F-s-QPTZ pos-
sessing different substitution patterns (1 and 3) in order to
investigate the impact of the steric hindrance on the nitrogen
inversion. Considering electronic device applications for the
QPTZ core, it is important to know whether the inversion of
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201803140.
DOI: 10.1002/adfm.201803140
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Figure 1. Molecular structures of the investigated molecules. a) 1, b) 2, and c) 3 and their relationships obtained by molecular modelling (optimized
structures with b3lyp/6-31g(d)). The view angle chosen to highlight the phenothiazine flip shows the two phenyl rings of the acridine moiety and the
two sigma bonds of the spiro bridge as superimposed (different views of 2N can be found Figure S48 in the Supporting Information). Sulfur atom
R
in yellow and nitrogen atom in blue.
the nitrogen atom can occur or not and, if it can, how quickly
and at which temperature.
1-phenyl-9-fluorenone for 1, 9-fluorenone for 2, 3-phenyl-
9-fluorenone for 3 (Scheme 1). The corresponding fluorenols
were not isolated and were involved in an intramolecular elec-
trophilic cyclisation reaction to give the three targets 1–3 in
good yields.
2
. Results and Discussion
After synthesis, the H and 13C NMR spectra of the three
compounds 1–3 provide the first piece of interesting data (see
Figures S49–S68 in the Supporting Information). Indeed,
3 displays a complex spectrum, characteristic of a mixture of
diastereoisomers, very different to that of the unsubstituted 2
and that of the 1-substituted 1.
1
Compounds 1–3 were readily obtained through a short and
efficient synthetic approach by coupling the halogeno-aryl
1
0-(2-bromophenyl)-10H-phenothiazine (obtained from the
copper catalysed Goldberg coupling of phenothiazine and
-bromoiodobenzene), with the corresponding 9-fluorenone:
2
Scheme 1. Syntheses of compounds 1–3.
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The presence of diastereoisomers in 3 suggests at least
two stereogenic centers, identified in our case as the nitrogen
atom and the spiro carbon (the fluorene being dissymmet-
rical). These stereochemical features have been confirmed by
chiral high pressure liquid chromatography (HPLC) studies
phenothiazine core. Indeed, the QPTZ moiety is not flat due
to the intense deformations induced by the phenothiazine and
acridine cores (Figure 2). In fact, the nitrogen atom is pyramidal
with values of nitrogen torsion angle found between 157° for
3N C and 3N C and 162° for 3N C (see Figure S17 and Table
R
S
S
R
S S
(see Figures S2, S4, and S6 in the Supporting Information),
S3 in the Supporting Information), as expected for nitrogen with
[13]
which have allowed to separate and isolate each diastereoi-
one or more aromatic substituents. This shows that the triva-
lent nitrogen in these compounds is in an intermediate hybridi-
somer of 3: 3N C , 3N C , 3N C , and 3N C (Figure 1c).
R
R
S
S
R
S
S R
2
3
X-Ray crystallography of the two diastereoisomers 3N C and
zation state between sp and sp . Thus, the phenothiazine can
move from one side of the fluorene to the other through a
structural reorganization of the acridine, much like a wing flap-
ping. The same kind of stereoisomers would
S
S
3
N C (Figure 2c) clearly confirms the inversion of the nitrogen
R S
atom in the QPTZ fragment, which is caused by a flip of the
have been expected for 1 which displays the
same two stereogenic centres. However,
only two stereoisomers (enantiomers) have
been isolated by chiral HPLC: 1N CS and
R
1
N C (Figures 1a and 2a). Indeed, the steric
S R
hindrance induced by the pendant phenyl
ring in position 1 of the fluorene prevents
the flipping of the phenothiazine above dis-
cussed and hence the formation of the other
pair of enantiomers. This structural feature
provides an efficient diastereocontrol and
can be easily visualized on the X-ray struc-
tures of the two enantiomers 1N CS and
R
1
N C (Figure 2a). The phenyl ring in C1
S R
displays two important characteristics. First,
it is almost orthogonal to the fluorene with
a dihedral angle between the mean plane of
the phenyl ring in C1 and that of the fluorene
of 83.3° for 1N CS and of 84.1° for 1N CR
R
S
suggesting a strongly sterically hindered
environment (see Figures S13 and S14 in
the Supporting Information). This structural
characteristic is at the origin of the emer-
gence of one-substituted fluorenes recently
[14]
reported in literature. In addition, the pen-
dant phenyl ring is very close to the QPTZ
fragment (three C/C intramolecular distances
are indeed shorter than the sum of the Van
der Walls radii, see Figures S15 and S16
in the Supporting Information).
Finally, the case of 2 provides a different
example. For this molecule, in which the
unsubstituted fluorene is symmetrical,
the only stereogenic centre is the nitrogen
atom. This feature, clearly observed in the
X-ray structure of the racemic mixture of
2
leads to two enantiomers 2NS and 2NR
(
Figure 2b), which have been successfully
isolated by chiral HPLC (see Figure S2 in
the Supporting Information). The assign-
ment of the absolute stereochemistry was
performed for each enantiomer of 1 and
3
by crystallography (Figure 2a,c and see
Figures S7–S10,S12 and Tables S1–S2 in the
Supporting Information) and confirmed by
the comparison of experimental and theo-
retical circular dichroism (CD) spectra (see
Figure 2. X-ray structures of a) 1N C and 1N C , b) 2N and 2N , and c) 3N C and 3N C The
R
S
S
R
R
S
R
S
S S.
view angle chosen to highlight the phenothiazine flip shows the two phenyl rings of the acridine
moiety and the two sigma bonds of the spiro bridge as superimposed. Sulfur atom in yellow
and nitrogen atom in blue.
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Figures S31–S33 in the Supporting Information). In the case of
hindrance induced by the presence of the phenyl ring in
position 1. Finally, it should be stressed that 1–3 present the
same electronic properties.
2
N and 2N , the stereochemistry has been assigned, thanks to
S R
their CD spectra (Figure 3).
These spectra are mirror images of each other and in good
coherence between experimental and theoretical studies. The
CD spectrum of 2N (2N ) displays i) a small negative (positive)
band at 357 nm, ii) a small positive (negative) band at 325 nm,
iii) an intense negative (positive) band at 268 nm, iv) an intense
positive (negative) band at 251 nm, v) a small negative (positive)
band at 225 nm, and vi) an intense negative (positive) band at
The key feature of this work is related to the nitrogen inver-
sion energy barrier in the constrained structure of the QPTZ
fragment, which is high enough to enable the isolation at room
temperature of the different stereoisomers of the presented
compounds. In order to evaluate this barrier, the mechanism
of the nitrogen inversion has been studied by a combina-
tion of experimental (NMR and HPLC) and theoretical tech-
niques. Thus, the kinetic investigations of the nitrogen inver-
sion of 3N C into its diastereoisomer 3N C have been first
S
R
2
03 nm (Figure 3).
To sum up, compound 2 displays only the nitrogen atom as
R
R
S R
stereogenic center (and thus two enantiomers), 3 displays both
performed by measuring through chiral HPLC the evolution of
the spiro carbon and the nitrogen atom as stereogenic centers
the concentration of a solution of 3N C in ethanol over time at
R
R
(
leading to two pairs of enantiomers and thus four diastere-
351.15 K (Figure 4a).
This study gives a kinetic constant k of 0.0029 min , a free
−
1
oisomers) and finally 1 also displays both stereogenic centers
but leading to only one pair of enantiomers due to the steric
1
≠
−1
energy of activation ΔG of 115.5 kJ mol (calculated from the
Eyring equation, see Table S7 in the Supporting Information)
and a half-life of inversion t1/2 of 112 min for the N inversion
from 3N C to 3N C (Table 1). Note that the conformer 3N C
R
R
S
R
R R
is slightly more stable than its diastereoisomer 3N C , with an
S
R
−
1
energy difference of 0.4 kJ mol , value in accordance with that
−
1
found by calculations (0.4 kJ mol , see below).
1
Kinetic investigations have also been performed through H
NMR studies (Figure 5). Indeed, the acridine protons in β posi-
tions of the spiro carbon (Ha/Hb for 3N C +3N C and Hc/
R
R
S S
Hd 3N C +3N C ) are sensitive to the nitrogen’s configura-
tion (Figure 5) and were used as efficient probes to determine
S
R
R S
the [3N C +3N C ]/[3N C +3N C ] ratio. These signals are
R
R
S
S
S
R
R S
detected as doublets of doublets at 6.57 and 6.38 ppm respec-
tively for 3N C and 3N C and 6.52 and 6.33 ppm for 3N C
S
R
R
S
R R
and 3N C with no overlap.
S
S
Thus, the decrease of the concentration [3N C +3N C ] in
S
R
R S
deuterated N,N-dimethylsulfoxide (DMSO-d ) has been meas-
6
ured for the two protons Hc and Hd at five temperatures
between 348 and 373 K (see Figures S22–S26 in the Supporting
Information). At 348 K, starting with 97% of 3N C +3N C , the
S
R
R S
amount of their corresponding diastereoisomers 3N C +3N C
R
R
S S
gradually increases (15% after 50 min, 35% after 200 min) to
reach 48% after 760 min. Indeed, the final state is composed
of 3N C +3N C at 48% and 3N C +3N C at 52% which cor-
S
R
R
S
R
R
S S
−
1
responds to a stabilization of 0.3 kJ mol for 3N C +3N C
R
R
S S
compared to 3N C +3N C This provides a kinetic con-
S
R
R S.
−
1
≠
stant k of 0.00275 min , a free energy of activation ΔG of
−
1
1
14.6 kJ mol at 348 K and a half-life of inversion t1/2 of 132 min
(
from 3N C +3N C to 3N C +3N C ).
S
R
R
S
R
R
S S
The rate of inversion increases with the temperature. The
resulting free energies are gathered in Table S8 in the Sup-
porting Information for the five temperatures. The van’t Hoff
plot of ln K versus 1/T gave us the enthalpy of activation of
≠
−1
the inversion ΔH = 106 kJ mol and the entropy of activa-
≠
−1 −1
tion of the inversion ΔS = −24 J.mol .K (Figure 6 and see
Figure S27 and Table S9 in the Supporting Information). Thus,
the free energy of activation ΔG is found to be 113 kJ mol
at room temperature (298 K). This value being higher than
00 kJ mol , it explains why all the isomers can be isolated
≠
−1
−
1 [8]
1
at room temperature. Furthermore, we calculated a value of
Figure 3. a) Experimental (CH OH at 298 K) and b) theoretical (time-
3
≠
−1
ΔG of 114.9 kJ mol from 3N C at 315.15 K, slightly lower
dependent density functional theory (TD-DFT), PBE0/6-311+g(d,p)) circular
R R
dichroism spectra of enantiomers 2N and 2N .
than the one obtained by chiral HPLC studies in ethanol. The
S
R
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Figure 4. Evolution of the concentration of a) 3N C or b) 2N in ethanol through time at 351.15 K.
R
R
S
nitrogen inversion barrier is therefore found to be higher in
ethanol than in DMSO, which could be explained by a stabi-
lization of the ground state through hydrogen bonding, even
though this solvent effect seems weaker in our case than the
ones reported in the literature.[7a] Furthermore, thermogravi-
metric analysis (TGA) investigations show decomposition
temperatures, corresponding to a 5% mass loss, recorded
between 572 K (for 3N C +3N C ) and 587 K (for 3N C )
depending on the sample constitution (Figure S38, Sup-
porting Information). Differential scanning calorimetry (DSC)
does not allows us to determine the melting and the glass
transition temperatures for each enantiopur form, because
the nitrogen inversion begins at lower temperature than these
characteristic temperatures (Figures S44–S47, Supporting
Information).
579 K for the enantiopur 1N C (see Figure S37 in the Sup-
R
S
porting Information). Thus, the packing mode seems to have
an effect on the decomposition temperatures. Moreover, as
the mass loss appears total around 633 K for both samples,
we believe that the sublimation occurs at this temperature
without a real decomposition of the compounds. DSC meas-
urements were performed for both 1 and 1N CS between
R
253 and 513 K. At the first heating cycle (see Figures S40
and S41 in the Supporting Information), the two com-
pounds present a sharp endothermic peak at 522 and 491 K
respectively, associated with the melting of the two compounds
S
R
R
S
R S
(T : 521 and 488 K respectively, determined from the peak
m
onset). When both liquids were cooled down at the same rate
from 513 to 253 K, no recrystallization was detected; hence,
the cooling leads to amorphous solids, which is highly ben-
eficial for further device incorporation. At the second heating
cycle, a glass transition phenomenon was observed at 378 K
for 1 and at 377 K for 1N C (T : 376 and 374 K, respectively
Compound 1 displays a drastically different behavior. Indeed,
the heating of 1 to 443 K leaves the 1H NMR spectrum
unchanged, clearly confirming that even with the help of
thermal input the nitrogen inversion is impossible due to
the steric hindrance induced by the phenyl ring in C1. Fur-
thermore, TGA investigations of 1 and 1N C show decom-
R
S
g
determined from the peak onset), which is comfortable high
enough for future device application. Finally, the heating of 1 or
its enantiopur counterparts does not induce any degradation or
nitrogen inversion.
R
S
position temperatures of 548 K for the racemic mixture and
It is important to mention that kinetic investigations through
H NMR study cannot be performed in the case of 2 since the
inversion takes place between two enantiomers that are hence
indistinguishable from each other through NMR.
The kinetic investigations of the nitrogen inversion of 2NR
Table 1. Experimental thermodynamical data for the N inversion at
1
3
51.15 K.
ΔG [kJ mol−1]
≠
k [min−1
]
Compounds
t1/2 [min]
133
Solvent
Ethanol
2
NR
115.9a)
0.00260
0.00288
0.00356
0.00331
0.00392
into its enantiomer 2N have therefore been performed by chiral
S
3
NRCR
115.5a)
112
Ethanol
HPLC in similar conditions than those above mentioned (in
ethanol at 351.15 K, Figure 4b). The inversion between the two
enantiomers (from 2N to 2N ) provides very similar values (k =
1
14.9b)
97
DMSO-d6
Ethanol
115.1a)
R
S
3
NSCR
112
−1
≠
−1
0
.0026 min , ΔG = 115.9 kJ mol and a half-life of inversion
1
14.6b)
88
DMSO-d6
of 133 min) than those exposed above for compound 3. Indeed,
the phenyl ring in position 3 is too far from the nitrogen atom to
have an influence on the kinetics of its inversion. Furthermore,
a)
HPLC measurements (in ethanol, T = 351.15 K); b)Calculated ΔG≠ at 351.15 K
from the values of ΔH and ΔS≠ obtained by NMR measurements in DMSO-d6.
≠
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a)
-
-
-
-
-
-
13.5
14.0
14.5
15.0
15.5
16.0
0
.00265
0.00270
0.00275
0.00280
0.00285
0.00290
1
/T
b)
-
-
-
-
-
-
-
13.0
13.5
14.0
14.5
15.0
15.5
16.0
Figure 5. a) Evolution of the integration of Ha, Hb, Hc, and Hd through
time. b) Nomenclature of the investigated protons Ha, Hb, Hc, and Hd,
1
c) Evolution of the H NMR spectra of 3N C +3N C in DMSO-d6 at
S
R
R S
3
48 K.
TGA investigations of 2 and 2N show decomposition tempera-
R
0.00265
0.00270
0.00275
0.00280
0.00285
0.00290
tures recorded at 548 K for the racemic mixture and 500 K for
the enantiopur 2NR (see Figure S39 in the Supporting Infor-
mation), again showing the effect of the packing mode on the
stability. One can also note that these decomposition tempera-
tures are lower than those exposed above for compounds 1 and 3,
showing the influence of the pendant phenyl ring in raising the
thermal stability. As for compound 3, the melting and glass tran-
sition temperatures could not be determined by DSC for each
enantiopur form because of the nitrogen inversion (Figures S42
and S43, Supporting Information).
Finally, the mechanism of the nitrogen inversion has been
studied by theoretical calculations. Thus, the molecular struc-
tures of the two enantiomers of 2 in their ground state and
planar-shaped transition state, corresponding to the nitrogen
inversion process, were modelled (Figure 7a). Nitrogen torsion
1
/T
Figure 6. Evolution of Ln(k/T) in function of 1/T a) for 3N C +3N C and
S
R
R S
b) for 3N C +3N C .
R
R
S S
at the origin of the chirality of QPTZ, the transition state cor-
responding to the conversion of 3N C (nitrogen torsion angle
of 194°) into 3N C (nitrogen torsion angle of 166°) has also
been modelled according to the same procedure (Figure 7b and
see Figure S28 and S29 in the Supporting Information). The
nitrogen inversion barrier is thus evaluated at 120.4 kJ mol
from 3N C and 120.0 kJ mol from 3N C (3N C being sta-
bilized by 0.4 kJ mol compared to 3N C as experimentally
observed). Therefore, experimental and theoretical data are in
good agreement and provide similar values for both 2 and 3,
highlighting that the presence of the phenyl ring on the flu-
orene backbone in position 3 has no influence on the nitrogen
inversion but induces the formations of the diastereoisomers.
R
R
S
R
−1
−
1
R
R
S
R
R R
−
1
S
R
angles were calculated at 165° for 2N , 180° for the transition
R
state and 195° for 2N . The nitrogen inversion leaves the flu-
S
orene unchanged, whereas the curvature of the acridine part
is reversed and the phenyl of the phenothiazine unit moves
from below to above the mean plane of the acridine. Thus, the
theoretical-free energy change along the nitrogen inversion of
3
. Conclusion
2
was evaluated at 121.0 kJ mol−1 (see Figure S20 in the Sup-
porting Information for the plot of the theoretical-free energy
change against the nitrogen torsion angle). This feature being
To conclude, this work presents the first examples of optically
pure QPTZ-based molecules. In these systems, the flipping
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Figure 7. Calculated structures of a) 2N and 2N and b) 3N C and 3N C and their transition states of N inversion in ethanol and the variation of
R
S
R
R
S R
free energy at 351.15 K under 1 atm (B3LYP/6-31g(d,p)). TS = transition state.
of the phenothiazine unit leads to a nitrogen inversion with a
high barrier energy (≈115 kJ mol−1 at 351.15 K), which allows
to successfully isolate each isomer at room temperature. Con-
straining the nitrogen atom in a pyramidal configuration
appears hence as an efficient strategy to generate enantiopure
compounds. In addition, the introduction of a phenyl ring in
position 1 of the fluorene completely prevents the flipping of
the phenothiazine and hence the formation of the other pair
of enantiomers providing an efficient diastereocontrol. As the
efficiency of the QPTZ fragment has recently been highlighted
in phosphorescent OLEDs (and is therefore an interesting can-
didate for other organic electronics applications),[ we believe
that the present work may open new avenues in the use of this
electron-rich fragment in its chiral form.
(C.Q.) and for a PhD grant (L.S.). The authors also thank Dr L. Favereau
(Rennes) for fruitful discussions.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
diastereocontrol, nitrogen inversion, quinolinophenothiazine, spiro
architecture
5]
Received: May 7, 2018
Revised: July 3, 2018
Published online:
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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