Perfluorocyclohexene bridges in inverse DiArylEthenes: Synthesis through Pd-catalysed C-H bond activation, experimental and theoretical studies on their photoreactivity

Article abstract of DOI:10.1039/c3cc43754j

The palladium-catalysed direct di-heteroarylation of 1,2- dichloroperfluorocyclohexene with a variety of heteroarenes gives rise in to a new family of 1,2-di(heteroaryl)perfluorocyclohexenes. These derivatives do not exhibit photoreactivity and this unexpected outcome is explained by calculations demonstrating the lack of reactive isomers.

Full text of DOI:10.1039/c3cc43754j

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Perfluorocyclohexene Bridge in Inverse DiArylEthenes: One Step Synthesis through
Pd-Catalysed C-H bond Activation, Joint Experimental and Theoretical Studies on
Their Photoreactivity
Kedong Yuan,a Julien Boixel,a Hubert Le Bozec,a Abdou Boucekkine,a Henri Douceta*, Véronique
5
Guerchais,a* Denis Jacqueminb,c*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
50 thiazole derivatives,11-13 ii) their photoreactivity, and, iii)
The palladium-catalysed direct di-heteroarylation of 1,2-
theoretical studies rationalizing their photochemical behaviour.
10 dichloroperfluorocyclohexene with a variety of heteroarenes gives
rise in one step to
a new family of 1,2-di(heteroaryl)
The new C6-DAE 1-5 were prepared by Pd-catalysed C-H bond
activation/vinylation using the appropriate heterocycles with the
commercially available 1,2-dichlorooctafluorocyclohexene used as
55 the coupling partner (Scheme 2). The catalytic system Pd(OAc)2
associated to PCy3 in the presence of KOAc in CMPE14,15 allows
the regioselective C5 functionalization of 2-ethyl-4-methylthiazole
giving rise to the formation in one step of compound 1 in 46%
yield. The use of 3-methylbenzothiophene led to the formation of 2
perfluorocyclohexenes. These derivatives do not exhibit any
photoreactivity and this unexpected outcome is explained by
theoretical calculations demonstrating the lack of reactive isomers.
15 Photochromic compounds have attracted considerable interest for
optical data storage. Among them, much attention has been focused
on DiArylEthene (DAE) derivatives due to their thermal stability
and fatigue resistance.1 The photochromic reactivity depends on
several factors: conformation of the open-ring isomer, electron- 60 in a similar yield of 40%. Then, the three thiophene-based
20 donor/acceptor substituents, conjugation length of the heteroaryl
groups, and the architecture and also varies between normal (N-
type)2 versus inverse (I-type)3,4 DAE (see Scheme 1). In addition,
the nature of the ethene bridge can be diversely modified. Beside
derivatives 3-5 were obtained in 45-80% yields using
PdCl(C3H5)(dppb) or Pd(OAc)2 / PCy3 as catalysts. The high yield
in 5 is certainly due to the presence of a bromo substituent at C3. 3-
Bromothiophenes are known to be very reactive for Pd-catalysed
the ubiquitous cyclopentene, perfluorinated or not, various 65 direct arylations.12d It should be noted that no cleavage of the C-Cl
25 heterocycles have been successfully employed and most of them
and C-Br bonds of the thiophene derivative was observed in the
course of the synthesis of and 5, allowing further
contains C=C bond within
a
a
five-membered ring.5 The
4
photochromic properties in which the ethene bridge is incorporated
transformations. In all cases, no significant amount of the mono-
into a six-membered ring have been also demonstrated for several
heteroarylated octafluorocyclohexene was detected by GC/MS
normal C6-DAE.6 In contrast, the use of inverse DAE is less 70 analysis. To our knowledge, these couplings represent the first
30 explored, and to the best of our knowledge, no example of inverse
C6-DAE (2-thienyl/5-thiazolyl) species has been reported yet. This
communication fills this gap.
examples of direct heteroarylations of a dichlorocyclohexene.
PdCl(C3H5)(dppb)
H
or
F8
Pd(OAc)2 / PCy3
S
F8
+
normal DAE
inverse DAE
CPME, KOAc,
120°C, 20 h
S
S
R
R'
N-type
R'
R'
1-5
I-type
Z
Cl
Cl
R
R
Z
Z
S
F8
Z = CH, N
F6
F6
S
S
S
S
F8
F8
F8
Z
Z
Z
Z
S
S
S
S
S
S
N
S
C6
C5
Z = CH, N
N
O
O
Scheme 1. Chemical structures of DAE derivatives: normal (N-
35 type) and inverse (I-type) incorporating a five- (C5) and six
membered-ring (C6) as central ethene bridge
1 46%
2 40%
3 66%
F8
F8
We have been investigating the chemistry and the
photochromic behavior of normal and inverse C5-DAE derivatives
and we have studied their geometric and electronic structures by
40 theoretical calculations.7 Some of us also recently demonstrated
that Pd-catalysed coupling of 1,2-dichlorohexafluorocyclopentene
with heteroarenes via C-H bond activation8,9 is an efficient method
to prepare inverse diarylperfluorocyclopentenes (C5-DAE).10 As an
extension of these works, and with the aim of understanding the
45 factors that control the photoreactivity, we decided to study for the
first time the impact of the six-membered ring,
perfluorocyclohexene, as the ethene bridge in inverse C6-DAE.
S
S
S
S
Cl
Cl
Br
Br
O
O
O
O
O
O
4 45%
5 80%
Scheme 2. One step access to I-type C6-DAE derivatives through
Pd-catalysed C-H bond activation
75
Surprisingly, these derivatives do not exhibit any
photoreactivity. The electronic spectra (CH2Cl2, 298 K) of
compounds 1-5 in their open forms exhibit an intense absorption
band at λ 300-350 nm which are ascribed to intraligand (IL) π→π*
transitions of the thiophene/thiazole rings (See ESI, Table S1,
Herein, we describe i) the one step access to I-type C6-DAE
species through Pd-catalysed C-H bond activation of thiophene and
80 Figures S2 and S3). UV irradiation (λ = 350 nm, CH2Cl2) of all
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synthesized compounds 1-5 does not induce any photochromic
response, that is we could not observe changes of the UV-vis nor
1H NMR spectra irrespective of the irradiation time. For 2 that
the conformational blend is strongly dominated by c (AP), e (P)
and f (P) all being photochromically inactive. In fact, the
photoactive isomers represent less than 2.5% of the total, i.e. are
absorbs at rather short wavelength, we have tested that a more 55 present in negligible quantities in the experimental pot. In addition,
their CC distances of 3.67 and 3.74 Å are significantly larger than
the CC distance in the corresponding C5-DAE structure (3.59 Å at
the same level of theory for the reactive AP structure) and it is
known that longer CC distance in the open isomer tends to lead to
60 smaller quantum yield of cyclization. For the inverse C5-DAE
model (similar to 1), the same computational protocol indicates a
strong domination of a reactive AP conformer (90.0%, CC distance
of 3.59 Å), an outcome consistent with previous investigations on
model inverse C5 DAE,7b,7c clearly highlighting the difference
65 between inverse C5 and C6 DAE.
5
energetic irradiation (λ = 325 nm) does not yield photochromic
response. These observations are certainly puzzling compared with
the results reported for DAE having an azulene bridge that are
photochromic only for 4-thiazolyl and not for 3-thienyl groups,16 as
a result of steric repulsion. As expected for non-photochromic
10 DAE, we could observe fluorescence and the emission data are
listed in Table S1 in ESI. Several factors have been considered to
rationalize the peculiar non-photochromic behavior of inverse C6
DAE.1,7,17 In particular, we show below that the ground-state
conformation has a dramatic effect on the photocyclization
15 reaction, consistently with previous works.17
Table 1. Relative G (see ESI), percentage of presence (computed
at room temperature following Boltzmann equation) and distance
We have used theoretical calculations to investigate the inverse
C6 DAE (see ESI for computational details). As the lack of
photoreactivity is independent of the nature of the substituents, we
have used a model 1 in which the side ethyl groups have been
20 removed to allow the use of accurate models for determining
energies (MP4, see ESI). Previous investigations have shown that
three main factors can explain non-photochromic reactivity of
DAE: energetic, conformational and electronic.1d,7a For the second
aspect, it is well known that most DAE present two groups of open
25 isomers, namely parallel (P) and anti-parallel (AP) conformers,
only the latter possibly allowing the ring-closure reaction.1 Indeed,
in the parallel DAE the thiophene rings are not oriented in a way
allowing electrocyclization. In most normal perfluorinated C5-
DAE, the ratio between parallel and anti-parallel structures is close
30 to 1/1, and this accounts for the limiting quantum yield of 50%
often observed experimentally if the AP/P interconversion is slow.1
On the contrary, in inverse C5-DAE, it has been shown by both
NMR and theoretical studies that the ratio is in significant favour
of the anti-parallel form.7b,7c,18 In DAE there are often a large series
35 of conformers that need to be explored,18 and we have performed
such extensive exploration of the potential energy surface for the
model molecule. Eight conformers could be identified (see Figure
1): four P and four AP.
Conformer
G
(kcal.mol-1)
2.83
Ratio
(%)
0.7
1.6
79.2
0.0
11.5
7.0
0.0
CC Dist.
(Å)
AP (C2) Chair
AP (C2) Chair
AP (C2) Chair
AP (C1) Boat
P (C1) Chair
P (C1) Chair
3.67
3.74
4.66
5.13
3.99
3.91
4.06
4.00
a
b
c
d
e
f
g
h
2.30
0.00
6.29
1.15
1.44
5.98
6.15
P (C1)
P (C1)
Boat
Boat
0.0
between reactive carbon atoms for the eight conformers. The
relative orientation of the two thiazole rings as well as the
70 conformation of the perfluorocyclohexene are indicated as well.
See Figure 1 for graphical representation.
In Figure 2, we display the frontier molecular orbitals
computed for the model inverse C6 DAE (b conformer) and its C5
75 counterpart. The topology of these frontier orbitals are very similar
for the two compounds. More specifically, both LUMO present
electronic density on the reactive carbon atoms as well as a
bonding character for the to-be-formed CC σ bond. Therefore,
from this orbital point of view,1d no lack of photoreactivity could
80 have been expected: the use of perfluorocyclohexene instead of a
perfluorocyclopentene bridge is not enough to significantly pull out
the electrons from the reactive five member rings.
a
e
b
c
d
h
f
g
40 Figure 1. Representation of the eight conformers found for the
model inverse C6 DAE.
Figure 2. HOMO (bottom) and LUMO (top) of the model inverse
85 C6 (left) and C5 (right) DAE. Computed with the CAM-B3LYP
functional (see ESI).
The main features of all these eight conformers are listed in
Table 1. As can be seen, both pseudo-chair and pseudo-boat
conformations of the cyclohexene ring have been considered, the
45 latter yielding much less stable structures, as expected. The only
structures that would allow photochromism are conformers a and
b. Indeed, in addition to presenting an AP conformation, a
conformer needs to have a CC distance below the 4.2 Å threshold
to allow cyclization, at least in the solid state,19 and both c and d
50 are significantly above this threshold. As can be seen, inverse C6-
DAE are strongly dominated by conformers impeding cyclization:
In conclusion, we have demonstrated that the palladium-
catalysed direct coupling of various heteroarenes with 1,2-
dichloroperfluorocyclohexene offers a fast and direct access to
90 symmetric 1,2-di-(heteroaryl)perfluorocyclohexenes. Replacing the
perfluorocyclopentene by the related six-membered ring, the
perfluorocyclohexene as the ethene bridge in inverse DAE has a
2
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DOI: 10.1039/C3CC43754J
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Acknowledgment. K. Y. is grateful to the French ministère for
research for a PhD grant. This work was supported by BRESMAT.
D.J. acknowledges the European Research Council (ERC) and the
Région des Pays de la Loire for financial support in the framework
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