Stereoselective and quantitative [2 + 2] photodimerization of a symmetrical octafluoro stilbene in the solid state: Face-to-face stacking of the fluorinated rings in trans-1,2-bis(2,3,5,6-tetrafluorophenyl)ethylene

Article abstract of DOI:10.1016/j.jfluchem.2016.05.010

Graphical abstract The symmetrical octafluoro stilbene undergoes a stereoselective and quantitative [2 + 2] photodimerization in the solid state. C-H?F interactions and face-to-face stacking dominate the self-assembly of the alkene in the solid. The resul

Full text of DOI:10.1016/j.jfluchem.2016.05.010

Journal of Fluorine Chemistry 188 (2016) 5–9
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Stereoselective and quantitative [2 + 2] photodimerization of a
symmetrical octafluoro stilbene in the solid state: Face-to-face stacking
of the fluorinated rings in trans-1,2-bis(2,3,5,6-tetrafluorophenyl)
ethylene
Michael A. Sinnwella, Benjamin J. Ingenthronb, Ryan H. Groenemanb,
,
*
Leonard R. MacGillivraya,
*
a
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
b
Department of Biological Sciences, Webster University, St. Louis, MO 63119, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
The symmetrical octafluoro stilbene trans-1,2-bis(2,3,5,6-tetrafluorophenyl)ethylene (1) undergoes a
stereoselective and quantitative [2 + 2] photodimerization in the solid state. The olefin self-assembles via
C-Hꢀ ꢀ ꢀF and face-to-face interactions of the fluorinated phenyl rings into a geometry for a topochemical
photodimerization. The crystal structure and stereochemistry of the photoproduct rctt-1,2,3,4-tetrakis
(2,3,5,6-tetrafluorophenyl)cyclobutane (2) has been determined.
Received 17 March 2016
Received in revised form 23 May 2016
Accepted 25 May 2016
Available online 25 May 2016
Keywords:
ã 2016 Elsevier B.V. All rights reserved.
[2+2] photodimerization
Supramolecular chemistry
Self-assembly
1. Introduction
reported. Halogen bonds (Scheme 1c) have also been used to
sustain photoreactivity in the solid state in more limited cases [12].
Understanding supramolecular interactions of organic fluorine
continues to be important for advancing the design and syntheses
of fluorinated materials [1]. Liquid crystals [2], pharmaceutics [3],
and fluorinated metal-organic frameworks [4] have demonstrated
attractive physical properties [5] related to integrations of fluorine
atoms. In this context, noncovalent interactions involving fluorine
have been exploited to facilitate chemical reactivity in the solid
All of these examples illustrate how supramolecular interactions
involving fluorine atoms can facilitate the syntheses of small
molecules via [2 + 2] photodimerizations in solids.
Our interests lie in achieving chemical reactivity in the solid
state using fluorine-functionalized olefins. Recently, we described
the use of C6F5ꢀ ꢀ ꢀC6F5 interactions, in combination with argento-
philic forces, to direct a head-to-head intermolecular [2 + 2]
photodimerization in the solid state (Scheme 2a) [13]. We more
recently reported the ability of a rigid bipyridine to direct an
intermolecular photodimerization via halogen bonds [14,15]. In
particular, co-crystallization of trans-1,2-bis(4-iodotetrafluoro-
phenyl)ethylene (2I-1) with 1,8-di(4-pyridyl)naphthalene afforded
a four-component supramolecular assembly sustained by halogen
bonds (Scheme 2b). UV-irradiation of the solid generated
rctt-1,2,3,4-tetrakis(1-iodo-2,3,5,6-tetrafluorophenyl)cyclobutane
(4I-2) stereoselectively and in quantitative yield [16]. The inherent
photostability of 2I-1 as a pure solid required use of the bipyridine
to achieve the solid-state photodimerization to form 4I-2.
state.
Perfluorophenyl-phenyl
(C6F6ꢀ ꢀ ꢀC6H6)
interactions
(Scheme 1a) have, thus, been shown to support the syntheses of
fluorine-containing cyclobutanes via the photoinduced [2 + 2]
cycloaddition reaction, with reactions occurring in single- and
multi-component solids [6]. Perfluorophenyl-perfluorophenyl (C6F
6ꢀ ꢀ ꢀC6F6) [7] (Scheme 1b) interactions, as well as analogous forces
involving polyfluorophenyl rings [8], have been used to support the
parallel stacking of carbon-carbon double bonds (C C) required for
¼
[2 + 2] photodimerizations. In related work, fluorine-directed
solid-state photoreactions of fluorocoumarins [9], fluorobenzyli-
denepiperitones [10], and diarylbutadienes [11] have been
To this end, we endeavored to investigate how the removal of
the terminal iodide atoms from 2I-1 in the form of trans-1,2-bis
(2,3,5,6-tetrafluorophenyl)ethylene 1 would effectively influence
the self-assembly of 1 in the solid state. Terminal C-H groups of
polyfluoroarenes are useful synthons in organic synthesis
* Corresponding authors.
E-mail addresses: ryangroeneman19@webster.edu (R.H. Groeneman),
len-macgillivray@uiowa.edu (L.R. MacGillivray).
http://dx.doi.org/10.1016/j.jfluchem.2016.05.010
0022-1139/ã 2016 Elsevier B.V. All rights reserved.

6
M.A. Sinnwell et al. / Journal of Fluorine Chemistry 188 (2016) 5–9
Scheme 1. Integration of fluorine in supramolecular chemistry: (a) C6F6ꢀ ꢀ ꢀC6H6 interactions, (b) C6F6ꢀ ꢀ ꢀC6F6 interactions, and (c) halogen bonds.
Scheme 2. Photoactive assemblies: (a) C6F5ꢀ ꢀ ꢀC6F5 interactions and argentophilic forces and (b) halogen bonding.
(e.g. arylations), which would also make a resulting cyclobutane an
attractive synthetic target [17]. To our knowledge, the unsym-
metrical alkene trans-1-(4-aminophenyl)-2-(2,3,5,6-tetrafluoro-
The alkene 1 crystallizes in the monoclinic space group P 21/n.
The olefin sits around a crystallographic inversion center, with
one-half of the molecule being present in the asymmetric unit
(Fig. 1a). An analysis of the extended structure reveals 1 to self-
assemble in the solid state via offset C-Hꢀ ꢀ ꢀF forces (Hꢀ ꢀ ꢀF (Å): 2.61,
Cꢀ ꢀ ꢀF (Å): 3.330(2), C-Hꢀ ꢀ ꢀF (ꢁ): 134.9, calc. 98% of sum of van der
Waals radii) to form alternating 2D sheets within the crystallo-
graphic bc-plane (Fig. 1b) [19]. The sheets interact via offset face-
to-face stacking of the polyfluoro rings, with metrics similar to
analogous architectures [20]. The assembly of the sheets afford
infinite stacks of 1 along the a-axis. As a consequence of the
phenyl)ethylene represents the only documented case of
a
solid-state photodimerization involving a polyfluoroarene that
possesses a terminal C-H group [12].
Here, we report the structure and photoreactivity of 1 in the
solid state. We show 1 to undergo a [2 + 2] photodimerization
stereoselectively to generate rctt-1,2,3,4-tetrakis(2,3,5,6-tetra-
fluorophenyl)cyclobutane (2) in quantitative yield (Scheme 3).
Face-to-face stacking involving the polyfluorophenyl rings, in
combination with C-Hꢀ ꢀ ꢀF forces, dominate the assembly of 1 in the
solid.
assembly process, the C C bonds are parallel and separated by
¼
3.81 Å, which conforms to the criteria of Schmidt for a [2 + 2]
photodimerization in a solid (Fig. 1c) [21].
2. Results and discussion
To determine the photoreactivity, a powder crystalline sample
of 1 was placed between two glass plates and exposed to
UV-radiation (broadband medium-pressure Hg lamp) for a period
of approximately 30 h. A 1H NMR spectrum revealed a photo-
dimerization to occur in the solid stereoselectively and in
quantitative yield (Figs. S1–S4). Specifically, the formation of a
cyclobutane ring was evidenced by the complete disappearance of
The synthesis of 1 was accomplished using a Wittig reaction of
triphenyl-(2,3,5,6-tetrafluorobenzyl)phosphonium bromide and
2,3,5,6-tetrafluorobenzaldehyde [18]. Single crystals of 1 suitable
for X-ray diffraction studies in the form of colorless needles were
obtained via slow evaporation from a toluene solution overnight
(Table 1).
Scheme 3. Photodimerization of 1 to form 2 in the solid state.

M.A. Sinnwell et al. / Journal of Fluorine Chemistry 188 (2016) 5–9
7
Table 1
General and crystallographic data for 1 and 2.
Compound
1
2
CCDC number
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
1451758
C14H4F8
324.17
295(2)
monoclinic
P21/n
1451759
C28H8F16
648.34
100(2)
triclinic
P 1
a/Å
b/Å
c/Å
3.8106(2)
6.4160(3)
23.9985(10)
90
94.353(3)
90
585.04(5)
2
7.5147(5)
8.1356(6)
10.0065(6)
97.382(4)
104.242(4)
104.357(4)
562.62(7)
1
a ꢁ
/
b ꢁ
g ꢁ
/
/
Volume/Å3
Z
D
calc/g/cm3
1.840
0.197
320
1.914
0.205
320
m
/mm-1
F(000)
range for data collection/ꢁ
2
u
6.574 to 50.062
4.288 to 55.366
Index ranges
Reflections collected
Independent reflections
ꢂ4 ꢃ h ꢃ 4, ꢂ7 ꢃ k ꢃ 7, ꢂ28 ꢃ l ꢃ 28
ꢂ9 ꢃ h ꢃ 9, ꢂ10 ꢃ k ꢃ 10, ꢂ12 ꢃ l ꢃ 12
8292
1037
9604
2592
R
int = 0.0210
Rint = 0.0263
2592/0/199
1.087
Data/restraints/parameters
1037/0/100
1.037
Goodness-of-fit on F2
Final R indexes [I ꢄ 2
s
(I)]
R1 = 0.0320
wR2 = 0.0835
R1 = 0.0420
wR2 = 0.0917
R1 = 0.0386
wR2 = 0.0967
R1 = 0.0465
wR2 = 0.1021
Final R indexes [all data]
Fig. 1. X-ray structure of 1: (a) ORTEP view (ellipsoids 50% probability level), (b) 2D sheet, and (c) stacks of alkenes between 2D sheets showing parallel alignment of C C
¼
bonds.
the single olefinic proton at
d
= 7.36 ppm and appearance of a single
An X-ray diffraction analysis confirmed the stereochemistry of
the cyclobutane 2. Specifically, the photoproduct 2 crystallizes in
the triclinic space group P ı with one-half of the molecule in the
asymmetric unit. The tetrafluorophenyl moieties of the cyclo-
butane ring exhibit a rctt- stereochemistry [C-C (Å): 1.558(2) and
1.577(2)], which is consistent with the geometry in the crystal
cyclobutane proton at = 5.33 ppm.
d
To confirm the stereochemistry of the photoproduct 2, a solid
sample was recrystallized from toluene:ethanol (50:50 v:v,
4.0 mL). The recrystallization afforded colorless plates suitable
for single-crystal X-ray analysis (Table 1).

8
M.A. Sinnwell et al. / Journal of Fluorine Chemistry 188 (2016) 5–9
Fig. 2. X-ray structure of 2: (a) ORTEP view (ellipsoids 50% probability level), (b) C-Hꢀ ꢀ ꢀF, C-Fꢀ ꢀ ꢀpF, and face-to-face stacking, (c) Fꢀ ꢀ ꢀF contacts, and (d) interdigitation within
2D layers.
structure of the pure olefin (Fig. 2a). The extended structure of
`` 2 reveals the cyclobutane to self-assemble within the crystallo-
graphic bc-plane via a combination of C-Hꢀ ꢀ ꢀF interactions [Hꢀ ꢀ ꢀF
(Å): 2.57, Cꢀ ꢀ ꢀF (Å): 3.474(2), C-Hꢀ ꢀ ꢀF (ꢁ): 158.3, calc. 96% of sum of
van der Waals radii], C-Fꢀ ꢀ ꢀpF [Fꢀ ꢀ ꢀcentroid (Å): 3.67] and face-to-
face stacking of polyfluoro rings (Fig. 2b). Short Fꢀ ꢀ ꢀF contacts [Fꢀ ꢀ ꢀF
(Å): 2.897(2) and 2.932(2), calc. 99% of sum of van der Waals radii]
are propagated along the a-axis (Fig. 2c). As a consequence of the
assembly process, 2 lies interdigitated with adjacent sheets
assembling via Fꢀ ꢀ ꢀF contacts [Fꢀ ꢀ ꢀF (Å): 2.847(2), calc. 97% of
sum of van der Waals radii] and C-Fꢀ ꢀ ꢀpF interactions [Fꢀ ꢀ ꢀcentroid
(Å): 3.210] (Fig. 2d). The close-packing adopted by 2 contrasts that
of 4I-2, which forms a channel-type inclusion solid [14].
photodimerization in the solid state. The resulting cyclobutane 2
possesses polyfluoro phenyl groups in a rctt-configuration, as
confirmed by the X-ray structure of the photoproduct. We are now
investigating the scope of the stacking in solid-state reactions
involving congeners of 1, as well as the ability of 2 to serve as a hub
for downstream chemical transformations in post-synthetic
modifications involving the C-H groups.
4. Experimental
4.1. Materials
All reagents and solvents used were commercially available. The
reagents 2,3,5,6-tetrafluorobenzyl bromide and 2,3,5,6-tetrafluor-
obenzaldehyde were purchased from Oakwood Products while
triphenylphosphine was purchased from STREM Chemical. All
solvents including toluene, chloroform, and dimethylformamide
were purchased from Fisher Scientific. All reagents and solvents
were used without further purification.
3. Conclusions
In this report, we have shown C-Hꢀ ꢀ ꢀF forces and face-to-face
stacking to support the ability of the symmetrical stilbene 1 to
undergo
a
stereoselective
and
quantitative
[2 + 2]

M.A. Sinnwell et al. / Journal of Fluorine Chemistry 188 (2016) 5–9
9
4.2. Methods
Photoreactions were conducted using UV-radiation from a
H.G.) as well as the National Science Foundation (L.R.M., DMR-
1408834).
450 W medium-pressure mercury lamp in an ACE Glass photo-
chemistry cabinet. Single crystals of 1 were finely ground using a
mortar and pestle, and then placed between a pair of Pyrex glass
plates. The powdered sample was irradiated in six-hour intervals.
1H NMR spectra were recorded using a Bruker AVANCE-300 NMR
spectrometer operating at 300 MHz using DMSO-d6 as NMR
solvent. 19F NMR spectra were recorded using a Bruker AVANCE-
500 NMR spectrometer operating at 471 MHz using DMSO-d6 as
NMR solvent and trifluoroacetic acid as internal standard. All data
were processed with MestReNova suite of software programs.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/j.
jfluchem.2016.05.010.
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Acknowledgements
We are grateful for funding from Webster University in the form
of both Faculty Research Grants and Faculty Development Fund (R.