Regiospecific topochemical reactions controlled by trifluoromethyl directing groups

Article abstract of DOI:10.1039/b413898h

A parallel, offset-stacked orientation was found in the crystal packing of E,E-1,4-di(o-trifluoromethyl)phenyl-1,3-butadiene. UV-irradiation of the powered crystalline sample resulted in a quantitative conversion to a single [2 + 2] cycloaddition product.

Full text of DOI:10.1039/b413898h

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Regiospecific topochemical reactions controlled by trifluoromethyl
directing groups{
Jin Liu* and Kelly Jo Boarman
Received (in Corvallis, OR, USA) 8th September 2004, Accepted 8th October 2004
First published as an Advance Article on the web 29th November 2004
DOI: 10.1039/b413898h
3-butadiene 1. To synthesize 1, E-(o-CF3)–C6H4CHLCHCHO,
which was prepared by the C2-extension of o-CF3–C6H4CHO with
(C6H5)3PLCHCHO, was coupled in a Horner–Wadsworth–
Emmons olefination with o-CF3–C6H4CH2P(O)(OC2H5)2 using
sodium hydride as a base. The colorless crystals of 1 were obtained
from ethyl acetate in an overall yield of 50%.
A parallel, offset-stacked orientation was found in the crystal
packing of E,E-1,4-di(o-trifluoromethyl)phenyl-1,3-butadiene.
UV-irradiation of the powered crystalline sample resulted in a
quantitative conversion to a single [2 + 2] cycloaddition product.
A classic example of topochemical reaction control in the solid
state is the photodimerization of cinnamic acid discovered by
Schmidt et al. in the 1960s.1 Through their pioneering work2 as
well as many other studies,3,4 a topochemical principle for the
[2 + 2] cycloaddition reactions of olefinic molecules in the solid
state was outlined. According to Schmidt’s topochemical principle,
products can be formed only if the reactants lie parallel and are
A view of the X-ray crystal structure of 1 determined at 173 K
(Fig. 1a) indicates that the molecule is related by an inversion
center so that only half the structure is crystallographically
unique.{ The crystal structure shows that the diene unit is coplanar
with the absolute torsional angles ranging from 179 to 180u, and
the two phenyl rings are slightly twisted from the conjugated 1,3-
butadiene unit with a torsional angle (y4.0u, C1–C2–C3–C8). As
expected, the offset-stacked orientation between the phenyl rings of
neighboring molecules has been found in the crystal packing
(Fig. 1b). Compound 1 crystallizes to a layered structure in which
˚
separated by 3.5–4.2 A. With this ordering principle in mind,
chemists have used a variety of innovative approaches to organize
molecules to allow such a reaction to occur. Chloro,5 fluoro,6 and
acetoxy3 were found to have the topochemical controlling effects
…
˚
olefins of neighboring layers are separated by 3.50 A. As a result,
on the crystal lattices of some olefinic reactants. Also, p p stacking
interactions7 were used to construct ideal architectures for [2 + 2]
photodimerization and photopolymerization.6,8 Moreover, during
the past ten years, templates have been successfully designed to
organize all-trans-bis(4-pyridyl)polyenes9,10 and other short poly-
enes11 for [2 + 2] cycloaddition. However, it is still a challenge to use
a template to assemble E,E-1,4-diphenyl-1,3-butadiene (DPB) that
contains no charged ring systems and is photostable in the solid
state.3 Herein, we report the first application of a trifluoromethyl
group as a new handle to direct DPB molecules in arrangements
suitable for [2 + 2] cycloaddition reactions.
the double bonds of the diene units conform to the topochemical
principle. Further analysis of the crystal structure (1) suggests that
…
…
˚
the shortest distance of H F bonds (C9–F H7: 2.67 A) is
identical to the sum of the van der Waals radii for H and F
(H 5 1.20 A, F 5 1.47 A).15 Thus, hydrogen bonding is considered
to be negligible in this case. Also, the trifluoromethyl groups of the
neighboring molecules in the crystal lattice tend to come closer to
˚
˚
…
Although non-covalent p p interactions between perfluoro-
phenyl rings have been reported in different cases,12,13 very little is
…
p p stacking, either
known about substituent effects in
theoretically or experimentally.14 It is notable that fluorine plays
…
a critical role in the formation of p p stacking orientations. It has
occurred to us that a trifluoromethyl group may have the same
attractive prospect as fluorine. Therefore, we chose to examine the
crystal
packing
of
E,E-1,4-di(o-trifluoromethyl)phenyl-1,
Fig. 1 (a) An ORTEP diagram of 1 (top); (b) the molecules (1) stack in
an offset arrangement along the unique (b) axis, and the distance between
˚
the layers of the molecules is 3.50 A.
{ Electronic supplementary information (ESI) available: spectroscopic
data. See http://www.rsc.org/suppdata/cc/b4/b413898h/
*jin.liu@murraystate.edu.
340 | Chem. Commun., 2005, 340–341
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…
…
F
˚
each other at a distance of 2.93 A (F1 F3), due to the weak F
spectrum of each photoproduct, the appearance of a pair of d
and dd in the range of d 5 6.60–6.00 ppm showed a strong
resemblance to the proton absorptions characterized for the
olefinic hydrogens of 2. All the structural data strongly supports
that 3 and 4 indeed undergo a single [2 + 2] cycloaddition,
analogous to 1. The results further corroborate that trifluoro-
methyl is capable of juxtaposing DPB molecules in a parallel
interactions.6 Because of the weakness of the short-distance
…
contacts, we believe that the noncovalent p p stacking interaction
is mainly responsible for the observed self-assembling of 1.
UV-irradiation of 1 in solution using a 200 W Hg medium-
pressure lamp with a Pyrex filter resulted in the conversion of 1 to
its Z,E isomer in .95% yield. On the other hand, UV photolysis of
the powdered crystalline sample 1 using the same light source at
room temperature for 15 h afforded a single product (2) in 100%
yield. The cyclobutane ring of 2 was characterized by two proton
signals at 4.45 and 4.15 ppm (1H NMR). Also, as evidenced by a
doublet (d) and a doublet of doublets (dd) at 6.71 and 5.91 ppm,
respectively, the existence of carbon–carbon double bonds was
suggested in 2. The spectroscopic data indicated that two DPB
molecules underwent a single [2 + 2] addition reaction to form 2.
This observation is in accordance with reactivity studies involving
1,3-butadienes in solid.2 In addition, it was noticed, interestingly,
that photoisomerization of the CLC bonds in 2 was undetected in
solution16 upon direct irradiation (Pyrex). To unequivocally
elucidate the stereochemistry of 2, crystallographic data was
obtained (Fig. 2). The central four-membered ring of 2 was found
to be the crystallographic centre of inversion. Examination of the
crystal structure indicates that the C2–C1 double bond of 1 (see
structure of 1 and Fig. 1b) reacted with the C19–C29 double bond
of the neighboring molecule. The regiospecific addition is due to
fashion for an intermolecular [2 + 2] cycloaddition by means of a
…
p
p stacking interaction. We are now investigating potential
applications of the trifluoromethyl group to other systems for
stereo-controlled reactions in the solid state.
We are very grateful to Dr Victor G. Young, Jr. at the X-Ray
Crystallographic Laboratory, University of Minnesota for the
determination of the crystal structures. J. Liu thanks Kentucky
NSF EPSCoR and the Howard Hughes Medical Institution for
financial support.
Jin Liu* and Kelly Jo Boarman
Department of Chemistry, Murray State University, Murray, Kentucky,
42071, USA. E-mail: jin.liu@murraystate.edu.; Fax: +1 270-762-6474;
Tel: +1 270-762-6626
Notes and references
{ Crystal data. Compound 1, C18H12F6, M 5 342.28, monoclinic,
˚
a 5 10.3758(19), b 5 4.9215(9), c 5 15.280(3) A, a 5 90, b 5 107.108(2),
3
˚
c 5 90u, U 5 745.8(2) A , T 5 173(2) K, space group: P21/c, Z 5 2, m(Mo–
Ka) 5 0.140 mm21, 6437 reflections measured, 1690 unique (Rint 5 0.0285)
which were used in all calculations. The final wR(F2) 5 0.1131 (all data).
Compound 2, C36H24F12, M 5 684.55, triclinic, a 5 7.9904(9),
˚
the closest distance (3.72 A) between the C2–C1 and C19–C29
˚
bonds, while the distance (4.92 A) between two neighboring C2–
˚
C1 double bonds is exceeded for the topochemical reaction. It
…
b 5 10.0114(11), c 5 10.6547(12) A, a 5 73.214(2), b 5 68.806(2),
3
˚
¯
c 5 84.573(2)u, U 5 760.77(15) A , T 5 173(2) K, space group: P1, Z 5 1,
m(Mo–Ka) 5 0.138 mm21, 7718 reflections measured, 2700 unique
(Rint 5 0.0368) which were used in all calculations. The final
wR(F2) 5 0.1257 (all data). CCDC 249913 and 249914. See http://
www.rsc.org/suppdata/cc/b4/b413898h/ for crystallographic data in .cif or
other electronic format.
appears that the p p stacking interaction produced by trifluor-
omethyl groups directed the geometry of crystallization to yield
photoactive crystals of 1.
To further probe the generality of trifluoromethyl in exerting
direct control of the crystal packing of DPB molecules, we
prepared E,E-1,4-di(m-trifluoromethyl)phenyl-1,3-butadiene 3 and
E,E-1,4-di(p-trifluoromethyl)phenyl-1,3-butadiene 4 by the proce-
dure described above. UV irradiation of powdered crystalline
samples 3 and 4 for 15 h at 25 uC afforded a single photoproduct
in 50–60% yield, respectively. The yields remained the same after
irradiation for 15 h. Each photoproduct isolated from the
unreacted starting material by way of silica-gel column chromato-
graphy (hexanes as an eluant) was suggested by MS to be a dimer
from a single [2 + 2] reaction (see ESI{). Also, as evidenced by a
broad proton signal at around 4.00 ppm and two carbon signals in
the range of d 5 47.0–45.0 ppm, the cyclobutane ring in each
photoproduct was confirmed. Moreover, in the 1H NMR
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Fig. 2 A thermal ellipsoid drawing of 2.
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Chem. Commun., 2005, 340–341 | 341