Phenyl-perfluorophenyl stacking interactions: Topochemical [2+2] photodimerization and photopolymerization of olefinic compounds

Article abstract of DOI:10.1021/ja974072a

The face-to-face stacking interaction between phenyl and perfluorophenyl groups is emerging as a common noncovalent interaction. To explore the generality of this supramolecular synthon, the solid-state packing structure and reactivity of several monoolefins and diolefins substituted with phenyl and perfluorophenyl groups was investigated. Of the seven crystalline or cocrystalline materials investigated, six were found to undergo a photochemically induced [2+2] reaction in the solid state. By determining the stereochemistry of the photoproduct and/or X-ray structural analysis of the olefinic precursors, the stacked interaction between phenyl and perfluorophenyl groups in the photoactive crystals were revealed.

Full text of DOI:10.1021/ja974072a

J. Am. Chem. Soc. 1998, 120, 3641-3649
3641
Phenyl-Perfluorophenyl Stacking Interactions: Topochemical [2+2]
Photodimerization and Photopolymerization of Olefinic Compounds
Geoffrey W. Coates,*,† Alex R. Dunn,‡ Lawrence M. Henling,‡ Joseph W. Ziller,§
Emil B. Lobkovsky,† and Robert H. Grubbs*,‡
Contribution from the Department of Chemistry, Baker Laboratory, Cornell UniVersity, Ithaca,
New York 14853, The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of
Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, and
Department of Chemistry, UniVersity of California, IrVine, IrVine, California 92717
ReceiVed December 1, 1997
Abstract: The face-to-face stacking interaction between phenyl and perfluorophenyl groups is emerging as a
common noncovalent interaction. To explore the generality of this supramolecular synthon, the solid-state
packing structure and reactivity of several monoolefins and diolefins substituted with phenyl and perfluorophenyl
groups was investigated. Of the seven crystalline or cocrystalline materials investigated, six were found to
undergo a photochemically induced [2+2] reaction in the solid state. By determining the stereochemistry of
the photoproduct and/or X-ray structural analysis of the olefinic precursors, the stacked interaction between
phenyl and perfluorophenyl groups in the photoactive crystals were revealed.
Introduction
Scheme 1. Olefin Photodimerization and Diolefin
Photopolymerization
The photodimerization of organic molecules in the solid state
has been known since the end of the last century.1 Through
the pioneering work of Schmidt and co-workers,2,3 as well as
many other studies,4-6 the prerequisites for photochemical [2+2]
reactions in the solid state are now well established. With few
exceptions, olefins arranged in a parallel orientation that have
center-center separations of 3.5-4.2 Å undergo the photocy-
cloaddition reaction.4 Diolefin molecules whose crystals have
these same intermolecular olefin parameters typically react to
form polymers in the solid state (Scheme 1).7-9 One of the
intriguing aspects of these photocycloaddition and photopo-
lymerization reactions is that the stereochemistry of the products
is determined by the relative arrangement of the molecules in
the crystal.10-12 Therefore, topochemical reactions hold great
potential for the synthesis of molecules of defined stereochem-
istry. Unfortunately, it is impossible at the current time to
rationally design a molecule that will, with any degree of
certainty, crystallize in the necessary arrangement for reaction
in the solid state.13,14 The most common approach to the
identification of photoactive olefinic compounds is simply a
process of trial and error.
The identification and detailed understanding of commonly
observed interactions in molecular crystals is crucial to the
advancement of the field of crystal engineering. Through the
study of these interactions (recently termed “supramolecular
synthons”),15 it should be possible to design molecules that have
a high probability of packing in the desired crystalline arrange-
ment. For example, chloro-,16 fluoro-,17 alkoxyaryl groups,18
as well as electron donor/acceptor19 and hydrogen bonding
interactions,20 have been used to align olefinic molecules in the
crystal for [2+2] photodimerization.
† Cornell University.
‡ California Institute of Technology.
§ University of California, Irvine.
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Noncovalent interactions between aromatic groups have been
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Published on Web 04/02/1998

3642 J. Am. Chem. Soc., Vol. 120, No. 15, 1998
Coates et al.
from van der Waals forces and favorable electrostatic attractions
between the molecules, not from charge-transfer interac-
tions.37,44,45 The theoretical electrostatic potential map of
benzene46 reveals concentrations of negative potential above and
below the molecular plane, and a ring of positive potential in
the plane focused on the hydrogens. Hexafluorobenzene
possesses the reverse charge distribution. Stated differently,
both benzene and hexafluorobenzene have large molecular
quadrupole moments that are similar in magnitude, yet opposite
in sign.37 Further support for the importance of electrostatics
in such interactions comes from Cozzi and Siegel’s elegant
studies of rotational barriers in 1,8-diarylnaphthalenes47,48 and
from recent theoretical studies.49-53
We have recently reported the use of the phenyl-perfluo-
rophenyl stacking interactions to align diacetylene molecules
for reaction in the crystalline state.54 These studies revealed
that the phenyl-perfluorophenyl arrangement first seen in
benzene/hexafluorobenzene may be a fairly general supramo-
lecular motif. The distances between phenyl centroids in these
diacetylenes varies from 3.6 to 3.8 Å, and are very close to the
separation of the benzene-hexafluorobenzene cocrystal, 3.77
Å.43 Since these distances are within the range necessary for a
solid-state [2+2] photocycloaddition, we investigated the use
of the phenyl-perfluorophenyl interaction to orient olefinic
molecules in the crystalline state. In this paper we report the
application of this interaction for the topochemical photocy-
cloaddition of olefins and photopolymerization of diolefins.
Figure 1. Solid-state packing arrangements of the benzene dimer and
the benzene/hexafluorobenzene dimer.
science.21 These stacking interactions contribute substantially
to the stabilization of DNA,22,23 and are thought to be intimately
involved in the tertiary structure and recognition events of
proteins.24,25 The aggregation of porphyrins and aromatic
macrocycles in solution has been attributed to aromatic-
aromatic interactions,26,27 as has the liquid-crystalline behavior
of some arene-containing molecules.28 In addition, arene-arene
interactions are important in molecular recognition events29-31
and have been shown to influence selectivity in both the solution
phase32-34 and the solid state.35,36
Although a considerable amount of work concerning the
stacking of aromatic rings has concentrated on phenyl-phenyl
interactions, there is a growing interest in the interactions of
phenyl and perfluorophenyl groups.37-40 Patrick and Prosser
first recognized that a 1:1 mixture of benzene (mp 5.5 °C) and
hexafluorobenzene (mp 4 °C) forms a complex that melts at 24
°C.41 Although pure benzene adopts an edge-to-face structure
in the solid state,42 it has been determined that the structure of
the lowest temperature phase of the benzene/hexafluorobenzene
material consists of face-to-face stacks of alternating molecules
(Figure 1).37,43 This stacking arrangement is thought to result
Results and Discussion
Photocycloaddition Reactions. Photodimerizations of trans-
cinnamic acid and its derivatives are among the most studied
topochemical reactions. Two photoactive polymorphs of cin-
namic acid are known: the R-form and the â-form. The R-form
produces R-truxillic acid upon photolysis, while the â-form
yields â-truxinic acid. The â-form is metastable and undergoes
a phase change to the R-form above room temperature. Due to
the electrostatic attraction between coplanar phenyl and per-
fluorophenyl groups, we anticipated that cinnamic acid (1) and
pentafluorocinnamic acid55 (2) might cocrystallize to give a
â-type arrangement. Crystallization of a 1:1 mixture of 1 (mp
133-4 °C, R-form) and 2 (mp 154-6 °C) from ethanol yielded
a colorless, microcrystalline material that melted at 169-71 °C.
The small crystal size did not permit X-ray structure analysis.
1H NMR spectroscopy revealed a 1:1 ratio of 1:2 in the solid.
The 1•2 cocrystal was subjected to photolysis at room temper-
ature; after 7 h, only 13% of the material remained unreacted.
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photostable in the solid state.

Phenyl-Perfluorophenyl Stacking Interactions
J. Am. Chem. Soc., Vol. 120, No. 15, 1998 3643
Scheme 2. Solid-State Photocycloaddition Reactions of 1•2,
4•5, and 7
Figure 2. Molecular packing diagram of the trans-stilbene (4)/trans-
decafluorostilbene (5) cocrystal. The fluorine atoms are shaded.
1H NMR revealed the formation of only one cyclobutane product
(3), which has been assigned the stereochemistry depicted in
Scheme 2 based on two pieces of information. First, reaction
of 3 with P2O5 yields the cyclic anhydride, which means the
two acid groups of the product must exhibit a cis relationship
on adjacent carbons of 3. Assuming that the trans dispositions
of the acid and phenyl groups of 1 and 2 are retained in the
product, then the stereochemistry of 3 can be assigned as shown
in Scheme 2. Second, the 19F NMR (376.5 MHz) spectrum
exhibits a coalescence of the ortho-fluorine shifts at -50 °C.
From the variable-temperature NMR data, a rotational barrier
for the pentafluorophenyl group of 9.3 ( 0.3 kcal/mol (at -50
°C) can be calculated, which is consistent with the cis relation-
ship of the phenyl and perfluorophenyl groups in 3. The
stereochemistry of 3, high photolysis yield, and complete
absence of homodimerization products is strong evidence that
1 and 2 pack sequentially in the crystal, in a head-to-head
arrangement. To probe the generality of the phenyl-perfluo-
rophenyl interaction in the design of photoactive crystals, the
cocrystallization of trans-stilbene (4) and trans-decafluorostil-
bene (5) was investigated. Cocrystallization of a 1:1 mixture
of 4 (mp 122-4 °C) and 5 (mp 103 °C) from ethanol yielded
colorless needle-shaped crystals that melted at 139 °C. Again,
1H NMR revealed the 1:1 ratio of 4:5 in the cocrystal.
diffraction. As can be seen in Figure 2, 4•5 is stacked in vertical
columns in an alternating fashion. Molecules of 4 and 5 are
situated on centers of inversion, which results in the equal
spacing of adjacent olefins. Significantly, the relative arrange-
ment of 4 and 5 confirms the stereochemistry of 6 as shown in
Scheme 2. The angles between reactive olefin carbons in 4
and 5 are 68.4° (C1-C2-C2′), 69.9° (C1-C1′-C2′), 112.2°
(C1′-C2′-C2), and 109.6° (C2-C1-C1′), and the olefin-
olefin separation is 3.784 (C1-C2) and 3.749 Å (C1′-C2′),
consistent with the photochemical reactivity of the crystal.
trans-Pentafluorostilbene (7) has been reported to undergo a
photochemical dimerization reaction in the solid state.57 In the
absence of X-ray structural data, the stereochemistry of the
cyclobutane product (8) was not unambiguously determined.
The photolysis of 7 (crystallized from diethyl ether/ethanol) was
carried out for 24 h under conditions identical to those for 4•5;
a quantitative yield of a single product (8) was determined by
1H NMR. The stereochemistry of 8 was assigned in the same
manner as for 6. As with cyclobutanes 3 and 6, variable-
temperature 19F NMR reveals the barrier to rotation of the
pentafluorophenyl group of 8 is 9.2 ( 0.3 kcal/mol at the
coalescence temperature (-55 °C, 376.5 MHz).
To confirm the stereochemistry of 8, the structures of 7 and
8 were determined by single-crystal X-ray diffraction. As can
be seen in Figure 3, 7 is stacked in vertical columns, and the
molecules are packed in a head-to-tail fashion. Although 7 at
first glance appears to be packed equally spaced in its stacks,
the molecules above and below a given molecule of 7 are related
by different inversion centers. The angles between reactive
olefin carbons in 7 are 109.4° (C1′-C2′-C1), 70.5° (C2′-C1′-
C2), 97.0° (C2′-C1′-C2′′), and 82.9° (C1′-C2′-C1′′); how-
ever, the olefin-olefin separation is nearly identical at 3.707
(C1-C2′) and 3.700 Å (C2′-C1′′). These geometrical param-
eters are consistent with the observed photochemical reactivity
of the crystal. To establish that the packing geometry of 7
determines the stereochemistry of the photoproduct, the X-ray
structure of 8 was determined. As shown in Figure 4, the
stereochemistry of 8 (as predicted by the packing geometry of
7 and NMR spectroscopic information) is confirmed.
Photolysis of the 4•5 cocrystal at ambient temperature for 5
h yielded a new product in quantitative yield. Note that neither
1
456 nor 557 are photoactive as the pure crystal. The H NMR
spectrum revealed the complete absence of shifts attributable
to 4 or 5, and is consistent with a sole cyclobutane product (6)
as depicted in Scheme 2. The assignment of the stereochemistry
of 6 is based on the coupling constants of the 1H NMR spectrum
(see Experimental Section). In addition, the cis relationship of
the H5- and F5-phenyls of 6 was confirmed by variable-
temperature 19F NMR. At the coalescence temperature of -43
°C (376.5 MHz), the barrier to rotation of the perfluorophenyl
group is 9.5 ( 0.3 kcal/mol. This value is consistent with the
barrier determined for 3.
To determine the packing arrangement of 4 and 5 in the
cocrystal, the structure was determined by single-crystal X-ray
(56) Hoekstra, A.; Meertens, P.; Vos, A. Acta Crystallogr. 1975, B31,
2813-2817.
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P. IzV. Akad. Nauk SSSR, Ser. Khim. 1981, 2106-2109.
A particularly unusual feature of the photodimerization of
4•5 and 7 is that both reactions proceed in virtually quantitative

3644 J. Am. Chem. Soc., Vol. 120, No. 15, 1998
Coates et al.
monoalkenes (Scheme 1). To determine whether the phenyl-
perfluorophenyl interaction could be used to design crystals for
photocycloaddition polymerizations, diolefins containing these
groups were synthesized and their photoreactivity was inves-
tigated.
The packing of 7 in the solid state suggested that extension
of this conjugated structure by the appropriate type of styryl
group might yield a diolefin capable of photopolymerization.
Two such molecules are possible: trans,trans-1,4-bis(2-phe-
nylethenyl)-2,3,5,6-tetrafluorobenzene (9) and trans, trans-1,4-
bis(2-pentafluorophenylethenyl)benzene (10). Dienes 9 and 10
were synthesized by a Heck coupling of the corresponding 1,4-
diiodobenzenes and styrenes. Compound 9 is a colorless,
crystalline solid, while 10 forms very light-green crystals.
Figure 3. Molecular packing diagram of the trans-pentafluorostilbene
(7). The fluorine atoms are shaded.
Photolysis of 9 for 20 h yielded a white powder that was
virtually insoluble in typical organic solvents. Extraction of
the solid with boiling toluene gave a soluble fraction (15%);
gel-permeation chromatography revealed that this soluble mate-
rial was composed of oligomers of 9 (predominantly dimer to
tetramer). Presumably, the main insoluble fraction (85%)
consists of higher molecular weight polymer. 1H NMR
spectroscopy demonstrated that the shifts of the cyclobutane
protons of the soluble product closely matched those of model
compound 8, suggesting that 9 packs in slanted stacks as
depicted in Scheme 1. Single-crystal X-ray structure analysis
confirmed this hypothesis (Figure 5). Molecules of 9 are
positioned on centers of inversion; therefore all reacting olefins
are equally spaced in the crystal. The distance between reactive
olefins is 3.823 Å (C1-C2′), and the angles between reactive
carbon atoms are 76.4°(C1-C2-C1′) and 103.6° (C2-C1-
C2′).
Diolefin 10 was found to be completely photostable, even
after photolysis for 24 h. An X-ray structural analysis of 10
(Figure 6) revealed that although the molecules pack in slanted
stacks, the olefin separation is 4.86 Å (C1-C2′). This distance
is clearly too large for an allowed photochemical reaction. It is
unknown why 10 packs in this unanticipated geometry, although
it should be mentioned that the stacking is structurally related
to the low-energy, parallel-slipped benzene dimer.59
Figure 4. Molecular structure of cyclodimer 8 from the photolysis of
trans-pentafluorostilbene (7). The fluorine atoms are shaded.
yield. Photodimerization reactions typically give less than 100%
yields, which has been attributed to olefins that are isolated
between cyclobutane molecules in the crystal matrix. Therefore
for evenly spaced olefin stacks in the crystal, the theoretical
maximum yield is 1 - 1/e2 ) 86.5%.58 From the X-ray
structure of 7, it can be seen that the molecules essentially pack
in pairs, explaining the quantitative yield of photodimer 8.
However in the structure of 4•5, the molecules are not packed
in pairs, but are packed in perfectly spaced stacks. At the current
time, it is not clear why this photodimerization proceeds in
quantitative yield. Possible explanations include the follow-
ing: perturbation of the olefin spacing following reactions of
adjacent olefins; an equilibration between isolated olefins and
the cyclodimer; and a heterogeneous photodimerization process
where the reaction occurs unevenly from the outside of the
crystal inward (along the stacking axis).
Molecular pairs that have complementary arrangements of
fluorinated/nonfluorinated aromatic groups typically form coc-
rystals, where the molecules pack alternately in well-ordered
stacked columns. For example, the mated molecular couples
benzene/hexafluorobenzene,43 naphthalene/octafluoronaphtha-
lene,60 biphenyl/decafluorobiphenyl,61 trans-stilbene/trans-de-
cafluoroazobenzene,62 4/5, and diphenyldiacetylene/decafluo-
Photopolymerization Reactions. In 1967 the first to-
pochemical photopolymerization of a diolefinic molecule was
reported. Subsequently, many studies have revealed that the
geometrical parameters that dictate reactivity in diolefin systems
(parallel alkenes, d < 4.2 Å) are the same as those of the
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Phenyl-Perfluorophenyl Stacking Interactions
J. Am. Chem. Soc., Vol. 120, No. 15, 1998 3645
Figure 5. Molecular packing diagram of diolefin 9 (the fluorine atoms are shaded) and the proposed structure of poly(9).
Figure 6. Molecular packing diagram of diolefin 10. The fluorine atoms are shaded.
rodiphenylacetylene54 pack in alternating layers of molecules.
Since dienes 9 and 10 exhibit complementary electrostatic
surfaces, the cocrystallization of these two molecules was
explored. Compounds 9 (mp 216 °C) and 10 (mp 208 °C) (1:1
solution in tetrahydrofuran) were cocrystallized by solvent
evaporation. 1H NMR revealed that the resultant fine, light
green needles (mp 227 °C) contained a 1:1 ratio of 9:10.
Photolysis of 9•10 for 20 h gave a polymeric product in
quantitative yield. The soluble fraction (67% in boiling toluene)
was analyzed by gel permeation chromatography, which con-
firmed the presence of oligomers. 1H NMR spectroscopy
revealed an average degree of polymerization of 22, and that
the shifts of the cyclobutane protons closely matched those of
model compound 8. Based on this information alone, we
propose that the 9•10 cocrystal consists of columns of alternate
molecules, which yields a zigzag type polymer structure63 upon
photolysis (Figure 7). Crystals of the 9•10 cocrystal suitable
for X-ray analysis have not yet been obtained.
Cocrystallization of trans,trans-1,4-Bis(2-pentafluorophe-
nylethenyl) benzene with Aromatic Solvents. 2,3,4,5,6-
Pentafluorobiphenyl,64 2,3,4,5,6-pentafluorodiphenylacetylene,65
2,3,4,5,6-pentafluorodiphenyldiacetylene,54 7, and the cocrystals
listed in the previous section are all known to stack in columns
of alternating phenyl-perfluorophenyl groups. One common
trait to each of these compounds is the 1:1 ratio of H5-phenyl:
F5-phenyl groups in these structures. One noticeable feature
of photostable diene 10 is the uneven ratio (2:1) of fluorinated
arene groups to nonfluorinated arene groups. Therefore, the
augmentation of the hydrocarbon arene component of 10 was
explored by attempting cocrystallization with a variety of
(64) Brock, C. P.; Naae, D. G.; Goodhand, N.; Hamor, T. A. Acta
Crystallogr., Sect. B 1978, 34, 3691-3696.
(65) Henling, L. M.; Dunn, A. R.; Coates, G. W.; Grubbs, R. H.
Unpublished results.
(63) Hasegawa, M. AdV. Phys. Org. Chem. 1995, 30, 117-171.

3646 J. Am. Chem. Soc., Vol. 120, No. 15, 1998
Coates et al.
Figure 7. Proposed solid-state packing and polymerization stereochemistry of the 9•10 cocrystal.
Figure 8. Molecular packing diagram of the cocrystal of 10 with o-xylene (11). The fluorine atoms are shaded.
unfluorinated aromatic solvents. Recrystallization of 10 from
both benzene and toluene yielded photostable crystals with no
included solvent. However recrystallization from o-xylene (11)
produced a 1:1 cocrystal, which upon photolysis yielded a
soluble cyclobutane product. X-ray structural analysis of the
10•11 cocrystal revealed a staggered, vertical stack of 10 where
vacant spaces between the diene “bricks” are filled by o-xylene
molecules (Figure 8). The angles between reactive olefin
carbons in 10•11 are 73.0° (C1′-C2′-C1), 107.0° (C2′-C1′-
C2), 80.0° (C2′-C1-C2′′), and 99.9° (C1′-C2′-C1′′), and the
olefin-olefin separation is approximately 3.6 Å (3.638 Å (C1-
C2′) and 3.598 Å (C2′-C1′′)). An interesting feature of this
structure is the edge-to-edge interaction between the para
fluorine atom of 10 and the ortho hydrogen of the adjacent
xylene (d ) 2.47 Å). This intermolecular C-F‚‚‚H-C interac-
tion66,67 was observed in the structures previously reported in
this paper, and is common among structures that contain phenyl
suggesting that a rearrangement occurs during the reaction with
xylene loss, allowing a photopolymerization of 10 to occur.
Conclusions
The current results indicate that the alternating, stacked
phenyl-perfluorophenyl arrangement first seen in benzene-
hexafluorobenzene is a general supramolecular motif. In
monoolefinic and diolefinic systems, these aromatic associations
orient the reactive olefinic moieties such that a photochemical
cycloaddition reaction is possible (olefins parallel, separated by
less than 4.2 Å). Of the seven crystalline olefinic materials
investigated in this paper, all but one (diene 10) undergoes the
anticipated photochemical reaction. In this one exceptional case,
the diene packs with a slipped-stacked orientation of the
aromatic groups, which is known to be a fairly low energy
interaction for the benzene dimer. In the other cases, the
stereochemistry of the photoproduct and/or X-ray structural
analysis of the olefinic precursors reveals the stacked interaction
between phenyl and perfluorophenyl groups in the crystals. This
face-to-face interaction of phenyl/perfluorophenyl groups ap-
pears to direct the geometry of crystallization to yield photo-
active crystals. Due to the ease of incorporating phenyl and
perfluorophenyl groups into molecules, we believe this novel
intermolecular interaction will stimulate new developments in
43,54
and pentafluorophenyl groups.
Molecules of 10 are predisposed in the crystal for simple
dimerization; however upon photolysis xylene loss decreases
the yield and selectivity of the coupling reaction. The resultant
product is virtually insoluble in ordinary organic solvents,
(66) Dunitz, J. D.; Taylor, R. Chem. Eur. J. 1997, 3, 89-98.
(67) Teff, D. J.; Huffman, J. C.; Caulton, K. G. Inorg. Chem. 1997, 36,
4372-4380.

Phenyl-Perfluorophenyl Stacking Interactions
J. Am. Chem. Soc., Vol. 120, No. 15, 1998 3647
The organic layer was washed three times with 50 mL of 1 M HCl
and dried over MgSO4. The organic phase was concentrated under
reduced pressure and purified by column chromatography (silica gel/
hexanes) yielding 5 as a white, fluffy powder (0.560 g, 81%). Spectral
data and physical properties matched those reported in the literature:69
1H NMR (CDCl3, 400 MHz) δ 7.32 (s, 2H); 19F NMR (CDCl3, CFCl3,
376.5 MHz) δ -141.7, -153.4, -161.9; MS m/z (MH+) calcd for
C14H2F10 359.9997, found 359.9998; mp 103 °C.
4•5 Cocrystal. trans-Stilbene (4) and trans-decafluorostilbene (5)
(1:1) were cocrystallized from a saturated, hot absolute ethanol solution.
The solution was allowed to cool slowly to room temperature, during
which colorless, transparent crystals were formed. After crystallizing
overnight, the cocrystals were collected by filtration over a medium
pore frit, then dried in vacuo to give 4•5. 1H NMR confirmed a 1:1
ratio of 4•5. Mp 139 °C.
1r,2â,3â,4r-1,2-Bis(pentafluorophenyl)-3,4-diphenylcyclobu-
tane (6) from the Photolysis of 4•5. Crystals of 4•5 were placed in
a quartz reaction tube and photolyzed for 5 h at ambient temperature.
1H NMR revealed quantitative conversion to cyclodimer 6: 1H NMR
(CDCl3, 400 MHz) δ 7.21 (m, 10 H), 5.09 (d, J ) 9.1 Hz, 2 H), 4.551
(d, J ) 9.1 Hz, 2 H); 19F NMR (CDCl3, CFCl3, 376.5 MHz) δ -142.1,
-155.5, -162.2; variable-temperature 19F NMR data: coalescence
temperature 230 K, ∆ν ) 1930 Hz for the ortho fluorine shifts at 213
K; MS m/z (M - H)+ calcd for C28H13F10 539.0857, found 539.0876.
trans-2,3,4,5,6-Pentafluorostilbene (7). Compound 7 was synthe-
sized with use of a modified literature preparation.70 Tripropylamine
(4.0 mL) and pentafluorostyrene (2.34 g, 12.0 mmol) were separately
filtered over silica and then combined with iodobenzene (2.45 g, 12.0
mmol) in a 10 mL Schlenk tube. Palladium acetate (27.8 mg, 0.124
mmol) was added to the solution and the mixture was freeze-pump-
thawed twice under argon. The reaction mixture was heated to 100
°C for 20 h. During this time a white precipitate formed and the
solution turned red. The reaction mixture was cooled to room
temperature and diluted with Et2O (100 mL) and DI H2O (50 mL).
The aqueous phase was washed with Et2O (50 mL). The combined
organic fractions were washed twice with DI H2O (25 mL) and once
with 1 M HCl (25 mL). The solvent was then removed from the organic
phase under reduced pressure yielding a light yellow solid. Chroma-
tography (SiO2/hexanes) afforded 7 as a white solid (1.70 g, 52.4%).
Spectral data and physical properties matched those reported in the
literature:70 1H NMR (CDCl3, 400 MHz) δ 6.99 (d, J ) 16.8 Hz, 1 H),
7.36 (m, 2 H), 7.50 (d, J ) 16.8, 1 H), 7.55 (m, 3 H); 19F NMR (CDCl3,
CFCl3, 376.5 MHz) δ -142.6, -156.4, -162.9; MS m/z (M+) calcd
for C14H7F5 270.0466, found 270.0468; mp ) 138 °C.
areas of science where the ability to control the solid-state
geometry of molecules is important.
Experimental Section
General Considerations. All manipulations involving olefin or
diolefin synthesis were performed with Schlenk line techniques.
Tripropylamine was dried over activated 4 Å molecular sieves, toluene
and tetrahydrofuran were passed over solvent purification columns,68
styrene and 2,3,4,5,6-pentafluorostyrene were passed over a 2 cm plug
of silica gel immediately prior to use, and other reagents and solvents
were purchased from Aldrich and used as received.
1H and 19F NMR spectra were obtained on a JEOL GX400
spectrometer; 13C NMR spectra were recorded with a General Electric
QE-300 spectrometer. Gel permeation chromatography was performed
by using an Altex Model 110A pump, an American Polymer Standards
Am Gel column (10 µm linear porosity) with chromatography grade
CH2Cl2 as eluent, and a Kratos Spectroflow 757 UV absorbance
detector. Gas chromatography/mass spectroscopy was performed on
a Hewlett-Packard 5890A gas chromatograph coupled with a Hewlett-
Packard 5970 Series Mass Selective Detector. Low- and high-resolution
mass spectra were obtained at the Chemistry and Biology Mass
Spectrometry Facility at Caltech or at the Southern California Mass
Spectrometry Facility at the University of California, Riverside.
Polymerization reactions were carried out in quartz or Pyrex reaction
tubes under argon with use of a quartz-jacketed 450 W Hanovia
medium-pressure mercury vapor lamp at ambient temperature.
Procedures: 1•2 Cocrystal. trans-Cinnamic acid (1, 1.00 g, 6.75
mmol) and trans-2,3,4,5,6-pentafluorocinnamic acid (2, 1.61 g, 6.75
mmol) were dissolved in 8 mL of hot absolute ethanol. The solution
was allowed to cool slowly to room temperature, during which fine
white crystals were formed. After crystallizing overnight, the cocrystals
were collected by filtration over a medium pore frit, then dried in vacuo
to give 1•2 (2.45 g, 94%). 1H NMR confirmed a 1:1 ratio of 1•2. Mp
169-171 °C.
1r,2r,3â,4â-3-Pentafluorophenyl-4-phenyl-1,2-cyclobutanedicar-
boxylic Acid (3) from the Photolysis of 1•2. Crystals of 1•2 (20.0
mg, 0.0517 mmol) were placed in a quartz reaction tube and photolyzed
for 7 h at ambient temperature. 1H NMR spectroscopy revealed
unreacted 1•2 (13%) and the cyclodimer 3 (87%): 1H NMR (CDCl3,
400 MHz) δ 8.5 (br, s, 1H), 7.22 (m, 3H), 7.05 (m, 2H), 4.77 (m, 1H),
4.36 (m, 2H), 4.05 (m, 1H); 13C NMR {1H} (CDCl3, 100 MHz) δ 36.6,
41.9, 44.3, 44.5, 126.0, 126.5, 128.5, 137.3, 178.6, 179.1, and a series
of broad peaks in the aromatic region (Ar C-F); 19F NMR (CDCl3,
CFCl3, 376.5 MHz) δ -141.6, -156.4, -163.2; variable-temperature
19F NMR data: coalescence temperature 223 K, ∆ν ) 1530 Hz for
the ortho fluorine shifts at 213 K; MS m/z (MH+) calcd for C18H12F5O4
387.07, found 387.1.
1r,2r,3â,4â-1,3-Bis(pentafluorophenyl)-2,4-diphenylcyclobu-
tane (8) from the Photolysis of 7. Crystals of 7 were placed in a
quartz reaction tube and photolyzed for 24 h at ambient temperature.
1H NMR revealed quantitative conversion to cyclodimer 8: 1H NMR
(CDCl3, 400 MHz) δ 4.88 (m, 4 H), 7.2 (m, 10 H); 19F NMR (CDCl3,
CFCl3, 376.5 MHz) δ -141.2, -155.7, -162.4; variable-temperature
19F NMR data: coalescence temperature 218 K, ∆ν ) 1235 Hz for
the ortho fluorine shifts at 213 K; MS m/z (M+) calcd for C28H14F10
540.0936, found 540.0937, 270.0 (from cyclobutane fragmentation);
mp ) 118 °C.
Dehydration of 3. Crude 3 (17 mg, 0.044 mmol) was dissolved in
CDCl3 (1 mL) and ca. 50 mg of P2O5 was added. The solution was
briefly heated to boiling, then allowed to sit overnight. The solution
was decanted from the excess P2O5 and analyzed by NMR: 1H NMR
(CDCl3, 400 MHz) δ 7.2 (m, 5H), 4.59 (m, 1H), 4.49 (m, 1H), 4.23
(m, 1H), 4.12 (m, 1H); MS m/z (MH+) calcd for C18H10F5O3 369.06,
found 369.1.
trans,trans-1,4-Bis(2-phenylethenyl)-2,3,5,6-tetrafluorobenzene (9).
Tripropylamine (3.0 mL) and styrene (1.76 g, 16.9 mmol) were
separately filtered over silica and then combined with 1,4-diiodotet-
rafluorobenzene (3.40 g, 8.46 mmol) in a 10 mL Schlenk tube.
Palladium acetate (37.9 mg, 0.169 mmol) was added to the solution
and the mixture was freeze-pump-thawed twice under argon. The
reaction mixture was heated to 100 °C for 20 h. During the course of
the reaction the reaction mixture turned brown. The reaction mixture
was cooled to room temperature, causing a white solid to precipitate,
and diluted with Et2O (800 mL) and 1 M HCl (100 mL). The organic
fraction was washed with 1 M HCl (100 mL) and solvent was then
removed in vacuo. Chromatography (SiO2/hexanes, ethyl acetate)
afforded 9 as a white solid, which was recrystallized from a saturated
chloroform solution (0.567 g, 18.9%): 1H NMR (CDCl3, 400 MHz) δ
7.09 (d, J ) 16.8 Hz, 2 H), 7.32 (m, 2 H), 7.39 (m, 4 H), 7.51 (d, J )
trans-Decafluorostilbene (5). Compound 5 was synthesized with
use of a modified literature preparation.69 Triphenylphosphine (1.01
g, 3.85 mmol) and 2,3,4,5,6-pentafluorobenzylbromide (0.59 mL, 1.01
g, 3.90 mmol) were measured into a dry 100 mL Schlenk flask under
argon. The reactants were dissolved in 25 mL of dry toluene and
refluxed for 90 min, resulting in an air-stable white powder upon
filtration (1.22 g, 61%). The phosponium salt (1.06 g, 2.02 mmol)
was dissolved in 40 mL of dry THF and deprotonated with KOtBu
(0.215 g, 1.92 mmol) over 4.5 h. Next, pentafluorobenzaldehyde (0.380
g, 1.94 mmol) was added to the reaction mixture, which was then stirred
at room temperature for 14 h. The reaction mixture was then quenched
with several drops of distilled H2O and diluted with 100 mL of Et2O.
(68) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(69) Muszkat, K. A.; Castel, N.; Jakob, A.; Fischer, E.; Luettke, W.;
Rauch, K. J. Photochem. Photobiol. A: Chem. 1991, 56, 219-226.
(70) Fuchikami, T.; Yatabe, M.; Ojima, I. Synthesis 1981, 365-366.

3648 J. Am. Chem. Soc., Vol. 120, No. 15, 1998
Coates et al.
16.8 Hz, 2 H), 7.55 (m, 4 H); 19F NMR (CDCl3, CFCl3, 376.5 MHz)
δ -144.4; MS m/z (M+) calcd for C22H14F4 354.1032, found 354.1044;
mp ) 216 °C.
) 97.186(2)°, â ) 92.450(2)°, γ ) 93.592(3)°, V ) 570.38(12) Å3, Z
) 2, Fcalcd ) 1.573 g cm-3, 2θmax ) 41.02°, T ) 293 K. SMART
CCD Area Detector System (Mo KR, λ ) 0.71073 Å) frames were
integrated with the Siemens SAINT program to yield a total of 1977
reflections, of which 1173 were independent (Rint ) 3.10%) and 851
were above 4σ(F). Data were corrected for absorption with the
SADABS program. Structure solution and refinement: direct methods
(SHELXS-86),72 full-matrix least-squares refinement on F2,73 using
anisotropic displacement parameters for all non-hydrogen atoms, 172
parameters, R1 ) 0.0542, wR2 ) 0.1330 with I > 2σ(I), R1 ) 0.0761,
wR2 ) 0.1482 for all data, ∆Fmax ) 0.164 e Å-3, ∆Fmin ) -0.226 e
Photopolymerization of 9. Crystals of 9 (43 mg) were placed in a
Pyrex reaction tube under argon and photolyzed for 20 h at ambient
temperature. The resulting white powder was extracted with 100 mL
of refluxing toluene, yielding 6.5 mg of soluble material upon filtration
(15% yield). This soluble fraction was analyzed by NMR: 1H NMR
(CDCl3, 400 MHz) δ 4.75 (m, ca. 8 H), 7.15 (m, ca. 20 H), 7.35 (m,
ca. 6 H), 7.50 (m, ca. 4 H); 19F NMR (CDCl3, CFCl3, 376.5 MHz) δ
-142.6, -143.1, -144.1. Gel-permeation chromatography showed that
oligomers were present.
Å-3
.
trans, trans-1,4-Bis(2-pentafluorophenylethenyl)benzene (10). Com-
pound 10 was synthesized with use of a modified literature prepara-
tion.70 Tripropylamine (12.0 mL) and pentafluorostyrene (3.97 g, 20.5
mmol) were separately filtered over silica and then combined with
toluene (30 mL) and 1,4-diiodobenzene (3.25 g, 9.85 mmol) in a 100
mL Schlenk tube. Palladium acetate (50.2 mg, 0.224 mmol) was added
to the solution and the mixture was freeze-pump-thawed twice under
argon. The reaction mixture was heated to 100 °C for 20 h. During
the course of the reaction a large amount of white precipitate formed.
The reaction mixture was cooled to room temperature, causing a white
solid to precipitate, and diluted with Et2O (600 mL) and DI H2O (300
mL). The organic fraction was washed with 1 M HCl and solvent
was then removed in vacuo, giving a reddish-brown solid. The crude
product was washed with CH2Cl2, giving a pale yellow-green solid
which was dissolved in tetrahydrofuran and passed over a short column
of silica gel. Removal of solvent in vacuo yielded 10 (2.40 g, 52.7%).
Spectral data and physical properties matched those reported in the
literature:71 1H NMR (CDCl3, 400 MHz) δ 7.03 (d, J ) 16.8 Hz, 2 H),
7.44 (d, J ) 16.8 Hz, 2 H), 7.56 (s, 4 H); 19F NMR (CDCl3, CFCl3,
376.5 MHz) δ -142.4, -155.9, -162.6; MS m/z (M+) calcd for
C22H8F10 462.0466, found 462.0458; mp ) 208 °C.
Single-Crystal X-ray Diffraction Analysis of 7. Crystal structure
data for 7 (single crystal from diethyl ether/ethanol): formula C14H7F5,
M ) 270.20, crystal dimensions 0.17 × 0.37 × 0.41 mm, triclinic,
space group P1h, a ) 6.058(2) Å, b ) 7.329(3) Å, c ) 12.386(5) Å, R
) 85.71(4)°, â ) 87.60(3)°, γ ) 86.39(3)°, V ) 546.9(4) Å3, Z ) 2,
Fcalcd ) 1.641 g cm-3, 2θmax ) 56°, CAD-4 diffractometer (Mo KR, λ
) 0.7107 Å, graphite monochromator), ω scan, T ) 160 K, 5431
measured reflections, all 2633 independent reflections used in the
refinement, Lorentz and polarization factors were applied, µ ) 1.54
cm-1. Structure solution and refinement: direct methods (SHELXS-
86),72 full-matrix least-squares refinement of F2 (CRYM),74 200
parameters, hydrogen atoms were refined isotropically, R ) 0.036 with
2
2
2
Fo > 3σ(Fo ) and 0.044 with Fo > 0, wR ) 0.083, ∆Fmax ) 0.31 e
Å-3, ∆Fmin ) -0.29 e Å-3
.
Single-Crystal X-ray Diffraction Analysis of 8. Crystal structure
data for 8 (single crystal from pentane): formula C28H14F10, M )
540.39, crystal dimensions 0.60 × 0.50 × 0.30 mm, monoclinic, space
group P21, a ) 12.888(3) Å, b ) 15.828(3) Å, c ) 13.117(3) Å, â )
116.81(3)°, V ) 2388.1(8) Å3, Z ) 4, Fcalcd ) 1.503 g cm-3, 2θmax
)
41.54°, T ) 293 K. SMART CCD Area Detector System (Mo KR, λ
) 0.71073 Å), frames were integrated with the Siemens SAINT
program to yield a total of 8263 reflections, of which 3887 were
independent (Rint ) 5.62%) and 2879 were above 4σ(F). Data were
corrected for absorption with the SADABS program. Structure solution
and refinement: direct methods (SHELXS-86),72 full-matrix least-
squares refinement on F2,73 using anisotropic displacement parameters
for all non-hydrogen atoms, 696 parameters, R1 ) 0.1233, wR2 )
0.3289 with I > 2σ(I), R1 ) 0.1453, wR2 ) 0.3598 for all data, ∆Fmax
Attempted Photolysis of 10. Crystals of 10 grown from benzene,
tetrahydrofuran, ethanol, CH2Cl2/hexanes, and chloroform were deter-
1
mined to be completely photostable by H NMR.
9•10 Cocrystal. Dienes 9 and 10 (1:1) were cocrystallized from a
saturated, hot mixture of toluene and ethanol (40:60). The solution
was allowed to cool slowly to room temperature, during which very
fine, light green needles were formed. After crystallizing overnight,
the cocrystals were collected by filtration over a medium pore frit, then
dried in vacuo to give 9•10. 1H NMR confirmed a 1:1 ratio of 9•10.
Mp 227 °C.
Photopolymerization of 9•10. Crystals of 9•10 (145 mg) were
placed in a quartz reaction tube and photolyzed for 20 h at ambient
temperature. The resulting white powder was extracted with 100 mL
of refluxing toluene, yielding 97 mg of soluble material upon filtration
(67% yield). This soluble fraction was analyzed by 1H NMR. A broad
multiplet at 4.75 ppm attributable to protons attached to the cyclobutane
ring was present, consistent with the cyclobutane shifts of 8. Integration
of the olefin and cyclobutane resonances yielded an average degree of
polymerization of 22. In addition, gel permeation chromatography
showed the presence of multiple oligomers.
) 0.650 e Å-3, ∆Fmin ) -0.371 e Å-3
.
Single-Crystal X-ray Diffraction Analysis of 9. Crystal structure
data for 9 (single crystal from chloroform): formula C22H14F4, M )
354.33, crystal dimensions 0.50 × 0.05 × 0.03 mm, monoclinic, space
group P21/c, a ) 7.8076(11) Å, b ) 5.8086(8) Å, c ) 18.291(3) Å, R
) 90°, â ) 97.10(2)°, γ ) 90°, V ) 823.2(2) Å3, Z ) 2, Fcalcd ) 1.430
g cm-3, 2θmax ) 49.54°, T ) 173(1) K. SMART CCD Area Detector
System (Mo KR, λ ) 0.71073 Å), frames were integrated with the
Siemens SAINT program to yield a total of 3788 reflections, of which
1408 were independent (Rint ) 6.51%). Data were corrected for
absorption with the SADABS program. Structure solution and refine-
ment: direct methods (SHELXS-86),72 full-matrix least-squares refine-
ment on F2,73 using anisotropic displacement parameters for all atoms,
696 parameters, R1 ) 0.0620, wR2 ) 0.1397 with I > 2σ(I), R1 )
10•11 Cocrystal. Diene 10 was recrystallized from a hot, saturated
solution of o-xylene. Thin, light-green plates were formed. 1H NMR
confirmed a 1:1 ratio of 10•11. A melting point could not be measured
due to evaporative loss of xylene during the measurement.
0.1289, wR2 ) 0.1953 for all data, ∆Fmax ) 0.285 e Å-3, ∆Fmin
-0.271 e Å-3
)
.
Photolysis of 10•11. Crystals of 10•11 were placed in a quartz
reaction tube under argon with some o-xylene solvent and were
photolyzed for 10 h. Over the course of the reaction, the crystals turned
from light green to a white powder. This photolysis product is
practically insoluble in typical organic solvents; however a1H NMR
spectrum of a small soluble fraction showed a multiplet at 4.75 ppm,
indicative of the formation of a cyclobutane ring. Further attempts to
analyze this product by NMR and mass spectroscopy were unsuccessful.
Single-Crystal X-ray Diffraction Analysis of 4•5. Crystal structure
data for 4•5 (single crystal from ethanol): formula C28H14F10, M )
540.40, crystal dimensions 0.60 × 0.15 × 0.07 mm, triclinic, space
group P1h, a ) 6.0932(7) Å, b ) 7.5325(9) Å, c ) 12.5669(15) Å, R
Single-Crystal X-ray Diffraction Analysis of 10. Crystal structure
data for 10 (single crystal from toluene): formula C22H8F10, M )
462.28, crystal dimensions 0.04 × 0.24 × 0.26 mm, monoclinic, space
group P21/c, a ) 4.8334(9) Å, b ) 6.1473(8) Å, c ) 29.262(4) Å, R
) 90°, â ) 91.191(13)°, γ ) 90°, V ) 869.3(2) Å3, Z ) 2, Fcalcd
)
1.766 g cm-3, θmax ) 27.50°. A Siemens P4 diffractometer was used
(Mo KR, λ ) 0.71073 Å, graphite monochromator). The determination
of Laue symmetry, crystal class, unit cell parameters, and the crystal’s
(72) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(73) Sheldrick, G. M. SHELX93, Program for refinement of crystal
structures; University of Go¨ttingen, Germany, 1993.
(74) Duchamp, D. J., American Crystallographic Association Meeting,
Bozeman, Montana, 1964; paper B14, pp 29-30.
(71) Filler, R.; Gadomski, J. E. J. Fluorine Chem. 1990, 47, 175-178.

Phenyl-Perfluorophenyl Stacking Interactions
J. Am. Chem. Soc., Vol. 120, No. 15, 1998 3649
orientation matrix was carried out according to standard procedures.75
Intensity data were collected with use of a 2θ/ω scan technique, T )
158 K. The raw data were processed with a local version of CARESS76
which employs a modified version of the Lehman-Larsen algorithm
to obtain intensities and standard deviations from the measured 96-
step peak profiles. Subsequent calculations were carried out by using
the SHELXTL program.77 All 2218 data were corrected for Lorentz
and polarization effects and were placed on an approximately absolute
scale.
The structure was solved by direct methods and refined on F2 by
full-matrix least-squares techniques. The analytical scattering factors
for neutral atoms were used throughout the analysis.78 The molecule
is located about an inversion center. Hydrogen atoms were located
from a difference Fourier map and refined (x, y, z and Uiso). At
convergence, wR2 ) 0.1718 and GOF ) 1.068 for 162 variables refined
against all 1988 unique data (as a comparison for refinement on F, R1
) 0.0591 for those 1309 data with F > 4.0σ(F)).
triclinic, space group P1h, a ) 7.220(4) Å, b ) 7.926(3) Å, c )
23.118(13) Å, R ) 83.25(4)°, â ) 83.94(5)°, γ ) 67.11(4)°, V )
1207.7(11) Å3, Z ) 2, Fcalcd ) 1.563 g cm-3, 2θmax ) 50°, CAD-4
diffractometer (Mo KR, λ ) 0.7107 Å, graphite monochromator), ω
scan, T ) 160 K, 9381 measured reflections, 4238 independent
reflections used in the refinement, Lorentz and polarization factors were
applied, µ ) 1.44 cm-1
. Structure solution and refinement: direct
methods (SHELXS-86),72 full-matrix least-squares refinement of F2
(CRYM),74 433 parameters, hydrogen atoms were refined isotropically,
2
R ) 0.054 with Fo2 > 3σ(Fo ), 0.064 with Fo2 > 0, wR ) 0.119, ∆Fmax
) 0.34 e Å-3, ∆Fmin ) -0.43 e Å-3
.
Acknowledgment. Financial support of this work was
provided by the National Science Foundation and the AFOSR
through its MURI program. G.W.C. gratefully acknowledges a
Camille and Henry Dreyfus New Faculty Award, and A.R.D.
thanks the Camille and Henry Dreyfus Foundation for a Caltech
for a Summer Undergraduate Research Fellowship.
Single-Crystal X-ray Diffraction Analysis of 10•11. Crystal
structure data for 10•11 (single crystal from o-xylene): formula
C30H18F10, M ) 568.46, crystal dimensions 0.30 × 0.46 × 0.55 mm,
Supporting Information Available: Tables of crystal data,
structure solution and refinement, atomic coordinates, bond
lengths and angles, and anisotropic thermal parameters for 4‚5,
7, 8, 9, 10, and 10‚11 (62 pages, print/PDF). See any current
masthead page for ordering information and Web access
instructions.
(75) XSCANS Software Users Guide, Version 2.1; Siemens Industrial
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