Resorcinol-templated head-to-head photodimerization of a thiophene in the solid state and unusual edge-to-face stacking in a discrete hydrogen-bonded assembly

Article abstract of DOI:10.1021/ol4035403

A head-to-head photodimerization of a β-substituted thiophene stacked face-to-face in the solid state using a ditopic hydrogen-bond-donor template is reported. The face-to-face stacking is attributed to contributions of intertemplate forces, which contrasts an assembly wherein the same thiophenes stack edge-to-face yet maintain a discrete hydrogen-bonded structure.

Full text of DOI:10.1021/ol4035403

Letter
pubs.acs.org/OrgLett
Resorcinol-Templated Head-to-Head Photodimerization of a
Thiophene in the Solid State and Unusual Edge-to-Face Stacking in a
Discrete Hydrogen-Bonded Assembly
Kristin M. Hutchins,† Joseph C. Sumrak,† and Leonard R. MacGillivray*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
S
* Supporting Information
ABSTRACT: A head-to-head photodimerization of a β-substituted
thiophene stacked face-to-face in the solid state using a ditopic
hydrogen-bond-donor template is reported. The face-to-face stacking
is attributed to contributions of intertemplate forces, which contrasts
an assembly wherein the same thiophenes stack edge-to-face yet
maintain a discrete hydrogen-bonded structure.
Scheme 1. (a) Solid-State Packings of Thiophenes and (b)
Comparison of Head-to-Head vs Head-to-Tail Stacked
Olefins
upramolecular approaches to direct the assembly and
Spacking of molecules are of increasing importance to
control chemical reactivity in the solid state.1 In this context,
small molecules that function as hydrogen-bond-donor
templates have emerged as useful to overcome effects of
crystal packing to direct intermolecular [2 + 2] photo-
dimerizations in solids. For a photodimerization to occur in
the solid state, carbon−carbon double (CC) bonds are
generally required to be parallel and separated <4.2 Å.2 In this
context, we have reported resorcinol (res), and derivatives, as
small-molecule ditopic templates that confront the crystal
packing problem to reliably assemble olefins in solids into
favorable geometries for the cycloaddition reaction.3 Reactivity
is achieved within discrete hydrogen-bonded assemblies with
olefins preorganized into face-to-face π-stacked arrangements.
Successes of the template method are attested by the
stereocontrolled syntheses of molecules (e.g., ladderanes,
cyclophanes) that are difficult to achieve using more conven-
tional solution methods.1a
With this in mind, we report here the use of a res to direct a
solid-state photodimerization of a thiophene. Thiophenes are
integral components of organic semiconductor materials,4
comprising organic light-emiting diodes,5 organic thin-film
transistors,6 and photovoltaic cells.7,8 Moreover, we expected
that a res could overcome edge-to-face π-forces that dominate
the solid-state packings of thiophenes so as to enforce a face-to-
face geometry of two olefins in a head-to-head orientation to
undergo the photoreaction (Scheme 1). Photodimerizations of
thiophenes in both solution and the solid state, however, are
extremely rare,9 with attempts to gain levels of regiocontrol of
the photoreaction having been explored in very limited cases.
Here, 4,6-diiodo-res (4,6-diI-res) is used as a template to
assemble trans-1-(4-pyridyl)-2-(3-thienyl)ethylene (β-PTE)
into a discrete hydrogen-bonded assembly in a co-crystal of
composition (4,6-diI-res)·2(β-PTE). The olefin adopts a face-
to-face π-stacked geometry and undergoes an intermolecular [2
+ 2] photodimerization to generate head-to-head rctt-1,2-bis(4-
pyridyl)-3,4-bis(3-thienyl) cyclobutane (4p3tcb) regioselec-
tively and in near-quantitative yield (Scheme 2). We also
demonstrate the res congener 4,6-dichlororesorcinol (4,6-diCl-
res) to support β-PTE in an edge-to-face π-stacked geometry in
(4,6-diCl-res)·2(β-PTE), which is photostable. We are
unaware of an example wherein a res accommodates edge-to-
face π-stacking within a discrete hydrogen-bonded assembly.
Moreover, we attribute the photoreactivity of (4,6-diI-res)·2(β-
Received: December 6, 2013
Published: February 5, 2014
© 2014 American Chemical Society
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Letter
To orient β-PTE into a geometry for a photodimerization,
the olefin was co-crystallized with 4,6-diI-res in a 2:1 ratio in
acetonitrile. Colorless plate-shaped crystals formed upon slow
solvent evaporation after a period of 2 days. The formulation of
(4,6-diI-res)·2(β-PTE) was confirmed by H NMR spectros-
copy and single-crystal X-ray diffraction.
Scheme 2. Assembled Thiophene Using a res
1
A single-crystal X-ray analysis18 confimed 4,6-diI-res to
assemble β-PTE into a face-to-face π-stacked geometry in the
chiral space group P21. The asymmetric unit consists of two
molecules of 4,6-diI-res and four molecules of β-PTE that
comprise two three-component supramolecular assemblies (A
and B), each sustained by two intermolecular O−H···N
hydrogen bonds [O···N separation (Å): O(1)···N(1) 2.64(3),
O(2)···N(2) 2.72(1), O(3)···N(3) 2.66(1), O(4)···N(4)
2.69(1)] (Figure 2). Each β-PTE is disordered over two sites
PTE) to contributions of secondary I···O interactions involving
the templates that effectively act to compress the olefins into
requisite face-to-face π-stacked geometry in the solid.
Figure 2. X-ray structure of (4,6-diI-res)·2(β-PTE): (a) assembly A
and (b) assembly B. Highest occupancy is shown for each olefin.
Photodimerizations of styrylthiophenes pose an interesting
synthetic challenge. The parent olefin trans-1,2-di(3-thienyl)-
ethene does not photodimerize in solution,10 while there are
few reported styrylthiophenes that react to form cyclobutanes.
Very few of the olefins react in appreciable yields (i.e., >30%) in
either solution or the solid state.11 High yielding photo-
dimerizations (i.e., >70%) of thiophenes are extremely rare.12
To our knowledge, the only β-substituted thiophene reported
to undergo a photodimerization in high yield is (E)-1-2′,5′-
dibromo-3′-thienyl-2-pentafluoro-phenylethene, which gener-
ates a head-to-tail, as opposed to a head-to-head, photo-
product.13
We initially focused on the reactivity of β-PTE as a pure
solid. The olefin was synthesized via a Heck reaction of 4-
vinylpyridine and 3-bromothiophene.14 Colorless plate-shaped
single crystals of β-PTE were obtained from slow evaporation
of a saturated toluene solution over a period of 2 days.
A crystal structure analysis15,16 revealed β-PTE, which is
disordered over three sites [site occupancies: 0.66(1), 0.23(1),
and 0.11(1)], to adopt typical edge-to-face packing in the
acentric space group Pc. The edge-to-face packing places the
olefin into two orientations (labeled as ABA) along the b-axis
(Figure 1). The mean planes of β-PTE are canted at 62°,
[site occupancies: 0.81(1) and 0.19(1) and 0.79(1) and 0.21(1)
for A, 0.77(1) and 0.23(1) and 0.56(1) and 0.44(1) for B]. In
assembly A, the mean planes of the β-PTE molecules are
canted at 33° and olefins are separated by 4.30 Å, with the
thiophene rings of the major occupied sites pointing in the
same direction. In assembly B, the mean planes of β-PTE are
canted at 13° and olefins are separated by 4.15 Å, with the
thiophene rings of the major sites pointing in opposite
directions. The separations between olefins in adjacent
assemblies are 5.35 and 5.63 Å for assembly A and B,
respectively.
Since the olefins of (4,6-diI-res)·2(β-PTE) lie in a geometry
suitable for a [2 + 2] photodimerization, a powdered crystalline
sample (50 mg) was irradiated with UV radiation (medium
pressure Hg). A photodimerization was confirmed by the
emergence of a cyclobutane peak (4.6 ppm) and upfield shifts
1
(0.2 ppm) of the pyridine peaks in the H NMR spectrum.
After a period of 100 h, photodimerization occurred in up to
90% yield.19
To confirm the stereochemistry of the photoproduct, single
crystals of the reacted solid were obtained via a recrystallization
from acetonitrile/methanol/toluene (1:1:1 v/v). Slow solvent
evaporation over a period of approximately 3 days afforded
colorless plate-shaped crystals of (4,6-diI-res)·(4p3tcb)
suitable for single-crystal X-ray analysis.
A single-crystal X-ray analysis20 revealed a discrete two-
component assembly of a 1:1 co-crystal of (4,6-diI-res)·
(4p3tcb), which lies in the centrosymmetric space group P21/c.
In the solid, head-to-head 4p3tcb [C−C bond lengths (Å):
C(12)−C(13) 1.56(1), C(12)−C(23) 1.58(1), C(13)−C(24)
1.56(1), C(23)−C(24) 1.53(1)] (Figure 3) interacts with 4,6-
diI-res via O−H···N hydrogen bonds [O···N separation (Å):
O(1)···N(1) 2.66(1), O(2)···N(2) 2.71(1)]. The thiophene
rings of the photoproduct are splayed about the cyclobutane
ring at 74°, with the mean planes being canted by 56°.
During our attempts to employ a res to assemble β-PTE into
a face-to-face geometry, we discovered an unusual co-crystal
assembly. When β-PTE was co-crystallized with 4,6-diCl-res in
acetonitrile via slow evaporation, pale yellow rod-shaped
Figure 1. Edge-to-face packing of β-PTE along the a-axis (highest
occupancy).
similar to unsubstituted oligothiophenes.17 The CC bonds
are separated by 6.15 Å, with the edge-to-face packing placing
β-PTE in a photostable arrangement according to the
principles of Schmidt.2 When a sample in the form of a
crystalline powder was exposed to broad-band UV irradiation
1
(20 h), β-PTE was determined to be photostable from H
NMR spectroscopy.
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Organic Letters
Letter
Figure 3. X-ray structure of (4,6-diI-res)·(4p3tcb).
crystals of (4,6-diCl-res)·2(β-PTE) formed after a period of 2
days. The formulation of (4,6-diCl-res)·2(β-PTE) was
confirmed by single-crystal X-ray analysis and 1H NMR
spectroscopy.
A single-crystal X-ray analysis21 revealed the components of
(4,6-diCl-res)·2(β-PTE) to crystallize in the centrosymmetric
space group P1. In contrast to (4,6-diI-res)·2(β-PTE), the
̅
stacked olefins are oriented in an edge-to-face geometry, being
sustained by intermolecular O−H···N hydrogen bonds [O···N
separation (Å): O(1)···N(1) 2.71(1), O(2)···N(2) 2.74(1)]
(Figure 4). The mean planes of the β-PTE molecules are
Figure 5. Extended packing of linear columns: (a) (4,6-diI-res)·2(β-
PTE) and (b) (4,6-diCl-res)·2(β-PTE). Dashed black lines show I···
O interactions.
edge-to-face π-stacked geometry involving β-PTE was realized
with the stacking in a discrete hydrogen-bonded structure. We
are currently investigating the scope of employing templates to
enforce face-to-face π-stacking and direct the reactivity of
additional thiophenes, with a goal to construct complex
molecules and materials with increasing applications in organic
synthesis and materials science.
Figure 4. X-ray structure of (4,6-diCl-res)·2(β-PTE) highlighting
edge-to-face stacking.
canted at 67°, similar to the pure component. The edge-to-face
stacking is manifested by the C−H···π forces involving the
thiophene rings (Cα−H(thiophene)···C(thiophene) 3.71 Å).
The pyridine groups are also engaged in edge-to-face π-forces
(C−H(pyridine)···C(pyridine) 3.76 Å). As a consequence of
the assembly process, the CC bonds of the discrete assembly
are separated by 4.73 Å. The distance between olefins of
adjacent assemblies is 6.33 Å.2 Indeed, when a crystalline
powder was exposed to broad-band UV radiation for a period
of 50 h, (4,6-diCl-res)·2(β-PTE) was photostable.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Experimental details, H NMR, and single-crystal X-ray data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: len-macgillivray@uiowa.edu.
The co-crystal (4,6-diCl-res)·2(β-PTE) provides a novel
example of aromatics stacked edge-to-face in a discrete
hydrogen-bonded assembly based on a res. The observation
is suprising given the strong propensity of a res to promote
face-to-face π-stacked geometries of reactive olefins in solid-
s.1a,22 That (4,6-diI-res)·2(β-PTE) and (4,6-diCl-res)·2(β-
PTE) exhibit different π-stackings, and photoreactivities,
involving β-PTE can be attributed to secondary interactions
involving the templates. In both solids, adjacent assemblies
pack as linear columns (Figure 5). In the case of photoreactive
(4,6-diI-res)·2(β-PTE), adjacent assemblies of each column
interact via I···O interactions (3.47 and 3.38 Å for assembly A
and B, respectively) involving the templates.23 The intertem-
plate forces effectively serve to ‘compress’ β-PTE molecules of
the stacked assemblies to promote the face-to-face π-stacking.
Conversely, in (4,6-diCl-res)·2(β-PTE), the halogen-to-oxygen
separations are appeciably longer (nearest: Cl···O: 5.75 Å),
with the interassembly packing being dominated by the edge-
to-face π-forces akin to β-PTE as a pure solid.
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
L.R.M. acknowledges the National Science Foundation (DMR-
1104650) for support.
■
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In this report, we have described a head-to-head [2 + 2]
photodimerization of a β-substituted thiophene using a ditopic
hydrogen-bond donor template based on a res. A surprising
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monoclinic, space group Pc, a = 6.335(1) Å, b = 7.494(1) Å, c =
9.767(1) Å, α,γ = 90°, β = 94.26(1)°, V = 462.36(9) Å3, 2θ = 56°, Mo
Kα radiation (λ = 0.7107 Å) for Z = 2 and R = 0.0540 for I > 2σ(I).
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(18) Crystal data for (4,6-diI-res)·2(βPTE): crystal size 0.16 × 0.09
× 0.01 mm3, monoclinic, space group P21, a = 9.283(1) Å, b =
34.548(4) Å, c = 9.898(1) Å, α,γ = 90°, β = 117.72(1)°, V = 2810.2(5)
Å3, 2θ = 51°, Mo Kα radiation (λ = 0.7107 Å) for Z = 4 and R =
0.0614 for I > 2σ(I).
(19) That the photodimerization occurs in a 90% yield may reflect
the presence of reactive centers wherein disordered olefins lie in a
criss-cross geometry.
(20) Crystal data for (4,6-diI-res)·(4p3tcb): crystal size 0.105 × 0.09
× 0.01 mm3, monoclinic, space group P21/c, a = 17.753(2) Å, b =
10.170(1) Å, c = 16.281(2) Å, α,γ = 90°, β = 108.20(1)°, V =
2792.5(5) Å3, 2θ = 51°, Mo Kα radiation (λ = 0.7107 Å) for Z = 4 and
R = 0.0470 for I > 2σ(I).
(21) Crystal data for (4,6-diCl-res)·2(βPTE): crystal size 0.32 ×
0.08 × 0.07 mm3, triclinic, space group P1, a = 9.398(1) Å, b =
̅
10.280(1) Å, c = 13.792(2) Å, α = 84.15(1)°, β = 89.570(5)°, γ =
76.85(1)°, V = 1290.6(2) Å3, 2θ = 50°, Mo Kα radiation (λ = 0.7107
Å) for Z = 2 and R = 0.0513 for I > 2σ(I).
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