Intramolecular [2 + 2] Photodimerization Achieved in the Solid State via Coordination-Driven Self-Assembly

Article abstract of DOI:10.1021/acs.orglett.5b00527

(Figure Presented) An intramolecular [2 + 2] photocycloaddition is achieved in the organic solid state via self-assembly of Ag(I) ions and an endo-ditopic bipyridine. The cations aide to organize carbon-carbon double (C=C) bonds attached to the bipyridine for the cycloaddition reaction. The C=C bonds react regioselectively and quantitatively to afford a photoproduct with edge-sharing four-, five-, and six-membered rings. Our study demonstrates the first use of a metal-organic template to direct an intramolecular [2 + 2] photodimerization in the organic solid state.

Full text of DOI:10.1021/acs.orglett.5b00527

Letter
pubs.acs.org/OrgLett
Intramolecular [2 + 2] Photodimerization Achieved in the Solid State
via Coordination-Driven Self-Assembly
†
†
Rebecca C. Laird, Michael A. Sinnwell, Nam P. Nguyen, Dale C. Swenson, S. V. Santhana Mariappan,
and Leonard R. MacGillivray*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242-1294, United States
*
S Supporting Information
ABSTRACT: An intramolecular [2 + 2] photocycloaddition is achieved in the organic solid state via self-assembly of Ag(I) ions
and an endo-ditopic bipyridine. The cations aide to organize carbon−carbon double (CC) bonds attached to the bipyridine for
the cycloaddition reaction. The CC bonds react regioselectively and quantitatively to afford a photoproduct with edge-sharing
four-, five-, and six-membered rings. Our study demonstrates the first use of a metal−organic template to direct an intramolecular
[
2 + 2] photodimerization in the organic solid state.
or a [2 + 2] photocycloaddition to occur in the organic
tions differed by only ∼0.30 kJ/mol, which would allow 1 to
7
F
solid state, carbon−carbon double (CC) bonds are
crystallize in the syn conformation and react in a solid.
expected to adhere to Schmidt’s topochemical postulate, being
The bis(4-pyridine) 1 was, thus, synthesized using a modified
procedure based on a Heck reaction. To determine the
photoreactivity of pure 1, single crystals (5 mg) of the diene
were grown by slow evaporation in MeCN (10 mL). Colorless
blade-like crystals grew within a period of ca. 1 h.
The asymmetric unit consists of two full molecules (A and
̅
B) of 1 that crystallize in the centrosymmetric space group P1.
1
aligned parallel and separated by <4.2 Å. To overcome the
effects of packing that hinder olefins from assembling into
geometries to react in solids, small-molecule and metal-based
2
,3
templates have been reported. The operation of a template
relies on noncovalent forces to assemble and position olefins
into multicomponent self-assembled structures for photo-
reaction. To date, templates have been applied exclusively to
intermolecular photocycloadditions, enabling access to archi-
tecturally rich molecules difficult to obtain in solution.
The 4-pyridyl groups are splayed by 16.3° and 25.3° in A and B,
with the pyridine rings rotated by 16.8° and 20.7° and the ring
centroid-to-centroid distances separated by 5.2 and 4.9 Å,
respectively (Figure 2). Each pair of CC bonds in A and B,
however, adopts an anti-conformation, with inner/outer C-to-C
In this study, we sought to determine whether the 1,8-
naphthalene diene 1,8-bis[(E)-2-(4-pyridyl)ethenyl] naphtha-
lene (1) undergoes an intramolecular [2 + 2] photodimeriza-
8
separations of 2.8 Å/4.6 Å (A) and 2.8 Å/4.5 Å (B). The anti-
9
4
relationship means that 1 adopts a unique axially chiral
tion in the solid state (i.e., in absence of a template) to give cis-
conformation with the CC bonds being related by an
di(4-pyridyl)naphthocyclobutane (2) (Figure 1). The 1,8-
5
,6
idealized C rotation axis (Figure 2a). The CC bonds of the
substitution pattern was expected to place the CC bonds
of the 4-vinylpyridine groups in close enough proximity to
support an intramolecular reaction to generate 2. A successful
reaction would proceed via endo-ditpoic 1 in a syn conformation
wherein the CC bonds adopt a parallel orientation. From
semiempirical PM3 calculations, the syn- and anti-conforma-
2
diene 1, thus, do not conform to the topochemical postulate of
1
Schmidt for a [2 + 2] photodimerization in a solid.
Molecules A and B of 1 are of the same handedness in the
solid state. The dienes form chiral columns along the a-axis
with adjacent molecules organized head-to-tail (Figure 2b).
Dienes of neighboring columns lie head-to-head, being
sustained by C−H···N forces (2.7 Å). Chiral layers, thus,
form within the ab-plane with the stacked layers related by a
center of inversion. The closest CC bond separation of
adjacent dienes is 4.5 Å, which is beyond the limit of Schmidt.
When a crystalline powder of 1 was exposed to broad-band UV-
Figure 1. Crystallization of 1 in syn conformation for an intramolecular
Received: May 12, 2015
Published: June 16, 2015
[
2 + 2] photocycloaddition.
©
2015 American Chemical Society
3233
DOI: 10.1021/acs.orglett.5b00527
Org. Lett. 2015, 17, 3233−3235

Organic Letters
Letter
Figure 4. X-ray structure of 1a (210 K): (a) discrete dinuclear
assembly and (b) extended packing.
Figure 2. X-ray structure of 1: (a) C rotation axis of chiral anti
2
conformer (right: schematic of anti-conformation) and (b) 2D chiral
layer in ab-plane.
adopt an antiparallel conformation. We note that both sets of
CC bonds of 1a, however, are disordered at room
temperature (C11/C12 95% antiparallel/5% parallel; C18/
C24 91% antiparallel/9% parallel), with no disorder present at a
lower temperature (210 K). These observations are consistent
with the CC bonds of the dinuclear rectangles being
dynamically disordered in the solid. Metrics associated with the
X-ray structure of 1b are comparable to 1a, with the Ag···Ag
separation being 3.80 Å. A higher R value for 1b prohibits
observations of disorder.
radiation (500 W medium-pressure Hg lamp) for a period of ca.
4
0 h, 1 was determined to be photostable.
To address the photostability of 1 as a pure form, we turned
10
to Ag(I) ions. Whereas Ag(I) has been utilized by us, and
1
1
others, to assemble CC bonds within acyclic complexes for
intermolecular cycloadditions, we reasoned that a self-assembly
process involving 1 and Ag(I) could generate a discrete self-
assembled macrocycle in the form of a dinuclear Ag(I)-based
1
2
rectangle with CC bonds positioned to react (Figure 3).
Figure 3. Use of dinuclear Ag(I) rectangle to direct intramolecular [2
When 1a−c was exposed to UV-radiation (500 W medium-
1
pressure Hg lamp) for a period of ca. 40 h, H NMR
spectroscopy (DMSO-d ) revealed the bipyridine 1 to undergo
6
an intramolecular [2 + 2] photodimerization to generate a
cyclobutane photoproduct in up to quantitative yield
(
conversions: 1a, 47%; 1b, 59%; 1c, 100%). For 1a and 1b,
the photoreaction was incomplete with no side products having
been formed. In contrast to pure 1, photoreactions were clearly
evidenced by up to the complete disappearance of olefinic
peaks at 8.2 and 7.0 ppm and emergence of peaks at 4.7 and 4.0
ppm in the cyclobutane region (see Supporting Information
(SI), Figure S5). We attribute the generation of 2 in the solid
state to the CC bonds undergoing pedal motions to form the
+
2] photodimerization in solid state.
Self-assembled coordination complexes involving endo-ditopic
building blocks are of an ongoing interest in supramolecular
1
2b
chemistry,
yet have not been exploited to direct an
14
intramolecular [2 + 2] photodimerization in a solid.
cyclobutane rings, with the auxiliary template components
supporting an environment that accommodates the disorder.
To confirm the structure of the photoproduct, the cyclo-
butane was isolated via extraction from reacted 1c using CHCl3
and NaOH. The extraction afforded 2 as a cream-colored
powder, which was characterized by NMR spectroscopy. Single
crystals of a salt of 2 (8 mg) were synthesized from slow
evaporation of a MeCN solution (10 mL) in the presence of
HBr added dropwise. Pale yellow laths of [2H-2][2Br] grew
within a period of ca. 2 d.
−
Crystalline powders of [Ag (1) ][(X) ] (where X
=
2
2
2
−
−
−
CF SO3 (1a), CH C H SO (1b), and ClO3 (1c)) were,
3
3
6
4
3
thus, grown by slow solvent evaporations of solutions of 1
0.024 mmol) and the appropriate Ag(I) salt (0.048 mmol) in
(
MeCN (15 mL). Single crystals of 1a and 1b were obtained as
colorless rods and laths, respectively, from slow evaporations
from MeCN. While numerous conditions were employed for
1
c, single crystals suitable for X-ray diffraction could not be
obtained.
Single-crystal X-ray diffraction confirmed the formation of
The asymmetric unit consists of one-half of a 2H-dication of
1
2
−
Ag(I)-based rectangles. The asymmetric unit of 1a consists of
two Ag(I) ions, two coordinated molecules of 1, and
corresponding anions that form dinuclear rectangles in the
centrosymmetric space group P1 (Figure 4). The assembly is
sustained by argentophilic forces (Ag····Ag 3.45 Å), with each
Ag(I) ion being coordinated to two dienes of opposite
handedness (Figure 4a). The vinylpyridine groups lie less
splayed (11.6°) and rotated (6.4°) compared to free 1 while the
CC bonds lie in closer proximity, displaying inner/outer C-
to-C separations of 2.8 Å/4.4 Å and a centroid-to-centroid
distance of 4.0 Å. As with pure 1, however, the CC bonds
2 and one Br ion that crystallize in the space group C2/c
+
−
(Figure 5). Adjacent ions participate in N −H···Br hydrogen
bonds [N···Br separations (Å): N(14)···Br(1) 3.20(1) and
N(14′)···Br(1) 3.21(1)] (Figure 5a). The dication, which is
disordered over two sites (occupancies: 50/50), assumes a
pseudochair geometry with the pyridine rings oriented opposite
to the naphthalene ring through the cyclobutane ring [C−C
bond distances (Å) C17−C17′ 1.57(1), C17−C18 1.57(1),
C18−C18′ 1.58(1), C18′−C17′ 1.58(1)] (Figure 5b). The
ions assemble head-to-tail within the bc-plane (Figure 5c). The
structure of the cyclobutane ring supports the intramolecular [2
12
̅
3
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DOI: 10.1021/acs.orglett.5b00527
Org. Lett. 2015, 17, 3233−3235

Organic Letters
Letter
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13
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(
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(
9
ASSOCIATED CONTENT
Supporting Information
■
*
S
Details of syntheses, NMR studies, and X-ray structure
1
6
solutions. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b00527.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: len-macgillivray@uiowa.edu.
Author Contributions
†
M.A.S. and N.P.N. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (L.R.M.
DMR-1408834) for funding. R.C.L. acknowledges the Uni-
versity of Iowa Graduate College for financial support in the
form of the Presidential Graduate Research Fellowship.
3
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DOI: 10.1021/acs.orglett.5b00527
Org. Lett. 2015, 17, 3233−3235