[2+2] photodimerization of 1-aryl-4-pyridylbutadienes through cation-π interactions

Article abstract of DOI:10.1016/j.tetlet.2014.03.036

Irradiation of 1-aryl-4-pyridylbutadienes in the presence of 1 equiv of HCl produced syn and anti head-to-tail dimers, among a number of possible dimers, whereas irradiation in the absence of HCl gave a complex mixture. This indicated that the acid serves as a catalyst for the regio- and stereoselective [2+2] photodimerization of 1-aryl-4-pyridylbutadienes through cation-π interactions between the pyridinium and aromatic rings. The produced synHT dimers underwent Cope rearrangement to produce cyclooctadienes, and they were in equilibrium at a ratio of 85:15 in CDCl₃.

Full text of DOI:10.1016/j.tetlet.2014.03.036

Accepted Manuscript
[2+2] Photodimerization of 1-aryl-4-pyridylbutadienes through cation-π inter-
actions
Shinji Yamada, Yuka Azuma, Kanae Aya
PII:
S0040-4039(14)00434-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.03.036
TETL 44358
Reference:
Tetrahedron Letters
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Revised Date:
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1 February 2014
5 March 2014
7 March 2014
Please cite this article as: Yamada, S., Azuma, Y., Aya, K., [2+2] Photodimerization of 1-aryl-4-pyridylbutadienes
through cation-π interactions, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.03.036
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[2 + 2] Photodimerization of 1-aryl-4-pyridylbutadienes
through cation-π interactions
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Shinji Yamada ∗, Yuka Azuma, Kanae Aya

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
[2 + 2] Photodimerization of 1-aryl-4-pyridylbutadienes through cation-π
interactions
Shinji Yamada∗, Yuka Azuma, Kanae Aya
Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Irradiation of 1-aryl-4-pyridylbutadienes in the presence of 1 equiv of HCl produced syn and
anti head-to-tail dimers, among a number of possible dimers, whereas irradiation in the
absence of HCl gave a complex mixture. This indicated that the acid serves as a catalyst for the
regio- and stereoselective [2 + 2] photodimerization of 1-aryl-4-pyridylbutadienes through
cation-π interactions between the pyridinium and aromatic rings. The produced synHT dimers
underwent Cope rearrangement to produce cyclooctadienes, and they were in equilibrium at a
ratio of 85:15 in CDCl3.
Available online
Keywords:
Cation-π interaction
Cope rearrangement
[2 + 2] Photodimerization
Catalysis
2014 Elsevier Ltd. All rights reserved.
cyclooctadiene
Introduction
Figure
1
shows possible head-to-head and head-to-tail
cycobutane dimers,7 each of which accompanies both syn and
anti stereoisomers. To attain high selectivities in the [2 + 2]
photodimerization of dienes, various methods for the
preorientation of substrates8 using templates,9 hosts10 or metal
complexes11 have been extensively developed. We have
previously reported the regio- and stereoselective [2 + 2]
photodimerization of styrylpyridines in an acidic solution,12
where cation-π interactions between the pyridinium and aromatic
rings play a key role in arranging the substrate molecules in a
head-to-tail fashion. In this letter, we report that 1-aryl-4-
pyridylbutadienes undergo [2 + 2] photodimerization to give a,b-
HT-type of dimers in an acidic solution through cation-π
interactions (Scheme 1).13
The [2 + 2] photodimerization reaction of alkenes is one of the
most important reactions in synthetic organic photochemistry1
and the reaction has been applied to various fields of chemistry.
2,3
In the case of the photochemical reactions of unsymmetrical
dienes, having two reactive sites, a number of regio- and
stereoisomeric cyclobutane dimers4 and E/Z isomers5 as well as
intramolecularlly-reacted products6 are produced. Therefore,
controlling the regio- and stereoselectivities for dienes is more
challenging than that of monoalkenes.
Scheme 1. Photodimerization of 1-aryl-4-pyridylbutadiene
through cation-π interactions.
Figure 1. Possible cyclobutane dimers obtained by the photodimerization of
unsymmetrical dienes.
———
Results and discussion
∗ Corresponding author. Tel.: +81-3-5978-5349; fax: +81-3-5978-5349.
E-mail address: yamada.shinji@ocha.ac.jp

2
Tetrahedron
We employed the 1-aryl-4-pyridylbutadienes 1a,14 1b15 and 1c
The characteristic features in these [2 + 2] photodimerization
reactions are summarized as follows: (a) addition of HCl is
particularly effective in enhancing the formation of dimers, (b)
the cyclobutane dimers 2b and 3b were selectively produced
among a number of possible dimers, (c) a methoxy group is
particularly effective in promoting the [2 + 2] photodimerization
reactions. These features strongly suggest that the positively
charged pyridinium ring and aryl moiety came into close
proximity with each other through cation-π interactions to form
preorganized head-to-tail type molecular dimers A and B as
shown in Scheme 3. As the double bonds of the molecular dimer
A are close together, synHT dimer 2b can be readily produced.
On the other hand, as the two molecules of B are situated clossed
orientation, the molecules would rotate oppositely with respect to
the long axis so that the π-orbitals overlap each other, which
would lead to the formation of antiHT dimer 3b. It has been
reported that the ladder-like dimers shown in Figure 1 were
obtained in solid-state reactions of cocrystals with templates8 and
host-guest complexes.5 However, no such products were
produced in the present reactions. This can be explained by the
fact that the formation of a cyclobutane leaves the two alkene
moieties separated from each other in the absence of a template
and a host.
as substrates for the photodimerization reactions (Scheme 2). The
results are shown in Table 1. Although the photochemical
reaction of 1-phenyl-4-pyridylbutadienes (1a) was recently
investigated in an acidic water solution with CB8,5 we initially
investigated the photochemical reaction of 1a in methanol under
neutral conditions.13 Irradiation of 1a with a 450W high-pressure
mercury lamp for 15h through a Pyrex filter gave a complex
mixture. When irradiation of 1a was carried out in the presence
of 1.0 equiv of HCl, syn head-to-tail (synHT) dimer 2a and anti
head-to-tail (antiHT) dimer 3a were produced in 3% and 4%
isolated yields, respectively (entry 2). These results were
inconsistent with those reported in the literature,5 possibly due to
differences in the solvent and the concentration of the solution.
On the other hand, the photochemical reaction of 1b afforded
dimer 3b even in neutral conditions with a significantly lower
yield (entry 3). The addition of 0.5 equiv of HCl dramatically
improved the yields to give 2b and 3b in 17.5 and 15.5 % yields,
respectively (entry 4). Increasing the amount of HCl loading to
1.0 equiv led to an increase in the syn and anti HTdimers (entry
5). When the concentration of the solution was increased from
0.1 to 0.5 M, the yields were improved to produce a 1:1 mixture
of dimers in 58% isolated yield (entry 6). When using water as a
solvent, the stereoselectivity was changed to give the synHT
dimer as a major product (entry 7). This can be explained by the
hydrophobic effect;16 the transition state of the dimerization
leading to 2b is more compact than that leading to 3b, which
would be responsible for the selective formation of 2b. On the
other hand, irradiation of 1c having an electron-withdrawing
group produced a complex mixture both in neutral and acidic
conditions (entries 8 and 9).
Scheme 3.
interactions.
Preorientation of dienes through cation-π
The structures of dimers 2b and 3b were determined by 1H and
13C NMR spectra and MS spectra (see, Supporting data). The
NMR spectra showed that they have an unsymmetric dimer
structure with a four-membered ring and two trans double bonds.
The MS spectra for the HBF4 salts of 2b and 3b show a
characteristic fragment peak of m/z 211.09 with 67% and 70%
intensities, respectively, which corresponds to a CH3OC6H4-CH-
CH-C5H4N moiety, strongly suggesting that 2b and 3b are both
head-to-tail dimers (See, Supporting data). The isolated 2b was
gradually isomerized to 4b by Cope rearrangement, and they are
in equilibrium in solution as shown in Scheme 4. In contrast, 3b
did not undergo Cope rearrangement. As the isolated 4b gave
crystals suitable for X-ray structural analysis, the X-ray
crystallographic analysis was carried out. The structure is shown
in Figure 2,17 which revealed the stereochemistry of 2b to be syn,
as a result 3b was assigned as the anti isomer.
Scheme 2. Photodimerization of 1a-1c.
Table 1. Photodimerization of 1a-1c in various conditions
aIsolated yield unless otherwise noted.
bDetermined by 1H NMR spectrum.
Scheme 4. Equilibrium between 2b and 4b.

3
stereoselective photodimerization of dienes.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas ‘Advanced Molecular
Transformations by Organocatalysis’ from MEXT.
Supplementary Data
Figure 2. X-ray structure of 4b.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/
Figure 3 shows the plots for the yield of 2b in the Cope
rearrangement of 4b as a function of the time of heating at 50°C
References and notes
in CDCl3. As the time of heating was increased, the y
of
2b increased through consumption of 4b. After heating for 30h,
the yield of 2b was almost unchanged, indicating that 2b and 4b
are in equilibrium at a ratio of 85:15. Similarly, rearrangement of
2b to 4b reached to an equilibrium at the same ratio. 2a and 4a
were also in equilibrium at the same ratio in CDCl3.
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7. In this paper, the structure of dimers is designated as follows: a,a-
HT indicates that the two double bonds assigned a reacted
together with head-to-tail manner.
Figure 3. Yields of 2b and 4b by time of heating.
It has been reported that similar synHT dimers undergo Cope
rearrangement to give cyclooctadienes.18 The stereochemistry of
4b was in agreement with that reported;18d the two double bonds
adopted a Z-configuration with all pyridyl and phenyl rings being
syn-displaced. This stereochemical outcome would be a result
from the endo-boat transition state (Scheme 5). This can be
explained by theoretical studies of the Cope rearrangement,19 in
which the endo-boat is the most stable of all possible transition
states. As the Cope rearrangement of 1,2-diallylbutane is applied
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13. This work has been reported in the 91th Annual Meeting of the
Chemical Society of Japan; Aya, K.; Azuma, Y.; Yamada, S.,
“Stereoselective [2+2] photocycloaddition of 1-aryl-4-
pyridylbutadienes using cation-π interactions”, 2011, 3PA-195.
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C 2008, 112, 8429-8437.
Scheme 5. Formation of cis-cyclooctadiene through the endo-
boat transition state.
Conclusion
16. Lindström, U. M. Chem. Rev. 2002, 102, 2751-2772.
17. CCDC number for 4b: 983828.
In summary, we reported that cation-π interactions between
the pyridinium and benzene rings of 1-aryl-4-pyridylbutadienes
are effective for the formation of preorganizied head-to-tail type
molecular dimers, irradiation of which produced synHT and
antiHT dimers. As the photodimerization of dienes in solution
generally produces a number of products, the present procedure
using an acid as a catalyst would afford a practical method for the
18. (a) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc., 1977, 99, 877-
883. (b) Berson, J. A.; Dervan, P. B. J. Am. Chem. Soc. 1972, 94,
7597-7598. (c) Berson, J. A.; Dervan, P. B. ; Jenkins, J. A. J. Am.
Chem. Soc. 1972, 94, 7598-7599. (d) Green, B. S.; Lahav, M.;
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M.; Schmidt, G. M. J. J. Chem. Soc. B, 1967, 312-317.
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20. Priscilla, C.-K.; Snapper, M. L. Org. Lett. 2001, 3, 2819-2821.

products (%) a
entry
compd
conc (M) HCl (eq)
solv
time (h) conv (%)
2
3
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1b
1b
1b
1c
1c
0.3
0.9
0.1
0.1
0.1
0.5
0.3
0.1
0.1
0.0
1.0
0.0
0.5
1.0
1.0
1.0
0.0
1.0
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
H2O
15
15
15
6
90.0
93.7
100
complex mixture
3.2
0
4.0
5.5
80.0
80.2
86.2
92.8
100
17.5 b
21.0
28.8
39.1
15.5 b
20.0
29.0
18.5
6
6
6
MeOH
MeOH
15
15
complex mixture
complex mixture
100