Role of cation - π interactions in the photodimerization of trans-4-styrylpyridines

Article abstract of DOI:10.1021/ja074874y

In order to explore the contribution of the cation-π interaction in the photocyclodimerization of styrylpyridines, the effects of the acid concentration and the substituent on the aryl ring on the product distribution were investigated. The structures of

Full text of DOI:10.1021/ja074874y

Published on Web 09/19/2007
Role of Cation-π Interactions in the Photodimerization of
trans-4-Styrylpyridines
Shinji Yamada,* Naoko Uematsu, and Kaori Yamashita
Department of Chemistry, Faculty of Science, Ochanomizu UniVersity, Bunkyo-ku, Tokyo 112-8610, Japan
Received July 21, 2007; E-mail: yamada.shinji@ocha.ac.jp
Scheme 1. Selective Formation of syn-HT Dimer in the Presence
of HCl
Controlling the regio- and stereoselectivities during the photo-
cyclodimerization of alkenes is one of the key subjects in synthetic
organic photochemistry.1 To this end, much effort has been directed
toward the use of various organized media and supramolecular
environments, such as crystals,2 clays,3 cyclodextrines,4 zeolites,5
biomolecules,6a micelles,6b hydrogen-bonding templates,6c and self-
assembled cages6d and hosts.6e Another approach is the employment
of a molecule having a self-assembling property; however, there
are few successful examples in solution except for the case using
supramolecular assistance.7
Table 1. Product Distribution of Photodimerization of 1a-1c
We were interested in a reported [2 + 2]-photodimerization
reaction of trans-styrylpyridines, in which a syn head-to-tail (HT)
dimer is obtained as the major product only in an acidic solution.8,9
However, this remarkable effect of the acid on the product
distribution still remains unexplained. On the basis of our recent
studies regarding the application of a pyridinium-π interaction
toward organic synthesis,10,11 we envisioned that the pyridinium-π
interactions could be significantly responsible for the remarkable
acid effect on the stereoselectivity during the photodimerization of
the trans-styrylpyridines.
Products (%)a,b
HCl
conv
(%)
entry
compd
(equiv)
2
3
4
5
6
In this communication, we describe that the pyridinium-π
interaction between substrates plays an essential role in the selective
formation of the syn-HT dimer during the photolysis of trans-4-
styrylpyridines in acidic media (Scheme 1). Furthermore, a revised
structure for one of the product dimers was also proposed. In order
to explore the contribution of the cation-π interaction12 to the
product selectivity, the effects of the acid concentration and the
substituent on the aryl ring on the product distribution were
investigated along with the structural confirmation of the product
dimers by an X-ray crystallographic analysis. In addition, the
differences in the X-ray packing structures of 1a and 1a‚HCl were
clarified.
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1b
1c
0
1
2
3
5
10
3
3
81
91
90
96
95
96
94
94
20
50
58
64
67
71
27
95
14
10
8
13
9
7
24
2
14
14
12
13
15
14
0
0
0
0
0
0
0
6
0
52
26
22
10
9
8
43
3
0
a Determined by HPLC and 1H NMR. b The structures of 2-4 were
determined by X-ray crystallographic analyses; see Supporting Information.
The photochemical reaction of trans-4-styrylpyridine was first
investigated under neutral conditions. Irradiation of trans-4-
styrylpyridine 1a in a 1.66 M methanol solution with a 450 W high-
pressure mercury lamp for 17 h afforded cis-4-styrylpyridine 6a
as the major product with three minor dimers 2a-4a as shown in
Table 1 (entry 1). The photolysis in the presence of 1 equiv of
concentrated hydrochloric acid resulted in remarkable changes of
the product distribution (entry 2); the syn-HT adduct 2a increased
to become a major product along with a significant decrease in 6a.
As the amount of HCl loading increased, the syn-HT product 2a
dramatically increased and the cis-isomer 6a decreased (entries
2-6), whereas the acid exerted little effect on the yields of 3a and
4a. Figure 1 clearly shows the product distribution dependence on
the HCl amount. The product distribution was almost constant for
more than a 3 equiv loading of HCl. This is in agreement with the
1H NMR studies in which more than 3 equiv of HCl is required
for protonation of the substrates. These observations indicate that
the intermediate styrylpyridinium salt enhances the formation of a
preorganizing head-to-tail type molecular dimer as shown in Scheme
Figure 1. Product distribution dependence on the HCl amount.
1. The decrease in the cis-isomer 6a can be explained as a result
of the accelerated formation of 2a.
Although the structures of the dimers have already been
1
determined by H NMR and MS spectral analyses,9 we reinvesti-
gated them based on an X-ray structural analysis for the first time.
9
12100
J. AM. CHEM. SOC. 2007, 129, 12100-12101
10.1021/ja074874y CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
The excitation of a CT complex is also a powerful method for
the selective photocycloaddition.16 A charge-transfer absorption is
often observed in related systems.17 However, no CT bands were
observed in the absorption spectra of the HCl salts of 1a-1c in
methanol. Recently, we elucidated the origin of the interaction
between the pyridinium and the aromatic rings by ab initio
calculations,13 in which the long-range interactions, such as the
electrostatic and inductive interactions, predominate. Therefore,
even if a charge-transfer interaction is involved in this system, the
major contributor would be a cation-π interaction.
All of these results described here lead to the conclusion that
pyridinium-π interactions govern the alignment of the trans-
styrylpyridinium cation in solution, the irradiation of which would
result in the selective formation of the syn-HT dimer.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (B) (No. 17350046) from the Japan Society
for the Promotion Science.
Figure 2. X-ray packing structures for (a) 1a and (b) 1a‚HCl. Hydrogen
atoms were omitted for clarity. The nitrogen atoms are indicated with a
red color.
Supporting Information Available: Experimental details and
1
characterization of new compounds. H NMR spectra for 4a, 2b, 3b,
5b, 2c, and 3c. X-ray crystallographic data and CIF files for 1a, 1a‚
HCl, 2a, 3a, and 4a. This material is available free of charge via the
Internet at http://pubs.acs.org.
The X-ray structures proved that 2a and 3a were syn-HT and syn-
HH dimers, respectively, which are in agreement with those reported
in the literature.9 More important, the X-ray analysis of 4a provided
a revised structure from the reported anti-HT to anti-HH.
Irradiation of compound 1b having a strong electron-withdrawing
CF3 group in the presence of 3 equiv of concentrated HCl resulted
in a significantly lower selectivity compared with the case of 1a
(entry 7); the relative yields of 2b and 6b are 27 and 43%,
respectively. On the other hand, the photodimerization of 1c
possessing a methoxy group resulted in a significantly higher
selectivity; the 95% formation of the syn-HT dimer 2c was observed
along with a small amount of 3c (entry 8). The fact that the
selectivity of the syn-HT dimer 2 is on the order of 1c > 1a > 1b
and that of the cis-isomer 6b is in the opposite order clearly shows
the important role of the electron density around the π-component
in the orientation of the dimerization and the acceleration of the
formation of 2. Because an electrostatic interaction is the major
force of the cation-π interaction,12,13 the substituent effects strongly
suggest the significant contribution of the cation-π interaction in
this reaction. Although an explanation was proposed for this
selectivity, in which the contribution of the charge repulsion
between the pyridinium cations is the main factor,9 this cannot
satisfy the observed acceleration of the formation of 2. Moreover,
this charge repulsion model cannot explain the fact that the syn-
HH dimer 3 decreased in the order of 3b > 3a > 3c.
References
(1) (a) Griesbeck, A. G.; Mattay, J. Synthetic Organic Photochemistry; Marcel
Dekker: New York, 2005. (b) Coyle, J. C. Photochemistry in Organic
Synthesis; The Royal Society of Chemistry: London, 1986. (c) Horspool,
W. M. Synthetic Organic Photochemistry; Plenum Press: New York, 1984.
(2) Tanaka, K.; Toda, F. In Organic Solid-State Reactions; Toda, F., Ed.;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002; pp 109-
158.
(3) Takagi, K.; Shichi, T. In Solid State and Surface Photochemistry;
Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2001;
pp 31-110.
(4) Yang, C.; Inoue, Y. In Cyclodextrin Materials Photochemistry, Photo-
physics and Photobiology; Douhal, A., Ed.; Elsevier: Amsterdam, 2006;
Chapter 11.
(5) Ramamurthy, V.; Shailaja, J.; Kaanumalle, L. S.; Sunoj, R. B.; Chan-
drasekhar, J. Chem. Commun. 2003, 1987.
(6) For selected papers, see: (a) Nishijima, M.; Wada, T.; Mori, T.; Pace, T.
C. S.; Bohne, C.; Inoue, Y. J. Am. Chem. Soc. 2007, 129, 3478. (b) Takagi,
K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten, D. G. J. Am.
Chem. Soc. 1986, 108, 7865. (c) Varshney, D. B.; Gao, X.; Friscic, T.;
MacGillivray, L. R. Angew. Chem., Int. Ed. 2006, 45, 646. (d) Karthikeyan,
S.; Ramamurthy, V. J. Org. Chem. 2007, 72, 452. (e) Yang, J.; Dewal,
M. B.; Shimizu, L. S. J. Am. Chem. Soc. 2006, 128, 8122.
(7) Amirsakis, D. G.; Elizarov, A. M.; Garcia-Garibay, M. A.; Glink, P. T.;
Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed.
2003, 42, 1126.
(8) Williams, J. L. R.; Webster, S. K.; Van Allan, J. A. J. Org. Chem. 1961,
26, 4893.
(9) (a) Li, X.-H.; Wu, L.-Z.; Zhang, L.-P.; Tung, C.-H. Org. Lett. 2002, 7,
1175. (b) Zhang, W.-Q.; Zhang, X.-H.; Zheng, Y.; Shen, G.; Zhuang,
J.-P. Ganguang Kexue Yu Guang Huaxue 2000, 18, 144. (c) Quina, F.
H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 877.
A comparison of the X-ray structures of 1a and its HCl salt
supported the existence of the pyridinium-π interaction during the
photodimerization. The packing diagrams of 1a and its salt are
shown in Figure 2a and 2b. Figure 2a clearly shows that the phenyl
and the pyridyl groups are separated from each other and no
stacking orientation is observed. On the contrary, the molecules of
the HCl salts are alternately packed in an antiparallel alignment.
The phenyl and the pyridinium rings are arranged face-to-face with
the distance between them being 3.295 Å, suggesting the existence
of the intermolecular cation-π interaction between them. It has
been reported that the photolysis of this salt in the solid state
provided the syn-HT dimer 2a in high yield.14 This indicates the
similarity of the alignment structures in crystal and in solution. The
related pyridinium systems also show a similar head-to-tail type
alignment.15
(10) (a) Yamada, S.; Inoue, M. Org. Lett. 2007, 9, 1477. (b) Yamada, S.;
Yamamoto, J.; Ohta, E. Tetrahedron Lett. 2007, 48, 855. (c) Yamada, S.;
Saitho, M.; Misono, T. Tetrahedron Lett. 2002, 43, 5833. (d) Yamada,
S.; Morita, C. J. Am. Chem. Soc. 2002, 124, 8184.
(11) For a review, see: Yamada, S. Org. Biomol. Chem. 2007, 5, 2903.
(12) For a review, see: Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97,
1303.
(13) Tsuzuki, S.; Mikami, M.; Yamada, S. J. Am. Chem. Soc. 2007, 129, 8656.
(14) Horner, M.; Hu¨nig, S. J. Am. Chem. Soc. 1977, 99, 6120.
(15) (a) Yamada, S.; Morimoto, Y. Tetrahedron Lett. 2005, 46, 5673. (b)
Yamada, S.; Morimoto, Y.; Misono, T. Tetrahedron Lett. 2006, 47, 5557.
(16) (a) Saito, H.; Mori, T.; Wada, T.; Inoue, Y. Org. Lett. 2006, 8, 1909. (b)
Saito, H.; Mori, T.; Wada, T.; Inoue, Y. J. Am. Chem. Soc. 2004, 126,
1900. (c) Saito, H.; Mori, T.; Wada, T.; Inoue, Y. Chem. Commun. 2004,
1652.
(17) For examples, see: (a) Kim, E. K.; Lee, K. Y.; Kochi, J. K. J. Am. Chem.
Soc. 1992, 114, 1756. (b) Ishida, T.; Ibe, S.; Inoue, M. J. Chem. Soc.,
Perkin Trans. 2 1984, 297. (c) Verhoeven, J. W.; Dirkx, I. P.; de Boer.
T. J. Tetrahedron 1969, 25, 4037.
JA074874Y
9
J. AM. CHEM. SOC. VOL. 129, NO. 40, 2007 12101