Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 14–18
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Journal of Photochemistry and Photobiology A:
Chemistry
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Short note
Photo-dehydro-Diels–Alder reaction of
1-phenyl-2-(pyridyl)acetylenes in acidic aqueous solution
Junpeng Zhuang ∗ , Shuguang Zhang, Haijun Hao, Long Jiang
Department of Organic Chemistry, Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history:
Received 3 May 2013
Received in revised form 29 June 2013
Accepted 8 July 2013
Available online 23 July 2013
Keywords:
Photo-dehydro-Diels–Alder reaction
Photodimerization
Diarylacetylene
Naphthalene
Head-to-tail
a b s t r a c t
The photocycloaddition of 1-(4-R-phenyl)-2-(4-pyridyl)acetylenes (R = H, Br, CH3 , Cl) and 1-(4-Rphenyl)-2-(2-pyridyl)acetylenes (R = H, Br, Cl) was carried out in acidic aqueous solution. The
unexpected photo-dehydro-Diels–Alder reaction of these monomers was observed, and the results
show that two monomers react in a head-to-tail manner and lead to the formation of 2-phenyl-1,3di(pyridyl)naphthalene derivatives. This reaction presents a direct metal-free method to construct the
1,2,3-triaryl substituted naphthalenes from diarylacetylenes.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, photo-dehydro-Diels–Alder (PDDA) reaction has
been reported, which provides a new method to construct a considerable variety of substituted naphthalene derivatives [1,2]. The
diene moiety in PDDA reaction contains a C C triple bond and a C C
double bond, which differs from the diene moiety in the classical
Diels–Alder reaction.
Wessig has reported the extensive studies on the intermolecular
and intramolecular [4 + 2] PDDA reaction. The synthesis of phenylnaphthalenes [3–5], 1,1 -binaphthyls [6,7], N-heterocyclic analogs
of phenylnaphthalenes [8], and highly strained naphthalenophanes
[9] are accomplished by the PDDA reaction using 3-arylynones
and arylacetylenes in organic solvents upon the irradiation of UV
light. Though the PDDA reaction is an efficient route to synthesize
naphthalenes, the starting materials always contain arylynones or
arylacetylenes and the PDDA reaction of other functional molecules
is seldom reported. Herein, the unexpected PDDA reaction of 1phenyl-2-(pyridyl)acetylene is presented.
It is well-known that the photodimerization of diarylethenes
will form a cyclobutane ring, and among these diarylethenes,
stilbenes and styrylpyridine derivatives are the most studied
ones [10–20]. In the past few years, we have been studying
the photodimerization of some N-heteroarylethenes, such as
∗ Corresponding author. Tel.: +86 10 64412948.
E-mail address: zhuangjp@mail.buct.edu.cn (J. Zhuang).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.07.004
styrylpyridines, styrylbenzoxazoles and styrylphenyloxazoles in
acidic aqueous solution [21–24].
In this paper, we synthesized 1-phenyl-2-(4-pyridyl)acetylene
to explore its photochemical properties in 1 M HCl aqueous solution, and to verify whether the heteroarylacetylene has a similar
photo property as heteroarylethene. To our surprise, a new photodimer with the structure of 1,2,3-triaryl substituted naphthalene
is formed as the main product. A literature survey revealed that the
unexpected photoreaction of 1-phenyl-2-(4-pyridyl)acetylene is a
PDDA reaction. To the best of our knowledge, this is the first PDDA
reaction of N-heteroarylacetylenes in acidic aqueous solution and
this reaction is a direct method to construct 1,2,3-triaryl substituted
naphthalene structure without metal catalyst or organic solvents
(Scheme 1).
2. Results and discussion
The synthesis of 1,2,3-triphenylnaphthalene derivatives from
diphenylacetylenes by mesoporous graphitic C3 N4 catalyst [25], by
RhCl(PPh3 )3 -based catalyst [26] or by other conditions have been
reported [27–31]. In contrast to these method, the PDDA reaction
of 1-phenyl-2-(pyridyl)acetylene can be carried out in mild conditions without the presence of any metal-catalyst and without inert
atmosphere. Compared with the arylynones and arylacetylenes
reported by Wessig et al. [1], the diarylacetylene monomers used in
this paper is simple and easy to prepare. The PDDA reaction of such
monomers extends the scope of PDDA reactions from monoarylacetylenes to diarylacetylenes and from organic solvent to aqueous
solution.
J. Zhuang et al. / Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 14–18
15
Scheme 1. The PDDA reaction of 1-phenyl-2-(pyridyl)acetylenes 1a–1g in acidic aqueous solution.
The photoreaction of 1-phenyl-2-(4-pyridyl)acetylenes have
rarely been studied. Alabugin and coworkers have studied
the triplet acetylenes photoreactions as synthetic equivalents of 1,2-bicarbenes. Several diarylacetylenes derivatives
react with 1,4-cyclohexadiene to form 1,5-diaryl substituted
tetracyclo[3.3.0.02,8 .04,6 ]octanes in acetonitrile upon UV irradiation [32,33].
The 1a–1g monomers were synthesized from styrylpyridines
by bromination with Br2 in acetic acid and then debromination
with KOH in refluxing ethanol in moderate yield (from 40% to 70%)
according to the literature method [34]. And the styrylpyridines
were synthesized from 4-methylpyridine or 2-methylpyridine and
benzaldehyde derivatives in refluxing acetic anhydride or in DMF
in the presence of KO-tBu at 80 ◦ C [35].
When 1-phenyl-2-(4-pyridyl)acetylene 1a was dissolved in
1 M HCl solution and irradiated for several hours, TLC monitoring showed that a new spot was observed on the TLC plate
without obvious side reaction. We used 1-(4-bromophenyl)-2-(4pyridyl)acetylene 1b to carry out the same reaction, two hours
irradiation led to the formation of a sole product 2b with an isolated yield of 47%. The single crystal structure of 2b shows that
a triarylnaphthalene is formed, which means the photo-dehydroDiels–Alder reaction takes place between the two diarylacetylene
monomers.
We carried out the experiments of 1a again with inner irradiation for about 20 h. 230 mg 2a and 680 mg unreacted 1a were
isolated by column chromatography using 1 g 1a dissolved in
400 mL 1 M HCl solution. The yield of 2a based on the converted
1a is 72%, which means this photocycloaddition reaction in acidic
aqueous solution is a new type of reaction with good yield and
selectivity. The 1 H NMR spectrum and single crystal structure
show that 2a has the same structure as 2b. The 4-methyl, 4-chloro
substituted derivatives (1c and 1d) of 1a and 1-(4-R-phenyl)-2-(2pyridyl)acetylenes (1e, 1f, 1g; R = H, Br, Cl) were synthesized, the
photocycloaddition of these monomers also led to the formation of
triarylnaphthalene.
In order to carry out the reaction more efficiently, we designed a
photo reactor. As shown in Fig. 1, the photo reactor consists of two
glass tubes and there is only narrow space between the two tubes.
The solution of monomer is filled between the two tubes and is
compressed to form a thin film. Each time we use only 2–3 mg
monomer dissolved in several milliliter solution. Because the
surface area of the solution exposed to UV light is largely increased
and the thickness of the solution film is decreased, the conversion of
monomer to photodimer is very quick and even finishes in several
minutes.
The photoreaction was monitored by HPLC using the photo
reactor. Fig. 1 shows that 2a is the main product of 1a in acidic
aqueous solution with UV irradiation, only trace amount of side
product is observed. The chromatograms of 1b, 1c and 1d were also
determined (supporting information). The results show that the
PDDA reaction of such 1-phenyl-2-(4-pyridyl)acetylene derivatives
is very efficient and the conversion of monomers to photodimers
is very high in dilute solution in a thin film.
Compared with the high yields of photodimerizations of
4-styrylpyridines [19], 2-styrylbenzoxazoles [21] and 5-phenyl2-styryloxazoles [22] in acidic aqueous solution, the isolated
yields of 2a–2e using several hundred milligrams to 1 g of
1a–1e dissolved in several hundred milliliters of acidic aqueous
solution are only 15–47% (Scheme 1 and supporting information) which indicated that the reactivity of this PDDA reaction
in the preparative level is low. With prolonged irradiation
several undetermined byproducts were formed gradually and
the reaction was then terminated. It was found that 1a–1d
containing 4-pyridyl were more active than 1e–1g containing
Fig. 1. Photoreactor and the time-resolved HPL chromatogram of 1a irradiated for
0, 5, 10, 15, 25, 45 and 60 min.
16
J. Zhuang et al. / Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 14–18
Fig. 2. Crystal structure of 2a.
2-pyridyl. The substituents on the phenyl ring also affected the
reactivity of the diarylacetylenes. We have synthesized 1-(4trifluoromethylphenyl)-2-(4-pyridyl)acetylene and found it was
inert to the UV irradiation. Therefore, the electron-withdrawing
groups decreased the reactivity and the electron-donating groups
facilitated the reaction. It was found that 1b and 1f containing Br
have high photoreactivity than the other monomers which was
ascribed to the heavy atom effect of Br.
We determined five single crystal structures of 2a, 2b, 2e, 2f
and 2g. The structure of 2a as a typical example is shown in Fig. 2.
Two pyridyl groups are situated on the 1,3 positions of the naphthalene ring, which indicates that the photocycloaddition of the
two monomers take place in a head-to-tail manner just like the
photodimerization of styrylpyridines in acidic aqueous solution.
The dipole interaction between the protonated pyridyl ring and the
phenyl ring facilitate the monomers to approach to each other in
a head-to-tail manner. No other triarylnaphthalene isomers with a
head-to-head configuration were observed in this reaction because
of the repulsion interaction between the protonated pyridyl rings.
The phenyl ring and the adjacent C C triple bond in one monomer
act as the diene moiety and the C C triple bond in the other
monomer act as the dienophile moiety to form the naphthalene ring
by the PDDA reaction. The dihedral angles in 2a between the naphthalene ring and 1-(4-pyridyl), 2-phenyl and 3-(4-pyridyl) rings are
67.2◦ , 66.4◦ and 44.8◦ , and the corresponding dihedral angles in 2b
are 79.5◦ , 67.7◦ and 42.7◦ , and that in 2e are 64.3◦ , 56.0◦ and 53.8◦ ,
respectively.
As shown in Fig. 3(a) and (b), the maximum absorptions of free
1a–1d in dichloromethane range from 282 to 304 nm due to the
␲–␲* transition of the conjugated diarylacetylene molecules. The
Fig. 3. UV spectra of 1a–1d and 2a–2d. (a) Free 1a–1d in CH2 Cl2 , 5 × 10−5 mol/L; (b) protonated 1a–1d in HCl aqueous solution, 5 × 10−5 mol/L; and (c) free 2a–2d in CH2 Cl2 ,
5 × 10−5 mol/L.
J. Zhuang et al. / Journal of Photochemistry and Photobiology A: Chemistry 270 (2013) 14–18
17
References
Fig. 4. The proposed mechanism of photocycloaddition of 1a in acidic aqueous
solution.
maximum absorption wavelengths of 1a–1d in acidic aqueous solution were shifted to about 330 nm because of the protonation of the
pyridyl ring, which increase the absorption of the monomers in near
ultraviolet region. When monomers form the naphthalene photodimer upon irradiation, the conjugated system of the monomer
was changed, the maximum absorption wavelength of 2a–2d in
CH2 Cl2 shifted to about 255 nm.
We have tried to carry out the reaction by heating without UV
irradiation. The monomer was unchanged even in the condition of
refluxing for several hours. Then, we tried to use a mixed solvent
of water and methanol or a pure solvent of methanol instead of
the acidic aqueous solution to carry out this reaction upon irradiation, there was also no product formed. These results show that the
protonation of the monomer and the UV irradiation are the main
factors that lead to the formation of triarylnaphthalene.
According to the literatures, the photocycloaddition of 1phenyl-2-(pyridyl)acetylenes in acidic aqueous solution is a typical
photo-dehydro-Diels–Alder reaction [1,2]. The proposed mechanism is illustrated in Fig. 4. Two protonated monomers in solution
approach to each other in a head-to-tail manner by intermolecular
dipole interaction. Upon UV irradiation, a C–C single bond between
the two alkyne moieties is formed providing a 1,3-butadiene-1,4diyl biradicals. Then the radical center connected to the pyridinium
ring attacks an ortho position of the phenyl ring in the other
monomer to give the highly strained cyclic allene. Then the migration of hydrogen in the cyclic allene leads to the formation of
naphthalene ring.
In summary, the PDDA reaction of 1-phenyl-2-(pyridyl)
acetylene derivatives was carried out. This work is a valuable
extension of the PDDA reaction to the diarylacetylene derivatives in acidic aqueous solution, which provides a new method
for the synthesis of 1,2,3-triarylnaphthalenes. The study of other
diarylacetylene derivatives containing N-heteroaryl groups, such
as benzoxazole and phenyl-oxazole, is being carried out.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (20902008).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
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