Visible light catalyzed aromatization of 1,3,5-triaryl-2-pyrazolines by platinum(II) polypyridyl complex under oxidant-free condition

Article abstract of DOI:10.1007/s11426-016-5554-7

With visible light (λ=450 nm) irradiation of a catalytic amount of platinum(II) terpyridyl complex, 1,3,5-triaryl-2-pyrazolines can be smoothly converted to their corresponding pyrazoles and hydrogen in quantitative yields with no use of any oxidant at ro

Full text of DOI:10.1007/s11426-016-5554-7

SCIENCE CHINA
Chemistry
February 2016 Vol.59 No.2: 1–5
doi: 10.1007/s11426-016-5554-7
• ARTICLES •
· SPECIAL TOPIC · Organic Photochemistry
doi: 10.1007/s11426-016-5554-7
Visible light catalyzed aromatization of 1,3,5-triaryl-2-pyrazolines
by platinum(II) polypyridyl complex under oxidant-free condition
Pan Ye1,2, Deng-Hui Wang2, Bin Chen2*, Qing-Yuan Meng2, Chen-Ho Tung1,2 & Li-Zhu Wu2*
1School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
2Key Laboratory of Photochemical Conversion and Optoelectronic Materials; Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
Received November 27, 2015; accepted December 21, 2015
With visible light (=450 nm) irradiation of a catalytic amount of platinum(II) terpyridyl complex, 1,3,5-triaryl-2-pyrazolines
can be smoothly converted to their corresponding pyrazoles and hydrogen in quantitative yields with no use of any oxidant at
room temperature.
pyrazolines, pyrazoles, photoredox catalysis, visible light, platinum(II) terpyridyl complex
Citation:
Ye P, Wang DH, Chen B, Meng QY, Tung CH, Wu LZ. Visible light catalyzed aromatization of 1,3,5-triaryl-2-pyrazolines by platinum(II)
polypyridyl complex under oxidant-free condition. Sci China Chem, doi: 10.1007/s11426-016-5554-7
hexachloroanitimonate [15], Pd/C [16], activated carbon
1 Introduction
[17], FeCl3 [18]. However, some of them suffer from draw-
backs such as the use of toxic and expensive reagents, strict
reaction conditions, and difficulty in purification. Photoox-
idation has afforded an alternative way in oxidative aroma-
tization. Direct ultraviolet irradiation [19] and dye-sensi-
tized photooxidation [20] of various aryl-substituted 1,3-
diphenyl-2-pyrazolines also led to dehydrogenation to the
corresponding pyrazoles in the presence of molecular oxy-
gen, but some of them may incorporate oxygen with ring
destruction to yield carbonyl compounds. A mild and effi-
cient photoaromatization of aryl- and hetarylpyrazolines
through a photoinduced electron transfer (PET) has also
been developed, with a considerable excess of carbon tetra-
chloride as the second reactant [21].
Recently, visible light photoredox catalysis with transi-
tion metal complexes has found broad utility in organic
synthesis [22]. Among them, the square-planar platinum(II)
polypyridyl complex is appealing at the forefront of photo-
chemistry due to its unique properties of open axial coordi-
nation site, strong visible absorptions, long excited-state
Pyrazoles are an important class of five-membered hetero-
cyclic compounds that often exist in biologically active nat-
ural products and synthetic compounds of agricultural and
pharmaceutical interest [1]. Furthermore they are useful
ligands for the new generation of transition metal catalysts
for carbon-carbon coupling reactions [2]. The synthesis of
pyrazoles is by all means an attractive area in heterocyclic
chemistry [3]. Among them, the oxidative aromatization of
1,3,5-trisubstituted pyrazolines provides an efficient access,
because 1,3,5-trisubstituted pyrazolines can be conveniently
prepared from phenylhydrazine and chalcone derivatives
[4]. Actually, various oxidizing reagents have been em-
ployed to oxidize pyrazolines to the corresponding pyra-
zoles such as HgO [5], MnO2 [6], AgNO3 [7], Zr(NO3)4 [8],
KMnO4 [9], Pb(OAc)4 [10], p-chloranil [11], trichloroiso-
cyanuric acid [12], iodobenzenediacetate [13], iodic acid/
iodine pentoxide [14], tri(4-bromophenyl)aminium (TBPA+)
*Corresponding authors (email: chenbin@mail.ipc.ac.cn; lzwu@mail.ipc.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2016
chem.scichina.com link.springer.com

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Ye et al. Sci China Chem February (2016) Vol.59 No.2
lifetime, high luminescent quantum yield, and good chemi-
cal stability [23]. With visible-light irradiation, for example,
pyridine derivatives [24], 3,4-diarylpyrroles [25], and
3,4-diarylthiophenes [26] have been successfully obtained
respectively, in the absence of any oxidants. To develop a
photocatalytic approach for the preparation of 1,3,5-triaryl
pyrazoles from synthetic point of view, we expected that
platinum(II) complex may act as a catalyst to conduct such
photochemical transformation. In the present work, we re-
port that upon irradiation with visible light (=450 nm), a
catalytic amount of platinum(II) terpyridyl complex 1 is
capable of converting 1,3,5-triaryl-2-pyrrazolines (2) to the
corresponding pyrazoles (3) and hydrogen (H2) with no use
of any oxidant (Scheme 1).
Figure 1 (a) UV-Vis absorption spectra of 1, 2a and 3a in CH3CN; (b)
absorption spectral change of 2a (1.7×105 mol L1) and 1 (9×106 mol L1)
in degassed CH3CN as a function of time with irradiation at >450 nm.
Inset: the differential absorption spectra with irradiation time of 0, 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 25, 30, and 40 min, respectively
(color online).
2 Experimental
General procedure for the photoreaction: the 1,3,5-triaryl
pyrazolines 2 (0.017 mmol, 1 equiv.) and platinum(II) com-
plex 1 (0.5 mol, 0.03 equiv.) were dissolved in deoxygen-
ated CH3CN (1 mL) in a 10 mL reaction tube equipped with
a rubber septum and magnetic stir bar, and then the argon-
purged solution was irradiated by blue LEDs (max=450 nm)
at room temperature. The generated hydrogen was analyzed
by GC using a 5 Å molecular sieve column with thermal
conductivity detector. After the substrate was completely
converted (monitored by TLC), the solvent was removed
under reduced pressure, then the products 1,3,5-triaryl py-
razoles (3) were isolated by extraction with ethyl acetate or
purified by column chromatography on silica gel eluting
lexes 1 displays broad visible light absorption assigned to a
mixture of metal-to-ligand and ligand-to-ligand charge
transfer transitions ranging from 400 to 500 nm, while 2a
does not absorb in this region, so a glass filter was used to
cut off light below 450 nm with 500 W high-pressure Ha-
novia mercury lamp, thus only 1 was excited. Irradiation of
1 and 2a in degassed CH3CN solution quickly decreased the
absorbance at 295–400 nm for 2a, accompanied by a growth
with a maximum at 260 nm, typical absorption of 1,3,5-
triphenylpyrazole (3a) in CH3CN (Figure 1(b)). The well-
defined isosbestic point at 295 nm suggests that both of 2a
and 3a are present in the solution. In addition, 1 is stable
without any decomposition. This process was much clearer
in the differential absorption spectra (inset, Figure 1(b)). As
shown in Figure 2, the pyrazoline 2a displayed intense
fluorescence with max at 460 nm in CH3CN at room tem-
perature which originates from an intramolecular-charge-
1
with ethyl acetate/petroleum ether and identified by H
NMR, 13C NMR and MS.
3 Results and discussion
To begin this study, 1,3,5-triphenyl-2-pyrazoline (2a) was
chosen as the standard substrate for the desired photocata-
lytic reaction. As shown in Figure 1(a), platinum(II) comp-
Figure 2 Fluorescence change of 2a as a function of time for the photo-
reaction of 2a (1.7×105 mol L1) and 1 (9×106 mol L1) in degassed
CH3CN (ex=360 nm). The time interval is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14, 16, 18, 20, 25, 30, and 40 min, respectively (color online).
Scheme 1 Oxidant-free photocatalytic aromatization of pyrazolines to
pyrazoles (color online).

Ye et al. Sci China Chem February (2016) Vol.58 No.2
3
Table 1 Photocatalytic reaction of 1,3,5-triarylpyrrazolines by plati-
num(II) terpyridyl complex 1 a)
transfer, where the pyrazoles 3a are non-emissive. Upon
irradiation with >450 nm, the typical fluorescence of 2a
disappeared when the reaction was finished, that means the
reaction could be pursued up to complete conversion of 2a
to 3a.
1
This observation is also in line with H NMR spectral
change before and after irradiation by blue LEDs (max=450
nm). As shown in Figure 3, the three typical alkyl protons in
the 4- and 5-position of the pyrazoline ring (Ha, Hb, Hc in
2a, respectively at 3.13, 3.91, 5.4 ppm) decreased with time,
while new signal appeared at 6.98 ppm, which was in ac-
cord with the typical alkene proton in the 4-positon of py-
razole ring (H4 in 3a). Meanwhile, all aryl protons down-
field shifted. In spite of the conversion changing with irra-
diation time, no secondary byproduct except H2, which was
confirmed by GC analysis, was detected throughout the
reaction. The final 1H NMR spectrum coincided completely
with the pure 3a prepared by the oxidation. On the basis of
the consumption of the starting material of 2a, the conver-
sion of the photoreaction was up to 100%.
In contrast, irradiation of 2a in the absence of complex 1
in CH3CN at =450 nm resulted in no product formation.
Moreover, no products could be obtained when the reaction
was carried out in the dark. Evidently, both light and 1 are
essential for the dehydrogenation. In addition, only a cata-
lytic amount of 1 (3 mol% equiv.) significantly resulted in
the photochemical reaction to form 3a, and this transfor-
mation can be of preparative interest, because it occurs with
high concentration (up to 102 mol L1).
Conv.
Entry Substrate
Ar1
Ar2
Time (h)
H2 (%) c)
(%) b)
36
1
2
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
Ph
Ph
Ph
6
35
98
65
–
Ph
12
12
24
12
24
12
24
12
24
12
24
12
24
100
68
3
Ph
p-CH3C6H4
p-CH3C6H4
p-OCH3C6H4
p-OCH3C6H4
o-ClC6H4
o-ClC6H4
p-NO2C6H4
p-NO2C6H4
2-furyl
4
Ph
100
60
5
Ph
60
–
6
Ph
100
48
7
Ph
42
–
8
Ph
85
9
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
55 d)
90 d)
26
0
10
11
12
13
14
0
15
–
2f
2-furyl
66
2g
2g
2-thienyl
2-thienyl
36
30
–
100
a) Conditions: 2 (0.017 mmol), 3 mol% 1 (0.5 mol), in CH3CN (1 mL)
under an argon atmosphere, irradiation with 3 W blue LEDs, r.t.; b) con-
version of 3 were determined by H NMR; c) GC yields using pure me-
thane as an internal standard; d) trace amount of corresponding amino
1
derivative 3i were observed, Ar2=p-NH2C6H4.
stituents on pyrazolines caused the rate of dehydrogenation
to decrease. With prolonged reaction time, electronic effects
of the substituents on phenyl seemed not to affect the yield
of the reaction very much, corresponding pyrazoles (3b–3g)
was formed in good to excellent yield. In all cases the reac-
tion was rather clean, except 2e (Table 1, entries 9, 10), no
H2 obtained, and 3e formed accompanied by a minor
amount of the reduced amino derivative 3i. The reaction
time listed in Table 1 roughly indicated that the substituent
of aryl groups Ar2 in 5-position of 2-pyrazolines seemed to
have a higher influence on the reactivity compared to Ar1 in
4-position of 2-pyrazolines.
With the optimized reaction conditions in hand, a variety
of 1,3,5-triaryl-2-pyrazolines (2b–2g) were explored, and
the results obtained are summarized in Table 1. The sub-
To understand the primary process of the photocatalytic
reaction, we examined the interaction between 2 and com-
plex 1. Complex 1 is emissive with max at 628 nm in dega-
ssed CH3CN at room temperature (Figure 4), which is read-
ily quenched by 1,3,5-triaryl-2-pyrazoline of 2. The quench-
ing process follows Stern-Volmer kinetics (Figure 4) and
the quenching constants (kq) were calculated from emission
intensity measurements to be on the order of 109 L mol1 s1
(Table 2). The emission quenching is due to electron trans-
fer from the 1,3,5-triaryl pyrazoline substrate to the excited
1 since the singlet energy transfer from 2a–2g to the excited
1 is thermodynamically impossible with the energy of the
singlet excited state of 1 much lower than that of 2a–2g.
Calculation of the free energy change (G) by the Rehm-
Weller equation reveals that the electron transfer process
from 2a–2g to the triplet 1 is exergonic (Table 2). As only
Figure 3 1H NMR spectra of 1 and 2a in argon-flushed CD3CN before (a)
and after (b) irradiation. For the trace amount of 1, the spectra only show
the disappearance of 2a and the appearance of 3a.

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Ye et al. Sci China Chem February (2016) Vol.59 No.2
only 1 was excited, the PET from 2a to 1 indeed takes
place. According to the lifetimes of 1 with and without 2a in
CH3CN, the rate constant for the PET process can be con-
sequently obtained as kET=10.5×109 L mol1 s1, close to the
diffusion limit. Affected by the strong fluorescence bleach-
ing of 2a, the lifetime of the newly formed intermediate
species by photoinduced electron transfer could not be de-
termined, nor the rate (kCR) for the back electron transfer
process. Prolonged irradiation of 1 and 2 in CH3CN led to
no permanent change, indicating the reduced platinum spe-
cies is stable.
Careful re-examination of this reaction for the hydrogen
production revealed that the molar ratio of 3 to H2 is ap-
proximate to 1:1 (Table 1), and the material balance is
greater than 99%. On the basis of the above results, we may
deduce that when 2 is used as a quencher, electron transfer
pathway followed by proton-coupling pathway leading to
five coordinated platinum(III)-H species are possible. In
fact, the formation of 3 and H2 by 1 supports an effective
intramolecular H abstraction. On the basis of the above re-
sults, we proposed that the photocatalytic reaction is initi-
ated by photoinduced electron transfer followed by pro-
ton-coupling process (Scheme 2). Under the irradiation
condition, 2 not only serves as the electron donor to the ex-
cited complex 1 but also acts as proton donors to the photo-
reduced platinum center of 1. The generated platinum(III)-H
species would consecutively react with the pyrazoline
radical to form 3 and H2 and simultaneously to regenerate
Figure 4 Emission spectra of complex 1 in degassed CH3CN as a func-
tion of concentration of 2a. The inset shows the Stern-Volmer plot for
luminescence intensity quenching of 1 by 2a.
Table 2 Data of kq and G for the photocatalytic conversion
2a
7.3
2b
8.9
2c
2d
9.9
2e
4.8
2f
2g
8.0
kq (×109 L
mol1 S1)
Eox (V) a)
11.6
10.1
1.11
1.16
1.12
1.16
1.17
1.10
1.16
G (V) a) 0.52 0.47 0.51 0.47 0.46 0.53 0.47
a) G was calculated by equation: G=Eox–Ered–E0,0–e2/a, where
Eox and Ered versus NHE are oxidative and reductive potentials of 2 and
1, respectively, which were measured by cyclic voltammetry in degassed
CH3CN solution with 0.1 mol L1 nBu4NPF6 as supporting electrolyte.
Ered is 0.53 V for 1 and e2/a is 0.05 V in CH3CN. E0,0 refers to the
lowest excited energy of 1 in CH3CN (2.11 V).
the complex 1 was excited, it is therefore reasonable to con-
sider that the photoinduced electron transfer from 2 to the
excited 1 is responsible for luminescence quenching and the
products generation.
Nanosecond transient absorption spectroscopy further
confirms that the emission quenching is due to the electron
transfer from 2 to the excited 1. Figure 5 shows the
time-resolved transient absorption spectra observed follow-
ing 355 nm excitation of 1, 1 with 2a, 2a, respectively. The
spectrum exhibits strong transient absorption of the lowest
3MLCT state for 1 throughout all wavelengths examined
(370–800 nm) and decays with a lifetime of 270 ns, match-
ing that of the emission. When 2a was introduced into 1, the
lifetime of 1 decreased and the 3MLCT absorption of 1 was
replaced by a new bleaching in the region of 400–600 nm
(Figure 5(b)). It is apparent that a strong similar bleaching
for 2a emerged immediately under the same condition (Fig-
ure 5(c)). However, the pyrazoline-quenched sample does
exhibit a maximum at 520 nm in the differential absorption
spectra (Figure 5(d)) that is clearly not present in the tran-
sient absorption spectra of 2a. With reference to the detailed
spectroscopic work by our group [24–26] and Schmehl et
al. [27] on platinum(II) complexes, the observed intense
peak around 520 nm was possibly resulted from the inter-
mediate species formed by electron transfer. As in this case
Figure 5 (a) Transient absorption spectra of 1 in CH3CN at room tem-
perature; (b) transient absorption spectra of 1 (5.0×105 mol L1) with 2a
(7.7×104 mol L1); (c) transient absorption spectra of 2a in CH3CN; (d)
transient absorption spectra of 1 and 2a (triangles, blue) in CH3CN after
subtraction of the 2a spectrum. ex=355 nm (color online).

Ye et al. Sci China Chem February (2016) Vol.58 No.2
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Acknowledgments This work was supported by the Ministry of Science
and Technology of China (2013CB834505, 2013CB834804), the National
Natural Science Foundation of China (21390404, 91427303, 21402217),
and the Key Research Programme of the Chinese Academy of Sciences
(KGZD-EW-T05).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online
at http://chem.scichina.com and http://link.springer.com/journal/11426.
The supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains
entirely with the authors.
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