Visible light-induced highly selective transformation of olefin to ketone by 2,4,6-triphenylpyrylium cation encapsulated within zeolite Y

Article abstract of DOI:10.1039/b515137f

2,4,6-Triphenylpyrylium cation encapsulated within zeolite Y promotes highly selective transformation of olefins to ketones with molecular oxygen, under visible light (λ > 400 nm) irradiation at room temperature. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515137f

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Visible light-induced highly selective transformation of olefin to ketone
by 2,4,6-triphenylpyrylium cation encapsulated within zeolite Y{
Yasuhiro Shiraishi,* Naoya Saito and Takayuki Hirai
Received (in Cambridge, UK) 25th October 2005, Accepted 8th December 2005
First published as an Advance Article on the web 6th January 2006
DOI: 10.1039/b515137f
through the narrow window, thus affording TP+ encapsulated
2,4,6-Triphenylpyrylium cation encapsulated within zeolite Y
within zeolite Y (TPY; Fig. 1A). Preliminary experiments revealed
that TPY is applicable to several photoreactions, such as
isomerization of cis-stilbene8a and decomposition of pollutants.8d,e
Herein, we describe that photoactivated TPY promotes highly
selective transformation of olefin to ketone with O2, while
suppressing dimerization of olefin. This unprecedented activity is
triggered simply by a structural feature of TPY: the small zeolitic
cavity suppresses the formation of large dimers. This is a catalytic
system achieving the highest ketone selectivity among those
proposed so far.2–7 Photooxygenation of olefins within zeolite
have been studied extensively;9 however, none of the systems
achieves selective ketone production. Size-selective phototransfor-
mation of molecules is promoted within several zeolitic systems,10
where substrate activities and product selectivities are controlled by
the pores.11 However, none of the systems had been utilized as a
reactor for suppression of dimer formation leading to selective
oxygenation of molecules.
promotes highly selective transformation of olefins to ketones
with molecular oxygen, under visible light (l . 400 nm)
irradiation at room temperature.
Selective transformation of olefins to ketones is one of the most
important functional group transformations in organic synthesis.1
Catalytic transformation processes driven by molecular oxygen
(O2) have attracted a great deal of attention;2 however, most of the
processes employ homogeneous catalysts which are rarely recycl-
able. Heterogeneous catalytic processes have also been proposed,3
but they require relatively expensive oxidants such as H2O2. To the
best of our knowledge, there is only one report of a successful
heterogeneous catalytic system driven by O2, which employs a Ru-
bound Al2O3,4 but it needs severe reaction conditions (550 K).
Development of an economical heterogeneous system capable of
promoting selective transformation of olefin to ketone with O2 is
now the focus of attention.
TPY was prepared according to the literature procedure,8a
where the amount of TP+ within TPY is 6.8 wt% (0.22 mmol g21
of TPY).{ Photooxygenation was carried out by photoirradiation
(.400 nm)12 of a suspension of TPY in O2-saturated MeCN with
the respective olefins.§ Table 1 summarizes the results of
photooxygenation of terminal olefins. Reaction of styrene 1 with
TPT gives benzaldehyde 2 via CLC bond cleavage, along with a
large amount of dimers (3 and 4) (77%). In contrast, TPY gives 2
with excellent selectivity (.99%) without the production of
dimers. Reactions of TPT with a-substituted styrenes, such as
Photooxygenation is a promising process because it can be
operated at room temperature. So far, photocatalytic processes
using a semiconductor, titanium dioxide, have been studied
extensively,5 but the processes lack sufficient selectivity and require
UV light for catalyst activation. Some homogeneous photo-
sensitizers can promote oxygenation of olefins by visible light
(.400 nm) activation, but the ketone selectivity is quite low.6 2,4,6-
Triphenylpyrylium tetrafluoroborate (TPT) is one of these
photosensitizers,7 which is photoexcited by ,470 nm irradiation.
The excited TPT leads to an electron transfer (ET) from olefin to
+
?
TPT and produces a radical cation of olefin (olefin ), while
producing neither singlet oxygen (1O2) nor superoxide anion
(O2 2). As a result of this, oxygenation proceeds via reaction of
?
+
?
olefin with O2, thus inevitably promoting undesirable dimeriza-
tion of olefin.
TP+ cation can easily be immobilized onto a zeolitic support by
a ‘‘ship-in-a-bottle’’ method.8 Reaction of acetophenone and
chalcone with zeolite Y leads to a formation of TP+ within its
supercage (13 s diameter 6 8 s window),8a where Brønsted acid
sites of zeolite act as the counterions. Bulky TP+ cannot pass
Research Center for Solar Energy Chemistry, and Division of Chemical
Engineering, Graduate School of Engineering Science, Osaka University,
Toyonaka, 560-8531, Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp;
Fax: +81-6-6850-6273; Tel: +81-6-6850-6271
{ Electronic Supplementary Information (ESI) available: DR UV–vis
spectrum (Fig. S1); thermogravimetric profile (Fig. S2); relationship
between average diameter of olefins and the ratio of kq(TPY)/kq(TPT)
(Fig. S3); fluorescence decay profile (Fig. S4); E1/2 and kq data (Table
S1); photooxygenation results of b-substituted styrenes (Table S2). See
DOI: 10.1039/b515137f
Fig. 1 (A) Schematic representation of TP+ incorporated in zeolite Y
supercage. (B) Maximum length (l) and its cross-sectional length (w) of
molecule 3.
ox
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Table 1 Results of olefin photooxygenation on various sensitizersa
reactions of 1, 5, and 9 still produces dimers (Table 1), where the
product distributions are similar to those obtained with only TPT.
This rules out an acid site of zeolite as the factor for selective
transformation.13 Bulky TPT cannot pass through the small zeolite
window;8a hence, in a TPT + Y system, ET from olefin to TPT and
Product select. (%)
Reactant
conv. (%)
Ketone
Dimers
olefin + oxygenation by O2 must occur in a bulk solution, as is also
?
+
?
the case in the TPT system. In contrast, within TPY, olefin
formed via ET is stabilized as a charge-compensating cation of
zeolite and hence its diffusion is much suppressed;8a therefore,
+
?
olefin oxygenation by O2 must occur within the zeolite cavity.
TPY
TPT
TPT + Y
7.9
18.6
12.1
.99
23
26
68
67
9
7
These suggest that suppression of the dimer formation within TPY
may be attributable to the narrow zeolite cavity.
TP+, of a planar structure with 13 s length, is encapsulated on
an equatorial plane of the spherical supercage (13 s diameter), as
+
depicted in Fig. 1A.8a Olefin must therefore be formed and
?
stabilized within the hemisphere space (13 6 6.5 s). The size of
dimers was checked to clarify whether dimers can form within this
space. By MO calculations (MNDO-PM3),11i the maximum length
(l) of molecules and their cross-sectional length (width, w) were
determined (Fig. 1B). As summarized in Table 2, the widths of the
dimers are clearly larger than the hemisphere height (6.5 s), while
the widths of the ketones are smaller. These findings clearly
indicate that the narrow TPY hemisphere suppresses olefin
dimerization, thus allowing selective ketone production.
TPY
TPT
TPT + Y
3.7
18.8
18.5
.99
45
48
48
44
7
8
TPY also promotes highly selective transformation of 4-chloro-
methylstyrene 14 and 4-chlorostyrene 17 to the corresponding
aldehydes (Table 1), while both TPT and TPT + Y systems show
much lower selectivity. As well known, these Cl-substituted
aldehydes are indispensable materials in organic synthesis.1,2e,3b,4
The present TPY system therefore has the potential to become a
very powerful tool to synthesize these materials economically.
The disadvantages of TPY must be noted. (i) Conversions are
lower than those obtained by TPT (Table 1). This is because ET
from the olefin to TP+ is suppressed by the narrow zeolite cavity.
This is confirmed by fluorescence quenching experiments on TP+:
the quenching rate constants, kq(TPY), by olefins are much lower
(,17%) than kq(TPT) (Table S1{). The ratio kq(TPY)/kq(TPT)
tends to decrease with an increase in the size of the olefin (Fig. S3{),
indicating that diffusion of larger size olefins is much more
suppressed. (ii) Transformation of aliphatic olefins is not favored
thermodynamically.6,7 This is because ET from these olefins to
TP+ is poor due to the relatively positive oxidation potential of
these olefins.7a (iii) Transformation of b-substituted olefins is
unsuccessful (Table S2{). Reactions of cis- and trans-stilbene with
TPY
TPT
TPT + Y
6.2
25.4
14.4
.99
73
77
18
14
8
7
1
2
TPY
TPT
TPT + Y
9.3
52.1
25.8
.99
49
56
51
44
TPY
TPT
TPT + Y
a
4.2
18.0
14.6
.99
51
60
49
40
MeCN, 15 ml; olefin, 750 mmol; O2, 1 atm; photoirradiation time,
5 h; TPY or Y, 34 mg; TPT, 7.5 mmol.
Table 2 Maximum length (l) and cross-sectional length (width, w) of
molecules determined by MO calculations
l/s
w/s
l/s
w/s
a-methylstyrene 5 and 1,1-diphenylethylene 9, also produce large
amounts of dimers [(7 and 8) and (11, 12, and 13)], respectively. In
contrast, TPY affords the corresponding ketones (6 and 10) with
very high selectivity (.99%).
1
2
7.42
6.61
9.91
8.16
7.46
7.44
9.72
8.79
9.17
8.98
5.33
5.39
6.53
7.71
5.54
5.33
7.01
7.16
5.78
5.93
11a
12a
13a
14
11.24
12.04
12.41
8.89
8.06
10.5
8.48
7.69
9.80
11.14
8.96
9.07
5.42
5.49
8.29
5.36
5.39
8.10
3a
4a
5
In the TPY system, material balance between olefins and
ketones agrees reasonably well (.97%). When TPY recovered
after the photoreaction was dissolved in hydrofluoric acid and
extracted with CH2Cl2, GC analysis did not detect dimers or
ketones. This implies that dimers neither adhered to the cavity wall
nor were confined within the cavity, and therefore are not formed
within the cavity at all. Use of TPT with zeolite Y (TPT + Y) for
15
6
16a
17
7a
8a
9
18
19a
10
a
Dimerized products shown in Table 1.
774 | Chem. Commun., 2006, 773–775
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TPY give isomerized olefins (.94%), while reaction with TPT
+
gives 2 with .10% selectivity.8a Olefin is stabilized within the
?
cavity as a charge-compensating cation of zeolite.8a These diaryl
olefins have relatively negative oxidation potential (Table S1{)
and, hence, are stabilized more than monoaryl olefins. The strong
stabilization may strongly suppress the CLC bond cleavage by O2,
thus leading to lower ketone production. Reaction of trans-
b-methylstyrene with TPY affords allylbenzene (50%) together
with 2 (Table S2{). This is because the carbocation of trans-
b-methylstyrene is transformed into a more stable allylic cation via
proton transfer catalyzed by acid sites within the zeolite.14
In conclusion, we found that TPY promotes the highly selective
transformation of aryl olefins with a terminal CLC bond to the
corresponding ketones with O2 by visible light irradiation. This is
triggered simply by suppression of dimer formation within the
narrow zeolite cavity. The proposed system still contains several
problems; we are now addressing issues (i) and (iii) by varying the
cavity size and number of acid sites on the zeolite. Nevertheless,
the proposed concept will contribute to the development of a more
efficient photosensitizing system for selective transformation of
molecules.
We are grateful for financial support by Grants-in-Aid for
Scientific Research (No. 15360430) and on Priority Areas
‘‘Fundamental Science and Technology of Photofunctional
Interfaces (417)’’ (Nos. 15033244 and 17029037) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (MEXT).
Notes and references
{ TPY was synthesized using HY zeolite (Tosoh Corp.; HSZ-331HSA;
SiO2/Al2O3 5 6; BET surface area, 650 m2 g21; particle size, 4 mm), as
follows: a mixture of chalcone (100 mg) and acetophenone (50 mg)
dissolved in isooctane (50 ml) was refluxed for 12 h with HY zeolite (1.0 g).
The obtained yellowish solid was washed by Soxhlet extraction with
CH2Cl2 for 18 h and dried under vacuum. The diffuse reflectance UV–vis
spectrum of TPY is identical to the absorption spectrum of TPT dissolved
in CH2Cl2 (Fig. S1{). The TP+ content within TPY was determined by the
loss of weight at 523–973 K on a thermogravimetric profile (Fig. S2{).
§ TPY (34 mg containing 7.5 mmol TP+) was suspended in dry MeCN
(15 ml) containing each olefin (750 mmol) within a Pyrex glass tube
(id 10 mm; capacity, 20 ml), and each tube sealed using a rubber septum
cap. TPY was dispersed well by ultrasonication for 5 min, and O2 was
bubbled through the solution for 10 min at 273 K to avoid the evaporation
of solvent. The tube was photoirradiated with magnetic stirring by a 2 kW
Xe lamp (USHIO Inc.) filtered through an aqueous NaNO2 solution
(3 wt%) to give light wavelengths of l . 400 nm (light intensity at 400–
470 nm: 477 mW m22). The solution temperature during photoirradiation
was 313 K. The resulting solution was recovered by centrifugation and
analyzed by GC-FID. For the TPT experiment, a MeCN solution (15 ml)
containing TPT (7.5 mmol) was used for the reaction.
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