Chemoselective Epoxidation of Allyloxybenzene by Hydrogen Peroxide Over MFI-Type Titanosilicate

Article abstract of DOI:10.1002/ejoc.202000249

The chemoselective synthesis of 2-(phenoxymethyl)oxirane from allyloxybenzene is achieved with over 90 % yield in a sustainable reaction system using titanium-substituted silicalite-1 (TS-1) as a catalyst, hydrogen peroxide (H₂O₂) as an oxidant, and a mixture of MeOH/MeCN as a solvent at 40 °C. No acid-catalyzed side reactions prompted by the Lewis acidity of the Ti active site in TS-1 are observed. The TS-1 catalyst can also promote the formation of oxiranes from various p-substituted allyloxybenzenes in good yields. The reaction mechanism is investigated through the reaction with other allyloxy compounds. The results, which are supported by DFT calculations, indicate that an active species of Ti peroxides formed from the reaction of TS-1 with H₂O₂ selectively oxidizes the allyloxybenzene to 2-(phenoxymethyl)oxirane.

Full text of DOI:10.1002/ejoc.202000249

DOI: 10.1002/ejoc.202000249
Communication
Epoxidation
Chemoselective Epoxidation of Allyloxybenzene by Hydrogen
Peroxide Over MFI-Type Titanosilicate
Yoshihiro Kon,*[a] Takuya Nakashima,[a] Dachao Hong,[a] Xinyi Ji,[b] Ryota Osuga,[b]
Satoru Ito,[a,c] Tadahiro Fujitani,[a] Kazuhiko Sato,[a] and Toshiyuki Yokoi*[b]
Abstract: The chemoselective synthesis of 2-(phenoxy- catalyst can also promote the formation of oxiranes from vari-
methyl)oxirane from allyloxybenzene is achieved with over ous p-substituted allyloxybenzenes in good yields. The reaction
90 % yield in a sustainable reaction system using titanium-sub- mechanism is investigated through the reaction with other ally-
stituted silicalite-1 (TS-1) as a catalyst, hydrogen peroxide loxy compounds. The results, which are supported by DFT cal-
(H2O2) as an oxidant, and a mixture of MeOH/MeCN as a solvent culations, indicate that an active species of Ti peroxides formed
at 40 °C. No acid-catalyzed side reactions prompted by the Le- from the reaction of TS-1 with H2O2 selectively oxidizes the
wis acidity of the Ti active site in TS-1 are observed. The TS-1 allyloxybenzene to 2-(phenoxymethyl)oxirane.
good yields (26 % and 5 %, respectively) because 2a generally
suffers from ether bond cleavage to form phenol under acidic
conditions.[9] The Claisen rearrangement of 2a can occur as a
side reaction under acidic conditions (Scheme 1).[10] Moreover,
the obtained product 1a can decompose to form 1-methoxy-3-
phenoxypropan-2-ol (3a),[6] resulting in low yields. Since hydro-
phobic fields in zeolites have been reported to inhibit the for-
mation of by-products in oxidation reactions,[8b] the utilization
of “hydrophobic pockets” in zeolites[1,11,12] can be expected to
suppress the side reactions observed in the epoxidation of 2a.
In this work, we describe the design and preparation of differ-
ent types of zeolites containing Ti active sites for the chemose-
lective epoxidation of allyloxybenzene.
Selective oxidation of alkenes to epoxides is a fundamental
transformation in organic synthesis, and is one of the most fre-
quently utilized processes in chemical industries.[1] Especially,
2-(phenoxymethyl)oxiranes (1) are major constituents of epoxy
resins,[2] which are known as versatile materials used in lamina-
tion of circuit boards and encapsulation of electronic compo-
nents owing to their excellent thermal and chemical resistances
as well as their workability and durability.[2,3] Several catalysts
have been reported to selectively oxidize allyloxybenzenes (2)
to produce 1 in high yields. For example, 2-(phenoxymethyl)ox-
irane (1a) was reported to be produced from allyloxybenzene
(2a) by using phosphotungstic acid-immobilized catalysts and
Oxone® or tert-butyl hydroperoxide (tBuOOH) as an oxidant.[4]
However, these reactions inevitably lead to the formation of
wastes such as acids and tBuOH.
The use of H2O2 as an oxidant for the epoxidation of 2a to
produce 1a is one of the most promising approaches because
water is the only by-product.[5–8] Sheldon's group showed that
the H2O2 oxidation of 2a gave 1a when catalyzed by HReO4/
Ph2AsMe in trifluoroethanol solutions.[6] Strukul and co-workers
reported the green H2O2 oxidation of 2a to 1a by a PtII complex
catalyst.[7] Unfortunately, these reactions did not give 1a in
Scheme 1. Possible reaction scheme from 2a.
[a] Interdisciplinary Research Center for Catalytic Chemistry, National Institute
of Advanced Industrial Science and Technology (AIST),
Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
E-mail: y-kon@aist.go.jp
We found that the chemoselective oxidation of 2 to produce
1 proceeded in high yields with good selectivity using a tita-
nium-substituted silicalite-1 (TS-1) catalyst and H2O2 as an oxid-
ant. This reaction system was applicable to the production of
various kinds of allyloxy compounds. The TS-1 catalyst could be
reused for the epoxidation of 2a for at least five cycles with
99 % selectivity. We also successfully obtained 1a from the
chemoselective oxidation of 2a on a gram-scale under opti-
mized conditions; 5.30 g of 2a was oxidized by TS-1 to produce
[b] Nanospace Catalysis Unit, Institute of Innovative Research, Tokyo Institute
of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
E-mail: yokoi@cat.res.titech.ac.jp
http://www.nc.iir.titech.ac.jp/
[c] Institute for Energy and Material/Food Resources, Technology Innovation
Division, Panasonic Corporation
3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202000249.
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Communication
5.21 g of 1a in 93 % isolated yield [Equation (1), see the detailed 2a and the selectivity of 1a were strongly correlated with the
procedure in Supporting Information (SI)].
MeOH:MeCN ratio (Figure 1).
The TS-1 catalyst, which was prepared by the YNU method
(see, SI), is known to catalyze the epoxidation of small and sim-
ple alkenes.[13] We characterized its crystal structure by XRD,
DR-UV, FT-IR, 29Si MAS NMR, and pyridine-adsorbed FT-IR spec-
troscopy (Figures S1–S5).[13] The screening of the catalyst and
the reaction conditions for the oxidation of 2a to 1a with H2O2
were investigated, and the results are summarized in Table 1.
Figure 1. Correlation between MeOH/MeCN ratio and reactivity of 2a.
Table 1. Screening of conditions for the oxidation of allyloxybenzene (2a)
with H2O2.[a]
The addition of MeOH resulted in higher conversion of 2a
and higher yield of 1a. The yield of 1a decreased as the solvent
alcohol size increased in the order MeOH < EtOH < iPrOH <
tBuOH (86 %, 43 %, 19 %, and 20 % yield, respectively; Table
S6). The alcohol size dependence of the yield of 1a is consistent
with that previously reported for propene epoxidation in a gas
flow reaction.[11,12] Considering this, a five-membered ring Ti–
OOH active species would be the most plausible Ti active site
because MeOH enhanced the conversion of 2a.[11,18] On the
other hand, 3a was obtained in 10 % yield as a by-product
when using net MeOH as solvent, which was suppressed by
adding MeCN into MeOH. The MeCN solvent is known to form
hydrogen bonding with MeOH;[19] therefore, the addition of
MeCN would reduce the Brønsted acidity of the solution
through hydrogen bonding between MeOH and MeCN, thereby
inhibiting the formation of 3a.
The H-ZSM-5 catalyst, which contains Al instead of Ti,
showed no reactivity in the H2O2 oxidation of 2a. Ti-MWW gave
1a in 4 % yield despite having almost the same Si/Ti ratio and
BET surface area as TS-1. The low yield for Ti-MWW can be
ascribed to be the less accessibility of 1a to the catalytic sites
due to the lower hydrophobicity of Ti-MWW compared to that
of TS-1.[20] Meanwhile, Ti-ꢀ[21] and Ti-MCM-41,[22] which are
known as catalysts for the oxidation of small alkenes, gave 1a
in negligible yields (3 % and 0 %, respectively), due to their
lower partition coefficient in MeOH as compared to that of TS-
1.[16] Besides, Ti-MCM-41 was decomposed by H2O2 during the
reaction. Control experiments using Ti-containing solid catalysts
were performed by employing a physical mixture of TS-1 and
TiO2 (TS-1/TiO2 = 9:1) or pristine TiO2 as catalysts to examine
the function of internal Ti atoms. In the case of the TS-1/TiO2
mixture, the yield of 1a decreased to 77 %, and pristine TiO2
yielded only 1 % of 1a (Table 1). The TS-1 catalyst was easily
separated from the reaction solution and could be reused after
calcination at 500 °C for 3 h. Thus, the yield of 1a was main-
tained as high as 90–95 % with 99 % selectivity even after five
cycles (Figure 2 and Table S7). The oxidation reaction of 2a with
H2O2 took place on the surface of TS-1 as evidenced by a hot-
filtration test, in which the epoxidation of 2a ceased after TS-1
Catalyst [mg]
Si/Ti
ratio
Conversion
Yield of 1a
Selectivity
[%][c]
of 2a [%][b]
[%][b]
none
TS-1 (10)
TS-1 (20)
TS-1 (30)
–
0
0
0
59
59
59
59
59
59
–
77
270
50
–
73
91
93
63
99
97
0
7
11
3
73
90
91
62
91
86
0
4
3
0
1
> 99
99
98
98
92
89
0
57
27
0
TS-1 (20)[d]
TS-1 (20)[e]
TS-1 (20)[f]
H-ZSM-5 (20)
Ti-MWW (20)
Ti-ꢀ (20)
Ti-MCM-41 (20)
TiO2 (20)
5
86
20
90
[g]
TS-1 + TiO2
–
77
[a] Reaction conditions: catalyst, 40 °C, 67 h, mixture of MeOH (1 mL) and
MeCN (1 mL) as solvent, 1.0 mmol of 2a, 2.0 mmol of 35 wt.-% H2O2 aqueous
solution (aq). [b] Determined by GC analysis based on 2a. [c] Selectivity =
yield/conversion × 100. [d] Using 1.0 mmol of 35 wt.-% H2O2 aq. [e] Reaction
at 50 °C. [f] Using MeOH (2 mL) without MeCN. [g] Using TS-1 (18 mg) with
TiO2 (2 mg).
The yield of 1a at 67 h was increased from 73 % to 91 % by
increasing the amount of TS-1 catalyst from 10 to 30 mg (see
also Tables S1 and S2), suggesting that TS-1 can stabilize the
steady state of an intermediate generated in the oxidation of
2a. The turnover numbers (TONs) were calculated to be 260
(10 mg), 160 (20 mg), and 110 (30 mg) for TS-1.[14] Since the use
of large amounts of TS-1 (30 mg) also induced the formation of
3a, we employed 20 mg of TS-1 for further screenings. The
reaction requires double molar amount of H2O2 to reach a 90 %
yield of 1a due to partial decomposition of H2O2 induced by
TS-1 (Table S3). The efficiency of H2O2 consumption for the ep-
oxidation was measured to be 64 %, and unavoidable decom-
position occurred by using 2 equiv. of. H2O2. When using 10 mg
of TS-1, the efficiency was increased to 96 %; however, the yield
of 1a was decreased to 73 % (Table S2).[15] The optimal reaction
temperature is important to obtain 1a with good selectivity
(Table S4) because decomposition of 1a to 3a occurred above
40 °C (Table S5). The TS-1 catalyst used in the reaction exhibited
few defects and low OH density on its surface (Figures S2–S4),
which might suppress the acid-catalyzed side reaction at 40 °C.
Solvents are known to affect the accessibility of the alkene was separated from the reaction solutions at 6 h reaction time
to TS-1.[12,16,17] Accordingly, we observed that the conversion of (Table S8).
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Table 2. TS-1-catalyzed oxidation of allyloxybenzenes (2) with H2O2.[a]
Figure 2. Conversions and yields of the production of 1a catalyzed by TS-1
in the repeated experiments.
Oxidation of various allyloxy p-substituted benzenes (2b–2i)
was effectively achieved by the TS-1 catalyst, producing the
corresponding epoxides (1b–1i) in up to 93 % isolated yields
(Table 2).
The reactivities for aryl halide derivatives 2d–2f were similar
to that for 2a, and 1d–1f were obtained in 85–93 % isolated
yields. Both electron-rich and electron-poor alkenes (2b, 2c, and
2g–2i) afforded the corresponding epoxides in 56 %–77 % iso-
lated yields. In the case of 1-(allyloxy)-3-chlorobenzene (2j) and
1-(allyloxy)-2-chlorobenzene (2k), the corresponding oxiranes
were obtained in 32 % and 0 % yields, respectively. The reaction
of [(3-methylbut-2-en-1-yl)oxy]benzene (2l) having a bulky allyl
moiety showed a low conversion (31 %) and a 22 % yield of the
corresponding oxirane 1l. The steric hindrance of substrates has
been reported to prevent the oxidation of the alkene in the
presence of TS-1 catalysts.[11] Accordingly, the drastic decrease
of the yield in the order of allyloxy 4-, 3-, and 2-chlorosubsti-
tuted benzenes suggests the effect of steric hindrance between
2 and TS-1. In pyridine-adsorbed FT-IR measurements, the char-
acteristic band of pyridine adsorbed on the Brønsted acid
site at 1545 cm–1 was not observed, and a Lewis acid site at
1445 cm–1 was detected (Figure S5).[17] The Brønsted acid sites,
which facilitated Claisen rearrangement of 1 or hydrolysis of 2,
were not observed on the surface of TS-1.
In the case of other ethers such as 1-(allyloxy)octane (4) and
[(allyloxy)methyl]benzene (5), TS-1 also showed good reactivity
to give the corresponding oxiranes 6 and 7 in 62 % and 49 %
yields, respectively, together with the formation of 1-octanol
and benzyl alcohol in 11 % and 10 % yields (Scheme 2).
The use of TS-1 catalysts has been reported to oxidize aryl
compounds,[23,24] which could be a potential side reaction in
the reaction of 2 by TS-1. DFT calculations showed that the
HOMO (–6.41 eV), HOMO-1 (–6.47 eV), and HOMO-2 (–6.69 eV)
orbitals of 2a were distributed in the aryl moiety, and the
HOMO-3 orbital was located on the allyl moiety (–7.57 eV)
(Table S9 and Figure S6). The van der Waals radius of the allyl
moiety and the benzene ring in 2a were calculated to be 4.25
and 6.53 Å, respectively (Figure S7). Thus, the steric repulsion
between the bulky benzene ring of 2a and TS-1 can prohibit
the access of the benzene ring of 2a to the Ti active sites in TS-
1. These results are consistent with those previously reported
for a bulky alkoxide-substituted benzene ring whose contact
with the Ti active sites in TS-1 was inhibited.[11,23]
[a] Reaction conditions: 20 mg of TS-1 (Si/Ti = 59), 40 °C, 67 h, mixture of
1 mL of MeOH and 1 mL of MeCN as solvent, 1.0 mmol of 2, 2.0 mmol of
H2O2 (35 % in aqueous solution). [b] Determined by GC analysis based on
2a. [c] Isolated yield. [d] Selectivity = yield/conversion × 100. [e] 40 mg of TS-
1, 90 h.
Scheme 2. TS-1-catalyzed epoxidation of allyloxy compounds to oxiranes.
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In summary, we have demonstrated the selective H2O2 oxid-
ation of 2 to 1 in over 90 % yield with excellent selectivity not-
withstanding the multifunctionality of 2. The Brønsted acid sites
of TS-1 was not observed in pyridine-adsorbed FT-IR, and the
side reactions such as the Claisen rearrangement and hydrolysis
were inhibited. Furthermore, the acid-catalyzed ether bond
cleavage at 2 was not observed in the reaction because Ti Lewis
acidic sites were present only in the framework. The Lewis acid-
ity enabled the formation of Ti–OOH as active sites for the ep-
oxidation of 2 at 40 °C.
Keywords: Epoxidation · Sustainable chemistry · Allylic
compounds · Hydrophobic effect · Zeolites
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Received: February 24, 2020
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Communication
Epoxidation
A H2O2 chemoselective epoxidation of
allyloxybenzene was developed using
titanium-substituted silicalite-1 (TS-1)
as catalyst. The gram-scale synthesis of
2-(phenoxymethyl)oxirane was per-
formed using 5.30 g of allyloxyben-
zene to produce 5.21 g of 2-(phenoxy-
methyl)oxirane in 93 % isolated yield.
MeOH/MeCN as mixed solvent facili-
tated the desired epoxidation even at
40 °C. The developed TS-1 catalyst is
reusable, productive, and has a broad
substrate scope.
Y. Kon,* T. Nakashima, D. Hong,
X. Ji, R. Osuga, S. Ito, T. Fujitani,
K. Sato, T. Yokoi* ................................ 1–5
Chemoselective Epoxidation of
Allyloxybenzene by Hydrogen Per-
oxide Over MFI-Type Titanosilicate
DOI: 10.1002/ejoc.202000249
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