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10
[Ru]-3
In(OTf)3 (40)
89
83
4
We decided to study the oxidation of alkenes by water using
a catalyst combination of a pincer ruthenium complex and an
acid. Obviously, two side reaction pathways are required to be
overcome for achieving the desired transformation: 1) homo-
coupling and polymerization of alkenes under acidic condi-
tions,10 2) transfer hydrogenation of alkenes.11 We chose sty-
aReaction conditions: 1a (0.30 mmol), [Ru] (1.5 mol%), acid, 1,4-di-
oxane (2.0 mL), water (1.0 mL), heated in a sealed tube at 150 oC
(silicon oil bath temperatur) at reflux for 36 h. The reaction conver-
sion and yields were determined by GC using mesitylene as the in-
ternal standard. n.d. = not detected.
1
2
3
4
5
6
7
8
In order to understand the reaction mechanism, experi-
ments were carried out to get an idea about the role of the two
co-catalysts. The hydration reaction of styrene was first stud-
ied in the absence of dehydrogenation catalysts (Scheme 2a).
It has been reported that In(OTf)3 is a highly efficient catalyst
for the homo-coupling of styrene in 1,4-dioxane.12 However,
only 44% conversion of , 19% to 1-phenylethanol ( ) and
17% to the homo-coupling product (the rest likely being
polymerized styrene) were observed when 40 mol% of
In(OTf)3 was used in the mixed solvent system of 1,4-dioxane
and water (2:1). This result indicated that water suppressed
the homo-coupling of styrene. In the next step, dehydrogena-
5a
tion of the secondary alcohol by complex
ied in the absence of acids (Scheme 2b). An excellent reaction
yield (92%) of the ketone product was obtained along with
the observation of hydrogen gas liberation. Efforts have also
been made to identify the complex intermediates during the
dehydrogenation process. According to 31P{1H} NMR analy-
1a
rene ( ) as the model substrate and complex
[Ru]-3
as the
acceptorless dehydrogenation catalyst to start the investiga-
tion. In a mixed solvent system of 1,4-dioxane and water
(2:1), representative Brønsted acids and Lewis acids were
tested upon external heating at 150 oC in a sealed tube at reflux
(Table 1, entries 1-6). The hydrogen transfer product
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12
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15
16
17
18
19
20
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22
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24
25
26
27
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30
31
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33
34
35
36
37
38
39
40
41
42
43
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1a
5a
3a
ethylbenzene ( ) was observed in all the reaction systems.
4a
The co-catalyst indium(III) triflate gave the best result for
1a 2a
Markovnikov oxidation of to by water (Table 1, entry
6). The Lewis acids FeCl3 and AlCl3 were also tested un-
der the same conditions but no desired product was ob-
served. As expected, very low substrate conversion and no de-
sired product were observed in the absence of acid catalysts
(Table 1, entry 7). The catalytic efficiency of other pincer ru-
thenium(II) complexes was also investigated. Compared with
[Ru]-3
was stud-
2a
[Ru]-3
complex
dine-based PNP-Ru complex (
along with a large amount of the hydrogen transfer product
3a [Ru]-2
, the coordinatively saturated, aromatic acri-
[Ru]-2
) afforded a lower yield,
[Ru]-3
[Ru]-3
sis, the signal of complex
was the major peak in the re-
5a
, which may be formed upon HCl elimination from
(Table 1, entry 8). In the case of the bipyridine-based PNN-
[Ru]-1
action mixture of
and 10 eq. at room temperature
or after heating (see the Supporting Information, Figure S2
[Ru]-3
Ru complex (
), much lower reaction selectivity and ef-
for details). It indicates that complex
is the resting
ficiency were observed, likely due to deactivation of this acid-
sensitive dearomatized complex (Table 1, entry 9). Notably,
quite good reaction selectivity and efficiency were obtained
5a
state during the dehydrogenation of . Combining the reac-
tion results in Schemes 2a and 2b, complex
In(OT)3 are likely to work synergistically in the Markovnikov
oxidation of styrene by water with H2 liberation. Additionally,
an isotopic labeling experiment was carried out to confirm
2a
that the oxygen atom of originates from water. When a mix-
ture of dioxane (0.50 mL) and H2 O (0.25 mL, 97% O-la-
belled) was used as solvents, 67% yield of was obtained and
[Ru]-3
and
[Ru]-3
by using 1.5 mol% of
and 40 mol% of In(OTf)3 (Table
1, entry 10). After cooling to room temperature, hydrogen gas
was detected by GC analysis in 72% yield (See the Supporting
Information, Figure S1 for details). The effect of changing the
18
18
[Ru]-3
loading of
was also investigated (See the Supporting
2a
Information, Table S1 for details).
18
the product was detected by GC-MS to be 94% O-labelled
(Scheme 2c; see the Supporting Information, Figure S3 for
details). The kinetic isotope effect (KIE) of O-H bond cleav-
age was studied by performing in two parallel reactions using
H2O and D2O, resulting in about kH/kD = 2.0, indicating that
O-H bond cleavage is likely involved in the rate-determining
step (Scheme 2d,; see the Supporting Information, Scheme
S1 for details).
Table 1. Optimization of the Catalytic System for the Oxi-
dation of Styrene by Watera
cat. [Ru]
cat. acid
O
H2
↑
or
+
Ph
1a
Ph
3a
dioxane/H2O = 2:1
Ph
2a
yield (%)
entry
[Ru]
acid (mol%)
conv. (%)
2a
39
44
55
3a
11
13
8
1
2
3
4
5
6
7
8
9
[Ru]-3
[Ru]-3
[Ru]-3
[Ru]-3
[Ru]-3
[Ru]-3
[Ru]-3
[Ru]-2
[Ru]-1
HOTf (20)
HNTf2 (20)
62
74
71
5
Scheme 2. Mechanistic Studies
TsOH·H2O (20)
PhCOOH (20)
Sc(OTf)3 (20)
In(OTf)3 (20)
none
n.d.
50
1
8
73
82
9
63
7
n.d.
53
5
In(OTf)3 (20)
In(OTf)3 (20)
87
47
28
2
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Ben-David, Yehoshoa
