C-H bond activation by water on a palladium or platinum metal surface

Article abstract of DOI:10.1055/s-2007-983738

A water molecule is partially cleaved on a palladium or platinum metal surface under hydrothermal conditions to form an active platinum species. The species is effective for C-H bond functionalization which can be applied for H/D-exchange reactions, C-C bond-forming reactions, and C-N bond-forming reactions. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-983738

SPECIAL TOPIC
2055
C–H Bond Activation by Water on a Palladium or Platinum Metal Surface
C
S
–H
B
ond
A
c
e
tivation by
i
W
ater
o
j
n a Pa
i
lladium
r
or Plati
o
num Metal Surfac eM atsubara,* Keisuke Asano, Yuichi Kajita, Mitsuru Yamamoto
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoudai-katsura, Kyoto 606-8501, Japan
Fax +81(75)3832461; E-mail: matsubar@orgrxn.mbox.media.kyoto-u.ac.jp
Received 14 April 2007
This means that the oxidation of platinum with water is
Abstract: A water molecule is partially cleaved on a palladium or
nearly impossible to complete. The value of the redox po-
tential, however, refers to the redox process of the metal
platinum metal surface under hydrothermal conditions to form an
active platinum species. The species is effective for C–H bond func-
tionalization which can be applied for H/D-exchange reactions, C–
C bond-forming reactions, and C–N bond-forming reactions.
bulk. It is well known that the reaction vessel of a hydro-
thermal reactor suffers from corrosion; these noble metals
5
will be oxidized on their surface. As a catalytic process,
Key words: platinum, deuterium, carbazole, water, hydrothermal,
hydrogenation
partial dissociation of water (Scheme 2) on the metal sur-
face will be important. Thiel and Madey reported that DH
for partial dissociation on the platinum surface is only 25
6
kJ/mol and on a palladium surface is –5 kJ/mol. On a pal-
In the fundamental course of organic chemistry, we have
learned that the reactivity of organic compounds can be
classified corresponding to the acidity constant K (pK ).
ladium or platinum surface, partial dissociation of water
under hydrothermal condition is not unreasonable. Thus,
the partial dissociation of water on a palladium or plati-
num surface may form the cationic species 2, which may
be effective for C–H bond activation. When complete dis-
sociation is performed, the dihydride species will afford a
hydrogen complex, which allows a hydrogenation reac-
tion of alkenes. As complete dissociation of a water mol-
ecule on the surface of palladium or platinum is not easy
according to the reported enthalpy change (Pd, 58 kJ/mol;
Pt, 118 kJ/mol), simple heating of alkenes in water with
platinum or palladium would not give the alkanes; the
procedure results in the isomerization of the double bond
a
a
Compounds with low pK values will give a carbanion
a
equivalent by treatment with a base that has the corre-
sponding strength. Although activation of the weakest ac-
ids, hydrocarbons, with base would be very difficult
according to this explanation, the recent development of
direct C–H bond functionalization with transition-metal
1
catalysts has shown us this additional possibility. This
development has been performed in the case of C–H acti-
2
vation at sp -hybridized carbon atoms. Direct C–H bond
3
functionalizations at sp -hybridized carbon atoms without
assistance by functional groups are difficult to perform ef-
ficiently, although some examples have shown high po-
(
Table 1), which is thought to be promoted by C–H bond
2
activation.
tential. Among them, the method by Fujiwara, which can
be applied to a practical reaction, is notable. In this meth-
partial
dissociation
complete
dissociation
od, a saturated hydrocarbon such as cyclohexane is carbo-
adhesion
H
3
nylated by the use of a palladium catalyst.
H
According to Fujiwara’s report, the reactive species for
C–H bond activation was thought to be a palladium(II)
cationic species ([PdOAc] ). We considered whether the
O
O
H
O
H
H H
+
metal surface
metal surface
metal surface
same kind of cationic species could be made from water
and metal. As shown in Scheme 1, an oxidative insertion
of water into metal will give metal hydride 1, which may
Scheme 2 Oxidative insertion of water on a metal surface
4
ionize into 2 in a polar medium such as water.
However, the addition of a deoxygenating reagent, such as
triphenylphosphine, would promote the formation of a
metal–hydrogen complex from water and metal. As
shown in Table 2, treatment of dodec-1-ene with hot wa-
ter and palladium or platinum metal in the presence of
M
+
H2O
HO-M-H
[M-H]+[OH]–
1
2
Scheme 1 Oxidative insertion of water and the formation of cat- triphenylphosphine afforded dodecane.
ionic species
The cationic species 2 will activate the C–H bond to af-
ford organometallic species 3 (Scheme 3). Unfortunately,
reductive elimination of the metal from 3, which is
thought to be a fast process, will afford the starting mate-
rial. When the substrate is an alkene this may be accom-
panied by migration of the double bond. However, the use
of deuterium oxide will alter the situation completely.4 It
was reported that the hydride on a transition metal can be
replaced with the hydrogen atom of the surrounding wa-
The redox potentials of palladium and platinum are too
high to react with water. For example, the redox potential
of Pt /Pt is 0.98 V under both basic and acidic conditions.
2
+
,7
SYNTHESIS 2007, No. 13, pp 2055–2059
x
x.
x
x
.
2
0
0
7
Advanced online publication: 18.06.2007
DOI: 10.1055/s-2007-983738; Art ID: C02907SS
Georg Thieme Verlag Stuttgart · New York
©

2
056
S. Matsubara et al.
SPECIAL TOPIC
Table 1 Treatment of Oct-1-ene with Hot Water in the Presence of
a Palladium or Platinum Catalyst
Table 3 H/D-Exchange Reaction of Alkanes, Alkenes, and Al-
kanones with Hot Deuterium Oxide and Palladium-on-Carbon Cata-
lysta
a
catalyst (10 mol%)
octane + octenes
1
0 wt% Pd/C ( 2 mol%)
D2O, 250 °C
H2O,
fully deuterated
substrate (ClDmOn)
substrate (ClHmOn)
250 °C, 4 MPa, 2 h
Catalyst
Pd/C
Product ratio
Octane
Entry Substrate
Time (h) D incorpora- Yield (%)b
tion (%)
Oct-1-ene
Oct-2-ene
Oct-3-ene
and oct-4-ene
1
2
3
4
5
6
7
8
9
0
cyclooctene
4
>95
>95
>95
>95
76
80
84
84
99
98
86
83
87
99
93
2
3
<1
<1
46
52
52
45
cyclododecene
cyclododecane
cyclopentadecane
n-pentadecane
cyclooctanone
cyclodecanone
cyclododecanone
4
Pt/C
6
a
Reaction conditions: oct-1-ene (2.0 mmol), Pd/C (5 wt% on char-
coal, 10 mol%) or Pt/C (5 wt% on charcoal, 10 mol%), degassed H O
4
2
(
20 mL), autoclave (30 mL), 250 °C (4 MPa), 2 h.
16
10
12
4
>95
>95
>95
>95
>95
Table 2 Treatment of Dodec-1-ene with Palladium or Platinum Cat-
alyst in the Presence of Triphenylphosphine in Hot Water
a
catalyst (5 mol%)
PPh3
dodec-1-ene
dodecenes + dodecane
cyclopentadecanone 10
dodecan-2-one 11
H2O,
50 °C, 4 MPa, 6 h
2
1
Entry
Catalyst Additive
Product ratio
a
Substrate (5.0 mmol), Pd/C (10 wt%, 100 mg, 2 mol% Pd), D O
2
Dodecenes Dodecane
(20.0 g).
Isolated yield.
b
1
2
Pd/C
Pd/C
Pt/C
PPh (1.0 equiv)
13
21
2
87
79
98
3
PPh (0.2 equiv)
3
catalytic efficiency.10 Hydrogen atoms on the benzene
ring are exchanged with deuterium atoms more efficiently
with the platinum catalyst. In these cases, platinum oxide
also showed catalytic activity. Although the valence of
platinum in the reactive species has not been defined, it
has already been shown that the formation of a platinum
hydride species was observed in the catalyst species.9k
3
PPh (1.0 equiv)
3
a
Reaction conditions: dodec-1-ene (2.0 mmol), Pd/C (5 wt% on char-
coal, 5 mol%) or Pt/C (5 wt% on charcoal, 5 mol%), degassed H O
2
(
20 mL), autoclave (30 mL), 250 °C (4 MPa), 2 h.
ter, the hydrogen atom on a hydrocarbon can be ex-
changed with the deuterium atom of deuterium oxide in
the presence of species 2. Considering the relative stabil-
ity of the C–D bond compared to the C–H bond, all the C–
H bonds will be converted into C–D bonds.
As shown in Figure 1, several heterocyclic compounds
were converted into completely deuterium-labeled deriv-
atives in the presence of a catalytic amount of platinum
oxide under hydrothermal conditions with deuterium ox-
ide. In these transformations, C–Pt bond formation is in-
dispensable. To form this bond, not only direct C–H bond
functionalization but also electrophilic aromatic substitu-
8
4
,7,9
H
C
M-H
C
+
–
11
+
[M-H] [OH]
tion is also a possible route.
H
H
2
+ H O
Reconsidering the proposed reactive species shown in
Scheme 3, displacement of hydride on metal with C or
with a heteroatom prior to the reductive b-hydride elimi-
nation will open the way to C–C or C–heteroatom bond-
forming reactions instead of the H/D-exchange reaction.
As shown in Table 5, 1-aminobiphenyl (4) was treated
with platinum or palladium catalyst under hydrothermal
conditions; the use of platinum-on-carbon as a catalyst
gave carbazole (5) in good yield.12
2
3
Scheme 3 Activation of C–H bonds with [Mtl–H] [OH]–
+
As shown in Table 3, the treatment of alkenes with hydro-
thermal deuterium oxide in the presence of a catalytic
amount of palladium-on-carbon resulted in a complete
H/D-exchange reaction. Considering the mechanism as
shown in Scheme 3, the H/D-exchange reaction will occur
even with saturated hydrocarbons. Actually, alkanes (en-
tries 3–5) and alkanones (entries 6–10) were efficiently
converted into the corresponding perdeuterated product.4
The reaction pathway can be rationalized as shown in
Scheme 4. The C–H bond functionalization in 2-biphenyl-
+
amine by [Pt–H] species, which is formed by the oxida-
tion of platinum metal with hydrothermal water, is
thought to be the initial step of the transformation. Dis-
As shown in Table 4, treatment of butylbenzene with pal-
ladium and platinum compounds was examined to find the
Synthesis 2007, No. 13, 2055–2059 © Thieme Stuttgart · New York

SPECIAL TOPIC
C–H Bond Activation by Water on a Palladium or Platinum Metal Surface
2057
Table 4 Metal-Salt Catalyzed Deuteration of Butylbenzene under Hydrothermal Conditionsa,b
CAr
C1 C2 C3
C4
H
H
D
D
D2O (10 mL)
H
2 2 2 3
CH –CH –CH –CH
D
CD2–CD2–CD2–CD3
catalyst (0.04 mmol)
250 °C
H
H
D
D
(
2.0 mmol)
Entry
Catalyst
Time (h)
Product ratio
CAr
C1
67
61
32
28
42
58
75
95
C2
C3
49
50
24
13
32
48
65
95
C4
43
42
16
5
1
2
3
4
5
6
7
Pd/C
2
2
20
2
42
38
14
10
26
49
59
94
PdO
Pd black
Raney Ni
PtO2
2
<5
3
2
2
28
65
81
96
30
40
56
83
PtO2
4
PtO2
6
8
PtO2
14
a
Substrate (2.0 mmol), catalyst (2 mol% metal), D O (20.0 g).
The ratios were determined by H NMR, H NMR, and MS.
2
b
1
2
Table 5 Preparation of Carbazole (5) from 2-Biphenylamine (4)
placement of hydride on the platinum catalyst with an in-
tramolecular amine group, which was followed by
reductive elimination, afforded carbazole (5).
catalyst
Not only a C–N bond-forming reaction but also a C–C
bond-forming reaction was demonstrated in the oxidative
cyclization of diphenylamine (6) (Table 6). In this case,
C–H bond functionalization at the ortho position by a [Pt–
H2O, 250 °C
12 h
NH
NH2
4
5
+
H] species should be the initial step of the transforma-
Catalyst
Pd/C
Amount (mol%) Yield of 5 (%) Recovery (%)
tion.
5
5
5
3
7
90
93
< 3
77
We have shown our recent examples of C–H bond func-
tionalization reaction by platinum and palladium catalysts
in hydrothermal water. The reactive species is thought to
be formed by water on these metals. We can show their re-
activity for C–H bond functionalization, but have not
Pd(OAc)2
Pt/C
5
76
16
Pt/C
NH2
oxidation with water
H
+
–
OH
Pt
H
H
OH
4
C–H bond activation
HOH
H
HN
Pt(0)
Pt-H
H
N
H
N
Pt
5
H–H
reductive elimination
Scheme 4 A plausible pathway from 2-aminobiphenyl (4) to carbazole (5)
Synthesis 2007, No. 13, 2055–2059 © Thieme Stuttgart · New York

2
058
S. Matsubara et al.
SPECIAL TOPIC
D
D
D
D
D
D
D
N
D
D
D
N
N
D
D
D
D
D
D
D
D
D
N
D
8
4%
72%
99%
D
D
D
D
D
D
D
D
D
N
D
N
D
D
D
D
D
D
D
D
N
D
D
D
D
N
80%
N
D
D
D
9
6%
87%
D
D
D
D D D D
D
D
D
D
D D
D
N
D
D
D
D
D D
N
N
D
D
D D
N
D
D
96%
93%
Figure 1 The results of H/D-exchange reactions using platinum oxide catalyst under hydrothermal deuterium oxide conditions (250 °C,
MPa)
4
Table 6 Carbazole (5) formation by oxidative coupling of diphenyl-
amine (6)
Preparation of Carbazole
2-Biphenylamine (2.0 mmol), Pt/C (10 wt% Pt; 100 mg, 0.1 mmol
Pt), and distilled H O (15 mL) were heated at 250 °C for 12 h in a
2
®
3
0 mL Teflon -lined stainless steel autoclave (see ref. 4). After it
was cooled to r.t., the obtained mixture was extracted with CHCl .
The Teflon vessel absorbs organic compounds; thus, to extract the
catalyst (3 mol%)
3
®
NH
2
H O, 250 °C
product completely, the vessel was washed with CHCl , H O, and
3
2
12 h
NH
then EtOAc using ultrasonic cleaner, each for 30 min. H O used for
2
6
5
washing was extracted with EtOAc. The combined organic layers
were dried over Na SO and concentrated in vacuo. The obtained
2
4
Catalyst
none
Yield of 5 (%)
Recovery (%)
product was purified by chromatography over a short silica gel col-
1
2
umn. The product was analyzed by GC, H and H NMR spectros-
copy, and mass spectrometry. Carbazole was obtained in 76% yield.
< 1
69
48
12
8
> 95
11
Pt/C
Acknowledgment
PtO2
30
This work was supported financially by the Japanese Ministry of
Education, Culture, Sports, Science, and Technology and by Kyoto
University, International Innovation Centre.
Pd/C
NiCl2
70
84
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