Mechanistic studies on fluorocyclohexene conversion to fluorobenzene under Pd-catalyzed dehydrogenation

Article abstract of DOI:10.1016/j.molcata.2011.01.021

The chemical reactivity and reaction mechanism of fluorobenzene formation by dehydrogenation of fluorocyclohexenes was investigated. 1-Fluorocyclohexene reacted with oxidants such as nitrobenzene and oxygen to give fluorobenzene in good yields under moderate conditions in the presence of a Pd catalyst. A detailed comparison of oxidative dehydrogenation with non-oxidative dehydrogenation proved that the oxidants effectively suppressed isomerization and disproportionation, and offered a selective synthesis of fluorobenzene.

Full text of DOI:10.1016/j.molcata.2011.01.021

Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Mechanistic studies on fluorocyclohexene conversion to fluorobenzene under
Pd-catalyzed dehydrogenation
a,b,∗
c
a
Masahiro Tojo
, Shinsuke Fukuoka , Hiroshi Tsukube
a
Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
Chemistry and Chemical Process Laboratory, Asahi KASEI Chemicals Corporation, 2767-11 Niihama, Kojima-Shionasu, Kurashiki, Okayama 711-8510, Japan
New Business Development, Asahi KASEI Corporation, 1-105 Kanda Jinbocho, Chiyoda-ku, Tokyo 101-8101, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2010
Received in revised form 17 January 2011
Accepted 22 January 2011
Available online 28 January 2011
The chemical reactivity and reaction mechanism of fluorobenzene formation by dehydrogenation of flu-
orocyclohexenes was investigated. 1-Fluorocyclohexene reacted with oxidants such as nitrobenzene and
oxygen to give fluorobenzene in good yields under moderate conditions in the presence of a Pd catalyst. A
detailed comparison of oxidative dehydrogenation with non-oxidative dehydrogenation proved that the
oxidants effectively suppressed isomerization and disproportionation, and offered a selective synthesis
of fluorobenzene.
Keywords:
Fluorocyclohexene
Fluorobenzene
©
2011 Elsevier B.V. All rights reserved.
Dehydrogenation
Oxidative dehydrogenation
Palladium catalyst
1
. Introduction
Fluorocyclohexenes are alternative precursors in the syn-
thesis of fluorobenzene and can be converted to fluoroben-
zenes by catalytic non-oxidative and oxidative dehydrogenation
[10,11]. The fluorocyclohexenes are easily synthesized from
gem-difluorocyclohexane [12], which can be obtained from cyclo-
hexanone by various methods [13,14]. Although unsubstituted
cyclohexene was reported to be catalytically disproportionated
into benzene and cyclohexane or oxidatively dehydrogenated to
benzene [15], fluorinated cyclohexenes were not discussed in
detail. Herein, we report the chemical reactivity and reaction
mechanism of the dehydrogenation of fluorocyclohexenes in the
Fluoroaromatics are widely used in the synthesis of pharma-
ceuticals [1], agrochemicals [2], functional organic materials [3],
and high-performance polymers [4]. In particular, fluorobenzene
is the most widely used fluoroaromatic as a starting material for
commercial scale manufacturing processes. Among the reported
synthetic methods [5], the most general method is the Schiemann
reaction [6], which involves unstable diazonium salts. Nucleophilic
displacement of aryl chloride by fluoride is known to generate
◦
chloride as waste, and requires a high temperature (450 C) for
◦
unsubstituted benzene [7]. Direct fluorination of benzene using F2
and some metal fluorides are limited in use because of their poor
selectivity [7]. 1-Alkyl-4-fluoro-1,4-diazoniabicyclo [2.2.2]octane
salts (SelectfluorTM) are more selective electrophilic fluorinating
reagents, but only fluorinate activated aromatics [7,8]. Copper(II)
fluoride also reacts directly with benzene and is regenerated after
treating the copper metal with HF under molecular oxygen [9].
However, high reaction temperatures are necessary (fluorination:
presence of catalyst at moderate temperature (ca. 100–200 C). 1-
Fluorocyclohexene yielded mainly benzene and cyclohexane via
non-oxidative dehydrogenation, but gave fluorobenzene in good
yields with oxidants, e.g., nitrobenzene and oxygen.
2. Experimental
2.1. General remarks
◦
◦
4
3
50–550 C; regeneration: 400 C) and conversion is less than
0%.
Unless otherwise noted, chemical reagents were commercial
samples and used without further treatment. Catalysts were pur-
chased from N.E. Chemcat Corporation. Quantitative analysis was
carried on a gas chromatograph with a FID detector. The chemical
structure of the product was identified by instrumental analysis.
The metal analysis of the reaction mixtures was conducted using
ICP-AES.
∗ Corresponding author at: Chemistry and Chemical Process Laboratory, Asahi
KASEI Chemicals Corporation, 2767-11 Niihama, Kojima-Shionasu, Kurashiki,
Okayama 711-8510, Japan. Tel.: +81 86 458 6601; fax: +81 86 458 3335.
E-mail address: tojo.mb@om.asahi-kasei.co.jp (M. Tojo).
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.021

9
0
M. Tojo et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
Table 1
Non-oxidative dehydrogenation of 1-fluorocyclohexene 1.
a
F
F
F
+
+
+
+
Catalyst
Additive
1
6
7
8
9
10
.
Additiveb
Temperature ( C)
◦
Time (h)
Conversionc (%)
Selectivityd (%)
Catalyst activitye (mmol/m2 h)
Entry
Catalyst
6
7
8
9
10
1
2
3
4
5
6
7
8
9
Pt-C
Ru-C
Pd-C
None
None
None
None
None
None
None
None
None
KF
90
90
90
90
90
1
1
1
1
1
1
0.25
0.25
0.25
0.25
0.25
0.25
38
3
7
3
6
0
11
22
10
6
12
23
0
63
70
69
61
0
59
52
64
73
76
69
0
2
2
3
0
0
0
0
0
0
0
0
0
32
21
25
33
0
30
26
25
21
9
0
0
0
0
0
0
0
0
0
2
3
0
0.010
0.020
0.009
0.19
0.0
0.58
2.4
2.1
2.5
4.4
5.4
0.0
27
35
31
0
44
23
44
86
77
49
0
Pt-black
Ru-black
Pd-black
Pd-black
Pd-black
Pd-black
Pd-black
Pd-black
Pd-black
90
90
120
150
150
150
150
1
1
1
0
1
2
PhNH2
tert-BuOK
7
0
a
b
c
d
e
f
Reaction was carried out with 30 mmol of 1 and 0.10 g of catalyst in a stainless steel closed reactor (volume: 10 mL) under N2.
0 mmol was used.
Determined by gas–liquid chromatography.
3
Based on converted 1.
f
6
yield per unit metal surface area per hour.
2
2
2
2
2
2
Pt-black 30 m /g; Ru-black 90 m /g; Pd-black 25 m /g; Pt-C 30 m /g; Ru-C 290 m /g; Pd-C 350 m /g.
2.2. Synthetic procedure
2.3. Catalytic reaction
2
1
.2.1. 1-Fluorocyclohexene 1 by dehydrofluorination of
,1-difluorocyclohexane 2
2.3.1. Conversion of 1 to fluorobenzene 6 by non-oxidative
dehydrogenation (Table 1)
2
was
synthesized
from
cyclohexanone
via
bis
In a closed stainless steel reactor (volume: 10 mL) equipped with
pressure relief valve were placed 1 (30 mmol), catalyst (0.10 g), and
additive (30 mmol) under nitrogen atmosphere. The reactor was
(
trifluoroacetoxy)cyclohexane 3 according to literature proce-
dures [14 g]: colorless liquid, bp 99–101 C/760 mmHg (lit. [12a]
9–100 C/760 mmHg). H NMR (CDCl ): ı 1.9–2.3 (m, 10H).
MS (EI): m/z 120 (M , 9%), 100 (63), 80 (12), 57 (98), 41 (100).
Aluminium fluoride (2.5 g) was packed in a fixed bed flow reactor.
To the reactor, N2 gas was fed (200 mL/h). After the reactor was
heated to 400–410 C, gaseous 2 was fed to the reactor (7.5 g/h)
for 10 h. All the reaction mixture was cooled and collected into a
mixture of ice and saturated aq. NaHCO . The resulting mixture
was extracted with diethyl ether. The combined extracts were
washed with water, dried over MgSO , and the solvent was
evaporated. The crude product was distilled under atmospheric
pressure to give 1 [16b,17,18]: yield 60.1 g (96%). Colorless liquid,
bp 95–96 C/760 mmHg (lit. [17] 95–96 C/760 mmHg). MS (EI):
m/z 100 (M , 47%). H NMR (CCl ): ı 1.7–2.0 (m, 4H), 2.1–2.4 (m,
◦
◦
1
◦
9
shaken at 90–150 C for 0.25–1 h. All the reaction mixtures were
3
+
analyzed with toluene as an internal standard using gas–liquid
chromatography. The yields of 6–10 were based on converted
1. Products 6, 7 and 9 were separated by preparative gas–liquid
chromatography. All products 6–10 were characterized spectro-
scopically and showed identical MS and 1H NMR spectra to the
commercial reagents. The gas phase of the reaction mixture was
analyzed using gas–liquid chromatography. Molecular hydrogen
was detected in the gas phase of the reaction mixture (Entries
1–11). The Pd was not detected in reaction mixtures (Entries 1–11)
by ICP-AES method (<0.1 ppm).
◦
3
4
◦
◦
+
1
4
4
H), 5.1 (d, 1 H).
2.3.2. Conversion of 1 to fluorobenzene 6 by oxidative
dehydrogenation (Table 2)
2
.3.2.1. Liquid phase reaction (Entries 1–6). In a closed stainless
2
.2.2. 3-Fluorocyclohexene 4 [19]
To diethylaminosulfur trifluoride (DAST, 0.05 mol) in a 50-mL
steel reactor (volume: 10 mL) equipped with pressure relief valve
were placed 1 (30 mmol), Pd-Black (0.10 g), and nitrobenzene
(24 mmol) under nitrogen atmosphere. The reactor was shaken
at 90–150 C for 0.25–8 h. All the reaction mixtures were ana-
lyzed in the same manner as described above. The Pd was not
detected in reaction mixtures (Entries 1–6) by ICP-AES method
◦
polyethylene bottle cooled at −50 C, 2-cyclohexen-1-ol (0.05 mol)
◦
◦
was added slowly at −50 C with stirring under nitrogen atmo-
sphere, and the mixture was further stirred for 3 h while warming
to room temperature. The reaction mixture was poured into a
mixture of ice and saturated aq. NaHCO . The upper layer was
(<0.1 ppm). It was confirmed that aniline and H O were formed
3
2
separated, washed with ice water, and dried over MgSO in a refrig-
via reduction of nitrobenzene. A large-scale repetition of Entry 5
was carried out, increasing the quantity by 10. The reaction mix-
4
erator to give crude 4: 63% yield. 1H NMR (CCl ): ı 1.5–2.2 (m,
4
2
H), 4.9 (m, 1 H), 5.8 (m, 1 H), 6.0 (m, 1 H). The obtained prod-
ture was washed with saturated aq. NaHCO . The upper layer was
3
1
uct decomposed slowly at room temperature [19] to give HF and
separated, washed with water, and dried over MgSO to give crude
4
1
,3-cyclohexadiene 5, which was identified spectroscopically.
4, which was fractionated through a 60-cm spinning band col-
umn under atmospheric pressure to give fluorobenzene 6: yield
◦
2
6.2 g (91%). Purity 99.6%. Colorless liquid, bp 84–85 C/760 mmHg.
+
1
MS (EI): m/z 96 (M , 100%), 70 (18), 50 (8). H NMR (CDCl ): ı
3
1
Caution: Since HF is corrosive, the reaction mixture containing HF should be
6
.9–7.5 (m, 5H).
handled in a well ventilated hood with protection.

M. Tojo et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
91
Table 2
a
Oxidative dehydrogenation of fluorocyclohexenes.
F
F
+
Oxidant
+
Hydrogenated-Oxidant b
Pd-catalyst
6
1
3
-Fluorocyclohexene 1
-Fluorocyclohexene 4
.
Temperature Timed (h)
( C)
Conversione
(%)
Selectivity (%)
f
Catalyst activityg
Entry
Substrate
Oxidant
equiv.)
Phase of
reaction
Catalyst
(weight ratio )
c
◦
(mmol/m2 h)
(
6
7
8
9
10
1
2
3
4
5
6
7
8
9
1
1
1
1
1
4
1
1
1
PhNO2 (1.2)
PhNO2 (1.2)
PhNO2 (1.2)
PhNO2 (1.2)
PhNO2 (1.5)
PhNO2 (1.2)
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
Gas
Pd-Black (0.01)
Pd-Black (0.01)
Pd-Black (0.01)
Pd-Black (0.01)
Pd-Black (0.02)
Pd-Black (0.01)
Pd–SiO2 (0.05)
Pd–SiO2 (0.05)
Pd–SiO2 (0.05)
90
120
150
150
150
150
150
150
200
0.25
0.25
0.25
2
6
10
32
76
97
100
95
97
98
99
99
99
99
0.6 99
93
88
82
0
0
0
0
0
0
0
0
0
0
0
0.2
0.3
0.6
0
0
0
0
0
0
0.2
0.4
0.6
0
0
0
0
0
0
0
0
0
9.4
15.8
50.7
15.0
4.8
1.0
1.6
0.40
0.40
8
0.25
0.25
0.5
i
O2 (1.2)
6
11
17
O2 (1.2)i
O2 (2.4)i
Gas
Gas
0.5
100
aReaction was carried out with 30 mmol of 1 or 4 and catalyst in a 10-mL closed stainless steel reactor under N2 (Entries 1–6); gaseous 1 was fed to a fixed bed flow reactor
packed with catalyst (Entries 7–9).
b
Aniline and H2O were formed when PhNO2 was used as oxidant; H2O was formed when O2 was used as oxidant.
Catalyst to substrate weight ratio (Entries 1–6) or supported metal to carrier weight ratio (Entries 7–9).
Reaction time (Entries 1–6) or residence time (Entries 7–10) calculated from a reciprocal of LHSV (Liquid Hourly Space Velocity).
Determined by gas–liquid chromatography.
c
d
e
f
Based on converted 1.
g
h
i
h
6
yield per unit metal surface area per hour.
2
2
Pd-black 25 m /g; Pd-SiO2 100 m /g.
Mixed gas [O2/N2 mol ratio: 10/90 (Entries 7 and 8) or 20/80 (Entry 9)] was used.
2
.3.2.2. Gas phase reaction (Entries 7–9). The catalyst was packed
ratios of cyclohexane to benzene for Pd catalyst were estimated to
◦
◦
in a fixed bed flow reactor. The weight ratio of supported metal to
carrier was 0.01. To the reactor, mixed gas [O /N2 mol ratio: 10/90
be 2 at below 70 C and 0 at over 250 C. 8 was also oxidatively
dehydrogenated to benzene 7 in the presence of oxidant (Eq. (3) in
Scheme 1) [15a].
If the reaction of 1 to 6 similarly occurred in the absence of
oxidant, the selectivity of 6 might range between 33% (via dispro-
portionation: Eq. (1) in Scheme 1) and 100% (via dehydrogenation:
Eq. (2) in Scheme 1). Since the yield obtained here was never more
than 22% (Entry 7), dehydrofluorination and other side reactions
must also occur.
2
(
Entries 7 and 8) or 20/80 (Entry 9)], which was used as carrier gas
◦
having oxidant, was fed. After the reactor was heated to 150–200 C,
gaseous 1 was fed to the reactor. Residence time calculated from a
reciprocal of LHSV (Liquid Hourly Space Velocity) was 0.25–0.5 h.
All the reaction mixtures were analyzed in the same manner as
described above. Although H O2 was not detected, H O was found
2
2
in the oxidative dehydrogenation.
Scheme 2 shows a possible reaction sequence including sev-
eral dehydrogenation paths. The oxidative dehydrogenation path
will be discussed later. When oxidant was not employed, molecu-
lar hydrogen was detected in the gas phase of the reaction mixture.
This means that simple dehydrogenation (Eq. (2) in Scheme 1) actu-
ally proceeded, but the amount of the generated hydrogen was
insufficient to ascribe to formation of 6. Thus, 6 was formed by
both simple dehydrogenation and disproportionation of 1. Fluoro-
cyclohexane 10 was not detected in Entries 1–9 in Table 1, but the
intermediate 8 was formed in addition to 7 and 9 (Entries 1–3), sug-
gesting that 10 was dehydrofluorinated to cyclohexene 8. When 10
was independently synthesized and heated, it easily released HF
in the absence of base and did not eliminate HF in the presence of
base. This side reaction explains the higher yields of 7 and 9 but not
the low selectivity of 6. The isomerized 3-fluorocyclohexene 4 [20]
was also confirmed to eliminate HF to yield 1,3-cyclohexadiene 5
besides being disproportionated to 6 and 10. Dehydrofluorinated
products 8 and 5 are subject to disproportionation and/or dehy-
drogenation to yield 7 and 9. Another possible dehydrofluorinated
product, cyclohexyne 11, was not formed perhaps because 11 is
highly strained [21]. Consequently, the selectivity of fluorobenzene
3
. Results and discussion
3.1. Non-oxidative dehydrogenation of fluorocyclohexenes
We first characterized the catalytic non-oxidative dehy-
drogenation (disproportionation or simple dehydrogenation) of
fluorocyclohexenes. 1-Fluorocyclohexene 1 was treated with het-
erogeneous Pd, Pt, and Ru catalysts, which were chosen from the
viewpoint of easy separation and recovery. Reaction conditions and
selected results are summarized in Table 1.
Comparing catalyst activities (yield of 6 per unit metal sur-
face per hour), the Pd-black was the most active catalyst
2
(
0.58 mmol/m h) among those studied in generating fluoroben-
zene 6 (Entries 1–6), but the major product was benzene 7, followed
by cyclohexane 9 (all entries). Examination of temperature depen-
dency (Entries 7–9) showed that a relatively higher selectivity of
◦
6
(22%) was observed at lower temperature, e.g., 90 C (Entry 7).
This differs from the results of unsubstituted cyclohexene 8 [15b].
As shown in Scheme 1, 8 was reported to be disproportionated
into one-third equivalents of 7 and two-third equivalents of 9 at
a temperature less than 70 C (Eq. (1) in Scheme 1), and dehydro-
genated with the evolution of hydrogen at a temperature greater
than 250 C (Eq. (2) of Scheme 1). The reported data (Fig. 7 in
Ref. [14b]) indicated that disproportionation and dehydrogenation
concurrently occurred at intermediate temperatures [15b], i.e., the
6
from 1 is lower than that of unsubstituted benzene 7 from 8.
Addition of base increased the selectivity of 6 in Entries 10 and
1 in comparison with Entry 9 (no additive), and fluorocyclohex-
◦
1
◦
ane 10 appeared in the reaction mixture. Since the selectivity of 6
was less than 33%, dehydrofluorination must still occur. The addi-

9
2
M. Tojo et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
Disproportionation
1/3
+
+
+
2/3
(1)
Pd catalyst, < 70 °C
7
7
9
Dehydrogenation
2 H2
(2)
(3)
Pd catalyst, > 250 °C
8
Oxidative dehydrogenation
Oxidant,Pd catalyst
7
2 H O
2
Scheme 1. Reaction modes of unsubstituted cyclohexene.
tion of a weak base depressed autocatalytic HF elimination but a
stronger base such as tert-BuOK terminated the reaction entirely
6 was not temperature dependent (98–99%) and the activation
−
1
energy Ea was low, 35 kJ mol
(calculated from catalyst activ-
(
Entry 12). These results support the assumption of the existence
ity of 6 formation). With longer reaction time, 6 was obtained
almost quantitatively (Entry 5). In addition to the main prod-
uct 6, small amounts of 7 (6–17%), 8 (less than 1%), and 9 (less
than 1%) were formed as by-products when molecular oxygen was
used (Entries 7–9), but they were not detected at all in the pres-
ence of 12 (Entries 1–5). Catalyst activity of gas-phase reaction
of side reactions as described above.
3
.2. Oxidative dehydrogenation of fluorocyclohexene
The oxidative dehydrogenation of fluorocyclohexenes was car-
−
2
−1
in Entry 8) is lower than that of liquid-phase
(
0.4 mmol m
h
ried out with nitrobenzene 12 or molecular oxygen as an oxidant.
Table 2 indicates that fluorobenzene 6 was obtained as the major
product in 98–99% selectivity with nitrobenzene 12 (Entries 1–5)
and 82–93% with molecular oxygen (Entries 7–9). Examination of
temperature dependency (Entries 1–3) showed that selectivity of
−
2
−1
reaction (4.8 mmol m
h in Entry 5) even at same tempera-
ture. Both lower substrate concentration at gas-phase reaction and
weaker oxidant than nitrobenzene, oxygen, would be responsible
for this.
F
F
+
Hydrogenated Oxidant
c
t
6
6
n
a
id
x
Formation of
Fluorobenzene
o
F
ith
w
a, b
withou
1
F
t
o
x
id
H2
a
+
+
n
t
10
d
-HF
e
b
d
d
-HF
F
Side Reactions
-HF
11
4
5
7
8
9
b
+
Scheme 2. Possible reaction sequence of fluorobenzene formation by dehydrogenation of 1. aSimple dehydrogenation. bDisproportionation. cOxidative dehydrogenation.
d
Dehydrofluorination. eIsomerization.

M. Tojo et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
93
Scheme 3. Possible reaction mechanism of fluorobenzene formation. aInsertion of palladium metal (A) into the allylic C–H bond of 1 to form a ␲-allyl Pd-hydride (B). b␤-
c
d
Hydrogen elimination to give fluorocyclohexadienes (13 and 14) and Pd hydride species (D, E). Reductive elimination of H2. Insertion of O2 into the Pd–H bond producing
a peroxy intermediate (C, D). Reductive elimination of H2O2. fMigratory insertion to form an alkyl Pd-hydride species (F). Isomerization of ␲-allyl Pd-hydride (B, G).
e
g
h
i
Reductive elimination of 10. Reductive elimination of 1 and 4.
When non-oxidative dehydrogenation (Entry 7 in Table 1) and
12 suppressed isomerization and disproportionation completely.
oxidative dehydrogenation (Entry 1 in Table 2) were compared,
they showed comparable yields of 6 but marked differences in
the selectivity of 6 [5% yield in the former (23% conversion and
3.3. Mechanism for fluorobenzene formation
2
2% selectivity) and 6% yield in the latter (6% conversion and 98%
A possible reaction mechanism of 6 formation is shown in
selectivity)]. The yields of by-products 7 and 9 of the latter is
much less than those of the former, suggesting that the oxidant
suppressed the isomerization and non-oxidative dehydrogenation
Scheme 3. The postulated steps for non-oxidative dehydrogenation
are: insertion of palladium metal into the allylic C–H bond of 1 to
form a ␲-allyl Pd-hydride (step a); ␤-hydrogen elimination to give
fluorocyclohexadienes (13 and 14) and Pd-hydride species (step b);
reductive elimination of H2 (step c); migratory insertion to form
an alkyl Pd-hydride species (step f); isomerization of ␲-allyl Pd-
hydride (step g); reductive elimination of 10 (step h); and reductive
elimination of 1 and 4 (step i). Formed 1-fluoro-1,3-cyclohexadiene
(
disproportionation and simple dehydrogenation in Scheme 2).
Since molecular hydrogen was not detected in the gas phase of the
latter reaction mixture, simple dehydrogenation was depressed.
Independently synthesized 4 reacted with nitrobenzene to yield 7
almost quantitatively (Entry 6), showing that the isomerization of
4
was also depressed in the presence of oxidant. Fig. 1 shows time-
13 and 2-fluorocyclohexadiene 14 would react to give fluoroben-
courses of 6 formation in which nitrobenzene 12 was employed at
a less than stoichiometric amount (8% based on 1). The yields of 7
and 9 increased after 12 was consumed at ca. 0.1 h, confirming that
zene 6 in a similar manner.
The postulated steps for oxidative dehydrogenation additionally
include: insertion of O2 into the Pd–H bond, producing a peroxy

9
4
M. Tojo et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 89–94
5
4
3
2
1
0
0
0
0
0
0
50
40
30
20
10
0
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7
6
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
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TM
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intermediate (step d) and reductive elimination of H O2 (step e).
2
H O was not observed, which was thought to be decomposed
2
2
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2
2
(
paths can be drawn: via E and via C. Since our experimental results
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migrates into a ␲-allyl moiety and isomerizes. Thus, the oxidant
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(
0
nitrobenzene. Since nitrobenzene (redox potential E : 0.8 V [23a])
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0
is a stronger oxidant than oxygen (E : 0.68 V [23b]), the Pd-hydride
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[
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(
4
. Conclusions
4
[
[
[
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ate conditions in the presence of Pd catalyst. A detailed comparison
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the isomerization and disproportionation of 1 and gave high selec-
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(
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[
[
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(