Gold-catalyzed benzylic oxidation to carbonyl compounds

Article abstract of DOI:10.1016/j.tet.2008.12.055

The gold-catalyzed, mild and general benzylic oxidation toward carbonyl compounds with TBHP as oxidant is described. Corresponding products are obtained in moderate to excellent yields. Crown Copyright

Full text of DOI:10.1016/j.tet.2008.12.055

Tetrahedron 65 (2009) 1856–1858
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Tetrahedron
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Gold-catalyzed benzylic oxidation to carbonyl compounds
Huanrong Li a,*, Zhiping Li a,*, Zhangjie Shi b,c,*
a
Department of Chemistry, Renmin University of China, Beijing 100872, China
Beijing National Laboratory of Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Green
b
Chemistry Center, Peking University, Beijing 100871, China
c
State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2008
Received in revised form 2 December 2008
Accepted 2 December 2008
Available online 24 December 2008
The gold-catalyzed, mild and general benzylic oxidation toward carbonyl compounds with TBHP as
oxidant is described. Corresponding products are obtained in moderate to excellent yields.
Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
1. Introduction
by bidentate N-donor ligands.7,8 Obviously, the oxidation of alkene
was also accomplished by gold complexes catalyzed oxygen
Gold catalysis has rapidly become a hot topic in chemistry in the
past decade. Gold species are equally effective as heterogeneous or
homogeneous catalyst, which showed excellent results in di-
versified reactions.1 Gold complexes have been employed as
a highly efficient catalyst for the formation of C–C, C–O, C–N, C–S,
transfer process. Herein we described the first general gold-cata-
lyzed benzylic oxidation with tert-butyl hydroperoxide (TBHP).
2. Results and discussion
2
and C–F bonds starting from alkenes and alkynes. Recently great
progress for direct functionalization of the C–H bond of electron-
rich arenes to form new C–C bonds mediated by gold has been
achieved.3 However, research in homogeneous gold catalysis
moved relatively slow for a long time. For example, gold was be-
lieved to be a rare element, but it is more abundant than palladium,
platinum, rhodium, and many other precious metals. Gold com-
pounds are easily reduced, but hard to oxidize due to the necessary
Our preliminary results showed that gold(I) complexes sup-
ported by neocuproine (Neo.) could catalyze the oxidative cleavage
of C]C double bond to produce two carbonyl compounds when
9
TBHP was used as the oxidant. Interestingly, some alcohols could
also be oxidized to carbonyl compounds under the same conditions
with a 1:1 ratio of gold–Neo. In this study, our initial screening
3
started from the direct oxidation of sp C–H bonds. Diphenyl-
1
a
change of oxidation states. This changed in 1987 when Haruta
methane (1a) was chosen as a substrate for the optimization of the
catalysis protocol. Generally, 5 mol % of catalyst was employed in
our primary investigations with TBHP as the oxidant (Table 1). In
the presence of 5 mol % of AuCl supported by Neo., different sol-
vents, such as water and toluene were tested. Under these condi-
tions, the product could be obtained in 39% yield (Table 1, entries 1
and 2). It seemed that the reaction was not sensitive to temperature
since the results of the reaction didn’t change significantly when
et al. reported the low-temperature selective oxidation of CO with
4
O
2
catalyzed by solid-supported gold nanoparticles. Also, many
previous results showed that gold nanoparticles loaded on different
solid supports showed unique reactivities in the oxidation of al-
cohol under basic conditions. However, the exact mechanism has
5
not been clearly understood yet. In contrast, selective oxidation via
gold catalysis in homogeneous solution has rarely been reported.
There is only one case in this field, which was given by Hill et al.,
who firstly found the oxidation of sulfide to sulfoxide with dioxy-
ꢀ
the temperature was raised to 110 C (Table 1, entry 3). Moreover,
the product was isolated in only 46% yield even using 3 equiv of
6
ꢀ
gen catalyzed by gold(III) salt in aqueous solution. Very recently,
TBHP at 110 C (Table 1, entry 4). AuCl(PPh
3
) or KAuCl
4
$0.5H
2
O in
we and the other groups also showed that alcohols can be selec-
tively oxidized to carbonyl compounds in toluene or in aqueous
solution with dioxygen catalyzed by gold(I) complexes supported
the presence of Neo. could also promote this oxidation, but the
efficiency was very low (Table 1, entries 5–7). Absence of ligand
made the efficiency of this reaction much lower (Table 1, entries 8
and 9). Other nitrogen ligands were investigated. The efficiency of
oxidation was poor when TMEDA and pyridine were used as ligands
in toluene (Table 1, entries 10 and 11). However, when the reaction
was performed in pyridine for 10 h, the product was obtained in
84% yield (Table 1, entry 12). During this process, pyridine may play
*
Corresponding authors.
E-mail addresses: hrli@ruc.edu.cn (H. Li), zhipingli@ruc.edu.cn (Z. Li), zshi@
pku.edu.cn (Z. Shi).
0
040-4020/$ – see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.12.055

H. Li et al. / Tetrahedron 65 (2009) 1856–1858
1857
Table 1
a
Oxidation of diphenylmethane with TBHP catalyzed by gold under different conditions
O
cat. (5 mol %), L (5 mol %)
TBHP, solvent, T, 24 h
N
N
Neo.
1
1a
ꢀ
1a (%)b
Entry
Cat.
L
TBHP (equiv)
Solvents
T ( C)
1
2
3
4
5
6
7
8
9
AuCl
AuCl
AuCl
AuCl
Neo.
Neo.
Neo.
Neo.
Neo.
Neo.
Neo.
d
2
2
2
3
2
2
2
2
2
2
2
2
2
H
2
O
90
90
110
110
90
90
90
90
90
90
90
90
90
39
39
44
46
51
57
24
19
37
23
49
84
99
Toluene
Toluene
Toluene
Toluene
Toluene
AuCl(PPh
3
)
KAuCl
KAuCl
KAuCl
KAuCl
KAuCl
KAuCl
KAuCl
KAuCl
4
4
4
4
4
4
4
4
$0.5H
$0.5H
$0.5H
$0.5H
$0.5H
$0.5H
$0.5H
$0.5H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
H
H
2
O
O
2
d
Toluene
Toluene
Toluene
Pyridine
Pyridine
1
0
TMEDA
Pyridine
Pyridine
Pyridine
1
1
1
2c
13
a
All the reactions were carried out in the scale of 0.5 mmol of 1.
Isolated yields.
The reaction was performed in pyridine for 10 h.
b
c
dual roles: ligand and solvent. We observed that the reaction could
be completed to reach 99% yield if it took longer time (Table 1, entry
13).
Less active substrates with one annelated aryl group in a cyclic
system led to benzylic oxidation products with yields ranging be-
tween 57 and 65% (Table 3, entries 1–3). Dihydroisobenzofuran and
tetrahydroisobenzopyran produced the lactone in good efficiency.
Acyclic compounds bearing one aryl group were oxidized in the
benzylic position with moderate yields. It is important to note that
the benzyl bearing with silyl group and acetoxyl group and even
free toluene were over-oxidized to produce the benzoic acid in
moderate yields (Table 3, entries 4–6). Interestingly, by oxidation of
1,2-diphenylethane and benzyl ether, the corresponding carboxylic
acid was obtained in 46% and 40% yields, respectively (Table 3,
entries 7 and 8). By-products were not detected.
Under the optimized reaction conditions, various diarylmethane
substrates were subjected to oxidation (Table 2 and 3). Most dia-
rylmethylene derivatives gave the corresponding products in ex-
cellent yields (up to >99%; Table 2, entries 1–6). Interestingly, the
electronic effect of the substituents on aromatic ring of diaryl-
methylene derivatives was not observed. For example, the sub-
strates bearing an electron-withdrawing group on the aromatic
ring (Table 2, entries 3–5) produced products with 95–96% yields.
Table 3
Table 2
Benzylic oxidation of mono-benzylic derivatives
Benzylic oxidation of diarylmethane derivatives
Entry
1
Substrate
Product
Isolated yield (%)
58
O
O
KAuCl4 0.5 H O (5 mol %) pyridine (0.5 mL)
2
TBHP (2eq.), air, 24h, 90 °C
O
Entry
1
Substrate
Product
Isolated yield (%)
99
2
3
4
5
6
7
O
65
57
58
50
49
46
O
O
O
O
O
O
O
2
3
4
>99
95
CH3
COOH
H CO
H CO
3
3
F
COOH
COOH
COOH
OAc
Si
F
O
96
F
F
F
F
O
5
6
96
66
Cl
Cl
O
COOH
O
8
40

1858
H. Li et al. / Tetrahedron 65 (2009) 1856–1858
3
. Conclusions
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1
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.2. Representative procedure for the benzylic oxidation with
2
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4
2
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7
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We are grateful to National Science Foundation of China and
Renmin University of China for financial support.
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