COMMUNICATION
DOI: 10.1002/asia.201300003
ꢀ
Redox Inactive Metal Ion Promoted C H Activation of Benzene to Phenol
with PdII
AHCTUNGTRENNUNG
Huajun Guo, Zhuqi Chen, Fuming Mei, Dajian Zhu, Hui Xiong, and Guochuan Yin*[a]
ꢀ
Activation and functionalization of the C H bonds in al-
kanes and aromatic compounds has been a long challenge in
the academic community, and selective transformations of
methane to methanol and benzene to phenol are most at-
tractive.[1,2] Among the versatile redox metal ions, palladiu-
m(II) is the one most investigated.[3] Up to now, at least two
long standing challenges in PdII-catalyzed benzene hydroxyl-
ation exist: 1) efficient regeneration of the active PdII spe-
cies from the reduced Pd0 form when oxygen is used as an
oxidant, and 2) avoidance of the formation of biphenyl
charge of a redox metal ion accelerates its rate of electron
transfer with increased redox potential.[9] In fact, redox inac-
tive metal ions are extensively applied to modify the reactiv-
ity of a redox metal oxide in heterogeneous oxidations, and
they also play significant roles in enzymatic oxidations.[10,11]
Inspired by these works, herein, we report an early example
of benzene hydroxylation to phenol with an in situ-generat-
ed PdII
ACTHNUTRGNE(NUG bpym) catalyst (bpym: 2,2’-bipyrimidine) promoted
by a redox inactive metal ion. Through ligation with bpym,
the redox inactive AlIII ion can greatly promote the C H ac-
ꢀ
through coupling, which is very common in Pd-catalyzed C
tivation capability of the PdII ion in PdII
(bpym), which is
ꢀ
H activation.[4] To accelerate the regeneration of the active
PdII ion, the general protocols include the use of a redox co-
catalyst to assist the oxidation of Pd0 and/or the use of a spe-
cific ligand to stabilize the Pd0 atom to impede its aggrega-
tion.[4–6] The presence of the ligand may simultaneously in-
crease the steric hindrance surrounding the PdII ion, thus
preventing the activation of the second benzene by the PdII-
Ph intermediate, which would lead to biphenyl formation
through reductive elimination. The drawback of using
originally inactive in benzene functionalization.
Here, catalytic oxidations were performed in trifluoroace-
tic acid (TFA)/trifluoroacetic anhydride (TFAA) media
(95:5, v/v). Reactions were carried out in an oil bath at
1008C under stirring, and pressured oxygen was used as the
oxidant (Table 1). In the presence of four equivalents of tri-
Table 1. Lewis acid-promoted benzene hydroxylation by the catalyst PdII-
AHCTUNGTREGUN(NN bpym) in the absence or presence of additives.
ꢀ
a ligand is that it apparently reduces the C H bond activa-
Entry
Catalyst
Additive
Yield [mmol]
phenol
tion capability of the PdII ion, thus causing a loss in its activ-
ity towards benzene hydroxylation. For example, even when
1,10-phenanthroline (phen) was used as the ligand of the
PdII ion, phenol was reported as the sole product.[7] Howev-
er, owing to the reduced oxidizing capability, the reaction
had to be carried out at the high temperature of 1808C. Ap-
parently, novel catalysts still need to be designed for effi-
cient hydroxylation of benzene to phenol.
biphenyl
1
2
3
4
5
6
7
8
PdII
–
–
0.027
–
0.20
–
PdII +bpy
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpym
PdII +bpy
0.009
0.212
0.100
0.053
0.013
0.006
0.015
0.056
–
0.022
0.054
0.032
0.047
0.027
0.022
0.022
0.110
–
Al
Bi(OTf)3
(OTf)3
Yb(OTf)3
Zn(OAc)2
Mg(OAc)2
NaOTf
Sc(OTf)3
Al(OTf)3
(OTf)3
ACHTUNGTRENNUNG
YACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Recently, it was found that Lewis acids like Sc3+, Y3+
Zn2+, Ca2+, and even BF3 could promote triphenylphos-
phine oxygenation by MnV
(TAML)(O) analogues, sulfoxida-
tion by FeIV
(N4Py)(O), electron transfer and hydrogen ab-
,
9
ACHTUNGTRENNUNG
10
11
12
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
G
0.001
0.010
ACHTUNGTRENNUNG
Reaction conditions: PdACHTNUGTRNEUNG(OAc)2, 0.02 mmol; additive, 0.04 mmol;
CF3SO3H, 0.04 mmol; TFA, 4 mL; TFAA, 0.2 mL; benzene, 5.6 mmol;
O2, 20 atm; oil bath, 1008C; 16 h.
V
ꢀ
ꢀ
straction by (TPB8Cz)Mn (O), C H activation by MnO4 or
FeO4 , and catalytic oxidation of alkane by [OsVI(N)Cl4]ꢀ/
ꢀ
FeCl3.[8] In clarifying the relationships of the reactivity of
active metal intermediates with their physicochemical prop-
erties in oxidations, we also found that increasing the net
fluoromethanesulfonic acid, simple PdACTHNURTGNENG(U OAc)2 as catalyst
provides dominantly biphenyl as product with a small quan-
tity of phenol (0.20 mmol vs. 0.027 mmol, entry 1). Similar
product distributions were observed in most free PdII ion-
catalyzed benzene functionalizations.[4] For example, Ishii
[a] H. Guo, Z. Chen, F. Mei, D. Zhu, H. Xiong, Prof. G. Yin
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074 (P. R. China)
et al. reported that the reaction with PdACHTUNGTRENNUNG(OAc)2/
Fax : (+86)27-87543632
PVxMo12ꢀxO40 as catalyst affords biphenyl in 10.9% yield to-
gether with 0.3% of phenol.[4a] Here, the addition of either
Supporting information for this article is available on the WWW
2,2’-bipyridine (bpy) or bpym as a ligand to the PdACHTUNGTRENNUNG(OAc)2
Chem. Asian J. 2013, 8, 888 – 891
888
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim