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S. Mita et al. / Tetrahedron Letters 46 (2005) 7729–7732
Table 1. Hydroxylation of 1 to 2 by HPMo11V combined with Pd/C
under various conditionsa
Table 2. Hydroxylation of 1 to 2 by various heteropoly acids
combined with 5 wt % Pd/Ca
Run Temperature (°C) Conv. (%)
Yield (%)b
Run Catalyst
Conv.
of 1 (%)
Yield (%)
2
3
2
3
1
120
120
120
120
120
120
120
120
120
130
150
100
12
3.6
9.9 (53:18:29) [83] 1.3
1.5 (50:19:31) [42] 1.5
No reaction
1
2
3
4
5
6
7
8
H4PMo11VO40Æ31H2O 12
H3PMo10V2O40Æ25H2O 8.6
9.9 (53:18:29) [83] 1.3
6.9 (49:22:29) [42] 1.4
7.1 (52:20:28) [72] 1.0
6.2 (53:19:28) [85] 0.9
No Reaction
3.2 (53:19:28) [80] 0.6
0.2 (38:23:38) [50] 0.1
0.6 (51:20:29) [67] 0.3
2c
3d
4e
5f
H6PMo9V3O40Æ20H2O
H7PMo8V4O40Æ25H2O
H3PMo12O40Æ23H2O
H5PW10V2O40Æ21H2O
VO(acac)2
9.9
7.3
2.3
11
13
11
1.6 [70]
6.2 (57:7:36) [56]
1.5
6g
7h
8i
9.4 (54:20:26) [72] 1.1
8.8 (52:18:30) [80] 1.3
No reaction
4.0
0.4
0.9
NaVO3
9j
No reaction
a The reaction was run under the same conditions as Run 1 in Table 1.
10
11
12
13
15
10
9.7 (53:19:28) [75] 1.2
9.8 (54:20:26) [65] 1.3
8.8 (51:21:28) [88] 1.1
Table 2 shows the hydroxylation of 1 under air/CO (15/
5 atm) catalyzed by several heteropoly acids or vana-
dium compounds combined with 5% Pd/C under the
same conditions as run 1 in Table 1.
a 1 (15 mmol) was reacted in the presence of H4PMo11VO40Æ31H2O
(20 lmol), 5% Pd/C (2 mg), and AcONa (0.1 mmol) under air
(15 atm) and CO (5 atm) in AcOH/H2O (19/1, 7 mL) at 120 °C for
2 h.
b Numbers in parenthesis and blanket show the ratio of o-, m-, p-cresol
and the selectivity of 1 to 2, respectively.
c In the absence of Pd/C.
The hydroxylation of 1 to 2 was found to be also pro-
moted by H3+nPMo12ꢀnVnO40ÆxH2O (n = 2–4)8 to give
a mixture of o-, m-, and p-2 in the yields of 6.2–9.9%,
but H3PMo12O40Æ23H2O lacking the vanadium atom
did not catalyze the hydroxylation of 1 at all. The cata-
lytic activity of H5PW10V2O40Æ21H2O was low com-
pared with that of the corresponding molybdenum
compound, H5PMo10V2O40Æ25H2O. Almost no reaction
was induced by simple vanadium compounds like VO-
(acac)2 and NaVO3. On the basis of these results, the
real active vanadium species promoting the hydroxyl-
ation of 1 is thought to be a certain vanadium species
in molybdovanodophosphoric acids.
d In the absence of H4PMo11VO40Æ31H2O.
e In the absence of H2O.
f In the absence of AcONa.
g Air/CO (15/7 atm).
h Air/CO (15/1 atm).
i In the absence of CO.
j H2 (5 atm) was placed instead of CO.
In order to confirm optimal reaction conditions, 1 was
reacted under various reaction conditions and represen-
tative results are shown in Table 1. The reaction of 1
under air (15 atm) and CO (5 atm) by HPMo11V in the
presence of a very small amount of 5% Pd/C in a mixed
solvent (7 mL) of acetic acid and water (19:1) at 120 °C
for 2 h afforded 2 (o-2:m-2:p-2 = 53:18:29) in 9.9% yield
at higher selectivity (83% selectivity) and a small amount
of benzaldehyde (3) (1.3%) at 12% conversion of 1 (run
1). The reaction indicated the ortho and para orientation
and the ratio of o-2 to p-2 was roughly 2, which is statis-
tically satisfactory. The result seems to be the first suc-
cessful direct synthesis of 2 from 1 with air and CO,
which can be applied to industrial chemistry. The reac-
tion of 1 by HPMo11V alone led to 2 in low yield, but
no reaction was induced by Pd/C in the absence of
HPMo11V (runs 2 and 3). It was found that water is
an essential component to activate the HPMo11V cata-
lyst, since the reaction was markedly retarded in anhy-
drous acetic acid (run 4). The reaction in the absence
of NaOAc resulted in a decrease of the selectivity of 1
to 2 (56%) (run 5). The reaction was influenced by the
partial pressure of air and CO. For instance, under air
(15 atm) and CO (7 atm), the selectivity of 2 was slightly
lowered than that of CO (5 atm) (run 6). Under low CO
pressure (1 atm), the yield of 2 was slightly decreased
(run 7). However, when CO was removed from the reac-
tion system, no reaction was induced at all (run 8). No
reaction took place when H2 was used in place of CO
as the reducing agent (run 9). The reaction at 100,
130, and 150 °C was not considerably different from that
at 120 °C (runs 10–12).
To obtain information on the role of a Pd species in the
present hydroxylation, it may be interesting to compare
the reaction of CO with air by HPMo11V in the presence
of Pd/C with that in the absence of Pd/C under the same
conditions as the hydroxylation (Table 3).
The oxidation of CO with air was most smoothly pro-
moted by HPMo11V combined with Pd/C to form
5 mmol of CO2, while the reaction afforded only
1.6 mmol of CO2 by HPMo11V (runs 1 and 2). Similarly,
the formation of CO2 by Pd/C alone was also low (run
3). It is difficult to ascertain the role of the Pd/C in the
present reaction at the present stage. However, it does
not seem that the Pd/C promotes directly the present
reaction as the key catalyst, since the hydroxylation 1
to 2 was induced to some extent by HPMo11V alone
but not by Pd/C as shown in runs 2 and 3 in Table 1.
In addition, we showed that the hydroxylation of benz-
ene to phenol was efficiently catalyzed by molybdova-
Table 3. Generation of CO2 from CO and air by several conditionsa
Run
Catalyst
CO2 (mmol)
1
2
3
HPMo11V–Pd/C
5.0
1.6
2.0
HPMo11
V
Pd/C
a The reaction was run without 1 under the same conditions as run 1 in
Table 1.