Tetrahedron Letters 51 (2010) 2225–2227
Tetrahedron Letters
Novel oxidation of toluenes catalyzed by reusable vanadyl(IV) sulfate
under mild conditions with molecular oxygen
*
Takeo Nakai , Toshiyuki Iwai, Masatoshi Mihara, Takatoshi Ito, Takumi Mizuno
Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Efficient oxidation system using reusable vanadyl(IV) sulfate catalyst was established. Toluenes were eas-
ily oxidized under molecular oxygen (0.1 MPa) at 100 °C catalyzed by vanadyl(IV) sulfate to afford the
corresponding benzoic acids in excellent yields. The recovered catalyst could be reused without loss of
activity.
Received 4 January 2010
Revised 8 February 2010
Accepted 9 February 2010
Available online 20 February 2010
Ó 2010 Published by Elsevier Ltd.
Table 1
Benzoic acids are synthesized by the oxidation of the corre-
sponding toluenes,1,2 and used as important raw materials for var-
ious chemicals. For example, huge amount of terephthalic acid
from p-xylene is provided for fiber and plastic production. Among
many methods for the oxidation of toluenes, oxidation using nitric
acid or heavy metal peroxides as oxidant is unavoidable toxic
waste formation. Because of these problems, oxidation with molec-
ular oxygen (autoxidation) is preferable as a more environmentally
friendly method. For the autoxidation in the presence of Co2+ cat-
Solvent effect on the autoxidation of 4-tert-butyltoluene
Entry
Solvent
Catalyst
Temp (°C)
Yield (%)
1
2
3
4
5
6
7
8
AcOEt
AcOMe
CH3CN
DMF
DMSO
t-BuOH
AcOH
1
1
1
1
1
1
1
2
80
70
80
100
100
80
100
100
0
0
0
0
0
0
21
22
AcOH
alysts and using ht, a lot of preparative methods had been re-
ported.3–8 Also, few V2O5-catalyzed oxidation of toluenes was
also shown.1 However, V2O5 oxidation requires severe reaction
conditions (high temperature). Therefore, with our strategy for
environmentally friendly aerobic oxidation using commercially
available and inexpensive vanadium compounds, we herein wish
to report an oxidation of toluenes catalyzed by reusable vana-
dyl(IV) sulfate (2) under mild conditions with molecular
oxygen.9,10
Reaction conditions: 4-tert-butyltoluene (297 mg, 2 mmol), 1 (27 mg, 0.1 mmol) or
2 (22 mg, 0.1 mmol), NaBr (21 mg, 0.2 mmol), O2 (0.1 MPa), 20 h.
(entries 1–6). It was suggested that VO(IV) catalyst only works un-
der acidic condition in this oxidation. However, under acidic condi-
tion, catalyst
1 easily decomposed and gave acetylacetone.
Vanadyl(IV) sulfate (2), which is fairly stable and does not decom-
pose in acidic solution, shows almost the same reactivity toward
the present oxidation, compared with catalyst 1 (entry 8).
CH3
COOH
VO2+(1) or (2) / HBr
Next, the control reactions for the effects of acid and bromide
anion were performed (Table 2). Surprisingly, addition of 1 mL of
aq H2SO4 (1 mol/L) and 1 mmol of NaBr provided 4-tert-butylben-
zoic acid in 76% yield (entry 1). This result clearly exhibited that
addition of acids is essential for the present oxidation, and it was
encouraged by us to check the concentrations and amounts of sul-
furic acid (entries 1–6 and 10). More than 2 mol/L of concentration
and less than 1 mL amount of sulfuric acid were effective for this
reaction. Also, suitable amount of NaBr was more than 1 mmol.
High temperature (110 °C, close to boiling point) was not suitable
for the oxidation and gave 4-tert-butylbenzoic acid in unsatisfac-
tory yield, due to low solubility of oxygen gas (entry 11). The
necessity of vanadyl(IV) catalyst was confirmed in entry 12. Final-
ly, we successfully found that additive HBr remarkably worked as
both acid and bromide anion source to obtain the product in al-
most quantitative yield (entry 13).
O2, 100ºC
X
X
O
ð1Þ
O
O
O
O
V
VOSO4
2
1
A variety of solvents were tested for the oxidation of 4-tert-butyltol-
uene as a model, using vanadyl(IV) acetylacetonate (1) catalyst un-
der oxygen atmosphere (0.1 MPa) (Eq. 1 and Table 1). Only acetic
acid slightly brought about the oxidation to afford 4-tert-butylben-
zoic acid (entry 7). In non-acidic solvents, the oxidation was not
performed at all, and unchanged 4-tert-butyltoluene was recovered
* Corresponding author. Tel.: +81 6 6963 8051; fax: +81 6 6963 8049.
0040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.