Mendeleev Commun., 2008, 18, 196–197
4
was obtained only when the medium contained AcOH in
–
1
OMe
OMe
+
addition to the AcO ion.
MeO OAc MeO OAc MeO OAc
AcO–
3
–
It was considered that radical 2 (Nu = AcO ) is unstable.
For this reason, the acetoxylation of DMB in AcOH containing
acetate proceeds via the stages 1 ® 1' ® 4' ® 4 (Scheme 1).
However, it is hard to explain from this point of view the
AcO–
–
e
– e
+
R
R
R
R
R
OAc
1
1
2
3
5
influence of AcOH on the effectiveness of acetoxylation. On
AcO–
the other hand, it can be suggested, for example, that inter-
mediate 3 is decomposed faster than its ipso-interaction with
acetate (but not azolate) ion occurs; therefore, product 5 is
OMe
OMe
+
OMe
OAc
H
AcO
H
–
not formed (3 is probably less stable with Nu = AcO than with
OAc
AcO–
E+
–
+
Nu = Az ). Nevertheless, AcOH, which does not affect the stage
OAc
–
AcOH
– AcOE
of ipso-interaction, may assist electrophilically the abstraction of
the acetoxy group from intermediate 3'. As a result, the rearrange-
ment of cation 3 to cation 4' is catalyzed and its kinetics starts
dominating over cation 3 decomposition rate. From this point
of view, the acetoxylation mechanism may be described by
Scheme 1.
R
R
R
4
4'
3'
R = H, OMe, OAc
+
E = AcOH, ZnCl2
Scheme 2
Thus, the suggested acetoxylation mechanism is based on
an idea concerning the role of acidic components working as
electrophiles and promoting the transformation of cation 3 to
cation 4'. The correctness of this conclusion may be easily con-
firmed via changing AcOH for other electrophiles, for example,
Lewis acids. We specially studied the acetoxylation of various
substituted arenes in MeCN containing the acetate ion with
is a more effective electrophilic catalyst of the acetoxylation
reaction than AcOH.
In general, it follows from the table that, in the anodic
acetoxylation of arenes, a change of AcOH for ZnCl in all
2
cases leads to an increase of the product yields. These results
1
in conjunction with previous data allow us to describe the
ZnCl additives.
The anodic acetoxylation of substituted arenes leading to
mechanism of acetoxylation of arenes using Scheme 2.
2
+
The electrophilic components of the medium (E = AcOH,
3
,4–6
ortho-substitution products
with AcONa additives or in MeCN containing AcOH and its salts
Table 1, runs 1–8). For comparison, the table also contains data
is usually carried out in AcOH
ZnCl ) not only assist the abstraction of the acetoxy group
2
from the ipso-position of intermediate 3' but also make the
–
(
ortho-attack by the AcO ion easier due to bond polarization.
on the acetoxylation of the same arenes but in the absence of
This makes possible the rearrangement of arenonium cation 3'
into cation 4', which undergoes deprotonation leading to final
product 4.
AcOH, using the MeCN/ZnCl /Et NOAc system (runs 1'–8').
2
4
It is seen in the table (runs 1, 1', 8, 8') that the products of
DMB acetoxylation did not form in MeCN containing only an
acetate salt as a nucleophile. However, the addition of electro-
philic species to this system changed the situation dramatically.
After the addition of AcOH, DMB acetoxylation product was
obtained in 21% yield (run 2) and 4-methoxyphenyl acetate
acetoxylation product, in 23% yield (run 7). A change of AcOH
On the other hand, the presence of electrophiles such as
AcOH or ZnCl does not affect the ipso-interaction of cation 3
2
–
with the nucleophile (AcO ). This is the reason why ipso-bis-
addition products 5 were never observed experimentally (Table 1).
In general, the results allow us to conclude that the anodic
acetoxylation of arenes, as well as the anodic N-dimethoxy-
phenylation of azoles, follows the same mechanism including
the formation of arenonium cations 3 as key intermediates
(Schemes 1 and 2).
for ZnCl led to a substantial increase of acetoxylation product
2
yield to 70% in case of DMB (run 2') and 29% in case of
4
-methoxyphenyl acetate. These results are consistent with the
–
instability of corresponding (Nu = AcO , Scheme 1) arenonium
cations 3 and show that ZnCl (taking in account its low con-
centration in the reaction medium: < 1.0 mol per 1 mol of arene)
This work was supported by the Council of the President
of the Russian Federation (Programme for State Support of
Leading Scientific Schools of Russia, grant no. NSh 5022.2006.3)
and the Russian Academy of Sciences (programme no. 01)
2
Table 1 Yields of ortho-acetoxylation products after electrolysis (Pt anode)
1
,3,6
of arenes in systems containing AcOH salt/AcOH (including data ) or
AcOH salt/ZnCl mixture.
2
References
MeCN containing
Systems containing
a
1
2
3
4
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2
4
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Run
Nucleo- Yieldc
Medium
Yieldc
(%)
phileb
(%)
0
1
2
3
4
5
6
7
1,4-DMB MeCNd Et NOAc
1
1'
2'
3'
4'
5'
6'
7'
0d
70
70
19
12
30
29
4
1
e
1,4-DMB MeCN
Et NOAc 21
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4
3
1,4-DMB AcOH
AcONa
68
3
9
3
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AcONa
AcONa
5
6
7
8
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6
Anisole
AcOH
27
e
4-Methoxy- MeCN
Et NOAc 23
4
phenyl
acetate
9
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8
4-Methoxy- MeCNd Et NOAc Traces 8'
Tracesd
4
10 D. Magdziak, A. A. Rodriguez, R. W. Van Der Water and T. R. R. Pettus,
phenyl
acetate
Org. Lett., 2002, 4, 285.
a
b
Less than 1.0 mol per 1 mol of arene due to low solubility in MeCN. 1.5–
c
d
2
.0 mol per 1 mol of arene. On the basis of consumed arene. Without
e
AcOH or ZnCl additives. With 1.5 mol AcOH additive per 1 mol of arene.
2
Received: 18th January 2008; Com. 08/3070
197 –
–