Oxidation of benzyltins by oxovanadium(V) compound and molecular oxygen

Article abstract of DOI:10.1016/S0040-4039(01)00083-1

Benzyltin compounds were oxidized by oxovanadium(V) compound under an oxygen atmosphere to afford the corresponding aromatic aldehydes (ketone) and/or carboxylic acids.

Full text of DOI:10.1016/S0040-4039(01)00083-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1961–1963
Oxidation of benzyltins by oxovanadium(V) compound and
molecular oxygen
Toshikazu Hirao,* Chihiro Morimoto, Takashi Takada and Hidehiro Sakurai
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
Received 9 January 2001; accepted 17 January 2001
Abstract—Benzyltin compounds were oxidized by oxovanadium(V) compound under an oxygen atmosphere to afford the
corresponding aromatic aldehydes (ketone) and/or carboxylic acids. © 2001 Elsevier Science Ltd. All rights reserved.
Organotin compounds undergo facile oxidation to gen-
erate other functional groups via carbonꢀtin bond scis-
sion.1 Among these reactions, transformation to the
oxygen-containing group is one of the most important
reactions from a synthetic viewpoint. In particular,
allylic and benzylic tin compounds are readily con-
verted to the corresponding alcohols or their derivatives
by metallic oxidants such as Mn(IV),2 Ce(IV),3 or
Tl(III),4 organic oxidants such as MCPBA,5 or photo-
induced oxidation.6 In the course of our study on
oxovanadium(V)-induced oxidation of Group 14 metal
compounds,7,8 we found that benzylic tin compounds
can be oxidized directly to the corresponding aromatic
aldehydes (ketone) and/or carboxylic acids by oxovana-
dium(V) compound under an oxygen atmosphere.9
When 4-methylbenzyltributyltin (1a) was treated with
10
3 molar amounts of VO(OCH2CF3)Cl2 in t-BuOH
under an argon atmosphere, tolualdehyde 2a, benzyl
alcohol 4a, and benzyl chloride 5a were obtained in
38, 5 and 35% yields, respectively. The yield of alde-
hyde 2a was increased to 60% together with carboxylic
acid 3a (19% yield) when the reaction was performed
under an oxygen atmosphere (Eq. (1)).
SnBu3
O
O
OH
Cl
3 VO(OCH2CF3)Cl2
OH
H
+
Me
+
+
t-BuOH, 50 °C, 3 h
Me
Me
Me
Me
(1)
1a
2a
3a
4a
5a
Ar atmosphere
O2 atmosphere
38%
60%
0%
19%
5%
trace
35%
trace
Table 1. Oxidation of 1a by oxovanadium(V) compound under an oxygen atmospherea
Entry
Oxovanadium (mol%)
Time (h)
Yields (%)
3a
Recovery of 1a (%)
2a
4a
1
2
3
4
5
6
7
VO(acac)2
(300)
3
3
3
3
3
5
20
36
64
60
40
–
–
–
–
19
19
–
4
37
45
12
Trace
3
–
75
20
–
–
–
VO(OEt)3
(300)
(300)
(300)
(300)
(50)
VO(OPri)3
VO(OPri)2Cl
VO(OCH2CF3)Cl2
VO(OPri)2Cl
VO(OCH2CF3)Cl2
72
36
29
–
(50)
100
a All reactions were performed at 50°C.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00083-1

1962
T. Hirao et al. / Tetrahedron Letters 42 (2001) 1961–1963
Table 2. Oxovanadium(V)-catalyzed oxidation of benzyltins
50 mol% oxovanadium reagent, O atmosphere
2
ArCH2SnBu3 ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ ArCHO+ArCOOH
t-BuOH, 50°C
1
2
3
Entry
Ar
Oxovanadium
Time (h)
Total yield (%) 2+3 (2/3)
Recovery of 1 (%)
1
2
3
4
5
6
7
8
4-MeO-C6H4
4-MeO-C6H4
4-Me-C6H4
2-Me-C6H4
C6H5
4-Cl-C6H4
4-NC-C6H4
1-Naphthyl
VO(OCH2CF3)Cl2
VO(OPri)2Cl
24
72
36
72
72
72
72
24
59 (55/4)a,b
78 (63/15)b
100 (0/100)
100 (0/100)
62 (41/21)
95 (64/31)
25 (25/0)
–
–
–
–
23
4
VO(OCH2CF3)Cl2
VO(OCH2CF3)Cl2
VO(OCH2CF3)Cl2
VO(OCH2CF3)Cl2
VO(OCH2CF3)Cl2
VO(OCH2CF3)Cl2
18
–
86 (58/38)c
a The reaction was carried out at room temperature.
b Benzyl alcohol 4 was also obtained in 9% yield.
c Benzyl alcohol 4 and benzyl chloride 5 were obtained in 5 and 4% yields, respectively.
tributyltin was converted to acetophenone in 88% yield
as a sole isolable product.
The reaction conditions were optimized and the repre-
sentative results are shown in Table 1. When a weak
oxidant such as VO(acac)2, VO(OPri)3 or VO(OEt)3 was
used, the reaction did not proceed smoothly with the
low yield of 2a (entries 1–3), indicating that at least one
chloride ligand was required on oxovanadium(V) com-
pound. When VO(OPri)2Cl was employed, the total
yield of 2a and 3a was up to 83%, although the
formation of 4a could not be suppressed (entry 4). Use
of VO(OCH2CF3)Cl2 resulted in the selective formation
of 2a and 3a (79% total yield, entry 5). It should be
noted that the amount of oxovanadium(V) compound
could be reduced to 0.5 molar amount. Catalytic activ-
ity of VO(OPri)2Cl was not enough to accomplish the
reaction, and 1a was recovered in 29% yield even after
72 h. On the other hand, VO(OCH2CF3)Cl2 showed the
superior catalytic activity, giving only 3a quantitatively
(entries 6 and 7). Oxovanadium(V) compound (50
mol%) can induce a direct conversion of the benzyltin
to the corresponding aromatic aldehyde and/or acid
under oxygen atmosphere, in which at least 4 equiva-
lents of oxidant should be required.
O
50 mol% VO(OCH2CF3)Cl2
O2 atmosphere
SnBu3
(2)
t-BuOH, 50 °C, 72 h
88%
The precise reaction mechanism is unsolved, but the
following results suggest a direct conversion to car-
bonyl/carboxyl compound. It is noteworthy that ben-
zylic alcohol 4 is not involved as a key intermediate.12
When 4-methoxybenzyl alcohol was treated under simi-
lar oxidation conditions as employed above, the reac-
tion became very complicated, giving a complex
mixture including the alcohol and the aldehyde in 11
and 14% yields, respectively. On the other hand, ben-
zaldehydes 2 were quantitatively converted to the acids
3 under similar conditions. Therefore, the present oxo-
vanadium-O2 oxidation system appears to induce a
transformation of benzyltins, not to the alcohols but to
the aldehydes or their vanadium complexes directly,
and the thus-formed species may be converted to the
acids, especially effectively in the catalytic reaction.
Table 2 shows the examples for the reaction of various
benzyltins under the oxovanadium-catalyzed oxidation
conditions.11 Since 4-methoxybenzyltin was too suscep-
tible to oxidative conditions, the use of a milder oxi-
dant, VO(OPri)2Cl, was more suitable to afford the
aldehyde 2 and carboxylic acid 3 in 63 and 15% yields,
respectively (entries 1 and 2). As shown in entries 3–8,
the better yield was obtained with the more electron-
rich benzyltins, although the reactivity of non-substi-
tuted benzyltin was somewhat low (entry 5). In
particular, 4-methyl- and 2-methyl-benzyltins under-
went facile oxidation to give the acids 3 quantitatively
(entries 3 and 4).
As described above, a useful synthetic method for direct
catalytic oxidation of benzyltins to the corresponding
aldehydes/acids was achieved by the oxovanadium(V)-
induced oxidation. These findings are considered to
permit a versatile catalytic system for oxidative trans-
formation of organometallic compounds.
Acknowledgements
The use of the facilities of the Analytical Center, Fac-
ulty of Engineering, Osaka University, is acknowl-
edged. This work was partly supported by
a
This transformation can be applied to secondary ben-
zyltin compounds, giving the corresponding ketones
selectively. As shown in Eq. (2), 1-phenylethyl-
Grant-in-Aid for Scientific Research (No. 11450341)
from the Ministry of Education, Science, and Culture,
Japan.

T. Hirao et al. / Tetrahedron Letters 42 (2001) 1961–1963
Chem. Rev. 1997, 97, 2707.
1963
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