A simple and convenient manganese dioxide oxidation of benzyl halides to aromatic aldehydes under neutral condition

Article abstract of DOI:10.1246/cl.2005.194

A simple, convenient and inexpensive benzylic oxidation reaction of arylmethylhalides under neutral conditions by manganese dioxide to the corresponding aldehydes has been described. Copyright

Full text of DOI:10.1246/cl.2005.194

194
Chemistry Letters Vol.34, No.2 (2005)
A Simple and Convenient Manganese Dioxide Oxidation of Benzyl Halides
to Aromatic Aldehydes under Neutral Condition
Shyamaprosad Goswami,ꢀ Subrata Jana, Swapan Dey, and Avijit Kumar Adak
Department of Chemistry, Bengal Engineering and Science University Shibpury, Howrah -711 103, West Bengal, India
(Received October 4, 2004; CL-041171)
A simple, convenient and inexpensive benzylic oxidation re-
action of arylmethylhalides under neutral conditions by manga-
nese dioxide to the corresponding aldehydes has been described.
Table 1. Yield and products of MnO2 oxidation of benzyl
halides
Entry
Substrate
Time (/h)
Products
Yield (/%)a
Since the discovery of the Sommelet reaction1 in 1913, the
oxidation of benzyl halides to the carbonyl compounds repre-
sents one of the important fundamental reactions in organic syn-
thesis.2 Several other reaction procedures have been reported for
this transformation which include the Hass Bender reaction,3
pyridine followed by p-nitrosodimethylaniline,4 dimethyl sulf-
oxide,5 dimethylselenoxide-dipotassium hydrogen phosphate,6
bis(tetrabutylammonium)dichromate,7a potassium chromate-
HMPT in presence of crown ether,7b certain amine N-oxides,8
and also photooxidation9 of organic halides to the corresponding
carbonyl compounds. Recently, sodium periodate in refluxing
dimethylformamide,10 3,6-bis(triphenylphosphonium)cyclohex-
ene in refluxing acetonitrile-water,11 H2O2 catalyzed by V2O5
and Aliquat 336 in boiling water,12 and also microwave-expedit-
CHO
Br
Cl
1
2
2
6
80
40
CHO
CHO
Br
3
4
4
4
74
70
O2N
O2N
CHO
Br
EtO2C
EtO2C
CHO
Br
5
6
4
4
75
60
CO2Et
Br
CO2Et
CHO
O
O
N
H
N
H
ed solvent free synthesis using pyridine-N-oxide13a and mont-
CHO
Br
7
8
4
4
60
48
13b
Br
Br
morillonite K10-HIO3
have been used for such conversion.
However, most of these procedures suffer from strong oxidizing
conditions, longer reaction time, high reaction temperature, and
also the cost of the reagents used. Therefore, the need for a con-
venient and mild reaction procedure still stimulate a continuous
research effort.
Br
MeO
CHO
MeO
Cl
CHO
9
4
5
60
40
In our previous study,14 we showed that Co(I)(PPh3)3Cl is
an effective and versatile catalyst for the synthesis of aromatic
aldehydes from arylmethyl bromides in the presence of aerial
oxygen under very mild conditions. Manganese dioxide in
chlorinated hydrocarbons has been widely applied for the oxida-
tion of ꢀ,ꢁ-unsaturated alcohols (e.g. allylic, benzylic alcohols),
and also been employed in a variety of other organic reactions.15
Manganese dioxide is a readily available and inexpensive oxi-
dizing agent, however, its use in the conversion of benzyl halides
to the carbonyl compounds remains relatively unexplored.
In continuation of our work for the synthesis of aromatic and
heteroaromatic benzyl bromides,16 herein, we wish to describe a
convenient and inexpensive method for the oxidation of benzylic
halides to the aromatic aldehydes in moderate to good yields un-
der mild reaction conditions (manganese dioxide in chloroform,
reflux, 2–6 h) (Scheme 1).
CHO
Br
10
CHO
CHO
Br
11
6
58
48
12
13
6
6
Br
O2N
O2N
N
N
N
N
Br
Br
Br
CH3
40
aIsolated yields are of chromatographically pure material.
well for a wide variety of benzyl bromides (Entries 1–12), which
can be directly converted to the corresponding aldehydes where
a number of different functional groups like nitro, carbethoxy,
pivaloyl, methoxy, etc. survived. This method is also efficient
for the synthesis of polynuclear aromatic aldehydes (Entries 9
and 10).
The results of our investigation are presented in the Table 1.
As can be seen from this Table, it is clear that our method works
MnO2,
CHCl3
However, under identical condition, 2,3-bis(bromomethyl)-
quinoxaline (Entry 13) surprisingly gave 2-bromomethyl-3-
methylquinoxaline in 40% yield with the recovery of some
unreacted starting material, although attempts to oxidize 2,6-
bis(bromomethyl)pyridine, 2-N-acetylamino-6-bromomethyl-
CH2X
CHO
R
R
reflux, 2 - 6h
40 - 80%
X = Br, Cl
Scheme 1. Oxidation of benzyl halides to aldehydes.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
195
H
.
O
edged. S. J. and A. K. A. thank CSIR, Govt. of India for research
fellowships.
H
O
.
O
Mn
X
X
Mn
O
References and Notes
H
O
y
1
2
Previously named as Bengal Engineering College (Deemed
University).
CHO
O
Mn
X
Mn(OH)X
+
A
a) M. Sommelet, C. R. Acad. Sci., 157, 852 (1913). b) M.
Sommelet, Bull. Soc. Chim. Fr., 13, 1085 (1913).
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by B. M. Trost and S. V. Ley, Pergamon Press, Oxford
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Scheme 2. A plausible reaction mechanism for the MnO2
oxidation of benzyl halides.
pyridine, 2-pivaloyl-6-bromomethylpyridine, and 2-N-phthali-
mido-6-bromomethylpyridine were unsuccessful. Interestingly,
in the case of 1,4-bis(bromomethyl)benzene (Entry 7) only one
bromomethyl group got oxidized while the other remained unaf-
fected under the same conditions.
Although, at this point, no rigorous mechanistic studies
have been done, a working hypothesis may be suggested on
the basis of the observation of the products (Scheme 2). The la-
bile benzyl halide is probably displaced thermally to form a ben-
zyl radical by activated interaction with MnO2 under refluxing
conditions, which immediately couples with the oxygen radical
3
4
5
.
of OMn(X)(O) to form a possible intermediate (A). This under-
goes subsequent loss of acidic benzylic hydrogen followed by
reductive cleavage of oxygen–manganese bond in (A) with loss
of oxomanganese halide resulting in the formation of aromatic
aldehydes and probably MnO as a salt Mn(OH)X as the oxide
in reduced MnII state is basic.17
6
7
The reason for the formation of 2-bromomethyl-3-methyl-
quinoxaline (Table 1, Entry 13), as an exception to the normal
benzylic oxidation to aldehyde, may possibly be that the benzyl
radical gets quenched by hydrogen capture from solvent chloro-
form giving the observed product. We are currently extending
this methodology onto other heterocyclic as well as other aro-
matic systems having poly(bromomethyl) substituents.
Representative procedure for the oxidation of 4-nitrobenzyl
bromide: A mixture of 4-nitrobenzyl bromide (216 mg, 1 mmol)
and activated manganese dioxide (480 mg, 5 mmol, Aldrich
21,764-6) in chloroform (10 mL) was refluxed for 4 h at 60 ꢁC.
The progress of the reaction was monitored by TLC using petro-
leum ether–dichloromethane (3:1) as eluent. The reaction mix-
ture was filtered and washed with chloroform. The combined fil-
trates after evaporation and silica gel (60–120 mesh) purification
afforded 4-nitrobenzaldehyde (112 mg, 74%) along with recov-
ery of unreacted (15%) starting material.18
In conclusion, we have developed a very simple, efficient,
mild, and inexpensive procedure for the oxidation of benzyl hal-
ides to the aromatic aldehydes. We have found that this proce-
dure (MnO2 in refluxing chloroform) is even more controlled
at least in the case of oxidation of simple benzylbromide itself
than our previous one (aerial oxygen, Co(I)(PPh3)3Cl),14 though
it required lower temperature (Co(I), aerial oxygen is thus more
reactive). In the latter case, benzoic acid was obtained appreci-
ably from benzyl bromide itself (benzaldehyde was further oxi-
dized) unlike the present case where the yield of benzaldehyde is
excellent.
8
9
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17 J. D. Lee, in ‘‘Concise Inorganic Chemistry,’’ 5th ed.,
Chapman and Hall, London (1996) pp 736–742.
18 All products are characterized by FT-IR, 1H NMR and also
comparison of the melting points with authentic samples
when available.
Financial assistance from the Council of Scientific and
Industrial Research (CSIR), Govt. of India is gratefully acknowl-
Published on the web (Advance View) January 15, 2005; DOI 10.1246/cl.2005.194