Selective oxidation of benzylic alcohols and ethers and oxidative cleavage of benzylic tetrahydropyranyl and trimethylsilyl ethers to their carbonyl compounds by dinitrogen tetroxide-impregnated activated charcoal (N 2O4/charcoal)

Article abstract of DOI:10.1081/SCC-200058001

Benzylic alcohols and ethers were oxidized to their corresponding carbonyl compounds using dinitrogen tetroxide impregnated on activated charcoal (N ₂O₄/ charcoal) in CH₂Cl₂ at room temperature. Efficient oxidative cleavage of trimethylsilyl (TMS) and tetrahydropyranyl (THP) ethers to their corresponding carbonyl compounds was also performed by this reagent. High selectivity was observed for oxidation of benzylic alcohols and ethers and also trimethylsilyl ethers in the presence of tetrahydropyranyl ethers. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200058001

This article was downloaded by: [Fordham University]
On: 07 October 2013, At: 09:27
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic
Organic Chemistry
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/lsyc20
Selective Oxidation of
Benzylic Alcohols and Ethers
and Oxidative Cleavage of
Benzylic Tetrahydropyranyl
and Trimethylsilyl
Ethers to Their Carbonyl
Compounds by Dinitrogen
Tetroxide–Impregnated
Activated Charcoal (N2O4/
Charcoal)
Nasser Iranpoor a , Habib Firouzabadi a & Ali Reza
Pourali a
a Department of Chemistry, Shiraz University, Shiraz,
Iran
Published online: 17 Dec 2010.
To cite this article: Nasser Iranpoor , Habib Firouzabadi & Ali Reza Pourali (2005)
Selective Oxidation of Benzylic Alcohols and Ethers and Oxidative Cleavage of
Benzylic Tetrahydropyranyl and Trimethylsilyl Ethers to Their Carbonyl Compounds
by Dinitrogen Tetroxide–Impregnated Activated Charcoal (N2O4/Charcoal), Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 35:11, 1527-1533, DOI: 10.1081/SCC-200058001

To link to this article: http://dx.doi.org/10.1081/SCC-200058001
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Synthetic Communicationsw, 35: 1527–1533, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200058001
Selective Oxidation of Benzylic Alcohols and
Ethers and Oxidative Cleavage of Benzylic
Tetrahydropyranyl and Trimethylsilyl
Ethers to Their Carbonyl Compounds by
Dinitrogen Tetroxide–Impregnated
Activated Charcoal (N2O4/Charcoal)
Nasser Iranpoor, Habib Firouzabadi and Ali Reza Pourali
Department of Chemistry, Shiraz University, Shiraz, Iran
Abstract: Benzylic alcohols and ethers were oxidized to their corresponding carbonyl
compounds using dinitrogen tetroxide impregnated on activated charcoal (N2O4/
charcoal) in CH2Cl2 at room temperature. Efficient oxidative cleavage of trimethylsilyl
(TMS) and tetrahydropyranyl (THP) ethers to their corresponding carbonyl compounds
was also performed by this reagent. High selectivity was observed for oxidation of
benzylic alcohols and ethers and also trimethylsilyl ethers in the presence of tetra-
hydropyranyl ethers.
Keywords: Benzylic alcohol, benzylic ether, charcoal, oxidation, tetrahydropyranyl
ether, trimethylsilyl ether, N2O4
Formorethanacentury, gaseousorliquiddinitrogen tetroxide (N2O4)hasfound
wide applications in organic synthesis as an oxidizing, nitrating, and nitrosating
agent.[1,2] Apart from difficulties in handling and the corrosive nature of N2O4,
another limitation of utilizing this versatile reagent in organic synthesis is its
high reactivity, which usually causes undesired side reactions and requires
low temperature-controlled conditions.[2a] To remove these drawbacks, we
Received in the UK October 27, 2004
Address correspondence to N. Iranpoor and H. Firouzabadi, Department of
Chemistry, Shiraz University, Shiraz 71454, Iran. Fax: þ98(711)2280926; E-mail:
iranpoor@chem.susc.ac.ir; firouzabadi@chem.susc.ac.ir
1527

1528
N. Iranpoor, H. Firouzabadi, and A.-R. Pourali
have prepared and used different organic, inorganic, and polymeric complexes
of N2O4 for nitrosation and nitration reactions.[3] Recently, we reported N2O4
impregnated on activated charcoal (N2O4/charcoal) as an efficient, cheap,
and safe reagent for the selective and heterogeneous oxidation of sulfides to
sulfoxides and also disulfides to thiosulfonates.[3c] This reagent is stable in
refrigeration for months without loosing its activity.
Now we report the use of N2O4/charcoal as a mild reagent for the
oxidation of benzylic alcohols and ethers, and also THP and TMS ethers to
their corresponding carbonyl compounds.
Using this reagent, heterogeneous oxidation of benzylic alcohols to their
corresponding carbonyl compounds was performed in good to excellent yields
at room temperature in CH2Cl2 without any overoxidation to carboxylic acids
(Scheme 1, Table 1).
We observed that this method is very convenient for oxidation of primary
and secondary benzylic alcohols in the presence of aliphatic ones.
The chemoselectivity of the method was shown by performing a competi-
tive reaction between benzyl alcohol and cyclohexanol. It was observed that
benzyl alcohol was selectively converted to benzaldehyde with N2O4/char-
coal in excellent yield (95%); however, cyclohexanol remained intact (98%).
The oxidative cleavage of ethers to give aldehydes or ketones is an attrac-
tive synthetic route for organic chemists. However, this transformation is
usually carried out under forceful reaction conditions using a wide range of
oxidizing agents such as chromic acid/acetic acid,[4] cerium(IV) sulfate/
acetic acid,[5] silver(II) oxide,[6] uranium hexafluoride,[7] cerium(IV)
ammonium nitrate coupled with sodium bromate,[8] trityl tetrafluoroborate,[9]
silica gel supported metal nitrates, [10] and N2O4 in the presence of silica gel.[11]
We investigated the applicability of the N2O4/charcoal system as a new
source of N2O4 for oxidation of benzylic ethers. It was found that oxidative
cleavage of benzylic ethers to their corresponding carbonyl compounds can
be performed with this reagent in CH2Cl2 at room temperature under
aprotic and neutral conditions (Scheme 2, Table 2). This transformation is
again highly selective for benzylic ethers and alkyl ethers remain unreacted.
Because of the importance of direct conversion of trimethylsilyl (TMS)
and tetrahydropyranyl (THP) ethers to carbonyl compounds,[12b,13–15] we
studied the applicability of using N2O4/charcoal for this transformation
(Scheme 3, Table 3). This reagent converted various benzylic TMS ethers
Scheme 1.

Selective Oxidation of Benzylic Alcohols
1529
Table 1. Oxidation of benzylic alcohols to the corresponding aldehydes and ketones
with N2O4/charcoal in CH2Cl2 at room temperaturea
Isolated
yield %
Entry
R
R0
Time/h
1
2
H
4-Me
2-MeO
4-MeO
2-Cl
4-Cl
2-NO2
4-NO2
4-Ph
H
H
H
4
3
90
96
89
91
85
86
75
80
90
95
98
86
70
90
70
75
3
H
3
4
H
2.5
3.5
4
5
H
6
H
7
H
8
8
H
8
9
H
5
10
11
12
13
14
15
16
Me
Et
3.5
3.5
7.5
8
H
H
Ph
H
PhCO
CH2OH
COOH
Ph
H
6
H
4-Cl
10
10
aAll the products are known compounds and were identified by their spectral data.
to their corresponding aldehydes and ketones in CH2Cl2 at room temperature.
However, because of the resistance of THP ethers, their oxidative deprotec-
tions required refluxing CHCl3.
Selective oxidation reactions were also studied by this reagent using
binary mixtures. Excellent selectivity was observed for the oxidation of
benzylic alcohols (Table 4, entries 1, 2), trimethylsilyl ethers, and benzylic
ethers in the presence of THP ethers (Table 4, entries 3–7).
In conclusion, using N2O4/charcoal as a cheap, safe, efficient, and easily
prepared reagent provides a practical method for the heterogeneous oxidation
of benzylic alcohols, ethers, and TMS and THP ethers to their carbonyl
compounds without any overoxidation reactions under mild and neutral
reaction conditions. The products can be easily obtained by a simple filtration
and evaporation of the solvent. In addition, this reagent shows high selectivity
Scheme 2.

1530
N. Iranpoor, H. Firouzabadi, and A.-R. Pourali
Table 2. Oxidative cleavage of benzylic ethers with N2O4/charcoal in CH2Cl2 at
room temperaturea
Isolated
yield %
Entry
Ar
R
R0
Time/h
1
2
C6H5
C6H5
C6H5
H
H
Me
Bu
3
3
93
96
98
90
92
95
96
70
75
3
H
C6H5 CH2
Me
4
4
4-MeO-C6H5
4-MeO-C6H5
4-MeO-C6H5
4-MeC6H5
C6H5
H
2.5
2
5
H
Bu
4-MeO-C6H5 CH2
6
H
2
7
H
Me
Bu
3.5
8
8
CH3
C6H5
C6H5
9
10
C6H5
C6H5
Me
Ph2CH
10
10
b
—
aAll the products are known compounds and were identified by their spectral data.
bNo reaction.
for oxidation of benzylic alcohols and ethers and also trimethylsilyl ethers in
the presence of THP ones.
EXPERIMENTAL
General
Chemicals were purchased from Merck and Fluka Chemical Companies. The
products were characterized by comparison of their physical data with those of
known samples or by their spectral data. IR spectra were run on a Shimadzu
model 8300 FT-IR spectrophotometer. NMR spectra were recorded on a
Bruker Avance DPX-250. Mass spectra were recorded on a Shimadzu
GCMS-QP 1000 EX. The purity of the products and the progress of the
reactions were accomplished by TLC on silica-gel polygram SILG/UV254
plates or by a Shimadzu Gas Chromatograph GC-14A instrument with a
flame ionization detector.
Scheme 3.

Selective Oxidation of Benzylic Alcohols
1531
Table 3. Oxidative deprotection of TMS ethers in CH2Cl2 and THP ethers in CHCl3
with N2O4/charcoala
Isolated
yield %
Entry
R
R0
R00
Condition
Time/h
1
2
C6H5
4-MeO-C6H5
4-MeC6H5
4-ClC6H5
4-NO2C6H5
C6H5
H
H
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
THP
rt
rt
0.5
0.2
0.2
0.5
3
96
98
99
95
90
96
98
95
98
95
70
90
85
3
H
rt
4
H
rt
5
H
rt
6
CH3
C6H5
H
rt
1
—
b,c,d
7
C6H5
C6H5
rt
8
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
1
9
4-MeO C6H5
4-MeC6H5
4-NO2C6H5
C6H13
H
THP
THP
0.5
0.25
3.5
2.5
3
10
11
12
13
H
H
THP
THP
CH3
C6H5
C6H5
THP
aAll the products are known compounds and were identified by their spectral data.
bThe product is diphenyl methanol.
cImmediately.
dIf the reaction is stirred for 8 h, benzophenone is produced in 83% yield.
Table 4. Selective oxidations of different binary mixtures using N2O4/charcoal at
room temperature
Entry
1
Substrate
Producta
Time/h
Yield %
4-MeC6H4CH2OH
C6H5CH2OTHP
4-MeC6H4CHO
C6H5CH2OTHP
4
90
98
2
3
4
5
6
7
4-ClC6H4CH2OH
C6H13CH(Me)OTHP
4-ClC6H4CHO
C6H13CH(Me)OTHP
6
1
1
2
6
5
84
95
4-MeC6H4CH2OTMS
C6H5CH2OTHP
4-MeC6H4CHO
C6H5CH2OTHP
98
99
4-ClC6H4CH2OTMS
4-NO2C6H4CH2OTHP
4-ClC6H4CHO
4-NO2C6H4CH2OTHP
98
100
C6H5CH(Me)OTMS
C6H5CH2OTHP
C6H5COMe
C6H5CH2OTHP
92
98
(PhCH2)2O
C6H5CH2OTHP
PhCHO
C6H5CH2OTHP
95
96
PhCH2OCH3
C6H13CH(Me)OTHP
PhCHO
C6H13CH(Me)OTHP
92
99
aAll the products are known compounds and were identified by their spectral data.
bGC yield using internal standard.

1532
N. Iranpoor, H. Firouzabadi, and A.-R. Pourali
Oxidation of 4-chlorobenzyl alcohol; typical procedure: A solution of
4-chlorobenzyl alcohol (0.285 g, 2 mmol) in CH2Cl2 (4 mL) was prepared
and N2O4/charcoal (0.48 g, 2.2 mmol) was added to this solution at room
temperature with stirring. After completion of the reaction (4 h), the mixture
was filtered. Evaporation of the solvent followed by chromatography on a
short column of silica gel using petroleum ether–acetone (9:1) afforded
the 4-chlorobenzaldehyde (0.242 g, 86%); mp 42–448C [Lit.[3g] mp
42–458C].
Oxidative cleavage of di(4-methoxybenzyl) ether; typical procedure: To
a stirring solution of di(4-methoxybenzyl) ether (0.258 g, 1 mmol) in CH2Cl2
(5 mL) was added N2O4/charcoal (0.66 g, 3 mmol) at room temperature. After
completion of the reaction (2 h), the mixture was filtered. Evaporation of the
solvent followed by chromatography on a short column of silica gel using
petroleum ether–acetone (9:1) afforded the 4-methoxybenzaldehyde
(0.258 g, 95%); bp 247–2508C/760 Torr [Lit.[3g] bp 245–2488C/760 Torr].
Oxidative deprotection of 4-methoxybenzyl trimethylsilyl ether; typical
procedure: To a solution of 4-methoxybenzyl trimethylsilyl ether (0.388 g,
2 mmol) in CH2Cl2 (6 mL) was added N2O4/charcoal (0.88 g, 4 mmol) and
the mixture was stirred at room temperature. After completion of the
reaction (12 min), the mixture was filtered. The filtrate was applied on a
short column of silica gel which was eluted with petroleum ether–acetone
(9:1). The solvent was evaporated to give 4-methoxybenzaldehyde (0.267 g,
98%); bp 247–2488C/760 Torr [Lit.[14] bp 247–2488C/760 Torr].
Oxidative deprotection of benzyl tetrahydropyranyl ether; typical proce-
dure: To a solution of benzyl tetrahydropyranyl ether (0.384 g, 2 mmol) in
CHCl3 (6 mL) was added N2O4/charcoal (0.88 g, 4 mmol) while stirring
gently. The reaction was refluxed for 1 h. After completion of the reaction,
the mixture was filtered. Chromatography of the crude product on a short
column of silica gel eluted with petroleum ether–acetone (9 : 1) gave benzal-
dehyde (0.202 g, 95%) as colorless liquid; bp 176–1788C/760 Torr [Lit.[3g] bp
175–1788C/760 Torr].
ACKNOWLEDGMENT
The authors are thankful to Shiraz University Research Council for the partial
support of this work.
REFERENCES
1. (a) Riebsomer, J. L. Chem. Rev. 1945, 36, 157; (b) Ogata, Y. In Oxidation in
Organic Chemistry; Trahanovsky, W. S., Ed.; Academic Press: New York,
1978; Part C; pp. 295–342; (c) Paquette, L. A. Encyclopedia of Reagents for
Organic Synthesis; John Wiley: Chichester, 1995; pp. 3741–3743.

Selective Oxidation of Benzylic Alcohols
1533
2. (a) Field, B. O.; Grundy, J. J. Chem. Soc. 1955, 1110; (b) Addison, C. C.;
Sheldon, J. C. J. Chem. Soc. 1956, 2705; (c) Bosch, E.; Kochi, J. K. J. Org.
Chem. 1995, 60, 3172; (d) Bosch, E.; Kochi, J. K. J. Chem. Soc., Perkin Trans.
1996, 1, 2731; (e) Bosch, E.; Rathore, R.; Kochi, J. K. J. Org. Chem. 1994, 59,
2529; (f) Bozell, J. J.; Hoberg, J. O. Tetrahedron Lett. 1998, 39, 2261;
(g) Kim, Y. H.; Shinhama, K.; Fukushima, D.; Oae, S. Tetrahedron Lett. 1978,
1211; (h) Oae, S.; Kim, Y. H.; Fukushima, D.; Shinhama, K. J. Chem. Soc.,
Perkin Trans. 1978, 1, 913; (i) Oae, S.; Shinhama, K.; Fujimori, K.; Kim, Y. H.
Bull. Chem. Soc. Jpn. 1980, 53, 775; (j) Oae, S.; Kunieda, N.; Tagaki, W.
Chem. Ind. (London) 1965, Oct. 23, 1790.
3. (a) Iranpoor, N.; Firouzabadi, H.; Pourali, A. R. Tetrahedron 2002, 58, 5179;
(b) Iranpoor, N.; Firouzabadi, H.; Pourali, A. R. Synthesis 2003, 1591;
(c) Iranpoor, N.; Firouzabadi, H.; Pourali, A. R. Synlett 2004, 347;
(d) Iranpoor, N.; Firouzabadi, H.; Heydari, R. Phosphorus, Sulfur Silicon 2003,
178, 1027; (e) Firouzabadi, H.; Iranpoor, N.; Heydari, R. Phosphorus, Sulfur
Silicon 2002, 177, 631; (f) Firouzabadi, H.; Iranpoor, N.; Heydari, R. Synth.
Commun. 2001, 31, 2037; (g) Iranpoor, N.; Firouzabadi, H.; Zolfigol, M. A.
Bull. Chem. Soc. Jpn. 1998, 71, 905; (h) Firouzabadi, H.; Iranpoor, N.;
Zolfigol, M. A. Synth. Commun. 1998, 28, 1179.
4. Harrison, I. T.; Harison, S. J. Chem. Soc., Chem. Commun. 1966, 752.
5. Olah, G. A.; Gupta, B. G. B.; Fung, A. P. Synthesis 1980, 897.
6. Synder, C. D.; Rapoport, H. J. Am. Chem. Soc. 1972, 94, 227.
7. (a) Olah, G. A.; Welch, J. J. Am. Chem. Soc. 1978, 100, 5396; (b) Olah, G. A.;
Welch, J.; Ho, T.-L. J. Am. Chem. Soc. 1976, 98, 6717.
8. Musgrave, O. C. Chem. Rev. 1969, 69, 499.
9. Barton, D. H. R.; Magnus, P. D.; Strreckert, G.; Zurr, D. J. Chem. Soc., Chem.
Commun. 1971, 1109.
10. (a) Nishiguchi, T.; Bougauchi, M. J. Org. Chem. 1989, 54, 3001;
(b) Nishiguchi, T.; Bougauchi, M. J. Org. Chem. 1990, 55, 5606.
11. Nishiguchi, T.; Okamoto, H. J. Chem. Soc., Chem. Commun. 1990, 1607.
12. (a) Olah, G. A.; Ho, T.-L. J. Org. Chem. 1977, 42, 3097; (b) Olah, G. A.; Ho, T.-L.
Synthesis 1976, 609.
13. (a) Muzart, J. Synthesis 1993, 11 and references cited therein; (b) Firouzabadi, H.;
Mohammadpour-Baltork, I. Synth. Commun. 1994, 24, 1065; (c) Firouzabadi, H.;
Shirini, F. Synth. Commun. 1996, 26, 649; (d) Mohammadpour-Baltork, I.;
Nourozi, A. R. Synthesis 1999, 487; (e) Mohammadpour-Baltork, I.;
Kharamesh, B. J. J. Chem. Res., Synop. 1998, 146.
14. Firouzabadi, H.; Iranpoor, N.; Zolfigol, M. A. Bull. Chem. Soc. Jpn. 1998, 71,
2169.
15. Firouzabadi, H.; Mohammadpoor-Baltork, I.; Kolagar, S. Synth. Commun. 2001,
31, 905.