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S. Mannam, G. Sekar / Tetrahedron Letters 49 (2008) 1083–1086
oxidant molecular oxygen and TEMPO with 30% H2O2,
the reaction produced a 38% isolated yield of carboxylic
acid in 24 h at room temperature (entry 2). When 30%
H2O2 was replaced by anhydrous tBuOOH (5 M in decane)
all the starting material was consumed in just 6 h at room
temperature and the reaction provided a 91% isolated yield
of acid (entry 3). To our surprise, the real catalyst in this
reaction is the ligand free CuCl. CuCl alone without DAB-
CO gave an identical result (entry 4). Replacement of anhy-
drous tBuOOH (5 M in decane) by 70% tBuOOH (in water)
also provided almost the same results at room temperature.
is very simple, and works efficiently at room temperature
without any additives or ligands.
Acknowledgements
We thank DST (SERC Fast Track Research Project
No.: SR/FTP/CS-19/2004) New Delhi, for the financial
support. S.M. thanks CSIR, for a Junior Research
Fellowship.
t
References and notes
The cheaper cost of aqueous BuOOH compared with
anhydrous tBuOOH encouraged us to carry out the oxida-
tion reaction with 70% tBuOOH (in water). Reaction with-
out CuCl provided only a trace amount of acid (entry 7).
This clearly indicates that CuCl is the catalyst.
1. Hudlicky, M. Oxidations in Organic Chemistry; American Chemical
Society: Washington, DC, 1990; p 174.
2. Hainess, A. H. Methods for the Oxidation of Organic Compounds;
Academic: New York, 1988; p 221, 423.
3. Ogliaruso, M. A.; Volfe, F. A. In Updates From the Chemistry of
Functional Groups. In Synthesis of Carboxylic Acids Esters and their
Derivatives; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1991; p
357.
4. Ganem, B.; Heggs, R. P.; Biloski, A. J.; Schwartz, D. R. Tetrahedron
Lett. 1980, 21, 685.
We then screened the reaction with several solvents and
acetonitrile turned out to be the best solvent providing the
highest yield of acid in the shortest time (entry 6). Similarly
CuCl became the choice of Cu salt in view of the yield and
reaction rate.
5. Nwauka, S. O.; Keehn, P. M. Tetrahedron Lett. 1982, 23, 3131.
6. Benjamin, R. T.; Sivakumar, M.; Hollist, G. O.; Borhan, B. Org. Lett.
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M.; Noyori, R. Tetrahedron Lett. 2000, 41, 1439.
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Hyaric, M. Synthesis 1993, 295.
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11. Heaney, H.; Newbold, A. J. Tetrahedron Lett. 2001, 42, 6607.
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M. J. Mol. Catal. 2001, 175, 277.
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5–8.
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17. Heaney, H. Top. Curr. Chem. 1993, 1, 164.
18. Sreedevi, M.; Alamsetti, S. K.; Sekar, G. Adv. Synth. Catal. 2007, 349,
2253.
19. Sreedevi, M.; Sekar, G. Tetrahedron Lett., submitted for publication.
20. Typical experimental procedure: To a solution of CuCl (4.95 mg,
0.05 mmol) in 2 mL of CH3CN and aldehyde (1 mmol) was slowly
added dropwise aqueous tBuOOH (0.142 mL, 1 mmol, 70% in water)
over 5 min. The resulting reaction mixture was stirred at room
temperature until the disappearance of starting material (TLC). After
completion of the reaction, the solvent was evaporated and to the
resulting crude reaction mixture water was added. The pH was
adjusted to 8.0–8.5 with saturated NaHCO3 and then the reaction
mixture was extracted with ethyl acetate. The aqueous layer was
acidified to pH 2.0 using 2 N HCl and extracted with ethyl acetate.
The organic layer was concentrated and purified by silica gel column
chromatography to give the carboxylic acid. All the products gave
satisfactory spectral data.
To determine the scope of this new catalytic system, a
wide range of aldehydes were oxidized under the optimized
conditions (5 mol % CuCl, 1 equiv of 70% tert-butyl hydro-
peroxide (in water) in acetonitrile at room temperature)
and the results are summarized in Table 2. It is clear that
all the aromatic, vinylic and aliphatic aldehydes were easily
oxidized to the corresponding acids in short times (1–9 h)
with almost quantitative conversion and excellent isolated
yields. Heteroatom-containing aldehydes such as furan-2-
carbaldehyde and picolinaldehyde produced the corres-
ponding acids, furan-2-carboxylic acid and picolinic acid
in good yields, respectively (entries 12 and 14). Aromatic
dialdehydes such as phthaldehyde and terephthaldehyde
were also oxidized to the corresponding diacids, phthalic
acid and terephthalic acid at room temperature in just 2.5
and 3 h, respectively, with excellent yields (entries 10 and
11). Allylic aldehydes such as cinnamaldehyde, crotonalde-
hyde and acrylaldehyde were also oxidized to the corres-
ponding acids in short times and in good to excellent
yields (entries 15–17). To our surprise, aliphatic aldehydes
such as cyclohexanecarbaldehyde, palmitaldehyde and the
aliphatic dialdehyde glutaraldehyde were also oxidized to
the corresponding acids and diacids (entries 18–21). A
detailed mechanistic study of this CuCl catalyzed oxidation
and its application in the field of total synthesis of biologi-
cally active natural products is under progress.
In conclusion, we have developed a new procedure for
the oxidation of aldehydes to acids using a catalytic
t
amount of CuCl and 1 equiv of 70% BuOOH (in water)
in acetonitrile under very mild conditions. This procedure