wil N

aticH scapaondis gr

een of contemporary interestanic synthesis and industrialne of the fundamental reac-used is Jones reagent for suchction inditiot be ab-basedental h

mentally benign reagents shall always remain an attractive avenuein this area. Similar catalytic transformation of the aldehydes tocarboxylic acids has been reported with SeO2.8 Our recent resultshighlight the oxidation of aldehydes to carboxylic acid using 30%H2O2 as the oxidant in the presence of catalytic amounts ofAgNO3.9 Our continued interest in studying catalytically activeenvironmentally benign processes compelled us to investigate

the capability of Bi(III) reagents towards oxidation. It must benoted that Bi2O3 is reported to catalyze the oxidation of a-hydroxyketones.10

2. Results and discussion

tion in much shorter time. With 5 equiv 5 M t-BuOOH (decane),the reaction was found complete in 2.5 h with 90% isolated yield.The reaction tookmuch longer time for completion (20 h)whenper-formedwith 5 equiv 30% H2O2 in EtOAc and yielded 82% of product.Among the different solvents used for optimization (Table 1, entries19), EtOAc yielded best results. The other Bi(III) salts (Table 1,entries 1012) were found to be inferior as compared to Bi2O3.

Having realized the correct conditions for oxidation, we contin-ued our quest with a variety of aromatic and aliphatic substrates(Table 2). The scope of our catalytic system is applicable for a widerange of aromatic, conjugated and aliphatic substrates. These

* Corresponding author. Tel.: +91 44 22574223; fax: +91 44 22574202.

Tetrahedron Letters 51 (2010) 35213523

Contents lists availab

ro

.e lE-mail address: dchakraborty@iitm.ac.in (D. Chakraborty).that have been used successfully include Oxone,4 calciumhypochlorite5 and 2-hydroperoxyhexauoro-2-propanol.6 Excel-lent catalytic methods using metals have been developed usingoxidation reactions. Interesting methodologies for metal-mediatedtransformation of the aldehyde functionality to carboxylic acidhave been reported recently.7 The above-mentioned reagents andmethods involved have one or more limitations which includethe use of superstoichiometric amounts of expensive compounds,employment of highly basic or acidic reaction conditions and hightemperature. The search for catalytic processes involving environ-

The conversion of 4-methoxybenzaldehyde to 4-methoxyben-zoic acid is extremely facile in EtOAc under reux conditions inthe presence of 10 mol % Bi2O3 and 5 equiv 70% t-BuOOH (water)as the oxidant (Table 1, entry 1). Oxidation with t-BuOOH (water)alone in EtOAc was found to be negligible (

Table 2Bi2O3-catalyzed oxidation of aldehydes to carboxylic acidsa

R H

O 10 mol% Bi2O35 equiv. 70% t-BuOOH (water)

R OH

O

EtOAc

Entry Aldehyde Acid Timeb

(h)Yieldc

(%)

1 CHO COOH 2.5 90

2 CHOMeO COOHMeO 2 97

3CHO

OMe

COOH

OMe3 93

4CHO

MeO

COOH

MeO6.5 90

5CHOMeO

MeO

COOHMeO

MeO4 90

6 CHOMeO

OMe

MeO

COOHMeO

OMe

MeO

3 92

7 CHOHO COOHHO 3 87

8 CHON COOHN 6 89

9 CHOCl COOHCl 2 89

edron Letters 51 (2010) 35213523aldehydes were converted to the corresponding carboxylic acids ingood isolated yields in reasonable time (Table 2). It is pertinent tomention here that mild halogenic oxidants such as hypochlo-rites,5,11 chlorites12 and NBS13 are not suitable for substrates withelectron-rich aromatic rings, olenic bonds and secondary hydro-xyl groups. Substitutions at different position on the phenyl ringdo not hinder the reaction, although the reaction time is affected.The reaction is much faster with substrates containing electron-donating substituents in the aromatic ring (Table 2, entries 18vs entries 913). Substitution of extremely electron-withdrawinggroup like NO2 retards the oxidation reaction considerably (Table2, entries 1113). Our catalytic system is mild and shows sufcientselectivity in carrying out the expected oxidation without affecting

Table 1Optimization of the reaction conditions for the conversion of 4-methoxybenzalde-hyde to 4-methoxybenzoic acid with different solvents, 5 equiv 70% t-BuOOH (water)and 10 mol % Bi(III) salts

H

OBi salt, t-BuOOHsolvent, reflux

OH

O

MeOMeO

Entry Catalyst Solvent Timea (h) Yieldb (%)

1 Bi2O3 EtOAc 2 972 Bi2O3 MeCN 15 903 Bi2O3 Toluene 20 804 Bi2O3 CH2Cl2 18 845 Bi2O3 DMF 4 876 Bi2O3 DMSO 5 857 Bi2O3 THF 32 708 Bi2O3 EtOH 17 809 Bi2O3 CH3NO2 10 82

10 BiCl3 EtOAc 30 7111 BiBr3 EtOAc 27 8012 Bi(NO3)35H2O EtOAc 36 79

a Monitored using TLC until all the aldehyde was found consumed.b Isolated yield after column chromatography of the crude product.

3522 P. Malik, D. Chakraborty / Tetrahother functionalities such as phenol and amine (Table 2, entries 7and 8). Oxidation of a, b unsaturated derivatives (Table 2, entry15) resulted in the formation of the expected acid in very goodyield.

The kinetic studies of the oxidation with 4-methoxybenzalde-hyde, 4-nitrobenzaldehyde, furan-2-carboxaldehyde and butyral-dehyde were explored next. High-pressure liquid chromatography(HPLC) was used to determine the various starting materials andproducts present as a function of time. The concentration of reac-tant and product for the oxidation of 4-methoxybenzaldehyde isshown in Figure 1.

The concentration of the aldehyde decreases steadily while thatof the carboxylic acid increases. We have calculated the rate of suchreactions. As an example let us consider the conversion of 4-methoxybenzaldehyde to 4-methoxybenzoic acid. Vant Hoff differ-entialmethodwas used to determine the order (n) and rate constant(k). FromFigure 1, the rate of the reaction at different concentrationscan be estimated by evaluating the slope of the tangent at each pointon the curve corresponding to that of 4-methoxybenzaldehyde.With these data, log10(rate) versus log10(concentration) is plotted.The order (n) and rate constant (k) are given by the slope of the lineand its intercept on the log10(rate) axis. It is clear that this reactionproceedswith second-order kinetics (n = 2.15) and the rate constantk = 0.2134 L mol1 min1. For the other substrates namely 4-nitro-benzaldehyde, furan-2-carboxaldehyde and butyraldehyde, theorder of the reaction n 2 with rate constants (k) is 6.41 103L mol1 min1, 1.76 102 L mol1 min1 and 6.75 102 L mol1min1, respectively (see Supplementary data for details).

10CHOCl

Cl

COOHCl

Cl5.2 88

11CHO

NO2

COOH

NO242 84

12CHO

O2N

COOH

O2N40 85

13 CHOO2N COOHO2N 41 82

14O CHO O COOH

10 91

15 PhCHO

PhCOOH 3 90

16 CHO COOH 8 85

17 CHO COOH 7 90

a Reactions performed in EtOAc with 10 mol % Bi2O3 and 5 equiv 70% t-BuOOHunder reux conditions.

b Monitored using TLC until all the aldehyde was found consumed.c Isolated yield after column chromatography of the crude.

hydes to carboxylic acids with a table top reagent such as14

References and notes

1. (a) Hollingworth, G. J.. In Comprehensive Organic Functional GroupTransformations; Katritzky, A. R., Meth-Cohn, O., Rees, C. W., Pattenden, G.,Eds.; Elsevier Science: Oxford, 1995; Vol. 5, p 23; (b) Hudlicky, M. InOxidations in Organic Chemistry. In ACS Monograph Series; American ChemicalSociety: Washington DC, 1990; Vol. 186, p 174; (c) Larock, R. C. InComprehensive Organic Transformations: A Guide to Functional GroupPreparations, 2nd ed.; Wiley-VCH: New York, 1999; p 1653; (d) Smith, M.B.; March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure,5th ed.; Wiley-Interscience: New York, 2001; (e) Sheldon, R. A.; Van Bekkum,H. Fine Chemicals through Heterogeneous Catalysis; Wiley-VCH Verlag GmbH &Co.: Weinheim, 2001.

2. (a) Bowden, K.; Heilbron, I. M.; Jones, E. R. H.; Weedon, B. C. L. J. Chem. Soc.1946, 3945; (b) Heilbron, I.; Jones, E. R. H.; Sondheimer, F. J. Chem. Soc. 1949,604607; (c) Bladon, P.; Fabian, J. M.; Henbest, H. B.; Koch, H. P.; Wood, G. W. J.Chem. Soc. 1951, 24022411; (d) Curtis, R. G.; Heilbron, I.; Jones, E. R. H.;Woods, G. F. J. Chem. Soc. 1953, 457464; (e) Bowers, A.; Halsall, T. G.; Jones, E.R. H.; Lemin, A. J. J. Chem. Soc. 1953, 25482560; (f) Djerassi, C.; Engle, R. R.;

0.1

0.2

0.3

0.44-OMeC6H4COOH 4-OMeC6H4CHO

Conc

entra

tion

(mol/

L)

with 10 mol % Bi2O3 and 5 equiv 70% t-BuOOH (water) in EtOAc under reuxconditions.

P. Malik, D. Chakraborty / Tetrahedron Letters 51 (2010) 35213523 3523Bi2O3. It is noteworthy to mention that this method does notuse ligands and other additives.

Acknowledgements

This work was supported by Department of Science andTechnology and Council of Scientic and Industrial Research,3. Conclusions

In summary, we have developed a simple, efcient, chemoselec-tive and inexpensive catalytic method for the oxidation of alde-0 20 40 60 80 100 1200.0

Time (min)Figure 1. Concentration versus time in the oxidation of 4-methoxybenzaldehydeNew Delhi. The services from the NMR facility purchased underthe FIST program, sponsored by the Department of Science andTechnology, New Delhi, are gratefully acknowledged.

Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.tetlet.2010.04.101.Bowers, A. J. Org. Chem. 1956, 21, 15471549.3. Cainelli, G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer:

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14. Typical experimental procedure: To a stirred suspension of Bi2O3 (68 mg,0.10 mmol) and aldehyde (1 mmol) in 2.5 mL EtOAc was added 70% t-BuOOH(water) (0.90 mL, 5 mmol). The reaction mixture was heated to reux. Theprogress of the reaction was monitored using TLC until all aldehyde was foundconsumed. The crude product was treated with saturated NaHCO3 solution.This was extracted with ethyl acetate. Finally, the aqueous layer was acidiedusing 2 N HCl and extracted with ethyl acetate. The organic layer wasconcentrated and subjected to column chromatography. The spectral data ofthe various carboxylic acids were found to be satisfactory in accordance withthe literature (see Supplementary data for details).

Bi2O3-catalyzed oxidation of aldehydes with t-BuOOHIntroductionResults and discussionConclusionsAcknowledgementsSupplementary dataReferences and notes