La<sub>2</sub>O<sub>3</sub> Catalyzed Oxidation of Alcohols

doi:10.4236/ijoc.2011.12008

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International Journal of Organic Chemistry, 2011, 1, 41-46

doi:10.4236/ijoc.2011.12008 Published Online June 2011 (http://www.SciRP.org/journal/ijoc)

La2O3 Catalyzed Oxidation of Alcohols

Ravikumar R. Gowda, Debashis Chakraborty*

Department of Chemistry, Indian Institute of Technology Madras, Chennai, India

E-mail: [email protected]

Received April 8, 2011; revised April 28, 2011; accepted May 12, 2011

Abstract

A variety of aromatic, aliphatic and conjugated alcohols were transformed to the corresponding carboxylic

acids and ketones with a quantitative conversion in high yields with 70% t-BuOOH solution is water in the

presence of catalytic amounts of La2O3. This method possesses a wide range of capabilities since it can be

used with other functional groups which may not tolerate oxidative conditions, involves fairly simple method

for work-up, exhibits chemoselectivity and proceeds under ambient conditions. The resulting products are

obtained in good yields within reasonable time.

Keywords: Oxidation, Alcohol, Carboxylic Acid, La2O3, t-BuOOH

1. Introduction

The oxidation of alcohols has been of contemporary in-

terest due to diversified potentials in organic chemistry

and industrial manufacturing, and is recognized a fun-

damental reaction [1-5]. The oxidation of primary alco-

hols yields aldehydes which may be further oxidized to

give carboxylic acids. The most popular and widely used

reagent for oxidation is Jones reagent [6-11]. However,

the reaction is stoichiometric and is performed under

highly acidic conditions. Substrates having acid sensitive

functionalities may not tolerate such acidity. In addition,

the generation of Cr-based side products may be viewed

as a potential environmental hazard [12]. Other reagents

that have been used successfully include Oxone [13],

calcium hypochlorite [14] and 2-hydroperoxyhexafluoro-

2-propanol [15]. Excellent catalytic methods using met-

als have been developed using oxidation reactions. In-

teresting methodologies for metal mediated transforma-

tion of aldehydes to carboxylic acids have been reported

recently [16-27]. The catalytic oxidation of alcohols em-

ploying transition metals such as Ru, Co, Mo, Pd, V and

W have been reported [28-39]. In addition, 2,2,6,6-

tetramethylpiperidinyl-1-oxyl often referred to as TEMPO

along with NaClO has been an efficient combination for

such oxidations [40-46]. The above reagents and meth-

ods have one or more limitations which include the use

of superstoichiometric amounts of expensive reagents

and use of highly basic or acidic reaction conditions. The

search for catalytic processes that use environmentally

benign reagents is always an attractive avenue. Our re-

cent results highlight the oxidation of aldehydes to car-

boxylic acid using 30% H2O2 as the oxidant in the pres-

ence of catalytic amounts of AgNO3 [47]. However, we

were unable to convert alcohols to ketones and carbox-

ylic acids under similar conditions. We were inspired to

venture into the area of La(III) catalyzed oxidation and

explore the possibility of using such compounds as cata-

lysts for oxidation reaction.

2. Results and Discussion

We decided to explore the possibility of converting pri-

mary alcohols to carboxylic acids and secondary alcohols

to ketones with the various La(III) salts. The optimiza-

tion of reaction conditions for the oxidation of primary

alcohols to the corresponding carboxylic acids was per-

formed with (4-nitrophenyl)methanol as a suitable sub-

strate in the presence of different solvents, oxidants and

5 mol% of La(III) salts (Table 1).

The oxidation of (4-nitrophenyl)methanol to 4-nitro-

benzoic acid takes place rapidly in the presence of 5

mol% La2O3 and 5 equiv. 70% t-BuOOH (water) using

MeCN as a suitable solvent (Table 1, Entry 1). In the

presence of 2 equiv. 70% t-BuOOH (water), only 30%

product could be isolated. With 5 equiv. 5M t-BuOOH

(decane), the reaction was found complete in 26 h with

90% isolated yield. Various trials were done in the pres-

ence of different solvents (Table 1, Entries 1-9) and dif-

ferent La(III) salts (Table 1, Entries 10 and 11). Best

results were obtained with 70% t-BuOOH (water) as the

oxidant and 5 mol% of La2O3 as the catalyst in MeCN.

R. R. GOWDA ET AL.

Table 1. Optimization of the reaction conditions for the

conversion of (4-nitrophenyl)methanol to 4-nitrobenzoic

acid with different solvents, 5 equiv. 70% t-BuOOH (water)

and 5 mol% La(III) salts.

Entry Catalyst Solvent Time (min)a Yield (%)b

1 La2O3 MeCN 18 98

2 La2O3 EtOAc 24 75

3 La2O3 toluene 24 27

4 La2O3 CH2Cl2 24 71

5 La2O3 DMF 24 42

6 La2O3 DMSO 24 82

7 La2O3 THF 24 25

8 La2O3 EtOH 24 5

9 La2O3 MeNO2 24 88

10 LaCl3 MeCN 24 65

11 LaBr3 MeCN 24 52

aTime required for complete conversion; bIsolated yield after column

chromatography of the crude product with ethyl acetate and hexane.

We proceeded with investigation the oxidation of vari-

ous aromatic and aliphatic substrates (Scheme 1, Table 2).

Again we see that La2O3 actively catalyzes the trans-

formation of different primary alcohols to the corre-

sponding benzoic acid with variety of different sub-

strates. Substitutions at different positions on the phenyl

ring do not hinder the reaction, although the reaction

time is affected. Our catalyst shows sufficient selectivity

in this oxidation without disturbing functional groups

like phenol and amine (Table 2, Entries 7 and 8). Oxi-

dation of





unsaturated derivatives (Table 2, Entry 15)

resulted in the formation of the expected acid in good

yield. In addition, the transformation of secondary al-

cohols to ketones is extremely facile as indicated by

Entries 17-20 of Table 2.

It is pertinent to mention here that mild halogenic

oxidants like hypochlorites [14,48,49], chlorites [50,51]

and NBS [52,53] are not suitable for substrates with

electron rich aromatic rings, olefinic bonds and secon-

dary hydroxyl groups.

The kinetic studies of the oxidation with (4-methoxy-

phenyl)methanol and (3-nitrophenyl)methanol were ex-

plored next. High-pressure liquid chromatography (HPLC)

was used to determine the various starting materials,

products and aldehyde intermediates for alcohol oxidation

present as a function of time. The concentration of reactant,

intermediate and product for the oxidation of (4-meth-

oxyphenyl)methanol is shown in Figure 1.

The concentration of the alcohol decreases steadily

while that of the carboxylic acid increases. The concentra-

tion of the intermediate aldehyde increases, achieves a

steady state and then progressively converts itself to the

acid. The curve showing (4-methoxyphenyl)methanol is

zero-order in substrate. We have calculated the rate of

such reactions. As an example let us consider the conver-

sion of (4-methoxyphenyl)methanol to 4-Methoxy-

benzoic acid. The Van’t Hoff differential method was used

to determine the order (n) and rate constant (k) (Figure 2).

Scheme 1. La2O3 catalyzed oxidation of alcohols.

0510 15 20 25

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Concentration(mol/L)

4-OMeC

ime(h)

6H4COOH

4-OMeC6H4COH

4-OMeC6H4CH2OH

r details).

Figure 1. Van’t Hoff differential plot for the oxidation of

(4-methoxyphenyl)methanol with 5 mol% La2O3 and 5

equiv. 70% t-BuOOH in MeCN under ambient condition.

From Figure 1, the rate of the reaction at different

concentrations can be estimated by evaluating the slope

of the tangent at each point on the curve corresponding

to that of 4-Methoxybenzylalcohol. With these data,

log10(rate) versus log10(concentration) is plotted. The

order (n) and rate constant (k) is given by the slope of the

line and its intercept on the log10 (rate) axis. From Figure

2 it is clear that this reaction proceeds with second-order

kinetics (n = 2.01) and the rate constant k = 9.29 × 10–1

L·mol1·h1. For the other substrate namely

(3-nitrophenyl)methanol, the order of the reaction n =

2.02 with rate constants (k) 1.61 × 10–4 L·mol1·h1 re-

spectively (see supporting information fo

3. Conclusions

In summary, we have developed a simple, efficient,

chemoselective and inexpensive catalytic method for the

oxidation of primary alcohols to carboxylic acids and

secondary alcohols to ketones using a table top reagent

such as La2O3. It is noteworthy that this method does not

use ligands and other additives.

4. Experimental Section

4.1. General Reagents and Equipments

All the substrates along with t-BuOOH, used in this

R. R. GOWDA ET AL.43

Table 2. La2O3 catalyzed oxidation of alcoholsa.

Entry Alcohol Product Time (h)bYield (%)c

COOH

10 88

CH2OHMeO

COOHMeO

23 90

OMe

COOH

OMe

12 96

MeO

COOH

MeO

15 87

OHMeO

MeO

COOHMeO

MeO

17 89

OHMeO

OMe

MeO

COOHMeO

OMe

MeO

25 90

OHHO

COOHHO

27 87

OHN

COOHN

33 85

OHCl

COOHCl

25 88

OHCl

COOHCl

28 86

COOH

40 82

COOH

48 85

OHO

COOHO

18 98

OCH

OCOOH

25 89

Ph CH

Ph COOH

25 87

COOH

55 83

Ph Ph

6 93

Ph Me

17 90

4 92

22 89

R. R. GOWDA ET AL.

--1 -1-1 -0 -01.6 .4.2 .0.8.6-0.4

-3.9

-3.6

-3.3

.3-0

-3

-2

-1

-0

-0 Y = -0.03198 + 2.0112 X

log10 (rate)

log10 C

Figure 2. Van’t Hoff differential plot for the oxidation of

(4-methoxyphenyl)methanol with 5 mol% La2O3 and 5

equiv. 70% t-BuOOH in MeCN under ambient condition.

study were purchased from Aldrich and used as received.

The solvents used were purchased from Ranchem, India

and purified using standard methods. 1H and 13C spectra

were recorded with a Bruker Avance 400 instrument.

Chemical shifts were referenced to residual solvent

resonances and are reported as parts per million relative

to SiMe4. CDCl3 was used for NMR spectral measure-

ments. HPLC analysis was done with Waters HPLC in-

strument fitted with Waters 515 pump and Waters 2487

dual λ absorbance detector.

4.2. Typical Procedure for the Oxidation of

Primary Alcohol to Carboxylic Acid in MeCN

Under a nitrogen atmosphere, to a stirred solution of

La2O3 (16.29 mg, 0.05 mmol) and primary alcohol (1

mmol) in 2.5 mL MeCN was added 70% t-BuOOH (wa-

ter) (0.64 mL, 5 mmol). The progress of the reaction was

monitored using TLC until all the alcohol was consumed.

All the volatiles were removed using a rotary evaporator.

The crude product was treated with saturated NaHCO3

solution. This was extracted with ethyl acetate. Finally,

the aqueous layer was acidified using 2N HCl and ex-

tracted with ethyl acetate. The organic layer was concen-

trated in vacuum and subjected to column chromatogra-

phy with ethyl acetate and hexane. The spectral data of

the various carboxylic acids were found to match satis-

factory in accord with the literature.

4.3. Typical Procedure for the Oxidation of

Secondary Alcohol to Ketone in MeCN

Under a nitrogen atmosphere, to a stirred solution of

La2O3 (16.29 mg, 0.05 mmol) and secondary alcohol (1

mmol) in 2.5 mL MeCN was added 70% t-BuOOH (wa-

ter) (0.64 mL, 5 mmol). The progress of the reaction was

monitored using TLC until all the ketone was found

consumed. All the volatiles were removed using a rotary

evaporator. The residue was quenched with 2 mL water

and extracted with ethyl acetate. The organic layer was

concentrated in vacuum and subjected to column chro-

matography with ethyl acetate and hexane. Spectral

characterization of the various ketones was found to

match with the literature.

–

0.3

–

0.6 Y = –0.03198 + 2.0112X

–

0.9

–

1.2

–

1.5

–

1.8

–

2.1

–

2.4

–

2.7

–

3.0

–

3.3 5. Acknowledgements

–

3.6

–

3.9

This work was supported by the Department of Science

and Technology, New Delhi.

–

1.6 –1.4 –1.2 –1.0 –0.8 –0.6 –0.4

6. Electronic Supplementary Material

The online version of this article contains supplementary

material

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