Oxidation of alcohols by iodine in the presence of nitroxyl radicals generated electrochemically

Article abstract of DOI:10.1134/S1070427210040324

Oxidation of primary and secondary alcohols in the two-phase system of methylene chloride-aqueous solution of sodium hydrocarbonate in the presence of the mediator system of potassium iodide-nitroxyl radical was studied. It is supposed that under these co

Full text of DOI:10.1134/S1070427210040324

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 4, pp. 745−747. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.Sh. Kagan, V.P. Kashparova, I.Yu. Zhukova, I.I. Kashparov, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 4,
pp. 693−695.
BRIEF
COMMUNICATIONS
Oxidation of Alcohols by Iodine in the Presence
of Nitroxyl Radicals Generated Electrochemically
E. Sh. Kagan, V. P. Kashparova, I. Yu. Zhukova, and I. I. Kashparov
South Russian State Technical University (Novocherkassk Polytechnical Institute), Novocherkassk, Russia
Received April 28, 2009
Abstract—Oxidation of primary and secondary alcohols in the two-phase system of methylene chloride–aqueous
solution of sodium hydrocarbonate in the presence of the mediator system of potassium iodide–nitroxyl radical
was studied. It is supposed that under these conditions the iodonium ion generated on a platinum electrode is the
primary oxidizing agent.
DOI: 10.1134/S1070427210040324
Selective oxidation of alcohols up to aldehydes is one of
the oldest problems of organic synthesis.An examination
of published data shows that the most promising is the
method of alcohols oxidation using a catalytic system
containing a nitroxyl radical of the 2,2,6,6-tetramethyl-
piperidine series and an inexpensive primary oxidizing
agent, for example a hypochlorite or other oxidizing
agents including anodic oxidation of a nitroxyl radical [1,
2]. The pathway of the oxidation of organic compounds
using nitroxyl radicals as catalysts is shown in scheme 1.
of this method is a considerable iodine consumption (2
mol of iodine per 1 mol of alcohol) and the formation of
coproducts (potassium iodide) [3].
In the present work we have offered the electrochemi-
cal procedure for the oxidation of primary and secondary
alcohols up to corresponding carbonyl compounds with
application of the two-mediator system of nitroxyl radical
(I-III) - potassium iodide:
Scheme 1.
It is known that in the case of electrochemical oxida-
tion of alcohols in the presence of only potassium iodide
primary alcohols are oxidized up to ethers of acids and
secondary alcohols - up to corresponding ketones. In this
case the iodonium ion is the oxidizing agent [4, 5]. It was
found that the presence of a catalytic amount of a nitroxyl
radical of the 2,2,6,6-tetramethylpiperidine series changes
radically the reaction direction. Primary alcohols in this
case are oxidized only up to corresponding aldehydes. A
possible mechanism of the reaction of alcohols oxida-
tion by the system nitroxyl radical - potassium iodide is
shown in Scheme 2.
However in some cases the application of such strong
oxidizing agents as NaOCl is unacceptable, as it can give
rise to the oxidation of other functional groups or to the
halogenation of a substrate [3]. The oxidation of alcohols
can be carried out in mild conditions in the presence of
crystalline iodine and a nitroxyl radical. A disadvantage
745

746
KAGAN et al.
Concentration of reagents, M: 0.05 alhohol/0.005 (10 mol %)
Scheme 2.
(II)/0.01 (20 mol %) KI
Yielda of aldehyde of ketone
(with respect to initial alcohol),
%, at Q, Φ mol–1
Run
Alcohol
no.b
2
4
95 (85)b
62.5
1
Benzyl
60.3
43.6
23.0
48.7
30.0
25.0
27.0
42.0
46.5
38.7
90 (75) b
2
Phenylethyl
"
3
72
"
4
52
85 (80) c
5
Cyclohexanol
"
6
65
It is possible that the role of the primary oxidizing
"
7
67
agent can be also played by other products of the electro-
chemical transformations of the iodide anion (I2 and I·),
which can oxidize the nitroxyl radical up to the oxoam-
monium cation [6].
Electrochemical oxidation of alcohols in presence of
the mediator system of potassium iodide–4-acetylamino-
2,2,6,6-tetramethylpiperidine-l-1-oxyl (II) in the two-
phase system of methylene chloride–aqueous solution
of 0.6 M NaHCO3 (pH 8.6.)
The data on the electrochemical oxidation of alcohols
with application of the mediator system of nitroxyl radical
(I)–(III)– potassium iodide are presented in the table. In
all cases the structure of the nitroxyl radical practically
did not affect the carbonyl compound yield. Differences
in the yields of products did not exceed 5%.
The oxidation reaction was carried out in a two-phase
system of organic solvent–water. As organic phases we
used methylene chloride, toluene, and carbon tetrachlo-
ride. No essential differences in the reaction yields de-
pending on the organic phase were observed.
The rate of the oxidation reaction substantially de-
pends on the рН of medium. In an alkaline medium
(NaHCO3 solution) the reaction passes faster than in
the neutral (see the table, experiments nos. 4 and 7) that
confirms the mechanism proposed earlier for the oxidation
of alcohols by oxoammonium salts [7]. It is connected
with the fact that in the alkaline medium alcohol enters
the reaction as an alkoxide.
The oxidation of alcohols in the presence of a smaller
amount of the nitroxyl radical and potassium iodide
(see the table, experiments nos. 3, 6, and 8) results in
a decrease in the yield of the corresponding carbonyl
compound (in terms of an initial alcohol).
"
8
57
9
1-Butanol
1-Pentanol
2-Propanol
85
10
11
89
80
a
By GLC data
b Experiments nos. 3 and 6: concentrations of reagents, M, is 0.05
alhohol/0.005 (10 mol %) (II)/0.005 (10 mol %) KI; experiments nos.
4 and 7: рН of electrolyte is 6.86 (buffer solution 0.05 M Na2HPO4
and 0.05 M KH2PO4); experiment no. 8: concentrations of reagents,
M, is 0.05 alhohol/0.0005 (1 mol %) (II)/0.001 (2 mol %) KI.
c
Yield of isolated product.
EXPERIMENTAL
In the experiments we used “Aldrich” reagents;
2,2,6,6-tetramethylpiperidine nitroxyl radicals were syn-
thesized according to the procedures [8] by the oxidation
of corresponding sterically-hindered piperidines. Reac-
tion products were analyzed by the method of gas-liquid
chromatography. The data of the chromatographic analy-
sis (gas chromatograms) were obtained on an LKhM-8
MD chromatograph (detector katharometer; gas-carrier
helium; column length 1.5 m, column diameter 3 mm;
carrying agent Chromaton-N-AW; immobile phase PEG-
4000; throughput rate of helium 4 ml/10 s; sensitivity
1 : 3). The content of products was determined by peak
areas in the approximation of equal sensitivity coefficients
of the detector to analyzed substances. Accuracy of mea-
suring areas was 5 rel %. The thin-layer chromatography
was carried out onAl2O3 (II degree of activity after Brock-
man, eluent CH2Cl2, developer I2 vapor).
The technique of the electrochemical oxidation of
alcohols using the mediator system of nitroxyl radical
(I)–(III)–potassium iodide in the two-phase system of
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 4 2010

OXIDATION OF ALCOHOLS BY IODINE
747
methylene chloride–NaHCO3 aqueous solution (pH 8.6)
was as follows.
ary alcohols using the mediator system of a nitroxyl radi-
cal of the 2,2,6,6-tetramethylpiperidine series–potassium
iodide results in the formation of aldehydes and ketones,
respectively, with a high yield of the compounds.
An alcohol, 0.05 mol, dissolved in 30 ml of methy-
lene chloride, NaHCO3, 5 g (0.06 mol), and KI, 2 g
(0.01 mol), dissolved in 70 ml of distilled water were
poured in a 150 ml diaphragmless electrolyzer supplied
with a mechanical stirrer, a water jacket, and two plati-
num electrodes (anode 20 cm2, cathode 15 cm2). Then
0.005 mol of the nitroxyl radical was added. Electrolysis
conditions: temperature 20–25°, current strength of 0.5A
(anode current density 0.025Acm–2), maximal voltage on
the electrolyzer 10 V, and intensive stirring. The synthesis
was finished after passing 4 Ф mol–1 of electricity. After
electrolysis termination the organic layer was separated.
The aqueous layer was extracted with methylene chloride
(2 × 50 ml), organic extract were merged and dried by
anhydrous sodium sulfate. According to TLC, reaction
mixtures contained two substances, an initial alcohol and
an oxidation product. Contents of alcohol and aldehyde
(ketone) were determined by the GLC method.
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