Quinaldinium Chlorochromate Supported on Alumina: A New and Efficient Reagent for the Oxidation of Alcohols

Article abstract of DOI:10.1007/s00706-003-0063-8

The new, mild chromium(VI) oxidizing agent, quinaldinium chlorochromate supported on neutral alumina, was prepared as a stable yellow solid. The reagent is suitable to oxidize various primary and secondary alcohols to the corresponding carbonyl compounds and anthracene to anthraquinone in good yields.

Full text of DOI:10.1007/s00706-003-0063-8

Monatshefte f€ur Chemie 134, 1565–1569 (2003)
DOI 10.1007/s00706-003-0063-8
Quinaldinium Chlorochromate Supported
on Alumina: A New and Efficient
Reagent for the Oxidation
of Alcohols
ꢀ
ꢁ
¨
i and Beytiye Ozgu¨n
˘
Nebahat Degirmenbas°
€
Gazi Universitesi, Fen-Edebiyat Fakultesi, Kimya Bolumu, Teknikokullar,
€
€ € €
€
06500 Ankara, Turkiye
Received April 16, 2003; accepted April 22, 2003
Published online September 18, 2003 # Springer-Verlag 2003
Summary. The new, mild chromium(VI) oxidizing agent, quinaldinium chlorochromate supported on
neutral alumina, was prepared as a stable yellow solid. The reagent is suitable to oxidize various
primary and secondary alcohols to the corresponding carbonyl compounds and anthracene to anthra-
quinone in good yields.
Keywords. Quinaldinium chlorochromate; Oxidation; Alcohols; Carbonyl compounds.
Introduction
Oxidation of alcohols to carbonyl compounds is one of the most important reac-
tions in organic chemistry [1–2]. Chromium(VI) reagents are widely utilized in the
oxidation of alcohols to carbonyl compounds. Some of the important entries in the
list of the reagents are pyridinium chlorochromate [3], pyridinium dichromate [4],
pyridinium fluorochromate [5], pyridinium bromochromate [6], quinolinium fluoro-
chromate [7], and prolinium chlorochromate [8]. However, most of these reagents
that have been developed so far suffer from at least one of the drawbacks such as
high acidity, photosensitivity, instability, tedious work-up procedures, or require-
ment of large excess of reagent.
Over several years our group has been involved in developing reagents allowing
oxidations to be performed under mild conditions [9]. In extension of our studies
on development of new reagents based on chromium(VI) we present here quinal-
dinium chlorochromate (QnCC) on alumina as a mild and efficient oxidant for
alcohols and anthracene.
ꢁ
Corresponding author. E-mail: beytiyeozgun@hotmail.com

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N. Degirmenbas°i and B. Ozgun
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Results and Discussion
The reagent QnCC can be easily prepared in good yield (72%) by reaction of
quinaldine with chromium trioxide in 6 M hydrochloric acid. The structure of
the product was confirmed by elemental analysis and its IR (KBr) spectrum. The
infrared absorption frequencies for the chlorochromate group at ꢀꢀ¼ 949, 876, and
466 cmꢂ1 in quinaldinium chlorochromate are attributable to ꢀꢀasym (Cr¼O), ꢀꢀsym
(Cr¼O), and ꢀꢀ (Cr–Cl); these assignments are in accord with those found for
KCrO3Cl [10]. It is soluble in DMF and DMSO, sparingly soluble in chloroform,
dichloromethane, acetonitrile, and water, and insoluble in carbontetrachloride, ben-
zene, toluene, and ether. These results are indicative of the ionic nature of QnCC.
The compound is diamagnetic. It is a 1:1 electrolyte (LM ¼ 150 mho cm2 molꢂ1, in
acetonitrile). The pH of 0.01 M solutions of PCC, zinc chlorochromate monohy-
drate [11], and QnCC were found to be 1.75, 2.30, and 3.79. A higher pH value of
QnCC compared to its companion reagents attests its far less acidic character.
Preliminary investigations demonstrated that some primary and secondary alcohols can be oxidized
to the corresponding aldehydes and ketones with QnCC. Although oxidation of benzyl alcohol to
benzaldehyde smoothly proceeded using this reagent in CH2Cl2, the oxidation of other benzylic alcohols
did not occur in good yield. Other primary and secondary alcohols did not give considerable amounts of
the corresponding carbonyl compounds. For example, when 1-octanol was subjected to oxidation with
QnCC, only 22% of octanal were obtained. Since organic synthesis by solid phase methods is a powerful
tool [12] and chromium oxidants like pyridinium chlorochromate adsorbed on alumina [13], chromic
acid on silica [14], and chromyl chloride on silica-alumina have been reported to give better yields under
milder conditions as compared to the parent oxidants, we tried to follow this road for QnCC.
Thus, QnCC adsorbed on alumina was prepared by adding alumina to a solu-
tion of QnCC in CH2Cl2 and evaporation to dryness. The yellow solid was kept in
vacuum at room temperature and stored in the dark before use.
The effect of solvent in the oxidation reaction was evaluated by carrying out the
oxidation in a series of solvents with varying polarity. Oxidation of benzyl alcohol
with QnCC supported on alumina in a 1:1.5 ratio was carried out in n-hexane,
carbon tetrachloride, dichloromethane, acetone, and dimethylformamide (Table 1).
Use of more polar solvents such as DMF and acetone (in which the reagent is
soluble) resulted in moderate yields. The best results were obtained with the other
solvents. Dichloromethane was chosen as the standard solvent.
The stability and activity of this supported reagent was compared with that of
the unsupported QnCC by carrying out the oxidation of benzyl alcohol with both
Table 1. Oxidation of benzyl alcohol in different solvents using QnCC=alumina
Solvent
Substrate=Oxidant
(molar ratio)
Reaction period=h
Yielda=%
n-Hexane
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
2
2
2
2
2
97
95
94
77
69
Carbon tetrachloride
Dichloromethane
Acetone
Dimethylformamide
a
Yields refer to isolation of 2,4-DNP derivative

Quinaldinium Chlorochromate Supported on Alumina
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Table 2. Oxidation of benzyl alcohol using QnCC and QnCC=alumina stored for different periods
Oxidant
Storage
Substrate=Oxidant
(molar ratio)
Reaction
Yielda=%
period=week
period=h
QnCC
1
2
4
6
1:1.5
1:1.5
1:1.5
1:1.5
4
4
4
4
78
76
71
62
QnCC=alumina
1
2
4
6
1:1.5
1:1.5
1:1.5
1:1.5
2
2
2
2
94
92
91
89
a
Yields refer to isolation of 2,4-DNP derivative
Table 3. Oxidation of organic substrates with QnCCa=alumina
Entry Substrate
Product
Time=h Yield=%b 2,4-DNP (m.p.=ꢃC)
Obs. Ref. [15]
1
2
3
1-Octanol
Octanal
2
2
2
71
94
73
103–105 106
235–236 237
253–254 254
Benzyl Alcohol
Benzaldehyde
4-Methoxybenzyl 4-Methoxybenzaldehyde
Alcohol
4
5
6
4-Methylbenzyl
Alcohol
4-Methylbenzaldehyde
4-Chlorobenzaldehyde
4-Nitrobenzaldehyde
Cyclohexanone
2
2
2
73
61
54
231–233 233
263–265 265
318–319 320
4-Chlorobenzyl
Alcohol
4-Nitrobenzyl
Alcohol
7
8
Cyclohexanol
2
2
53
60
160–162 162
155–156 156
4-tert-Butylcyclo- 4-tert-Butylcyclo-
hexanol
hexanone
9
10
11
Benzhydrol
Benzoin
Benzophenone
Benzil
2
2
3
72
236–238 238
92c
89c
–
–
–
–
Anthracene
Anthraquinone
a
Oxidations were carried out in dichloromethane with a substrate to oxidant ratio 1:1.5 (1:3 for
anthracene) at room temperature; b Yields refer to isolated 2,4-DNP derivatives identified by melting
points; c Yields refer to isolated benzil and anthraquinone whose melting points were taken directly
and confirmed by comparison with authentic samples (IR, TLC, and NMR)
reagents after different periods of storage. It was found that the supported reagent
was more stable and active than the unsupported QnCC (Table 2).
In order to ascertain the efficacy of the reagent as an oxidant, it was tested on a
wide array of alcohols in dichloromethane at room temperature.
Thus, QnCC=alumina readily oxidizes primary (Table 3, entries 1–6) and sec-
ondary alcohols (Table 3, entries 7–10) to their corresponding aldehydes and
ketones (Scheme 1) in good yields.

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N. Degirmenbas°i and B. Ozgun
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Scheme 1
QnCC=alumina in dichloromethane also oxidizes anthracene to anthraquinone
in good yield (Table 3, entry 11).
In conclusion, QnCC=alumina is an efficient reagent for the oxidation of var-
ious types of alcohols into the corresponding carbonyl compounds. QnCC=alumina
has advantages in terms of ease of preparation, lower acidity, reaction period, and
yield of products. We envisaged that this oxidant system, in addition to its mild-
ness, would have the additional advantage of trapping the reduced chromium spe-
cies in the alumina matrix for safe disposal and making isolation of products of
oxidation easier. Moreover, this reagent is very stable, and can be stored for longer
periods without much loss in its activity and hence turns out to be a very useful
reagent in synthetic organic chemistry.
Experimental
Synthesis of Quinaldinium Chlorochromate (QnCC)
A solution of 10.0g of CrO3 (0.10 mol) in 18.4cm3 of 6 M HCl (0.10 mol) was cooled to 0ꢃC and to
this 13.9cm3 of quinaldine (0.10 mol) were added dropwise during 30 min. The reaction mixture was
cooled for 2 h. The resulting yellow solid was collected and washed with ether, kept under suction until
moderately dry, and placed under vacuum pump pressure until it became a dry powder, yield 72%, mp
139ꢃC dec. Anal. calc. for C10H9NHCrO3Cl: C 42.93, H 3.58, N 5.01, Cr 18.60%; Found: C 42.96, H
3.56, N 5.02, Cr 18.94%.
Preparation of QnCC=Alumina
To an ice cold mixture of 15g of CrO3 (0.15 mol) and 25cm3 of 6 M HCl (0.15 mol), 45g of Al2O3
(neutral, Brockmann II–III) were added with stirring. Quinaldine, 22cm3 (0.15 mol), was then added
dropwise and the resulting yellow solid was filtered, washed with water and cold acetone, and dried
in vacuo for 3 h. The average capacity of the reagent as estimated by iodometry was found to be
1.2–1.6 mmol of QnCC per g of Al2O3.
General Procedure for Oxidation
The oxidations were all conducted in dry apparatus and under effꢁcient stirring. To a thoroughly stirred
suspension of QnCC and QnCC=alumina (1.5–3 mmol) in 10 cm3 of CH2Cl2 1 mmol of the substrate
dissolved in a small amount of the solvent was added. The mixture was stirred at room temperature
for the period indicated in Table 1. The progress of the reaction was followed by TLC
(n-hexane:ethylacetate¼ 2:1). After the completion of the reaction the solid was filtered off and
washed with 10 cm3 of CH2Cl2, and the filtrate was evaporated on a rotary evaporator to furnish the
product which was isolated as the 2,4-dinitrophenyl hydrazone (2,4-DNP).

Quinaldinium Chlorochromate Supported on Alumina
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Acknowledgements
The authors are grateful to the Research Foundation of Gazi University for supporting this study.
References
[1] Cainelli G, Carbillo G (1984) Chromium Oxidation in Organic Chemistry, V19, Springer, Berlin
[2] Hudlicky M (1990) Oxidation in Organic Chemistry. American Chemical Society, Washington
[3] Corey EJ, Suggs JW (1975) Tetrahedron Lett 16: 2647
[4] Corey EJ, Schmidt G (1979) Tetrahedron Lett 20: 399
[5] Bhattacharjee MN, Chaudhuri MK, Dasgupta HS, Roy N (1982) Synthesis 7: 588
[6] a) Narayanan N, Balasubramanian TR (1986) Indian J Chem 25B: 228; b) Durai S, Ravikumar V,
Narayanan N, Balasubramanian TR, Mohan S (1987) Spectrochim Acta 43A: 1191
[7] Murugesan V, Pandurangan A (1992) Indian Journal of Chemistry 31B: 377
[8] Mamaghani M, Shirini F, Parsa F (2002) Russ J Org Chem 38: 1113
€
€
˘
˘
€
€
[9] a) Ozgun B, Degirmenbas
°
ꢁ N (1996) Synth Commun 26: 3601; b) Ozgun B, Degirmenbas
°
ꢁ N
ꢁ N, Ozg€un B (2002) Monatsh Chem 133: 1417;
°ꢁ N, Ozg€un B (2002) J Inst Sci Techn Gazi Univ 15: 657
€
˘
(1999) Synth Commun 29: 763; c) Degirmenbas
°
€
˘
d) Degirmenbas
[10] Stammreich H, Sala G, Kawal K (1961) Spectrochim Acta 17: 226
[11] Firouzabadi H, Sharifi A (1999) Synthesis 6: 999
[12] Cheng YS, Liu WL, Chen S (1980) Synthesis 3: 223
[13] Sengh RP, Subbaro HN, Dev S (1979) Tetrahedron 35: 1789
[14] Sanfillipo JR, Chem CI (1977) J Org Chem 42: 2182
[15] Furniss BS, Hanaford AJ, Smith WG, Tatchell AR (1989) Textbook of Practical Organic
Chemistry, 5th ed, John Wiley, New York