Quinaldinium fluorochromate and quinaldinium dichromate: Two new and efficient reagents for the oxidation of alcohols

Article abstract of DOI:10.1007/s00706-003-0079-0

Two new mild oxidizing agents, quinaldinium fluorochromate and quinaldinium dichromate, were prepared and characterized. These reagents are suitable to oxidize various primary and secondary alcohols to the corresponding carbonyl compounds and anthracene to antraquinone in good yields. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-003-0079-0

Monatshefte f€ur Chemie 135, 407–410 (2004)
DOI 10.1007/s00706-003-0079-0
Quinaldinium Fluorochromate
and Quinaldinium Dichromate:
Two New and Efficient Reagents
for the Oxidation of Alcohols
ꢀ
ꢁ
¨
i and Beytiye Ozgu¨n
˘
Nebahat Degirmenbas°
€
Gazi Universitesi, Fen-Edebiyat Fakultesi, Kimya Bolumu, Teknikokullar,
€
€ € €
€
06500 Ankara, Turkiye
Received May 30, 2003; accepted June 5, 2003
Published online February 5, 2004 # Springer-Verlag 2004
Summary. Two new mild oxidizing agents, quinaldinium fluorochromate and quinaldinium dichro-
mate, were prepared and characterized. These reagents are suitable to oxidize various primary and
secondary alcohols to the corresponding carbonyl compounds and anthracene to antraquinone in good
yields.
Keywords. Quinaldinium fluorochromate; Quinaldinium dichromate; Oxidation; Alcohols; Carbonyl
compounds.
Introduction
Oxidation of primary and secondary alcohols to the corresponding carbonyl com-
pounds is most frequently accomplished by Cr(VI) reagents [1]. Some of the impor-
tant entries in the list of reagents are the Collins reagent [2], pyridinium
chlorochromate [3], pyridinium dichromate [4], pyridinium fluorochromate [5], pyr-
idinium bromochromate [6], and 2,20-bipyridinium chlorochromate [7]. In recent
years, significant improvements have been achieved by the introduction of new
oxidising agents, such as 3,5-dimethylpyrazolium fluorochromate [8], 2,6-dicarbox-
ypyridinium chlorochromate [9], quinolinium fluorochromate [10], quinolinium
dichromate [11], imidazolium fluorochromate [12], and pyrazinium dichromate [13].
Over several years our group has been involved in developing new reagents
allowing oxidations to be performed under mild conditions [14]. In continuation of
our studies on the development of halochromates and dichromates with heterocy-
clic bases we now report the preparation, characterisation, and synthetic utility of
quinaldinium fluorochromate (QnFC) and quinaldinium dichromate (QnDC) with a
ꢁ
Corresponding author. E-mail: beytiyeozgun@hotmail.com

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N. Degirmenbas°i and B. Ozgun
408
view to minimising the existing difficulties encountered in the oxidation of organic
compounds with Cr(VI) based oxidants.
Results and Discussion
The two new chromate complexes can be prepared easily in good yields (85%) and
(88%) by addition of aqueous solutions of CrO3 to quinaldine; the use of 40%
aqueous hydrofluoric acid causes the formation of the fluorochromate species,
while the dichromate species is formed when distilled water is used. The structures
of the products were confirmed by elemental analysis and their IR (KBr) spectra.
Thus, the IR frequencies of the fluorochromate group at ꢀꢀ ¼ 948, 870, and
617 cmꢂ1 in quinaldinium fluorochromate are attributable to ꢀꢀasym (Cr¼O), ꢀꢀsym
(Cr¼O), and ꢀꢀ (Cr–F); these assignments are in accord with those found for
KCrO3F [15]. The IR frequencies of the dichromate group at ꢀꢀ ¼ 948, 891, and
785 cmꢂ1 in quinaldinium dichromate are attributable to ꢀꢀasym (CrO3), ꢀꢀsym (CrO3),
and ꢀꢀsym (Cr–O–Cr). These results are in close agreement with those found for the
dichromate ion in (NH4)2Cr2O7 [16]. Solubility tests of QnFC and QnDC in var-
ious solvents showed that the two complexes are highly soluble in DMSO and
DMF, and sparingly soluble in CHCl3, CH2Cl2, acetonitrile, and water and insol-
uble in CCl4, benzene, toluene, and ether. These results are indicative of the ionic
nature of QnFC and QnDC; they are diamagnetic. QnFC is a 1:1 electrolyte
(LM ¼ 120 mho cm2 molꢂ1, in acetonitrile) and QnDC is a 2:1 electrolyte
(LM ¼ 295 mho cm2 molꢂ1, in acetonitrile). The pH values of 0.01 M aqueous solu-
tions of PCC, PFC, QFC, QxDC, QnFC, and QnDC were found to be 1.75, 2.45,
3.35, 2.30, 3.92, and 3.80 [17]. The higher pH values of QnFC and QnDC com-
pared to their companion reagents attest to their far less pronounced acidic char-
acters. Both QnFC and QnDC are stable for prolonged periods of time (at least 18
months) when stored dry and in the absence of light.
In order to ascertain the efficacy of the reagents as oxidants, they were tested on
a wide array of alcohols in CH2Cl2 at room temperature. Thus, QnFC and QnDC
readily oxidize primary (Table 1, entries 1–6) and secondary alcohols (Table 1,
entries 7–10) to their corresponding aldehydes and ketones (Scheme 1) in good to
excellent yields. QnFC and QnDC in CH2Cl2 also oxidize anthracene to anthra-
quinone in good yields (Table 1, entry 11). The results (Table 1) also show that
QnFC is a milder oxidation reagent than QnDC.
In conclusion, the easily accessible new reagents QnFC and QnDC are mild,
efficient, and stable oxidizing agents. Their controlled acidities makes them
suitable reagents for the oxidation of acid sensitive compounds. The reduced
Scheme 1

Quinaldinium Fluorochromate and Dichromate
409
Table 1. Oxidation of organic substrates by QnFCa and QnDCa
Entry
Substrate
Product
QnFC
QnDC
Time=h
Yield=%b
Time=h
Yield=%b
1
2
1-Octanol
Octanal
4
3
2
2
3
3
3
2
3
2
3
75
79
94
90
74
69
70
60
84
82c
81c
4
3
3
3
4
3
4
4
3
3
3
62
70
77
75
69
65
64
61
72
71c
77c
Benzyl Alcohol
Benzaldehyde
3
4-Methoxybenzyl Alcohol
4-Methylbenzyl Alcohol
4-Chlorobenzyl Alcohol
4-Nitrobenzyl Alcohol
Cyclohexanol
4-Methoxybenzaldehyde
4-Methylbenzaldehyde
4-Chlorobenzaldehyde
4-Nitrobenzaldehyde
Cyclohexanone
4
5
6
7
8
4-tert-Butylcyclohexanol
Benzhydrol
4-tert-Butylcyclohexanone
Benzophenone
9
10
11
Benzoin
Benzil
Anthracene
Anthraquinone
a
Oxidations were carried out in dichloromethane with a substrate to oxidant ratio 1:1.5 (1:3 for anthracene) at room
b
c
temperature; Yields refer to isolated 2,4-DNP derivatives identified by melting points; Yields refer to isolated benzil
and anthraquinone whose melting points were taken directly and confirmed by comparison with authentic samples (IR, TLC,
and NMR)
chromium species can be trapped on a silica gel column for safe disposal. Based on
all the results hitherto obtained with these reagents it may be stated that QnFC and
QnDC are valuable additions to the existing palette of oxidizing agents.
Experimental
Results of elemental analysis of QnFC and QnDC agreed favourably with the calculated values.
Synthesis of Quinaldinium Fluorochromate
CrO3 (20 g, 0.2 mol) was dissolved in 25cm3 of H2O in a polyethene beaker and 11cm3 of 40%
hydrofluoric acid (0.2mol) were added with stirring at room temperature. To the resultant clear orange
red solution, 28 cm3 of quinaldine (0.2mol) were added dropwise with stirring. The mixture was heated
on a water bath for about 15 min, then cooled to room temperature, and allowed to stand for 1 hour.
The bright yellow, crystalline quinaldinium fluorochromate was isolated by filtration. It was recrys-
tallized from H2O and dried in vacuo for about 2 h. Yield 85%; mp 146–148ꢃC.
Synthesis of Quinaldinium Dichromate
Quinaldine (28 cm3, 0.2mol) was slowly added to a cooled solution of 20 g of CrO3 (0.2mol) in 20cm3
of H2O. After 0.5 h, the reaction mixture was diluted with 40 cm3 of acetone and cooled to ꢂ15ꢃC. The
yellow crystals were collected and washed with acetone. It was recrystallized from H2O and dried in
vacuo for about 2 h. Yield 88%; mp 150–151ꢃC.
General Procedure for Oxidation
The oxidations were conducted in a dry apparatus and under efficient stirring. To a thoroughly stirred
suspension of 1.5–3 mmol of QnFC or QnDC in 10cm3 of CH2Cl2 1 mmol of substrate dissolved in a
small amount of the solvent was added. The mixture was stirred at room temperature for the period

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N. Degirmenbas°i and B. Ozgun: Quinaldinium Fluorochromate and Dichromate
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indicated in Table 1. The progress of the reaction was followed by TLC (solvent, n-hexane:ethylacetate,
2:1, v=v). After the completion of the reaction the solid was filtered off, 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-dinitrophenylhydrazone (2,4-DNP).
Acknowledgements
The authors are grateful to the Research Foundation of Gazi University for supporting this study.
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