Quinoxalinium dichromate: A new and efficient reagent for the oxidation of organic substrates

Article abstract of DOI:10.1007/s00706-002-0476-9

A new chromium(VI) reagent, quinoxalinium dichromate ([C₈H₆N₂H]₂Cr₂O ₇, QxDC), was prepared by reacting quinoxaline with CrO₃ in water. QxDC oxidizes primary and secondary alcoho

Full text of DOI:10.1007/s00706-002-0476-9

Monatshefte f u r Chemie 133, 1417±1421 ꢀ2002)
DOI 10.1007/s00706-002-0476-9
Quinoxalinium Dichromate: A New
and Ef®cient Reagent for the Oxidation
of Organic Substrates
| and Beytiye Ozg uÈ nÃ
È
Nebahat De gÆ irmenbas
°

Gazi Universitesi, Fen-Edebiyat Fak u ltesi, Kimya B o l u m u , TR-06500 Ankara, Turkey
Received February 2, 2002; accepted ꢀrevised) April 3, 2002
Published online July 15, 2002 # Springer-Verlag 2002
Summary. A new chromiumꢀVI) reagent, quinoxalinium dichromate ꢀ[C H N H] Cr O , QxDC),
2 7
8
6
2
2
was prepared by reacting quinoxaline with CrO in water. QxDC oxidizes primary and secondary
3
alcohols and oximes to the corresponding carbonyl compounds, and anthracene to anthraquinone.
Keywords. Quinoxalinium dichromate; Oxidation; Alcohols; Oximes; Carbonyl compounds.
Introduction
In recent years, chromiumꢀVI)-based oxidizing reagents have been extensively
worked on, leading to the development of a number of reagents which have found
wide application in many types of oxidative transformations [1±2]. Pyridinium
chlorochromate ꢀPCC) [3], pyridinium dichromate ꢀPDC) [4], pyridinium ¯uoro-
chromate ꢀPFC) [5], pyridinium bromochromate ꢀPBC) [6], quinolinium ¯uoro-
chromate ꢀQFC) [7], and 3,5-dimethylpyrazolium ¯uorochromate ꢀDmpzHFC) [8]
have turned out as the most signi®cant chromiumꢀVI) oxidants. As part of a program
aiming at new reagents allowing both oxidations and brominations to be performed
under mild conditions we have recently prepared four new reagents: quinolinium
bromochromate ꢀQBC) [9], benzimidazolium bromochromate ꢀBIBC) [10], quinal-
dinium bromochromate ꢀQnBC) [11], and isoquinolinium bromochromate ꢀIQBC)
[
12]. The chromium-based reagents developed so far all suffer from at least one of
the following drawbacks: high acidity, photosensitivity, instability, tedious workup
procedures, or requirement of large excess of reagent. Therefore, the search for a
new reagent persisted which has now led to the synthesis of quinoxalinium dichro-
mate ꢀ[C H N H] Cr O , QxDC).
8
6
2
2
2 7
à Corresponding author. E-mail: beytiyeozgun@hotmail.com


N. De gÆ irmenbas
°
ꢀ and B. Ozg u n
1
418
Results and Discussion
Quinoxalinium dichromate ꢀQxDC) can be easily prepared in good yield ꢀ78%) by
addition of quinoxaline to a solution of chromium trioxide in water in a molar ratio
of 1:1. QxDC is a yellow, non-hygroscopic, and stable solid compound which can be
stored in the darkness for months without losing its activity. The structure of
the product was con®rmed by elemental analysis and its IR spectrum ꢀnujol). The
À 1
infrared absorption frequencies for the dichromate group at 950, 900, and 787 cm
in QxDC are attributable to ꢀasymꢀCrO ), ꢀ ꢀCrO ), and ꢀ ꢀCr±O±Cr); these
sym
3
sym
3
assignments are in accordance with those found for ꢀNH ) Cr O [13]. It is soluble
4
2
2 7
in dimethyl formamide and dimethylsulfoxide, sparingly soluble in dichloro-
methane, acetonitrile, and chloroform, and insoluble in benzene, toluene, ether,
nitrobenzene, and ethyl acetate. These results are indicative of the ionic nature of
ꢀ
QxDC. The reagent melts sharply at 115±116 C. The compound is diamagnetic. It is
2
À 1
a 2:1 electrolyte ꢀà ˆ 210 mho Á cm Á mol , acetonitrile), showing a pH of 2.30
M
in a 0.01 M aqueous solution. The pH values of 0.01 M solutions of PCC, PFC, and
QFC were found to be 1.75, 2.45, and 2.65 [3, 5, 7]. Thus, the acidity of QxDC is less
pronounced than that of PCC, but comparable to that of PFC and QFC.
In order to ascertain the ef®cacy of the reagent as an oxidant, it was tested on a
wide array of substrates in dichloromethane at room temperature. QxDC readily
oxidizes primary ꢀTable 1, entries 1±6) and secondary alcohols ꢀTable 1, entries
7
±10) to their corresponding aldehydes and ketones ꢀScheme 1) and was found to
a
Table 1. Oxidation of organic substrates with QxDC
b
d
Entry
Substrate
Product
Uncatalyzed
Catalyzed
c
c
t=h
Yield=%
t=h
Yield=%
1
2
3
4
5
6
7
8
CH ꢀCH ) CH OH
3
CH ꢀCH ) CHO
2 6
4
692
69
9
71
2 6
2
3
4
4
4
4
4
4
4
2
71
2
2
9
2
84
69
64
62
68
2
87
2
71
2
2
2
65
64
70
9
4
79
2
81
ꢀcontinued)

Quinoxalinium Dichromate as a Versatile Oxidant
1419
Table 1 ꢀcontinued)
b
d
Entry
Substrate
Product
Uncatalyzed
Catalyzed
c
c
t=h
Yield=%
t=h
Yield=%
1
0
4
71
73
2
73
1
1
1
4
2
75
2
3
6
6
74
73
3
3
74
78
1
a
Oxidations were carried out in dichloromethane with a substrate to oxidant ratio of 1:1.5 ꢀ1:3 for
b
anthracene) at room temperature; all products were identi®ed by comparison of their physical and
c
d
spectroscopic data with those of authentic samples; isolated yields; in the presence of anhydrous
acetic acid as catalyst
Scheme 1
be an effective reagent in the oxidative deoximation of several oximes to their
parent carbonyl compounds ꢀScheme 1) in reasonably good yields ꢀTable 1, entries
12, 13). QxDC in dichloromethane also oxidizes anthracene to anthraquinone in
good yield ꢀTable 1, entry 11).
The oxidation reactions for all the substrates were also performed in dichloro-
methane at room temperature in the presence of anhydrous acetic acid as catalyst
ꢀ
Table 1). The results clearly demonstrated the catalytic effect of anhydrous acetic
acid. The reactions are in all cases is markedly accelerated, and the reaction time is
reduced. In most cases, there is some improvement in the overall yield of the
product as well.
The stability and activity of QxDC was investigated during different periods of
its storage. The results are summarized in Table 2.
In conclusion, the easily prepared new reagent QxDC has a number of advan-
tages over naphthyridinium dichromate ꢀNapDC) [14], PCC [3], quinolinium di-
chromate ꢀQDC) [15], and PDC [4] in considering the amount of oxidant, acidity,
percent yield, and reaction time. The reduced chromium species can be trapped on
a silica gel column for safe disposal. Moreover, it is stable and can be stored for
long periods without much loss in its activity and hence turns out to be a very
useful reagent in synthetic organic chemistry.


1
420
N. De gÆ irmenbas
°
ꢀ and B. Ozg u n
a
Table 2. Oxidation of benzyl alcohol using QxDC after various storage periods
b,c
Yield =%
Oxidant
Storage period=week
Substrate=Oxidant
molar ratio)
Reaction
ꢀ
period=h
QxDC
1
2
4
6
1:1.5
1:1.5
1:1.5
1:1.5
4
4
4
4
69
66
62
56
a
b
Oxidations were carried out in dichloromethane at room temperature; all products were identi®ed
c
by comparison of their physical and spectroscopic data with those of authentic samples; isolated
yields
Experimental
Preparation of quinoxalinium dichromate
3
A solution of 26.4g quinoxaline ꢀ0.2mol) in 60cm H O was slowly added to a cooled solution of
2
3
3
2
1.0g CrO ꢀ0.2mol) in 20cm H O. After 0.5 h, the reaction mixture was diluted with 40cm
3 2
ꢀ
acetone, cooled to À 15 C, and the orange solid was ®ltered, washed with acetone, and dried in vacuo.
ꢀ
M.p.: 115±116 C; yield: 78%; calcd. for [C H N H] Cr O : C 40.32, H 2.94, N 11.76, Cr 21.73;
8
6
2
2
2 7
found: C 40.17, H 2.93, N 11.72, Cr, 21.76.
General procedure for the oxidation of substrates
The oxidations were conducted in a dry apparatus and under ef®cient stirring. To a thoroughly stirred
3
suspension of 1.5±3 mmol QxDC in 10 cm CH Cl , 1 mmol of substrate dissolved in a small amount
2
2
of solvent was added. The mixture was stirred at room temperature for the period indicated. The
progress of the reaction was followed by TLC ꢀn-hexane:ethylacetate ˆ 2:1). After completion of the
reaction, diethyl ether was added to the mixture which was then passed trough a previously prepared
short column of silica gel. Elution of the carbonyl compound was effected with diethyl ether. Evapora-
tion of the solvent afforded the pure product in 62±92% yield.
3
In the case of catalyzed oxidation reactions, 100mm of anhydrous acetic acid were added to the
well stirred solution of QxDC in CH Cl . After stirring for at least 5 min, the substrate was added in
2
2
neat form to the oxidant-catalyst solution. The further procedure is same as described above.
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, vol 19. Springer,
Berlin
[
[
[
[
[
2] Hudlicky M ꢀ1990) Oxidation in Organic Chemistry. American Chemical Society, Washington
3] Corey EJ, Suggs JW ꢀ1975) Tetrahedron Lett 2647
4] Corey EJ, Schmidt G ꢀ1979) Tetrahedron Lett 399
5] Bhattacharjee MN, Chaudhuri MK, Dasgupta HS, Roy N ꢀ1982) Synthesis 588
6] a) Narayanan N, Balasubramanian TR ꢀ1986) Indian J Chem 228; b) Durai S, Ravikumar V,
Narayanan N, Balasubramanian TR, Mohan S ꢀ1987) Spectrochim Acta 43A: 1191
7] Murugesan V, Pandurangan A ꢀ1992) Indian J Chem 31B: 377
[

Quinoxalinium Dichromate as a Versatile Oxidant
1421
[
[
8] Bora U, Chaudhuri MK, Dey D, Kalita D, Kharmawphlang W, Mandal GC ꢀ2001) Tetrahedron
7: 2445
5

9] Ozg u n B, De gÆ irmenbas
°
ꢀ N ꢀ1996) Synth Commun 26ꢀ19): 3601
ꢀ N ꢀ1999) Synth Commun 29ꢀ5): 763

[
[
10] Ozg u n B, De gÆ irmenbas
11] De gÆ irmenbas
°

°
ꢀ N, Ozg u n B ꢀ2002) Journal of the Institute of Science and Technology, Gazi
University ꢀsubmitted)

kuner O, De gÆ irmenbas

[
12] Cos°
°
Technology, Gazi University ꢀsubmitted)
ꢀ N, Ozg u n B ꢀ2002) Journal of the Institute of Science and
[13] Stammerich H, Bassi D, Sala O, Seibert H ꢀ1958) Spectrochim Acta 13: 19 2
[14] Harry BD, Roger MS, William WP, Garry LG ꢀ1984) Heterocycles 22: 2029
[15] Balasubramanian K, Prathiba V ꢀ1986) Indian J Chem 25B: 326