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Article

Efficient, Solvent-Free Oxidation of Organic Compounds with Potassium Dichromate in the Presence of Lewis Acids

Department of Chemistry, Isfahan University, Isfahan 81744, Iran
*
Author to whom correspondence should be addressed.
Molecules 2001, 6(11), 900-908; https://doi.org/10.3390/61100900
Submission received: 9 August 2001 / Revised: 25 September 2001 / Accepted: 22 October 2001 / Published: 31 October 2001

Abstract

:
The synthetic utility of potassium dichromate in the presence of Lewis acids under solid phase conditions is described. This reagent efficiently oxidizes alcohols, acyloins, oximes and semicarbazones to their corresponding carbonyl compounds, while trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and ketals undergo oxidative deprotection to produce carbonyl compounds efficiently.

Introduction

The total syntheses of complex molecules demand new methods in different areas of organic chemistry. Therefore, the development of reagents is always rewarding to synthetic organic chemistry. Oxidation is one of the most important classes of organic reactions from different points of view and effecting them under non-aqueous and aprotic conditions has found valuable applications in modern organic syntheses [1].
Potassium dichromate has been used extensively for the oxidation of different organic compounds under varied aqueous acidic conditions [1,2,3,4,5,6], but some of these methods suffer from disadvantages such as high reaction temperature, overoxidation of primary alcohols to carboxylic acids, long reaction times, low yields of the products and tedious workup. Its use as an oxidant under neutral conditions is also limited by its very low solubility in most organic solvents.
Heterogeneous reactions facilitated by reagents supported on various solid inorganic surfaces have gained special attention in recent years [7,8,9]. The advantages of these methods over conventional homogeneous reactions are typically higher selectivity, enhanced reaction rates, milder reaction conditions, cleaner products and manipulative simplicity. As a continuation of our ongoing work on development of environmentally benign methods using solid supports [10], we now report a convenient method for the oxidation of different classes of organic compounds with potassium dichromate in the presence of Lewis acids under solid phase conditions.

Results and Discussion

Potassium dichromate is a readily available and inexpensive reagent. The catalytic effects of several Lewis acids upon the activity of this oxidant were thoroughly studied. For this purpose the oxidation of benzhydrol to benzophenone was investigated in the presence of AlCl3, FeCl3, BiCl3, NiCl2, CeCl3, ZrCl4, SnCl2.2H2O, CuCl2.2H2O, MnCl2.4H2O and CoCl2.6H2O. The experimental results show AlCl3 to be the most effective catalyst for this purpose (Table 1).
Table 1. Percent Conversion of Benzhydrol to Benzophenone with K2Cr2O7 in the Presence of Different Lewis Acids Under Solid Phase Conditions
Table 1. Percent Conversion of Benzhydrol to Benzophenone with K2Cr2O7 in the Presence of Different Lewis Acids Under Solid Phase Conditions
Time (Min)
Lewis acid5101530
AlCl3100---
FeCl365757780
BiCl380859090
NiCl230506060
CeCl350657580
ZrCl470808585
SnCl2.2H2O30608590
CuCl2.2H2O60758090
MnCl2.4H2O25507580
CoCl2.6H2O60657075
Reagent ratio: Ph2CHOH: K2Cr2O7: AlCl3= 1:1:1
Primary and secondary benzylic alcohols are oxidized to their corresponding aldehydes and ketones with potassium dichromate in the presence of AlCl3 in high yield (Table 2). No overoxidation of primary alcohols to the corresponding carboxylic acids was observed under these conditions.
Table 2. Oxidation of Alcohols to Carbonyl Compoundsa Molecules 06 00900 i001
Table 2. Oxidation of Alcohols to Carbonyl Compoundsa Molecules 06 00900 i001
EntryR1R2Time (Min)Yields (%)b
1C6H5H595
22-MeOC6H4H395
33-MeOC6H4H490
44-MeOC6H4H398
53,4-(MeO)2C6H3H295
62-NO2C6H4H3072
73-NO2C6H4H3074
84-NO2C6H4H2575
94-ClC6H4H1093
104-BrC6H4H1587
115-MeFurylH488
12C6H5CH31095
134-ClC6H4CH31593
144-BrC6H4CH31590
154-PhC6H4CH31090
163,4-(MeO)2C6H3CH3398
17C6H5C6H5593c
182-PyridylC6H51580
19C6H5CH=CHH3065d
20C6H5C6H5CO1095
214-CH3OC6H44-CH3OC6H4CO396
22 Molecules 06 00900 i0021090
23 Molecules 06 00900 i0033010
24CH3(CH2)6H3010
a All of the products are known compounds and were identified by comparison with authentic samples. b Isolated yields. c In the absence of AlCl3, benzhydrol was converted to benzophenone in only 10% yield after 30 min with K2Cr2O7. d 10% of benzaldehyde was obtained from the reaction mixture.
α-Hydroxy ketones are converted to α-diketones in excellent yields without any carbon-carbon bond cleavage (entries 20, 21). Saturated alcohols such as cyclohexanol and 1-heptanol are resistant towards oxidation with this reagent and the corresponding carbonyl compounds are obtained in poor yields (entries 23, 24). The oxidation of larger amounts (5-10 mmol) of some alcohols was also investigated. The results were comparable to those of the small scale experiments, therefore, it seems that the methodology is also applicable for medium to large scale operations. In order to stress the selectivity of this method, an equimolar mixture of 4-methoxybenzyl alcohol and 1-heptanol was treated with an equimolar of potassium dichromate and aluminium chloride. The experimental results show that only 4-methoxybenzyl alcohol was oxidized selectively. It is noteworthy that many of the other chromium (VI) based oxidants either do not display such selectivity [11,12,13,14,15,16,17,18,19] or they require much longer reaction times [20,21]. Therefore, this selectivity represents a useful practical achievement in such oxidation reactions.
Potassium dichromate in the presence of aluminium chloride is also able to convert benzylic oximes and semicarbazones to their corresponding aldehydes and ketones in high yields (Table 3).
Table 3. Conversion of Oximes and Semicarbazones to Carbonyl Compoundsa Molecules 06 00900 i004
Table 3. Conversion of Oximes and Semicarbazones to Carbonyl Compoundsa Molecules 06 00900 i004
EntryR1R2GTime(Min)Yield(%)b
1C6H5HOH1092
22-MeOC6H4HOH590
34-MeOC6H4HOH595
43,4-(MeO)2C6H3HOH595
53-NO2C6H4HOH3075
64-NO2C6H4HOH3085
75-MeFurylHOH1085
8C6H5CH3OH1590
94-BrC6H4CH3OH1595
104-ClC6H4CH3OH1591
114-BrC6H4CH2BrOH585
124-PhC6H4CH3OH1586
13C6H5C6H5OH1585
14 Molecules 06 00900 i002OH1077
15 Molecules 06 00900 i003OH3010
16C6H5HNHCONH21090
174-CH3OC6H4HNHCONH2595
184-NO2C6H4HNHCONH23077
194-ClC6H4HNHCONH21087
203,4-(MeO)2C6H3HNHCONH2595
215-MeFurylHNHCONH2580
22C6H5CH3NHCONH21088
234-BrC6H4CH3NHCONH21085
244-ClC6H4CH3NHCONH21585
254-BrC6H4CH2BrNHCONH21092
264-PhC6H4CH3NHCONH22093
27C6H5C6H5NHCONH21090
28 Molecules 06 00900 i002NHCONH23088
29 Molecules 06 00900 i003NHCONH2305
a All of the products are known compounds and were identified by comparison with authentic samples. b Isolated yields.
Further oxidation of aldehydes to their carboxylic acids was not observed. Under the same reaction conditions, saturated oximes and semicarbazones are converted to their corresponding carbonyl parent compounds in poor yields (Table 3, entries 15, 29).
In contrast to the method described in this paper, deoximation using pyridinium chlorochromate (PCC) suffers from long reaction times (12-94 h), low yields and low selectivity [22]. PCC-H2O2 system is not sutiable for aldoximes and overoxidation products are usually produced and show low selectivity as well [23]. Therefore, this method is superior to PCC and PCC-H2O2 system in terms of reaction times, yields and selectivity.
In order to further assess the capabilities of this reagent, we decided to also perform oxidative deprotection of trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and ketals. The treatment of a variety of TMS and THP ethers, ethylene acetals and ketals with potassium dichromate in the presence aluminium chloride under solid phase conditions afforded the corresponding carbonyl compounds in high yields (Table 4 and Table 5).
Table 4. Conversion of TMS and THP Ethers to Carbonyl Compoundsa Molecules 06 00900 i005
Table 4. Conversion of TMS and THP Ethers to Carbonyl Compoundsa Molecules 06 00900 i005
EntryR1R2R3Time(Min)Yield (%)b
1C6H5HTMS1590
22-MeOC6H4HTMS1595
34-MeOC6H4HTMS1095
42-NO2C6H4HTMS3080
54-NO2C6H4HTMS3081
6C6H5CH=CHHTMS3065c
7C6H5CH3TMS2082
84-ClC6H4CH3TMS2585
94-BrC6H4CH3TMS2590
104-PhC6H4CH3TMS2577
113,4-(MeO)2C6H3CH3TMS1096
12C6H5C6H5TMS2595
134-ClC6H4C6H5TMS2090
14 Molecules 06 00900 i002TMS1585
15 Molecules 06 00900 i003TMS305
16C6H5HTHP595
172-CH3OC6H4HTHP595
194-CH3OC6H4HTHP396
203-NO2C6H4HTHP3075
214-NO2C6H4HTHP3080
224-ClC6H4HTHP1090
233,4-(MeO)2C6H3HTHP396
24C6H5CH3THP1087
254-ClC6H4CH3THP1080
264-PhC6H4CH3THP1590
27C6H5C6H5THP587
284-ClC6H4C6H5THP1590
29 Molecules 06 00900 i003THP305
a All of the products are known compounds and identified by comparison with authentic samples. b Isolated yields. c 15% of benzaldehyde was obtained from the reaction mixture.
Table 5. Conversion of Ethylene Acetals and Ketals to Carbonyl Compoundsa Molecules 06 00900 i006
Table 5. Conversion of Ethylene Acetals and Ketals to Carbonyl Compoundsa Molecules 06 00900 i006
EntryR1R2Time(Min)Yields(%)b
1C6H5H592c
23-MeOC6H4H590
34-MeOC6H4H395
43,4-(MeO)2C6H3H298
53-NO2C6H4H3075
64-ClC6H4H1090
7C6H5CH31095
84-ClC6H4CH31590
94-BrC6H4CH31586
104-PhC6H4CH32090
113,4-(MeO)2C6H3CH3396
124-BrC6H4CH2Br1092
13C6H5C6H52088
14 Molecules 06 00900 i0021585
15 Molecules 06 00900 i0033015
a All of the products are known compounds and identified by comparison with authentic samples. b Isolated yields. c 2-phenyl-1,3-dioxolane remained intact after 30 min in the presence of K2Cr2O7 without AlCl3

Conclusions

We present new methodology for the oxidation of organic compounds under solid phase conditions. In addition, the commercial availability and low cost of the reagent, high yields of the products, mild reaction conditions, easy workup and short reaction times are noteworthy advantages of this method and make this reagent practical bench-top oxidant.

Experimental

General

All of the starting materials used in this work are commercially available or were prepared according to published procedures [24,25,26,27]. All of the products are known compounds and were identified by comparison of their physical and spectral data with those of authentic samples. Reported yields refer to isolated pure products.

General Procedure for the Oxidation of Organic Compounds

A mixture of substrate (1 mmol), potassium dichromate (1-2 mmol) and aluminium chloride (1 mmol) in a mortar was ground with a pestle for the time specified in Table 2, Table 3, Table 4 and Table 5. The progress of the reaction was monitored by TLC or GLC. The mixture was extracted with CH2Cl2. The solvent was evaporated and the resulting crude material was purified on a silica-gel plate or silica-gel column with appropriate eluent to afford the pure product (Table 2, Table 3, Table 4 and Table 5). In oxidation of alcohols, 1 mmol K2Cr2O7 and in other cases, 2 mmol K2Cr2O7 was used.

Acknowledgements

We are thankful to the Isfahan University Research Council for partial support of this work.

References

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  • Sample Availability: All products reported in this paper are available from the authors.

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MDPI and ACS Style

Mohammadpoor-Baltork, I.; Sadeghi, M.M.; Adibi, A.-H. Efficient, Solvent-Free Oxidation of Organic Compounds with Potassium Dichromate in the Presence of Lewis Acids. Molecules 2001, 6, 900-908. https://doi.org/10.3390/61100900

AMA Style

Mohammadpoor-Baltork I, Sadeghi MM, Adibi A-H. Efficient, Solvent-Free Oxidation of Organic Compounds with Potassium Dichromate in the Presence of Lewis Acids. Molecules. 2001; 6(11):900-908. https://doi.org/10.3390/61100900

Chicago/Turabian Style

Mohammadpoor-Baltork, Iraj, Majid M. Sadeghi, and Abol-Hassan Adibi. 2001. "Efficient, Solvent-Free Oxidation of Organic Compounds with Potassium Dichromate in the Presence of Lewis Acids" Molecules 6, no. 11: 900-908. https://doi.org/10.3390/61100900

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