Oxidative transformation of alcohols and organic halides in aqueous solution

Article abstract of DOI:10.1039/c4nj00393d

An efficient and new methodology for a one pot conversion of organic halides and alcohols to their corresponding aldehydes or ketones has been developed using quinolinium chlorochromate in an aqueous medium without adding any cosolvent. The same protocol has been applied for the oxidative conversion of 2-(bromomethyl)-5,7-dinitro-1H-indole to its corresponding aldehyde in the presence of sodium hydroxide.

Full text of DOI:10.1039/c4nj00393d

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Oxidative transformation of alcohols and organic
halides in aqueous solution†
Cite this: New J. Chem., 2014,
38, 3749
Neeraj Gupta,* Apoorva Thakur and Pushpa Bhardwaj
An efficient and new methodology for a one pot conversion of organic halides and alcohols to their
corresponding aldehydes or ketones has been developed using quinolinium chlorochromate in an
aqueous medium without adding any cosolvent. The same protocol has been applied for the oxidative
conversion of 2-(bromomethyl)-5,7-dinitro-1H-indole to its corresponding aldehyde in the presence of
sodium hydroxide.
Received (in Montpellier, France)
15th March 2014,
Accepted 24th April 2014
DOI: 10.1039/c4nj00393d
www.rsc.org/njc
transformations was Cr based oxidants such as the Jones, Sarrett
and Collins reagents.21 Most of these are non-selective, undergo
Introduction
New and alternative methods for chemical synthesis1 are in rapid deactivation and produce carboxylic acids as an end
great demand to properly harness the benefits of chemistry. product. The controlled oxidation of alcohols or alkyl halides
A few alternatives currently available include the use of a new class of to an aldehyde is a difficult task; in order to overcome these
solvents such as ionic liquids, catalytic and atom economic reactions limitations organic derivatives of Cr(VI) have been prepared and
together with efficient energy techniques such as microwaves and are being extensively used in non aqueous solvents21 for
ultrasonic irradiation. Adding to this, organic transformations in different synthetic endeavors.
aqueous media2 have gained much attention in the last two decades.
Chlorochromate, fluorochromate and dichromate derivatives of
As water is a readily available, economical and harmless solvent, pyridine22 and quinoline23 are commonly used organic derivatives
it has been the focus of synthetic chemists in recent times; a lot of Cr(VI). These derivatives were found to be better in their reactivity
of chemical reactions previously considered impossible in water and selectivity compared to the traditional Cr oxidants, and offer a
are feasible today, including benzoin condensation,3 allylation benefit of solubility in organic solvents. Recently, guanidinium
reactions,4,5 Suzuki Coupling,6 Barbier type alkylation,7 Friedel– chlorochromate [GCC] has been used in the presence of a phase-
Crafts8 and Mannich reactions.9
transfer catalyst24 for the oxidation of different benzylic and
The oxidative transformation of halides and alcohols to their aliphatic alcohols in water. Pyridinium chlorochromate [PCC] has
corresponding carbonyl compounds is of great importance during been used to oxidize 5,6-dihydropyrans25 to the corresponding
the synthesis of fine or special chemicals, such as medicinally or anhydromevalonolactone in dichloromethane in good to excellent
biologically active compounds, perfumes and agrochemicals.10 yields. Quinolinium fluorochromate [QFC] was used for the oxida-
Traditionally, the oxidation of alcohols was carried out using metal tion of alcohols and phenanthrene26 in dichloromethane. It was
based oxidizing reagents11 such as those of chromium12 [Cr(VI)] and also used for the oxidative conversion of oximes, tosylhydrazones
manganese13 [Mn(VII)]. In addition to these, other reagents such and N,N-dimethylhydrazones27 to corresponding carbonyl com-
as tertiary butyl hydroperoxide14 (TBHP), hydrogen peroxide15 pounds in good to excellent yields. Another analogue, quinolinium
and sodium hypochlorite16 have recently been utilized for these chlorochromate [QCC], was applied to the mild and selective
reactions, particularly in association with catalysts.17 On the oxidation of alcohols,28 aromatic anils,29 lactic and glycolic acids30
contrary, oxidation of organic halides to the corresponding and methionine,31 all in non-aqueous organic solvents.
aldehydes and ketones is relatively difficult.18 The classical
QCC is a yellow to brown solid, which is not very sensitive to
methods for this transformation are the Hass–Bender19 and moisture and is stable32 in aqueous solutions for a considerable
Sommelet reactions.20 The obvious choice for these oxidative amount of time. Quinolinium ions in QCC provide selectivity28
and stability to the reagent in comparison to pyridinium and
Department of Chemistry, Faculty of Basic Sciences, Shoolini University, Bajhol,
isoquinolinium analogues such as PCC and isoquinolinium
chlorochromate [IQCC]. In fact, quinolinium ions are better at
providing selective oxidation of secondary alcohols in the
Solan, H.P, India-173229. E-mail: neerajgupta@shooliniuniversity.com,
gupta_nrj@yahoo.co.in; Fax: +91-1792-308000; Tel: +91-867-971-2395
† Electronic supplementary information (ESI) available: Spectroscopic data of
presence of primary alcohols and can be used for the oxidation
of a wide variety of substrates. This selectivity is probably a result
indole derivatives and the methodology used for the removal of chromium. See
DOI: 10.1039/c4nj00393d
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of the different charge densities on the chromium center in
various chlorochromate analogues. Quinolinium ions, being
bigger in size in comparison to pyridinium ions, have a lower
positive charge per unit area and hence stabilize the negative
charge in the chromium center to a lesser extent. The reagent is
even more selective than IQCC, which is a result of different
orientations of the nitrogen atom28 in the respective cations.
Kinetic studies during the oxidation of different substituted
benzaldehydes, under pseudo-first-order conditions using QCC,
have revealed that both electron-releasing and electron-
withdrawing substituents can increase the rate of the reaction.
This indicates that the reaction operates through two different
mechanisms and the reagent is effective in performing the
oxidation of electron rich and electron deficient substrates with
equal ease. Certain oxidation reactions such as the oxidation of
3-methyl-2,6-diphenyl-piperidin-4-ones33 are accelerated by the
presence of electron releasing substituents, which may be due to
the increase in electron density in the reaction center. QCC is also
employed for the oxidation of benzaldehyde34 in the presence of
cetyl trimethylammonium bromide in acidic aqueous solutions.
Kinetic studies have revealed that the reaction rate is increased by
the presence of surfactant due to the formation of micelles.
Furthermore, organic acids such as acetic acid decrease the rate
of reaction in aqueous solutions.35
Most of the oxidation processes are either performed in
organic solvents or in aquo-organic mixtures. Furthermore,
methodologies describing oxidation in aquo-organic mixtures are
mainly focused on kinetic studies and do not provide synthetic
details. In addition, there is no work which addresses the problem of
waste generation at the end of a reaction. The work described in this
paper has the potential to develop a solventless synthetic procedure
using QCC in aqueous solution, it is equally effective for natural
product synthesis and it generates clean waste at the end of reaction.
Due to the wide demand for the development of new synthetic
methodologies in aqueous media, and our continuing endeavor
to use new and alternative media for organic synthesis,36,37 we
are reporting to the best of our knowledge, the first use of QCC
for the oxidation of different alcohols and halides in an aqueous
medium, without using any co-solvent.
Scheme 1 Oxidation of alcohols with QCC.
benzyl alcohol, at an ambient temperature for 3 h, and observed
that its conversion to benzaldehyde was 50%, as determined by
1H NMR analysis. We were excited to see that the reagent worked
effectively in pure water, and our focus was then to increase the
solution yield further without adding any co-solvent. We there-
fore further increased the reaction temperature to 45–50 1C, but
no appreciable change was observed. Increasing the reaction
time to 12 h did not result in a change in the yield. Finally, we
were able to achieve an increased yield of benzaldehyde by taking
1.5 mol equivalents of QCC at an ambient temperature (Fig. 1).
We were able to oxidize a variety of alcohols (1 mmol) with QCC
(1.5 mmol) in water by stirring the contents at 25–30 1C for
4–14 h, and the various results obtained are compiled in
Table 1. The progress of the reaction was monitored by TLC
and the isolated products were characterized by IR and 1H NMR
spectroscopy. Water immiscible liquid products, such as
benzaldehyde, formed a distinct layer on the water at the end
of the reaction and were recovered by distillation without using
any organic solvent for the extraction.
Oxidation processes in water are facilitated by the polar
nature and stoichiometric use of QCC, which has the tendency to
go into water and make the medium suitable for organic reactions
by increasing the solubility. Mechanistically, the oxygen atom in
alcohol coordinates to the Cr(VI) atom forming a chromate ester
by displacing chlorine, which then acts as a base, abstracting a
proton and finally resulting in the oxidation of alcohol to the
respective carbonyl compound. Non-bonded electron pairs in
organic halides may show a similar interaction with the Cr centre
(Fig. 2), as shown by the oxygen atom in alcohols, resulting in
the formation of an intermediate analogous to chromate esters.
Results and discussion
Organic reactions in aqueous media have attracted great attention
in recent years because of the benefits water offers as a solvent,
such as cost effectiveness, non-toxicity and simplified work up.
Furthermore, the miscibility of different organic salts such as
ionic liquids38 and chromium salts, can be controlled by varying
the cationic or anionic parts. Prompted by this thought, we have
utilized a mild and selective reagent QCC for developing a new
methodology for oxidative transformations in water. Herein, we
are reporting the oxidation of different alcohols (Scheme 1) and
halides without using any co-solvent in water at an ambient
temperature using QCC.
Initially, we prepared QCC by the reported method23 and tested
its effectiveness in water for the oxidation of 1 mol equivalent of
Fig. 1 Oxidation of 1 mmol of benzyl alcohol with different quantities
of QCC.
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Table 1 Comparative yields during oxidation of alcohols with QCC
S. no.
R
R0
Time (h)
Yield (%)
1
2
3
4
5
C6H5
4-(CH3O)C6H4
4-(Cl)C6H4
4-(NO2)C6H4
Cyclohexyl (including C
in general formula)
C2H5
H
H
H
H
—
4
4
4
4
14
62
81
76
65
64
Scheme 2 Oxidation of halides with QCC.
6
7
CH3
H
14
14
77a
52
C5H11
in water (3 mL), at a temperature of 45–50 1C for 2.0–2.5 h,
followed by the addition of QCC (1.5 mmol) at room tempera-
ture (Scheme 2). The results obtained are compiled in Table 2.
The progress of the reactions was monitored via TLC and the
isolated products were characterized with the help of IR and
a
Product was not isolated and only a percentage conversion from
1H NMR is reported.
This can prove beneficial for the one pot conversion of halides 1H NMR spectroscopy. It is likely that the reaction involves the
directly to their corresponding carbonyl compounds (aldehyde or nucleophilic displacement of a halide with a hydroxyl group
ketone), therefore we next attempted the conversion of benzyl (generated from NaOH) to form an alcohol, which was used
chloride to benzaldehyde in water using QCC, by keeping all the in situ for the oxidation.
parameters constant. Contrary to our predictions, no conversion
To ascertain the efficacy of the process for organic synthesis
to benzaldehyde was reported in this case. We also performed the and promote its use in process chemistry, we further applied
reaction at elevated temperatures up to refluxing and by adding a the methodology to the oxidation of an indole derivative,
co-solvent THF, but no conversion to aldehyde was reported.
namely 2-(bromomethyl)-5,7-dinitro-1H-indole (3), which was
The addition of an equimolar quantity of a hydroxide base may synthesized by Fischer indole synthesis.39,40 This oxidation will
result in nucleophilic substitution of the halide to the corre- in turn, open a route for the synthesis of 2-substituted indole
sponding alcohol, which may be used in situ for the generation aldehydes from the corresponding halide in aqueous media,
of benzaldehyde. Therefore, we next performed the reaction in a which can be used further for the generation of a chiral centre.
basic medium using one mole equivalent of NaOH at a tempera- In the first stage we prepared 1-bromopropane-2-one by the
ture between 45–50 1C, followed by the addition of QCC at room reported method,41 by adding bromine to acetone in the presence
temperature that resulted in a much lower conversion (o10%) of of CaCO3 at 0–5 1C, and then stirring the contents at room
halide to aldehyde. Next, we doubled the molar equivalents of the temperature for 3 h to afford the crude product, which was then
base with respect to the halide and an increase in the product purified by vacuum distillation.
yield up to 38% was observed. Further optimization revealed that
We next synthesized the corresponding hydrazone 1-(1-bromo-
aromatic halides are converted into the corresponding carbonyls propan-2-yl)-2-(2,4-dinitrophenyl) hydrazine (2) by reacting
in 7–8 h, whereas no conversion for aliphatic chlorides such as equimolar amounts of 2,4-dinitrophenyl hydrazine (1) with
chlorohexane and 2-chlorobutane was observed under similar 1-bromopropane-2-one in ethanol in the presence of catalytic
conditions. Thinking that solubility might hinder the oxidation amounts of acetic acid, which was recrystallized in ethanol to
process, we also attempted it in a mixture of water–THF (8 : 2), but afford the pure product. Finally, the corresponding halide 3
no change was reported in this case.
was obtained by cyclisation of 2 at an elevated temperature
We were therefore able to convert the respective halide (115–125 1C) with polyphosphoric acid (PPA) and it was also
(1 mmol) into an aldehyde by stirring it with NaOH (2 mmol) purified by re-crystallisation in ethanol.
Fig. 2 Plausible complex formation by the interaction of alcohols and halides with QCC.
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Table 2 Comparative yields during oxidation of halides with QCC
S. no.
R
R0
Time (h)
Yield (%)
1
2
3
C6H5
4-(Cl)C6H4
Cyclohexyl (including C
in general formula)
C2H5
H
H
—
8
8
18
38
35
37
4
5
CH3
H
18
18
—
—
C5H11
After the synthesis of 2-(bromomethyl)-5,7-dinitro-1H-indole
(3) we moved our attention towards its oxidation with QCC
in an aqueous medium under the optimized conditions
(Scheme 3). We were successful in obtaining the corresponding
carbonyl derivative 4 by reacting 3 with QCC in an alkaline
medium. Product formation at different stages was also
ascertained by UV-visible absorption spectroscopy where the
formation of an indole nucleus in 3 was marked with a shift
Fig. 3 Comparative UV-visible absorption spectrum of 1, 2, 3 and 4.
towards higher wavelengths (Fig. 3). This shift is observed due from water. Initially, we determined the amount of Cr6+ leached
to extending conjugation as we move from compound 2 to 3. into water49 by dissolving a definite amount of QCC into the
Furthermore, the mass spectrum of 3 has two peaks of equal water. We found that 60% of the QCC remained suspended in
intensity at m/z 298 and 300.
water and 40% gets dissolved in it. The heterogeneous mixture
In order to increase the scope of the developed methodol- was filtered to remove undissolved QCC. The obtained filtrate
ogy, we were interested in removing the chromium ions was reduced with FeSO4 and coagulated by adding Ca(OH)2.
completely from the aqueous waste. Cr+6 has been considered The precipitates were removed by filtration to remove solid
as the toxic form whereas Cr3+ has been considered as an waste and the aqueous solution was checked for traces of
essential nutrient component for metabolizing42,43 fats, sugars chromium ions that were found to be absent.
and proteins. In the literature several methods are available for
Once we were sure that the method was capable of removing
the removal of chromium, such as ion exchange,44 reverse Cr6+ ions from water, after reduction with FeSO4, we treated the
osmosis,45 adsorption46 and the most common method for waste generated at the end of the reaction. The reaction mass
the removal of Cr6+ is its reduction to Cr3+, followed by its was first filtered to remove undissolved solid and the filtrate was
coagulation and filtration. Various reducing agents including then reduced with inexpensive FeSO4. The solid precipitates
metal salts and metals themselves, including ferrous sulfate,47 obtained after coagulation with Ca(OH)2 were removed by filtra-
iron48 etc. can be used for this purpose.
tion and the mother liquor obtained was found to be acidic
The aqueous waste obtained in the reaction was first (pH = 3.97). It was made neutral by adding 0.1 N Ca(OH)2 and
reduced to a Cr3+ species which was subsequently removed was finally discarded.
Scheme 3 Oxidation of 2-(bromomethyl)-5,7-dinitro-1H-indole with QCC.
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Conclusions
Notes and references
We have developed a new protocol for the oxidative transforma-
tion of alcohols and organic halides to aldehydes using QCC in
aqueous solution. The method is efficiently used under solvent-
less conditions for the oxidation process and generates clean
waste after its reduction with FeSO4. This work demonstrates an
efficient use of QCC in natural product synthesis and provides
an insight into the reactions that need chromium species
exclusively for their transformations, such as 5-hydroxymethyl
preparation50 (5-HMF) from biomass resources.
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Experimental
(a) General procedure for the oxidation of alcohols
Alcohol (5 mmol) was added to water (15 mL) at room tempera-
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Acknowledgements
We thank Dr Sudhanshu Pratap Singh and Dr Dilbag Rana for
assisting in titrations and providing timely literature.
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