Mild oxidative conversion of nitroalkanes into carbonyl compounds in ionic liquids

Article abstract of DOI:10.1080/00397910903267921

Basic hydrogen peroxide and sodium perborate were found to be cheap and efficient alternatives for the conversion of primary and secondary nitro to carbonyl compounds (Nef reaction) in ionic liquids. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903267921

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Mild Oxidative Conversion of
Nitroalkanes into Carbonyl Compounds in
Ionic Liquids
Olga Bortolini a , Antonio De Nino a , Angelo Garofalo a , Loredana
Maiuolo a & Beatrice Russo a
a Department of Chemistry , Università della Calabria , Rende, Italy
Published online: 26 Jul 2010.
To cite this article: Olga Bortolini , Antonio De Nino , Angelo Garofalo , Loredana Maiuolo & Beatrice
Russo (2010) Mild Oxidative Conversion of Nitroalkanes into Carbonyl Compounds in Ionic Liquids,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 40:16, 2483-2487, DOI: 10.1080/00397910903267921
To link to this article: http://dx.doi.org/10.1080/00397910903267921
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Synthetic Communications1, 40: 2483–2487, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903267921
MILD OXIDATIVE CONVERSION OF NITROALKANES
INTO CARBONYL COMPOUNDS IN IONIC LIQUIDS
Olga Bortolini, Antonio De Nino, Angelo Garofalo,
Loredana Maiuolo, and Beatrice Russo
`
Department of Chemistry, Universita della Calabria, Rende, Italy
Basic hydrogen peroxide and sodium perborate were found to be cheap and efficient
alternatives for the conversion of primary and secondary nitro to carbonyl compounds
(Nef reaction) in ionic liquids.
Keywords: Hydrogen peroxide; ionic liquids; Nef reaction; perborate
The conversion of nitroalkanes into their corresponding carbonyl compounds (the
Nef reaction) is an important synthetic transformation that overturns the polarity
of the adjacent carbon atom from nucleophilic to electrophilic.[1] Several methods
employing oxidative or reductive conditions as well as nitrosating agents have been
proposed for this reaction over the years, with increasingly high levels of chemoselec-
tivity. Permanganate and similar modified reagents, such as KOH=MeOH=KMnO4,
are the oxidants most widely used,[2] together with peroxide-based systems such as
dimethyldioxirane,[3,4] bis-(trimethylsilyl)-peroxide,[5] sodium percarbonate,[6] or
carbonate=H2O2,[7] O3,[8] and 1O2.[9] However, only some of these methods are
successful for the conversion of primary nitroderivatives to aldehydes, because
further transformation to carboxylic acid is frequently observed or the reaction
results in an intractable mixture.[10]
Basic aqueous H2O2 is a well-known, inexpensive, and environmentally
benign reagent mostly used for the transformation of electron-deficient olefins to
the corresponding epoxides (Weitz–Scheffer reaction). Recently, we have adapted
this reaction to ionic liquids (ILs),[11] a novel reaction medium and alternative to
traditional volatile and carcinogenic solvents, which allows for easy product recov-
ery, different product selectivity, and, most important, solvent recycling. Also,
sodium perborate (SPB) and sodium percarbonate (SPC) are solid peroxygen com-
pounds with great storage stability and no shock sensitivity, which function as a
source of moderately alkaline H2O2.[12,13] They are cheap, safe, easy to handle,
and readily available, being present in many domestic washing formulations. In this
Received August 3, 2009.
Address correspondence to Olga Bortolini, Department of Chemistry, Universita della Calabria,
via Bucci 12 C, 87036 Rende, CS, Italy. E-mail: o.bortolini@unical.it
`
2483

2484
O. BORTOLINI ET AL.
communication, we describe the conversion of different primary and secondary
nitroalkanes with basic hydrogen peroxide, from different precursors in ionic liquids,
leading to the corresponding carbonyl compounds as efficient alternatives to the tra-
ditional Nef transformation. A comparison of the different oxidants used (i.e.,
H2O2=OHꢀ,[14] SPB) and a simple procedure for the recovery and reuse of the ILs
are included.
Nitroalkanes are among the most versatile intermediates in organic synthesis
in view of their high reactivity and of the number of possible further transforma-
tions. The Kornblum reaction of an alkyl halide with potassium or sodium nitrite,
in dipolar solvents, is the most direct route to nitroalkanes.[15] The conversion
of nitro compounds 1–8 to carbonyl derivatives with basic H2O2 or SPB in
bmim(OTf) (bmim ¼ 1-butyl-3-methyl imidazolium; OTfꢀ ¼ CF3SO3ꢀ)[16] proceeds
in acceptable reaction times (3–7 h) with the formation of the corresponding alde-
hydes or ketones in conversions usually greater than 85% and yields in the range
60–96% (Table 1).
Table 1. Conversion of nitroderivates to carbonyl compounds with basic hydrogen peroxide and sodium
perborate in bmim(OTf) (1 mL); substrate=oxidant=base 1:1.5:2
Conv. (%)a
Entry
Substrate
Oxidant
Product
Time (h) [Yield (%)]b
1
2
H2O2=OHꢀ
SPB
3c
3d
97 (95)
[98 (95)]
3
4
H2O2=OHꢀ
SPB
3c
3d
98 (96)
[98 (95)]
5
6
H2O2=OHꢀ
SPB
5c
7d
97 (93)
[98 (94)]
7
H2O2=OHꢀ
SPB
3
6
85 (82)
[90 (78)]
8
9
10
H2O2=OHꢀ
SPB
4
6
75 (72)
[80 (75)]
11
12
H2O2=OHꢀ
SPB
4
6
70 (65)
[70 (60)]
13
14
H2O2=OHꢀ
SPB
3
65 (60)
15
16
H2O2=OHꢀ
SPB
5
18
60 (40)
[67 (51)]
aDetermined by GC.
bIsolated yield.
cRoom temperature.
d70 ꢁC.

NITROALKANES TO CARBONYL COMPOUNDS IN ILs
2485
The reaction is effective with simple cyclic and acyclic nitro compounds and
particularly efficient for the conversion of primary nitroderivatives to aldehydes,
which are obtained in yields ranging from very good to reasonable. For oxidations
employing SPB, heating is required (70 ꢁC), because this oxidant is less effective than
H2O2=OHꢀ. The reaction workup is fast and simple, consisting of the direct extrac-
tion of the carbonyl derivative from the ionic liquid with ether.
One criticism that has been applied to the use of ionic liquids is that often a
volatile organic solvent is employed in the workup or product isolation. In this
and other cases, however, a single extraction allowed the quantitative recovery of
both the product and the IL and the reuse of the latter for further transformations.
The results, reported in Table 2, support this important point. The reaction of nitro-
cyclopentane with H2O2=OHꢀ in bmim(OTf) afforded the corresponding cyclopen-
tanone, extracted with Et2O (2 mL), in 95% isolated yield. The remaining IL, without
any treatment, was added to fresh portions of substrate and oxidant for new reaction
cycles, which proceeded without great reduction of efficiency.
Nitrosubstituted ketones smoothly underwent conversion to diketo derivatives
in moderate yields of about 50% with both oxidants, although with different reaction
times (entries 15 and 16, Table 1). By studying the reaction of a set of primary nitro-
derivatives to aldehydes with H2O2=OHꢀ or SPB, we observed a decrease of the sub-
strate conversion by increasing its lipophilicity (entries 7–12, Table 1). Consequently,
we decided to investigate the role of the IL on this process, focusing our attention on
some hydrophilic and hydrophobic ILs, that is, bmim(OTf), bmim(PF6), and methyl
pyridinium triflate [mp(OTf)].[17] All the ILs were prepared in our laboratories
following published procedures with appropriate modifications.[16,18] In particular,
bmim(OTf) and mp(OTf) were obtained from commercially available N-butyl
imidazole and pyridine, respectively, by an unprecedented direct reaction with
methyl triflate (CF3SO2OCH3) in the absence of solvent (see the experimental
section). Contrary to our expectations, the reactions of 1-nitro heptane, used as a
prototype, with basic hydrogen peroxide in the hydrophobic bmim(PF6) and in
the moderately hydrophilic mp(OTf) were slower than in bmim(OTf), with substrate
conversions of about 40% after 4 h. Longer reaction times promote the formation of
products other than heptanal, which is never observed in bmim(OTf).
In summary, the H2O2=OHꢀ or SPB oxidation of nitroderivatives in ILs
provides a mild and efficient alternative to the traditional Nef reaction. This trans-
formation is particularly recommended for the conversion of primary nitro
compounds to aldehydes. Good yields, low cost, and easy workup, together with
the straightforward recovery and reuse of the IL, characterize this process.
Table 2. Conversion of nitrocyclopentane to cyclopentanone with
H2O2=OHꢀ in recycled bmim(OTf)
Run
Time (h)
Conversion (%)a
Yield (%)a
1
2
3
3
3
3
97
96
95
95
93
92
aBoth conversion and yield were determined by GC analysis.

2486
O. BORTOLINI ET AL.
EXPERIMENTAL
Synthesis of Nitroalkanes
1-Nitro heptane 6 was synthesized from the corresponding 1-bromo heptane
(19.5 mmol) by reaction with (Bu)4N(NO2) (23.4 mmol) in CH2Cl2 at 0 ꢁC.[15] After
3 h, the reaction was diluted with water, extracted with Et2O, and dried over anhy-
drous Na2SO4, and the solvent was evaporated. The 1-nitro heptane was purified by
1
distillation (bp 105–111 ꢁC) at 16 mm=Hg. H NMR (CDCl3, 300 MHz): d ¼ 4.37
(t, J ¼ 7.0 Hz, 2H), 2.00 (m, 2H), 1.27 (m, 8H), 0.88 ppm (t, J ¼ 6.1, 6.7 Hz, 3H);
IR (NaCl): n ¼ 2970, 2876, 2829, 1577, 1457, 1350 cmꢀ1; (ESI-MS) [M þ H]þ m=z
216.1. 2-Nitroethylbenzene 7 was prepared according to the same procedure and
purified by column chromatography, eluent hexane–Et2O 7:3, oil. 1H NMR (CDCl3,
300 MHz): d ¼ 7.26 (m, 5H), 4.61 (t, J ¼ 7.3, 7.4 Hz, 2H), 3.32 ppm (t, J ¼ 7.4, 7.4 Hz,
2H); IR (NaCl): n ¼ 2997, 2881, 1553, 1480, 1443, 1403, 1357 cmꢀ1; (ESI-MS)
[M þ H]þ m=z 152.2.
Synthesis of Ionic Liquids
Solvent-free synthesis of 1-butyl-3-methyl imidazolium triflate,
bmim(OTf). In a three-necked, 250-mL, round-bottomed flask equipped with reflux
condenser, 100-mL dropping funnel, and magnetic stirrer, 38.1 mL of N-butyl imi-
dazole (0.29 mol) were placed under N2. The equipment was placed in an ice-cooled
bath, and 33.42 mL (0.3 mol) of methyl trifluoromethanesulfonate were slowly added
dropwise under vigorous stirring. The solution was left to react for 24 h. The reaction
was washed with dichloromethane, and the residual liquid was concentrated using a
1
rotavapor to remove traces of solvent. Yield 95%. H NMR (acetone-d3, 300 MHz,
d ppm relative to TMS): 9.11 (s, 1H), 7.77 (t, 1H, J ¼ 2.2 Hz), 7.71 (t, 1H, J ¼ 2.2 Hz),
4.35 (t, 2H, J ¼ 9.3 Hz), 4.04 (s, 3H), 2.00–1.90 (m, 2H), 1.38–1.29 (m, 2H), 0.87
(t, 3H, J ¼ 9.2 Hz).
Solvent-free synthesis of N-methyl pyridinium triflate, mp(OTf). The
same procedure was used for the preparation of this IL by reacting pyridine
(0.29 mol) and methyl trifluoromethanesulfonate (0.3 mol) for 24 h at room tempera-
1
ture. Yield 93%. H NMR (DMSO-d6, 300 MHz, d ppm relative to TMS): 6.47 (d,
2H, J ¼ 6.2 Hz), 6.07 (t, 1H, J ¼ 7.8 Hz), 5.62 (dd, 2H, J ¼ 6.2 Hz, J ¼ 7.8 Hz), 1.85
(s, 3H).
1-Butyl-3-methyl imidazolium hexafluorophosphate, bmim(PF6). This
compound was obtained by standard procedures.[19] The identity of the reaction
products, all known compounds, were obtained by comparison of the pertinent
spectroscopic data with literature reports.
ACKNOWLEDGMENT
The authors are grateful to the University of Calabria for financial support.

NITROALKANES TO CARBONYL COMPOUNDS IN ILs
2487
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