The question of the regiodirection of the [2+3] cycloaddition reaction of triphenylnitrone to nitroethene

Full text of DOI:10.1007/s10593-009-0318-3

Chemistry of Heterocyclic Compounds, Vol. 45, No. 6, 2009
THE QUESTION OF THE REGIODIRECTION
OF THE [2+3] CYCLOADDITION REACTION
OF TRIPHENYLNITRONE TO NITROETHENE
R. Jasiński1*
Keywords: nitroalkenes, nitrones, [2+3] cycloaddition, regioselectivity.
It was reported in paper [1] that at room temperature the [2+3] cycloaddition of triphenylnitrone (1) to
nitroethene (2) led to the formation of 4-nitro-2,3,3-triphenylisoxazolidine (3) while at 110°C a mixture of
compounds was formed, consisting of the nitroisoxazolidine 3, 5-nitro-2,3,3-triphenylisoxazolidine (4), and
2,3,3-triphenylazetidinone (5) in a ratio of ~1:1:0.5. However, it should be noted that only the isoxazolidine 3
was isolated in a pure state. The compounds, named by the authors the isoxazolidine 4 and the azetinone 5 were
isolated from the product mixture by column chromatography in the form of mixture so that their structures were
completely unverified.
Ph
NO2
~20°C
Ph
N
H
H
NO2
H
Ph
Ph
Ph
Ph
O
3
C
172
C
172
+
+
C
N
173
173
–
Ph
Ph
Ph
Ph
Ph
O
2
1
110°C
+
3
+
N
N
Ph
NO2
O
O
5
4
Since we had shown previously that in reactions of the nitrone 1 with other conjugated nitroalkenes (1-nitro-
propene-1 [2, 3], 3,3,3-trichloro-1-nitropropene-1 [3, 4], 2-methoxycarbonyl-1-nitroethene [3]) no 5-nitro-
isoxazolidines were formed at all, whereas the 4-nitroisoxazolidines were practically the only heterocyclic
reaction products, the temperature dependence of the regioselectivity represented in [1] seemed to us debatable.
Therefore we have repeated the experiment as described in paper [1]. In the first place we have studied the
course of the reaction using HPLC at room temperature. As would be expected, in the reaction mixture,
_______
* To whom correspondence should be addressed, e-mail: radomir@chemia.pk.edu.pl.
1T. Kostyushko Krakov Polytechnical Institute, 31-155 Krakov, Poland.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.6, 932-934, June, 2009. Original article
submitted September 16, 2008. Revised version submitted April 6, 2009.
748
0009-3122/09/4506-0748©2009 Springer Science+Business Media, Inc.

independently of the course of the reaction, we observed only one product, the structure of which was
established as the nitroisoxazolidine 3 (Rf = 11.7 min) on the basis of elemental analysis and spectral data.
Monitoring the reaction at a temperature of 110°C showed that on increasing the temperature cycloaddition of
nitrone 1 to the nitroalkene 2 also led to a single cycloadduct which was shown to be the same nitroisoxazolidine
3. Benzophenone was also formed in small amounts (Rf = 8.4 min). Under the reaction conditions it gradually
decomposes forming preferebly the initial materials. We showed in separate experiments that the
nitroisoxazolidine is a thermally unstable compound. We also confirmed that nitroethene polymerized at high
temperatures [3, 5]. Simultaneously nitrone 1 was slowly converted into benzophenone [6, 7]. Its presence in the
reaction mixture was confirmed by HPLC.
Thus our investigations showed that the cycloaddition of nitrone 1 to nitroalkene 2 occurs
regioselectively independently of the temperature. The fractions isolated from the product mixture by the
authors of paper [1] contained most likely the nitroisoxazolidine 3 and benzophenone, contaminated by
oligomers of nitroethene, but none of the expected nitroisoxazolidine 4 and the azetidinone 5. The presence of
benzophenone , caused the apperance of the C=O stretching band (ν 1750 cm-1) in the IR spectrum, which the
authors of paper [1] attributed as a sign of the presence of azetidinone 5.
It should be underlined that the observed regioselectivity of the reaction correlates well with the diagram
of the HMO interactions of the substrates [8].
Monitoring of the reactions and control of the purity of compounds was carried out using a Knauer liquid
chromatograph (detector UV Smartline 2500, Lichrospher 100-10 RP18 4×240 mm column, eluent 50% (by
volume) THF, rate of flow of eluent 1.3 cm3/min, λ 254 nm). Melting points were measured with a Boetius
1
PHMK-05 apparatus. IR spectra of KBr tablets were measured on a Bio-Rad FTS 175 C apparatus, H NMR
spectra of CDCl3 solutions with TMS as internal standard with a Tesla BS-567C (80 MHz) spectrometer. Electron
impact mass spectra were measured with a Varian MAT-112 instrument (ionization energy 70 eV). UV spectra
were recorded with a Specord 75 instrument. Elemental analyses were carried out with a Perkin Elmer PE-2400
CHN analyzer.
Triphenylnitrone 1 and nitroethene 2 were prepared by known methods [9, 10].
4-Nitro-2,3,3-triphenylisoxazolidine (3). Mp 132-135°C (dec). IR spectrum, ν, cm-1: 1562 and 1374
1
(NO2), 932 and 918 (isoxazolidine ring). UV spectrum (MeOH), λmax, nm (log є): 211 (4.29), 243 (3.80). H
NMR spectrum, δ, ppm (J, Hz): 6.08 (1H, dd, J = 5.6, J = 7.6, H-4); 5.11 (1H, dd, J = 5.6, J = 10.0, H-5); (1H,
dd, J = 7.6, J = 10.0, H-5'). Mass spectrum, m/z (Irel, %): 346 [M]+ (50), 273 [M-H2C=CHNO2] (29), 257
[M-O–H2C=CHNO2] (5), 180 [M-O–H2C=CHNO2-C6H5] (32), 91 [C6H5N] (100). Found, %: C 72.82; H 5.24;
N 8.09. C21H18N2O3. Calculated, %: C 72.71; H 5.12; N 8.10.
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