ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 10, pp. 1996–2000. © Pleiades Publishing, Ltd., 2010. Original Russian Text © E.A. Ishmaeva, Ya.A. Vereshchagina, D.V. Chachkov, S.V. Makarenko, K.S. Kovalenko, V.M. Berestovitskaya, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 10, pp. 1686–1690.

1996

Structure of Alkyl 2,3-dibromo-3-nitroacrylates in Solution E. A. Ishmaevaa, Ya. A. Vereshchaginaa,b, D. V. Chachkovc,

S. V. Makarenkod, K. S. Kovalenkod, and V. M. Berestovitskayad a Kazan State University, ul. Kremlevskaya 18, Kazan, Tatarstan, 420008 Russia

elenishmaeva@gmail.com b Kazan State Technological University, Kazan, Tatarstan, Russia

c Joint Supercomputer Center, Kazan Branch, Russian Academy of Sciences, Kazan, Tatarstan, Russia d Herzen Russian State Pedagogical University, St. Petersburg, Russia

Received January 14, 2010

Abstract―The structure of the new representatives of polyfunctional bromonitroalkenes, alkyl 2,3-dibromo-3-nitroacrylates, was examined by means of dipole moments, [B3LYP/6-31G(d)] quantum-chemical methods, and IR spectroscopy. They were found to have nontrivial Z-configuration in a solution.

The published information on dihalogenonitro-alkenes is not abundant, and dibromonitroethenes are mentioned in isolated instances [1, 2]. Recently Makarenko et al. [3] developed a sufficiently simple procedure for the synthesis of original functionalized dibromonitroethenes, which contain in the vicinal position to the nitro-group an alkoxycarbonyl function. By the XRD analysis the Z-configuration of 1,2-dibromo-1-nitro-2-ethoxycarbonylethene (ethyl 2,3-dibromo-3-nitroacrylate) was established where the steric strain was surmounted due to the displacement of the ester group out of the plane of the C=C bond.

Analyzing the data of the dipole moments, IR spectroscopy, and quantum-chemical calculations of 2-ethoxycarbonyl(trichloromethyl)-1-nitro- and 1-bromo-1-nitroethenes, we established earlier that these mole-cules had a configuration with the trans-location of nitro- and ester (or trichloromethyl) groups. Moreover, the s-cis-orientation of multiple bonds C=C and C=O is characteristic of nitroacrylates [4].

It was interesting to follow the influence on the structure of 2-alkoxycarbonyl-1-bromo-1-nitroethenes of the introduction of the second bromine atom.

In the present work the experimental and theoretical conformational analysis of alkyl 2,3-dibromo-3-nitroacrylates was performed using the methods of dipole moments, IR spectroscopy, and quantum chemistry [B3LYP/6-31(d)]. The coefficients of

calculation equations, the orientational polarizations, and experimental dipole moments (benzene, 25°C) of methyl and ethyl 2,3-dibromo-3-nitroacrylates I and II are presented in Table 1. The relative energies cal-culated according to vectorial-additive diagram, the theoretical and experimental dipole moments for com-pounds I and II are given in Tables 2 and 3. At ex-amining Table 2 it should be taken into consideration that in the structures Ia–Id the dihedral angle between the planes Br–C=C–S and O=C–O–C is equal to 90°. In the structure Ie the double bonds C=C and C=O are located in one plane, and NO2-group is perpendicular to this plane.

In the analysis of the data from Table 3 the following special features of the structures studied should be considered. In the structures IIa, IIg–IIl the bromine atoms have trans-orientation, while in the structures IIb–IIf they are cis-arranged. In this case in the structures IIa–IIg, IIj–IIl the dihedral angle between the planes Br–C=C and O=C–O–C is equal to 90°, while in the structures 8 and 9 both double bonds are located in one plane, NO2-group is perpendicular to this plane. Moreover, the given E and Z-forms of conformers are distinguished by the turn of ethyl group relative to O=C–O fragment.

The comparison of all given data (Tables 1–3) makes it possible to conclude that the studied compounds exist in a solution as Z-isomers. The

DOI: 10.1134/S1070363210100191

quantum-chemical study showed that in both com-pounds the energy minimum corresponded to the structures, where the bromine atoms were located on one side of the multiple bond, i. e., the molecules had a Z-configuration. Nitro-group was in the plane of the multiple bond, the torsion angles O–N–C=C equaled 179.6° for nitroethene I and 179.9° for nitroethene II. At the same time the ester fragment is practically perpendicular to this plane: torsion angles C=C–C=O equal 93.8° for compound I and 94.2° for II. Just these Z-structures in both compounds are energetically preferable. The remaining forms of compound I and the majority of the forms of compound II are less feasible by 12.16 to 36.31 kJ mol–1 (I) and from 12.35 to 37.88 kJ mol–1 (II). The dipole moments of these forms, calculated according to the vectorial-additive diagram (Tables 1–3) reproduce well for I and satisfactorily for II the experimental polarity of the investigated compounds. Their theoretical dipole moments also describe sufficiently well the experiment (Tables 1–3). For structures IIc, IId (Table 3) the relative energies are also small (1.60 and 0.95 kJ mol–1), which does not theoretically exclude the possibility of their participation in the equilibrium. However, since their calculated and theoretical moments are lower than the experimental values, it would be necessary to allow for the participation in the conformational equilibrium of trans-isomers, what contradicts the remaining obtained results. Moreover, the data of the IR spectra testify to the prevalence of one form.

The examination of the theoretical lengths of =C–Br bond from the side of the ester group (1.882 and 1.883 Å in nitroethenes I and II, respectively) and also the length of =C–Br bond from the side of NO2-group (1.871 Å in both compound) and the olefin bond =C–Br (1.89±0.01 Å [5]) shows the small shortening of bond lengths. However there are no reasons to speak of the conjugation between C=C bond and ester group, since the latter is moved out of the plane of the C=C bond. It should be noted that the shortening of the length of the =C–Br bond from the side of nitro group (~0.020 Å) is 2 times more than from the side of ester group

(~0.009 Å). However, also in this case this there is hardly any significant interaction of the lone electron pair of the bromine atom with the multiple bond C=C and the nitro-group, since the dipole moment calculated according to the vectorial-additive diagram (3.54 D) for the energetically preferable Z-isomer of nitroethene I practically coincides with its experi-mental value (3.47 D) (Table 2). In the case of nitroethene II the observed exaltation of the experimental (4.00 D), calculated (3.54 D) and theoretical (3.33 D) values of dipole moment allows for such interaction (Table 3).

Comp. no. α γ Роr, cm3 µ, D

I 4.559 0.026 246.256 3.47

II 5.858 0.104 327.638 4.00

Comp. no.

Conformers ΔE,

kJ mol–1 µcalc,

D µtheor,

D

Iа 15.78 3.02 4.16

Ib 0.00 3.54 3.15

Ib 22.49 – 4.81

Id 36.31 – 3.48

Ie 12.16 – 3.80

C

Br

C

N

Br C O

OCH3

O

O

C

Br

C

N

Br

C O

OCH3

O

O

C

Br

C

N

Br

C O

O

CH3

O

O

C

Br

C

N

Br CO

O

CH3

O

O

C

Br

C

N

Br C O

OCH3

O

O

Table 1. Coefficients of calculation equations and experi-mental

Table 2. Relative energies calculated by vectorial-additive diagram and theoretical dipole moments (D) for methyl 2,3-dibromo-3-nitroacrylate I (µexp 3.47 D)

STRUCTURE OF ALKYL 2,3-DIBROMO-3-NITROACRYLATES IN SOLUTION

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1997

Table 3. Relative energies calculated by vectorial-additive diagram and theoretical dipole moments (D) for ethyl 2,3-dibromo-3-nitroacrylate II (µexp 4.00 D)

Comp. no.

Conformers ΔE,

kJ mol–1 µcalc,

D µtheor,

D

IIа 15.58 3.03 4.44

IIb 0.00 3.54 3.33

IIc 1.60 – 3.44

IId 0.95 – 3.10

IIe 21.53 – 4.88

IIf 23.76 – 5.04

IIg 16.99 – 4.47

Comp. no.

Conformers ΔE,

kJ mol–1 µcalc,

D µtheor,

D

IIh 12.35 – 4.08

IIi 13.62 – 4.04

IIj 16.22 – 4.35

IIk 34.90 – 3.84

IIl 37.88 – 3.89

IIm 36.74 – 3.86

C

Br

C

N

Br C O

OCH2

O

O

CH3

C

Br

C

N

Br

C O

O H2C

O

OCH3

C

Br

C

N

Br

C O

O H2C

O

OCH3

C

Br

C

N

Br

C O

OCH2

O

O

H3C

C

Br

C

N

Br

C O

O

H2C

O

O CH3

C

Br

C

N

Br

C O

O

H2C

O

OCH3

C

Br

C

N

Br CO

O

CH2

O

O

H3C

C

Br

C

N

Br C O

O

CH2

O

O

CH3

C

Br

C

N

Br C O

O

H2C

O

OCH3

C

Br

C

N

Br CO

OCH2

O

O

CH3

C

Br

C

N

Br CO

O

CH2

O

O

CH3

C

Br

C

N

Br CO

OCH2

O

O

CH3

C

Br

C

N

Br C OO

H2C

O

OCH3

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ISHMAEVA et al. 1998

We emphasize that the deviation of C(O)Alk group from the plane of C=C bond in both compounds (>90°) obtained from the theoretical calculations is sufficiently well consistent with X-ray structural data for the crystalline nitroethene II (108.9°).

To increase the calculation accuracy for the bromine “heavy” atoms we made an attempt to enlarge the basis set by adding 6D-functions to the standard basis 6-31(d) and by using pseudopotentials for the bromine atoms (Table 4). However, dis-regarding the type of the basis used the theoretical dipole moments of energetically preferable conformers of both compounds I, II differ little. For compound I the maximal difference Δμ is 0.14 D, while for compound II, 0.08 D.

The study of the IR–Fourier spectra of compounds I and II in mineral oil and in methylene chloride solution up to the concentration 10–3 M does not principally contradict the possible presence of other conformers, as shown by the presence of a certain asymmetry in the stretching vibrations bands ν(C=O) (1748–1751 cm–1) and ν(NO2) [1551–1558 (νas), 1316–1318 (νs) cm–1] in both compounds. However, the relative energies of other conformers are considerably higher (Tables 2, 3), therefore the presence of them in any perceptible amount is highly improbable. The prevalence of only one conformer also follows from the analysis of the form of bands ν(C=O), ν(NO2), ν(С=С) (1588 cm–1 in compound I and 1590 cm–1 in nitroethene II).

For calculating the dipole moments by vectorial-additive diagram the following bond angle values were used: ∠ССBr = 121° [6], ∠СС=O = 120°, ∠СС–O = 120°, ∠ССN = 121° [7], ∠ССH1 = 118° [7], ∠ССH2 = 117° [7], and also the bond and groups moments: m(Csp2–Br) = 0.66 D calculated from μexp CH2=CHBr [8], m(Csp2–NO2) = 2.81 D (μexp CH2=CHNO2) [8], m(H–Csp2) = 0.70 D [9], m(Csp3–Csp2) = 0.78 D (μexp C3H6) [8], m(C=O) = 1.94 D [10], m(C-O) = 0.18 D [μexp CH3C(O)OEt], m(Et–O) = 1.11 D [8].

EXPERIMENTAL

Alkyl 2,3-dibromo-3-nitroacrylates I and II were prepared according to procedure [3], mp 34–36°С (I) and 35–37°С (II). Their individuality was proved by TLC and 1H NMR spectroscopy [3].

The IR Fourier-spectrograms of nitroethene I were registered from mineral oil, KBr pellets, and CH2Cl2

solution, and of nitroethene II, from mineral oil and CH2Cl2 solution on a Bruker Vector-22 spectrometer in the range of 4000–400 cm–1 (resolution 1 cm–1); the operating layer thickness 0.162 mm, solution concentration 6.1×10–3 M (I) and 7.3×10–3 M (II).

The physicochemical measurements of the quantitative characteristics of electrical properties of compounds investigated were performed for a series of four benzene solutions of these compounds at 25±0.2°С. The solvents were prepared directly before the measurements according to the standard procedure, given in [10]. For determining the experimental values of dipole moments we used the second Debye method based on the measurement of the dielectric constant of the dilute solutions of the substance in a nonpolar solvent [9]. The dielectric constant of solutions was determined on an instrument IDM-2 [12] operating by the beat method. The error in the measurement of dielectric constant comprises ±0.05 D.

The refractive indices of solutions were determined on IRF-23 refractometer with the accuracy ±0.00001 for the sodium D-line.

The coefficients of the calculated equations and the experimental dipole moments of the investigated compounds are presented in Table 1. The accuracy of the experimental dipole moments determination is ±0.05 D.

The quantum-chemical calculations were per-formed by means of Gaussian 03 program [13] by DFT B3LYP hybrid method using the basis set 6-31G(d) with the total optimization of geometry. In all cases the correspondence of the obtained stationary points to the energy minima was proved by the absence of the negative eigenvalues of the second derivatives matrix. The calculations were performed in the Kazan Branch of Joint Supercomputer Center of Russian Academy of Sciences (http://wt.knc.ru).

Table 4. Theoretical dipole moments (D) for compounds I and II obtained in different basis

Basis I II

B3LYP/6-31G(d) 3.15 3.33

B3LYP/6-31G(d)//LanL2DZ//6D 3.04 3.27

B3LYP/6-31G(d)//SDD//6D 3.01 3.25

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1999

ACKNOWLEDGMENTS

The authors are grateful to A.V. Chernova for the registration and aid in the interpretation of the IR spectra.

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