CRYSTAL STRUCTURES OF (E)-1-(3,4,5-TRIMETHOXYPHENYL)-2-NITROETHENE AND (E)-1-(3,4,5-TRIMETHOXYPHENYL)-2-NITROPROPENE

¹Instituto de Física e Química de São Carlos, USP, C.P. 369, 13560-970, São Carlos, SP,
Brazil.
²Facultad de Ciencias Médicas, U. de Santiago de Chile, Casilla 442, Correo 2, Santiago,
Chile.
³Departamento de Química, Facultad de Ciencias, U. de Chile, Casilla 653, Santiago, Chile.
(Received: February 24, 1999 - Accepted: July 30, 1999).

ABSTRACT

The crystal and molecular structures of (E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene (1), C₁₁H₁₃NO₅, M_r = 239.2, and (E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene (2), C₁₂H₁₅NO₅, M_r = 253.3, were determined by X-ray diffraction. Compound 1: monoclinic, space group P2₁/c, a = 7.429(2), b = 9.828(3), c = 32.766(2) Å, b = 95.65(2)°, V = 2381(1) ų, Z = 8. Compound 2: monoclinic, space group P2₁/c, a = 8.888(3), b = 13.650(3), c = 10.369(3) Å, b = 94.04(2)°, V = 1255(1) ų, Z = 4. Compound 1 exhibits no unusual structural features. Compound 2 shows a side chain out of the ring plane and with expanded valence angles.

KEY WORDS: X-ray crystallography, substituted nitrostyrenes, conformation.

RESUMEN

Las estructuras cristalinas y moleculares de (E)-1-(3,4,5-trimetoxifenil)-2-nitroeteno (1), C₁₁H₁₃NO₅, M_r = 239,2 y (E)-1-(3,4,5-trimetoxifenil)-2-nitropropeno (2), C₁₂H₁₅NO₅, M_r = 253,3, fueron determinadas por difracción de rayos X. El compuesto 1 es monoclínico, grupo espacial P2₁/c, a = 7,429(2), b = 9,828(3), c = 32,766(2) Å, b = 95,65(2)°, V = 2381(1) ų, Z = 8. El compuesto 2 es monoclínico, grupo espacial P2₁/c, a = 8,888(3), b = 13,650(3), c = 10,369(3) Å, b = 94,04(2)°, V = 1255(1) ų, Z = 4. El compuesto 1 no presenta rasgos inusuales en su estructura molecular. El compuesto 2 tiene la cadena lateral fuera del plano del anillo y con los ángulos de valencia expandidos.

PALABRAS CLAVES: Cristalografía de rayos X, nitroestirenos sustituidos, conformación.

*To whom correspondence should be addressed.

INTRODUCTION

b-Nitrostyrenes (1-aryl-nitroalkenes) are useful synthetic intermediates for which fungistatic, antibacterial, antiprotozoal, antitumor, and insecticidal activities have been reported¹⁾, and for which quantitative structure-activity relationships have been found involving measures of their electrophilicity and the Hückel bond index of the nitrovinyl double bond^2,3). Earlier crystallographic studies on (E)-1-aryl-2-nitropropenes have shown unusually large values for the side chain bond angles^1,4,5), explicitly attributed to steric compression between the methyl group and a hydrogen atom at one of the ortho positions of the benzene ring in the single case studied by us in which the side chain is practically coplanar with the aromatic ring⁵⁾. Nevertheless, in the other two cases, the flat, conjugated side chain forms a fairly large dihedral angle (about 25°) with the median plane of the ring, which might be expected to relieve steric strain at least in part. In an attempt to clarify this situation, we have now carried out a comparative study of (E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene (1) and (E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene (2), differing only in the absence or presence of the side chain methyl group presumed to clash with one of the ortho hydrogen atoms.

RESULTS AND DISCUSSION

(E)-1-(3,4,5-Trimethoxyphenyl)-2-nitroethene, 1, crystallizes with two distinct molecules in the unit cell. In both conformers, the benzene ring and the nitrovinyl side chain are coplanar, and the bond angles in the side chain lie within the expected range, with no significant expansion of the C(6)-C(1)-C(7) and C(2)'-C(1)'-C(7)' angles, and the C(1)-C(7)-C(8) and C(1)'-C(7)'-C(8)' angles attaining values of 126.0(2)° and 124.9(3)°, respectively. As is usually the case in molecules with three neighboring methoxyl groups bonded to a benzene ring, the central methoxyl oxygen-carbon bond is nearly perpendicular to the median ring plane, forming angles of -93.7(3)° in one molecule and -74.8(3)° in the other. In the (E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene (2) molecule, the benzene ring and the nitrovinyl side chain form a dihedral angle of 12.3(3)°(or 13.0°, defining the planes as for the 4-methoxy analogue 5 ¹⁾) and the bond angles at C(1) and in the side chain are appreciably distorted, with the C(2)-C(1)-C(7) angle spread to 124.3(2)°, and the C(1)-C(7)-C(8) and C(7)-C(8)-C(12) angles attaining 130.9(2)° and 130.6(2°) respectively. The oxygen-carbon bond of the sterically hindered, central methoxyl of 2 forms an angle of -74.1(3)° with the ring plane, as expected and found for the two conformers of 1 in the crystal phase.

Previously published X-ray structures of (E)-1-(substituted phenyl)-2-nitropropenes show similar side chain distortions. Using the same numbering system as for compound 2, the C(2)-C(1)-C(7), C(1)-C(7)-C(8), and C(7)-C(8)-C(12) valence angles, and also the C(2)-C(1)-C(7)-C(8) torsion angle, may be directly compared with those found for (E)-1-(4-hydroxy-3-methoxyphenyl)-2-nitropropene (3)⁴⁾, (E)-1-(4-dimethylaminophenyl)-2-nitropropene (4)¹⁾, and (E)-1-(4-methoxyphenyl)-2-nitropropene (5)⁵⁾:

TABLE I. Selected valence angles and dihedral angles of (E)-1-aryl-2-nitropropenes.

It may be seen that the expansion of the side chain valence angles is very similar for all four compounds. On the contrary, the torsion angles vary widely from the essentially flat (E)-1-(4-dimethylaminophenyl)-2-nitropropene molecule to the rather strongly twisted 4-hydroxy-3-methoxy and 4-methoxy analogues, with the 3,4,5-trimethoxy compound 2 occupying an intermediate position. Thus, the expected relief of steric strain by twisting the side chain out of the ring plane (with the corresponding sacrifice of resonance energy) is not reflected in a decrease of the bond angles to values closer to those found in the (E)-1-aryl-2-nitroethene.

The C(7)-C(8) double bond length in nitropropene 2 is 1.327(3) Å. In nitrostyrenes 3, 4 and 5, the C(7)-C(8) bond lengths are 1.340(3)⁴⁾, and 1.316(4)⁵⁾ Å, respectively, with the larger values corrsponding to the para-hydroxy and para-dimethylamino analogues, which are expected to be more highly conjugated. On the other hand, the C(1)-C(7) single bond lengths are 1.459(3), 1.465(4)⁴), 1.447(4)¹⁾, and 1.474(4)⁵⁾, respectively, where only the very highly conjugated nitrostyrene 4 shows significant bond shortening. The C(7)-C(8) bond lenghs for the two distinct (E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene (1) molecules are unexpectedly short: 1.317(4) and 1.308(4) Å, while the C(1)-C(7) distances show nornal values of 1.455(4) and 1.463(4) Å, respectively, which might be construed as an indication that these planar, undistorted molecules are less conjugated than their counterpart 2.

A few years ago the infrared spectra of six phenylnitroethenes and six phenylnitropropenes (including compounds 4 and 5) were assigned, and minimum energy conformations and rotational barriers around the C(1)-C(7) bond were calculated at the AM1 level which led to the conclusion that the nitroethenes are all flat molecules and the nitropropenes form a 45° interplanar angle between the median planes of the ring and the side chain⁶⁾. In order to determine to what extent the structural features observed in X-ray studies result from crystal packing forces and to what extent they are a consequence of the electronic structures of these compounds, higher-level quantum-chemical calculations will have to be carried out as well as additional spectroscopic studies.

EXPERIMENTAL

The title compounds were prepared by n-butylamine-catalyzed Knoevenagel condensation of 3,4,5-trimethoxybenzaldehyde with either nitromethane (for 1) or nitroethane (for 2), at reflux in acetic acid⁷⁾. Crystals grown in methanol were selected for X-ray studies. Crystals were cut to dimensions 0.27 x 0.47 x 0.55 mm³ [(E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene, 1] and 0.55 x 0.60 x 0.75 mm³ [(E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene, 2] and mounted on an Enraf-Nonius CAD4 diffractometer. The cell parameters were obtained by a least-squares fit of 25 reflections (11 < q < 19 for 1 and 9 < q < 22 for 2 ).

Data collection for the nitroethene (1) was performed in the monoclinic system with cell parameters a = 7.434(1), b = 9.829(2), c = 32.753(7) Å, b = 95.77(2)°; w-2 q scan technique, scan width (0.80 + 0.35 tanq)°, variable scan rate with maximum scan time of 10 s per reflection, and the range of measured hkl was 0£h £8, 0£ k £10, -35£ /£ 35. Data collection for the nitroprene (2) was performed in the monoclinic system with cell parameters a = 8.888(3), b = 13.650(3), c = 10.370 (3)Å, b = 94.04(2)°; the same scan technique, width and maximum scan time as for 1 were used for compound 2, and the range of measured hkl was 0£ h £10, 0 £k £16, -12£ / £12.

No significant decrease in intensities of two standard reflections was observed for either compound. 3301 unique reflections were collected for 1 of which 2889 were observed (I > 3s(I)), and 1944 unique reflections for 2 of which 1428 were observed. Scattering factors for neutral atoms were taken from the international Tables of X-ray Crystallography and corrected for both Df' and i(DF") terms. Intensity data were corrected for Lorentz and polarization effects, but no absorption correction was applied because of the low linear absorption coefficient. In both cases, the systematic absences were 0k0; k = 2n and h0I: I = 2n, consistent with the space group P2₁/c. Equivalent reflections were merged with R_int = 0.009 for compound 1 and R_int = 0.032 for compound 2.

Direct methods with SHELXS-86⁸⁾ solved the molecular structures for non-hydrogen atoms for both compounds. Using SHELXs-76⁹⁾, hydrogen atomic coordinates were given by Fourier difference methods or, in the case of methyl groups, by the refinement of carbon atoms as rigid CH₃ groups. The function minimized in the refinement was Sw(|F₀|-|F_c|)² with w = k|s²(F₀) + gF₀²|^-1, where s(F₀) is the esd for the observed amplitude based on counting statistics. All non-hydrogen atoms were refined with anisotropic thermal parameters. All the calculations were carried out on a VAX 6000-420 computer.

Interatomic distances (Å) C₁₁H₁₃NO₅ (1).

Bond angles (degrees) C₁₁H₁₃NO₅ (1)

Interatomic distances for compound 2: C₁₂H₁₅NO₅

Bond angles (°) for compound 2: C₁₂H₁₅NO₅

Dihedral angles betwen planes

(E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene (1)

Plane 1 2 3 4

Atoms C1/C2/C3/C4/C5/C6 N1/O1/O2 C1'/C2'/C3'/C4'/C5'/C6' N1'/O1'/O2'

Dihedral angle 1-2: 6.3(1.5) 3-4: 2.13(4.7)

(E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene (2)

 

Plane 1 2

Atoms C1/C2/C3/C4/C5/C6/O3/O4/O5/C7 C1/C7/C8/C12/N1/O1/O2

Dihedral angle 1-2: 12.3(3)

An alternative way of defining planes, similar to that used in the paper by Brito et al. (1994) gives the following dihedral angle:

1 2

C1/C2/C3/C4/C5/C6 C12/N1/O1/O2

1-2: 13.0(3)

Torsion angles

(E)-1-(3,4,5-trimethoxyphenyl)-2-nitroethene (1)

Molecule A		Molecule B
C6/C1/C7/C8 C2/C1/C7/C8 C7/C8/N1/O1 C7/C8/N1/O2 C3/C4/O4/C10	-4.0(4) 175.7(3) 177.5(3) -2.9(4) -93.7(3)	C6'/C1'/C7'/C8' C2'/C1'/C7'/C8' C7'/C8'/N1'/O1' C7'/C8'/N1'/O2' C3'/C4'/O4'/C10'	-177.1(3) 3,4(4) 2.2(4) -175.4(3) -74.8(3)

(E)-1-(3,4,5-trimethoxyphenyl)-2-nitropropene (2)

C6/C1/C7/C8 C2/C1/C7/C8 C7/C8/N1/O1 C7/C8/N1/O2

-169.0(3) 10.7(4) 2.4(4) -178.2(2)

C3/C4/O4/C10 C1/C7/C8/C12 C12/C8/N1/O1 C12/C8/N1/O2

-74.1(3) 0.8(5) -178.5(3) 0.9(4)

ACKNOWLEDGEMENTS

This work was made possible in part by the Presidential Chair in Sciences (BKC).

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