Crystal structure of (1E,3E)-4-methylthio-2-nitro-3-phenylsulfonyl-l-pyrrolidino-1,3-butadiene,C15H18N2O4S2

C15H18N2O4S2, monoclinic, P12i /c l (No. 14), a = 8.754(3) Â, b = 12.499(2) Â , c = 15.599(3) λ,β = 95.09(2)°, V = 1700.1 Â , Ζ = 4, Rgt(F) = 0.049, wRTef(F) = 0.128, Τ = 294 Κ. Source of material The title compound was synthesized in 87% yield f rom 3-nitro-4-(phenylsulfonyl)thiophene [1] with a proper modification of our ring-opening procedure of thiophene derivatives [2] and crystallized from ethanol. Mp 463.7-464.3°. Experimental details In the refinement, the atoms C6A and C6B were considered as isotropic and the sum of their site occupancy factors was tied to unity [final values: 0.613(7) and 0.387(7) respectively, with U{C6A) = U(C6B) = 0.070(1) Â]. No further constraints were imposed on heavy atoms. The high displacement parameters of C7 indicated its trend to disordered behaviour, but the resolution of the data was not sufficient to consider the atom split over two distinct positions. Several Η atoms were located by difference syntheses and refined without constraints; those bonded to C12 and C9 were subjected to a riding and to a rigid group refinement respectively. The positions of Η atoms bonded to C6A and C6B were calculated but not allowed to be refined. Discussion The crystal structure of the title compound proves an (E ,^ -con figuration which is of relevance for the study of the reactivity of derivatives obtained therefrom in cyclization processes [3]. The structure is affected by disorder, one atom of the pyrrolidine ring * Correspondence author (e-mail: libra@unige.it) being split over two different positions (C6A, C6B). For both of them the pyrrolidine moiety is in an envelope conformation, the ring asymmetry parameters [4,5] evidencing a pseudo-mirror plane through atom C7 in the "C6A-ring" [AC* = 0.002(2); atoms N l , C5, C6A, C8 coplanar within 0.002(6) Â], and another pseudo-mirror plane through atom C5 in the "C6B-ring" [ACj 0.017(3); atoms N I , C6B, C7, C8 coplanar within 0.022(9) Â], The out-of-plane atoms (C7 and C5, respectively) lie at distances of 0.500(5) Â and 0.468(3) Á from the relevant mean planes above. The internal strain imposed by the bulky pyrrolidine moiety reflects on a short 1,5-intramolecular contact [C3---C5 3.093(4) A] and, even more evidently, on a deviation of the C 1 = C 2 sp s y s t e m f r o m p l a n a r i t y , t he t o r s i o n a n g l e N 1 C 1 C 2 C 3 being as large as 22.6(4)°. Correspondingly a rather long C1=C2 bond distance [1.370(3) Â] is found. In order to ascertain the effect of packing forces on the molecular conformation, the geometry of the title compound was optimized with quantum mechanical calculations [6] at the HF/3-21G* level (263 basis functions). Starting data were the experimental coordinates, considering in separate calculations both the "C6A" and the "C6B" forms; they converged to the same molecular model. The resulting molecular conformation shows a remarkable increase of the C 1 C 2 C 3 C 4 torsion angle [from -114.6(3)° in the crystal to -93.0° in the isolated molecule], accompanied by a general relief of the steric hindrance, the N 1 C 1 C 2 C 3 torsion angle decreasing from 22.6(4)° to a mere 0.2° in the isolated molecule. Other changes in geometry are less dramatic: the C1=C2 bond distance decreases to 1.359°A and the short C 3 C 5 intramolecular contact increases to 3.152 A. These values indicate however a residual strain due to the molecular overcrowding. On the other side, in the crystal the intermolecular distances are in the normal range, the shortest contact (with respect to the sum of the involved van der Waals radii) being d(C50 3 ) = 3.162(3) Â ( 0 3 in -χ, 1-y, 1 -z). Therefore, we consider that crystal forces are effective in amplifying the internal strain, as observed in the experimental conformation. Table 1. Data collection and handling. pale yellow prism, size 0.22 χ 0.24 χ 0.48 mm Mo Ka radiation (0.71070 Â) 3.33 cm" Enraf-Nonius CAD4. ω 54.94° 3886,3886 /obs > 2 a(I0bs), 2596 244 NRCVAX [7], SHELXL-97 [8], PLATON [9] Crystal: