Crystal structures of phenyl-substituted cyclopropanes. IV. the crystal structure (at 21‡C and −100‡C) and the phenyl ring conformation in 4-cyclopropylacetanilide

Abstract

The crystal structure of 4-cyclopropylacetanilide was investigated at room temperature (21 ~ and at -100~ in order to determine the orientation of the phenyl ring with respect to the cyclopropane moiety and the effect of this substituent on the stereochemistry of the three-membered ring. The compound was chosen because it is one of the few species containing a simple phenyl ring as the sole cyclopropane ring substituent and whose crystals are suitable for X-ray diffraction at room temperature. The substance crystallizes in space group P21/c at either temperature (no phase transitions) with cell constants: (at 21~ a = 9.725(2), b = 10.934(3), and c = 9.636(2) .A., ~ = 106.13(1)~ V = 984.21 ~3 and d(calc; z = 4) = 1.182 g cm -3. The relevant parameters for the -100*C structure are a = 9.557(4), b ---I0.980(2), and c = 9.641(2) ,~,/~ = 106.34(3)~ V = 970.76 ,~3 and d(calc; z = 4) = 1.199 gcm -3. Final values were R(F) = 0.042, Rw = 0.035, using unit weights, and its nonhydrogen atoms were used to phase the low-temperature data, whose final discrepancy indices were R(F) = 0.051, Rw = 0.061. The phenyl substituent is almost exactly in the bisecting conformation with respect to the C--C--C angle at the point of attachment to cyclopropane and conjugative effects are clearly evident in the lengths of the cyclopropane ring [1.494(3), 1.498(3), and 1.474(4) A, the later being the distal bond]. If one omits the terminal methylene fragments at C 10 and C 11, the atoms comprising the acetanilide fragment and the substituted carbon of the cyclopropane ring lie in a nearly perfect plane. Molecular mechanics as well as semiempirical (AM 1) calculations were carried out in order to determine the structure of the energy-minimized configurations in the two computational environments. The molecular conformations thus obtained are close to that experimentally observed from the X-ray diffraction experiment. In both theoretical models, the lowest energy conformation is that in which the plane of the phenyl ring bisects the cyclopropane C--C--C angle as was experimentally observed. Finally, the shape of the conformational barrier as a function of the orientation of the plane of the phenyl ring was computed, giving a maximum barrier to rotation of 2.2 kcal/mol. Similar calculations were carried out for two other aryl cyclopropanes, whose rings (naphthalene and anthracene) cannot adopt the bisecting position. Comparisons of experimental geometrical parameters as well as of the barriers to rotation are presented.

Chameleonic Behavior of the α‐Methylcyclopropyl Group and Its Through‐Space Interactions: A Route to Stabilized Three Redox States in Diarylnitroxides

“…The electronic properties of the cyclopropyl group and the cyclopropylphenyl moiety are well documented in the literature . According to spectral and X‐ray experiments, arylcyclopropanes preferentially adopt the bisected conformation (providing there is a π‐conjugative interaction between the cyclopropane and arene) unless steric repulsion prevents the compound from reaching this state. Thermodynamic stabilization provided by this interaction (1.9 kcal mol −1[29] ) indicates that the cyclopropyl group can freely rotate at room temperature, with a slight preference for the bisected conformation.…”

Synthesis, Crystal Structure, and Rotational Energy Profile of 3-Cyclopropyl-1,2,4-benzotriazine 1,4-Di-N-oxide

“…22 In the bisected conformation the molecular orbital overlap between the cyclopropyl group and the arene π-system is maximal. The bisected conformation is exemplified, for example, by the crystal structures of cyclopropylbenzene 23,24 and of cyclopropyl acetophenone. 25 …”

Crystal structure of methyl 3-(2-cyano-2-ethoxycarbonyl-3- phenylcyclopropyl)prop-2-enoate, C17H17NO4

“…The cyclopropyl bond lengths are affected by the interactions with these groups. The proximate bonds (d(C1-C2) =1 .528(4) Å and d(C1-C3) = 1.556(4) Å)ofthe cyclopropane ring bearing the phenyl group are significantly elongated by 0.04 Å (mean values) compared to the distal bond (d(C2-C3) =1.491(4) Å)which has the normal bond length (1.497 Å) [7]. The difference between the proximal bond lengths C1-C2 and C1-C3 demonstrate not only the difference between the electronic effect of the vinyl group and the phenyl group with its synchronous orientation, but also the steric clash between the cyano group and these groups.…”