Conformational study of some 4'-substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones.

The conformational analysis of some 4'-substituted 2-(phenylselanyl)-2-(ethylsulfanyl)-acetophenones bearing the substituents NO2 (1), Br (2), H (3), Me (4) and OMe (5) was performed by ν(CO) IR analysis, B3LYP/6-31+G(d,p) and single point polarisable continuum model (PCM) calculations, along with NBO analysis for 1, 3 and 5. Calculations for 1-5 indicate the existence of three stable conformations, c1, c2 and c3, whose stability depends on the balance between electrostatic and orbital interactions that are strictly related to the geometrical arrangement. The comparison between the experimental IR spectra in solution and the computed data in gas phase for 1-5 allows the c1 conformer to be assigned to the less intense component at higher frequency of the carbonyl doublet and both the c2 and c3 ones to the more intense lower frequency component. The sum of the calculated molar fraction of c2 and c3 conformers decreases from 95% to 63% on going from 1 to 5 (in gas phase), and this trend compares well with the PCM calculations and the IR experimental data for the majority of the solvents for all compounds 1-5. The NBO analysis for 1, 3 and 5 shows that the sum of the relevant orbital delocalization energies for the c1, c2 and c3 conformers is almost constant and does not match the computed stability order. The lowest stability of the c1 conformer for 1-3 can be related to the small value of the α dihedral angle that enables a strong electrostatic destabilizing repulsion between the O(CO)(δ-)…S(δ-) atoms. The relative stability of the c1 conformer increases for 4 and 5 as the α dihedral angle enlarges and the repulsion is minimized. Moreover, the strong repulsive field effect between the C(δ+)=O(δ-) and C(δ+)-S(δ-) dipoles exerted to a greater extent on the c1 conformers of 1-3 with respect to 4 and 5, causes a major increase of the corresponding C=O bond orders and related carbonyl frequencies. For the c2 conformer, the electrostatic destabilizing repulsion between the O(δ-)…Se(δ-) atoms is weaker than that involving the O(δ-)…S(δ-) atoms in the c1 conformer and therefore has negligible effects on the conformer stability that is mainly determined by the sum of the orbital interactions. The c3 conformer has the shortest S(δ-)…Se(δ-) contact for all compounds and thus the related electrostatic repulsion seems to be the most important factor that affects its stability. In conclusion, the computed order of stability of the three conformers for 1-5 depends on the electrostatic repulsions between close charged atoms rather than on the sum of the orbital delocalization energies that are quite similar for all the conformers.

Spectroscopic and theoretical studies of some 3-(4'-substituted phenylsulfanyl)-1-methyl-2-piperidones.

The analysis of the IR carbonyl bands of some 3-(4'-substituted phenylsulfanyl)-1-methyl-2-piperidones 1-6 bearing substituents: NO2 (compound 1), Br (compound 2), Cl (compound 3), H (compound 4) Me (compound 5) and OMe (compound 6) supported by B3LYP/6-31+G(d,p) and PCM calculations along with NBO analysis (for compound 4) and X-ray diffraction (for 2) indicated the existence of two stable conformations, i.e., axial (ax) and equatorial (eq), the former corresponding to the most stable and the least polar one in the gas phase calculations. The sum of the energy contributions of the orbital interactions (NBO analysis) and the electrostatic interactions correlate well with the populations and the νCO frequencies of the ax and eq conformers found in the gas phase. Unusually, in solution of the non-polar solvents n-C6H14 and CCl4, the more intense higher IR carbonyl frequency can be ascribed to the ax conformer, while the less intense lower IR doublet component to the eq one. The same νCO frequency trend also holds in polar solvents, that is ν(CO)(eq)< ν(CO)(ax). However, a reversal of the ax/eq intensity ratio occurs going from non-polar to polar solvents, with the ax conformer component that progressively decreases with respect to the eq one in CHCl3 and CH2Cl2, and is no longer detectable in the most polar solvent CH3CN. The PCM method applied to compound 4 supports these findings. In fact, it predicts the progressive increase of the eq/ax population ratio as the relative permittivity of the solvent increases. Moreover, it indicates that the computed ν(CO) frequencies of the ax and eq conformers do not change in the non-polar solvents n-C6H14 and CCl4, while the ν(CO) frequencies of the eq conformer become progressively lower than that of the ax one going from CHCl3 to CH2Cl2 and to CH3CN, in agreement with the experimental IR values. The analysis of the geometries of the ax and eq conformers shows that the carbonyl oxygen atom of the eq conformer is free for solvation, while the O[CO]…H[o-Ph] hydrogen bond that takes place in the ax conformer partially hinders the approach of the solvent molecules to the carbonyl oxygen atom. Therefore, the larger solvation that occurs in the carbonyl oxygen atom of the eq conformer is responsible for the observed and calculated decrease of the corresponding frequency. The X-ray single crystal analysis of 2 indicates that this compound adopts the most polar eq geometry in the solid. In fact, in order to obtain the largest energy gain, the molecules are arranged in the crystal in a helical fashion due to dipole moment coupling along with C-H…O and C-H…π(Ph) hydrogen bonds.

3,3-Bis(4-bromo-phenyl-sulfan-yl)-1-methyl-piperidin-2-one.

In the title compound, C18H17Br2NOS2, the conformation of the piperidin-2-one ring is based on a half-chair with the methyl-ene C atom diagonally opposite the N atom being 0.649 (3) Šabove the plane of the remaining five atoms (r.m.s. deviation = 0.1205 Å). The S atoms occupy axial and bis-ectional positions, and the dihedral angle between the benzene rings of 59.95 (11)° indicates a splayed disposition. Helical supra-molecular chains along the b axis sustained by C-H⋯O inter-actions is the major feature of the crystal packing. The chains are connected into a three-dimensional architecture by C-H⋯Br and C-H⋯π inter-actions.

3,3-Bis[(4-meth-oxy-phen-yl)sulfan-yl]-1-methyl-piperidin-2-one.

The piperidone ring in the title compound, C(20)H(23)NO(3)S(2), has a distorted half-chair conformation with the central methyl-ene atom of the propyl fragment lying 0.696 (1) Šout of the plane defined by the other five atoms (r.m.s. deviation = 0.071 Å). One of the S-bound phenyl rings is almost perpendicular to the mean plane through the piperidone ring, whereas the other is splayed [dihedral angles = 71.95 (6) and 38.42 (6)°]. In the crystal, C-H⋯O and C-H⋯π inter-actions lead to the formation of supra-molecular layers in the ab plane.

1-Methyl-3,3-bis-(phenyl-sulfan-yl)piperidin-2-one.

The piperidone ring in the title compound, C(18)H(19)NOS(2), is in a distorted half-chair conformation, distorted towards a twisted boat, with the central methyl-ene C atom of the propyl backbone lying 0.606 (2) Šout of the plane defined by the other five atoms (r.m.s. deviation = 0.1197 Å). One of the S-bound phenyl rings is almost perpendicular to the least-squares plane through the piperidone ring, whereas the other is splayed [dihedral angles = 75.97 (6) and 44.21 (7)°, respectively]. The most prominent feature of the crystal packing is the formation of helical supra-molecular chains along the b axis sustained by C-H⋯O inter-actions. The chains are consolidated into a three-dimensional architecture via C-H⋯π inter-actions whereby one S-bound phenyl ring accepts two C-H⋯π contacts.

1-Methyl-3,3-bis-[(4-methyl-phen-yl)sulfan-yl]piperidin-2-one.

The piperidone ring in the title compound, C(20)H(23)NOS(2), has a half-chair distorted to a twisted-boat conformation [Q(T) = 0.5200 (17) Å]. One of the S-bound benzene rings is almost perpendicular to the least-squares plane through the piperidone ring, whereas the other is not [dihedral angles = 75.28 (5) and 46.41 (5) Å, respectively]. In the crystal, the presence of C-H⋯O and C-H⋯π inter-actions leads to the formation of supra-molecular layers in the ab plane.

1-(4-Bromo-phen-yl)-2-ethyl-sulfinyl-2-(phenyl-selan-yl)ethanone monohydrate.

In the title hydrate, C(16)H(15)BrO(2)SSe·H(2)O, the sulfinyl O atom lies on the opposite side of the mol-ecule to the Se and carbonyl O atoms. The benzene rings form a dihedral angle of 51.66 (17)° and are splayed with respect to each other. The observed conformation allows the water mol-ecules to bridge sulfinyl O atoms via O-H⋯O hydrogen bonds, generating a linear supra-molecular chain along the b axis; the chain is further stabilized by C-H⋯O contacts. The chains are held in place in the crystal structure by C⋯H⋯π and C-Br⋯π inter-actions.

3,3-Bis[(4-chloro-phen-yl)sulfan-yl]-1-methyl-piperidin-2-one.

The piperidone ring in the title compound, C(18)H(17)Cl(2)NOS(2), has a distorted half-chair conformation. The S-bound benzene rings are approximately perpendicular to and splayed out of the mean plane through the piperidone ring [dihedral angles = 71.86 (13) and 46.94 (11)°]. In the crystal, C-H⋯O inter-actions link the mol-ecules into [010] supra-molecular chains with a helical topology. C-H⋯Cl and C-H⋯π inter-actions are also present.

2-(4-Methoxy-phenyl-sulfin-yl)cyclo-hexan-1-one.

The cyclo-hexa-none ring in the title compound, C(13)H(16)O(3)S, is in a distorted chair conformation. The intra-molecular S⋯O(carbon-yl) distance is 2.814 (2) Å. Mol-ecules are connected into a two-dimensional array via C-H⋯O contacts involving the carbonyl and sulfinyl O atoms.

1-Methyl-3-phenyl-sulfonyl-2-piperidone.

The piperidone ring in the title compound, C(12)H(15)NO(3)S, has a slightly distorted half-chair conformation with the methyl, carbonyl and phenyl-sulfonyl ring substituents occupying equatorial, equatorial and axial positions, respectively. Mol-ecules are connected into centrosymmetric dimers via C-H⋯O inter-actions and these associate into layers via C-H⋯O-S contacts. Further C-H⋯O inter-actions involving both the carbonyl and sulfonyl O atoms consolidate the crystal packing by providing connections between the layers.