Spectroscopic and Theoretical Studies of Some 3-(4′-Substituted phenylsulfanyl)-1-methyl-2-piperidones

Jean Santos,Maurizio Colle,Paulo Olivato,Daniel Rodrigues,Elisângela Vinhato,Julio Zukerman-Schpector,Bruna Contieri,Carlos Cerqueira

doi:10.3390/molecules18077492

Abstract

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 νCO (eq) < νCO (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 νCO frequencies of the ax and eq conformers do not change in the non–polar solvents n-C6H14 and CCl4, while the νCO 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…πPh hydrogen bonds.

Highlights

The analysis of the infrared band and theoretical B3LYP/6-311++G(d,p) calculations of someN,N-diethyl-2-[(4′-substituted) phenylsulfanyl acetamides Et2NC(O)CH2SC6H4-Y (Y=OMe, Me, H, Cl, Br, NO2) [1] indicated the existence of two pairs of cis (c) and gauche (g) conformers in the gas phase, with the gauche conformers being significantly more stablethan the cis ones
The similarity of the carbonyl frequencies in the gas phase for the anti and syn pairs of the cis and gauche conformers originates a carbonyl doublet in solution for which the less intense, higher frequency component is ascribed to the pair of the cis conformers and the lower frequency component corresponds to the more stable pair of the gauche conformers
Aiming to throw more light on the nature of the orbital and electrostatic interactions that influence the stability of the cis-gauche conformers of 2-(4′-substituted)-phenylsulfanylamides [1], this paper reports the IR study of some six-member ring lactams, i.e., 3-(4′-substituted phenylsulfanyl)-1-methyl2-piperidones bearing in the 4′ position the substituents, i.e., Y = NO2, Br, Cl, H Me and OMe (Scheme 1) along with density functional theory (DFT), polarizable continuum model (PCM) and natural bond orbital (NBO)

Summary

Introduction

The analysis of the infrared (νCO) band and theoretical B3LYP/6-311++G(d,p) calculations of some. Our theoretical and X-ray single crystal studies of some 3-(4′-substituted phenylsulfonyl)-1-methyl-2-piperidones [7,8] have shown that, in these compounds, the 2-piperidone ring assumes a slightly distorted half-chair conformation. Aiming to throw more light on the nature of the orbital and electrostatic interactions that influence the stability of the cis-gauche conformers of 2-(4′-substituted)-phenylsulfanylamides [1], this paper reports the IR study of some six-member ring lactams, i.e., 3-(4′-substituted phenylsulfanyl)-1-methyl2-piperidones bearing in the 4′ position the substituents, i.e., Y = NO2 (compound 1), Br (compound 2), Cl (compound 3), H (compound 4) Me (compound 5) and OMe (compound 6) (Scheme 1) along with density functional theory (DFT), polarizable continuum model (PCM) and natural bond orbital (NBO). Calculations for 4 taken as the representative compound for the whole series, along with the X-ray diffraction analysis of 2 These compounds were chosen taking into account some conformational rigidity of the piperidone ring, which allows the 3-substituent to assume almost exclusively the quasi-equatorial and axial conformations. Atom labeling of 3-(4′-substituted phenylsulfanyl)-1-methyl-2-piperidones and definition of relevant torsion angles

Results and Discussion

Materials

IR Measurements

Data Collection and Processing

Theoretical Calculations

Conclusions