Long-range 19F-1H spin-spin coupling as an indication of conformational equilibrium of fluoro-substituted aryl vinyl sulfides

Abstract

Long-range 19 F-1 H spin-spin coupling with participation of vinyl group protons, observed in the 1 H NMR spectra of fluoro-substituted aryl vinyl sulfides, indicates that these compounds exist as equilibrium mixtures of s-cis and s-trans rotational isomers.I, R 1 = CH 2 =CHS, R 2 = F; II, R 1 = Cl, R 2 = F; III, R 1 = R 2 = CH 2 =CHS; IV, R = H; V, R = Me.Steric structure of vinyl sulfides having various substituents was studied by several physicochemical methods [1,2]. It was found that alkyl vinyl sulfides give rise to rotational isomerism related to internal rotation about the S-C α bond. According to the results of nonempirical quantum-chemical calculations with account taken of electron correlation in terms of the second-order perturbation theory using Pople's triplesplit basis set, the molecule of methyl vinyl sulfide adopts preferentially planar s-cis conformation [3]. The second, less energetically favorable conformer is represented by two quasi-degenerate nonplanar gauche structures separated by a low rotational barrier corresponding to the planar trans conformer [3]. Therefore, it can be regarded as quasi-s-trans conformer in which molecular fragments move with a large amplitude. Alkyl vinyl sulfides with branched alkyl groups are characterized by predominant s-trans (gauche) conformation [1, 2]. On the other hand, there are no published data on rotational isomerism in the series of aryl vinyl sulfides, though electronic structure of these compounds was examined on the basis of their 13 C NMR spectra [4]. Furthermore, 1 H and 13 C NMR studies on hetaryl vinyl sulfides revealed some conformation-related parameters which can be used to identify predominant spatial configuration [5,6].We previously synthesized several fluoro-substituted aryl vinyl sulfides [7,8] which were used as initial compounds in the synthesis of a large number of more complex derivatives possessing an analogous fragment [9][10][11][12][13]. These compounds clearly displayed in the 1 H NMR spectra long-range spin-spin couplings between 19 F nuclei and proton in the α-position of the vinyl group, and the observed couplings were used to identify regioisomers [10,11]. However, long-range 19 F-1 H coupling with other protons in the vinyl group was not observed previously, and stereochemical conditions ensuring observation of such coupling were not discussed in studies on the synthesis of fluoro-substituted aryl vinyl sulfides. The present communication reports on the use of long-range 19 F-1 H spin-spin couplings involving α-and β-protons in the vinyl group, as well as of other conformation-dependent 1 H and 13 C NMR parameters, in experimental study on rotational isomerism of fluorine-containing aryl vinyl sulfides I-III. The populations of different rotamers of I-III and unsubstituted phenyl vinyl sulfide IV were compared on the basis of other conformation-related parameters of their 1 H and 13 C NMR spectra.

Effect of aromatic ring anisotropy on the 1H NMR shielding constants and conformational equilibrium of sterically strained aryl vinyl ethers

“…The procedure for performing structural studies on organic molecules with the aid of NMR spectroscopy is attractive due to its accessibility and simplicity, for NMR chemical shift is one of the most readily and reliably measurable spectral parameters of a substance. Conformational equilibria in the series of fluorinated aryl vinyl sulfides [5] and selenides [6] were studied previously by analysis of their 1 H and 13 C NMR spectra. In the present study stereochemical relations inherent to 1 H and 13 C shielding constants were used to examine rotational isomerism in the series of sterically strained aryl vinyl ethers having a bulky substituent in the ortho position with respect to the vinyloxy group.…”