Mechanisms of long-range ¹³C, ¹³C spin–spin coupling in thioanisole and its derivatives. Conformational applications

Abstract

. Can. J. Chem. 66, 1229Chem. 66, (1988.The I3c nuclear magnetic resonance chemical shifts and the long-range I3C,l3c spin-spin coupling constants are reported for 23 thioanisole derivatives enriched in I3C at the methyl position. Forpara and metn substituted thioanisole derivatives, "J(C,C) (n being the formal number of bonds intervening between the coupled nuclei) can be related to functions of the angle by which the thiomethyl group twists out of the aromatic plane. For n = 3 , 4 , 5 , the ensuing relationships yield estimates of the twofold barriers to rotation about the C( l ) -S bond. The barrier is lower in ethyl phenyl sulfide than in thioanisole derivatives. Complications arise for ortho substituted thioanisole derivatives but estimates of the torsional motion about the C( 1) -S bond can be obtained from the observed "J(C,C). Among the complications is the expected fact that 3~( C , C ) , which is shown to be larger in the cis than the trans orientation of the intervening bonds ("anti-Karplus" behaviour), is perturbed by the substituent attached to the coupled nucleus. It is confirmed that in 2-hydroxythioanisole the thiomethyl group is oriented effectively perpendicular to the benzene plane, attributable to a stereospecific hydrogen bond between the hydroxyl group and the 3p lone-pair on the sulfur atom. In acetone-d6 solution, an equilibrium exists between this conformation and one in which an intermolecular hydrogen bond exists with solvent molecules. In the latter, the thiomethyl group prefers a coplanar orientation. In 2-aminothioanisole, the thiomethyl group twists out of the plane by about 60" so as to optimize the N-H. . .3p interaction. This twist angle is changed very little in acetone-d6 solution because the second N-H bond can hydrogen bond to the solvent molecules without disrupting the intramolecular N-H. . .3p interaction. It is also shown that the chemical shift of the I3C nucleus in the methyl group is a good conformational indicator in rneta and para substituted thioanisoles. Therefore it can be used as such for molecules in which "J(C,C) is difficult to find, in 1,4-dithiomethylbenzene, for example.On a mesurk les dkplacements chimiques en rLsonance magnktique nuclCaire du I3C ainsi que les constantes de couplage spin-spin A longue distance I3c,l3C pour 23 dkrivCs du thioanisole enrichis en I3C dans la position du mCthyle. Pour les dkrivks du thioanisole substitutts en positionspara ainsi que me'ta, les constantes 'J(C,C), pour lesquelles n correspond au nombre de liaisons formelles intervenant entre les noyaux qui sont couplks, peuvent Ctre relikes aux fonctions de l'angle de torsion du groupement thiomkthyle par rapport au plan aromatique. Pour n = 3 , 4 ou 5, les relations qui en dkrivent permettent d'kvaluer les banikres binaires A la rotation autour de la liaison C( 1) -S.La barrikre est plus faible dans le cas du sulfure d'ithyle et de phknyle que dans les cas des dkrivks du thioanisole. On fait face i des complications avec les dCrivks du thioanisole substituks en ortho; tout...

Optimization of the Antioxidant Activity of Hydroxy‐Substituted 4‐Thiaflavanes: A Proof‐of‐Concept Study

“…This behavior has been described previously by Schaefer et al [25] and has been explained by considering the poor tendency of alkylthio substituents to conjugate with the phenolic aromatic ring and their preference to form a stereospecific IHB using a 3p lone pair on sulfur that uses almost pure 3p orbitals to form its bonds. [26] As a matter of facts, compound 5 showed a smaller BDE (OH) and a larger k inh with respect to the corresponding o-alkylthio derivatives, because of the absence of an IHB in the parent phenol and the large conjugative stabilization of the phenoxyl radical by the S atom (see Figure 5).…”

Analysis of long‐range through‐space couplings via an intramolecular hydrogen bond

“…Polar solvents result in rapid equilibrium between intramolecular and intermolecular hydrogen bonding as in other cases. 26 This diminishes the ability of the intramolecular hydro- Could not be resolved.…”

Concerning the internal rotational barrier and the experimental and theoretical nJ(13C,13C) and nJ (1H,13C) in ethylbenzene-β-13C

“…pumpthaw cycles and flame-sealed to provide reasonably symmetric tops. The 'H nmr spectra were obtained with an AM 300 nmr spectrometer in a manner previously described (7,12). Acquisition times of the free induction decays ranged from 41 to 51 s for various response regions; as many as 64 decays were coadded in digital form and the digital resolution was 0.012 Hzlpoint or better.…”

“…It turns out that 6~(~,~) and 6~(~,~) are consistent with two magnitudes of the internal barrier, one high and corresponding to 3 as the conformer of lowest energy, the other much lower and consistent with 1 as most stable. INDO MO FFT (8) computations of long-range, u-T coupling constants have been reasonably successful in the prediction of their qualitative conformational dependence (7,9,10). The quantitative predictions are usually less successful, that is, those of the extrema in relationships such as "J = r'J90 sin20; hence the need for the use of model compounds in which the conformations are well defined (large barriers).…”