Sulfur hyperconjugation

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

The barriers for the rotation around the SS bond have been estimated from microwave data [40] and ab initio MO calculations [41, 42] as 34-36 kj mor (cis-barrier) and 24-25 kJ mol (trans-barrier), respectively. They are explained as the result of three effects First, the repulsion of the two 3p lone pairs at the sulfur atoms is lowest for r=90°. Second, the 3p lone pair electrons are partly delocalized into the antibonding a orbitals of the SH bonds originating from the neighboring sulfur atom. This hyperconjugation or 7i bond is at its maximum for r=90° but disappears for r=0° and 180°. Third,... 

This also proves an earlier conclusion on hyperconjugation in an 0CH20 fragment of the 1,3-dioxolane cation radical this conclusion was based on mass spectrometry (To-dres, Kukovitskii et al. 1981). As calculated, the carbon-hydrogen bonds corresponding to 0CH20 in the radical cation are weaker than those in the neutral molecule. For this reason, this site exhibits a maximal probability that deprotonation will result in the formation of the 2-yl radical (Belevskii et al. 1998). In experiments, photoirradiation of 1,3-dioxolane solutions in sulfur hexafluoride at 77 K really leads to formation of the cation radical of 1,3-dioxolane and the l,3-dioxolan-2-yl radical as a result of deprotonation. Consecutive ring... 

The a-deprotonation of conformationally constrained thioethers can proceed with high diastereoselectivity (Scheme 2.16) [54]. That equatorial protons are removed much more rapidly than axial protons suggests that stabilization of carbanions Qt -S-Q by sulfur is mainly a result of hyperconjugation between the carbanion lone pair and the antibonding S-Ca- orbital [55]. 

The effect of the sulfur atom is principally one of acidification - a phenylsulfanyl (PhS) substituent increases the acidity of a-protons by about 5-10 pATa units.79 This effect is now generally considered not to be a result of interaction with sulfur s d-orbitals, but rather of sulfur s polarisability80 and of hyperconjugation with the antiperiplanar S-C a orbital (110).81... 

Anderson and coworkers emphasized the polarizability of sulfur, which accounts for the regiochem-istry of C—H acidification by sulfur. Epiotis et al. developed a hyperconjugative model involving delocalization of the unshared pair on carbon into the low-lying adjacent S—R antibonding orbital," which accounts for the stereochemical aspects of C—H bond acidification by sulfur. 

A further interaction that allows to delocalize the positive charge also onto the two apical sulfur atoms was found to be the 3p 3a hyperconjugation as shown in Fig. 11. 

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