Oxygen-sulfur interactions

Optically active sulfonium and selenonium salts are well known and the stereochemistry of the isomers has been studied.1 3 Optically active cyclic diaryl(alkoxy)-sulfonium salts 14, 15, and 16, stabilized by intramolecular sulfur-oxygen interaction, were synthesized in 2000 by reacting optically active spirosulfuranes with trimethyloxonium tetrafluoroborate.29 The absolute configurations were assigned on the basis of the reaction mechanism. The sulfonium salts were hydrolyzed in KHC03aq. to yield optically active sulfoxides in over 86% ee (Scheme 7). 

A. Kucsman and I. Kapovits, Non-bonded sulfur-oxygen interaction in organic sulfur compounds, in Organic Sulfur Chemistry Theoretical and... 

The stability constants for the 2o/10 species seem to be mainly controlled by the activation energies and, in turn, rate constants for the dissociation of the three-electron bond (back reaction of eq. 40/40a). The respective values, for the two systems with all-methyl and all-i-propyl substitution are 57 and 17 kJ mol and 1.5x10 and 5.6x10 s . 6 xhe latter are certainly in line with the S S bond energies but, again, a direct correlation is not justified because in aqueous solution the reaction of interest is not simply the dissociation of the 3-e-bond but, in fact, a displacement process which involves also a water molecule. (See section on sulfur-oxygen interactions ). 

The >S 0(0)C- species touches on another aspect of general interest, namely, the establishment of a three-electron bond between two elements of very different electronegativity. Any sulfur-oxygen interaction, in principle, constitutes an extreme situation with respect to an asymmetry in the MO energy diagram. One of the most relevant examples is probably the association of a water molecule to an oxidized sulfur radical cation, as formulated in eq. 58.56... 

Clearly, with the sulfur-oxygen interactions we are approaching the limits of the 2a/la concept. 

This trend is fully corroborated by the experimental results obtained for differently substituted systems. The values listed in Table 2 reveal a dramatic effect of the substituents on the optical transition energies. The combined effect of four /-butyl groups no longer allows stabilization of the sulfur-sulfur three-electron bond. In this case (i.e. upon oxidation of /-BU2S) only the monomer /-Bu2S radical cation, or more precisely a water associate of it (see Section 3.5.2 on sulfur/oxygen interactions) which absorbs at 310 nm can be detected [96]. 

Although the R2S -type species are usually addressed as monomeric sulfide radical cations, some interaction with the solvent must be anticipated in polar liquids. This should generally be kept in mind when assessing and comparing properties of these species. It is of particular relevance for aqueous solutions. The specific situation in this environment will, however, not be addressed in this chapter but discussed in detail later in connection with sulfur-oxygen interactions in Section 3.5.2. 

A more stable sulfur-oxygen interaction can be achieved via i /ramolecular association in sterically rigid and structurally defined molecules as in structures 22and23[150, 151]. 

A general question arising in this context is whether these hydroxyl adducts, and possibly all (or at least some) of the sulfur-oxygen interaction species should be formulated as S.%0, that is, three-electron bonded species or perhaps better as sulfuranyl radicals, that is, >S -OH. The former is not unjustified because of many characteristics typical of a a weakened bond. On the other hand, the significant difference in electronegativity between these two heteroatoms drives the unpaired electron towards the more electropositive atom and, therefore, the >S -OH notation should perhaps be preferred since it describes the actual electronic situation more accurately. In this sense sulfur-oxygen (and in general X-Y) interactions approach the limits of the 2[Pg.184]

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

Facile isocyanide insertion reactions into metal-carbon, -nitrogen, -sulfur, -oxygen, - hydride, and - halide bonds have been found to readily occur. The insertion into metal-hydrides to give stable formimidines is particularly noteworthy since corresponding formyls (—CHO) are exceptionally difficult to synthesize and tend to be very unstable. There is a great deal of interest in carbon monoxide reductions, and the instability of the intermediate reduction products has made a study of the reduction process extremely difficult. Recently, however, the interaction of isocyanides with zirconium hydrides has allowed the isolation of the individual reduction steps of the isocyanide which has provided a model study for carbon monoxide reduction (39). 

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