Sulfoxide group

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Tosylhydra2one and 7-phenyl sulfoxide groups have also been introduced into cholesterol and eliminated to prepare the 5,7-diene (45,46). The method of choice is the aHyUc brornination—dehydrobrornination procedures, and the commercial yields ia converting cholesterol to 7-dehydrocholesterol are ia the range of 35—50%. 

PPL and Hpase from Pseudomonas sp. catalyze enantioselective hydrolysis of sulfinylalkanoates. For example, methyl sulfinylacetate (46) was resolved by Pseudomonas sp. Hpase in good yield and excellent selectivity (62). This procedure was suitable for the preparation of sulfinylalkanoates where the ester and sulfoxide groups are separated by one or two methylene units. Compounds with three methylene groups were not substrates for the Hpase (65). 

The sulfoxide group tends to occupy an axial rather than an equatorial position in the thiane S-oxides. 

The formation of a sulfoxide group is confirmed by infrared spectroscopy absorption bonds typical of a sulfonic group can be observed in 1120 -i- 1160 and 1310... 

The stereochemical course of the enantioface differentiation on the aldehyde is dictated by the configuration of the sulfoxide group sulfinyl-subsdtuted dihydroisoxazoles epimeric at C-5 (e.g., 18) provide aldol adducts 19 with the same configuration at the hydroxy-substituted carbon (C-2 ) independent of the absolute configuration at C-5, however, with different degrees of stereoselectivity23. 

However, it can be expected that anions of allyl sulfoxides maintain, at low temperature, the chiral integrity of the sulfoxide group and that reactions with electrophiles could have regio-and stereochemical implications. 

Mainly sulfoxide groups are introduced as chiral auxiliaries for the modification of a,/J-unsat-urated enones (see Section D.1.5.3.5.). Chiral imine derivatives have also been used (see Section D.1.5.3.1.). Various chiral alcohols, and in particular 8-phenylmenthol, have been successfully used as auxiliaries, mainly in two-fold Michael additions to a,/ -unsaturatcd esters. 

Several theoretical and experimental characteristics of the sulfone and sulfoxide groups are substantially modified when these are incorporated within a cyclic array. As a rule, the smaller the ring size the larger the deviation from the normal expected properties and behavior of the sulfone and sulfoxide groups. 

The first substituted thiirene dioxides21 and thiirene oxides22 (e.g. 4 x = 2 and x = 1, respectively) were synthesized and characterized by Carpino and coworkers, while the parent thiirene oxide and dioxide are not known to date. However, the successful syntheses of the substituted unsaturated systems 4 opened the door to an extensive research involving the theoretical and experimental aspects of this class of intriguing compounds2, particularly as far as the unique role and characteristics of their sulfone and sulfoxide groups are concerned. 

Since it is clear that the presence of an unshared pair of electrons on the sulfur of the sulfoxide group leads to no special instability in the case of the known thiirene oxides (i.e., 18a, 28a,b and the first alkyl-substituted thiirene oxide 30 recently synthesized60), the reduced antiaromatic properties of the thiirene oxides relative to that of thiirenes have been manifested experimentally. As far as the possibility of electron-attracting conjugative stabilization involving the sulfur atom in thiirene oxides is concerned, the experimental evidence accumulated so far is not decisive. Thus, the chemical shift of the vinylic carbon of... 

TABLE 4. Selected IR stretching frequencies of sulfone and sulfoxide groups in thiirane and thiirene dioxides and oxides... 

Miscellaneous (formation of complexes, and configuration induced by the sulfoxide group). 

The question arises whether there are any unique characteristics associated with the acidity of a-hydrogens when the sulfone or the sulfoxide group is incorporated within a three-membered ring system. 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

Both the sulfone and the sulfoxide groups are characteristically electrophilic based on the increasing electropositivity of the sulfur atom in proportion to its oxidation state. Therefore, the nucleophilicity of these groups can be discussed only in terms of the nucleophilicity of either the trivalent sulfur atom, still having a pair of nonbonding electrons, or the oxygen atom in the sulfoxides. 

In the following transformations, the nucleophilic oxygen of the sulfoxide group plays a... 

Formation of cyclic sulfoxide (R)-55 by t reatment of bromoarene (i )-54 with tributylstan-nane apparently proves that intramolecular homolytic substitution at the sulfur atom of the sulfoxide group proceeds with strict inversion of configuration (equation 50)103. 

If the reagent is optically active because of the presence of a chiral sulfoxide group, the reaction can be enantioselective. For a review of such cases, see Solladie, G. Chimia, 1984, 38, 233. 

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