Sulfoxides racemization

Another quite different-looking reaction is the Mi slow rearrangement 1.18 - 1.19, which is invisible because thermodynamically unfavourable but the ease with which it takes place explains why allyl sulfoxides racemize so much more readily than other sulfoxides. Here, one end of the C-S bond moves from the sulfur (S-11) to the oxygen atom (0-21) and the other end moves from C-l to C-3. This is therefore called a [2,3] shift, the bond marked in bold moving two atoms at one end and three at the other. 

Allylic sulfoxides are tdso of interest mainly because of the possible sulfoxide/sulfenate rearrangement described by Mislow when he observed that allyl p-tolyl sulfoxide racemized in a temperature range... 

Scheme 18.4 Dynamic kinetic resolution via allylic sulfoxide racemization-alkene hydrogenation and a non-r2.31 Rh(l)-catalyzed mechanism for sulfoxide racemization. ...

Whereas the barrier for pyramidal inversion is low for second-row elements, the heavier elements have much higher barriers to inversion. The preferred bonding angle at trivalent phosphorus and sulfur is about 100°, and thus a greater distortion is required to reach a planar transition state. Typical barriers for trisubstituted phosphines are BOSS kcal/mol, whereas for sulfoxides the barriers are about 35-45 kcal/mol. Many phosphines and sulfoxides have been isolated in enantiomerically enriched form, and they undergo racemization by pyramidal inversion only at high temperature. ... 

JR)-2-Bromooctane undergoes racemization to give )-2-bromooctane when treated with NaBr in dimethyl sulfoxide. Explain. 

The effect of an additional site of complexation, as well as the influence of additional stereogenic centers, was demonstrated with the diastereomeric sulfoxides 2 A -2D40. Addition of the racemic,... 

The anions derived from racemic alkyl and benzyl tert-butyl sulfoxides undergo 1,4-addition to a,/J-unsaturated esters to give adducts with high product diastereoselection (>5 1)10,11. Alkyl 4-methylphenyl sulfoxides were found to be less diastereoselective. In the case of 2-methyl-2-(methylsulfinyl)propanc the highly hindered 2,6-di-rer7-butyl-4-methylphenyl ester was required to prevent a competing acylation reaction. 

Addition of racemic allylic sulfoxide anions to 2(5//)-furanone gives y-1,4-addition adducts1. The simple and induced diastereoselectivities are completely analogous to that of 2-cyclopen-tenone described earlier. 

The addition of the anion of the racemic 2-methyl-2-propenyl sulfoxides, rac-2-methyl-3-(phenyl-sulfinylpl-propene and /w-3-(rerr-butylsulfinyl)-2-methyl-l-propene to 2-cyclopentenone gives mixtures of (E)- and (Z )-y-l, 4-addition products which are a mixture of diastereomers at sulfur2. The (T )-products usually predominate, with the relative proportions of the (Z)-product increasing as the reaction temperature is increased. No asymmetric induction originating from the stereocenter at sulfur was observed when the sulfoxide substituent was phenyl however, there was a marginal improvement in the case of the (Zi)-product when the sulfoxide substituent was ferf-butyl. 

The 1,4-addition of the anion of a racemic /1-oxo sulfoxide to racemic 2-cyclopentenone was reported to give a single diastereomeric adduct resulting from addition opposite to the y-ace-toxy group20, t he relative configuration of the exocyclic stereocenter was not determined. 

The reaction of the enantiomerically pure sulfoxide anion (0.5 equiv) with a racemic bicyclic enone allows for the kinetic resolution of the enone15b. 

Lithiated racemic (E)- and (Z)-l-(/er/-butylsulfinyl)-2-methyl-2-butenes undergo addition to give predominantly anti-(E)-y- 1,4-addition products2. The sole difference between the major and minor adducts from the (Z)-sulfoxide with those from the (Z)-sulfoxide is in the relative configuration at sulfur. 

The addition of the anions of racemic cyclic allylic sulfoxides to various substituted 2-cyclopentenones gives y-l,4-adducts as single diastereomeric products22. The modest yields were due to competing proton-transfer reactions between the anion and enone. The stereochemical sense of these reactions is identical to that for the 1,4-addition reaction of (Z)-l-(/erf-butylsulfinyl)-2-methyl-2-butene to 2-cyclopentenone described earlier. 

From studies on the addition of racemic allylic sulfoxide anions of 3-substituted l-(phenyl-sulfinyl)-2-propenes to racemic 4-tcrr-butoxy-2-cyclopentenone, it was found that (El-allylic sulfoxides give. vyw-products, and (Z)-allylic sulfoxides give anti-productsx. 

In contrast to allylic phosphine oxides, phosphonates, sulfones and sulfoxides, the chemistry of lithiated allylic sulfoximines has been less extensively developed25 27. The reaction of lithiated racemic A-phenyl-A -(4-rnethylphenyl)-S -(2-propenyl)sulfoximine with either 2-cy-clopentenone or 2-cyclohexenone gave a complicated mixture with 1,4-oc-ad ducts being slightly favored over the 1,4-7-adducts. The yields of these adducts were poor25. In contrast, lithiated racemic Ar-tert-butyldiphenylsilyl-5-phenyl-5,-(2-propenyl)sulfoximine gives mainly 1,4-y-ad-ducts on reaction with the same enones26. 

Racemic mixtures of sulfoxides have often been separated completely or partially into the enantiomers. Various resolution techniques have been used, but the most important method has been via diastereomeric salt formation. Recently, resolution via complex formation between sulfoxides and homochiral compounds has been demonstrated and will likely prove of increasing importance as a method of separating enantiomers. Preparative liquid chromatography on chiral columns may also prove increasingly important it already is very useful on an analytical scale for the determination of enantiomeric purity. 

The numerous examples of optically active sulfoxides reflect their configurational stability. Optically active sulfoxides resist thermal racemization by pyramidal inversion, so... 

Recrystallization of a 1 1 molecular complex formed from several sulfoxides and (R)-( + )-2,2 -dihydroxy-l, l -binaphthyl (7) allowed resolution of the former22. Conversely, using the optically pure sulfoxide, it was possible to resolve racemic bis-naphthol 7. 

Numerous reactions of racemic sulfoxides with chiral reagents have been accomplished2,12. These examples of kinetic resolution usually lead to sulfoxides of low enantiomeric purity, but there are some exceptions. 

Preparation of the appropriate optically active sulfmate ester is initially required for reaction with a Grignard or other organometallic reagent. If the method is to produce homochiral sulfoxides, the precursor sulfmate ester must be optically pure. An exception to this statement occurs if the reaction yields a partially racemic sulfoxide which can be recrystallized to complete optical purity. 

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