Sulfoxides, allyl stability

While the chemistry of alkyl and allylic sulfoxide anions is similar to that of phosphine oxides, phosphinates and sulfone stabilized anions (Sections 1.5.2.2.1 -2), the situation is further complicated by the additional stereogenic center at sulfur. Therefore in all cases, asymmetric induction may arise from the stereocenter at sulfur. 

In summary, the evidence described above demonstrates three main mechanistic features of the rearrangement of allylic sulfenates to sulfoxides (1) spontaneous and wholly concerted [2,3]-sigmatropic shift of allyl or a-substituted allyl esters (7 a, b) at one extreme (2) complete stability of the y-aryl and y,y-dialkyl substituted allyl sulfenates as well as... 

Also, the trimerization reaction was observed by using jr-allyl complexes of palladium stabilized by chelating ligands, such as 28 and 29 (38). The reaction was carried out at 70°C in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), and 1,3,6,10-dodecatetraene (27) was obtained in 60% yield at 30% conversion. 

The alkylation of sulfur-stabilized anions has been the subject of an excellent recent review633. Anions adjacent to a wide range of sulfur functionalities may be alkylated readily, the most common being sulfoxide and sulfone a-anions. In the synthesis of retinoic acid derivatives and vitamin A634-636, a-sulfonyl anions have been alkylated with an co-acetoxy-containing allyl halide in good yield (equation 96). 

Notice that arrows both start and stop on the sulfur atom, which changes from S(II) to S(IV) during the reaction. The new functional group with an S 0 bond is called a sulfoxide. This is j a good preparation of allylic sulfoxides. The ph/ product forms an anion stabilized by sulfur, which can be alkylated. 

The sulfoxide moiety is an excellent anion stabilizing group, i.e., allyl sulfoxides or vinyl sulfoxides with a hydrogen in a y-position are easily deprotonated (see Section D. 1.1.1.5.3.). Therefore, a-substituted allylic sulfoxides can either be obtained by alkylation of an unsubstituted allylic sulfoxide or by deprotonation-a-protonation of vinyl sulfoxides. 

The preparation of a-selenoketones, esters, nitriles and related compounds can easily be performed via alkylation of the corresponding enolates or stabilized carbanions [21]. These compounds have found many synthetic applications in radical chemistry. In Eq. (9), a typical example involving a ketone is depicted [22]. The stability of a-selenoketones such as 41 is remarkable. Similar reactions with lactones have been performed. For instance, this approach has been applied to the stereoselective synthesis of oxygen-containing rings to either faces of a bicyclic structure [23]. The approach based on a-selenenylation/radical allyla-tion compares favorably with classical enolate allylation procedures, which usually leads to mixture of mono- and diallylated compounds. Furthermore, this strategy is excellent for the preparation of quaternary carbon centers [24] as shown by the conversion of 43 to 45, a key intermediate for the synthesis of fredericamycin A, [Eq. (10)] [25]. Similar reactions with sulfoxides [26] and phosphonates [27] have also been reported. 

The 1,4-addition reaction of stabilized allylic carbanions with conjugated enones is normally complicated by the ambident reactivity of the carbanion. Clean 7-selective conjugate addition to cyclic enones, however, has been observed with lithiated allylic sulfoxides, phosphine oxides and phosphonates... 

A useful extension of sulfoxide-sulfenate rearrangements exploits the greater stability of allylic vs. vinylic sulfoxides.For example, the Knoevenagel product (202) is deconjugated under the conditions of its formation. This sets the stage for a 2,3-rearrangement in the same pot (equation 66). Optically active condensation products give hydroxy esters (201) with 64-72% 50-80% ee were reported for... 

A three-stage synthesis of allylic alcohols has been devised (Scheme 32)," which consists of (i) alkylation of a sulfone-stabilized allylic carbanion (ii) peroxy acid oxidation of the allylic sulfone to give a 2,3-epoxy sulfone and (iii) reductive elimination of the 2,3-epoxy sulfone to give the allylic alcohol. The overall strategy is similar to that of the Evans-Mislow allylic alcohol synthesis based on the 2,3-sig-matropic rearrangement of allylic sulfoxides. However, there are regiochemical advantages to the sul-... 

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