Sulfoxides, allylic isomerization

In contrast to the allylic sulfenates mentioned so far, cinnamyl trichloromethanesulfenate (9), prepared by the usual method, can be isolated and is relatively stable. Furthermore, its rearrangement to cinnamyl trichloromethyl sulfoxide (i.e., without allylic isomerization, equation 7), proceeds at a relatively slow rate (in CC14 at 80.0 °C, k = 3.90 x 10 5s 1). This result also contrasts with the observation mentioned earlier that cinnamyl arenesulfinate rearranges to a-phenylallyl aryl sulfone33,34. Similar behavior has been detected for y, y-dimethylallyl ester 11 which undergoes thermal isomerization to sulfoxide 12 (equation 8)36-38. 

As a means to generate allylic sulfoxides, the isomerization of vinyl sulfoxides was foimd to be an interesting alternative. As an example featuring this method, its application in efforts directed toward the total synthesis of gua-nacastepene A will be presented (Scheme 51) [ 125]. 

The reaction of the anion of an aryl allyl sulfoxide with benzaldehyde can take place via an a or y attack. The a attack leads to a product with three stercogcnic centers (four possible diastereomers) whereas the y attack results in a product which has only two stereogenie centers and geometric isomerism is possible. 

Owing to the reversible nature of the allylic sulfenate/allylic sulfoxide interconversion, the stereochemical outcome of both processes is treated below in an integrated manner. However, before beginning the discussion of this subject it is important to point out that although the allylic sulfoxide-sulfenate rearrangement is reversible, and although the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, desulfurization of the sulfenate by thiophilic interception using various nucleophiles, such as thiophenoxide or secondary amines, removes it from equilibrium, and provides a useful route to allylic alcohols (equation 11). 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

Similar to the well-known thio-Claisen rearrangement of allyl aryl sulfides211 and sulfonium salts212, the thio-Claisen rearrangement of allyl aryl sulfoxides has also been reported213. For example, heating of allyl 2-naphthyl sulfoxide (147) at 120 °C for 2h in dimethylformamide resulted in quantitative isomerization to the dihydronaphthothio-... 

The reactions of the corresponding propargyl sulfoxides and sulfones now resemble the chemistry of the other acceptor-substituted derivatives such as ketones and aldehydes (see Section 1.2.4). Compared with the thioethers, here much milder bases are sufficient apart from aluminum oxide, often triethylamine or potassium carbonate are used. Sometimes even a spontaneous isomerization takes place. The selective isomerization of one triple bond in the presence of a second triple bond in 126 [313] (Scheme 1.56) or an allyl sulfone in 129 [314] (Scheme 1.57) are just two examples out of a whole series [178, 304, 313, 315-331]. When, on the other hand, the in situ oxidation of 126 was carried out in an aprotic solvent, no isomerization at all was observed. 

The thermal racemization of optically active aryl allyl sulfoxides ArS(0)CHjCH=CH2 is orders of magnitude faster, and has a much lower activation energy, than that of aryl alkyl sulfoxides ArS(0)R (Bickart et ai, 1968). The reason is that the presence of the allyl group permits the sulfoxide to equilibrate with the isomeric, achiral sulfenate ester by a concerted, cyclic process (89) for whichis only about 20 kcal moM.The rates of racemiz-... 

The double bond of 9-allylcarbazoles can be moved into conjugation with the nitrogen by treatment with potassium rcrt-butoxide in dimethyl sulfoxide the cis-prop-l-enyl isomers were formed initially in studies with 9-allyl-carbazole, 9-allyl-3-chlorocarbazole, 9-allyl-3-nitrocarbazole, and 9-allyl-3,6-dichlorocarbazole. " Such isomerizations must proceed via a car-banion produced by proton abstraction from the saturated carbon on nitrogen. Indeed, when the red anion from 9-benzylcarbazole, formed using n-butyllithium, was quenched with deuterium oxide, the 9-CHDPh derivative was obtained. When the anions from 9-ally 1-carbazoles in the series 81... 

The isomerization of allyl ethers to 1-propenyl ethers, which is usually performed with potassium tert-butoxide in dimethyl sulfoxide, can also be carried out under milder conditions using tris(triphen-ylphosphine)rhodium chloride,208 and by an ene reaction with diethyl azodicarboxylate,209,210 which affords a vinyl ether adduct. Removal of an O-allyl group may be achieved by oxidation with selenium dioxide in acetic acid,211 and by treatment with N-bromosuccinimide, followed by an aqueous base.201,212... 

That sulfides coordinate to palladium(II) more strongly than the corresponding sulfoxides has received an interesting confirmation in a study of the complexes (19)-(21) (R = 4-MeQjHt).149 The complex (19) is the best catalyst both for cyclotrimerization of diphenylethyne and for isomerization of allyl ethanoates. In both processes a vacant metal coordination site is essential and the thioether sulfur is too strongly bound (in 20 and 21) for facile dissociation. 

It has been reported that treatment of methyl 1-methylsulfanylvinyl sulfoxides with sodium thiophenolate in methanol affords l-methylsulfanylalk-l-en-3-ols. A sequence has been proposed218 (see Scheme 45) in which the thiophilic base first causes an in situ isomerization of the vinyl sulfoxide moiety into an allylic sulfoxide which then undergoes a [2,3]-sigmatropic rearrangement and subsequent thiophilic cleavage of the intermediate sulfenic ester. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

The other major [2,3]-rearrangement, the Evans rearrangement,463 can be extended to synthesize functionalized allyl alcohols.464-465 The Evans rearrangement involves the rearrangement of an allylic sulfoxide to an allyl alcohol.463 The reverse reaction is also possible the conversion of an allyl alcohol to the analogous sulfoxide. The push-pull nature of the two rearrangements (Scheme 26.19), coupled with the transfer of chirality, provides a method to invert an allyl alcohol together with the isomerization of the alkene (Scheme 26.20)438,466... 

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