Sulfides, allylic 2,3-sigmatropic rearrangement

Scheme 3.56 Allyl sulfides imination/sigmatropic rearrangement sequence.

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

A more direct access to the unstable and non isolated sulfonium ylides 58a- c is the reaction of diisopropyl diazomethylphosphonate 57 with allylic sulfides, catalyzed by Cu(II), Rh(II) [39], or ruthenium porphyrins.[40] For example, the a-phosphorylated y,d-unsaturated sulfides 59-61 are obtained through the [2,3] -sigmatropic rearrangement of 58a-c. This method allows the use of a greater variety of starting allylic sulfide substrates, such as 2-vinyl tetrahydrothiophene, or propargylic sulfides (Scheme 15). 

Finally, Katsuki and coworkers [271] described an enantioselective Ru-catalyzed domino reaction, which includes a sulfamidation of an aryl allyl sulfide 6/3-111 using the chiral Ru(salen)-complex 6/3-115, followed by a 2,3-sigmatropic rearrangement of the formed 6/3-112 to give N-allyl-N-arylthiotoluenesulfonamides 6/3-113. On hydrolysis, 6/3-113 yielded N-allyltoluenesulfonamides 6/3-114 (Scheme 6/3.33). The enantioselectivity ranged from 78 to 83% ee. 

Uemura and co-workers (91) demonstrated that copper catalysts effectively transfer nitrenoid groups to sulfides generating chiral sulfimides. A complex obtained from CuOTf and 55d catalyzes nitrenoid transfer to prochiral sulfides to afford products such as 139 in moderate to poor enantioselectivities (<71% ee, Eq. 78). Nitrenoid transfer occurs selectively to the sulfur atom of allylic sulfides generating allylic sulfenamide (140) in moderate selectivity, after [2,3] sigmatropic rearrangement of the initial sulfimide 141, Eq. 79. 

Thienyl sulfides with the 2-position of the thiophene ring blocked, e.g., allyl 2-methyl-3-thienyl sulfide (51), also undergo sigmatropic rearrangement when heated in quinoline [Eq. (23)]. 2,6-Dimethyl-2,3-dihydro-thieno[3,4-Z ]thiophene (52) is formed, together with other products. 

Uemura and co-workers developed an interesting and unique catalytic system, in which the Rh(ii) intermediate is generated by Rh2(OAc)4-catalyzed reaction of conjugated ene-yne-carbonyl compounds (Equation (16)). The Rh(ii)-carbene is trapped by allyl sulfide to give [2,3]-sigmatropic rearrangement product 121 in good yields." ... 

Besides commonly used TsN=IPh, TsNs can also be employed as a nitrene source. Bach and Korber have recently reported an Fe(iii)-catalyzed imidation of allyl sulfides 200 with iV-/ //-butyloxycarbonyl azide (BocNs), followed by [2,3]-sigmatropic rearrangement (Scheme 21). " The azide reacts with FeCl2 to generate Fe(iv) nitrene complex 201, which then reacts with allyl sulfide 200, to give an intermediate sulfimide 203, presumably through Fe(iii) intermediate 202. 

The [3,3] sigmatropic rearrangement of allyl vinyl sulfides (thio-Claisen) is particularly easy activation enthalpies can be 10 kcal mob1 lower than for the oxygen analogues [486], and the reaction which was pioneered by Brandsma [487] occurs usually at room temperature and with excellent yields. 

Methylidenation of allylic thioethers. Methylidenation of an allylic phenyl-thioether with methylene iodide-diethylzinc is accompanied by a 2,3-sigmatropic rearrangement to a homologous allylic phenylthioether. The rearrangement is also initiated by ethylidene iodide. Cyclopropanation is not observed. The Simmons-Smith reaction with allylic sulfides results only in formation of an insoluble polymer. 

Asa direct application of this transformation, the combination of FeCl2 and BocN 3 was subsequently used for the imination of allyl sulfides, which undergo a [2,3]-sigmatropic rearrangement in the reaction media (Scheme 3.56) [172],... 

Enantioselective catalysis has also been used for the synthesis of optically active sulfimines [51]. By application of 5 mol % of the bisoxazoline-copper(I) catalyst 80, the sulfide 77 is oxidized catalytically to 78 which undergoes a [2,3]-sigmatropic rearrangement to give allyl amine 79 in 80% yield and with 58% ee (Eq. (20)). Other alkenes were found to give lower ee. 

Ketene, dichloroketene, and related compounds react with allylic ethers, sulfides, and selenides in a [3.3] sigmatropic rearrangement. This interesting reaction allows the synthesis of a number of medium and large ring compounds by expansion [72] [73] [74]. Dichloroketene has been prepared in situ by slow addi-... 

At first, the asymmetric [2,3]sigmatropic rearrangement of allylic sulfur ylides is surveyed. The first successful asymmetric [2,3]sigmatropic rearrangement involving sulfur ylides by Trost and Hammen appeared in 1973 [44]. The treatment of optically active sulfonium salt afforded the corresponding homo-allylic sulfide with 94% enantiomeric excess, in which the chiral information has been transferred from the sulfur atom to the carbon atom with excellent control (Scheme 28). Trost and Biddlecom also reported another approach to generate... 

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