Sulphur ylides, rearrangement

The synthesis of 3-benzylcyclobutanone (3) is an illustration of an overall intramolecular alkylation of an acyl anion equivalent (Section 5.9). The a,a>-dihalide is 2-benzyl-l,3-dibromopropane, and the acyl anion equivalent is methyl methylthiomethyl sulphoxide2 the product is 1-methylsulphinyl-l-methylthio-3-benzylcyclobutane which is obtained as a mixture of cis/trans isomers [(9) and (10)] (Expt 7.3). Aqueous acid hydrolysis in ethereal solution unmasks the carbonyl group. The possible mechanism of the reaction is via a Stevens-type rearrangement of the intermediate sulphur ylide, which may proceed in a pericylic, radical or ion pair fashion. 

Prenyl rearrangement. To date, only 4-prenyl-, but no 4-tert-prenylindoles have been synthesised. Rainier and co-workers exploited an interesting sequence of sulphur ylide Claisen rearrangement and Cope rearrangement to achieve C4-prenylation of 2-thioindoles (Scheme 51) [201]. Sulphur ylide intermediate... 

Scheme 51 Sequential sulphur ylide Claisen and Cope rearrangements leading to C4-prenylation of indole [201]...

Rainer has studied the sulfonium ylide-mediated thio-Claisen rearrangement. Rhodium carbenoid 443, generated from rhodium acetate and alkenyl diazoacetate 441, reacted with the 2-ethylthioindole 440 to reveal the sulphur ylide 445. Proton shift generated the ketene aminethio acetal 446 which underwent thio-Claisen rearrangement to yield indoline 442 in excellent yield. 

Thus, Simonneaux and co-workers have reported treatment of methyl allyl sulphide with 2-diazoethyl acetate at 25 in the presence of (SBF>4PRuCO as catalyst (homogeneous reaction, entry 1 in Table 4), which leads to ethyl-2-(methylthio)pent-4-enoate in 88 % yield with 8 % of dimers [56]. As presented in Scheme 21, the formation of ethyl-2-(methylthio)pent-4-enoate derives from the [2,3]-sigmatropic rearrangement of the sulphur ylide and that of the dimers (di-ethylfumarate and diethylmaleate) derives from 2-diazoethyl acetate dimerization. 

Treatment of the nitrogen ylides 74 (R1 = Me, Ph R2, R3 = alkyl) with sulphur in hot benzene gives the amines 75 ( 30% yield) (Eq. (11)). A mechanism for this Dimroth-type rearrangement has been proposed. The structure of the product 75 (R1 = Ph, R2 = R3 = Me) has been confirmed by X-ray crystallography (95JOMC127). 

Considerable attention has been focussed recently upon the reaction of car-benes with sulphur compounds, and there are now many examples of stable sulphonium ylides. The first example of a rearrangement within this series has been reported by Benati et aL, with formation of the dibenzothiepin (251). 

Stabilized ester ylides can undergo several possible reactions on heating. Heating (6a) to 155 led to a Steven s rearrangement product (25). Migration preference is shown when one group on sulphur is especially stabilized, in that (6c) afforded (25c) and (6d) afforded (25d). In the latter... 

Ylides of Sulphur, Selenium, Tellurium, and Related Structures 359 was proposed that sulphene formation was involved in each reaction, with the triphenylphosphine adding to the sulphene and displacing SO2 to afford the phosphonium ylide, which was protonated by the conjugate acid of the amine. Sulphenes have been reported to react with tropone to afford an adduct which rearranged on heating to an o-hydroxystyrene (lOQ. ... 

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