Allylic ylides, rearrangement

The allylic position of the sulfide group in the cycloadducts may be employed for 2,3-sigmatropic reactions, as demonstrated by the alkenation/sulfur ylide rearrangement sequence (144) (145). A... 

With the aid of a nitrogen ylide rearrangement, it is possible to transform an allylic secondary amine to an unsaturated carbonyl system, in certain circumstances with transfer of chirality (Scheme 53). ... 

Carbenoid attack at the heteroatom of allylic halides, ethers, amines, sulfides, and selenides generates an allylic ylide 23 that can undergo [2,3] sigmatropic rearrangement and, less frequently, [1,2] insertion (C-X insertion with and without allylic rearrangement, respectively). Similarly, an Af-acyl-2,5-dihydropyrrole could be cyclopropanated with ethyl diazoacetate in good yield. 

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

Sigmatropic rearrangements of allyl sulphonium allyl ylides (75) provide the basis of a repetitive one-pot ring-expansion sequence" for 2-vinyl thiocyclic compounds (Scheme 50). A paper" and a section in a review article" have described the utilization of allyl thiol dianions and allyl sulphide anions in carbonyl group umpolung . 

The catalytic effectiveness of ruthenium porphyrins for ylide generation in reactions of ethyl diazo acetate and diisopropyl diazomethylphosphonate with some allylic substrates was described for the first time by Simoimeaux and coworkers (Scheme 20) [194]. These reactions result in products of the [2,3]-sigmatropic rearrangement of intermediate allylic ylides. It was demonstrated that simple ruthenium porphyrins are highly effective catalysts for carbenoid reactions with alkyl allyl sulfides and alkyl allyl amines providing... 

There are a number of [2,31-sigmatropic rearrangements related to the Mislow-Evans rearrangement but differing in the heteroatom moiety. While these are not the main focus of the current chapter, they present unique opportunities for synthetic applications, and several recent examples are oudined in the following. Another closely related process, the [2,3]-Meisenheimer rearrangement of allylic IV-oxides, is discussed separately in Chapter IS on ammonium ylide rearrangements. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

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). 

Allyl vinyl ethers have been prepared using the ylide (101) but only from non-enolizable carbonyl compounds. The ethers rearrange on heating to give a-allyl aldehydes, e.g. (102). 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

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