Ammonium ylides rearrangements

The first example of an ammonium ylide rearrangement was encountered in 1928 by Stevens and co-workers who observed that a novel [l,2]-benzyl shift occurred when ammonium salt 1 was heated in dilute... 

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. 

Scheme 2.53 [2,3]-Stevens rearrangement of a vinylaziridine-derived ammonium ylide 219.

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. 

Ammonium ylides can also be generated when one of the nitrogen substituents has an anion stabilizing group on the a-carbon. For example, quaternary salts of /V-allyl a-aminoesters readily rearrange to y,8-unsaturated a-aminoesters.286... 

Scheme 6.18. Carbon-Carbon Bond Formation via [2,3]-Sigmatropic Rearrangements of Sulfonium and Ammonium Ylides...

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

Tandem ammonium ylide generation-rearrangement reactions... 

The key step in the total synthesis of (—)-epilupinine 253 involved the ring expansion of a proline-derived spirocyclic ammonium ylide to give 252 through a [1,2] Stevens rearrangement, as shown in Scheme 51 <1997T16565>. 

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

A ring enlargement process was used effectively to access the enantiopure pyrrolo [ 1,4]oxazepine-9a(7//)-carboxylate derivatives 142 and 143. The sequence involved copper (Il)-catalysed decomposition of an a-diazocarbonyl derivative attached to a chiral morpholinone, and a carbenoid, spiro-[5,6]-ammonium ylide, Stevens [1,2] rearrangement sequence. The Stevens and related rearrangements have considerable further potential for novel heterocyclic syntheses <00TA3449>. 

Table 2.21. Preparation of five-membered, nitrogen-containing heterocycles by rearrangement of chromium-carbene-derived ammonium ylides.

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Tertiary amines can react with electrophilic carbene complexes to yield ammonium ylides which usually undergo Stevens rearrangement (Figure 4.8) leading to products of a formal carbene C-N bond insertion. 

Fig. 4.8. Formation and Stevens rearrangement of ammonium ylides from acceptor-substituted carbene complexes.

Allylammonium ylides can undergo 2,3-sigmatropic rearrangement [1234]. With weakly nucleophilic amines, C-H bond insertion or hydride abstraction can compete efficiently with ammonium ylide formation. 

Experimental Procedure 4.2.5. Ammonium Ylide Formation and Stevens Rearrangement Diethyl 2-Benzyl-2- [(ethoxycarbonyl)mefiryl](methyI)amino malo-nate [1241]... 

Table 4.16. Generation and rearrangement of ammonium ylides from diazocarbonyl compounds and tertiary amines.

Wittig rearrangement of benzylic quaternary ammonium salts upon treatment with alkali metal amides via the ammonium ylide intermediates. 

Rearrangement Processes of Oxonium and Ammonium Ylides Formed by Rhodium(ll)-Catalyzed Carbene Transfer... 

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