Ammonium ylides, -sigmatropic

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. 

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. 

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]. 

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. 

Natural (-l-)-polyzonimine (19) has been synthesized by a reaction sequence using the asymmetric [2,3]sigmatropic rearrangement of the ammonium ylide to generate the chiral intermediate. The Homer-Emmons reaction of the ketone... 

It has been noted that for the ammonium ylide generation copper catalysts such as copper(ii) acetylacetonate [Cu(acac)2] and Cu(hfacac)2 are superior over Rh(ii) catalysts. Sweeney and co-workers have recently reported copper-catalyzed [2,3]-sigmatropic rearrangement of ammonium ylide generated from tetrahydropyridines 150 and diazo ester 129 (Equation (22)). A detailed study on the reaction conditions has revealed that Cu(acac)2 is the best catalyst for this reaction. 

A new synthesis of cr-substituted and a,a-disubstituted a-amino acid derivatives based on the ammonium ylide formation/[2,3]-sigmatropic rearrangement has been recently reported by Clark s group.Decomposition of a-diazo -keto ester 153 was studied in detail with Rh2(OAc)4, Cu(acac)2, and Cu(hfacac)2 as the catalyst. Cu(acac)2 and Cu(hfacac)2 gave similar results, but Rh2(OAc)4 turned out less effective (Equation (23)). 

Chem. Soc. Perkin Trans. 11999, 2439—2447. Blid, J. Somfai, P. Lewis add mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides. Tetrahedron Lett. 2003, 44, 3159-3162. 

II. AMMONIUM YLIDES A. Base-Promoted [1,2]-Sigmatropic Rearrangements... 

The [2,3]-sigmatropic reaction was found to proceed with high re-gioselectivity for the formation of the rearranged product. Thus, ammonium ylide 34 gave amine 35 with >95% trans-selectivity.24 This is undoubtedly a consequence of the concerted nature of the reaction. 

A valuable part of the [2,3]-sigmatropic rearrangement of ammonium ylides is the fact that stereochemical information can be transferred. For example, Kaiser and co-workers stereoselectively alkylated the C-6 position of penicillin using the nitrogen ylide 46 derived from lactam 45.28 Quatemization of 45 with allyl bromide followed by ylide generation using sodium hydride effected the [2,3]-rearrangement. This resulted in the exclusive formation of P-lactam 47 in 75% yield. 

The reaction of dichlorocarbene with A, A -diethyl-3-methyl-2-butenamine (100) produced A V-diethyl-4-methyl-2-pentenamide (103) as the major product.51 The formation of this material was attributed to the generation of ammonium ylide 101 followed by a [l,2]-allylic shift to give intermediate 102 which then hydrolyzed during workup to produce amide 103. No product resulting from a [2,3]-sigmatropic rearrangement of ylide 101 was detected in the crude reaction mixture. 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

Sigmatropic rearrangements of cyclic sulfonium and ammonium ylides have been extensively used in ring contraction and ring enlargement methods. An example is shown in Scheme 55. 

Allenes can be easily prepared using the 3,2-sigmatropic rearrangement of ylidic intermediates. This methodology also provides a general route towards the synthesis of cumulenes. Some examples using sulfonium and ammonium ylides are collected in Table 5. ... 

Sulfonium ylides (allylSR-CR2) and ammonium ylides (allylNR2-CR2) are also substrates for the [2,3] sigmatropic rearrangement. Both ylides are usually generated in situ by deprotonation of the sulfonium or ammonium salt. 

The [2,3]-sigmatropic rearrangement of ammonium ylides can lead to interesting heterocycles. Although it has been known for some time that the Simmons-Smith... 

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