Sommelet-Hauser Nitrogen

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]. 

Heterocyclic nitrogen-derived ylids behaviors have been studied.373 For instance, pyridine derivatives lead exclusively to [2,3]-sigmatropic rearrangement (Sommelet-Hauser) products. 

The Sommelet-Hauser rearrangement has rarely been used as a tool in organic synthesis and then only for very special systems [54], The reason is undoubtedly that the Hofmann elimination and the dealkylation reaction of quaternary nitrogen atoms are in direct competition with the ring expansion reactions of these substrates. 

Base catalyzed migration of one alkyl group from a quaternary nitrogen atom to the a -carbon atom of a second alkyl group or to an ortho aromatic position (see also Sommelet-Hauser) (via ammonium ylids, oroxonium ylids)7. 

Sommelet-Hauser rearrangement of the nitrogen ylide formed upon desilylation of oxazolinium derivatives affords benzoxazocines (67) and/or their putative tautomers (84) in low to modest yield... 

Although nitrogen ylides have been extensively used in organic transformations, especially those concerning the formation of nitrogen heterocycles,not many references are available for the use aza-sulfonium ylides and the Sommelet-Hauser rearrangement in the synthesis of indole derivatives. 

The Sommelet-Hauser reaction, a [2, 3] sigmatropic rearrangement of ammonium ylides, is also an attractive method for ring expansion of nitrogen heterocycles. Recently... 

Figure 13 Competition between the Sommelet-Hauser (top) and Stevens (bottom) rearrangements in the ring expansion of nitrogen heterocycles...

This is achieved with substitution about the nitrogen causing a sterically favored dissociation. On the other hand, the Sommelet-Hauser rearrangement can be promoted by delocalization of the double bond involved in the rearrangement, since this bond has to rotate (and effectively lose its double-bond character). This seems to be favored by electron-withdrawing substituents on the double bond. Refinement of these theoretical predictions should enable experimental verification. 

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