Ammonium ylids

SOMMELET HAUSER Ammonkjmyfld rearrangement Rearrangement of quaternary ammonium ylids to aminas by aryl trarafer. 

The proton in the thiazolium ring is relatively acidic (p Ta about 18) and can be removed by even weak bases to generate the carbanion or ylid an ylid is a species with positive and negative charges on adjacent atoms. This ylid is an ammonium ylid with extra stabilization provided by the sulfur atom. 

There are also vinylogues of the [2,3] rearrangement like the [4,5] shift in the ammonium ylid 5.90. This intermediate had available a [2,3] shift just... 

The key part of the molecule for reactivity is the thiazolium salt in the middle. The proton between the N and S atoms can be removed by quite weak bases to form an ylid. You saw sulfonium ylids in Chapter 46, and there is some resemblance here, but this ylid is an ammonium ylid with extra stabilization from the sulfur atom. The anion is in an sp2 orbital, and it adds to the reactive carhonyl group ofpyruvate. 

SOMMELET Aldehyde aynthesis 353 SOMMELET HAUSER Ammonium ylid rearrangement 354... 

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. 

Radical mechanisms are also known to be involved in 1,2-rearrangement reactions, such as the R alkyl group migration leading from ammonium ylids 16 to tertiary amines 19 (Scheme 3). Whilst the possible ion pair intermediate 17 is destabilized by the electron-withdrawing carbonyl group, the radical pair 18 is favored by the captodative stabilization of one of the radicals. 

Some of the first reports of an organocatalytic cyclopropanation were made by Gaunt and co-workers who used ammonium ylids as nucleophiles and reacted them into... 

Stevens rearrangement of carbonyl-stabilized ammonium ylids... 

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