SOMMELET HAUSER Ammonium ylid rearrangement

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

Fluoride ion-assisted desilylation has been extensively used to create an ylid from a /V-silyl methyl-quaternary ammonium salt. Its evolution to final produces) is variable and Sommelet-Hauser and Stevens rearrangement products were obtained (often as major products) in a ratio that can be shifted from one structure to another very close one, as in examples 1 and 2 dealing with //-benzyl salts.246,366 Differences in the solvents used are not significant because in the first example, HMPA does not reverse the ratio, yields and selectivity being just a bit lower, /so-toluene was proposed as an intermediate in example 1 it might also be the intermediate in example 2. Thus product partition reflects the relative ability of the C-H or the C-C bonds to be cleaved to produce aromatization with proton or a-amine carbocation migration. 

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

Sommelet-Hauser product 678. Small amounts of the para rearrangement product (679) and the direct displacement product (680) are also present. The ammonium salt precursor to the ylid (674) can generate either ylid (675 or 681), hut formation of 681 is sterically inhibited relative to 675. The Stevens product of 681 (amine 682) is more sterically crowded than 676 or 677. In polar aprotic solvents the major product is the Sommelet-Hauser product 678, hut in nonpolar solvents (hexane) the Stevens product 676 predominates. In DMSO, the enhanced nucleophilicity of the base causes the displacement product 680 (base attacks the methyl carbon and displaces the amine) to be formed in high yield. Alkoxide bases are not strong enough to generate the ylid from 674, and the displacement reaction (to give 680) dominates in those cases. 

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