Ammonium salts ylides

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

Highly Lewis basic and nucleophilic functional groups are not compatible with zinc carbenoids. The methylation or ylide formation of heteroatoms is one of the most important side reactions of these reagents. For example, amines, thioethers and phosphines readily react with the zinc reagents to generate ammonium salts", sulfonium" and phosphonium ylides" ". Terminal alkynes generally lead to a large number of by-products". ... 

A closely related reaction is formation of nitrogen ylides from quaternary ammonium salts (see 7-7) ... 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

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... 

Ylides lacking electron-withdrawing groups attached to the nitrogen atom also undergo this rearrangement. Rautenstrauch and co-workers demonstrated this point by transforming the symmetrical allylic ammonium salt 32 into amine 33. The overall yield for this process was 87%.23... 

Using simple allylic ammonium salts, Hill and co-workers showed that the rearrangement of rrans-(/ )-ammonium ylide 48 followed by... 

Propargylic groups are also known to participate in [2,3]-sigmatropic rearrangements and lead to the isolation of allenes as the major products. For example, the nitrogen ylide 65 was formed by treating the corresponding ammonium salt with NaH in DMSO afforded allene 66 in 86%... 

The reaction of l,4-diazabicyclo[2.2.2]octane (DABCO)-derived quaternary ammonium salts with electrophilic alkenes in the presence of a base opens a stereoselective route to highly substituted 2,3-dihydrofurans <2005S2188>. An ammonium ylide is initially formed that attacks the Michael acceptor. The resulting enolate finally expels DABCO in an O-alkylation reaction, affording fra j -2,3-disubstituted dihydrofurans as the major products (Equation 65) <2005S2188>. 

Ester-stabilized ammonium ylides of a-(dialkylamino)nitriles, formed by treatment of ammonium salts with DBU (l,5-diazabicyclo[5.4.0]undec-5-ene), undergo spontaneous fragmentation to give a,3-unsatu-rated nitriles. ... 

Another type of ylidic rearrangement, the Sommelet-Hauser rearrangement, was discovered later, in 1937, by Sommelet and studied extensively by Sommelet and by Hauser. This transposition involves the base-promoted rearrangement of non stabilized ammonium and sulfonium ylides. Thus, the ammonium salt (11), when heated with alkali, gave cleanly the substituted diphenylmethane derivative (12 Scheme 5). Wittig found that the dibenzyl ammonium salt (13) reacted with phenyllithium giving two... 

A second side reaction using this approach is the Hoffmann elimination of the sulfide or amine, during attempted deprotonation of the sulfonium or ammonium salt. This reaction is especially important in the case of ammonium ylides. Finally, another problem which surfaces during the deprotonation step is the formation of a mixture of ylides, e.g. (45) and (46) from (44), which undergo competing rearrangements to give (47) and (48 Scheme 12). ... 

The last, and probably most interesting, example of the approach towards chiral products derived from ylidic 3,2-rearrangements, involves the generation of a chiral ylide by enamioselective deprotonation of a prochiral sulfonium (ammonium) salt using a chiral base. Few attempts have been reported so far, the most noteworthy coming from the laboratory of Trost and involving the reaction of the sulfonium salt (321) with the chiral base (322 Scheme 81). The asymmetric induction is rather poor and represents the extent of prochiral recognition by the chiral base. 

Several limitations of this base-promoted rearrangement are worthy of mentioning (a) when structurally feasible, both Stevens and Sommelet-Hauser reactions will compete (b) certain substituents on the aromatic ring, e.g. Cl, CN, NO2, prevent the formation of the ylide and no rearrangement takes place (c) if a p-hydrogen is present on the ammonium salt, elimination becomes yet another undesirable side reaction, In an attempt to overcome these limitations, Sato has utilized the fluoride-induced desilylation of... 

The amine a-anions (38a-c) failed to rearrange under conditions employed for the analogous ethers. The corresponding ammonium salts, on the other hand, readily isomerize via the derived ylides (Stevens rearrangement) in a process analogous to the 1,2-Wittig rearrangement. ... 

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. 

Miscellaneous. Tetramethylphosphonium salts have been generated from methylenetrimethylphosphorane by treatment with acid in inert solvent, or with ammonium salts in inert solvents or in aqueous ethanol. Details have appeared of the preparation of ]8y-unsaturated ketones using lithiated phen-acylidenetriphenylphosphorane. Alkylation of the metallated ylide (32) has... 

Higher levels of diastereoselection are observed for the rearrangement of ylides derived from ammonium salts 128j 102. The relative configuration of the major products of this rearrangement were assigned based on an analysis of transition state conformations. 

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