Enolates acylammonium

Discussing the stereochemical outcome of the Claisen rearrangements, two aspects had to be considered. On the one hand, the relative configuration of the new stereogenic centers was found to be exclusively syn in 201 and 202, pointing out the passing of a chair-like transition state c-a and c-jS, respectively, including a Z-acylammonium enolate structure (complete simple diastereo-selectivity/internal asymmetric induction). 

The nucleophilic addition on substituted ketenes is a well-known method to generate a prochiral enolate that can be further protonated by a chiral source of proton. Metallic nucleophiles are used under anhydrous conditions therefore, the optically pure source of proton must be added then (often in a stoichiometric amount) to control the protonation. In the case of a protic nucleophile, an alcohol, a thiol, or an amine, the chiral inductor is usually present at the beginning of the reaction since it also catalyzes the addition of the heteroatomic nucleophile before mediating the enantioselective protonation (Scheme 7.5). The use of a chiral tertiary amine as catalyst generates a zwitterionic intermediate B by nucleophilic addition on ketene A, followed by a rapid diastereoselective protonation of the enolate to acylammonium C, and then the release of the catalyst via its substitution by the nucleophile ends this reaction sequence. 

The [2+2] cycloaddition also occurs between ketenes and aldehydes to give ox-etanones. This reaction is catalysed by tertiary amines which form acylammonium enolates in situ (Figure 8.8). 

The ketens used in cycloadditions with cyclopentenes are frequently generated in situ from an acid chloride and a tertiary amine, and the mechanism of these dehydro-halogenations has been investigated. The results suggested that the keten is formed via an acylammonium salt rather than via an enolate. Either species may be trapped under suitable conditions, indicating a complex series of equilibria. 

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