Nucleophile reactions azomethine ylide generation

N-Metalated azomethine ylides generated from a-(alkylideneamino) esters can exist as tautomeric forms of the chelated ester enolate (Scheme 11.8). On the basis of the reliable stereochemical and regiochemical selectivities described below, it is clear that the N-metalated tautomeric contributor of these azomethine ylides is important. Simple extension of the above irreversible lithiation method to a-(alkylideneamino) esters is not very effective, and cycloadditions of the resulting lithiated ylides to a,p-unsaturated carbonyl compounds are not always clean reactions. When the a-(alkylideneamino) esters bear a less bulky methyl ester moiety, or when a,P-unsaturated carbonyl compounds are sterically less hindered, these species suffer from nucleophilic attack by the organometallics, or the metalated cycloadducts undergo further condensation reactions (81-85). 

Besides the 1,3-dipolar cycloaddition of azomethine ylides to C60, the Bingel cycloprop anation reaction is widely used for regioselective functionalization of fullerenes. In principle, this versatile modification involves the generation of carbon nucleophiles from a-halo esters and their subsequent addition to C60 [19]. The addition takes place exclusively on double bonds between two six-membered rings of the fullerene skeleton, yielding methanofullerenes. As shown in Scheme 2, addition of diethylbromomalonate to C60, in the presence of an auxiliary base... 

Use of trimethylsilylmethyl triflate enables the effective formation of intermediate iminium salts in the reaction mixture because the counteranion, triflate ion, is nonnucleophilic both to carbon and silicon atoms. N-Silyl-methylation can also be performed with other alkylating agents, such as silymethyl chloride, bromide, and iodide. However, the resulting iminium salts desilylate immediately after they are formed by the attack of the halide counteranions, leading to a serious decomposition of the requisite iminium intermediates. The final step of desilylation generating azomethine ylides is effected by a fluoride anion which is selectively nucleophilic to a silicon atom. 

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