Silver fluoride, azomethine ylide generation

Padwa and co-workers have continued their long standing studies in the area of azomethine ylide generation and trapping, and now report that the N-[(trimethylsilyl)methyl]indoles (102) react with the electron-deficient alkenes (103), in the presence of silver fluoride, to afford the cycloadducts (104) regio- and stereo-specifically.7 The azomethine ylide(105) is a probable... 

The Pandey group has developed a silver fluoride-promoted desily lation of tertiary bis(silyl)amines as an interesting alternative method to access azomethine ylides (Scheme 2.10).18 Notably, this method allows the generation of nonstabilized azomethine ylides under essentially neutral conditions. The starting materials are prepared by a three-step process, sometimes coupled into a single operation. For example, Boc-protected pyrrolidine 36 can be sequentially deprotonated and silylated twice in a one-pot reaction (Scheme 2.10). Removal of the Boc group and alkylation of the free amine leads to bis(silyl)amine 37. When this compound is treated with 2 equiv of silver fluoride in the presence of phenyl vinyl sulfone, rapid formation of products 39 as single stereoisomers results. 

The same chiral auxiliary has been used in the cycloaddition of an optically active azomethine ylide to benzaldehyde and to l-nitro-2-(3,4-methylenedioxyphenyl)ethylene Ae ylide was generated in situ by treating (R)-(+)-/V-(l-phenylethyl)-A -cyanomethyl-A -trimethylsilylmethylamine (33) with silver fluoride. Unfortunately, no selectivity was observed in the first case and only a 3 2 preference was expressed in the second. Use of the azomethine ylide derived in the same manner from (/ )-(-)-N-(l-phenyl-2-methoxyethyl)-/V-cyanomethyl-N-trimethylsilylmethylamine (34) displayed a modest, but potentially useful, 4 1 diastereofacial selectivity in its reaction with l-nitro-2-(3,4-methylenedioxyphenyl)ethylene. The precise structure of the major and minor product was not determined (Scheme 25). 

Azomethine ylides can also be generated from a-cyano- or a-phenylthiomethylamines <85JOC4006, 86CB813). Silver fluoride promotes the elimination. The adducts from alkynes are 2,5-dihydropyrroles which can be oxidized to pyrroles by ddq (Scheme 82). 

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