Active methylene compounds tetrafluoroborate

Quaternary ammonium salts of heterocyclic compounds have been used in liquid-liquid phase-transfer syntheses. When these compounds are achiral, they show a behavior very similar to that of other quaternary ammonium salts. For example, 2-dialkylamino-l-alkylpyridinium tetrafluoroborates have been used by Tanaka and Mukayama282 in the alkylation of active methylene compounds PhCH2CN, PhCH(Et)CN, and PhCH(Me)COPh. However, comparative studies of the efficiency of the catalysts show that alkylpyridinium bromides283 or N-alkyl-Af-benzyl-piperidinium chloride284 have a smaller catalytic activity compared to tetraalkylammonium halides. McIntosh285 has described the preparation of azapropellane salts 186 as potential chiral phase transfer catalysts. 

Dinitrobenzenediazonium tetrafluoroborate can be used to form colored derivatives of activated methylene compounds such as malononitrile and related species. The products may be separated by TLC on silica gel layers [71]. 

A superior route to these compounds using the condensation procedures involves the use of alkoxycyclopropenyl cations (equation 73) . These can be prepared readily by alkylation of cyclopropenones with trialkyloxonium tetrafluoroborates . This process can be extended to a large number of active methylene compounds if the conditions of the reaction are controlled carefully and the tertiary non-nucleophilic base diisopropylethy-lamine is used Otherwise, a large number of side reactions are observed. 

It may be that the mixture of l-ethyl-2-chloropyridinium tetrafluoroborate and NaNs, recommended by Monteiro (1987 a) for diazo transfer to activated methylene compounds is essentially based on the primary formation of l-ethyl-2-azidopyridinium ions as transfer reagent. 

A new route to racemic a-amino-acids in general consists of an overall a-amination of simple carboxylic acid esters in a relative of the Japp-Klingermann reaction, a method which is usually only successful with active methylene compounds such.as malonates. Thus, 0-silyl-enolates of esters (464) condense with benzenediazonium tetrafluoroborate to give, after isomerization,... 

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