DIMETHOXYCARBENIUM TETRAFLUOROBORATE

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafluoroborate (H. Meerwein, 1960 R.F. Botch, 1968, 1969 from trimethyl orthoformate and BF3, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

In this case, a preliminary alkylation of the thioether group by triphenylmethyl hexafluorophosphate, Meerwein s salt, or dimethoxycarbenium tetrafluoroborate induces the process. The dimethylthiosulfonium complexes are versatile and easy-to-handle precursors to generate the corresponding (methylene)iron complex in situ, which can be used for the cyclopropanation of alkenes (Scheme 4-58). 

Related Reagents. Dimethoxycarbenium Tetrafluoroborate Dimethyliodonium Hexafluoroantimonate Dimethyl Sulfate O-Methyldibenzofuranium Tetrafluoroborate Methyl iodide Tri-methyloxonium Tetrafluoroborate. 

Purification highly pure oxonium salt is obtained from dimethoxycarbenium tetrafluoroborate and dimethyl ether. The resulting solid is vacuum dried at 50 °C/1 mmHg for 30 min. 

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