dicarbonylcyclopentadienyl(dimethylsulfoniummethylide)iron

synthetischethe preparation of Dicarbonyl(cyclopentadienyl)[(dimethylsulfonium)methyl]iron Tetrafluoroborate has gone through several revisions and improvements since its introduction in 1979. The most recently improved preparations[2] are satisfactory for large-scale applications and have been adapted for commercial use. In a one-pot operation, the stable, crystalline, dinuclear complex [Cp(CO)2Fe]2 is reductively cleaved with sodium dispersion to generate a THF solution of the highly nucleophilic sodium ferrate, Na[Cp(CO)2Fe]. This species is immediately alkylated with chloromethyl methyl sulfide, and the resulting iron alkyl, Cp(CO)2FeCH2SMe, is subjected to S-alkylation with methyl iodide. Finally, the resulting sulfonium salt, initially produced as an iodide, is subjected to in situ anion exchange and simultaneous crystallization by treatment with hot aqueous NaBF4, followed by cooling to give the reagent as a yellow-orange, or amber-colored, crystalline solid (eq 1) which has been characterized spectroscopically and by single-crystal X-ray diffraction.[3] The anion exchange is performed in order to assure that a nonnucleophilic counterion is present during the subsequent cyclopropanation reactions; a nucleophilic anion such as iodide may apparently intercept reactive intermediates on the pathway leading to the desired cyclopropane products. A remarkable property of the reagent is its unusually high stability compared to most other organometallic reagents. This point is especially clear from the fact that it can be recrystallized from hot water exposed to the air.