[1660-43-7] · C15H17BF4S · Isopropyldiphenylsulfonium Tetrafluoroborate · (MW 316.20)
Preparative Methods: this reagent is not commercially available and must be made prior to use. The ylide isopropylidenediphenylsulfurane, obtained by deprotonation with an appropriate base, is usually formed in situ and used immediately because of its extremely low stability.2 Two methods have been reported for the preparation of the sulfurane. Ethyldiphenylsulfonium tetrafluoroborate (formed by the reaction of diphenyl sulfide with triethyloxonium tetrafluoroborate, eq 1) is suspended in a solution of DME. The ylide is formed by addition of LiCHCl2 at -70 °C and reacted with MeI (1.1 equiv). The desired sulfonium salt precipitates from solution. The isopropylidenediphenylsulfurane is formed by addition of LDA to this suspension at -78 °C (eq 2).2
A second, but less efficient, method of preparation involves the reaction of diphenyl sulfide, AgBF4, and neat isopropyl iodide (eq 3).2
Three different base/solvent systems have been used to generate the isopropylidenediphenylsulfurane ylide: DME/LiCHCl2, t-BuOK/THF,5 and PhLi/THF.6 All methods have proven equally efficient. Different diastereomer ratios have been obtained with the t-BuOK/THF method compared to the other systems.7
Addition of 1 equiv of benzaldehyde to a solution of the ylide, formed as described above, in DME at -60 °C results in the formation of the oxirane (eq 4) in 75-80% isolated yield.2 Ketones may also react in good yields, but reaction times are somewhat longer.
Van Tamelen et al.8 used isopropylidenediphenylsulfurane to synthesize radiolabeled 4-norsqualene 2,3-oxide (eq 5) and homosqualene oxide.
If the carbonyl is a,b-unsaturated, reaction takes place at the alkene via addition/substitution to form gem-dimethyl-substituted cyclopropanes. For example, the gem-disubstituted cyclopropane was prepared from cyclohexenone in 74% yield (eq 6).9
Meyers et al. have reacted the ylide with a chiral bicyclic lactam to yield a tricyclic gem-disubstituted cyclopropane intermediate which, after four steps, gave deltamethric acid whose esters act as very potent insecticides (eq 7).10
In 1988, Krief et al. used isopropylidenediphenylsulfurane to synthesize (1R,3S),cis-hemicaronic aldehyde, a precursor to the (1R,3S),cis-dibromovinylchrysanthemic acid constituent of deltamethrin (eq 8).6,11
Wender et al. in 1989 used the ylide to synthesize an essential intermediate in the synthesis of phorbol (eq 9), a tumor promoter.12
Rapoport et al. developed an elegant synthesis of the carene ring system via reaction of 1-ethoxycarbonyl-1,3-cyclohexadiene with isopropylidenediphenylsulfurane (eq 10).4 The regiochemistry of the cyclopropane formation depends upon the solvent used in the reaction. This dependency of the regioisomer distribution provides a convenient route to either of the two carene ring systems.
Tony Durst & Robert N. Ben
University of Ottawa, Ontario, Canada