[64035-64-9] · C5H11F3O3SSi · Trimethylsilylmethyl Trifluoromethanesulfonate · (MW 236.32)
Alternate Name: trimethylsilylmethyl triflate.
Solubility: miscible with common organic solvents (ethers, halocarbons, hydrocarbons).
Form Supplied in: colorless oil.
Analysis of Reagent Purity: 1H NMR (CDCl3, ppm): d 0.19 (9 H, s) 4.27 (3 H, s).
Handling, Storage, and Precautions: can be stored indefinitely in the refrigerator under anhydrous conditions.
An imine is alkylated with the title reagent (TMSCH2OTf) to give a salt such as (1) which is not isolated, but is treated with Cesium Fluoride to generate the reactive azomethine ylide (2). In the presence of Dimethyl Acetylenedicarboxylate as the dipolarophile, (2) is trapped by [2 + 3] cycloaddition to afford a pyrroline (eq 1). Azomethine ylides (3)-(8)2,5 can be generated and trapped in the same way with external dipolarophiles, while (9)5h undergoes the analogous internal [2 + 3] cycloaddition process.5 Other desilylation approaches to azomethine ylides have been developed since the initial report in 1979,1 and the topic has been reviewed.2,6
The desilylation approach permits regiospecific methylide generation from sulfides or amines. If the ylide also contains an allylic group as a heteroatom substituent, then 2,3-sigmatropic rearrangement (eq 2) occurs smoothly. This process has been demonstrated for the ammonium ylide (10),2 and the sulfonium ylides (11)2 and (12).7 Rearrangement by 2,3-shift can also involve an aromatic double bond in ylide (13), as shown in eq 3.8 In this case, two nonaromatic intermediates can be detected, but chromatography affords the aromatized product 2,3-dimethyl(methylthiomethyl)benzene. Similar intermediates have been reported in the rearrangement of N-benzylammonium methylides generated by the desilylation method.9
Alkene formation is possible via sulfide alkylation with TMSCH2OTf, followed by ylide generation with CsF.2 The process involves a five-center elimination (eq 4). The same procedure leads to the ammonium ylide (14), and spontaneous fragmentation occurs at room temperature to give (E)-cyclododecene (70%).2 Sulfonium methylide generation and trapping by intramolecular attack at a C=O bond is also known, resulting in the formation of epoxides or fragmentation products.10 Treatment of Triphenylphosphine with TMSCH2OTf followed by desilylation with CsF in the presence of benzophenone results in the in situ generation and Wittig trapping of the phosphonium ylide Ph3P=CH2.2 Finally, TMSCH2OTf can also be used for the C-alkylation of a-lithio vinylcarbamates.11
University of Wisconsin, Madison, WI, USA