Silyl anions protonic acids

In the presence of trifluoroacetic acid, these aziridines undergo protonation at room temperature and ring opening at reflux in methanol, diethyl ether or hexane under nucleophilic attack of the trifluoroacetate anion followed by acyl exchange to give the corresponding a-silyl-(i-aminoalcoho] derivative.316... 

This reaction involves cleavage of SiR3 groups (R usually methyl) from aromatic rings, either (1) by acid (acid-catalyzed desilylation), in which the proton attacks the ring in the first and rate-determining step or (2) by base (base-catalyzed desilylation), in which the aromatic anion is produced by loss of the silyl group, and the proton reacts with the anion in the second and fast step. 

CB1981] and the same reaction of the related silyl enol ether (16) using polymer supported mandelic acid leads to (5)-mandelic acid in up to 94% e.e. [94TL2891]. Low temperature protonation of the anion derived from (17) produces mainly the cis product by kinetic control [94CB1495] and hydroboration of (18) gives the cis hydroxymethyl compound with high... 

Similar results were reported by Reed in 2009 previously, triflic acid, methyl triflate, and trial-kylsilyl triflate have been used as electrophilic sources of H+ for protonation, of CH3+ for alkylation, and of R3Si" for silylation, respectively. Replacement of these anions by [CHBuRjXgl (R = H, Me X = Cl, Br) increases the electrophilicity of the cations and avoids the nucleophilic chemistry of the anion this allows various reactions that are not possible with commonly used reagents. 

Polar vinyl monomers that contain labile hydrogen such as (meth)acrylic acid, hydroxyethyl methacrylate, and (meth)acrylamide cannot be used directly for anionic polymerization as they can act as a terminator via proton transfer to the reactive anions. They can be subjected to anionic polymerization only after appropriate protection of these functional groups into nonreactive groups toward anions, for example, by esterification or silylation. " A detailed list of protected monomers is given in various reviews. 

In 2008, we reported the use of chiral IV-trifyl thiophosphoimide to catalyze enantioselective protmiation of silyl enol ethers with various achiral proton sources (Fig. 13) [56]. It was found that neither the achiral acids nor stoichiometric amount of chiral catalyst alone can protonate the silyl enol ether substrate under such reaction conditions. We believe the combined BBA catalyst, which is an oxonium cation with chiral thiophosphoimide counteranion, is the reactive species for this protonation reaction. On the other hand, since the extremely bulky chiral counter anion cannot accomplish the desilylation step, presence of achiral proton source for catalyst regeneration turns out to be essential. 

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