What is your actual catalyst? TMS cleavage rates of diarylprolinol silyl ethers studied by in situ NMR

Abstract

Diarylprolinol silyl ethers are excellent and broadly applicable organocatalysts for various enamine and iminium-type synthetic transformations. However, their undesired degradation to the corresponding diarylprolinols and the subsequent formation of oxazolidines during reaction with aldehydes may significantly affect their catalytic performance. Therefore, in situ NMR was used to examine the TMS cleavage rate of diarylprolinol silyl ethers as a function of solvent properties, acidic/basic additives and the presence of water. Highly polar solvents with strong hydrogen bond acceptor properties and especially moderate acidic additives with pK_a (DMSO) values around 10 accelerate the deprotection significantly, whereas basic and highly acidic additives are not detrimental. Additional mechanistic studies reveal that the substitution reaction takes places at the silicon atom.