Diverse Site-Selective Transformation of Benzylic and Allylic Silyl Ethers via Organocatalytic Oxidation

We herein report nitroxyl-radical catalyst 1a, which recognizes the electronic properties of silyl ethers, thus enabling selective oxidation of benzylic and allylic silyl ethers, despite steric factors. A subsequent one-pot reduction accomplishes the formal deprotection to the corresponding benzylic and allylic alcohols. This catalytic system allows the direct oxidative desymmetrization of bis-benzylic and bis-allylic silyl ethers to access synthetically useful monoprotected conjugated aldehydes, which can be applied to one-pot enantioselective transformations. Mechanistic studies revealed that the highly electron-accepting oxoammonium species from 1a as well as the trifluoroacetate form of phenyl iodonium bis-(trifluoroacetate) (PIFA) play key roles for the selectivity-determining hydride transfer step. [GRAPHICS]