Umpolung, silyl enolates

In summary, the literature survey provides clear evidence for a-carbonyl radical intermediates but no convincing proof for further oxidation to a-carbonyl cation in the vast majority of silyl enol ether radical cation reactions. This suggests that for most cases, silyl enol ethers are more readily oxidized than the corresponding a-carbonyl radicals. Only in oxidations of -aryl substituted silyl enol ethers, a-carbonyl cation intermediates have been invoked. For example, one-electron oxidation of 87d with TTA" " in acetonitrile/MeOH afforded 76 in analogy to the a-Umpolung of ketones via enol radical cations (Scheme 4), and oxidation of 124 with FePHEN provided benzofuran 19 [171]. 

Other a-Umpolung reactions of ketones via oxidative reactions of silyl enol ethers can be accomplished by using MCPBA as oxidant following the Rubot-tom method [213], Pb OAc)4 [214], OSO4 [215], Ni(II)complex/oxygen [216] and iodosobenzene with BF3 [217] or Mn(III) [218] but certainly radical... 

Radical cations, generated by single electron oxidation of enamines that are generated in situ by the reaction of aldehydes with a chiral imidazolidinone, serve as a new class of electrophiles for silyl enol ethers (Scheme 3-1and allylsilanes. This novel Umpolung strategy allows the introduction of a chiral center at the a-position of aldehydes with these silicon reagents. The chiral imidazolidinone works as an... 

Overall, a-, / - and -functionalization is possible with the various silyl substrates reported in this section. In a formal sense, the relevant reactions discussed correspond to an umpolung of reactivity of the enolic or cyclopropyl systems, as depicted schematically below [22] ... 

Single electron oxidation of the non-activated carbonyl group, e.g. in aliphatic or aromatic aldehydes, ketones and carboxylic acid derivatives, is, on the other hand, much less feasible and only a handful of methods and synthetic applications are known. Useful methods for synthetic applications are chemical modifications to lower the oxidation potentials by peripheral donor substitution and a-silylation, or redox umpolung via oxidation of the corresponding carbonyl enols or enol ethers. 

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