Fleming (-Tamao) Oxidation
(Fleming-) Tamao (-Kumada) Oxidation
Parallel procedures for the oxidation of silyl groups to hydroxy groups were developed by Fleming and Tamao. The conversion of a dimethylphenylsilyl group, which involves a specific reaction mechanism, was pioneered by Fleming.
Mechanism of the Fleming-Tamao Oxidation
Silyl groups, which are non-polar electropositive groups without lone pairs, tolerate many chemical reactions that would not be possible in presence of hydroxy groups. The Fleming-Tamao Oxidation permits silyl groups to be used as "masked hydroxy groups", which has found broad application in total syntheses. In addition, enantioselective hydrosilylation of alkenes followed by Fleming-Tamao oxidation allows the preparation of chiral alcohols.
The first step of the Fleming Oxidation is the removal of the phenyl group in which the very stable phenylsilane group is converted into a more reactive halosilane after electrophilic aromatic substitution:
The removal may be done as separate step followed by the addition of the oxidation reagents or in one of the more convenient one-pot procedures (for an overview: I. Fleming, R. Henning, D. C. Parker, H. E. Plaut, P. E. J. Sanderson, J. Chem. Soc. Perkin Trans. 1, 1995, 317-337. DOI). The phenyl group can also be activated through bromination using excess bromine or a bromide source leading to phenyl bromide as by-product.
The displacement of the halide by peracetic acid leads to an intermediate that rearranges to give a silanol. Protic work-up gives the desired alcohol:
The Tamao Oxidation uses the more reactive fluoro- or chlorosilanes (RSiMenX(2-n)), in which the silicon is a stronger Lewis acid and shows more metallic character than the substrates used in the Fleming oxidation. Further activation by a fluoride ion then leads to a pentavalent intermediate which is able to bind hydrogen peroxide. The transition state is also stabilized through hydrogen bonding between fluorine and hydrogen:
For a computational study see: M. M. Mader, P.-O. Norrby, J. Am. Chem. Soc. 2001, 123, 1970-1976. DOI
Strained siletanes may also be used in the Tamao Oxidation instead of halosilanes; these intermediates offer a comparable Lewis acidity because coordination of the fluoride ion releases angle strain.
Lewis acidity enhanced by strain release
Asymmetric Synthesis of 1-Aryl-1,2-ethanediols from Arylacetylenes by Palladium-Catalyzed Asymmetric Hydrosilylation as a Key Step
T. Shimada, K. Mukaide, A. Shinohara, J. W. Han, T. Hayashi, J. Am. Chem. Soc., 2002, 124, 1584-1585.