Fleming—Tamao oxidation

Just as carbon-based alkyl groups can be oxidized, a silicon-based silane unit can be oxidized under certain conditions. Alkylsilanes can be converted to a hydroxy unit, but either an aryl group (R—SiR 2Ar) or another silyl group (RSi—SiR 3) must be attached to silicon. In an early version of this reaction, Fleming used a two-step process to transform the silane unit to an alcohol unit, treatment with mercuric acetate and peroxyacetic acid, followed by reduction with lithium aluminum hydride (see sec. 4.2.A for reductions with LiAlHq). Comins et al. used this procedure to convert dimethylphenylsilane (397) to alcohol 398 in 93% yield for the two steps, which was part of a synthesis of A-acetyl-A-methylphlegmarine.  

There were several cases when this original procedure failed, but it was discovered that if the silane units are treated with a fluorinating agent such as TBAF or CsF, the Ar or SiR 3 unit is replaced with F. Subsequent treatment with hydrogen peroxide or a peroxy acid gave the alcohol. This sequence is often called the 

Incorporation of the silyl unit can be done in several ways, including conjugate addition of silylcuprates to conjugated carbonyl compounds (see sec. 8.7.A.vi.). A brief discussion of silane carbanions will be presented in section 8.10. 

This reagent can also be used for the oxidation of methylated aryl derivatives to the corresponding aryl aldehyde, in what has become known as the Etard reaction. Tillotson and Houston found that the Etard reaction is catalyzed by small amounts of alkene, added to or present in the reaction medium. The reaction involves addition of chromyl chloride to a carbon disulfide or carbon tetrachloride solution of the arene. A dark brown, insoluble, and explosive intermediate usually precipitates. Dilute sulfurous acid is added to decompose the precipitate to the aldehyde. Toluene is converted to benzaldehyde and ethylbenzene was oxidized to phenylacetaldehyde with this reagent. 

Chromyl chloride can also produce ketones, but a common side reaction is formation of an a-chloro ketone, as in the conversion of cyclododecene to a-chlorocyclododecanone (401) in 79% yield. In a subsequent reaction, the chloride moiety in 401 was reduced to give cyclododecanone in 95% yield with zinc and acetic acid. Chromyl chloride also reacts with alkenes to give a trans-chlorohydrin rather than a carbonyl compound (see sec. 2.10.C for the preparation of chlorohydrins), as in the conversion of cyclohexene to tran5-2-chlorocyclohexanol. 5 It is interesting that in this particular case (the temperature was maintained at -78°C in dichloromethane), the cis-chlorohydrin was formed.  

---

Type I and II silicon-tethered carbonyl-ene reactions were first performed by Robertson.4,4a,4b One particularly striking application of this method is the conversion of isoserine derivative 3 (Scheme 1) into amino triol 4 via a carbonyl-ene reaction followed by Tamao-Fleming oxidation.5... 

Hunt and Roush <1995TL501> achieved a synthesis of swansonine 230 via a stereoselective allylation of a silyl allylboronate derived from tartaric acid followed by a Tamao-Fleming oxidation to introduce the C-2 hydroxy functionality (Scheme 54). 

This reaction, depending on the details of the reaction, is known as the Fleming Oxidation, the Fleming-Tamao Oxidation, the Tamao-Fleming Oxidation, or the Tamao-Kumada Oxidation. 

Like their carbon counterparts, silylcuprates and stannylcuprates readily participate in SN2 -substitution reaction with allylic electrophiles.43,51 For example, <37z//-diastereoselective allylic substitution of the vinyl oxiranes 147 with the lower-order cyanocuprate (PhMe2Si)Cu(CN)Li cleanly affords the allylsilanes 148 (Scheme 38).106 Products of this type can be converted into the corresponding diols with retention of configuration by Tamao-Fleming oxidation 53,53a,53b,107... 

The scope of synthetically interesting acceptors that can be functionalized in the rhodium/diene-catalyzed conjugate addition has been expanded considerably since the initial reports (Figure 8.10). Today, the range comprises not only a,P-unsaturated esters 105, which are particularly well suited for heterocychc-substi-tuted substrates [91], but also a,p-unsaturated Weinreb amides 106, which have found wide appHcation [92]. Both classes of acceptors allow straightforward modification of the resulting adducts. The use of P-silyl-substituted a,P-unsaturated carbonyl compounds 107 as acceptors is of special interest as these compounds can be transformed to P-hydroxyketones via Tamao-Fleming oxidation [93]. 

Tolerance of this novel Lewis acid-catalyzed methodology toward aryl-containing hydrosilanes allows for its broad application for the synthesis of alcohols via trans selective hydrosilylation/Tamao-Fleming oxidation sequence (see also eqs. (24-26)) (136). 

---