Alkoxide-catalyzed oxidation

For a general review on the zirconium alkoxide catalyzed oxidation reactions, see Krohn, K. Synthesis 1997,1115, and references therein. 

Alkoxide-catalyzed oxidation of secondary alcohols. Reverse of the Meerwein-Ponndorf-Verley reduction. 

The aluminum or potassium alkoxide-catalyzed oxidation of a secondary alcohol to the corresponding ketone (the reverse of the Meerwein-Ponndorf-Verlev reduction, q.v.) ... 

The oxidation of a hydroxyl group by an aluminum alkoxide-catalyzed hydrogen exchange with a receptor carbonyl compound is known as the Oppenauer oxidation. For oxidation of steroidal alcohols the reaction is generally... 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

The first zirconium-catalyzed oxidation reaction was reported by Kaneda, in which a zirconium oxide complex or zirconium alkoxide as the catalyst and t-BuOOH as an oxidant were employed to oxidize primary and allylic alcohols into aldehydes in high yields without formation of carboxylie acids [31]. 

RuCls-catalyzed alcohol oxidation of carveol (4, Table 4) similarly showed a higher rate with TBHP than with PHP, suggesting that the rate-limiting step in ruthenium catalyzed oxidation of alcohols may involve reaction of a ruthenium alkoxide with RO2H, resulting in formation of the carbonyl compound with simultaneous reoxidation of the ruthenium (Scheme 4). 

High molecular weight polyesters can be obtained only with special techniques because of the difficulty of obtaining complete water removal without loss of monomers. The reaction is self-catalyzed by carboxyl groups and can be catalyzed by other acids, for example, -toluenesulfonic acid and by compounds such as titanium alkoxides, dialkyltin oxides, and antimony pentafluoride. 

This reaction was first reported concurrently by Meerwein and Schmidt and Verley in 1925, and by Ponndorf in 1926, respectively. It is an aluminum alkoxide-catalyzed reduction of carbonyl compounds (ketones and aldehydes) to corresponding alcohols using another alcohol (e.g isopropanol) as the reducing agent or hydride source. Therefore, it is generally known as the Meerwein-Ponndorf-Verley reduction (MPV) or Meerwein-Ponndorf-Verley reaction. Occasionally, it is also referred to as the Meerwein-Ponndorf reduction, Meerwein-Ponndorf reaction, or Meerwein-Schmidt-Ponndorf-Verley reaction. About 12 years later, Oppenauer reported the reversion of this reaction in which alcohols were reversely oxidized into carbonyl compounds. Since then, the interchanges between carbonyl compounds and alcohols in the presence of aluminum alkoxide are generally called the Meerwein-Ponndorf-Oppenauer-Verley reduction or Meerwein-Ponndorf-Verley-Oppenauer reaction." ... 

As a replacement for alkynylanilines, Yamamoto has reported that indoles can he generated via the Pd(PPh3)4/CuCl-catalyzed coupling of 2-aIkynylaryUsocyanates with allylcarbonates (Scheme 6.18) [26]. In this case, fragmentation of the carbonate anion to an alkoxide upon oxidative addition to palladium allows conversion of the isocyanate into a carbamate for subsequent cydization. A number of substituted alkynes can participate in this reaction, and it can be performed with alcohols instead of allylcarbonates to form 3-unsubstituted indoles. A variant of this reaction involved the use of isocyanides in concert TMS-azide, providing a route to substituted N-cyanoindoles [27]. 

Hydrolysis is most rapid and complete when catalysts are employed [47]. Although mineral acids or ammonia are most generally used in sol-gel processing, other known catalysts are acetic acid, KOH, amines, KF, HF, titanium alkoxides, and vanadium alkoxides and oxides [47]. Many authors report that mineral acids are more effective catalysts than equivalent concentrations of base. However, neither the increasing acidity of silanol groups with the extent of hydrolysis and condensation (acidic silanols may neutralize basic catalysts [48]) nor the generation of unhydrolyzed monomers via base-catalyzed alcoholic or hydrolytic depolymerization processes have generally been taken into acount. 

With optically active tartrate esters present in the reaction system, titanium alkoxides catalyze a very stereoselective oxidation of allylic alcohols. This method must depend on specific interactions in the transition state by which the hydroxyl group controls the relationship between the double bond and approaching reagent and the tartrate establishes a chiral environment at the metal atom. The (+) and (-) enantiomers of diethyl tartrate give enantiomeric epoxides, each in >90% yield." ... 

Nucleophilic Reactions. Useful nucleophilic substitutions of halothiophenes are readily achieved in copper-mediated reactions. Of particular note is the ready conversion of 3-bromoderivatives to the corresponding 3-chloroderivatives with copper(I)chloride in hot /V, /V- dim ethyl form am i de (26). High yields of alkoxythiophenes are obtained from bromo- and iodothiophenes on reaction with sodium alkoxide in the appropriate alcohol, and catalyzed by copper(II) oxide, a trace of potassium iodide, and in more recent years a phase-transfer catalyst (27). 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

---