Oppenauer oxidation formates

The most common catalysts for the Meerwein-Ponndorf-Verley reduction and Oppenauer oxidation are Alm and Lnm isopropoxides, often in combination with 2-propanol as hydride donor and solvent. These alkoxide ligands are readily exchanged under formation of 2-propanol and the metal complexes of the substrate (Scheme 20.5). Therefore, the catalytic species is in fact a mixture of metal alkoxides. 

Olefin formation, by reductive elimination of 3-hydroxysulfones, 72, 2 Olefins, hydroformylation of, 56, 1 Oligomerization of 1,3-dienes, 19, 2 Oligosaccharide synthesis on polymer support, 68, 2 Oppenauer oxidation, 6, 5 Organoboranes ... 

Vicinal effects can also play a part in the course of the reaction utilizing Oppenauer conditions. 1,3-Diols or / -amino alcohols may not react, presumably on account of formation of an aluminum complex.5 5 46b> If oxidation were to take place it would probably be followed by dehydration to give an unsaturated ketone. Retro-aldol cleavage has been found to occur with a 17,21-dihydroxy steroid.32 The 11 -hydroxyl group which is generally inert to Oppenauer oxidation will react if a hydroxyl group is present on the... 

Normally, Oppenauer oxidations are performed employing Al3+ cations as catalyst because aluminium alkoxides possess a good balance of a desired high hydride transfer capability versus a low propensity to promote undesired base-induced reactions, like aldol condensations and Tischtschenko reactions. In the reaction, as originally described by Oppenauer, aluminium t-butoxide is used as catalyst,4 because its high basicity allows a very favourable equilibrium towards the formation of the aluminium alkoxide of the alcohol whose oxidation is desired. However,... 

The Oppenauer oxidation presents two important limitations on one side it is unable to oxidize certain alcohols because of unfavourable thermodynamics, and on the other side, base-induced reactions between the oxidant and the product may become dominant. That is why, it is seldom employed for the obtention of aldehydes because these compounds react readily under basic conditions. On the other hand, although aluminium alkoxides promote aldol condensations, many base-sensitive functional groups such as most esters but not formates—25 resist its action. 

Sometimes, reactions in which an alcohol is oxidized by hydride transfer to a metallic cluster, resulting in the formation of a metallic hydride that subsequently transfers a hydride to a sacrificial aldehyde or ketone, are described as Oppenauer oxidations.44 In the opinion of the authors, the name Oppenauer oxidation should be reserved for oxidation of alcohols in which a hydride is directly transferred from a metallic alkoxide to an aldehyde or ketone acting as oxidant. 

The Oppenauer oxidation of a ster-3-ol employing the strong oxidant p-quinone, instead of the more usual acetone, cyclohexanone or lV-methyl-4-piperidone, produces an over-oxidation resulting in the formation of a dienone, instead of the usual enone. 

Although aluminium alkoxides are able to promote base-induced reactions, the basic conditions involved are not extremely strong and many base-sensitive functional groups remain unaffected during Oppenauer oxidations, including alkyl halides,51 epoxides52 and most esters.53 On the other hand, the very sensitive formate esters are hydrolyzed under Oppenauer conditions and the resulting alcohols are oxidized in situ.25... 

A common side reaction during Oppenauer oxidations consists of the base-catalyzed condensation of the carbonyl compound, resulting from the oxidation, with the carbonyl compound used as oxidant. Sometimes, advantage is taken from this side reaction for synthetic purposes. For example, oxidation of primary alcohols with an aluminium alkoxide and acetone results in the formation of an intermediate aldehyde that condenses with acetone, resulting in a synthetically useful formation of an enone.59... 

The most common side reactions during Oppenauer oxidation consist of base-induced condensations of the aldehyde or ketone, generated during the oxidation, with the carbonyl compound used as oxidant.65 This side reaction is particularly prominent during the obtention of aldehydes because they are generally more reactive in aldol condensations than ketones. Furthennore, aldehydes very often suffer Tischtschenko condensations,66 resulting in the formation of dimeric esters during Oppenauer oxidations. That is why, the Oppenauer oxidation is seldom useful for the preparation of aldehydes. 

The addition of a-deprotonated alkyl halides to alkenes or carbonyl compounds can, because of the good leaving-group properties of halides, also lead to formation of cyclopropanes [292] or epoxides [187, 304, 306, 310], respectively. Because of the inherent instability of a-halo organometallic compounds, these intermediates should be handled carefully and on a small scale only. The ketone produced by the last reaction in Scheme 5.34 is probably formed by Oppenauer oxidation of the intermediate alcohol by the excess benzaldehyde [310],... 

The modified Oppenauer oxidation was used in the synthesis of estrone by P. Kocovsiw et al. The tetracyclic did was exposed to aluminum isopropoxide and A/-methyl-piperidine-4-one (oxidizing agent) to obtain the corresponding enone in good yield. The formation of the enone involved the migration of the initial p,y-double bond. The treatment of this enone with TsOH overnight in ether led to the formation of estrone by aromatization. 

Covalent Bonds R Al—0 / D Oxidation (Oppenauer Oxidation) Hydride Transfer (p-Hydride Elimination Meenwein-Ponndorf-Veriey Reduction) C-O Bond Formation (Tischenko Reaction) —Al—0 R... 

The transition states leading to the cis and trans alcohols differ substantially in size and the way in which they can be accommodated in the pores of zeolite Beta. That for the cis isomer is more or less linearly aligned with the pore axis and can easily be accommodated within the straight channels of the zeolite. The transition state for the formation of the trans isomer is more or less perpendicular to the channel wall and cannot be well accommodated within the micropores (Scheme 5). As required by this mechanism, the cis alcohol was found to undergo Oppenauer oxidation over zeolite Beta whereas activity for the trans isomer being negligible. 

Dehydroamination is performed in the presence of a hydrogenation-dehydrogenation catalyst and an alcohol. It has been proven that an aldehyde is formed as an intermediate. Formally, this transformation is obtained by three successive reac-tions-dehydrogenation of the alcohol (Oppenauer type oxidation), formation of an imine by nucleophilic attack then dehydration, and, finally, reduction of the imine (MPV-type reduction). In the last reaction step, it can be assumed that the dehydroamination pathway is similar to that of reductive amination. 

F.H. Oppenauer Oxidation. In oxidation reactions involving both Cr(VI) and DMSO reagents, the alcohol is converted to a complex in which the a-hydrogen of an alcohol can be removed as an acid. A classical and alternative method for the oxidation of alcohols focuses on the reversible reaction between ketones and metal alkoxides, which is especially effective when the metal is aluminuml34 xhe reversibility of the aluminum alkoxide reaction was first demonstrated by Verley 35 Ponndorf 36 for the reaction of a ketone with an aluminum alkoxide, which led to formation of a new aluminum alkoxide and a new ketone. In... 

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