Oppenauer-type oxidations

Interestingly, when analogous phosphine adducts were tested, very poor catalytic activity was obtained (Table 10.4, entries 6 and 7). On the other hand, the NHC-cationic complex [IrCp (ITM)(NCMe)2][OTf]2 25 proved highly efficient for the Oppenauer-type oxidation of a large range of secondary as well as primary alcohols in acetone (e.g.. Table 10.4, entries 4, 5, 8-10). 

Zr compounds are also useful as Lewis acids for oxidation and reduction reactions. Cp2ZrH2 or Cp2Zr(0 Pr)2 catalyze the Meerwein-Ponndorf-Verley-type reduction and Oppenauer-type oxidation simultaneously in the presence of an allylic alcohol and benzaldehyde (Scheme 40).170 Zr(C)1 Bu)4 in the presence of excess l-(4-dimethylaminophenyl) ethanol is also an effective catalyst for the Meerwein-Ponndorf-Verley-type reduction.1 1 Similarly, Zr(0R)4 catalyze Oppenauer-type oxidation from benzylic alcohols to aldehydes or ketones in the presence of hydroperoxide.172,173... 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

Oppenauer-type oxidation of secondary alcohols can be a convenient procedure for obtaining the corresponding carbonyl compounds. It was found recently [19], that Ir(I)- and Rh(I)-complexes of 2,2 -biquinoline-4,4 -dicarboxylic acid dipotassium salt (BQC) efficiently catalyze the oxidation of secondary alcohols with acetone in water/acetone 2/1 mixtures (Scheme 8.5). The reaction proceeds in the presence of Na2C03 and affords medium to excellent yields of the isolated ketones. The process is much faster in largely aqueous solutions, such as above, than in wet organic solvents in acetone, containing only 0.5 % water, low yields were observed (15 % vs. 76 % in case of cyclohexanol). 

The catalytic activity of Cp Ir(III) complexes in the Oppenauer-type oxidation of alcohols was considerably enhanced by the introduction of N-heterocyclic carbene ligands. Here, high turnover numbers (TONs) of up to 950 were achieved in the oxidation of secondary alcohols [40]. 

Hydrogen Transfer Oxidation of Alcohols (Oppenauer-Type Oxidation)... 

Hydrogen transfer reactions from an alcohol to a ketone (typically acetone) to produce a carbonyl compound (the so-caUed Oppenauer-type oxidation ) can be performed under mild and low-toxicity conditions, and with high selectivity when compared to conventional methods for oxidation using chromium and manganese reagents. While the traditional Oppenauer oxidation using aluminum alkoxide is accompanied by various side reactions, several transition-metal-catalyzed Oppenauer-type oxidations have been reported recently [27-29]. However, most of these are limited to the oxidation of secondary alcohols to ketones. 

Hiroi et al. also reported the Cp lr complex-catalyzed Oppenauer-type oxidation of primary alcohols in acetone and butanone [31]. These authors prepared a novel Ir-ligand bifunctional catalyst 6 having an amido-alkoxo ligand, the... 

Yamaguchi et al. also reported the use of another new Cp Ir NHC complex 9 bearing basic 2-(dimethylamino)ethyl group in a Cp ring as the catalytic precursor in Oppenauer-type oxidation (Scheme 5.7) [34]. Owing to the basic amino... 

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

Keywords Rhodium Iridium Catalysis Oppenauer-type oxidations ... 

Recent advances in alcohol oxidations by rhodium and iridium complexes have mainly focused on Oppenauer-type oxidations or reactions in which this type of oxidation is an intermediate step. An independent result is the oxidation of allyhc (Eq. 9) and benzyUc alcohols with f-BuOOH to the corresponding a,/l-unsaturated ketones [38] with [Rh2(p.-OAc)4]. The reactions were carried out at room temperature in dichloromethane and yields of up to 92% (by GC) in 24-48 h have been described. 

Since these are chemical equilibriiun reactions, by modifying the reaction conditions, i.e., using acetone as solvent instead of isopropanol, the reaction can be reversed, and therefore used for the oxidation (dehydrogenation) of alcohols (Oppenauer-type oxidation) [43]. Moreover, since acetone is the hy-... 

Water-soluble catalysts for Oppenauer-type oxidation of alcohols can be achieved by adding functionalized salts of classical ligands such as dipotassium 2,2 -biquinoline-4,4 -dicarboxylate (BQC) to acetone-water mixtures. In this way, the catalyst system [ Ir(ix-Cl)(cod) 2]/BQC is highly efficient for the selective oxidation of a wide range of alcohols such as benzylic. 

A sequence comprising an Oppenauer-type oxidation, intramolecular imine formation, and reduction, a process mediated by the iridium catalyst [Cp hG 2]2 and K2C03 in toluene at 120 °C for several days, afforded the structurally simple l-methyl-2,3,4,5-tetrahydro-177-benzo[l,4]diazepine in 68% yield from iV-(2-aminophenyl)-(Af-methylamino)propan-l-ol (Scheme 68) <2006TL6899>. 

Perillyl aldehyde (entry 11 in Table 13.3) is typically obtained from the corresponding alcohol via Oppenauer-type oxidation by using alkylboron compounds... 

Metal-catalyzed oxidation of alcohols to aldehydes and ketones is a subject that has received significant recent attention [21,56,57]. One such method that utilizes NHC ligands is an Oppenauer-type oxidation with an Ir or Ru catalyst [58-62]. These alcohol oxidation reactions consist of an equilibrium process involving hydrogen transfer from the alcohol substrate to a ketone, such as acetone (Eq. 5), or an alkene. Because these reactions avoid the use of a strong oxidant, the potential oxidative instability of NHC ligands is less problematic. Consequently, these reactions represent an important target for future research into the utility of NHCs. 

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. 

The zirconocene complex [2ZrH2] catalyses an Oppenauer-type oxidation of alcohols in the presence of an appropriate hydrogen acceptor. On oxidation of diols containing two primary alcohols, and of diols containing two secondary alcohol groups, one of the alcohol groups is selectively oxidized to form hydroxy-aldehydes and hydroxy-ketones respectively. This system... 

In 2012, Z.K. Yu et al. [158] reported on the transfer hydrogenation of ketones and Oppenauer-type oxidation of ketones. Ruthenium compounds containing an unsymmetrical NNC ligand, two phenylphosphines, and a chlorine atom 8.116 have exhibited highly catalytic activity for both reactions, as shown in Eqs. (8.39) and (8.40). 

The lanthanide (especially samarium) alkoxides serve as highly effective cata-lysts ° ° for Oppenauer-type oxidation of alcohols to aldehydes and ketones (Eq. 2.349) ... 

The use of NHCs as ancillary ligands in iridium-catalyzed Oppenauer-type oxidation of alcohols to carbonyls has led to some of the most active catalysts for this class of transformation. In 2005, Yamaguchi and co-workers reported the synthesis of a number of [(Cp )Ir] complexes featuring NHCs as the ancillary ligands.In addition to neutral complexes of the formula... 

A review describes the asymmetric epoxidation of allylic alcohols,369 another the role of metal oporphyrins in oxidation reactions.370 jhe TiiOPrMi, catalysed self-epoxidation of allylic peroxides proceeds via an intermolecular mechanism.371 Racemic allyl alcohols can be resolved by asymmetric epoxidation (eq.35).372 a Pd(II)/Mn02/benzoquinone system catalyses the oxidative ring-closure of 1,5-hexadienes (eq.36).373 propenyl phenols are oxidatively degraded to aryl aldehydes and MeCHO in the presence of Co Schiff-base catalysts.374 An Oppenauer-type oxidation with Cp2ZrH2/cyclohexanone converts primary alcohols selectively into aldehydes.375 co macrocycles catalyse the oxidation of aryl liydrazones to diazo compounds in high yields.376 similar Co complexes under CO oxidise primary amines to azo compounds.377 Arene Os complexes in the presence of base convert aldehydes and water slowly into carboxylic acids and H2.378... 

Shvo s catalyst 1 is a cyclopentadienone-ligated dimthenium complex, [Ru2(CO)4 (/t-H)(C4Ph4COHOCC4Ph4)]. It was first synthesized in 1984 by Shvo et al. [1, 2], Since then it has been widely applied in various hydrogen transfer reactions, including hydrogenation of carbonyl compounds [2, 3], transfer hydrogenation of ketones and imines [4,5], disproportion of aldehydes to esters [6], and Oppenauer-type oxidations of alcohols [7-9] and amines [10-12]. Shvo s complex 1 has also been found to be effective as a racemization catalyst for secondary alcohols and amines, and complex 1 has therefore been used together with enzymes in several dynamic kinetic resolution (DKR) protocols [13-18]. 

Although these systems will not be discussed further here, it should be noted that NHC-Ir complexes were also used to catalyze the Oppenauer-type oxidation of alcohols. ... 

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