Transfer hydrogenation homogeneous reactions

Asymmetric Transfer Hydrogenation of Ketones. The first reports on asymmetric transfer hydrogenation (ATH) reactions catalyzed by chiral metallic compounds were published at the end of the seventies. Prochiral ketones were reduced using alcohols as the hydrogen source, and Ru (274,275) or Ir (276) complexes were used as catalysts. Since then, many chiral catalytic systems for homogeneous ATH of ketones, imines, and olefins have been developed (37,38,256,257,277-289). The catalytic systems are usually based on ruthenium, rhodium, or iridium, and the ATH of aryl ketones is by far the most studied. Because of the reversibility of this reaction, at high conversions, a gradual erosion of the ee of the product has been frequently reported. An azeotropic 5 2 mixture of formic acid/triethylamine can be used to overcome this limitation. 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

In total syntheses where a homogeneously catalyzed transfer hydrogenation is applied, almost exclusively aluminum(III) isopropoxide is utilized as the catalyst. At an early stage in the total synthesis of (-)-reserpine (65) by Woodward [106], an intermediate with two ketone groups and two C-C double bonds is formed (66) by a Diels-Alder reaction of para-benzoquinone (67) and vinylacry-late (68). The two ketone groups were reduced with aluminum(III) isopropoxide... 

The hydration number (the number of water molecules intimately associated with the salt) of the quaternary ammonium salt is very dependent upon the anion. The change in the order of reactivity is thus believed to be due to the hydration of the anion the highly hydrated chloride and cyanide ions are less reactive than expected, and the poorly hydrated iodide fares better under phase transfer conditions than in homogeneous reactions. Methanol may specifically solvate the anions via hydrogen bonding, and this effect is responsible for the low reactivity of more polar nucleophiles in that solvent. 

It must be emphasized that the above considerations were made in the absence of reaction. Interfacial mass transfer followed by reaction requires further consideration. The Hatta regimes classify transfer-reaction situations into infinitely slow transport compared to reaction (Hatta category A) to infinitely fast transport compared to reaction (Hatta category H) [42]. In the first case all reaction occurs at the interface and in the second all reaction occurs in the bulk fluid. Homogenous catalytic hydrogenations, carbonylations etc. require consideration of this issue. An extreme example of the severity of mass transport effects on reactivity and selectivity in hydroformylation has been provided by Chaudari [43]. Further general discussions for homogeneous catalysis can be found elsewhere [39[. 

Meerwein-Pondorf-Verley reduction, discovered in the 1920s, is the transfer hydrogenation of carbonyl compounds by alcohols, catalyzed by basic metal compounds (e.g., alkoxides) [56-58]. The same reaction viewed as oxidation of alcohols [59] is called Oppenauer oxidation. Suitable catalysts include homogeneous as well as heterogeneous systems, containing a wide variety of metals like Li, Mg, Ca, Al, Ti, 2r and lanthanides. The subject has been reviewed recently [22]. In this review we will concentrate on homogeneous catalysis by aluminium. Most aluminium alkoxides will catalyze MPV reduction. 

Interesting reactions occur when the charge transfer at the electrode is associated with homogeneous reactions in solution that can precede or follow the electron transfer reaction at the electrode. A selection of possible schemes is shown in Table 6.1. Note the presence of many organic compounds the reduction or oxidation of these compounds involves, in many cases, the addition or removal of hydrogen, which... 

Like its homogeneous analogue, 3 shows high activity in both the oxidation of unsaturated hydrocarbons and the transfer hydrogenation of ketones, representative examples being shown in Table 1. For comparative purposes, reported yields for the analogous reactions using 6 are also shown. 

In many reactions molecular hydrogen may be replaced by other H-sources such as methanol, isopropanol, or formic acid even cyclic ethers such as dioxane or THF can be used in homogeneous transfer hydrogenation. The hydrogen donor coordinates to the metal and undergoes /3-hydrogen transfer ... 

In a very recent computational study, Diggle et al. have calculated the activation barriers for C(aryl)-X activation (X = H, F, OH, NH, CH3) as 0 (H), 9 (F), 12 (OH), 20 (NH ) and 21.3 kcal mol (CH3), respectively [155]. In comparison, the activation barrier for C(sp3)-H is 6.6 kcal moF [156]. C-X activation occurs under reaction conditions relevant for homogenous catalysis [157], but does not always result in decomposition as C-H activation is often reversible and can be exploited in catalytic transfer hydrogenations involving alcohols [156]. 

This book is about homogeneous reactions, that is, all kinds of reactions that occur within a single fluid phase. The term excludes reactions at interfaces, among them reactions of solids with fluids, heterogeneous catalysis, and phase-transfer catalysis. It does not exclude reactions in which a dissolved reactant is resupplied from another phase, as is the case, for example, in homogeneous hydrogenation or air oxidation reactions in the liquid phase in contact with a gas phase. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

To simulate the effects of reaction kinetics, mass transfer, and flow pattern on homogeneously catalyzed gas-liquid reactions, a bubble column model is described [29, 30], Numerical solutions for the description of mass transfer accompanied by single or parallel reversible chemical reactions are known [31]. Engineering aspects of dispersion, mass transfer, and chemical reaction in multiphase contactors [32], and detailed analyses of the reaction kinetics of some new homogeneously catalyzed reactions have been recently presented, for instance, for polybutadiene functionalization by hydroformylation in the liquid phase [33], car-bonylation of 1,4-butanediol diacetate [34] and hydrogenation of cw-1,4-polybutadiene and acrylonitrile-butadiene copolymers, respectively [10], which can be used to develop design equations for different reactors. 

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