Homogeneous catalysis reduction

Using supported rhodium as catalyst, 2-nitrostyrene is converted in benzene into skatole in 70y. yield, with CO/H2 (160 atm) at 160"CD2ll. However this reaction, conducted under hydrotormi1 ation conditions, involves -formation o-f 2-homogeneous catalysis, reduction o-f the nitro group by heterogeneous catalysis, then ring closure and thermal dehydration ... 

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

Multiple products are possible from C02 hydrogenation, but all of the products are entropically disfavored compared to C02 and H2 (Scheme 17.1). As a result, the reactions must be driven by enthalpy, which explains why formic acid is usually prepared in the presence of a base or another reagent with which formic acid has an exothermic reaction. Of the many reduction products that are theoretically possible, including formic acid, formates, formamides, oxalic acid, methanol, CO, and methane, only formic acid and its derivatives are readily prepared by homogeneous catalysis. 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

Sequential hydroformylation/reductive amination of dendritic perallylated polyglycerols with various amines in a one-pot procedure to give dendritic polyamines in high yields (73-99%). Furthermore, the use of protected amines provides reactive core-shell-type architectures after deprotection. These soluble but membrane filterable multifunctional dendritic polyamines are of high interest as reagents in synthesis or as supports in homogeneous catalysis as well as nonviral vectors for DNA-transfection (Scheme 18) [65]. 

The present procedure involving homogeneous catalysis is operationally simple and takes advantage of the easy availability of 2-(l -hydroxyalkyl )-acrylic esters. A two-step procedure Involving kinetic resolution of the racemic starting material with an optically active hydrogenation catalyst, followed by a further reduction with an achiral catalyst, leads to diastereomerically pure products in 4. 97t ee. 

Another example is butene dimerization catalyzed by nickel complexes in acidic chloroaluminates 14). This reaction has been performed on a continuous basis on the pilot scale by IFF (Difasol process). Relative to the industrial process involving homogeneous catalysis (Dimersol process), the overall yield in dimers is increased. Similarly, selective hydrogenation of diene can be performed in ionic liquids, because the solubility of dienes is higher than that of monoene, which is higher than that of paraffins. In the case of the Difasol process, a reduction of the volume of the reaction section by a factor of up to 40 can be achieved. This new Difasol technology enables lower dimer (e.g., octenes) production costs 14). 

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. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

A recent paper by Kwiatek and co-workers1891 discloses the use of a homogeneous catalysis system for the hydrogenation of epoxides. Reduction of cyclohexene oxide to cydohexanol, and of styrene oxide to 2-phenylethanol, was carried at atmospherio pressure, using Ks[Co(CN) ] to catalyze the reaction. 

This section has attempted to delineate the possible ways in which CO activation can be achieved by discrete metal complexes in order that homogeneous catalysis of CO reactions can be better understood. Principal means of activation are by significant bond order reduction and/or development of reactive charge distributions on the coordinated carbonyl. Oxidation or reduction of the CO ligand will transpire at carbon, and the primary mode of attack at that site will be by nucleophiles. 

These recent developments indicate the current level of activity in the homogeneous catalysis of CO reduction, and it is expected that these efforts will continue at least at this level in the near-term future. 

The second class of reactions, with CO and H20 serving as a source of two electrons, is not a catalytic reaction as such but is examined closely here since this is an integral step in the homogeneous catalysis of the water gas shift. The reaction may take two forms, with the reduction product being either a reduced metal center, (29), or a metal hydride, (30). 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

Elimination. Two kinds of elimination reaction are important in homogeneous catalysis. A catalytic cycle which has involved an oxidative addition generally ends with the reverse process of reductive elimination (equation 6). Rhodium-catalyzed hydrogenations end with this step. 

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