Oxidative-addition of hydrogen

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

After the precatalyst is completely converted to the active catalyst Xq, three steps are required to form the desired reduction product. The first step is the coordination of dehydroamino acid (A) to the rhodium atom forming adducts (Xi) and (Xi ) through C=C as well as the protecting group carbonyl. The next step is the oxidative addition of hydrogen to form the intermediate (X2). The insertion of solvent (B) is the third step, removing the product (P) from X2 and regenerating Xq. Hence, the establishment of the kinetic model involves these three irreversible steps. 

Stone and co-workers (16) recently announced that they had isolated and characterized a novel class of crystalline platinum(II) complexes prepared at room temperature by oxidative addition of hydrogen or of R3SiH (R3 = Me2Et, Me2CH2Ph, Cl3, or Et3)to (cyclo-C6H1, P Pt (C2H4. All these complexes were found to be excellent catalysts for hydrosilation... 

The catalytic asymmetric hydrogenation with cationic Rh(I)-complexes is one of the best-understood selection processes, the reaction sequence having been elucidated by Halpern, Landis and colleagues [21a, b], as well as by Brown et al. [55]. Diastereomeric substrate complexes are formed in pre-equilibria from the solvent complex, as the active species, and the prochiral olefin. They react in a series of elementary steps - oxidative addition of hydrogen, insertion, and reductive elimination - to yield the enantiomeric products (cf. Scheme 10.2) [56]. 

The k0bs-values are all to be interpreted as the sum of all rate constants for the oxidative addition of hydrogen, each multiplied by the mole fraction of the corresponding catalyst-substrate complex. Hence this gross-rate constant is dependent only on the ratio of intermediates, and not on their absolute concentrations. 

Reversible inhibition caused by materials that can function as ligand. Many compounds will bind to a metal this might be the solvent or impurities in the substrate or the solvent. It can also be a functional group in the substrate or the product, such as a nitrile. Too many ligands bound to the metal complex may lead to inhibition of one of the steps in the catalytic cycle. Likely candidates are formation of the substrate-catalyst complex or the oxidative addition of hydrogen. Removal of the contaminant will usually restore the catalytic activity. 

Oxidative addition of hydrogen to mononuclear Cp Ru-diborolyl complexes formed the dihydride species 24, which could not be isolated, and readily lose H2 in vacuum 30 detailed variable-temperature NMR studies were unable to distinguish between the possible classical Ru(H)2 and non-classical Ru(H2) structures. Acetonitrile, CO, and other electron donors add similarly and in some cases reversibly, leading to bent structures such as 25.29,30... 

The enantioselectivity determining step. Above we learnt that the oxidative addition of hydrogen is the rate-determining step. This step is irreversible and it also determines the enantioselectivity. This complex could still epimerise via substrate dissociation, but apparently it does not and migratory insertion is faster than epimerisation. We remember that two diastereomeric intermediates are involved, the major and the minor species and the minor species is the... 

The kinetics of hydroformylation by phosphine- or phosphite-modified complexes is even more complex than that of the cobalt-catalyzed reaction. Depending on the reaction conditions, either alkene complexation (Scheme 7.1, 6 to 7) or oxidative addition of hydrogen (Scheme 7.1, 9 to 10) may be rate-determining. 

The mechanism shown in Scheme 7, called the unsaturate route, is characterized by initial substrate coordination to metal followed by oxidative addition of hydrogen however, depending on the catalyst, substrate, and reaction conditions, the order of the individual steps can be... 

Hydrogen cyanide can be added across olefins in the presence of Ni, Co, or Pd complexes (Scheme 56) (123). Conversion of butadiene to adiponitrile is a commercial process at DuPont Co. The reaction appears to occur via oxidative addition of hydrogen cyanide to a low-valence metal, olefin insertion to the metal-hydrogen bond, and reductive elimination of the nitrile product. The overall reaction proceeds with cis... 

Hydrido complexes can be prepared by the oxidative addition of hydrogen to coordinatively unsaturated metals (e.g. Vaska s compound) or by reduction of higher valent compounds with borohydride or similar reagents. 

It might be possible to make a more active catalyst (per mole complex dissolved per unit volume of solvent) either by increasing the tendency towards the formation of coordinatively unsaturated complexes in solution or by increasing the rate constant of the rate determining step, i.e., the oxidative addition of hydrogen. 

This makes it easy to predict that solutions of HRh(PPh3)4 are more active catalysts than solutions of HRh(CO) (PPh3)3, either because the concentration of coordinatively unsaturated complexes increases or because the rate constant of the oxidative addition of hydrogen increases. A kinetic investigation of the hydrogenation of hexene catalyzed by solutions of hydridotetrakis( triphenylphosphine) rhodium (I) is reported here. 

Here n is 2 or 3 with the two hydrogenation cycles running in parallel. In this cycle the oxidative addition of hydrogen is the most probable rate determining step which suggests a rate expression ... 

Thus, it must be concluded that the former mechanism of Reaction 7 is the most probable and that the oxidative addition of hydrogen is rate determining. 

Oxidative addition. This reaction is quite general and normally involves the cleavage of a bond in the substrate and an increase of two in the formal oxidation state of the metal (equation 3). The most common example in catalysis is the oxidative addition of hydrogen. 

In their original studies, Wilkinson and co-workers postulated a mechanism involving oxidative addition of hydrogen to rhodium(I) to give a rhodium(III) dihydride, followed by coordination of alkene. The precise nature of the hydrogen transfer to the alkene was not clear at that time, but the general outlines of the mechanism were correct. 

The reaction of [Rh2Cl2(CgHi4)4] with 2-aminopyridine led to a species believed to be a cationic solvated rhodium(I)-aminopyridine complex. This was more active than [RhCl(PPh3)3] or [RuH(Cl)(PPh3)3] for the hydrogenation of cyclohexene. The mechanism was thought to involve oxidative addition of hydrogen to rhodium(I) prior to alkene coordination.143... 

Halpem and co-workers have carried out a detailed investigation of the mechanism of the asymmetric hydrogenation of methyl (MAC) and ethyl (EAC) (Z)-a-acetamidodnnamate by rhodium complexes of the ligands DIPAMP (50) and CHIRAPHOS (51).259 Coordination of alkene precedes the oxidative addition of hydrogen. For both ligands, one of the two possible diastereoisomers of the rhodium-diphosphine-alkene complex predominates in solution to a large extent. From the reaction of EAC with the S,S-CHIRAPHOS complex, this diastereoisomer has been isolated. Its structure is represented in (57).260... 

Halpern has shown that this predominant isomer exhibits negligible activity towards the oxidative addition of hydrogen. The minor isomer, which could be detected in solution for DIPAMP but not for CHIRAPHOS, reacts far more rapidly with hydrogen and is responsible for producing the major enantiomer of the hydrogenation product. The optical selectivity is thus due to this difference in reaction rates and not simply to the preferred manner of coordination of the alkene to the rhodium-diphosphine species.259,260 The precise reasons for this large difference in the rates of reaction of the two diastereoisomers with hydrogen are not yet known. The full mechanism is shown in Scheme 14. 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

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