Mechanisms of homogeneous

Antontsev S.N., Kazhikhov A.V., Monakhov V.N. (1990) Boundary value problems in mechanics of homogeneous fluid. Studies in Math. Their Appl. 22, North-Holland. 

T. C. Bruice and S. I Benkovic, Bioorganic Mechanisms, Vol. 1, W. A. Benjamin, New brk, 1966, pp. 1-258 W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New York, 1969 M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins, Wiley-Interscience, New York, 1971 C. Walsh, Enzymatic Reaction Mechanisms, W. H. Freeman, San Francisco, 1979 A. Fersht, Enzyme Structure and Mechanism, 2nd ed., W. H. Freeman, New York, 1985. 

M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins. Wiley-Interscience, New York, 1971. 

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

The generally accepted mechanism of homogeneous hydrogenation of simple ketones seems to occur by a [7r2 + cr2] pathway as depicted in Scheme 16. 

Bell, R. P. (1981). Proton Tunnelling in Chemistry , Chapman and Hall, London Bender, M. L. (1971). Mechanisms of Homogeneous Catalysis from Protons to Proteins . Wiley Interscience, New York... 

In the context of 1H chemical shifts and determination of the reaction mechanism of homogeneous hydrogenation catalysts, one usually tries to observe hydride-intermediates that typically resonate at high field (-5 to -30 ppm). Agostic bonds (see Fig. 11.1) also tend to have a hydride-like proton chemical shift. 

Today, many examples have been reported which demonstrate the potential of the PH IP technique as a powerful analytical tool to investigate the reaction mechanisms of homogeneously catalyzed hydrogenations [12]. 

Time-proven concepts for the reaction mechanisms of homogeneous hydrogenations follow two approaches which, according to Halperrfs step-wise analysis of hydrogenations using Wilkinsorfs catalysr [25] and the cationic catalyst DI-PHOS [26], respectively, can be grouped into the so-called dihydride or unsaturate routes [27] (Fig. 12.9). 

Whilst the metals of the cobalt group have provided valuable mechanistic information on the mechanism of homogeneous hydrogenation of arenes, there is little doubt that ruthenium forms the most active and versatile catalysts. 

The mechanism of the thermal 5 rnI reaction, using 4-nitrocumyl chloride and 2-nitropropanate ion as a model has been investigated. The results provided unambiguous evidence that a decrease in driving force is able to change the mechanism of homogeneous reductive cleavage reactions from stepwise to concerted. ... 

Scheme 7.7 Mechanism of homogenous catalysis of alkene hydrosilylation by [ Rh( i-OSiMe3)(cod) 2].

Isotopic labels (and especially enriched materials) have proven crucial in the investigation of the mechanisms of homogeneously catalyzed reactions [130]. Further, isotope effects on the rate or the equilibrium constant of a reaction can be diagnostic, and structural information can be provided by isotope-induced changes in the chemical shifts of neighbouring nuclei, and/or alterations in the coupling pattern of the detected spectra. The isotope- and position-specific information inherent to NMR techniques are ideally suited for the analysis of isotope effects in catalysis [131]. 

Figure 9.3 The mechanism of homogeneous hydrogenation using Wilkinson s catalyst.

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

Figure 9.5 The mechanism of homogeneous hydrogenation unsaturate route versus hydride route .

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

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