Ligand-promoted mechanism

Hering, J., and W. Stumm (1991), "Fluorescence Spectroscopic Evidence for Surface Complex Formation at the Mineral-Water Interface Elucidation of the Mechanism of Ligand-Promoted Dissolution," Langmuir7, 1567-1570. 

Adsorption of phosphate on Fe oxides involves a ligand exchange mechanism (Par-fitt and Russell, 1977 Sigg and Stumm, 1981) and appears to be promoted by increasing the ionic strength (Bowden et al., 1980). Spectroscopic studies have not provided an entirely consistent picture of the mode of phosphate adsorption, but the consensus from studies with a range of techniques is, that phosphate adsorbs on Fe oxides predominantly as a binuclear, bidentate complex. 

Some radical reactions occur under the control of transition metal templates. The first example of asymmetric creation of an asymmetric carbon with a halogen atom is shown by the a DIOP-Rh(I) complex-catalyzed addition of bromotrichloromethane to styrene, which occurs with 32% enantioselectivity (Scheme 99) (233). Ru(II) complexes with DIOP or BINAP ligands promote addition of arenesulfonyl chlorides to afford the products in 25-40% ee (234). A reaction mechanism involving radical redox transfer chain process has been proposed. 

Allouche L, Huguenard C, Taulelle F (2001) 3QMAS of three aluminum polycations space group consistency between NMR and XRD. J Phys Chem Solids 62 1525-1531 Amirbahman A, Gfeller M, Furrer G (2000) Kinetics and mechanism of ligand-promoted decomposition of the Keggin Afs polymer. Geochim Cosmochim Acta 64 911-919 Baes CF, Mesmer RE (1976) The Hydrolysis of Cations. John-Wiley, New York... 

The proposed mechanism for the ligand-promoted dissolution of aluminum oxide involves three general steps (1) ligand adsorption and surface complex formation, (2) slow detachment of a surface metal center (as a complex with the ligand), and (3) regeneration of the surface (shown schematically in Figure 1). 

Metal compounds possessing Lewis acid character are often used in the Michael addition reaction, and the methodology is reasonably applied to the asymmetric reaction in the presence of chiral ligands. The mechanism could involve either purely Lewis acidic activation of the Michael acceptor or generation of new orga-nometallic species by the transmetalation or C-H activation, although they were not clear in many cases. The system of Sn(OTf)2 and chiral (S)-diamine developed by Mukaiyama promoted the asymmetric addition of trimethylsilyl enethi-olate 68 to P-arylvinyl ketones (Scheme 13) [70, 71]. The diamine-coordinated tin enolate was considered to be involved, and slow addition of 68 was essential to inhibit the racemate formation process. 

It is widely accepted that square planar platinum(ll) complexes react by an 7, mechanism. BCinetic work on substitution reactions of other square complexes suggests that 7, processes predominate in these systems. When the entering ligand plays a role in determining the rate of a reaction, it is important to leam which ligands promote the most rapid reactions. 

Most of the mechanisms for heteroaromatic C—H bond activation by a transition-metal catalyst fall into one of these four categories (i) electrophilic aromatic metala-tion, (ii) carboxylate-ligand-promoted concerted metalation-deprotonation (CMD), (iii) base-assisted metalation and (iv) oxidative addition of C—H to the metal center. The type of mechanism operating in the cleavage of the C—bond depends on the electronics of the heterocycle (and therefore its substituents) and the reaction conditions being employed. 

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