Dissociative mechanism, lead-ligand interactions

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achieved. If a diphosphine ligand of C2 symmetry is used, two diastereomers of the enamide coordination complex can be produced because the olefin can interact with either the re face or the si face. This interaction leads to enantiomeric phenylalanine products via diastereomeric Rh(III) complexes. The initial substrate-Rh complex formation is reversible, but interconversion of the diastereomeric olefin complexes may occur by an intramolecular mechanism involving an olefin-dissociated, oxygen-coordinated species (18h). Under ordinary conditions, this step has higher activation enthalpies than the subsequent oxidative addition of H2, which is the first... 

An examination of the photolysis of trans-[Cr(NH3)4(H20)(NCS)Hg], produced from trtms-[Cr(NH3)4(H20)(NCS)f (1) by interaction with Mg, and of (1) itself, has shown that only cis-[Cr(NH3)4(H20)2] is formed. Emission activation energies have been determined for [Cr(NH3)5CN] and trans-[Cr(NH3)4(CN)2p and ligand field irradiation of cis-[Cr(NH3)4(CN)2] in acid solution leads to aquation of NH3 and CN in a process whose quantum yields are wavelength dependent. The product [Cr(NH3)3(H20)(CN)2] consists of a mixture of l,2-CN-3-H20 and l,2-CN-6-H20, the composition of which is determined by the ligand-field band excited. A E state is thought to be involved, and either a dissociative (symmetry restricted) or associative (edge displacement) mechanism operates. ... 

Examples of electrophilic interactions at the metal ions (as well as at the leaving and entering groups) have been demonstrated. These represent a formal oxidation of the metal and can lead to oxidative additions to the metal or, by transfer to a ligand, to a dissociation. On the other hand, at least some concerted oxidative additions, often described as electrophilic interactions, are seen in reality to resemble nucleophilic attack in their early stages and some of the two-step oxidative additions (most of which do conform to electrophilic attack) probably resemble the concerted mechanism in their initial interactions, the boundary being indistinct. Nucleophilic attack can, in some cases, precede electrophilic attack. 

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