Ligand transformations and reactivity

The attachment of a transition metal to an unsaturated hydrocarbon ligand transforms the reactivity properties of the ligand. Whereas the classic chemistry of alkenes and arenes is that of electrophilic addition and substitution reactions, the classic reactivity of multihapto-complexes is their reactions with nucleophiles. Such complexes need not be cationic to be good electrophiles, but it helps, and not surprisingly, the most powerful electrophilic multihapto-complexes have positive charge stabilized by the metal. 

Asymmetric catalytic hydrogenation is unquestionably one of the most significant transformations for academic and industrial-scale synthesis. The development of tunable chiral phosphorous ligands, and of their ability to control enantioselectivity and reactivity, has allowed asymmetric catalytic hydrogenation to become a reaction of unparalleled versatility and synthetic utility. This is exemplified in the ability to prepare en-antiomerically enriched intermediates from prochiral olefins, ketones, and imines through asymmetric hydrogenation, which has been exploited in industry for the synthesis of enantiomerically enriched drugs and fine chemicals. 

In stoichiometric and catalytic processes information from the directing ligands is normally transferred to structure and reactivity of the associates. The advantage of catalytic reactions is that, cycle by cycle, this information can be accumulated. Therefore, it has always been of interest to find out relevant parameters for properties of phosphanes and phosphites by which the induced control can be transformed into a numeric form with predictable power. 

The pH-dependent reactivity of 25 and 26 resembles that of related complexes with other polyamine ligands (55) and may be summarized as follows The peroxo complexes may be transformed into the dinuclear superoxo complexes plus a mononuclear halo species by addition of a suitable acid. In aqueous alkaline solution (pH > 12), the peroxo complexes are stable over several hours, whereas the superoxo complexes will react immediately even with dilute aqueous ammonia to reform the peroxo complexes. 

The use of ferrocene-based ligands 26 and 27 also effected the Pd-catalyzed formation of diaryl ethers,Eq. (193) [148,149]. The 27/Pd-catalyst is more reactive and mediates the transformation in better yield and at lower temperatures. 

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