Reactivity ligand complexes

Bonaccorsi ct al. [204 defined for the first time the molecular electrostatic potential (MEP), wdicli is dearly tfie most important and most used property (Figure 2-125c. The clcctro.static potential helps to identify molecular regions that arc significant for the reactivity of compounds. Furthermore, the MEP is decisive for the formation of protein-ligand complexes. Detailed information is given in Ref [205]. 

C6H2 (supermesityl, mes ) into the ligand such substituents sterically occlude vacant metal coordination sites and greatly increase the solubility of the complexes in common organic solvents. The subtle variations possible within P- and As-donor ligand complexes of the alkali metals lead to an almost bewildering array of structural types, many of which are not observed in other areas of alkali metal chemistry, and to wide variations in reactivity between complexes. 

This reaction has lent itself to the development of its asymmetric version (Scheme 88). The trick here is to remove the choride ligands from the coordination sphere of the platinum-chiral ligand complex. This makes the metal center more electrophilic, thus reactive reactions can be run at lower temperature. Interestingly, the best ligand was found to be the atropisomeric monophosphine (fJ)-Ph-BINEPINE.312 Enantiomeric excess up to 85% was observed. Very recently, enantioselectivity up to 94% ee has been achieved using [(AuCl)2(Tol-BINAP)] as pre-catalyst for the reaction of another enyne.313... 

The above reactions are rather slow because the insertion reactivity is directly related to the rate of dissociation of the THF ligand, which is also present as the solvent. The most reactive cationic complexes are therefore base-free, i. e. they do not contain coordinating solvents or ligands. Logically, anion coordination is a problem in such systems. Ion pairing reduces the reactivity of the cationic zirconocenes, but in the presence of B(C6F5)4, RB(C6F5)3 (R = H, Me), or BARF", this interaction is only weak. 

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