Aryl-metal complexes , kinetic stability

The reaction of kinetically stabilized germylenes with metal complexes has proven a useful and not surprising route to complexes containing a metal-germanium double bond. Thus, the reaction of germylenes containing bulky aryl groups with the metal carbonyls... 

The kinetic and thermodynamic selectivity for reactions of a titanium-imido complex with different types of C-H bonds has been determined. Reactions with substrates that possess primary and secondary C-H bonds occur selectively at the primary C-H bond. In addition, reactions with mixtures of alkanes and arenes occur selectively at the arene C-H bond. Like the stabilities of most low-valent, late metal complexes, the primary alkyl complex is thermodynamically more stable than the secondary alkyl complex, and the aryl complexes are more stable than the alkyl complexes. Activation of olefins at the ally-lie position occurs more slowly than reaction at the vinyl position, but when it does occur, the reaction generates a stable Ti -allyl complex. 

Generally, pentafluorophenyl complexes are more stable than perfluoro-alkyl analogues [241]. TUs enhanced stability presumably arises from an increased strength of the M-perfluoroaryl bond by donation of the metal electrons to 7i -orbitals of the aryl ligand. This increased bond strength would increase the kinetic stability of the compound by increasing the energy necessary for dissociation of the M—Rf bond. 

The reactivity of Pt(0) complexes toward aryl halides closely parallels that of the Ni(0) and Pd(0) analogs. The trans-oxidative addition product is obtained, and second-order kinetics are observed. The greater stability of the metal-aryl compared with the metal-alkyl bond in group VIII d °-M(0) systems is attributed to the greater electronegativity of the sp carbons in the aryl ligands . 

Deprotonation of ethers is another route to the a-alkoxy anions, but this pathway is often precluded by a kinetic barrier. Unless the a-carbon is benzylic [175], surmounting this barrier usually requires conditions that are not favorable to the survival of the anion [164]. Notable exceptions are the hindered aryl esters studied by Beak [176], Figure 3.13a, and the carbamates studied by Hoppe [177], shown in Figure 3.13b. In both cases, ec-butyllithium is required for deprotonation, and the carbonyls which direct the metalation by a complex-induced proximity effect [178] must be shielded from the base by large alkyl groups. Once formed, the organo-lithiums are chelated and stabilized by the heteroatom-induced dipole [179]. 

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