Agostic interactions acidity

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

CH(SiMe3)2] are pyramidal, being stabilized by agostic interactions of methyl groups with the highly Lewis acidic metal center, as shown in Fig. 18.4.6(b). 

Due to this lack of acidity, the internal carbon often does not activate upon metalation with many transition metals. This results in a novel form of porphyrin metal binding where the internal C-H bond(s) remain intact but lie close to the bound metal. Several metals exhibit this type of bonding, including manganese (25-27), rhodium (25), iron (29), cobalt (30 zinc (57), palladiiun (52), silver (55), cadmium, and mercury (34), The C-H bond can reside very close to the metal ion, often within 2.5 A. As a result of this proximity, this interaction has been referred to as agostic, however, the key measurement that would define the agostic interaction, the C- H coupling constant, has yet to be measured. 

The approach and insertion of an olefin molecule may or may not pass through a local minimum or coordination complex (first in brackets in eq. 16) recent theoretical work (128) indicates that the well, if it indeed exists, is very shallow. The insertion of the new molecule into the growing chain is represented in equation 13 as a structure intermediate between reactants and products. The mechanism for this apparently concerted reaction does not involve the participation of metal-based electrons, and can be considered to be a Lewis acid-assisted anionic attack of the zirconium alkyl (ie, the polymer chain) upon one end of a carbon-carbon double bond. The concept of this reaction pre-dates metallocene study, and is merely a variant of the Cossee-Arlman mechanism (129) routinely invoked in Ziegler-Natta polymerization. Computational studies indicate (130) that an a-agostic interaction (131) provides much needed stabilization during the process of insertion. 

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