Agostic interaction alkene insertion

Figure 6.4 Alkene insertion with a- and y-agostic interactions in the transition state 6.12 and intermediate 6.13. Such interactions are important for stereoregular polymerization by metallocene catalysts.

Specific examples of alkene insertions are the reactions of cationic hydrides [Cp2ZrH(L)]+ to give metal alkyls which may either be stabilized by agostic interactions, as in (21-XLII), or free of such interactions (21-XLIII), depending on L.180 The insertion of isobutene into M—C bonds of electron-deficient zirconium complexes was found to be reversible.109... 

Many theoretical studies of insertion concern the very important case of insertion of ethylene (or a-alkenes) into a growing polymer, following the Cossee-Arlman direct insertion mechanism (Scheme 6.2) [6], In this particular case, the resting states of the catalyst before and after each propagation step are similar, and contain an M-alkyl bond stabilized by an agostic interaction. The interactions with the solvent or external ligands must be also rather similar for both resting states. Thus, the values calculated must correspond only to the coordination and insertion of the alkene. 

The making and breaking of C-H and M-H bonds are key steps in important metal-catalyzed reactions, such as hydrogenations, hydrosilylations, C-H activations, and Fischer-Tropsch reactions. Insertion of an alkene into a metal-hydride bond and the reverse, /7-hydride elimination, are key steps in some of these processes (see Equation (3)). Occasionally, compounds exist as an agostic complex, which may be viewed as a key intermediate in these reaction paths. Brookhart has carried out extensive DNMR studies of these processes and agostic interactions,... 

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