A agostic interactions

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

Formation of metal-carbene complexes via a-agostic interaction... 

The current scope of the controlled monocarbotitanation of alkenes and alkynes is still very limited at least in part due to competitive side-reactions arising via ft- and a-agostic interactions, as alluded to Scheme 6. On the other hand, polymerization, also shown in Scheme 6, may be largely avoided or minimized in most cases. T o overcome some of the difficulties mentioned above, cyclic version of monocarbotitanation have been explored,25-27 as shown in Scheme 12. None of these reactions has as yet been widely used, but their further development might lead to synthetically useful methods. 

Figure 3. Modified Cossee mechanism for the polymerization of olefins with early transition metals. Green, Rooney and Brookhart introduced the presence of the adjuvant a-agostic interaction in the transition state.

Table 1. Geometries and energies of the H2Si(Cp)2ZrCH3 species. The angle 0 and the a-agostic interaction are defined in Figure 4. A geometry with a a-agostic interaction was not found at the HF level.

Figures. Minimum energy geometries of the H2Si(Cp)2ZrCH3(C2H4)+ system starting from the olefin-free H2Si(Cp)2ZrCH3+ species without, part a, and with, part b, a a-agostic interaction.

Since molecular mechanics cannot be used to calculate the energy of transition states, suitable models were adopted. These models are extremely similar to the Jt-olefin complex with an orientation of the growing chain rather similar to that adopted when a a-agostic interaction is present. They were often called pre-insertion intermediates because the insertion transition state could be reached from these intermediates with a minimal displacement of the reacting atoms. 

Proton/deuterium isotope effects on reaction rates are useful mechanistic probes. In the zirconocene-catalyzed alkene polymerization, the observed values of k .iH/feo(.2H determined by NMR fall in the range of 1.2-1.3 and support a transition state in which there is an a-agostic interaction (see 88) [132]. 

Cossee-Arlman mechanism, is based on the observed stereoselectivity and molecular modeling studies [Arlman and Cossee, 1964 Ewen, 1999 Rappe et al., 2000 Resconi et al., 2000]. A variation on this mechanism involves a lowering of the transition state barrier to insertion by an a-agostic interaction, specifically, an attractive interaction between titanium and a hydrogen on the first carbon attached to Ti. 

It has also been suggested that the experimental barrier of activation for propagation in olefin polymerisation is due to rearrangement of y-agostic species into those with a-agostic interactions in order to free the coordination site for the insertion of the next monomer molecule, rather than the actual insertion (Figure 3.19) [358],... 

The ratio of the molecular weights of polymers obtained from (O-l-pro-pene-l-d, and (Z)-l-propcnc-l-d, is found to be about 1.3. Can this be explained in terms of a-agostic interactions and kinetic isotope effect ... 

In Cs-symmetric metallocene catalysts syndiospecificity arises due to a-agostic interaction. Discuss the correctness of this statement. 

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