Theoretical studies and basic features of the insertion step

1 Theoretical studies and basic features of the insertion step 

The advances in computational chemistry in the last decade have provided insight into the intimate details of processes that cannot be experimentally observed. Several methods at different levels of sophistication can be applied, and reviews on them have appeared. More detailed consideration of these aspects can be found in the reviews of Niu and Hall [1], and Koga and Morokuma [2], which contain numerous references. The insertion in real systems is the result of many subtle influences throughout the process. These can be better calculated independently on model systems which help us to analyze the experimental results. For this reason we consider first the modem theoretical results before discussing the real systems. 

The insertion of alkenes into M-R bonds (whether M-H or M-C) is now considered as a migratory insertion process and requires a mutually cis arrangement of the alkene and the M-H or M-C bond. Since in many practical processes the alkene is an external ligand which needs to coordinate immediately prior to insertion, a few theoretical studies consider also the coordination step as part of the insertion process. 

The orbital interaction leading to the fission of the M-R bond and the formation of the new bonds is the typical synergetic interaction of transition metals with n-acceptor ligands. The a(M-R) and 7t(alkene) orbitals, which are occupied, interact in a four-centered transition state with the a (M-R) and 7t (alkene) orbitals, which are vacant. This interaction produces a mixing of the rr(alkene) and 7t (alkene) orbitals that polarizes the alkene Compared to the simple alkene coordination (Fig. 6.1, left), where the donor and acceptor alkene orbitals interact only with the metal orbitals (this is the classical Chatt-Dewar model), the interaction with o(M-R) and o (M-R) leading to insertion polarizes the donor alkene orbital towards C  

When the metal center lacks filled d orbitals (d metal ions), there is no possibility of M L back donation to the alkene, and the 7t (alkene) orbital is fully available to interact with the a(M-R) electrons, thus this looks an ideal interaction to make a C/3-R bond (Fig. 6.2). If one looks at the whole process, starting from the free alkene, the metal center acts as acceptor of an electron pair from the alkene. If one looks at the R migration step alone, the metal releases the electron pair of the M-R bond, and a vacant coordination site is produced compared to the alkene complex. Theoretical calculations support that this unoccupied d orbital is used to stabilize the new alkyl system by agostic interaction with the occupied a(C -R) orbital. 

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