Alkene complexes bonding models

Features (2) and (3) are explicable in terms of the Dewar-Chatt-Duncanson model for bonding in alkene complexes (Figure 3.63), which involves... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Having generated suitable (partially) cationic, Lewis acidic metal centers, several factors need to be considered to understand the progress of the alkene polymerisation reaction the coordination of the monomer, and the role (if any) of the counteranion on catalyst activity and, possibly, on the stereoselectivity of monomer enchainment. Since in d° metal systems there is no back-bonding, the formation of alkene complexes relies entirely on the rather weak donor properties of these ligands. In catalytic systems complexes of the type [L2M(R) (alkene)] cannot be detected and constitute structures more closely related to the transition state rather than intermediates or resting states. Information about metal-alkene interactions, bond distances and energetics comes from model studies and a combination of spectroscopic and kinetic techniques. 

Figure 3.55 Dewar Chatt Duncanson bonding model in metal alkene complexes.

The bonding in monometal alkyne complexes is usually interpreted in terms of the Dewar-Chatt-Duncanson model (293), since the alkyne molecule has a pair of n and n molecular orbitals which lie in the plane of the metal and the two carbon atoms. These two orbitals are denoted n and n, and are analogous to those in jr-bonded alkene complexes (394). There is also a pair of n and n molecular orbitals which lie perpendicular to the metal-carbon plane, denoted nL and n . These orbitals are illustrated in Fig. 14. Both sets of n and n orbitals have the correct symmetry to interact with metal d orbitals. The interaction... 

Schwartz s reasoning for optimizing these thermodynamic considerations led to the development of hydrozirconation. Hydride complexes of the late transition metals do not in general exhibit the hydrometallation reaction, probably because the alkene complexes are too stable. This may be understood from the Dewar-Chatt-Duncanson model for alkene bonding, wherein back donation of metal d-elec-trons to the alkene Tr -orbital is a major contributor. For metal centers with d -electron configurations, there should be substantial stabilization of (3) with respect to (2). Such metals are only found towards the left end of the Periodic Table, particularly Groups III A to VA. 

The mechanism of asymmetric hydrogenation of dehydroaminoacids has been studied by a combination of kinetic and spectroscopic methods, mainly by Halpern et al. [38] and Brown et al. [39]. It was proved that the substrate bound by both the double bond and the amide group. It was surprising to see that the major diastereomeric rhodium-alkene complex detected in solution was the less reactive one towards hydrogen. This showed the inaccuracy of previous models of the lock and key type between the prochiral double bond and the chiral... 

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