Allyl Model Catalysts and the Concept of Chronoselectivity

In agreement with these basic considerations, isolated / -allyl complexes 

Chain propagation If k = rates of butadiene insertion, statistics of placement = f ( kj / ) 

The name chronoselectivity has been coined to describe this phenomenon, which represents the first example of a synthetic catalytic code indeed, the complex simultaneously controls the regioselectivity, the stereoselectivity, and different statistics of placement (as a function of time) of the chain formation, in a non-thermodynamic manner. A preliminary mechanistic explanation has been suggested (Fig. 26), based on relative insertion and dynamic rearrangement rates in the complex (see also [6]). 

All of these experimental results fit very well with the previously proposed mechanisms, and strongly support them the detailed Idnetic and structural data obtained give a pretty clear view of the molecular course of the reaction, although the fundamental reasons underlying the regioselectivity control are still not very well understood. 

Other mechanistic pathways have been proposed, in particular a so-called outer-sphere mechanism (similar to a pericyclic allylic transposition) by Hugues and Powell. Although they are probably relevant to some stoichiometric reactions of -allyl complexes with dienes, they do not seem to fit the characteristic experimental features of these dienes coordination polymerizations. 

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