Leaving group aptitude

The reagent and the nucleofuge are the most important variables in controlling elimination reactions. The leaving group aptitude increases steeply from dialkylamino through alkoxide, fluoride, and chloride to bromide and iodide. 

After a decade of research the basic principles in the chemistry of decamethylsilicocene (1) seem to be understood. This compound shows the reactivity of a nucleophilic silylene due to the fact that the Tt-bonded pentamethylcyclopentadienyl ligands are easily transferred to a-bonded substituents during the reaction. The steric requirements of these substituents permit reactions with bulky substrates. The migratory aptitude and the leaving-group character of the pentamethylcyclopentadienyl groups... 

This frontier-orbital approximation does not give a complete picture of the influence of the leaving group on the stereochemistry. For instance, it does not account for the aptitude of the Si—X bond to be stretched under... 

The chemistry of selenones (11) has not been studied as extensively as that of selenoxides. However, it has been demonstrated that selenones function as good leaving groups, comparable to tosylates or hahdes. For example, they are readily solvolyzed in methanol and produce rearranged products when -substituents with high migratory aptitudes are present (equations 7 and 8). Vinyhc selenones also act as Michael acceptors, as in the first step of the process shown in equation (9). 

Migratory aptitude depends on configuration at C9, conformation, stereoelectronic factors, leaving group ability, solvation and acidity (protonation of bridgehead nitrogen). 

In order to account for the high stereoselectivity, the departure of the leaving group has to occur simultaneously with the migration. If the departure of X precedes the migration and a true carbene is formed, the anti/syn stereoselectivity would reflect the intrinsic migratory aptitudes of substituents instead of the stereoelectronic preferences (Figure 7.43). ... 

In the case of unsymmetrical glycols the product formation depends mainly on which OH group is lost to leave behind the most stable carbocation, and thereafter which group migrates. The order of migrating aptitude is Ar > H > R. 

Attention has been focused on two aspects of the reaction, namely the relative migratory aptitudes of different carbon groups and the effect of a change in the sulphonate leaving... 

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