Associative displacement, leaving group

We could imagine two types of nucleophilic attack upon a ligand. The first involves the formation of a new bond to the incoming nucleophile and the associated loss of some other leaving group. This corresponds to nucleophilic displacement. The second type of reaction does not involve the loss of any other species and corresponds to an addition reaction. There are a number of ways in which the polarisation of the ligand can enhance a... 

The different reaction paths described by the model are shown in scheme 8, For clarity, ionic charges and the proton transfer step that follows displacement by a hydroxylic molecule have been omitted. The intermediates that maintain association with the leaving group may be regarded as ion pairs. An unsolvated ion pair acquires solvation before formation of the substitution product and in its solvated form may undergo dissociation to yield a symmetrically solvated ion. The possibility of reaction at the stage of the solvated ion pair makes race-mization with partial inversion the expected stereochemical outcome. However, where a mechanism exists for preferential replacement of the counter-anion on the front side of the carbonium ion, in the manner 13... 

A stereochemical possibility unique to the associative mechanism is adjacent displacement in which the bond angle between entering and leaving groups is 90°. This possibility results from the ability of the pen-tacoordinate phosphorane intermediate to undergo pseudorotation (9,10) which, with certain restrictions (11) interchanges axial and equatorial ligands. Nucleophilic displacements on stable phosphoranes appear to proceed via an associative mechanism with hexacoordinated phosphorus intermediates (12, 13). The biochemical relevance of such displacements has not been established. 

The faster reaction of the aqua complex could be accounted for partially, it was argued, because ligand displacement by an entering nucleophile could be expected to depend to some extent on the lability of the leaving group even when the reaction follows an associative mechanism. 

The mechanism of reactions of silicon compounds has been extensively studied and is complicated.75 Detailed discussion is beyond the scope of this book a few illustrations will suffice. The displacement reactions of optically active methyl-l-naphthylphenylsilane usually proceed with inversion of configuration. For R3SiX in general, if the incoming nucleophile Y is more basic than X and X is a leaving group for which the conjugate acid has a pK < 6, then the displacement is by an associative pathway with inversion... 

The exception is the SN2 displacement reaction to cleave an oxacyclopropane. The release of bond-angle strain upon opening the three-membered ring overcomes the unfavorable energetics associated with producing a poor leaving group. 

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