Substitution reactions leaving group abilities

Leaving group The group of atoms (coded green in this text) displaced in a substitution reaction. Leaving-group ability The ease with which a group can be displaced in a nucleophilic substitution,... 

Alkenyl(phenyl)iodonium salts are highly reactive in vinylic nucleophilic substitution reactions because of the excellent leaving group ability of the phenyliodonium moiety. Only a few examples of non-catalytic alkenylation of carbon nucleophiles are known [50,51]. In most cases these reactions proceed with predominant retention of configuration via the addition-elimination mechanism or ligand coupling on the iodine [42,50]. 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

These reactions appear to be restricted to strongly basic nucleophiles, as we found no reaction with phenoxides and thio-phenoxides. This lack of reactivity is attributed to the reversibility of the nucleophilic substitution, promoted by the increased leaving group ability of the nucleophile, e.g. 

To evaluate whether a nucleophilic substitution reaction will occur, compare the leaving group ability of the incoming nucleophile and the departing leaving group, as shown in Sample Problem 22.2. 

Leaving group ability (Section 7.7) A measure of how readily a leaving group (Z) can accept the electron density of the C-Z bond during a substitution or elimination reaction. 

This equation also describes the overall reaction of either an 5 2 or a nucleophilic aromatic substitution process. In some cases, the only way to distinguish an reaction from these processes is that an is inhibited by radical inhibitors. Another distinguishing feature is that the order of the relative leaving group abilities of halides are opposite that found for nucleophilic aromatic substitution by the addition-elimination mechanism (see Chapter 3). 

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