Reactivity of the Leaving Group

The objective in selecting the reaction conditions for a preparative nucleophilic substitution is to enhance the mutual reactivity of the leaving group and nucleophile so that the desired substitution occurs at a convenient rate and with minimal competition from other possible reactions. The generalized order of leaving-group reactivity RSOj" I- > BF > CF pertains for most Sw2 processes. (See Section 4.2.3 of Part A for more complete data.) Mesylates, tosylates, iodides, and bromides are all widely used in synthesis. Chlorides usually react rather slowly, except in especially reactive systems, such as allyl and benzyl. 

With a given C group, the approximate order of reactivity of the leaving group (X) is as follows. 

These examples seem to indicate that the interaction of azines with bifunctional nucleophiles can result in the formation of cyclization products only if we deal with an azine in which the reactivity of the leaving groups is enhanced by aza-activation or by the presence of electron acceptors. It has been found indeed that diazines, triazines, etc., are more inclined to cyclization than are pyridines. 

The inhibitory capacity of an affinity label is related to its ability to form the reversible complex and to the reactivity of the leaving group. The reversible protein ligand complex dissociates to an active protein (e.g. enzyme), while the affinity labeled protein is either partly or totally inactivated. The degree of inactivation is dependent on the importance of the blcKked functional group to the biological (e.g. enzyme) activity of the affinity labeled protein. 

The leaving group also has a substantial effect on conversion to the desired product, and comments from Section I also apply to the main group elements. The difference in the reactivity of the leaving group(s) enables the chemist to rationally synthesize mixed aryl-alkyl derivatives. 

Halides are the dominating starting materials for carbonylation of aryl-X derivatives. This is not true for benzyl-X and allyl-X compounds. Here, alcohol derivatives [26] such as acetates, carbonates, ethers, and phosphates, and the alcohols themselves, are more important today. According to Murahashi et al. [27] the reactivity of the leaving group of allylic substrates decreases in the order... 

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