Structure Nucleophilicity Relationship of Carbon Free Radicals

Structure Nucleophilicity Relationship of Carbon Free Radicals 

The high positional and substrate selectivity of the homol5dic substitutions of protonated heteroaromatic bases with nucleophilic carbon free radicals is one of the main factors determining the S5mthetic success of these reactions. In this section it will be shown that the selectivity is mainly determined by the influence of polar effects. As the extent of these effects is much larger than that previously observed in all the other reactions of the same radicals, these reactions have provided very useful models for determining the structure nucleophilicity relationship of the most common carbon free radicals. 

Particularly suitable models were provided by the homolytic substitutions of 4-substituted pyridines and 2- and 4-substituted quinolines, which are selectively attacked respectively in the positions 2, 4 and 2 by nucleophilic carbon free radicals. The positional selectivity is in fact always determined by the protonated heterocyclic nitrogen, wliich has a much more activating effect than the common substituents. 

The method, by using these models, is based on the assumption that the radical-stabihzing effects are minimized and the strengths of the bonds formed between the free radicals and the heteroaromatic ring are not substantially affected by the fact that the substituents are in meta to the position of attack in the heterocyclic ring. Thus the sequence of the reactivities of the meta positions to the substituents can be considered as best reflecting the effect of the polar nature of the attacking radical. 

The results of the Table 17 show the substrate selectivity of the homolytic alkylation of 4-substituted pyridines by the simplest types of alkyl radicals  

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E. Structure Nucleophilicity Relationship of Carbon Free Radicals... 

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