Stereoelectronic and other factors

The transition state for disproportionation requires overlap of the (3 C—H bond undergoing scission and the /2-orbital containing the unpaired electron. This requirement rationalizes the specificity observed in disproportionation of radicals 29 (Section 1.4.2) and provides an explanation for the persistency of the triisopropylmcthyl radical (33) and related species (Section IA3.2). In the case of 33, the 3-hydrogens are constrained to lie in the nodal plane of the p-orbital due to stcric buttressing between the methyls of the adjacent isopropyls. 

It has been noted by a number of workers that the presence of a-substituents which delocalize the free spin favors combination over disproportionation, 

X = alkynyl alkenyl aryl nitrile keto ester alkyl exothermicity delocalization 

It has been suggested that benzylic radicals may form a dimeric association complex which may easily collapse to the combination product but be geometrically unfavorable for disproportionation. Even if this applies for the aralkyl radicals, it cannot account for the behavior of systems with other p-substituents. 

Another explanation follows from the above discussion on stereoelectronic factors.If overlap between the semi-occupied orbital and the breaking C-H bond favors disproportionation, then substituents which delocalize the free spin will serve to reduce this interaction and disfavor disproportionation. A proposal along these lines was made originally by Nelson and Bartlett w ho also noted that diminishment of the spin density at C could retard combination. However, it is not necessary that the two effects should cancel one another. 

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