Problem of Steric Restrictions

Ion-radical organic reactions of the Sj j l type are less sterically restricted than classical Sj reactions. Generally, the nucleophilic (not Sj j ) reactivity varies with the steric demand at the reaction center. The electron-transfer reactivity does not depend on steric effects. To illustrate this, one can compare electron transfer and nucleophilic reactivity between ketene silyl acetals and cationic electrophiles (Fukuzumi et al. 2001). Nevertheless, space strains may determine the overall results of these reactions if either intermediate radicals or forming products are sterically hindered. 

In the course of the reaction, the nitrite ion leaves the primary anion-radical. This produces the cyclohexyl radical in the pyramidal configuration. The vicinal methyl group sterically hinders the conversion of the pyramidal radical into the planar one. With a high concentration of the nucleophile, the rate of addition exceeds the rate of conversion, that is, Then the entering PhS group 

Dimerization of arylamine cation-radicals is a prominent example of the product s steric hindrance effects on the reaction course. Although stable in AN, the A,A-dimethyl-p-toluidine dimerizes if its uncharged counterpart is present in the solution (Goto et al. 2002, Oyama and Goto 2003). The cation-radical loses a proton, and the parent amine reacts as a base, accepting this proton. Coupling of benzylic radicals is an understandable result of the following reaction  

The reaction course principally changes as a result of CH3 relocation from the para to ortho position of the benzene ring. Because of steric hindrance at the benzylic radical center, the dimerization is directed to the free para position of the benzene ring and the corresponding benzidine occurred as the hnal product (Oyama and Kirihara 2004). 

The authors explain the difference through the ortho effect that hinders o-Wuester azathiacrown ether from one-electron oxidation but assists in the mercury coordination inside the macrocycle (Sibert et al. 2006). 

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