The Stereoelectronic Hypothesis

If it is assumed that S values are nearly equivalent, it is seen that Cy S is always favored, and this bias is greater the shorter the nnd length. On 

Since Tordo s calculations are very approximate, the differences found for the E.J. values are certainly too high. For instance, we will see in Section IV that introduction of alkyl substituents often modifies the ratio Cy5/Cy6 dramatically, much more than would be expected on the basis of these calculations. Nevertheless, the conclusion that the selectivity under kinetic control is mainly governed by the C(2)-X bond length has proved to be very useful. For instance, when X = PPh, Davis, Downer, and Kirby obtained what could be considered a puzzling result, namely exclusive formation of the Cy6 compound. This can be understood easily by a consideration of the large dc,2,-p bond length (1.83 A). Also, in the case of X = S [dc,j,.s = 1.81 A], it will be seen 

Finally, it should be noted that Baldwin, in a brief communication, has proposed some empirical rules governing ring closure reactions and although he is mainly concerned with nucleophilic cyclizations processes he has also considered free radical ones the origin of these rules appears similar to the one just discussed. 

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