20.13 : 自由基反应性：分子内与分子间

In intermolecular radical reactions, the radical and the radical trap are two different molecules.

The radical trap must be activated and present in high concentrations, and the radical source must have a weak carbon–heteroatom bond.

Now, consider an intramolecular radical reaction, in which a nucleophilic radical adds to a nucleophilic double bond, yielding a cyclized product.

Here, the radical trap is neither activated nor present in excess. The C–S bond is also relatively strong.

Still, such a reaction is feasible, giving a high product yield. Why so?

This is because, for intramolecular reactions, the radical and the radical trap are part of the same molecule and are always held closely, favoring rapid cyclization.

Consequently, the possibility of radical reduction by a hydride donor diminishes.

Moreover, the radical trap is neither highly reactive nor in excess, reducing its chances of reacting with the tin-hydride radical, irrespective of the latter's concentration.

Therefore, intramolecular radical reactions are very efficient and find applications in synthesizing five‐membered rings. Smaller ring sizes exhibit ring strain. Larger ring sizes are not preferred.