20.13:
Radical Reactivity: Intramolecular vs Intermolecular
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.
20.13:
Radical Reactivity: Intramolecular vs Intermolecular
Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak carbon–halogen bond.
In contrast, an intramolecular radical reaction involves a radical and a radical trap, which are parts of the same molecule. In such reactions, the radical trap is not activated and is also not present in excess. Moreover, the bonds in the radical starting material are relatively strong. But, still, intramolecular cyclization is enormously favored, with the product being formed in high yields. This is because the radical and the radical trap are part of the same molecule, due to which both are held close to each other. As a result, cyclization occurs rapidly. This rapid cyclization rules out the possibility of radical reduction by a hydride donor.
Lastly, intramolecular reactions are very powerful and efficient. These reactions are often used for synthesizing five‐membered rings over all other ring sizes.