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 elec…
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.
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Q1: What are the key differences between intermolecular and intramolecular radical reactions?
In intermolecular radical reactions, the radical and radical trap are separate molecules, requiring the trap to be activated and present in high concentration. In intramolecular reactions, both are part of the same molecule, held closely together, enabling rapid cyclization without needing activation or excess concentration. This proximity makes intramolecular reactions far more efficient.
Q2: Why do intramolecular radical reactions proceed efficiently despite having strong carbon-heteroatom bonds?
Intramolecular reactions are efficient because the radical and radical trap are part of the same molecule, keeping them in close proximity. This favors rapid cyclization and prevents radical reduction by hydride donors. The radical trap, being neither highly reactive nor in excess, resists competing reactions with tributyltin hydride radicals, ensuring high product yield.
Q3: What ring sizes are preferred in intramolecular radical cyclization reactions?
Five-membered rings are the preferred product of intramolecular radical cyclization. Smaller ring sizes experience significant ring strain, making them unfavorable. Larger ring sizes are also not preferred, as they reduce the efficiency of the cyclization process and lower product yields.
Q4: How does molecular proximity affect the outcome of intramolecular radical reactions?
Molecular proximity is crucial in intramolecular reactions because the radical and radical trap remain held closely together within the same molecule. This close association dramatically increases the rate of cyclization and minimizes side reactions. The result is rapid, efficient bond formation with minimal competing pathways.
Q5: What conditions must be met for intermolecular radical reactions to occur successfully?
Intermolecular radical reactions require three key conditions: the radical trap must be activated, present in high concentration, and the radical source must have a weak carbon-halogen bond. These requirements ensure sufficient collision frequency and reactivity between separate molecules to generate acceptable product yields.
Q6: How does the presence of a hydride donor affect intramolecular versus intermolecular radical reactions?
In intermolecular reactions, hydride donors can reduce radicals, competing with trap addition and lowering yields. In intramolecular reactions, rapid cyclization minimizes the opportunity for hydride reduction. Additionally, the radical trap's low reactivity and lack of excess concentration reduce its competition with hydride donors, preserving reaction efficiency.
Q7: Why is the concentration of the radical trap critical in intermolecular but not intramolecular radical reactions?
In intermolecular reactions, high radical trap concentration increases collision probability with the radical, driving product formation. In intramolecular reactions, the trap is already bound to the radical within the same molecule, eliminating the need for high concentration. This inherent proximity ensures efficient reaction regardless of bulk concentration.