Radical Chemistry 2 Flashcards
The following reaction is a conjugate addition to form intermolecular carbon-carbon bond forming reactions of radicals
What is the mechanism and product?
- Driven by the formation of a stronger I-SnBu3 bond
Why is no β-elimination observed for this reaction
No β-elimination of oxygen is observed, unlike in ionic reactions, due to the strong carbon-oxygen bond
(In ionic reactions we must also consider the ability of a leaving group to stablise a negative charge, which is high in the case of oxygen)
The following reaction is a intramolecular carbon-carbon bond forming reaction of radicals
What is the mechanism and product?
- 5-exo-trig cyclisation reaction
- Driving forcing is making a stronger C-C sigma bond from a weaker C-C sigma bond
C-Se is an example of another bond which will react with a radical
This reaction is an intramolecular C-C bond forming reaction of radicals
What is the mechanism and product
- Unusually this reaction reaction favours the formation of a 5-exo-trig cyclisation
Why is the 5-exo-trig cyclisation preferred over the alternative 6-endo-trig cyclisation
- Carbon 5 is closer than carbon 6 to react with the radical conformationally
- Less strain in the 5-membered transition state
- Both pathways are favoured according to Baldwin’s guidelines for ring closure, but the 5-exo-trig cyclisation is faster
Where does the nomenclature of these reactions come from?
- Exo/Endo - does the double bond form part of the ring (endo) or not (exo)
- Trig - trigonal planar, double bonds are sp² hybridised
Why is intramolecular C-C bond forming reactions of radicals favoured compared to the intermolecular processes
The radical and the double bond are more likely to come into contact with the intramolecular process
(however the precursors to these reactions are harder to get)
The following is an example of a 6-exo-trig radical cyclisation, which takes advantage of the ready sterochemical inversion of alkenyl radicals
What is the mechanism and the product?
Requires rotation around sp² carbon so radical and C-C double bond are closer together
Radicals can be used to make larger rings (macrocycles), where high dilution conditions are used to decrease the rate of premature quenching of the initially formed alkyl radicals by Bu₃SnH, giving the radical a chance to undergo macrocyclisation
What is the mechanism and product?
The real power of radical cyclisation is seen in domino reactions, where more than one carbon-carbon bond and several rings can be formed in one step
What is the first step of the mechanism?
The real power of radical cyclisation is seen in domino reactions, where more than one carbon-carbon bond and several rings can be formed in one step
How does this intermediate further react?
The high reactivity of radicals can be exploited to functionalise otherwise inert, unreactive bonds. The following example is a classic case of the Barton nitrite photolysis reaction, used to functionalise steroid frameworks
What is the mechanism
Driving force is the formation of strong N-O bond
SN2 reaction
This intermediate is then irradiated
What is the mechanism and the product?
The Hoffmann-Loffler-Freytag reaction is useful for forming five-membered rings satuarted nitrogen heterocycles (pyrrolidines) and involves the generation of highly reactive nitrogen-centred radical cations
What is the mechanism and product when we react with acid then irradiate
- Last step with NaOH in an SN2 reaction
What is the mechanism for the last step of the Hoffmann-Loffler-Freytag reaction
