A cycloaddition that destroys bonds: ozonolysis
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص906-907
2025-07-24
444
A cycloaddition that destroys bonds: ozonolysis
Our last type of cycloaddition is most unusual. It starts as a 1,3-dipolar cycloaddition but eventually becomes a method of cleaving π bonds in an oxidative fashion so that they end up as two carbonyl groups. The reagent is ozone, O3. Again, you met this reaction in Chapter 19, but we can now show you the full, remarkable details of the reaction mechanism. Ozone is a symmetrical bent molecule with a central positively charged oxygen atom and two terminal oxygen atoms that share a negative charge. It is a 1,3-dipole and does typical 1,3-dipolar cycloadditions with alkenes. The product is a very unstable compound. The O–O single bond (bond energy 140 kJ mol−1) is a very weak bond—much weaker than the N–O bond (180 kJ mol−1) we have been describing as weak in previous examples—and this heterocycle has two of them. It immediately decomposes—by a reverse 1,3-dipolar cycloaddition.
The products are a simple aldehyde on the left and a new, rather unstable looking molecule—a 1,3-dipole known as a carbonyl oxide—on the right. At least it no longer has any true O–O single bonds (the one that looks like a single bond is part of a delocalized system like the one in ozone). Being a 1,3-dipole, it now adds to the aldehyde in a third cycloaddition step. It might just add back the way it came, but it much prefers to add in the other way round, with the nucleophilic oxyanion attacking the carbon atom of the carbonyl group like this.
This compound—known as an ozonide—is the first stable product of the reaction with ozone. It is the culmination of two 1,3-dipolar cycloadditions and one reverse 1,3-dipolar cycloaddition. It is still not that stable and is quite explosive, so for the reaction to be of any use it needs decomposing. The way this is usually done is with dimethylsulfide or Ph3P, which attacks the ozonide to give DMSO and two molecules of aldehyde.
The ozonide will also react with oxidizing agents such as H2O2 to give carboxylic acids, or with more powerful reducing agents such as NaBH4 to give alcohols. Here are the overall transformations—each cleaves a double bond—it is called an ozonolysis.
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