Cycloadditions Pt6 Flashcards
Describe what is a 1,3-Dipolar cycloaddition?
- These involve the cycloaddition of a ‘1,3-dipole’ and a ‘dipolarophile’
- The reactions are 6-electron processes, and are [4+2] cycloadditions (same as Diels-Alder)
What is a 1,3-Dipole?
- When the resonance form is drawn, there is a negative charge formally on the carbon atom
- And the central nitrogen has four bonds to it
- And a terminal nitrogen which is positively charged with 3 bonds to it
- The two central resonance forms achieve 8 electrons, while the terminal resonance forms achieve 6 electrons - important because this is where the bonds are going to be formed
The 1,3-dipole supplies 4 electrons and the dipolarophile supplies 2 electrons
Complete the mechanism
Which is the dipole and which is the dipolarophile?
These reactions can be refered to [4+2] or (3+2) cycloadditions, what is the difference?
[ ] brackets refers to the number of electrons
( ) refers to the number of atoms
Draw this other resonance form for a nitrone
Draw this other resonance form for ozone
Draw this other resonance form for nitrile oxide
Draw this other resonance form for diazoalkane
Draw this other resonance form for an Azide
What is the major product of this reaction and why
- To avoid steric clash of the two dimethyl and ester substituenet (has the ability to react the other way round and them to be on adjacent carbons)
What is the major product of this reaction and why?
- Could have reacted the other way (i.e. attack on the trimethyl carbon) but sterics of the dimethyl and tri-methyl carbon would be too great
- Trans relationship with regards to central alkene, hence trans stereochemistry
What is a type I 1,3-dipole cycloaddition?
- HOMO 1,3-dipole
- LUMO dipolarophile
What is a Type II 1,3-dipolar cycloaddition?
- Borderline
- No clear preference in the HOMO-LUMO interaction
What is the product of this azide reacting with an alkene
What is the product of this reaction of a nitrone with an alkene
What is the product of this reaction of a nitrone with an alkene
Did you notice that despite these substituents having a different effect, the products still have the same regiochemistry
Why
This is due to them being type II, borderline reactions
One way to gain regioncontrol is intramolecular cycloadditions
What is the product and mechanism?
One way to gain regiocontrol is Lewis Acid coordination
- The two hydrogens on the alkene are cis and is translated into the product
- The reaction is on the top face because the hydroxyl group is pointing up, so when it tethers to the Mg, the substrate is bring it to the top face of alkene
When you add copper catalyst to the following reaction of azides reacting with alkynes has been dubbed a ‘click’ reaction meaning?
A reaction which occurs relatively very quickly under mild conditions
Another example of a click reaction is this ‘strain release’ promoted cycloaddition
What is the mechanism and product?
Ozone has a very low energy LUMO and will react with all alkenes
What is the mechanism for this reaction
- The intial product (acid) breaks apart in a reverse 1,3-dipolar cycloaddition to give you a different dipolararophile
- The O-O single bonds are quite weak which acts as the driving force for the reverse [4+2] dipolar reaction
- If you flip them they are react again to form a secondary arsenide
What happens if you then treat the product of this reactant with a reductant like dimethylsulphide (or triphenylphosphine)
- (the overall process is a very important synthetic transformation that produces two carbonyls from one alkene (ozoneolysis)
- Again attacking of weak O-O single bond
What is the product of this ozonolysis reaction
What is the product of this ozonolysis reaction?
What is the product of this ozonolysis reaction?
