Pericyclic Reactions Flashcards
Define a pericyclic reaction
A concetered process where bond orbitals overlap in a continuous cycle in the transition state.
All atoms provide orbitals to overlap and form a continuous ring.
Draw a set of MOs for a conjugated alkene (hexa-1,3,5-triene) without considering the orbital coefficients
Draw n lines where n is the number of conjugated p-orbitals.
Draw out the p-orbitals with an increasing number of nodes on each line.
Shade the orbitals according to the nodes and fill the orbitals with electrons.
Describe how to draw MOs to determine orbital coefficients and draw an example of butadiene.
Draw n lines where n is the number of p-orbitals conjugated on the molecule (4).
Draw n+2 dots on each line (6).
Draw a sine wave between the end dots on each line, with increasing numbers of nodes, starting at 0.
Draw the orbitals where the top of the orbital touches the curve.
How do you work out the orbital coefficients of a molecule with a electron donating/withdrawing substituent, for example ethene with an OMe group.
Draw the orbital coefficients the same way as normal, but once with an unsubstituted molecule, and the other with a CH2-/CH2+ group for the donating/withdrawing group.
The HOMO and LUMO coefficients are assumed to be an average between the two coefficients.
How do electron donating/withdrawing substituents affect the energies of MO’s?
Electron donating substituents make it easier to remove electrons (positive charge stabilised), hence increasing the HOMO and other orbital energies.
Electron withdrawing substituents make it easier to add electrons (negative charge stabilised), hence LUMO tends to decrease in energy.
In a Diels-Alder reaction between 1,3-butadiene and but-1-en-3-one, which molecule provides the HOMO and which provides the LUMO?
The HOMO and LUMO donating molecules is determined by the difference in energy between the two HOMO-LUMO pairs. One of the pairs will be closer in energy for different molecules.
1,3-butadiene has a closer HOMO-LUMO gap as there is more conjugation. But-1-en-3-one has an electron withdrawing group so its LUMO is modified to be lower in energy. This means the diene will provide the HOMO and the carbonyl will provide the LUMO.
How do you lay out a pericyclic reaction drawing to show the orbitals combining?
How do the orbital symmetries change when the product is formed?
Draw the molecules combining on two faces of a cube, showing the overlapping orbitals on the corners.
Orbital symmetry is conserved when moving from a starting material to a product.
Draw and describe how the frontier molecular orbital approach can be used to explain the electrocyclisation pictured.
(HOMO/LUMO are the frontier orbitals)
Draw lines with 5 points for each of the p orbitals, then draw the orbitals out without considering coefficients.
Work out which component will use HOMO and which will use LUMO for overlap. In this case it turns out to be impossible to work out which orbital to use, but we know that for electrocyclisation, the HOMO is used.
The phases of the orbitals can be seen to overlap when the bonds rotate inwards.
Give the Woodward-Hoffman rule for thermal pericyclic reactions, explaining its meaning.
A thermal pericyclic reaction is symmetry allowed if: (4q + 2)s + (4r)a = odd.
()s and ()a represent supra- and antarafacial components. Suprafacial components are those that react from the same face, antarafacial components react from the opposite faces.
(4q + 2) are components with 2, 6, 10, etc. electrons (double an odd number). Hence if a suprafacial componant has 2, 6, 10, etc. electrons, count in the equation.
Oppositely, (4r) are components with 4, 8, 12, etc. electrons (double an even number). Hence if an antarafacial component has 4, 8, 12, etc. electrons, count it in the equation.
Using Woodward-Hoffman rules, work out if the pictured reaction is thermally allowed.
First, draw the curly arrows for the reaction. Then identify and draw out the reacting components. Take care when drawing out components to make sure the bonds being formed are in the correct orientation (pointing at each other for sigma, pointing parallel for pi).
Work out the component electrons and if they’re reacting supra- or antarafacially. Then work out which components count towards the equation and use it to find if the reaction is thermally allowed.
What are the Woodward-Hoffman rules for photochemical reactions?
The opposite of thermal rules: (4q + 2)s + (4r)a = even.
This is opposite as one electron is promoted into the LUMO, creating two SOMOs.
A cyclisation reaction between benzene and an alkyne is possible under the correct conditions. Use Woodward-Hoffman rules to determing if it is thermally or photochemically.
Here benzene is acting as an alkene. Therefore the component only has two electrons despite the whole molecule being conjugated.
Name the four types of pericyclic reactions.
- Cycloadditions
- Electrocyclic reactions
- Sigmatropic rearrangements
- Group transfer reactions
What methods can you use to analyse cycloaddition reactions?
How are the reactions described?
Woodward-Hoffman and FMO can be used to analyse cycloadditions. When using FMO, usually the diene provides the HOMO and the dienophile provides the LUMO.
The reactions are described by [m + n] where m and n are the number of electrons in each component.
For the Diels-Alder reaction pictured, will the endo or exo product be formed and why?
There is a primary and secondary reaction between the reactants. The reaction is possible due to both aspects meaning the carbonyl must be in the endo position to form the secondary interaction.
In the rare case that the retero DA reaction is fast, the product will be under thermodynamic control and produce the exo product.
In the pictured Diels-Alder reaction, which of the two regioisomers will form and why?
By working out the orbital coefficients, the effect of the substituents can be seen. This shows that the orbitals in the HOMO and LUMO have one large and one small coefficient for each double bond. The large coefficients will therefore be favoured to overlap, preferentially forming the ortho isomer.
In a DA reaction between a substituted diene and a dienophile, what determines the position of the groups on the new 6 membered ring?
What stereoisomers will form?
The stereochemistry of the alkenes. The inside facing groups will be pushed up with the outside groups pushed down.
The opposite enatiomer will also form when the diene reacts from the bottom.
In this case only the major endo-products has been shown, but the two enantiomers of the minor exo-product will also form.
What are cycloadditions of the type pictured called?
They are 1,3-dipolar cycloadditions. The 1,3-dipolar compound can be an ozone or a nitrone.
Name and describe the mechanism and product of the reaction shown below.
It is a cheletropic reaction where two sigma bonds are formed from the same atom. The alkene pi orbitals rotate to form the new sigma bonds, producing a molecule with cis methyl groups.
Retro-cheletropic reactions can also occur.
Describe the two types of electrocyclisation reactions.
Why would each occur and what are the implications of one over the other?
Conrotatory and disrotatory.
Each will occur when is it favoured by the molecular orbitals (number of qualifiying componants). The stereochemistry will be controlled by the reaction that occurs.
