Pericyclic Reactions and Cycloadditions Flashcards
Pericyclic reactions differ from ionic reactions
Define ionic reactions?
- In ionic reactions electrons flow in one direction from the nucleophilic site/group to an electrophilic site/group
- ‘directionality’
Pericyclic reactions differ from ionic reactions
Define pericyclic reactions?
- The electron flow in a ‘circuit’ and there are no charged intermediates
- Reactions are ususally concerted
- Rearrangement of electrons, not really a sense of directionality
What are the 3 general types of pericyclic reactions?
1) Cycloaddition reactions (e.g. Diels-Alder, [2+2], 1,3-dipolar cycloadditions)
2) Electrocyclisation reactions
3) Sigmatropic rearragement reactions
What are the mechanism(s) and product(s) of this reaction?
- Note the regiochemistry and stereochemistry involve - which one is favoured
- This reaction is slow due to rotation around the diene
What is the product of this electrocyclisation
Notes the possibility for stereochemistry
What is the product of this Sigmatropic rearrangement reaction
Note the stereochemistry involve
(name derived from the fact we’re moving a sigma bond within a pi system)
Cycloaddition reactions: consider cyclopentadiene reacting with maleic anhydride
What is the mechanism and the two diasteromeric products that can be formed during this reaction
Why is the ENDO product the major product
- Due to the cyclopentane ring pointing away from the cyclohexene ring in the ENDO structure it is much more sterically hindered (thermodynamically unfavourable)
- But due to kinetics the ENDO product is favoured
- this reaction is ‘thermally allowed’
[2+2] cycloadditions are ‘photochemically allowed’
What is the mechanism and product of this reaction
what is an important thing to note about the compounds within these reactions?
- The reagents are neutral (i.e. the coulombic interactions are small)
- Hence the need to consider orbital interactions
In all reactions we are interest in HOMO-LUMO interaction
As the cycloaddition comprise two component (a DIENE and a DIENOPHILE) which HOMO and which LUMO do we consider?
We simply consider the HOMO and LUMO pair that are closest in energy
“Klopman’s equation” illustrates this
ΔE is this equation is equivalent to…
How favourable the reaction is
(We want this to be big)
What does this equation tell us about the HOMO-LUMO energy gap
The smaller the HOMO-LUMO energy gap, i.e. the denominator, the larger the value for ΔE and the more favourable the reaction is
What does this equation tell us about atomic orbitals?
- The coefficient/size of the atomic orbitals in the relevant molecular orbitals of one reacting component and another reacting component
- The bigger this is, the larger ΔE
What do we need to know to answer the Klopman equation
We need to know the HOMO and LUMO energies for each of the reactants - these can be calculated via ab initio methods, but the relative energies can also be estimated using some simple models (we are using the secondary)
The diagram below shows the relative energies of the molecular orbitals of the first few conjugated alkenes
Describe its components
- Pc - energy of a p orbital of an isolated carbon atom infinitely separated from anything else
- If we have two of those p orbitals approaching each other, once they get close enough they can interact
- Interaction in a constructive manner will lead to a bonding molecular orbital (same phase) Ψ1
- Overlap in a destructive phase will give an antibonding molecular orbital Ψ2*
Fill out the next diagram for the relative energies of the molecular orbitals for butadiene
- In butadiene you get four molecular orbitals with four pi electrons
Fill out the next diagram for the relative energies of the molecular orbitals for hexatriene
Fill our the next two diagrams for the relative energies of the molecular orbitals from the allyl cation and anion
When we have a odd number of carbons, it is the case we have non-bonding orbitals, which has the same energy as Pc
As the cycloadditions comprise of two components (a diene and a dienophile) which HOMO and which LUMO do we consider?
i.e. this example with ethene + butadiene
- We are looking for this distance with the smallest HOMO-LUMO gap
- In this case, it does not matter which pair of interactions are cosidered as they are equal
In reality this is a poor Diels-Alder reaction, and “forcing conditions” are required in order to obtain a reasonable conversion to product
What can we do?
In order to make the Diel-Alder reaction faster, we need to decrease the HOMO-LUMO energy difference
What is the first way we can decrease the HOMO-LUMO energy difference?
Raise the energy of the HOMO (diene) and lower the energy of the LUMO (dienophile)
What is the second way we can decrease the HOMO-LUMo energy difference
Raise the energy of the HOMO (dienophile) and lower the energy of the LUMO (diene)
How can we modulate the relative HOMO-LUMO energies of the diene and dienophile?
Use electron withdrawing and electron donating substituents
What is the effect of the following electron-withdrawing group on the charges on the alkene
What is the effect of the following electron-withdrawing group on the charges on the alkene
Complete the energy level diagram for a diene and a dienophile with an electron withdrawing group
Complete the energy level diagram for a dienophile and a diene with an electron withdrawing group
Complete the energy level diagram for a diene and a dienophile with an electron donating group
Complete the energy level diagram for a dienophile and a diene with an electron donating group
