Pericyclic reactions Flashcards
What is the most common type of organic reaction and what are the key features?
Ionic reactions, electrons move in pairs and in a clear direction.
What are the key features of radical reactions?
Electrons move one by one and use a ‘fish-hook’ curly arrow.
What is the definition of a pericyclic reaction?
Concerted processes in which bond orbitals overlap in a continuous cycle in the transition state.
Define the words concerted and continuous (applying to pericyclic reactions)
Concerted= all bond making and breaking occurs simultaneously (ie no intermediate). Continuous= no orbitals in between which don't interact.
What happens to an orbital phase at a node?
It inverts
What does the size of the orbital coefficient reflect (and not reflect)?
Reflects the electron density on the corresponding atom (not the size of the orbital)- ie the contribution of each orbital to the overall MO.
Explain how the conjugation length affects the HOMO-LUMO gap
The longer the conjugation length, the smaller the HOMO-LUMO gap. Adding electrons to the orbitals (with electron-donating substituents) makes it easier to remove them- hence HOMO and other orbitals tend to increase in energy. Removing electrons from the orbitals (with electron-withdrawing substituents) makes it easier to add new ones- hence LUMO tends to decrease in energy.
When is a rotation symmetry allowed in the Frontier molecular orbital approach?
If the phases of the overlapping orbitals match.
When is a thermal pericyclic reaction symmetry allowed according to the Woodward-Hoffman rule?
If (4q+2)s + (4r)a is an odd number
q and r = integers
What are s and a components?
Supra- and antarafacial components.
Supra- = reaction of the same face Antara= reaction of opposite faces
What counts as a component (W-H rule)?
Continuous group of connected atoms with the same type of orbital.
When is a photochemical pericyclic reaction allowed?
If (4q+2)s + (4r)a is an even number (opposite to thermal rules).
List the 4 types of pericyclic reactions
Cycloadditions, electrocyclic reactions. sigmatropic rearrangements, group transfer reactions
Are pericyclic reactions reversible?
Yes
What does [m+n] represent in pericyclic nomenclature?
m and n are the number of electrons in each component.
What is constructive orbital overlap?
Where the phases match.
Compare cis and trans dienes
trans= stable but unreactive, cis= les stable but reactive.
Very small barrier for rotation- interconvert.
Discuss exo- Vs endo- products in a Diels-Alder reactions
In a D-A reaction, a less thermodynamically stable product (endo- prod) is usually formed preferentially but if D-A reactions are very fast in both directions, the exo-product could dominate.
How do normal ad inverse Diels-Alder reactions differ?
Normal: diene= HOMO, dienophile= LUMO
Inverse (rare): diene= LUMO, dienophile= HOMO
How can you accelerate normal and inverse Diels-Alder reactions?
Normal: EW groups on the dienophile and ED groups on the diene.
Inverse: ED groups on the dienophile and EW groups on the diene.
What conditions to Diels-Alder reactions require?
High temp (as often reactions are quite slow).
How do achiral starting materials affect the products?
Reaction will form both enantiomers, ie product will be racemic mixture.
How can Lewis acid catalysis affect Diels-Alder reactions and how much LA is needed?
Lewis acid (eg AlCl3) can complex a heteroatom of EW substituents on a dienophile, lowering the LUMO energy hence improving overlap. Catalytic amounts of LA required.
How can you induce chirality in a Diels-Alder reaction?
Use a chiral Lewis acid.
What reacts in photochemical cycloadditions and what orbitals overlap?
Excited molecule will react with an unexcited molecule.
SOMO + HOMO and SOMO* + LUMO
What are cheletropic reactions?
Cycloadditions (and reverse reactions) with 2 sigma-bonds formed/broken to the same atom.
Define con- and disrotatory movement
con- = orbitals rotating in same direction. dis- = orbitals rotating in opposite direction.
Are all electrocyclisations allowed?
Yes (either con- or disrotatory) but they may be geometrically constrained.
What orbitals do you use in frontier MO (FMO) treatment for thermal and photochemical reactions?
Thermal= HOMO Photchemical= SOMO*
Name an example of an electrocyclisation
Nazarov cyclisation
Define a sigmatropic rearrangement and name some examples
Migration of a sigma bond from one end of a pi-system to the other.
Claisen, Cope, ox-Cope, hydride-shifts
What reactions occur in the Strychnine synthesis?
D-A followed by a retro D-A reaction (often difficult to recognise D-A reactions).
What sort of orbital overlaps are preferred in D-A reactions?
Large/large and small/small overlaps are favoured over large/small overlaps.
