Pericyclic reactions

Question 1

What is the most common type of organic reaction and what are the key features?

Answer 1

Ionic reactions, electrons move in pairs and in a clear direction.

Question 2

What are the key features of radical reactions?

Answer 2

Electrons move one by one and use a ‘fish-hook’ curly arrow.

Question 3

What is the definition of a pericyclic reaction?

Answer 3

Concerted processes in which bond orbitals overlap in a continuous cycle in the transition state.

Question 4

Define the words concerted and continuous (applying to pericyclic reactions)

Answer 4

Concerted= all bond making and breaking occurs simultaneously (ie no intermediate).
Continuous= no orbitals in between which don't interact.

Question 5

What happens to an orbital phase at a node?

Answer 5

It inverts

Question 6

What does the size of the orbital coefficient reflect (and not reflect)?

Answer 6

Reflects the electron density on the corresponding atom (not the size of the orbital)- ie the contribution of each orbital to the overall MO.

Question 7

Explain how the conjugation length affects the HOMO-LUMO gap

Answer 7

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.

Question 8

When is a rotation symmetry allowed in the Frontier molecular orbital approach?

Answer 8

If the phases of the overlapping orbitals match.

Question 9

When is a thermal pericyclic reaction symmetry allowed according to the Woodward-Hoffman rule?

Answer 9

If (4q+2)s + (4r)a is an odd number

q and r = integers

Question 10

What are s and a components?

Answer 10

Supra- and antarafacial components.

Supra- = reaction of the same face
Antara= reaction of opposite faces

Question 11

What counts as a component (W-H rule)?

Answer 11

Continuous group of connected atoms with the same type of orbital.

Question 12

When is a photochemical pericyclic reaction allowed?

Answer 12

If (4q+2)s + (4r)a is an even number (opposite to thermal rules).

Question 13

List the 4 types of pericyclic reactions

Answer 13

Cycloadditions, electrocyclic reactions. sigmatropic rearrangements, group transfer reactions

Question 14

Are pericyclic reactions reversible?

Answer 14

Yes

Question 15

What does [m+n] represent in pericyclic nomenclature?

Answer 15

m and n are the number of electrons in each component.

Question 16

What is constructive orbital overlap?

Answer 16

Where the phases match.

Question 17

Compare cis and trans dienes

Answer 17

trans= stable but unreactive, cis= les stable but reactive.

Very small barrier for rotation- interconvert.

Question 18

Discuss exo- Vs endo- products in a Diels-Alder reactions

Answer 18

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.

Question 19

How do normal ad inverse Diels-Alder reactions differ?

Answer 19

Normal: diene= HOMO, dienophile= LUMO

Inverse (rare): diene= LUMO, dienophile= HOMO

Question 20

How can you accelerate normal and inverse Diels-Alder reactions?

Answer 20

Normal: EW groups on the dienophile and ED groups on the diene.
Inverse: ED groups on the dienophile and EW groups on the diene.

Question 21

What conditions to Diels-Alder reactions require?

Answer 21

High temp (as often reactions are quite slow).

Question 22

How do achiral starting materials affect the products?

Answer 22

Reaction will form both enantiomers, ie product will be racemic mixture.

Question 23

How can Lewis acid catalysis affect Diels-Alder reactions and how much LA is needed?

Answer 23

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.

Question 24

How can you induce chirality in a Diels-Alder reaction?

Answer 24

Use a chiral Lewis acid.

Question 25

What reacts in photochemical cycloadditions and what orbitals overlap?

Answer 25

Excited molecule will react with an unexcited molecule.

SOMO + HOMO and SOMO* + LUMO

Question 26

What are cheletropic reactions?

Answer 26

Cycloadditions (and reverse reactions) with 2 sigma-bonds formed/broken to the same atom.

Question 27

Define con- and disrotatory movement

Answer 27

con- = orbitals rotating in same direction.
dis- = orbitals rotating in opposite direction.

Question 28

Are all electrocyclisations allowed?

Answer 28

Yes (either con- or disrotatory) but they may be geometrically constrained.

Question 29

What orbitals do you use in frontier MO (FMO) treatment for thermal and photochemical reactions?

Answer 29

Thermal= HOMO
Photchemical= SOMO*

Question 30

Name an example of an electrocyclisation

Answer 30

Nazarov cyclisation

Question 31

Define a sigmatropic rearrangement and name some examples

Answer 31

Migration of a sigma bond from one end of a pi-system to the other.
Claisen, Cope, ox-Cope, hydride-shifts

Question 32

What reactions occur in the Strychnine synthesis?

Answer 32

D-A followed by a retro D-A reaction (often difficult to recognise D-A reactions).

Question 33

What sort of orbital overlaps are preferred in D-A reactions?

Answer 33

Large/large and small/small overlaps are favoured over large/small overlaps.