Week 3: Pericyclic reactions

Question 1

What are three types of reactions in organic chemistry

Answer 1

Polar reactions: new bond formed between nucleophile and electrophile, with both electrons coming from the nucleophile

Radical reactions: new bond formed using one electron from each reactant

Pericyclic reactions: electrons are redistributed simultaneously through a cyclic transition state

Question 2

What are the 3 types of pericyclic reactions

Answer 2

Cycloadditions (diff sigma = 2)

Sigmatropic rearrangements (diff sigma = 0)

Electrocyclic reactions (diff sigma = 1)

Question 3

Outline a cycloaddition reaction

Answer 3

2 or more unsaturated molecules form a cyclic adduct with more single bonds

[ ] indicate number of electrons involved in formation of product

Reversible

Question 4

Explain frontier orbitals of cycloadditions

Answer 4

HOMO of diene and LUMO of dienophile

This is the smallest energy gap as electrons need to travel up to higher energy state

EDG to HOMO and EWG to LUMO will decrease energy gap

Question 5

Explain stereochemistry of a diene

Answer 5

Can be open chain or cyclic

MUST be in or able to convert to s-cis conformation

Question 6

Explain stereochemistry of cycloaddition products

Answer 6

Exo (kinetic): final double bond and other ring are closer to each other, more steric

Endo (thermo): final double bond and other ring are further from each other

The Hs where sigma bond forms are always cis to each other

Question 7

Explain Diels Alder regioselectivity

Answer 7

Use resonance structures of unsymmetrical reactants to see which isomer product will form

Want to match up negative charge from diene to positive from dienophile

terminal EDG on diene and EWG on dienophile will be ortho in product

internal EDG on diene and EWG on dienophile will be para to each other

Question 8

Explain the reasoning behind photochemical cycloaddition

Answer 8

UV light is absorbed, pi to pi* transition instead of sigma to sigma* occurs.

UV light changes HOMO of diene which will change the phases allowing proper overlap.

Irreversible because product has lost pi bonds to absorb UV light.

Usually occurs when one alkene bonded to a conjugating group as its HOMO will change but the other reactant will not.

Question 9

What are the Woodward Hoffman rules

Answer 9

If the total number of (4q+2)s and (4r)a components is odd, the reaction is thermal.

q and r are integers

Question 10

What is suprafacial + antarafacial and inversion + retention

Answer 10

Suprafacial (pi): forms new bonds on same face

Antarafacial (pi): forms new bonds on different faces

Retention (sp3): large lobe of sp3 orbital is involved in reaction

Inversion (sp3): small lobe of sp3 orbital is involved in reaction

Question 11

Outline a sigmatropic rearrangement

Answer 11

Pi bonds rearrange, 1 sigma bond breaks and 1 sigma bond forms

Reversible

Question 12

What is the [a,b] of sigmatropic notation

Answer 12

Label two atoms of breaking sigma bond as 1.
Count and label adjacent atoms (1 then 2 then 3 etc)
Small number is a and large is b where the new sigma bond forms

If only one side goes fully around just count from that 1

Question 13

List 4 common sigmatropic rearrangements

Answer 13

Claisen
Cope
Claisen-Cope (Aliphatic claisen)
[1,5] H shift

Question 14

Explain claisen and cope rearrangements

Answer 14

Claisen: [3,3] where there is an extra end step of a proton transfer to the O.

Cope: [3,3] most simple, rearrangement of only carbons in the ring

Question 15

Explain claisen-cope (aliphatic claisen)

Answer 15

[3,3], chair like transition state, no H transfer to O.

If one of the final double bonds has an R group, it will favour the trans (E) geometry and an equatorial position transition state.

Question 16

Explain a [1,5] H shift

Answer 16

where a H bond jumps over onto the next carbon because of the pi rearrangement.

Rapidly occurs and will therefore mean a molecule will isomerise to potentially multiple differing dienes.

The H moving can be called a suprafacial migration because the leaving group leaves and rejoins the same face. (thermal)

An antarafacial migration is basically impossible because it is too constricted for a H to do this between only 1 bond of the ring. (photochemical)

Question 17

Outline electrocyclic reactions

Answer 17

a new sigma bond is formed between the ends of a conjugated pi system (polyene), producing a cyclic compound.

reversible

Question 18

Suprafacial rules for electrocyclic reactions

Answer 18

(4n + 2) pi systems react suprafacially if in a thermal process and antarafacially in a photochemical process

(4n) pi systems react antarafacially if in a thermal process and suprafacially in a photochemical process

Question 19

Explain conrotatory and disrotatory for electrocyclic reactions

Answer 19

Conrotatory (thermal): the 2 groups on the carbons forming the new sigma bond rotate in the same way for orbital overlap.

Disrotatory (photochemical): one group rotates clockwise and the other anti-clockwise.