12 - PERICYCLIC REACTIONS

Question 1

things to keep in mind about resonance:

Answer 1

only electrons are moved
charges must be conserved
resonance isomers which have an octet around all atoms are more stable

Question 2

what is the core process of a diels-alder reaction?

Answer 2

diene+dienophile
the dienophile can also be an alkyne

Question 3

examples of synthesis where diels-alder reactions were used

Answer 3

RB Woodward’s synthesis of cortisone and gibberellic acid synthesis
or paraquinonic acid synthesis

Question 4

examples of diels-alder in nature?

Answer 4

lovastatin (cholesterol lowering agent) and spynosin A (insecticide)

Question 5

what is the core process of the claisen rearrangement

Answer 5

Question 6

what is an example in nature of a claisen rearrangement?

Answer 6

one step in the synthesis of phenylalanine is a claisen rearrangement

Question 7

what is the core process of the cope rearrangement?

Answer 7

Question 8

what is the core process of electrocyclic rearrangements?

Answer 8

an example of this is the synthesis of vitamin D

Question 9

what are pericyclic reactions and the three classes of pericyclic reactions?

Answer 9

concerted rearrangements involving a reorganization of bonding electrons in a cyclic fashion
three main classes:
- cycloadditions
- electrocyclic reactions
- sigmatropic shifts
one minor class:
- group transfer

Question 10

what are cycloadditions and examples?

Answer 10

addition of two pi systems (usually) to form a ring

Question 11

what are electrocyclic reactions, the rule and examples?

Answer 11

interconversion between acyclic polyenes and cyclic systems
the rule for this: if you make a new sigma bond it’s an electrocyclic reaction

Question 12

what are sigmatropic shifts, the rule and examples?

Answer 12

rearrangement where the location of the sigma bond moves across a pi system
rule= no new sigma bonds are made, the bonds just move

Question 13

how are cycloadditions named?

Answer 13

named by the number of reacting atoms in each component
only count the atoms that are part of the pi system that is part of the process
remember, the ATOMS not the ELECTRONS

Question 14

QUIZ: what would be the nomenclature of the cycloaddition of this reaction?

Answer 14

[3+2]

Question 15

what are some observations regarding [4+2] and [2+2] cycloadditions?

Answer 15

[4+2] additions go under thermal reactions
[2+2] additions are only observed under the influence of light
both reactions are thermodynamically possible, this means kinetics must play into this

Question 16

what is the effect of polar substituents on the diels-alder reaction?

Answer 16

if you put an EDG on the diene and an EWG on the dienophile, the reaction will increase
this still does not help [2+2] reactions however

Question 17

what does the fact that stereochemistry is conserved suggest?

Answer 17

stereochemistry in the diene and dienophile and product are conserved
this suggests that the reactions are concerted, everything happens at the same time so there is no time for twisting

Question 18

what conformation of diene does a diels-alder reaction require?

Answer 18

requires an s-cis diene conformation
that is why cyclopentadiene reacts really fast, it is locked into a s-cis conformation

Question 19

how is cyclopentadiene made from its dimer?

Answer 19

cyclopentadiene dimerizes with itself
heating cracks the dimer to produce the monomer which is distilled out

Question 20

how do orbitals affect reactions?

Answer 20

chemical reactions are characterised by the interactions of the electrons in the HOMO (nucleophile) of one molecule and the LUMO (electron acceptor) or another molecule
these interactions are significant when HOMO and LUMO are close in energy and when the symmetry of the orbitals allows bonding to occur
(the reason why thermal 2+2 cannot happen)