Pericyclic Reactions Flashcards
What 2 things define a pericyclic reaction?
1) Concerted Process - bonds are formed/broken simultaneously
2) Bond orbitals overlap in a continuous cycle
When determining orbital phases of a molecule, what does n define?
n = the number of p-orbitals in a molecule
For a tri-ene, what is n, and how many electrons does the molecule have?
n = the number of p-orbitals, hence n = 6.
The tri-ene has 6 electrons
When determining orbital coefficients, how many dots are drawn on each line?
n + 2, when n is the number of p-orbitals
Aside from the orbitals, what else is drawn when determining orbital coefficients?
A sine wave
What happens at each node when determining orbital coefficients?
The phase of the orbital changes
If a polyalkene has an EWG, what is the method when determining its orbital coefficients?
The average of the unsubstituted polyalkene and the polyalkene with the “ultimate EWG” are taken
What is the “ultimate” EWG?
CH2 +
What is the “ultimate” EDG?
CH2 -
True or False? When determining the orbital coefficients for the HOMO and LUMO of a substituted polyalkene, all p-orbitals must be taken into consideration?
False, only the p-orbitals involved during the reaction should be taken into consideration (e.g. not the lone pair on a ultimate EWG fragment –> only the alkene fragment)
How does a conjugated alkene effect the relative energies of molecular orbitals?
The longer the conjugation length, the smaller the HOMO-LUMO energy gap becomes
How does an EDG affect the relative energies of molecular orbitals?
EDG’s raise the energy of the orbitals (electrons are easier to remove due to increased electron density)
How does an EWG affect the relative energies of molecular orbitals?
EWG’s lower the energy of the orbitals (electrons are more difficult to remove due to decreased electron density)
Does a positive charge count as a p-orbital?
YES!
What determines if a reaction can occur in the frontier molecular orbital approach?
If the phases of the overlapping orbitals match
For Woodward-Hoffman rules, what determines if a THERMAL pericyclic reaction can occur?
If there are an ODD number of qualifying components
For Woodward-Hoffman rules, what determines if a PHOTOCHEMICAL pericyclic reaction can occur?
IF there are an EVEN number of qualifying components
For Woodward-Hoffman rules, what determines if a suprafacial components qualifies?
If the reacting orbitals have 2,6,10,14,… electrons
For Woodward-Hoffman rules, what determines if an antarafacial component qualifies?
IF the reacting orbitals have 4,8,12,16,… electrons
Define a component
A continuous group of connected atoms with the SAME TYPE of orbital
What are the three types of orbitals that are considered for Woodward-Hoffman rules?
pi, sigma and omega (lone pair)
Using Woodward-Hoffman rules, what are the 5 steps in identifying whether a reaction is thermally or photochemically allow?
1) Draw a curly arrow mechanism
2) Identify the components
3) Draw a 3D shape to identify the overlapping components
4) Assign components are supra- or antarafacial
5) Identify the number of qualifying components
What are the 4 types of pericyclic reaction?
1) Cycloadditons
2) Electrocylic reactions
3) Sigmatropic rearrangements
4) Group transfer reactions
What type of pericyclic reaction is Diels-Alder?
A Cycloaddition
What are the two molecules in a Diels-Alder reaction?
A dieneophile (alkene) and a diene
Why is the least stable endo-isomer sometimes the major product of a reaction?
Weak secondary reactions are present only with the endo product
Why might the more stable exo-isomer be the main product in a reaction?
If the reaction is reversible
In a normal electron demand Diels-Alder reaction, which molecule provides the HOMO and which the LUMO?
HOMO - Diene
LUMO - Dieneophile (alkene)
In an inverse electron demand Diels-Alder reaction, which molecules provides the HOMO and which the LUMO?
HOMO - Dieneophile (alkene)
LUMO - Diene
In a normal electron demand Diels-Alder reaction, how can the reaction be sped up by substituents and why?
An EDG on the diene and an EWG on the dieneophile (alkene) would decrease the HOMO-LUMO gap.
In an inverse electron demand Diels-Alder reaction, how can the reaction be sped up by substituents and why?
An EWG on the diene and an EDG on the dieneophile (alkene) would decreases the HOMO-LUMO gap.
How can Lewis Acids be used in pericyclic reactions?
Lewis Acids (like FeCl3 or AlCl3) can coordinate to an EWG to decrease electron density resulting in a decrease in the HOMO-LUMO gap.
In terms of the size of orbital coefficients, how do substituents effect the regiochemistry of a reaction?
Large orbital coefficients prefer to overlap with other large orbital coefficients, and likewise small with small. Drawing resonance forms show that the ionic charges align.
If the starting materials are not chiral, and there are no enantiomerically enriched compounds in the reaction mixture, then what will the products be?
The products will be racemic
How can enantionselectivity be induced in a Diels-Alder reaction?
By using an enantiomerically enriched compound, like a chiral Lewis acid catalyst
How do the reacting orbitals in a photochemical reaction differ to those in a thermal reaction?
In a thermal reaction, a HOMO and a LUMO orbital overlap. In a photochemical reaction a HOMO and excited SUMO orbital overlap (or LUMO and excited SUMO*)
What type of reagents are used in 1,3-dipolar cycloaddtions?
Nitrones or ozone
X=Y(+)-O(-)
Define a cheletropic reaction
Two or more sigma bonds are broken
What is an electrocylcisation reaction?
A ring formed from one molecule
Define conrotatory and disrotatory movement
Conrotatory: Both orbitals rotate in the same direction
Disrotatory: Orbitals rotate in different directions
Why is stereochemistry particularly important to consider during electrocylicastion reactions?
Con- or disrotartoy movement will move substituents in different directions resulting in different product stereochemistry
Why is equilibria rarely achieved in an electrocylication reaction?
Equilibria means the the forward and backward reaction must both have fast rates - which is rare under given condtions
Define a sigmatropic rearrangment
Migration of a sigma bond from one end of a pi-system to the other
What is the nomenclature in a sigmatropic rearrangement?
[m,n] where m is less than or equal to n
Match the SM: 1,5-diene, ether, carbonyl to the reaction: cope, oxy-cope and claisen
Cope: 1,5-diene
Oxy-cope: Carbonyl
Claisen: Ether
What is the best way to visualise sigmatropic rearrangments?
By drawing a chair conformation
Define a hydride shift?
The movement of a H atom and its electrons to a different position in a molecule
