Preece - Frontier MO's and Pericyclic Reactions Flashcards
What is a pericyclic transition state?
A reaction involving a cyclic transition state which is key to the product. Electrons do not flow in a general sense, movement is simeltaneous
Explain the specificity of a pericyclic reaction?
The reactions are stereospecific (the stereochemistry of the reactant is the same as the product) and regiospecific because of 1,3-syndiaxial interactions (equatorial on a chair)
When there’s a double bond in the ring, the chair is drawn in what way?
Linear carbon 2, double bond and carbon 4, then twist fashion of the remaining carbons
During a pecicyclic reaction, which product is formed? Kinetic or Thermodynamic?
Thermodynamic is preferred
In a migration reaction, which specificity is preferred, stereo or regio?
Stereo
What are the two generic starting materials for a pericyclic reaction?
Ene and Polyene Scenario’s:
1) Overlap of pi molecular orbitals
2) Overlap of sigma and pi MO
What is an electrocyclic reaction?
The formation of a sigma bond between the termini of a linear conjugated pi system by two of the pi electrons. The HOMO combination of pi orbitals rotate to overlap to form the new bond and retain stereochemistry
What are the two types of electrocyclic rotation?
Conrotatory - opposite face (orbitals oppositely aligned)
Disrotatory - Same face to same face (orbitals symmetrically aligned)
Methdod for calculating the stereochemistry in a ring opening reaction?
1) draw out the HOMO of the product without the substituents
2) draw out the transition state prior to C-C bond cleavage
3) Open the C-C bond to afford the HOMO of the product
4) Decide on how substituents move
Explain a cycloaddition reaction
A reaction that uses intermolecular pericyclic processes involving a formation of a ring from two independent conjugated systems (forming two new sigma bonds at the termini of the pi system). Reaction is stereospecific and regisoselective
what are the two types of cycloaddition?
Suprafacial (orbitals match up) and antarafacial (orbitals don’t match and have to swap)
What nomenclature is used to describe the Cycloaddition reaction?
πXs/a + πYs/a
Where X/Y = number of π electrons in each component
and S/A = the nature of attack, anti/suprarfacial
What is a diels-alder reaction?
Addition of a butadiene with another alkene group HOMO of butadiene and LUMO of other smaller alkene
In a Diels-Alder reaction, what does the introduction of electron withdrawing and electron donating groups do to the rate of reaction?
Electron withdrawing increase the rate of reaction because there’s a smaller HOMO LUMO energy gap
Electron donating decreases the rate of reaction because there’s a larger HOMO LUMO energy gap
What is Butadiene’s preferred form?
Z (trans)
