Orbitals and Organic Chemistry

Pericyclic Reactions

MOT

n The greater the number of nodes, the higher the energy.

n Double bonds are seen in the HOMO

Practice

n Show y₁, y₂, y₃, y₄* using the phase designators + and – for (2E,4Z,6E)-octatriene

n Note: Only the HOMO and LUMO are important for our purposes.

Symmetry: Frontier Orbitals

n Only orbitals that are in phase can form bonds

n For thermal reactions

n Only consider the ground state HOMO

n For photochemical reactions

n Consider the HOMO and LUMO

Bond/Orbital Rotation

n When bonds form, the bonds rotate so that overlapping orbitals are in phase.

How Can You Tell if its Dis or Con

n Even number of double bonds will give conrotatory

n Odd number of double bonds will give disrotatory

Problem

n What product will form from thermal cyclization of 2Z,4Z,6Z octatriene?

Allowed Symmetry

Photochemical Electrocyclic Reactions

n Electrons are promoted from the HOMO to the LUMO by absorption of a photon.

n The LUMO is converted to the HOMO and the reaction occurs based on this molecular orbital.

Example

n Electron promoted to the LUMO

n Is this Conrotatory or Disrotatory?

Problem

n What product will form in the photosynthetic cyclization of (2E,4Z,6E)-octatriene?

Hint

n Here is the excited state HOMO of the p network

Answer

n Can you do this without being given the orbital configuration?

n You need to be able to!

Practice Again

n Show y₁, y₂, y₃, y₄* using the phase designators + and –

n You need to be able to find HOMO and LUMO electron distributions.

Stereochemical Rules for Electrocyclic Reactions

Cycloaddition Reactions

n Diels-Alder Revisited. HOMO + LUMO

Suprafacial Cycloaddition

n Two possible ways for addition to occur

Antrafacial Additions

n Molecule must twist to achieve proper symmetry

4+2 p Addition

n HOMO and LUMO Overlap

2+2 p Addition Thermal

n HOMO + LUMO can not bond

2+2 p Addition Photosynthetic

n HOMO* + LUMO will bond

Stereochemcial Rules for Cycloaddition

Problem

n What type of reaction is involved?

n Photosynthetic or Thermal

n Antrafacial or Suprafacial

Answer

n Make one the HOMO and the other the LUMO

n Since 5 electron pairs involved, its suprafacial

Sigmatropic Rearrangements

n s bonded atom migrates through p rearrangement

Sigmatropic Rearrangements

Suprafacial and Antrafacial

n Both are possible

n Suprafacial is easier!

Stereochemical Rules for Sigmatropic Rearrangements

Sigmatropic Rearrangement

n Deuterium transfer

n Orbital View

Cope Rearrangement

n Start Organic Reactions Animations

n Go to the last file. Note: The program must be installed on the computer

Claisen Rearrangement

n The Claisen and Cope mechanisms are identical. (3,3) Suprafacial