Chapter 16: Conjugated Pi Systems & Pericyclic Reactions

Dienes

• A molecule with two carbon-carbon double bonds.

• Two conformations: cis and trans.

Is the cis or trans confirmation of a diene more stable?

• The trans confirmation is more stable because of steric hinderance.

1,2 Addition

• Hydrogen adds to the first carbon, halide adds to the second carbon.

• Under low conditions it is the major product.

• Kinetic control (Low E_a)

• Less stable because it is monosubstituted.

1,4 Addition

• Hydrogen adds to the 1st carbon, halide is added to the 4th carbon.

• At higher temperatures it is the major product.

• Thermodynamic control (High E_a)

• Product is formed more slower → more stability

• Lower energy because it is disubstituted.

• More stable

Pericyclic Reaction

• Occurs without ionic or free radical intermediates.

• Three types:

  • Cycloaddition

  • Electrocyclic

  • Sigmatrophic

• Is concerted.

Characteristics of Pericyclic Reactions

• Mechanism is concerted: no carbocation intermediates.

• Mechanism involves e^- ring moving around a closed loop.

• Polarity of the solvent affects reaction rate.

• Transition state is cyclic.

Cycloaddition: Sigma and Pi Bonds

• Gains +2 sigma bonds

• Loses -2 pi bonds

Electrocyclic: Sigma and Pi Bonds

• Gains +1 sigma bonds

• Loses -1 pi bonds

Systematic: Sigma and Pi Bonds

• Gains 0 pi bonds

• Loses 0 sigma bonds

Cumulated Diene

• Pi bonds are adjacent.

Conjugated Diene

• Pi bonds are separated by a single bond.

Isolated Diene

• Pi bonds are separated by one or more single bond.

Cycloaddition

• Twi pi bonds are converted into two sigma bonds.

• Results in the addition of 2 reaction to for a ring.

Diel’s Alder Reactions

• A conjugated diene reacts with a dienophile

• Are stereospecific

• Performed at low or moderate temperatures.

• Concerted

Dienophile

• “Diene-lover”

• Alkene with one double bond.

  • Alkynes can be dienophiles too.

• Electron-poor

Dienophiles contains…

• -COOH,CN,-COR (ketones),-CHO (aldehydes)

What products are formed in a diel-alder reaction?

• Two new bonds (pi bond, 2 sigma bonds), six-member ring

Alkynes as dienophiles

• Two double bonds are formed

Diene’s as a ring - Diene Alder Rxn’s

• If the diene starts off as a ring you get a bicyclic product.

• Called the endoproduct

What confirmation must dienes be in to react?

• S-cis confirmation.

• Will not react if the diene is in s-trans confirmation.

[2+2] Cycloaddition

• Two alkenes react to form a 4-member cyclobutane ring.

Electrocyclic Reaction

• A pericyclic reaction in which a conjugate polyene undergoes cyclization.

• Occurs under light conditions.

Electrocyclic Reaction Products

• A pi bond is converted to a sigma bond.

Conrotary

• The rotation of the orbital in the same direction

• Either all clockwise or all counterclockwise.

Disrotary

• Is when orbitals rotate in the clockwise and counterclockwise directions.

Highest Occupied Molecular Orbital for 4 Pi Conjugated

• Two orbitals filled at the top

• Two orbitals filled at the bottom

Electrocyclic: Four Pi Electrons Thermal

• Conrotatory

Electrocyclic: Six Pi Electrons Thermal

• Disrotatory

Photochemical: Four Pi Electrons

• Disrotatory

Photochemical: Six Pi Electrons

• Conrotatory

Sigma tropic Rearrangement

• Pericyclic reaction in which 1 sigma bond is replaced with another sigma bond.

Cope Rearrangement

• [3,3] Sigmatrophic rearrangement

• All 6 atoms in the cyclic transition state are carbon atoms.

• Occurs high thermal energy.

• No intermediates.

Claisen Rearrangement

• [3,3] Sigmatropic rearrangement

• Oxygen analouge of a cope rearrangement

• Occurs with an allylic vinylic ether.

• Equilibrium favors the double CO bond (carbonyl) - more stable