Chapter 16: Conjugated Dienes and Pericyclic Reactions

• Identify all vinylic locations where protonation can occur that will produce a allylic carbocation • Draw resonant structures for each location • Draw nucleophile attacking at these locations and consider stereochemistry

• Identify all allylic locations where protonation can occur • Draw resonant structures for each location • Draw nucleophile attacking at these locations and consider stereochemistry • Identify kinetic and thermodynamic products

Pericyclic reactions have 4 characteristic features: • The reaction mechanism is concerted; it proceeds without any intermediates • The mechanism involves a ring of electrons moving around a closed loop • The transition state is cyclic • The polarity of the solvent generally has no effect on the reaction rate (transition state has little, if any, charge)

The reactants are the diene and dienophile Dienophile needs to possess an electron withdrawing group or the reaction will require high temp, which does not favor product formation Only work with cis diens

When bicyclic systems form the terms ENDO and EXO are used to describe functional group positioning

Pericyclic process where a conjugated polyene undergoes cyclization Overall, a π bond is converted to a σ bond Conrotary- both sets of lobes rotate in the same direction Disrotary- both sets of lobes rotate in different directions

A pericyclic reaction in which one sigma bond is replaced with another The location of the π bonds changes as well Can be a [1,5] rearrangement or a [3,3]

A [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are carbon atoms Remember, equilibrium favors more stable (substituted) alkene π bonds must be separated by exactly three σ bonds

A [3,3] sigmatropic reaction that is the oxygen analogue of a Cope rearrangement Commonly occurs with allylic-vinylic ethers Also with allylic aryl ethers Equilibrium favors more stable carbonyl bond (C=O)

Compounds with conjugated π systems absorb UV or visible light More conjugation means lower energy gap • Wavelength of light needed can shift closer to visible range because smaller energy gap, the greater the λmax The group of atoms responsible for absorbing UV-Vis light is known as the chromophore