chapter 30 orbitals and organic chemistry: pericyclic reaction polar mechanism radical mechanism...
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Chapter 30 Orbitals and Organic Chemistry: Pericyclic Reaction
Polar mechanismRadical mechanismConcerted process
Electrocyclic reactionCycloadditionSigmatropic rearrangement
Woodward-Hoffmann rules
A pericyclic reaction can take place only if the symmetries of the reactant MOs are the same as the symmetries of the product MOs.In other words, the lobes reactant MOs must be of the correct algebraic sign for bonding to occur in the transition state leading to product.
If the symmetries of both reactant and product orbitals match up, or “correlate”, the reaction is said to be symmetry-allowed.
If the symmetries of reactant and product orbitals don’t correlate, the reaction is symmetry-disallowed.
Kenichi Fukui
Frontier orbitalsHighest occupied molecular orbital (HOMO)Lowest unoccupied molecular orbital (LUMO)
An electrocyclic reaction is a pericyclic process that involves the cyclization of a conjugated polyene. One bond is broken, the other bonds change position, a new bond is formed, and a cyclic compound results.
heat
a conjugated triene a cyclohexadiene
heat
(2E,4Z ,6E)-Octatriene cis-5,6-Dimethyl-1,3-cyclohexadiene
CH3
CH3
HH
CH3
CH3
H
H
CH3
CH3
H
H
(2E,4Z ,6Z )-Octatriene
heat
CH3
H
H
CH3
trans-5,6-Dimethyl-1,3-cyclohexadiene
h
Stereochemistry The most striking feature of electrocyclic reactions
Polyenes with an even( 偶数 )number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense, Whereas polyenes with an odd number of electron pairs undergo the same reactions in a disrotatory sense.