Lecture 19 - Aromatic Transition States: Cycloaddition and Electrocyclic Reactions. Cyclic conjugation that arises when p-orbitals touch one another can be as important for transition states as aromaticity is for stable molecules.
It is the controlling factor in "pericyclic" reactions. Regiochemistry, stereochemistry, and kinetics show that two new sigma bonds are being formed simultaneously, if not symmetrically, in the 6-electron Diels-Alder cycloaddition.
Although thermal dimerization of thymine residues in DNA is forbidden, photochemistry allows the 4-electron cycloaddition. "Electrocyclic" ring opening or closing chooses a conrotatory Mobius pathway, or a disrotatory Huckel pathway,
according to the number of electron pairs involved and whether light is used in the process. Dewar benzene provides an example of a very unstable molecule that can be formed photochemically and then persists because of unfavorable orbital overlap
in the transition state for ring opening. (from oyc.yale.edu)
Lecture 19 - Aromatic Transition States: Cycloaddition and Electrocyclic Reactions
Time
Lecture Chapters
[00:00:00]
1. Aromatic Ions
[00:05:59]
2. Pericyclic Reactions: Cycloaddition, the Diels-Alder Reaction, and Photochemistry