CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 19 - Professor J. Michael McBride
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 Möbius pathway, or a disrotatory Hückel
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.
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