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Pericyclic Reactions
Pericyclic reactions involve a concerted (one-step) mechanism and a cyclic
transition state, involving a simultaneous change in and bonds.
Cycloaddition: A reaction in which two - containing molecules come together to form two new - bonds and a new cycle.
Example: Diels-Alder.
Allowed and Forbidden Pericyclic
Reactions
Woodward-Hoffmann rules dictate which pericyclic reactions are “allowed”—have molecular orbital interactions that make them possible—and which are “forbidden”.
Woodward-Hoffman rules for cycloaddition:
Total number of -electron pairs:
odd Thermally allowed, photochemically forbidden.
even Photochemically allowed, thermally forbidden.
So [4+2] cycloaddition only requires heat,
h
(UV)
but [2+2] requires light.
[2 + 2] Cycloadditions Damage DNA,
Lead to Melanoma
h
(UV)
DNA strand
a thymidine dimer
thymidine (one of 4 DNA bases)
Electrocyclic Ring Opening and Closing
Electrocyclic reactions either form a ring from an acyclic starting material, or
open an existing ring into an acyclic product.
Ring Closing: An sp^3 - sp^3 bond is made.
Ring Opening: An sp^3 - sp^3 bond is broken.
- All electrocyclic ring reactions are allowed.
- Electrocyclic reactions are, in principle, reversible. In practice, thermodynamics typically favors one direction over the other.
Selectivity of Electrocyclic Ring Opening
Electrocyclic ring opening is controlled by the highest occupied molecular
orbital (HOMO) of the conjugated product.
Molecular orbital diagram:
E
HOMO =
Theory: Breaking bond becomes p components of HOMO; groups twist in same direction ( conrotatory ).
(^) but not
(E,Z) (Z,Z)
Woodward-Hoffman Rules for
Electrocyclic Reactions
Total number of reacting electron pairs:
odd
Thermal reaction pattern:
even
disrotatory
conrotatory
Photochemical reaction pattern:
conrotatory
disrotatory
