1 Chemistry 4420 Dr. Y. Zhao Topic 8 Pericyclic Reactions

Chemistry 4420 Dr. Y. Zhao
Other cycloadditions including [8 +2], [4 + 3] and [6 + 4]
cycloadditions are also known. A special class
cycloadditions where one of the component is a single
atom are called cheletropic reactions. For example, [2 +1]
and retro-[4 + 1] cycloadditions.
H
Cl
Cl
C
Cl Cl
H
[2 + 1] cycloaddition or carbene insertion
Ph
Ph
O
Ph
Ph
Ph
Retr o-[4 + 1] cycload dition
Ph
Ph Ph
(c) Sigmatropic Rearrangements
Sigmatropic rearrangements involve the cleavage of a σbond
connecting the end of one fragment with the end of
another, with concerted formation of another σ-bond at the
other ends of the fragments. Like cycloaddition,
sigamtropic rearrangement also uses [m + n] to indicate
the atoms involved in each component.
O
Ph
O
[ 3,3] sigmatropic rearrangement
or Oxy-Cope rearrangement
R O
R O
[3,3] sigmatropic rearrangement
or Claisen rearrangement
OBn
H
H
H
H
[1,5] sigmatropic rearrangement
Ph
H
OBn
Why [3,3]?
O
1 2
bond to be broken 1' 3' Ph bond to be formed
2'
R
HO
H
3
Cl
Cl
H
[1,2] sigmatropic rearrangement
or 1,2-alkyl shift
H
H
H
CH3
H3C
[1,3] sigmatropic rearrangement
(d) Ene Reactions
H
H
H
O
S
Ph
O
S
Ph
[2,3] sigmatropic rearrangement
The ene reaction is always a 6-electron reaction. It shares
some characteristics with the [4 + 2] cycloaddition and the
[1,5] sigmatropic rearrangement.
H
H
Alder ene reaction
H
O
Se Ph
O
H
H
retro-hetero-ene reaction
O +
Ph
H O
O
Se Ph
Summary of Pericyclic Reactions
electrocyclic reaction
one π bond one σ bond
sigmatropic rearrangement
oneσ bond new σ bond
H
O
Ph
retro-ene reaction
electrocyclic reaction
two π bond two σ bond
H H
ene reaction
oneπ bond one σ bond
andoneσ bond migrate
+
H
O
O
2