Download Number of Electron Pairs Allowed Sigmatropic Rearrangement

Chapter 22
Pericyclic Reactions
concerted
cyclic transition state of breaking and forming bonds
no electrophiles, nucleophiles, or radicals
very stereospecific
energy supplied by
heat, Δ, thermolysis or pyrolysis
or
light, hν, photolysis
conrotation: end carbons rotate in the same direction
disrotation: end carbons rotate in opposite directions
theoretical explanation (1965) developed by
R. B. Woodward and R. Hoffmann
and
K. Fukui
Fukui and Hoffmann shared the 1981 Nobel Prize
in chemistry for this work.
MO Theory for Conjugated Molecules
1. The number of MO’s equals the number of AO’s
combining to form them.
2. The energies of the MO’s are symmetrically placed
about the energy of an isolated p orbital
(arbitrarily taken as zero energy).
3. The energy of an MO increases as the number of nodes
increases.
4. Nodes are symmetrically placed in a molecule.
frontier MO’s = HOMO and LUMO
Electrocyclic Reactions
forms a sigma bond between the end atoms of a series
of conjugated pi bonds
product has one more sigma bond and one less pi bond than reactant
conrotation
Overlap of orbitals where new bond is formed must be
favorable (bonding) in the HOMO.
H3C
H3C
CH3
CH3
H3C
H3C
CH3
CH3
HOMO = π3nb
This reaction is thermally forbidden.
If HOMO has an odd number of nodes (π2, π4, π6 ...)
the thermal reaction proceeds with conrotation.
If HOMO has an even number of nodes (π1, π3, π5 ...)
the thermal reaction proceeds with disrotation.
Number of Electron Pairs
Disrotation
Conrotation
Odd
thermally
allowed
photochemically
allowed
Even
photochemically
allowed
thermally
allowed
Examples of Electrocyclic Reactions
allowed in either direction
two electron pairs (even)
thermally allowed reaction is conrotation
equilibrium favors the butadiene product
no angle strain
put light selectively into diene
(dienes absorb longer wavelength, lower energy light)
disrotation is photochemically allowed
6 electrons (3 pairs)
odd, so disrotation
is thermally allowed
4 pairs
3 pairs
Cycloaddition Reactions
two molecules
cyclized product
two pi bonds converted to two sigma bonds
classified according to the number of pi electrons in each component
thermal [2+2] cycloaddition reaction
photochemical [2+2] cycloaddition reaction
only one component is excited
thermal [4+2] cycloaddition reaction
Number of Electron Pairs
Allowed Cycloaddition
Odd
Thermal
Even
Photochemical
The Diels-Alder Reaction (a [4+2] Cycloaddition)
1950 Nobel Prize in chemistry
very useful, makes a six-membered ring
with excellent control of stereochemistry
stereochemistry:
syn addition on both components
conformation of the diene
more s-cis present
This diene is about 10x
more reactive.
unreactive
diene is held s-trans
H
O
O
H
O
intramolecular reactions are favorable
Other Cycloaddition Reactions
[2+2] cycloadditions
photochemically allowed
a [8+2] cycloaddition = 5 electron pairs
thermally allowed
Sigmatropic Rearrangements
intramolecular migration of a group along a conjugated pi system
To classify:
1. find the sigma bond broken in the reaction
2. assign number 1 to both atoms of this bond
3. number atoms of each component up to
where new sigma bond is formed
4. designate rearrangement by the numbers of the atoms
where the sigma bond is formed in the product
thermal [1,3] sigmatropic rearrangement
photochemical
[1,5] sigmatropic rearrangement
The [1,5] sigmatropic rearrangement is photochemically forbidden.
[3,3] sigmatropic rearrangement
[1,3]
4 electrons, 2 pairs, photochemically allowed
[1,5] and [3,3]
6 electrons, 3 pairs, thermally allowed
Number of Electron Pairs
Allowed Sigmatropic
Rearrangement
Odd
Thermal
Even
Photochemical
Examples of Sigmatropic Rearrangements
The [1,3] sigmatropic rearrangement is photochemically allowed.
The [1,5] sigmatropic rearrangement is thermally allowed.
[3,3] Sigmatropic rearrangements are thermally allowed.
Cope rearrangement
Claisen rearrangement
spontaneous
enolization
Rearrangements to Electron-Deficient Centers
one electron pair (odd)
thermally allowed
pinacol rearrangement
Beckmann rearrangement
Hofmann Rearrangement
Baeyer-Villiger Rearrangement
Summary
For disrotatory electrocyclic reactions, cycloadditions, and
sigmatropic rearrangements:
Number of Electron Pairs
Odd
Even
Allowed Reaction
Thermal
Photochemical
The rules for conrotatory electrocyclic are reversed.