Pericyclic Reactions
concerted reactions that have a cyclic, delocalized transition state
Categories of pericyclic reactions:
electrocyclic ring closure (or ring opening) -
formation (or breaking) of one new sigma bond connecting the ends of a delocalized pi system
(net change, one fewer pi bond, one more sigma bond)
example: cyclobutene ---> 1,3-butadiene
cycloaddition or (retrocycloaddition) -
formation (or breaking) of two new sigma bonds connecting the ends of two delocalized pi systems
(net change, two fewer pi bonds, two more sigma bonds)
example: a Diels-Alder reaction
sigmatropic rearrangement -
rearrangement of one sigma bond from one end a delocalized pi system to another
(no net change, same number of pi bonds, same number of sigma bonds)
example: a Cope rearrangement
Stereochemistry of pericyclic reactions:
suprafacial - attachment to the same side of a pi system (like syn addition)
antarafacial - attachment to opposite sides of a pi system (like anti addition)
disrotatory - rotation of p orbitals in the opposite direction (comparable to suprafacial)
conrotatory - rotation of p orbitals in the same direction (comparable to antarafacial)
There are also conventions for describing the size of each type of transition
state
example - [4+2] cycloaddition, [3,3]-sigmatropic shift
The Principle of Conservation of Orbital Symmetry (R.B. Woodward & R. Hoffmann)
A pericyclic reaction occurs readily (is "allowed") if the orbitals of the reactants convert to the orbitals of the products with conservation of orbital symmetry
This essentially means that there is maximal bonding through the transition state
Pericyclic reactions are typically highly stereospecific
- the stereochemistry of the reaction pathway is determined by following the location of the substituents as they move from reactants to products
- the underlying reason for a given reaction stereochemistry is determined by following the change in the MOs from reactants to products
Be able to follow both substituents and orbitals through a reaction
pathway
Orbital Correlation Diagrams
Woodward-Hoffmann method for correlating reactant orbitals with product orbitals
1) select an appropriate symmetry element
(e.g. mirror plane or rotation axis)
(the element should pass through at least one bond that is breaking or forming
in order to give useful information)
2) classify the reactant orbitals and the product orbitals with respect
to this element:
S = symmetric, A = antisymmetric
3) create an energy level diagram that connects orbitals of like symmetry
4) the reaction will be thermally allowed if there is no crossing to antibonding levels
Electrocyclic Reactions
example - the cyclobutene ring opening to butadiene (a 4-electron electrocyclic reaction)
correlation with a mirror plane of symmetry corresponds to disrotatory,
but the reactant orbitals are both S and the product orbitals are S and
A (symmetry-forbidden)
correlation with a C2 axis of symmetry corresponds to conrotatory,
the reactant orbitals are S and A, and so are the product orbitals (symmetry-allowed)
General rules for electrocyclic reactions:
with 4n electrons in the transition state, conrotatory is allowed
with (4n+2) electrons in the transition state, disrotatory is allowedFrontier Orbital method - just follow the preferred direction of the HOMO
photochemical reactions - a different orbital will be the HOMO, rules are usually reversed
Sigmatropic Rearrangements
example - [1,5] hydrogen shift
correlate orbital of the migrating group (1s) with the NBMO of the remaining
pi system (pentadienyl)
the symmetry and signs of the NBMO can be quickly estimated by the star
method
suprafacial is allowed
note that a [1,3]-H shift would have to be antarafacial (usually not observed unless acid or base catalyzed)
example - Cope rearrangement [3,3] shift
correlate NBMOs of two allyl groups (suprafacial on both is allowed)
Cycloaddition Reactions
example - [2+2] cycloaddition
use of symmetry-adapted combination orbitals from the two different pi
systems and
two different mirror planes to correlate their symmetry (corresponds to
suprafacial on both)
example - Diels-Alder reactions: [4+2] cycloadditions
correlate HOMO of one component with LUMO of the other
preferred Diels-Alder match is an electron-deficient dienophile (low-energy
LUMO) and diene HOMO
A Simple Summary Rule
count the number of electron PAIRS in the delocalized transition state
(even number of pairs = 4n electrons ; odd number of electron pairs = 4n+2 electrons)even / antara / con - - odd / supra / dis - - thermally allowed pathways (reverse for photochem)
- inversion of stereochemistry counts like antarafacial
- two antarafacial components count like suprafacial
Other Approaches
Huckel vs Mobius transition states
standard (Huckel) systems like 4n+2 electrons in a delocalized pi system (aromatic)
Mobius systems like 4n electrons (with one antarafacial or inversion)