Pericyclic Reactions Flashcards
Electrocyclic rxn
- pericyclic cyclization of conjugated polyenes
- specific streochemical outcomes realted to orbital symmetry
- rxns can occur by conrotatory or disrotatory process
Conrotatory
- used when there are 4n e- in the linear molecule
- groups rotate the same direction
Disrotatory
- used when there are 4n+2 e- in the linear molecule
- groups rotate in opposite directions
Woodward-Hoffman Rules
pericyclic rxns are symmetry controlled -symmetry of π-MOs dictates the reactivity and selectivity -can analyze the rxs using: Orbital symmetry correlation diagrams FMO symmetry analysis Transition state aromaticity
Molecular Orbital symmetry
- symmetry of orbitals is preserved through a rxn
- symmetry operations: rotation, reflection, inversion
- consider π and π* orbitals
Orbital symmetry correlation diagrams
C2 axis must be maintained for conrotatory opening
- occupied orbital in reactant MUST correlate with occupied orbital of like symmetry in pdt
- sigma plane must be maintained for disrotatory
Photochemical electrocyclic reactions
-electron promoted from the HOMO to the LUMO, can allow reactions to occur by allowing symmetry
General Rules for Thermal electrocyclic rxns
4n e-: conrotatory allowed, disrotatory forbidden
4n+2 e-: disrotatory allowed, conrotatory forbidden
General rules for photochemical electrocyclic rxn
4n e-: disrotatory allowed, conrotatory forbidden
4n+2 e-: conrotatory allowed, disrotatory forbidden
FMO approach
Consider symmetries of HOMO (thermal) and LUMO (photochemical) only
-determine the rotation that gives favourable in phase combinations on terminal carbons
Huckel/Mobuis Analysis
- analyze the TS for Huckel/Mobius aromaticity
- draw lowest energy MO with fewest nodes
- determine phase inversions in TS for con and disrotatory
- zero nodes are allowed for 4n+2 e- systems
- 1 node allowed for 4n e- systems
- RULES REVERSED FOR PHOTOCHEMICAL RXN
Torquoselectivity in electrocyclic rxns
- ring opening can occur in preferred direction
- dictated by orbitals of e- donor/acceptor substituent groups or sterics
- EWG rotate inwards during conrotatory opening
- EDG rotate outwards during conrotatory opening
- *sterics do not explain everything, EDG/EWG very important to selectivity
Meaning of WI DO acronym
- Withdraw inwards (EWG inward rotation)
- Donate outwards (EDG outwards rotation)
Cycloaddition rxns
- named after the # of mobile e- from each component
ie. 2+2 cycloaddition, 4+2 cycloaddition (diels alder) - no intermediates, all in a single step
Cycloaddition rxns - Orbital symmetry approach
- 2 mirror planes to consider for 2+2 cyclo
- thermally forbidden, photochemically allowed
- 1 mirror plane to consider for 4+2 cyclo
- thermally allowed, photochemically forbidden
Cycloaddition rxns - FMO approach
Consider HOMO of one reactant, LUMO of other
- does not matter which you choose for LUMO and HOMO
- if bonding interaction found, rxn is thermally allowed (matching phases)
Cycloaddition rxns-Huckel/Mobius Approach
Align fragment MOs with maximal bonding interactions
- 0 phase change (node) allowed for 4n+2
- 1 phase change (node) allowed for 4n
- *Rules reversed for photochemical rxns
Diels Alder RXN
- 1,3 diene adds to π-bond of a dieneophile
- diene must be s-cis configuration
- 2 new sigma bonds formed in place of 2 π-bonds
Diels Alder substituent effects
Most DA rxns involve HOMO of diene, LUMO of dienophile
- electron rich diene (EDG)
- electron poor dienophile (EWG)
Reverse electron Diels Alder
HOMO of electron rich dienophile
- LUMO of electron poor diene (with O or N)
- *look for when diene has a heteroatom
Diels Alder Regioselectivity
- controlled by FMO polarization
- Donors polarize HOMO towards unsubstituted carbon
- Acceptors polarize LUMO towards unsubstituted carbon
- look for resonance arguments
