Pericyclic Reactions

Mnemonics

TECA
TODS
POCA
PEDS
(Wait, Todd, stab the Tetoris guy)

C = Conrotatory
D = Disrotatory
A = Antarafacial
S = Suprafacial

Called the Woodward-Hoffman Rules

Electrocyclic Reactions

• intramolecular, new sigma bond forms between ends of conjugated system

• ring closure/opening if reversed

• - 1 pi bond; + 1 sigma bond

Follows Conrotary/Disrotary

Bond Count: number of pi bonds (+ 1 sigma bond if ring opening)

Favorability:

Closure is favored when 6-ring

Opening is favored when 4-ring

due to stability

Cycloaddition Reactions

• two separate compounds form 2 new sigma-bonds

• Count: all electrons involved

Possible Reactions:

[2+2] cycloaddition, [8+2] cycloaddition,

[4+2] cycloaddition (Diels-Alder)

Follows Suprafacial/Antarafacial

Antarafacial is not possible for smaller than 7-ring

Uses the HOMO of the donator and the LUMO of the receiver

Suprafacial retains stereo; Antarafacial inverts it

Diels-Alder Addition

[4+2] Addition

• Requires a diene and a dienophile

• diene is activated by EDG

• dienophile is activated by EWG

• only thermal is allowed

• favorable if diene is S-cis (both ends point in the same direction)

Stereochemistry: suprafacial retains stereochemistry (in terms of cis/trans)

But whether they go up or down is up to Exo/Endo

Exo and Endo Addition

Endo Addition: substituents are added under the diene

• pushes down so substituents become dashed

Exo Addition: substituents are added away from the diene

• pushes up so substituents become wedged

Endo is favored when substituents have pi-system due to pi-stacking

Exo is favored when substituents have no pi system due to steric

Sigmatropic Rearrangement

• old pi bond breaks, new one forms, pi bonds rearrange, bond count does not change

• Naming: start at the bond that will break, count away from it mirrored; until connection points (ex: [3,3])

Count: number of total electron pairs involved

Suprafacial: migrating group stays on same side of system

Antarafacial: migrating group moves to other side of system

Antarafacial requires 7-carbon or more ring or single methyl (since it can just flip) (will invert stereo though)

Specific Named Rearrangements

Cope Rearrangement:

• [3,3] sigmatropic rearrangement of a 1,5-diene

Claisen Rearrangement:

• [3,3] sigmatropic rearrangement of a allyl vinyl ether

Hydrogen Shifts

bond with hydrogen can break and shift in sigmatropic, doesn’t have to be R-Group

• pi-bond moves with it

[1,3] Hydrogen Shift: even, photo (thermal is antara)

[1,5] Hydrogen Shift: odd, thermal (photo is antara)

[1,7] Hydrogen Shift even, either thermal or photo