Cycloadditions Pericyclic Reaction Notes, Slides of Chemistry

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Pericyclic Reactions!

Cycloaddition! +

( )n

( )n

Electrocyclization!

π 1 - bonding

HOMO

π∗ 2 - antibonding

LUMO

p-orbitals

S

A

Ethylene π-Molecular Orbitals!

171 nm! 167 kcal/mol! 800 nm! visible!^ 400 nm! UV! 200 nm! IR!

214 nm! 133 kcal/mol!

1,3-Butadiene π-Molecular Orbitals!

800 nm! visible!^ 400 nm! UV! 200 nm! IR!

258 nm! 109 kcal/mol!

1,3,5-Hexatriene π-Molecular Orbitals!

LUMO HOMO S S S A A A 800 nm! visible!^ 400 nm! UV! 200 nm! IR!

HOMO LUMO S S A ! The Allylic System: Allyl Cation!

! The Allylic System: Allyl Radical! HOMO LUMO

S

S

A

Cycloaddition: Diels-Alder Reaction! An Allowed [4+2] Cycloaddition!

diene (^) dienophile HOMO π 2 LUMO π* 3 π* 4 π 1 π 1 - bonding HOMO π∗ 2 - antibonding LUMO butadiene ethylene S A A S S A

Diels-Alder Reaction:! The Effect of Electron Withdrawing Groups!

diene dienophile EWG EWG HOMO π 2 LUMO π* 3 π* 4 π 1 π 1 - bonding HOMO π∗ 2 - antibonding LUMO butadiene (^) deactivated ethylene EWG EWG S A A S S A

Diels-Alder Reaction: Mechanism! HOMO π 2 LUMO π* 3 π* 4 π 1 π 1 - bonding HOMO π∗ 2 - antibonding LUMO butadiene ethylene S A A S S A

Diels-Alder Reaction: The Endo Effect! ! O Secondary Effect!

Thermally Allowed Cycloadditions! 4n+2 Rule! ! HOMOs! LUMOs! A! S! 2! S! A! 4! S! A! 6! S! A! 8! [4+2]=6! [6+4]=10! [8+6]=14! 4n+2; n=1,2,3,4…..!

Photochemically Allowed Cycloadditions! 4n Rule! ! HOMOs! LUMOs! A! S! 2! S! A! 4! S! A! 6! S! A! 8! 4n; n=1,2,3,4…..! [2+2]=4! [4+4]=8! [6+6]=12! [8+8]=16! and [2+6]=8; [8+4]=12!

!

Electrocyclizations!

!

1,3-Butadiene-Cyclobutene!

! thermal hν hν (^) thermal trans cis E,E dis E,Z con dis con