1
What Kinds of Concerted Cycloadditions are Possible?
Concerted v. Stepwise
CH2
CH2
transition state
diradicalintermediate
ΔH(Pdt Int) ≈ ECCσ – Eallylic res ≈ 85 – 15 = 70ΔH(Rct Pdt) = -40ΔH(Rct Int) ≈ +30
ΔH* = 25
2
Periselectivity: Why Only Some Concerted Cycloadditions Obsd?
possible transitionstates
X
X
2
Stereoselectivity: Why Only Some Concerted (de)Cyclizations Obsd?
NO
NO
3
Fukui’s FMO Answer
• Concerted rxn possible (ALLOWED) if– Divide “reactants” into 2 pi systems– HOMO1 & LUMO2 achieve good overlap
• makes (H12)2/(E1-E2) significant
• More simply, concerted rxn allowed if significant orbital stabilizations
• Otherwise, concerted rxn is FORBIDDEN– stepwise pathway lower energy
CycloadditionsHOMO (top) – LUMO (bottom) Overlaps
2+2Forbidden
4+4Forbidden
4+2Allowed
4
(De)CyclizationsHOMO (π) & LUMO (σ*)
H3C
H
CH3
H
H3C
H
CH3
H
H3C
H
CH3
H
HCH3
H3CH
CH3H
H3CH
X
conrotatory
disrotatory
Woodward-Hoffmann Rules
• Concerted rxn possible (ALLOWED) if– after correlating Rct & Pdt orbitals according
to Principle of Conservation of Orbital Symmetry
– Rct ground state Pdt ground state
• Otherwise, concerted rxn is FORBIDDEN– stepwise pathway lower energy
5
Principle of Conservation ofOrbital Symmetry: Basis
• During rxn– Rct MOs get replaced by new MOs– New MOs = combinations of Rct MOs
• combinations btw MOs with same symmetry only• same symmetry (H12)2 ≠ 0 Rct MOs mix• different symmetry (H12)2 = 0 No mixing
Principle of Conservation ofOrbital Symmetry: Application
• Draw correlation diagram– Rct MOs by energy– Pdt MOs by energy
• Identify all symmetry operation(s) that are conserved during rxn– Label MO symmetries (S or A)
• Correlate MOs by energy & symmetry– “crossing” of correlation lines forbidden if MOs
have same symmetry
6
Diels-Alder: Draw MOs
unoccupied
occupied
σ*
π*
π
σsym. adaptedcombinations
Diels-Alder: Identify Conserved Symmetry Operation
plane of symmetry is conserved during entire reactionall MOs must be S or A wrt to this plane
7
Diels-Alder: Label (S v A) & Correlate MOs
unoccupied
occupied
Rct ground statePdt ground state
ALLOWED
2+2 Cycloaddition: Identify Symmetry Operation(s)
m1
m2
8
SS
SA
AS
AA
SA
AA
SS
AS
2+2 Cycloaddition: Draw MOs, Label & Correlate Symmetries
m1
m2Rct ground state
Pdt excited state
FORBIDDEN
(De)cyclizationConrotatory (2-fold rotation axis) v.
Disrotatory (plane of symmetry)
H3C
H
CH3
H
H3C
H
CH3
H
H3C
H
CH3
H
HCH3
H3CH
CH3H
H3CH
X
conrotatory
disrotatory
rotation
reflection
A
A
S
A
9
Conrotary (Left, ALLOWED) v. Disrotatory (Right, FORBIDDEN)
rotation reflection
Bottom Lines
• ALLOWED = Possible– other factors may prevent concerted rxn
• Fukui FMO– HOMO1 & LUMO2 must overlap– needn’t use dominant interaction
• Woodward-Hoffmann– correlation diagrams are tedious– still works even if molecules are not symmetric
• Other rules can be used too…
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