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…