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CHEM 311 > Pericyclic Reactions > Flashcards

Pericyclic Reactions Flashcards

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1
Q

Electrocyclic rxn

A
  • pericyclic cyclization of conjugated polyenes
  • specific streochemical outcomes realted to orbital symmetry
  • rxns can occur by conrotatory or disrotatory process
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2
Q

Conrotatory

A
  • used when there are 4n e- in the linear molecule

- groups rotate the same direction

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3
Q

Disrotatory

A
  • used when there are 4n+2 e- in the linear molecule

- groups rotate in opposite directions

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4
Q

Woodward-Hoffman Rules

A 
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
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5
Q

Molecular Orbital symmetry

A
  • symmetry of orbitals is preserved through a rxn
  • symmetry operations: rotation, reflection, inversion
  • consider π and π* orbitals
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6
Q

Orbital symmetry correlation diagrams

A

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
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7
Q

Photochemical electrocyclic reactions

A

-electron promoted from the HOMO to the LUMO, can allow reactions to occur by allowing symmetry

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8
Q

General Rules for Thermal electrocyclic rxns

A

4n e-: conrotatory allowed, disrotatory forbidden

4n+2 e-: disrotatory allowed, conrotatory forbidden

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9
Q

General rules for photochemical electrocyclic rxn

A

4n e-: disrotatory allowed, conrotatory forbidden

4n+2 e-: conrotatory allowed, disrotatory forbidden

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10
Q

FMO approach

A

Consider symmetries of HOMO (thermal) and LUMO (photochemical) only
-determine the rotation that gives favourable in phase combinations on terminal carbons

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11
Q

Huckel/Mobuis Analysis

A
  • 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
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12
Q

Torquoselectivity in electrocyclic rxns

A
  • 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
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13
Q

Meaning of WI DO acronym

A
  • Withdraw inwards (EWG inward rotation)

- Donate outwards (EDG outwards rotation)

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14
Q

Cycloaddition rxns

A
  • 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
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15
Q

Cycloaddition rxns - Orbital symmetry approach

A
  • 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
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16
Q

Cycloaddition rxns - FMO approach

A

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)
17
Q

Cycloaddition rxns-Huckel/Mobius Approach

A

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
18
Q

Diels Alder RXN

A
  • 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
19
Q

Diels Alder substituent effects

A

Most DA rxns involve HOMO of diene, LUMO of dienophile

  • electron rich diene (EDG)
  • electron poor dienophile (EWG)
20
Q

Reverse electron Diels Alder

A

HOMO of electron rich dienophile

  • LUMO of electron poor diene (with O or N)
  • *look for when diene has a heteroatom
21
Q

Diels Alder Regioselectivity

A
  • controlled by FMO polarization
  • Donors polarize HOMO towards unsubstituted carbon
  • Acceptors polarize LUMO towards unsubstituted carbon
  • look for resonance arguments

CHEM 311 (5 decks)

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