1. Pericyclic reactions Electrocyclisation Sigmatropic Cycloadditions
Cheletropic reactions…
Frontier orbitals
Correlation diagrams (MOs, States)
Aromaticity of the Transition State

2. Pericyclic reactions
A pericyclic reaction is a reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion.

3. Electrocyclic reaction ring closure of conjugated systems
An electrocyclic reaction is a pericyclic reaction where the net result is one p bond being converted into one s bond. electrocyclic reactions are photoinduced or thermal p p p s p p TS

4. Woodward- Hoffmann rules and symmetry conservation
Concern pericyclic reactions.
Tell about the mechanism passing through
the lowest activation barrier
Does not tell anything about the thermodynamic (reaction or reverse reaction)
Based on symmetry conservation

5. Why Woodward- Hoffmann rules are important?
Allows synthesis of compounds with determined asymmetric carbons.
First example of useful application of theory
Simplify sophisticated systems
Sample the main methods of analysis using theory
Prevails over alternative explanations such as steric effects.

6. Woodward- Hoffmann rules
Robert Burns Woodward
American, Nobel 1965
Roald Hoffmann 1937
American, Nobel 1981

7. Roald Hoffmann 1937 American, Nobel 1981

8. Conservation of Orbital Symmetry
H C Longuet-Higgins E W Abrahamson
Hugh Christopher Longuet-Higgins

9. What symmetry is preserved?
A mirror A C2 axis
W-H rules say that which symmetry element has to be preserved.

10. Up to now,
implicitly we have only considered the mirror symmetry

11. Electrocyclic reaction The orientation of CH3 depends on the symmetry conservation: when the mirror symmetry is preserved (here called disrotatory mode: rotation in opposite senses) we obtain the following reactions with asymmetric carbons
disrotatory
                                       
                                   
                
                                 
(2Z,4Z,6Z)-octatriene
disrotatory
The conrotation would give the opposite correspondance; the knowledge of the mechanism allows you to make the compound with the desired configuration.

12. It is the mechanism for photochemical process
The conservation of the C2 axis (here called conrotatory mode- rotation in the same sense) is not observed by thermal cyclization
It is the mechanism for photochemical process

13. Electrocyclic reaction ring closure of conjugated systems
An electrocyclic reaction is a pericyclic reaction where the net result is one p bond being converted into one s bond. Electrocyclic reactions are photoinduced or thermal

14. Sigmatropic reaction
Sigmatropic reaction is a pericyclic reaction wherein the net result is one s bond changed to another s bond. 1 1’ 3 2’
[2,3]
[3,3]

15. Sigmatropic reaction
Sigmatropic reaction is a pericyclic reaction wherein the net result is one s bond changed to another s bond.

16. Cope Rearrangement sigmatropic [3,3]
Oxy-Cope Rearrangement 1 1
Arthur Cope 1’ 3 1’ 3 3’ 3’

17. Claisen rearrangement sigmatropic [3,3]
Rainer Ludwig Claisen
( ) German 1 1 1’ 1’ 3 3 3’ 3’

18. cycloaddition
A cycloaddition is a reaction, in which two π bonds are lost and two σ bonds are gained. The resulting reaction is a cyclization reaction.

19. cycloaddition
This generates chiral compounds. Steric hindrance does not systematically explain.

20. Cheletropic reaction
A Cheletropic reaction is a pericyclic reaction where the net result is the conversion of a pi bond and a lone pair into a pair of sigma bonds; with both new sigma bonds adding into the same atom. .

21. Transition State Aromaticity (Dewar and Zimmermann)
Does not explicit MOs (does not require any calculation)
Based only on the signs of the overlaps
and on the count of the electrons involved.

22. Huckel annulene
The reaction goes through a ring

23. Huckel annulene
The reaction preserving a mirror symmetry goes through a ring

24. Hückel annulene S>0 S>0 S>0 S>0 S>0 S>0 S<0
By convention all the AOs are oriented in the same direction
(+ above the plane – below): The overlaps are positive:
S>0
S>0
S>0 + + + + + +
S>0
S>0
S>0
Reversing the sign of one AO still makes an even number of positive overlaps:
This defines Hückel annulene
For real unsaturated compound,
this is always the existing situation + + + + - +
S<0
S<0 +

25. Aromaticity according to Dewar
Michael J. S. Dewar (Michael James Steuart Dewar) English
born in Ahmednagar, India in 1918
The Dewar-Chatt-Duncanson model is a model in organometallic chemistry which explains the type of chemical bonding between an alkene and a metal (p-complex) in certain organometallic compounds. The model is named after Michael J. S. Dewar, Joseph Chatt and L. A. Duncanson .

26. Radical chain + C radical atom comparing the chain with the ring: Aromaticity
First order term. S A
E = 0
E = 0
E = 0
E = 0
0 for the ring
2/√(N-1) for the chain
4/√(N-1) for the ring
2/√(N-1) for the chain

27. Aromaticity according to Dewar
Radical chain + C radical atom comparing the chain with the ring: Aromaticity
Aromaticity according to Dewar S A
When the SOMO is antisymmetric
The ring is less stable than the chain
The polyene is ANTIAROMATIC
N-1 is odd
N = 4n
When the SOMO is symmetric
The ring is more stable than the chain
The polyene is AROMATIC
N-1 is even
N = 4n +2
The SOMO is once upon twice S (n=2n-1) or A (2n+1)

28. Hückel-type annulene
Aromatic 4n+2 electrons; antiaromatic 4n electrons

29. August Ferdinand Möbius
German
was a descendant of Martin Luther
by way of his mother.

30. Moebius rings: Aromaticity
One negative overlap. … … A A
p orbital binding through opposite lobes S A
E = 0
E = 0
E = 0
E = 0
4/√(N-1) for the ring
2/√(N-1) for the chain
0 for the ring
2/√(N-1) for the chain

31. Möbius annulene aromaticity rules are reversed
Aromatic 4n electrons; antiaromatic 4n+2 electrons

32. Aromatic systems have the largest HOMO-LUMO gap (the most stable ground state and the least stable first excited state) Antiaromatic systems have half filled degenerate non bonding levels (the smallest gap; the most stable first excited state and the least stable ground state ) 2b

33. Aromaticity rules for are the opposite for thermal and photochemical reaction
D hn

34. Electrocylic

35. Electrocyclic
C2 axis of symmetry
Mirror symmetry

36. Sigmatropic
Mirror symmetry
C2 axis of symmetry

37. Sigmatropic

38. Cycloadditions Suprafacial and antarafacial attack Suprafacial attack

39. Bond formation
Supra
Antara
Supra
Supra

40. Cycloaddition Supra-Supra Hückel Supra-Antara Möbius Antara-Antara
Mirror symmetry
C2 axis of symmetry
Mirror symmetry
unlikely

41. Transition State Aromaticity (Dewar and Zimmermann)
Does not explicit MOs
Based only on the sign of the overlaps
And on the electron count

42. For a thermal reaction (ground state) 4n (4) electrons one S<O 4n+2 (6) electrons all the S>O
For a photochemical reaction (excited state) 4n (4) electrons all the S>O 4n+2 (6) electrons one S<O

43. Bimolecular reactions: Favorable interaction in Frontier Orbitals
Bimolecular reactions: Favorable interaction in Frontier Orbitals. - this determines the conservation of a symmetry operation (axis or plane) Unimolecular reactions: Symmetry conservation of the HOMO - the HOMO accommodates the most mobile electrons - its amplitude is generally large at the reaction sites

44. Electrocyclic: symmetry conservation of the HOMO
The HOMO is U symmetric for C2 (antisymmetric for s)

45. The “conservation of the HOMO” requires calculating the HOMO
The “TS aromaticity” does not: it only look at the sing of overlaps and the # of electrons involved.

46. Symmetry alternates for MOs in a linear polyene HOMO symmetry switches according to N
G: three nodes
U: two nodes
G: one node
U: no node
For a mirror symmetry U=S an G=A, for a C2 symmetry U=S and G=A

47. Photochemical reaction 6-e Conrotatory

48. Electrocyclic Molecules with symmetric HOMOs give disrotatory ring-closure products.
Ring Closure With Symmetric HOMO
ground state 4n+2 electrons D
excited state 4n electrons hn
Molecules with symmetric HOMOs have the top lobe of one orbital in the same phase as the top lobe of the other orbital.

49. Electrocyclic Molecules with antisymmetric HOMOs give conrotatory ring-closure products.
Ring Closure With antiSymmetric HOMO
ground state 4n electrons D
excited state 4n+2 electrons hn
Molecules with antisymmetric HOMOs have the top lobe of one orbital in the same phase as the bottom lobe of the other orbital.

50. (2E,4Z,6E)-Octatriene ring closure is disrotatory, yielding cis-5,6-dimethyl-1,3-cyclohexadiene
The HOMO of (2E,4Z,6E)-octatriene is symmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6E)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO, and it has to be symmetric rather than antisymmetric.

51. (2E,4Z,6Z)-Octatriene ring closure is disrotatory, yielding trans-5,6-dimethyl-1,3-cyclohexadiene.
The HOMO of (2E,4Z,6Z)-octatriene is symmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6Z)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO, and it has to be symmetric rather than antisymmetric.

52. Photochemically induced (2E,4Z,6Z)-octatriene ring closure is conrotatory, yielding cis-5,6-dimethyl-1,3-cyclohexadiene.
The HOMO of (2E,4Z,6Z)-octatriene which has been excited by light is antisymmetric because MOs of linear conjugated pi systems alternate in symmetry starting with the lowest-energy MO being symmetric. (2E,4Z,6Z)-Octatriene has six MOs (from six atomic p orbitals overlapping), half of which (three) are filled in the ground state. The third-lowest-energy orbital has to be the HOMO in the ground state, and the fourth-lowest-energy orbital has to be the HOMO of the excited state.

53. (2E,4Z)-Hexadiene undergoes conrotatory ring closure to yield cis-3,4-dimethylcyclobutene.
The HOMO of (2E,4Z)-hexadiene has to be antisymmetric because this compound has to have four pi MOs, two of which are filled. The HOMO has to be the second-lowest-energy orbital. Since the lowest-energy orbital has to be symmetric, the HOMO has to be antisymmetric.

54. (2E,4E)-Hexadiene undergoes conrotatory ring closure to yield trans-3,4-dimethylcyclobutene.
The HOMO of (2E,4E)-hexadiene has to be antisymmetric because this compound has to have four pi MOs, two of which are filled. The HOMO has to be the second-lowest-energy orbital. Since the lowest-energy orbital has to be symmetric, the HOMO has to be antisymmetric.

55. Suprafacial and Antarafacial Sigmatropic Rearrangements
A sigmatropic rearrangement in which a migrating group remains on the same face of the pi system as it migrates is suprafacial. If the migrating group moves from one face of the pi system to the opposite face, the migration is antarafacial.
Suprafacial migrations normally occur when the HOMO of the p system is symmetric, and antarafacial migrations normally occur when the HOMO of the p system is antisymmetric and the migration transition state is a ring with seven or more atoms in it

56. Migration of Hydrogen
Since hydrogen's s orbital has only one phase, the phase of the lobe of the developing p orbital of the atom it migrates from and the lobe of the p orbital of the atom it migrates to must have the same phase. Thus, hydrogen is forced to migrate suprafacially in cases where there is an odd number of electron pairs involved in the migration (symmetric HOMO) and antarafacially in cases where there is an even number of pairs of electrons involved in the migration (antisymmetric HOMO).

57. Migration of Carbon Using One Lobe
Suprafacial and antarafacial migration of carbon with carbon using the same lobe to bond to its destination position that it uses to bond to its original position.
Caption
Notes
When carbon uses only one lobe to migrate in a sigmatropic rearrangement, it must migrate suprafacially when an odd number of electron pairs are involved in the migration (symmetric HOMO) and antarafacially when an even number of electron pairs are involved in the migration (antisymmetric HOMO). This type of migration results in retention of configuration at the migrating carbon

58. Migration of Carbon Using Both Lobes
Suprafacial and antarafacial migration of carbon with carbon using the opposite lobe to bond to its destination position from the one that it uses to bond to its original position.
When carbon uses both lobes to migrate in a sigmatropic rearrangement, it must migrate antarafacially when an odd number of electron pairs are involved in the migration (symmetric HOMO) and suprafacially when an even number of electron pairs are involved in the migration (antisymmetric HOMO). This type of migration results in inversion of configuration at the migrating carbon.

59. Cycloaddition 4+2 Supra-supra
The Diels-Alder reaction represents the prototype of cycloadditions. Besides the Grignard reaction, it is the most cited name reaction in chemical literature.
The reaction principle was discovered in 1928 by Otto Diels and his student Kurt Alder. Both were honored with the Nobel Prize for Chemistry in 1950.

60. The Diels-Alder reaction represents the prototype of cycloadditions
The Diels-Alder reaction represents the prototype of cycloadditions. Besides the Grignard reaction, it is the most cited name reaction in chemical literature.
Otto Diels
Kurt Alder
The reaction principle was discovered in 1928 by Otto Diels and his student Kurt Alder. Both were honored with the Nobel Prize for Chemistry in 1950.

61. Frontier orbital intractions: Diels-Alder 6-e Cycloaddition 4+2 Supra-supra

62. Cycloaddition 2+2 Supra-antara
Frontier MO analysis of a [2 + 2] cycloaddition reaction under thermal and photochemical conditions.
Under thermal conditions, this cycloaddition would have to be antarafacial, which is impossible for a [2 + 2] cycloaddition (forms a four-membered ring). Under photochemical conditions, this reaction allows suprafacial ring formation.

63. Cycloaddition 2+2 Supra-antara

64. Cycloaddition 2+2 Supra-antara
LUMO
Supra
HOMO
supra
HOMO
antara
LUMO
antara

65. The Alder rule diene with D ligand, dienophyle with A ligand The reverse Alder rule
LUMO
LUMO
HOMO
HOMO

66. The Alder rule The reverse Alder rule
LUMO
LUMO
HOMO
HOMO
It is better to have substituent with opposite properties.
Each pair favors one Frontier orbital interaction

67. Regioselectivity The Alder ruleor * *
Concerted reaction but not synchronous. The atoms with the largest coefficients bind first.

68. disrot conrot supra antara Supra Supra-supra Supra-antara D hn
Electrocyclic D
4n+2
disrot 4n
conrot hn
Sigmatropic
supra
antara
Supra
cycloaddition
Supra-supra
Supra-antara

69. Correlation of MOs Dewar - Zimmermann

70. Electrocyclic 4-e D C2 conservation hn s conservation

71. Electrocyclic 4-e D C2 conservation hn s conservation

72. Electrocyclic 4-e D C2 conservation
SA2A
sp2s*
p12p3p4
S2AS
p12p22
A2A2
s2p*2
S2S2 A
Barrier
S2AS
s2pp*
p12p2p3
A2SA S
S2A2
s2p2
p12p22
A2S2
No barrier
cyclobutene
butadiene
Diagram of states

73. Electrocyclic 4-e hn s conservation
p12p32
S2S2
p12p22
S2A2
S2A2
s2p*2 S
No barrier A
p12p2p3
A2SA
S2AS
s2pp* S
S2S2
s2p2
p12p22
S2A2
cyclobutene
butadiene
Diagram of states

74. Correlation of MOs Dewar – Zimmermann Electrocyclic 6 - e

75. Electrocyclic 6-e D s conservation hn C2 conservation

76. Cycloaddition 2+2 D C2 conservation hn s conservation

77. Cycloaddition 4+2 Diels-Alder
C2 symmetry
Supra: ethene in plane
Antara: butadiene Conrotatory rotation of the terminal AOs from the butadiene
Mirror symmetry
supra-supra
Disrotatory rotation of the terminal AOs from the butadiene

78. Cycloaddition 4+2 D s conservationp* A Y3 Y2 S A S p S Y1 A S S

79. Cycloaddition 4+2 hn C2 conservationp* S Y3 Y2 A S A p S Y1 A A S

80. Cycloaddition 6-e D s conservation
sS2sAp2sS*
S2AS2S A
Y12pY32s*
S2SA2A
Barrier
Y12p2Y2Y3 A
S2S2AS
sS2sA2pp*
S2A2SA
Y12p2Y22 S
S2S2A2
sS2sA2p2
S2S2A2
No barrier
Butadiene + ethene
2 s + 1 s
Diagram of states