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

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Presentation on theme: "1 Pericyclic reactions Electrocyclisation Sigmatropic Cycloadditions Cheletropic reactions… Frontier orbitals Correlation diagrams (MOs, States) Aromaticity."— Presentation transcript:

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

2 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 3 Electrocyclic reaction ring closure of conjugated systems An electrocyclic reaction is a pericyclic reaction where the net result is one  bond being converted into one  bond. electrocyclic reactions are photoinduced or thermal      TS

4 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 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 6 Woodward- Hoffmann rules Robert Burns Woodward American, Nobel 1965 Roald Hoffmann 1937 American, Nobel 1981

7 7 Roald Hoffmann 1937 American, Nobel 1981

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

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

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

11 11 Electrocyclic reaction The orientation of CH 3 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 The conrotation would give the opposite correspondance; the knowledge of the mechanism allows you to make the compound with the desired configuration. (2Z,4Z,6Z)-octatriene

12 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

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

14 14 Sigmatropic reaction Sigmatropic reaction is a pericyclic reaction wherein the net result is one  bond changed to another  bond. [2,3] 1 1’ 32’ [3,3]

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

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

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

18 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 19 cycloaddition This generates chiral compounds. Steric hindrance does not systematically explain.

20 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 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 22 Huckel annulene The reaction goes through a ring

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

24 24 Hückel annulene By convention all the AOs are oriented in the same direction (+ above the plane – below): The overlaps are positive: 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

25 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 (  - complex) in certain organometallic compounds. The model is named after Michael J. S. Dewar, Joseph Chatt and L. A. Duncanson.

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

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

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

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

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

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

32 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 ) 

33 33 Aromaticity rules for are the opposite for thermal and photochemical reaction  h

34 34 Electrocylic

35 35 Electrocyclic C2 axis of symmetry Mirror symmetry

36 36 Sigmatropic C2 axis of symmetryMirror symmetry

37 37 Sigmatropic

38 38 Cycloadditions Suprafacial attackAntarafacial attack Suprafacial and antarafacial attack

39 39 Bond formation Supra Antara

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

41 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 42 For a thermal reaction (ground state) 4n (4) electrons one S O For a photochemical reaction (excited state) 4n (4) electrons all the S>O 4n+2 (6) electrons one S

43 43 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 44 Electrocyclic: symmetry conservation of the HOMO The HOMO is U symmetric for C2 (antisymmetric for  )

45 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 46 Symmetry alternates for MOs in a linear polyene HOMO symmetry switches according to N U: no node G: one node U: two nodes G: three nodes For a mirror symmetry U=S an G=A, for a C2 symmetry U=S and G=A

47 47 Photochemical reaction 6-e Conrotatory

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

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

50 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 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 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 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 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 55 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  system is symmetric, and antarafacial migrations normally occur when the HOMO of the  system is antisymmetric and the migration transition state is a ring with seven or more atoms in it Suprafacial and Antarafacial Sigmatropic Rearrangements

56 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 57 Migration of Carbon Using One Lobe 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 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.

58 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 59 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 Cycloaddition 4+2 Supra-supra

60 60 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 Otto Diels Kurt Alder

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

62 62 Cycloaddition 2+2 Supra-antara 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. Frontier MO analysis of a [2 + 2] cycloaddition reaction under thermal and photochemical conditions.

63 63 Cycloaddition 2+2 Supra-antara

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

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

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

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

68 68 Electrocyclic  4n+2 disrot 4n conrot Electrocyclic h 4n+2 conrot 4n disrot Sigmatropic  4n+2 supra 4n antara Sigmatropic h 4n+2 antara 4n Supra cycloaddition  4n+2 Supra- supra 4n Supra- antara cycloaddition h 4n+2 Supra- antara 4n Supra- supra

69 69 Correlation of MOs Dewar - Zimmermann

70 70 Electrocyclic 4-e  C2 conservation h  conservation

71 71 Electrocyclic 4-e  C2 conservation h  conservation

72 72 Electrocyclic 4-e  C2 conservation  cyclobutene S2A2S2A2     S 2 AS     S2S2S2S2  A2S2A2S2 butadiene  A 2 SA  A2A2A2A2     SA 2 A  S 2 AS S A No barrier Barrier Diagram of states

73 73 Electrocyclic 4-e h conservation  cyclobutene S2S2S2S2     S 2 AS     S2A2S2A2  S2A2S2A2 butadiene  A 2 SA  S2A2S2A2  S2S2S2S2 No barrier A S S Diagram of states

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

75 75 Electrocyclic 6-e  conservation h C2 conservation

76 76 Cycloaddition 2+2  C2 conservation h  conservation

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

78 78 Cycloaddition 4+2   conservation       S S A S S S A S A A A A

79 79 Cycloaddition 4+2 h C2 conservation       A S A S A S S A S A S A

80 80 Cycloaddition 6-e  conservation  Butadiene + ethene S2S2A2S2S2A2 S 2 S 2 AS S 2 SA 2 A SASA S2S2A2S2S2A2  S 2 A 2 SA No barrier A A S Diagram of states           S   A    SASSAS S 2 AS 2 S Barrier


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