pericyclic reactions l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Pericyclic reactions PowerPoint Presentation
Download Presentation
Pericyclic reactions

Loading in 2 Seconds...

play fullscreen
1 / 80

Pericyclic reactions - PowerPoint PPT Presentation


  • 1967 Views
  • Uploaded on

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

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Pericyclic reactions' - Mia_John


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
pericyclic reactions
Pericyclic reactions
  • Electrocyclisation
  • Sigmatropic
  • Cycloadditions
  • Cheletropic reactions…
  • Frontier orbitals
  • Correlation diagrams (MOs, States)
  • Aromaticity of the Transition State
pericyclic reactions2
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.

electrocyclic reaction ring closure of conjugated systems
Electrocyclic reactionring 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

woodward hoffmann rules and symmetry conservation
Woodward- Hoffmann rulesand 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
why woodward hoffmann rules are important
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.
woodward hoffmann rules
Woodward- Hoffmann rules

Roald Hoffmann 1937

American, Nobel 1981

Robert Burns Woodward 1917-1979

American, Nobel 1965

conservation of orbital symmetry
Conservation of Orbital Symmetry

H C Longuet-Higgins E W Abrahamson

Hugh ChristopherLonguet-Higgins

1923-2004

what symmetry is preserved
What symmetry is preserved?

A mirror A C2 axis

W-H rules say that which symmetry element has to be preserved.

slide10

Up to now,

implicitly we have only considered the mirror symmetry

slide11

Electrocyclic reactionThe 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.

it is the mechanism for photochemical process
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

electrocyclic reaction ring closure of conjugated systems13
Electrocyclic reactionring 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

sigmatropic reaction
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]

sigmatropic reaction15
Sigmatropic reaction

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

cope rearrangement sigmatropic 3 3
Cope Rearrangementsigmatropic [3,3]

Cope Rearrangement

Oxy-Cope Rearrangement

1

1

Arthur Cope1902-1958

1’

3

1’

3

3’

3’

claisen rearrangement sigmatropic 3 3
Claisen rearrangementsigmatropic [3,3]

Rainer Ludwig Claisen

(1851-1930) German

1

1

1’

1’

3

3

3’

3’

cycloaddition
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.

cycloaddition19
cycloaddition

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

cheletropic reaction
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.

.

transition state aromaticity dewar and zimmermann
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.

huckel annulene
Huckel annulene

The reaction goes through a ring

huckel annulene23
Huckel annulene

The reaction preserving a mirror symmetry goes through a ring

h ckel annulene
Hückel annulene

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

+

aromaticity according to dewar
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 .

radical chain c radical atom comparing the chain with the ring aromaticity
Radical chain + C radical atomcomparing 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

radical chain c radical atom comparing the chain with the ring aromaticity27
Radical chain + C radical atomcomparing the chain with the ring: Aromaticity

Aromaticity accordingto 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)

h ckel type annulene
Hückel-type annulene

Aromatic 4n+2 electrons; antiaromatic 4n electrons

slide29

August Ferdinand Möbius

German 1780-1868

was a descendant of Martin Luther

by way of his mother.

http://www.youtube.com/watch?v=JX3VmDgiFnY

moebius rings aromaticity
Moebius rings: Aromaticity

S>0

S<0

A

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

0for the ring

2/√(N-1) for the chain

m bius annulene aromaticity rules are reversed
Möbius annulenearomaticity rules are reversed

2b

Aromatic 4n electrons; antiaromatic 4n+2 electrons

slide32

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 degeneratenon bonding levels (the smallest gap; the most stable first excited state and the least stable ground state )

2b

electrocyclic
Electrocyclic

C2 axis of symmetry

Mirror symmetry

sigmatropic
Sigmatropic

Mirror symmetry

C2 axis of symmetry

cycloadditions
Cycloadditions

Suprafacial and antarafacial attack

Suprafacial attack

Antarafacial attack

bond formation
Bond formation

Supra

Antara

Supra

Supra

cycloaddition40
Cycloaddition

Supra-Supra

Hückel

Supra-Antara

Möbius

Antara-Antara

Hückel

Mirror symmetry

Mirror symmetry

C2 axis of symmetry

unlikely

transition state aromaticity dewar and zimmermann41
Transition State Aromaticity (Dewar and Zimmermann)

Does not explicit MOs

Based only on the sign of the overlaps

And on the electron count

for a thermal reaction ground state 4n 4 electrons one s o 4n 2 6 electrons all the s o
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

slide43

Bimolecular reactions: Favorable interaction in Frontier Orbitals. - this determines the conservationof 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

electrocyclic symmetry conservation of the homo
Electrocyclic: symmetry conservation of the HOMO

The HOMO is U symmetric for C2 (antisymmetric for s)

slide45

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.

symmetry alternates for mos in a linear polyene homo symmetry switches according to n
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

electrocyclic molecules with symmetric homos give disrotatory ring closure products
ElectrocyclicMolecules 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.

electrocyclic molecules with antisymmetric homos give conrotatory ring closure products
ElectrocyclicMolecules 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.

2e 4z 6e octatriene ring closure is disrotatory yielding cis 5 6 dimethyl 1 3 cyclohexadiene
(2E,4Z,6E)-Octatriene ring closure is disrotatory, yielding cis-5,6-dimethyl-1,3-cyclohexadiene

The HOMO of (2E,4Z,6E)-octatriene issymmetric 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.

2e 4z 6z octatriene ring closure is disrotatory yielding trans 5 6 dimethyl 1 3 cyclohexadiene
(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.

slide52
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.

2e 4z hexadiene undergoes conrotatory ring closure to yield cis 3 4 dimethylcyclobutene
(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.

2e 4e hexadiene undergoes conrotatory ring closure to yield trans 3 4 dimethylcyclobutene
(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.

slide55

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

migration of hydrogen
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).

migration of carbon using one lobe
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.

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

migration of carbon using both lobes
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.

cycloaddition 4 2 supra supra
Cycloaddition 4+2Supra-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.

slide60

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 1876-1954

Kurt Alder. 1902-1958

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.

cycloaddition 2 2 supra antara
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.

cycloaddition 2 2 supra antara64
Cycloaddition 2+2Supra-antara

LUMO

Supra

HOMO

supra

HOMO

antara

LUMO

antara

the alder rule diene with d ligand dienophyle with a ligand the reverse alder rule
The Alder rulediene with D ligand, dienophyle with A ligandThe reverse Alder rule

LUMO

LUMO

HOMO

HOMO

the alder rule the reverse alder rule
The Alder ruleThe reverse Alder rule

LUMO

LUMO

HOMO

HOMO

It is better to have substituent with opposite properties.

Each pair favors one Frontier orbital interaction

regioselectivity the alder rule
RegioselectivityThe Alder rule

or

*

*

Concerted reaction but not synchronous.The atoms with the largest coefficients bind first.

electrocyclic 4 e d c2 conservation
Electrocyclic 4-eD 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

electrocyclic 4 e h n s conservation
Electrocyclic 4-ehn 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

cycloaddition 4 2 diels alder
Cycloaddition 4+2Diels-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

cycloaddition 4 2 d s conservation
Cycloaddition 4+2Ds conservation

A

Y4

S

A

A

p*

A

Y3

Y2

S

A

S

p

S

Y1

A

S

S

cycloaddition 6 e d s conservation
Cycloaddition 6-eD 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