quick reference to pericyclic reactions and photochemistry claude legault litterature meeting...
TRANSCRIPT
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Quick Reference to
Pericyclic Reactions and Photochemistry
Claude Legault
Litterature Meeting
December 13th, 2004
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Goal of the Presentation
Not a course, not a litterature meeting...
Small review of orbital symmetry rules
Simple tricks to determine permitted and forbidden processes
Classify Pericyclic Reactions into main classes
Explain the basics of photochemistry
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Definition of the Pericyclic Reactions
Definition Concerted reactions going through a cyclic transition state
Types
1. Cycloaddition (Diels-Alder, [3+2]-cycloaddition, ene reactions)Reactions with the formation of two sigma bonds between the extremity of twopi systems. (ene impliquates a sigma system)
2. Cheletropic reactionsFormation or breakage of two sigma bond between the extremity of a pi systemand one atom
3. Electrocyclic rearrangmentsFormation or breakage of a sigma bond between the extremity of one pi system
4. Sigmatropic rearrangments[1,j] : Migration of an atom along a pi system[i,j] : Migration of a sigma bond between two pi systems
5. Dyotropic rearrangmentsSimultaneous intramolecular migration of two sigma bonds
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Simplist View of Polyenes systems
Ψ1
Ψ2
Ψ3
Ψ4
S
A
S
A
S : SymmetricA : Antisymmetric
S
A
S
A
So a first simple thing to remember:
4n+2 (2,6,10,...) electrons polyenes: The HOMO will be symmetric4n (4,8,12,...) electrons polyenes: The HOMO will be antisymmetric
This is general...
Ψ1
Ψ2
Ψ3
Ψ4
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First Trick : SUPRA and ANTARA Nomenclature
Suprafacial Rxn happening on the same side of the pi system
Antarafacial Rxn happening on the opposite sides of the pi system
Suprafacial + Suprafacial = SUPRASuprafacial + Antarafacial = ANTARAAntarafacial + Suprafacial = ANTARAAntarafacial + Antarafacial = SUPRA
4n+2 electrons processes (2,6,10,14...) Only SUPRA permitted thermally4n electrons processes (4,8,12,16...) Only ANTARA permtted thermally
When two pi systemsreacts in a cycloaddition
Examples: [π2s+π2s] : 4 electrons SUPRA process -> Forbidden[π4s+π2s] : 6 electrons SUPRA process -> Permitted
[π6s+π4s] : 10 electrons SUPRA process -> Permitted
?
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DA and HDA and Definitions
Intermolecular
IntramolecularLevel 1
IntramolecularLevel 2
Diels-Alder relates only to [π4s+π2s]
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An Insight in the DA Process (Correlation Diag)
S
S
S
A
A
A
A
S
S
A
A
HOMO dienophile
LUMO dienophile
LUMO diene
HOMO diene
S
E
1 + 2
1 - 2
Rxn coord.
EWG
EWG EWGEWG
Spino, C. et al. Angew. Chem., Int. Ed. 1998, 37, 3262.
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The Klopman-Salem Equation
CoulombicInteractions
OrbitalsInteractions
Filled OrbitalsRepulsion
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Why the Observed Regio?
βab(ca+m)(cb+n) + βab(ca)(cb) = βab(ca+m)(cb) + βab(ca)(cb+n)
cacb + can + cbm + mn + cacb = cacb + cbm + cacb+ can
mn > 0
(ca+m)(cb+n) + (ca)(cb) = (ca+m)(cb) + (ca)(cb+n)
ca
ca+mcb+n
cb
ca
ca+m
cb+n
cb
vs
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Useful General Values
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[3+2] Cycloaddition
AB
C
D E
BA
D E
CB
A
D E
C
Fu, G. C. et al. J. Amer. Chem. Soc. 2003, 125, 10778.
Analoguous to the Diels-Alder Reaction [π4s+π2s]
Cu
EtO2C
NN
R
O
H
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[3+2] Cycloaddition
N N Bn
O
Stoltz, B. M. et al. J. Amer. Chem. Soc. 2003, 125, 15000.
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[3+2] Cycloaddition, Bond Strength Importance
AB
C
D E
BA
D E
CB
A
D E
C
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[3+2] Cycloaddition, Some Useful Values
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Ene Reaction : Special Case of Cycloaddition
IntramolecularLevel 1
IntramolecularLevel 2
RO
HHO
S
H
RO
HHO
S
H
RO
S
OH
H
HH
[(π2s+σ2s)+π2s] Usually facilitated by a enophile with a low lying LUMO
enophile
H H
H
H H
H
HHH
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Some Application...
O
O
H
H
OO
OH
OHH H O
O
O
O
a b
c
Inomata, K. et al. Org. Lett. 2004, 6, 409.
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Recent Use of a Ene/Retro-Ene Process
Corey, E. J. et al. Org. Lett. 2003, 5, 1999.
Halls, D. G. et al. J. Org. Chem. 2004, 69, 8429.
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Electrocyclic Rearrangments
Why?
Driving forces can be:
- Geometric tension- Aromatic stabilization (formation of an aromatic moiety)- Delocalization (opening or closing permits delocalization in an other
pi system)
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The Same Trick for Opening/Closing
How to know how it closes or open?
The simple trick: Take the open system (polyene) and apply the same principle as seen earlier by simply considering the HOMO of the polyene
4n+2 electrons HOMO (2,6,10,14...) Only Disrotatory opening/closing permitted thermally4n electrons HOMO (4,8,12,16...) Only Conrotatory opening/closing permtted thermally
Disrotatory (4n+2 e-)Conrotatory (4n e-)
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Torquoselectivity
If R is electrondonating, then thisfilled orbital would unfavorably interact with thebreaking sigma bond, so outward opening is favored.
If R is electronwithdrawing, then thisempty orbital would favorably interact with the
breaking sigma bond, so inward opening is favored.
R
R
Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5072.
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Torquoselectivity : Some Useful Values
Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5072.
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Recent Example of a Electrocyclic Rearrangment
Hsung, R. P. et al. J. Org. Chem. 2003, 68, 1729.
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[1,j] Shift, Definition and possibility
R R
Suprafacial + retention = SUPRASuprafacial + inversion = ANTARAAntarafacial + retention = ANTARAAntarafacial + inversion = SUPRA
4n+2 electrons processes (2,6,10,14...) Only SUPRA permitted thermally4n electrons process (4,8,12,16...) Only ANTARA permtted thermally
Hydrogen shift: The valence orbital of H being an 1S (spherical) orbital), inversion is impossible with this atom.
An antarafacial migration is only possible with a polyene of at least 6 carbons.
In cyclic systems, antarafacial migrations are impossible in smaller than 10 memberedrings.
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[i,j] Shift, Even More Possibility
So... Only 4n+2 (2,6,10,14...) electrons processes (SUPRA) are permitted thermally
Migration of a sigma along two pi systems. Only suprafacial-suprafacial migrations are allowed geometrically
Some nomenclature...
HO HO
OO
XX
Cope Rearrangment
Oxy-Cope Rearrangment
Claisen Rearrangment
Wittig[2,3] Rearrangment
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Recent Examples of [i,j] Shifts, Barriault
Oxy-cope
Claisen
Level 2 intramolecularene
Barriault, L. et al. J. Am. Chem. Soc. 2004, 126, 8569.
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Dyotropic Rearrangments
Type 1Type 2
Recent example of a type 2 dyotropic shift used in a total synthesis
Houk, K. N. et al. J. Am. Chem. Soc. 2003, 125, 5111.
Simultaneous intramolecular migration of two sigma bonds
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Now for Photochemistry
Advantages:
Non thermic activation (low temperature)
Give access to thermally forbidden processes
Photopumping (synthesis of high energy molecules)
Selective activation of chromophores:
R OMe
OO
R OMe
OO *
*
*
hυ
hυ
hυ
λ =300 nm
λ =254 nm
λ =180 nm
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Basics Explained
hυ
R
R
R
R
R
R
heat light
Quantum Yield: Φ = # molecules transformed / # photons absorbed
The way to measure it : Using an actinometer
The LUMO of the compound becomes a SOMO, the highest occupied orbital, so thereaction occurs through that orbital: A thermally fobidden process now becomes permitted
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Some Numbers (Activation Energy)
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First the Practical Stuff... (Absorbance)
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Jablonsky Diagram
Ca Cb Ca CbDouble bond Single bond
hυ
π
π*
π
π*
σ σ
Ca CaCb Cb
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Photodissociation
Ca Cb Ca Cb
hυ
Single bond Radical pair
σ σ
σ* σ*
Ca CaCb Cb
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Photosensibilization
Sens React
S0
S1T1
S1
T1
S0
Esens Ereact
Esens < Ereact
E*3sens > E*3
react
E
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Photosensibilization
Ph
O
Ph
O
Ph
O*1 *3
Ph
O *3
Ph
O *3
With acetophenone
With benzophenone
E*3norbonene = 309 kJ
E*3benzophenone = 288 kJ
E*3acetophenone = 309 kJ
hυ
hυ
ISC
ISC
Ph2CO Ph2CO*1 Ph2CO*3
O
Ph Ph
O
PhPh
O
PhPh
1) Spininversion
2) Cyclization
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Classical Reactions (Norrish Fragmentation)Norrish Type 1 fragmentation
R R'
O
R R'
O *1
R R'
O *3
R R'
O *3
R R'
O
R'
O
R
- CO2R R'R R'
hυ
Norrish Type 2 fragmentation
hυR
O*3R'
R
OR'H *3
R
OHR'
R
OHR' *3
R
OHR'
R
HO
R'
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Photopumping
Wavelength
A A
B
CO2Me
CO2Me
CO2Me
CO2Me
A B
hυ
low temp.