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Organic Photochemistry
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Organic Photochemistry Any chemical change bought about by light/electromagnetic
radiation
Chemical Change : Any event in the molecular level after absorbing aphoton.
No need for an overall chemical change
Wavelength range of e.m. radiation : 700-100nm (visible or uv)
Examples: Photosynthesis, vision etc..
Photosynthesis
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Basic Steps in Photochemistry
Absorption of photon (with uv or light frequency) by a moleculetakes it to an electronically excited state, which is the startingpoint for the subsequent reaction steps
Ground state: Electrons try to occupy the lowest possible
orbitals in pairs
Excited state: One electron occupy a higher energy orbital
than the lowest energy level available for it
(overall energy of the molecule also increases)
An electronically excited species (finite life time only) may havedifferent physical and chemical properties than the ground state.
An electronically excited state is more energetic than the groundstate and leads to more possibilities for the reaction
The electronic configuration of excited state is more favorable forproduct formation
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Chemical Processes by Excited
Molecules(A-B-C) A-B. + C. Simple Cleavage
(A-B-C) E + F Decomposition
(A-B-C) A-C-B Intramolecular Rearrangement
(A-B-C) A-B-C' Photoisomerization
(A-B-C) A-B-C-H + R. Hydrogen Atom AbstractionRH
(A-B-C) (ABC)2 Photodimerization
(A-B-C) ABC + A* PhotosensitizationA
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Quantum Yield A property relevant to most photo physical and
photochemical processes
A measure of efficiency with which absorbed radiationcauses the molecule to undergo a specified change
It is the number of product molecules formed for each
quantum of light absorbed
Definition: the number of moles of a stated reactantdisappearing, or the number of moles of a statedproduct produced, per einstein of monochromatic light
absorbed.(1 einstein = 1 mole of photons)
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So + hv --- S1 Excitation
S1v -- S1 + heat Vibrational Relaxation
S1 ----- So + hv Fluorescence
S1 ---- So + heat Internal Conversion
S1 --- T1 Intersystem Crossing T1
v -- T1 + heat Vibrational Relaxation
T1v -- So + hv Phosphorescence
T1 --- So + heat Intersystem Crossing
S1 + A (So) --- So + A (S1) Singlet-Singlet Energy Transfer
T1 + A (So) -- So + A (T1) Triplet-Triplet Energy Transfer
Jablonski Diagram
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Advantages of Photochemical Reactions
Overcome large kinetic barriers in a short amount oftime
Produce immense molecular complexity in a single step
Form thermodynamically disfavored products
Allows reactivity that would otherwise be inaccessibleby almost any other synthetic method
The reagent (light) is cheap, easily accessible, andrenewable
Drawbacks
Reactivity is often unpredictable
Many substrates are not compatible
Selectivity and conversion are sometimeslow
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PhotosensitizorEnergy transfer through photosensitization
D 1D
h
1D 3DISC
A + 3D D + 3A
3A Products
D = Donor
A = Acceptor
1 = Singlet
3 = Triplet
S0
S1
74 Kcal
.mole-1 69 Kcal/mole
T1
ISC
120 Kcal/mole
S0
T1
S1
60 Kcal/mole
Energy transfer
Benzophenone Butadiene
Ph2COh
1[Ph2CO]ISC 3[Ph2CO]
+ Ph2CO3
Dimeric products
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Organic Photochemical reactions
Photochemistry of simple Ketones Step I -Cleavage
Step II Decarbonylation
Step III Recombination
I. Norrish Type-I Reactions ( -Cleavage)
R
O
C
O
Ch
+
The carbonyl group accepts a photon and
is excited to a photochemical singlet state.Through intersystem crossing the tripletstate can be obtained. On cleavage of the -carbon carbon bond from either state, tworadical fragments are obtained.
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The fragments can simply recombine to the original carbonylcompound (path A). By extrusion of carbon monoxide in path B, two organicresidues can recombine with formation of a new carboncarbon bond When the carbon fragment has an -proton available it getsabstracted forming a ketene and a saturated hydrocarbon inpath C When the alkyl fragment contains a -proton it gets
abstracted with formation of an aldehyde and an alkene.
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Norrish Type II reactions
Cleavage of 1,4-biradicals formed by -hydrogen abstraction
The quantum yield for type II cleavage is only about 25%
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III. Alkene isomerization (cis-trans isomerization)
Example: cisand transstilbene.Both cis and trans-stilbene undergo * electron excitation by
absorption of uv light. A small proportion (6%) of the trans-S1 statefluoresces back to the trans-isomer.The stability of the stereoisomers of stilbene is due to a 62 kcal/molebarrier to rotation about the double bond produced by the -bond. Thisbonding is absent in the * excited state (magenta curve). These
local S1 states quickly relax by means of non-radiative internal conversion
to the transition state region of S0
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H
H
h
185 nm
sens
heath
h
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V. Photoenolization When the reactive carbonyl function and a -hydrogen are conjugated
via an aromatic ring or double bond, the 1,4-diradical created byhydrogen abstraction quickly relaxes to a conjugated enol tautomer
If an aromatic ring has been disrupted by the photoenolization, theenoltautomer is unstable and rapidly reverts to the initial aromaticcarbonyl compound.
This might appear to be a useless transformation, but it finds practicalapplication as a sunscreen ingredient
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VI. Patern-Bchi Reaction(Photochemical synthesis of oxetans)
O
O
O
EtO
OEt
CO2H
O N
N
O
OH
OH
N
N
NH2
O
O
NH2
NH
NH2
O
O
O
O
O
O
OAc
OR
H
OBz
OOAc OH
+
Paterno and Chieffi (1909), Buchi in 1954 mechanistic analysis
Insecticidal activity
Thromboxane A2 Oxetanocine
Bradyoxetin
Merrilactone A
Palitaxel
CHO
C
O
H
O
C C
O
C
C
OO
Reaction mechanism
h[PhCHO] S1
ISC[PhCHO] T1
(n-*)
Kisc aromatic >> K isc aliphatic (>>1010/s)
responsible
+
electrophile nucleophile
+
Major Minor
Biradical intermediate
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OO O
O
O
tBuO
O
tBu
OO
O
O
tBu
O
Ph PhO
O
C
O
O
C
Ph
Ph O
OO
Ph
Ph
+h
+
1 1.6
+h
1 atm O2
h, 11atm O2
+
lifetime = 1.6 ns
h
O O
CHOO OH
O O R
O
OO
R
O
O
O
R
OH
OMe
h
LAH
MeOHh
h
Intramolecular Paterno Buchi
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