organic photo chemistry

8/2/2019 Organic Photo Chemistry

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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

organic photo chemistry

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