Photochemistry

Organic Photochemistry

Introduction to PhotochemistryClassifications of Photochemical ReactionsApplication of Photochemistry in Organic Synthesis

Energies100 kcal/mol= 4.3 eV= 286 nm= 35000 /cm (near UV)nano= 10-9286 kcal/mol= 12.4 eV= 100 nm= 100000 /cm (far UV)Typical Bond Energies

C-H = 110 kcal/molC-C = 80C=C = 150C=O = 170Uv light 150 -40 nm wavelength, so this is sufficient energy to break bonds –knock electrons out of bonding orbitals (electronic excitation).

Chemically useful light is generally in the range of 200-400 nm

Often employ filters to regulate the wavelength of the radiation

n

*

n

*

n

*

n

*

n

*

ground state (S0) n-* (S1) n-* (T1) -* (S1) -* (T1)

A Jablonski diagram, named after the Polish physicist AleksanderJabłoński, is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Radiative transitions are indicated by straight arrows and nonradiative transitions by squiggly arrows.The vibrational ground states of each electronic state are indicated with thick lines, the higher rotational states with thinner lines.

Physical Processes Undergone by Excited Molecules

• So + hv --- S1 Excitation

• S1v -- S1 + heat Vibrational Relaxation

• S1 ----- So + hv Fluorescence

• S1 ---- So + heat Internal Conversion

• S1 --- T1 Intersystem Crossing

• T1v -- 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

Why Use Photochemistry

• Overcome large kinetic barriers in a short amount of time• Produce immense molecular complexity in a single step• Form thermodynamically disfavored products• Allows reactivity that would otherwise be inaccessible by almost

any other synthetic method• The reagent (light) is cheap, easily accessible, and renewable

• Drawback• Reactivity is often unpredictable• Many substrates are not compatible• Selectivity and conversion are sometimes low

Chemical Processes undergone 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

• 1) α-Cleavage (Norrish type I reaction). In solution the radicals undergo further reactions to give products.

• 2) Hydrogen Abstraction followed by cleavage = Norrishtype II cleavage.The radicals can abstract a Hydrogen atom from a donor. The resulting radicals can then undergo further reactions.

• An intramolecular example:

O

R1

R2

R2 R2R1

R1O

OR1

R2+

h, (300 nm)

solvent

1a–f

3f 4f2a–f

a b c d

R1

R2 CO2EtH

CO2Me

CO2Me

Ph H Ph

Ph

CO2EtH

HH

e f

+

O

R1 R2

R2

R2 R1

O

O

R2

25a–f

28a–f

1a–f

3a–f

17a–f

20a–f

22a–f

DPMODPM

R1O

O

R2

R1

R1

R2

R1

O

5

8