Organic photochemistry and pericyclic reactions CY50003 300

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Organic photochemistry and pericyclic reactions
(CY50003)3-0-0
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Course content

• Principles of photochemical reactions Excited
  states and their properties experimental set up
  for photochemical reactions(1) Several useful
  photochemical reactions and their applications in
  organic synthesis (isomerization, Patterno-Buchi
  reaction (1), Norrish type I and II reaction(1),
  Photoreduction, Rearrangements di-p-methane, oxa
  di-p- and aza di-p-methane rearrangements(2),
  Photocycloaddition (2), Photochemical aromatic
  substitution reaction (1), Reactions with singlet
  oxygen (1), Photochemical methods for protection
  and deprotection(2). Photochemistry of biological
  systems (photosensitized reactions of DNA/RNA,
  DNA damage and repair-1).

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Books

• CRC Handbook of Photochemistry and Photobiology.
  Eds by. William M. Horspool and Pill-Soon Song.
  1994. CRC Press. ISBN 0-8493-8634-9
• Synthetic organic photochemistry. Eds by. William
  M. Horspool, Plenum press. 1984. ISBN
  0-306-41449-X

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LIGHT ABSORPTION AND FATE OF EXCITATION ENERGY
Franck-Condon Principle
Ground state (E0) and two excited states (E1, E2)
of a molecule (vibrational and rotational levels
are not shown).  
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Criteria of an ideal sensitizer

• It must be excited by the irradiation to be used,
  small singlet triplet splitting. High ISC yield.
• It must be present in sufficient concentration to
  absorb more strongly than the other reactants
  under the condition.
• It must be able to transfer energy to the desired
  reactant, low chemical reactivity in Triplet
  state.

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Experimental set up for photochemical reactions

• Synthetic organic chemist (high intensity
• light source, easy to handle, various
• Flask size, specially designed systems)
• Physical chemist or physical-organic
• Chemist (mechanistic study)

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Short and Long cutoff filter solutions
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Immersion Well Reactors Components Lamps
Immersion wells Reaction flasks Standard
flasks Gas inlet flasks Flow-through flasks
Larger capacity standard flasks
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Non-Rotating AnnularPhotochemical Reactor
Large Quartz immersion well. 400 watt medium
pressure mercury lamp. Reactor base and
carousel assembly (non rotating), including
support rod and immersion well adjustable
clamp. set of sample tube support rings for
eight 25mm sample tubes Only the inner or the
outer tubes may be irradiated effectively at one
time UV Screen- consisting of three black
coated consisting of three black coated
aluminum sections. A light tight lid, a
removable front and back section, that are
joined by means of a light tight seal      
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Semi-Micro Reactor The semi-micro is a low cost,
easy-to-use device for irradiating a standard 1
cm cuvette (or small tube) with either 254nm or
350nm radiation for any preset time between 1
and 70 minutes. This reactor is ideal for
preliminary studies of small volumes of
solution.
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The multilamp reactors consist of a base, lid,
six or three lamp modules. Each module contains
two lamps. The base is hexagonal and provided
with a centrally located fan A number of
modules up to six or three may be operated.
Switches are provided to control the fan and lamp
modules. Supports from the lid hold samples
inside the reactors. Magnetic strips are used to
eliminate light leaks between the lamp modules.
Multilamp Reactors Six and Three Modules
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Complete photochemical reactor comprising
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Purity of solvent and gases

• Dilution (suppression of side reaction e.g.,
  polymerization and dimerization.)
• Spectral transmission of solvent ( solvents
  devoid of low-lying excited states are best)
• Purity of solvent (Oxygen free, impurities free)

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Transmission characteristics of various solvents
Measured for a 1cm path length of pure solvent
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Photochemical synthesis of oxetans

• Paternò-Büchi Reaction

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• Norish Type I Processes of Ketones Basic Concepts

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• Norish type II photoelimination of ketones
  Cleavage of 1,4-biradicals formed by ?-hydrogen
  abstraction

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JACS, 1977, 99, 3723-33
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Mechanism I
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Bioluminescence
Fireflies
Artistic rendering of bioluminescent Antarctic
krill
Firefly luciferin
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Image of bioluminescent red tide event of 2005 at
a beach in Carlsbad California showing
brilliantly glowing crashing waves containing
billions of Lingulodinium polyedrum
dinoflagellates
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Photochemical aromatic substitution reaction
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• Photochemical reactions with singlet Oxygen

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Photo removable protecting groups
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