The homolytic cleavage of covalent bonds in carbonyl compound under photochemical conditions known as Norrish Type Reactions They are divided into two types Norrish Type I Norrish Type II reaction

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Presented By
Nilesh S. More
Department of Medicinal Chemistry
MC/2019/14
राष्ट्रीय औषधीय शिक्षा एवं अनुसंधान संस्थान (नाईपर), हैदराबाद
National Institute of Pharmaceutical Education and Research (NIPER), HYDERABAD
Photochemistry of Carbonyl Compound,
Norrish type I and Type II Reaction

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APPLICATIONS
DISTINCTIVE FEATURES
NORRISH TYPE REACTION
DIFFERENT TYPES OF PHOTOCHEMICAL REACTIONS
INTRODUCTION

3.  Carbonyl compounds undergo various photochemical reactions in both gas and liquid phases.
 Carbonyl compound are best suited for Photochemical reactions because ketones are much
stable and undergo no. of interesting reactions
 The initiation of a photochemical reaction depends on the capacity of reactant(s) to absorb the
light from an emitting source.
 The light photon-energy must match with the energy necessary to excite one electron from
HOMO to LUMO of a ground state molecule - a quantized process - to its excited state
 INTRODUCTION
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Dantas JA. Photochemistry of Carbonyl Compounds: Application in Metal‐Free Reactions.
ChemPhotoChem. 2019 Jul;3(7):506-20.

4.  It involves an n →π* excitation of an oxygen lone pair
electron (n) to the π* lowest unoccupied molecular
orbital of the C═O chromophore
 excitation is from the S0 electronic ground state to the
first singlet excited state, S1, which correlates to excited
state products
 Dissociation on both T1 and S0 leads to ground-state
radical products
 Internal conversion (IC) from S1 or two ISC steps, S1 →
T1 → S0, can also lead to photolysis on the S0 ground
state
 ISC is spin forbidden, the geometries of S1 and T1
excited-state carbonyls are generally very similar, and
the energy separation between the states is small, leading
to fast S1 → T1 ISC rates
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Rowell KN. Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls.
The Journal of Physical Chemistry A. 2019 Nov 1;123(48):10381-96.

5.  The reactive excited states of saturated ketones are the n → π* states, whereas that of
conjugated ketones are π → π* states. Both these n → π* and π → π* transitions of carbonyl
compounds may occur by singlet or triplet excited states
 Both singlet and triplet excited states of a carbonyl compound react in different rates to give
same type of products in different ratios.
 Saturated ketones, the activation energies for singlet and triplet excited states are about 80–85
and 75–80 kcal/mol, respectively, and hence require UV light of wavelengths of about 270–
280 nm (near-UV region)
 for conjugated ketones, the activation energy for singlet and triplet excited states is below 80
kcal/mol, in the range 45–78 kcal/mol, and require light in the far-UV region (310–330 nm).
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Rowell KN. Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls.
The Journal of Physical Chemistry A. 2019 Nov 1;123(48):10381-96.

6.  DIFFERENT TYPES OF PHOTOCHEMICAL REACTIONS
OF CARBONYL COMPOUNDS
Norrish
type reaction
(I & II)
Cyclo-
Addition
Photo-
Reduction
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 NORRISH TYPE REACTION
 Kirkbride and Norrish identifying α-bond cleavage as the dominant photolysis pathway
in small carbonyls in 1931
 The homolytic cleavage of covalent bonds in carbonyl compound under photochemical
conditions known as Norrish Type Reactions
 They are divided into two types
Norrish Type
II
Norrish type I
Rowell KN. Structural Effects on the Norrish Type I α-Bond Cleavage of Tropospherically Important Carbonyls.
The Journal of Physical Chemistry A. 2019 Nov 1;123(48):10381-96.

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 Norrish Type I
 The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into
two free radical intermediates
 The carbonyl group accepts a photon and is excited to a photochemical singlet state
 Light energy is sufficient to break the alpha bond
 Through intersystem crossing the triplet state can be obtained
 On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained which
are alkyl or acyl radicle

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 Distinctive Features of Norrish type I reaction
 Reaction intermediates are free radicals – acyl, alkyl
 Reaction proceeds to form more stable free radicle
 Reaction is more effective on vapour state
 The SP2 and SP nature of alpha carbon never participated in Norrish Type I reaction
 Compound which are formed stable free radicals undergoes Norrish Type I reaction even in
Liquids state

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 Reaction Mechanism

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 Norrish Type II reaction
 Photochemical reaction of aldehydes or ketones bearing γ–
hydrogens
 An aldehyde or ketone bearing a γ -hydrogen atom can upon
irradiation undergo an intramolecular hydrogen shift by the so-
called Norrish type II reaction
 The resulting diradical species can undergo a subsequent ring
closure reaction to yield a cyclobutanol, or suffer fragmentation
to yield an enol and an alkene
 γ -hydrogens can undergo the intramolecular hydrogen
abstraction from the singlet excited (S1)-state as well as the
triplet excited (T1)-state
 β, γ – carbon always converted to olefins

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 Norrish type II reactions are very limited in carbonyl compound
 In Norrish type II reaction there are possibilities of formation of cyclobutanol derivative as by product

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 Reaction Mechanism

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 Distinctive Features of Norrish type II reaction
 [ n- π*]1 & [ n- π*]3 give rise to γ-
hydrogen transfer
 However, the singlet & triplet
reactions are quit distinguishable
 Photochemical elimination is
highly stereospecific when
intermediate biradical is [ n- π*]1
 Photochemical elimination is non-
stereoselective when biradical
intermediate is [ n- π*]3

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 Stabilization of 1,4- biradical
 It may undergo photo-elimination & form Olefins /
Enols
 It may directly cyclize & form cyclobutanol by-
product derivative
 It may convert into starting material which may due
to reversible formation of 1,4- biradical

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Liu W, Li CJ. Recent synthetic applications of catalyst-free photochemistry.
Synlett. 2017 Dec;28(20):2714-54.
16
o Disubstituted cyclohexanone converted into substituted cyclopentane
using N.T.-I reaction
o Reaction is successful due to high stability of radical intermediate of
N.T.-I reaction
 APPLICATIONS

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o N.T.-II reaction selectively activate remote methyl group
o Increase over-all product yield
Liu W, Li CJ. Recent synthetic applications of catalyst-free photochemistry.
Synlett. 2017 Dec;28(20):2714-54.

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Β-lactam synthesis from α-amino acid via photolysis
o Carried out by photo-wolf-rearrangement
o Generated ketene reacts with amine group without altering stereochemistry
Liu W, Li CJ. Recent synthetic applications of catalyst-free photochemistry.
Synlett. 2017 Dec;28(20):2714-54.

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Total synthesis of lineatin
o Under UV irradiation cycloadducts is formed by reacting furanone with
1,2-dichloroethylene
o The cycloadduct then converted to (+)-lineatin by functional group
manipulation.
Liu W, Li CJ. Recent synthetic applications of catalyst-free photochemistry.
Synlett. 2017 Dec;28(20):2714-54.

20.  CONCLUSION
Photochemically mediated reactions have proven to be highly useful for
construction of highly strained and complex molecular frameworks that can
otherwise be challenging to access via conventional processes.
In the future, it can also be used to expand our classical chemical
toolbox by offering improved reaction conditions and can facilitate the
discovery of new transformations for efficient bond construction.

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 REFERENCE
 Dantas JA, Correia JT, Paixão MW, Corrêa AG. Photochemistry of Carbonyl Compounds:
Application in Metal‐Free Reactions. ChemPhotoChem. 2019 Jul;3(7):506-20
 Rowell KN, Kable SH, Jordan MJ. Structural Effects on the Norrish Type I α-Bond Cleavage of
Tropospherically Important Carbonyls. The Journal of Physical Chemistry A. 2019 Nov
1;123(48):10381-96.
 Liu W, Li CJ. Recent synthetic applications of catalyst-free photochemistry. Synlett. 2017
Dec;28(20):2714-54.
 Laue T, Plagens A. Named Organic Reactions 2nd Edition.
 https://www.chemeurope.com/en/encyclopedia/Norrish_reaction.html#_ref-0/