Photochemistry of Carbonyl Compounds Explained

Photochemistry of Carbonyl Compounds

• Four main bands in saturated ketones in UV absorption Spectra.
• These bands are on 280, 195, 170 and 155nm, band at 280nm is
important for the photochemistry of carbonyl group.
• This band corresponds to n-π* transition.
• The photochemical reactions of carbonyl group is initiated by n-π*
transition.
• Electron promotion will lead to either singlet or triplet state and
photochemical reactions of carbonyl can either take place from
singlet or triplet state or by both the states

O
C °°
°°
O
C °°
°°
°
° O
C °°
°°
°
°
According to Kasha’s rule, only the lowest excited states will be involved in the primary
photochemical or photophysical processes of organic molecules in solution.
Carbonyl compounds give four type of reactions. These reactions include:
(i) α Cleavage
(ii) β Cleavage
(iii) Intramolecular and intermolecular hydrogen abstraction by carbonyl oxygen
(iv) Addition of carbonyl oxygen atom to carbon-carbon multiple bond
The four processes are competitive and the major process followed is sensitive to
structural variations in the ketones and the choice of the solvents

α Cleavage or Norrish Type I Process
Norrish type I process is given by three types of ketones:
(i) Saturated acyclic ketones
(ii) Saturated cyclic ketones
(iii)β,γ- unsaturated ketones
Norrish Type I process given by acyclic saturated ketones
Saturated carbonyl compounds undergo photoinduced de-
carbonylation in the gas phase. This process was first observed R.G.W
Norrish and is known as Norrish type I or α Cleavage process. Norrish
Type I process is commonly encountered in the gas phase. The
solution phase reaction of this type is uncommon

Primary Processes
Norrish type I process is characterized by initial cleavage of the
carbonyl carbon and alpha carbon bond to give an acyl and an alkyl
radical. This process is known as primary photochemical process.
R-CH2-C-CH-CH2-R” R-CH2-C + CH-CH2-R”
The initially formed acyl radical and alkyl radical is stabilized by one
of the secondary processes [a-c] as per scheme-2. similarly the alkyl
radical can be stabilized by recombination or disproportionation
O
R’
O
°
R’
°
Scheme-1

Secondary Process
(a) Decarbonylation of acyl radical to give carbon monoxide and an
alkyl radical. This alkyl radical can recombine to give an alkane or
can undergo intermolecular hydrogen abstraction to form an
alkane or alkenes.
R-CH2-C R-CH2 + CO (decarbonylation)
R-CH2 + R-CH2 R-CH2-CH2-R (recombination)
R-CH2 + CH-CH2-R” R-CH3 + R-CH=CH-R”
intermolecular hydrogen abstraction
O
°
°
° °
°
R’
°
Scheme-2a

(b) Intermolecular hydrogen abstraction by the acyl radical from the
alkyl radical to give an alkene [ Scheme 2(b)].
R-CH2-C + R’-CH-CH2-R” R-CH2-C-H + R-CH=CH-R”
This process can only be possible if alkyl radical has at least one β
hydrogen
O
° °
Scheme-2b
O

(c) Intermolecular hydrogen abstraction by the alkyl radical from the
alpha carbon of the alcyl radical to form a ketene and an alkane [
Scheme 2(c)].
R-CH2-C + R’-CH-CH2-R” R-CH=C=O + R’-CH2CH2-R”
The main secondary reaction of saturated acyclic ketones is
decarbonylation
O
° °
Scheme-2c

• Norrish type 1 is thus a two step radical mechanism . The first
step is a primary process and the second step is the secondary
process.
• Formation of alkyl and acyl radicals can be proved by the
trapping experiments, 2,2,4,4- tetramethylpiperidine-1-oxyl
radical was used for the trapping and the radical fragments
produced by the fission of 1,3-diphenylacetone, the radicals
formed were trapped as an ester and an ether as shown in
scheme 3

C6H5-CH2-C-CH2-C6H5 C6H5-CH2-C + CH2-C6H5
decarbonylation
+ C6H5-CH2-CH2-C6H5
- C-CH2-C6H5 -CH2-C6H5
O O
° °
N
O°
N
O
O
N
O
Scheme-3

The formation of radical intermediates is also readily demonstrated
by photolysis of a mixture of ketone (A) and (B) which gives
products from mixed radical combination.
Ph2-CH-C-CH-Ph2 + Ph-CH2-C-CH2-Ph
(A) (B)
Ph2-CH-CH-Ph2 + Ph-CH2-CH2-Ph + Ph2-CH-CH2-Ph + CO
cross product
O O

The cross over experiment also confirmed the two-step radical
mechanism.
Norrish type 1 process occurs from both the excited singlet and the
triplet states of n-π* transition. Photolysis of di-tertbutyl ketone
results in high yield of carbon monoxide (90%) from both the excited
singlet and triplet states.
The life time of singlet state is 4.5-5.6 X 10-9 Sec as compared with
0.11X 10-9 Sec for excited triplet state.
The type 1 process must occur about 100 times faster from triplet
than from singlet excited state. Studies with triplet quenchers such
as 1,3 cyclopentadiene have also shown that Norrish type 1
processes occurs from both triplet and singlet excited states.

Norrish type 1 cleavage is given mostly by those ketones whose n-π*
state is the lowest excited states. In most of the cases, the n-π* state
is the lowest excited states. However, α cleavage in arylalkyl ketones
and diaryl ketones is less efficient because n-π* excited state is not
the lowest excited state. In this case, there is a large barrier on the
reaction coordinate.
S2 (n,π*)
100kcal/mol S1 (n,π*)
76kcal/mol
Energy of diphenyl ketone

In Norrish type 1 reactions there is a preference for the formation of
most stable alkyl radical in case of unsymmetrical ketones.
CH3
CH3-C C-CH2 CH2-CH3
CH3
CH3
CH3-C + C-CH2 CH2-CH3 CH3
CH3 CH3-C C + CH2-CH2-CH3
CH3
O
O
°
°
O
° °

In this case only α bond undergoes cleavage. If both alkyl substituents
are same, then there is little selectivity of bond cleavage
CH3-CH2-CH2-C-CH2-CH3 CH3-CH2-CH2-C + CH2-CH3 +
CH3-CH2-CH2 + CH3-CH2-C
The Norrish type 1 process is mostly favoured by photolysis I the
vapour phase and is less pronounced for photolysis in the inert
solvents. In inert solvent formation of solvent cage takes place. This
facilitates the combination of the initially generated radical pair and
therefore low quantum yield of product.
O
°
O
° °
O
°

liq. Phase, hν
C6H5-CH2-C-CH2-C6H5 C6H5-CH2-C
+
C6H5-CH2
C6H5-CH2-CH2-C6H5 + CO
O O
°
°

Complete the reaction below:
C6H5-CH-C-CH2-C6H5 hν
C6H5
O

Norrish Type 1 Reaction of Cyclic Saturated Ketones
• Cyclic ketones in contrast to acyclic ketones show a greater
tendency to undergo α Cleavage to furnish acyl-alkyl biradicals.
This cleavage occurs in acyclic ketones from both singlet and
triplet excited state whereas in cyclic ketones it occurs
exclusively from triplet state due to rapid ISC.
• The triplet state is atleast 100 times more reactive than the
singlet excited state.
• Excited triplet state for Norrish type 1 processes is n-π* triplet
state

• This was demonstrated first for the irradiation of cyclopentanone in
both gases and in solution phases.
• Under both the conditions, the product is 4-pentenal,. Formation
of this product takes place at 313 nm and 254 nm .
• The formation of aldehyde can be quenched by 1,3-
cyclopentadiene. This quenching experiment confirms the
formation of triplet state.
• Norrish type 1 cleavage in cyclic ketones follow the similar
mechanism as in acyclic ketones, primary process followed by
secondary process involving decarbonylation/ intramolecular
hydrogen abstraction by the acyl radical/ intramolecular β
hydrogen abstraction from the acyl radical

(a) Decarbonylation
CH2
(CH2)n C S1 T1
CH2-CH2
CH2- C
(CH2)n
CH2-CH2
Acyl-Alkyl Biradical
O
O
°
°

CH2- C
(CH2)n
CH2-CH2
Acyl-Alkyl Biradical
CH2 CH3
(CH2)n CH2 (CH2)n
CH2 CH=CH2
Cycloalkane Alkene
O
°
°

(b) Intramolecular hydrogen abstraction by the acyl radical from the
β carbon of the alkyl radical to give an unsaturated aldehyde
CH2- C CH2- C-H
(CH2)n (CH2)n
CH2-CH2 CH=CH2
Acyl-Alkyl Biradical Unsaturated Aldehyde
O
°
°
O

(c) Intramolecular hydrogen abstraction from the acyl radical by the the
alkyl radical to give a ketene
βCH2- Cα CH= C=
(CH2)n (CH2)n
CH2-CH2 CH2-CH3
Acyl-Alkyl Biradical Ketene
the biradical of cyclic ketone an undergo either of the two hydrogen
transfer processes via a cyclic transition state in which a hydrogen atom
O
°
°
O

Is transferred to one radical centre from the atom adjacent to other
radical centre.
Photolysis of cyclic ketones in gas phase gives decarbonylation as well
as intramolecular hydrogen abstraction. Intramolecular hydrogen
abstraction lead to the formation of unsaturated aldehyde.
O
°
O
°

Decarbonylation intramolecular H abstraction
+ CO
Unsaturated Aldehyde
+
°
O
°
°
°
O
H

In solution phase biradical pair is not usually stabilized by
decarbonylation. In this case biradical is mainly stabilized by
intramolecular hydrogen atom transfer. This intramolecular
hydrogen atom transfer leads to the formation of either
unsaturated aldehyde or a ketene or both.
O
°
O
°
°
°
O
H
O

When photolysis is carried out in the presence of polar protic solvent
then the main species formed is ketene. This ketene then undergoes
solvent addition to give carboxylic acid (with water) or its derivatives
(ester with alcohol) as the only product
hν
ROH
ROH
O
°
O
°
O
O
OR
Ester

• There is also a possibility of a back-recombination reaction to
reform the starting material
hν Recombination
Use of suitably α substituted cyclic ketones show that recombination
reaction resulted in epimerization at the α carbon if α carbon is a
chiral
O
O
°
O
°

hν
(A) Epimer of (A)
Experimentally it has been found that photolysis product formation
via route [(a) to (c)] is faster than the photochemical interconversion
of the epimers. Since there is formation of epimeric, mixtures each
epimer affords the same mixture of products.
O
CH3
O
CH3

+ + +
O
CH3
CH3
O
CH3
CH3
CH3
CH3
CH3
CH3 CH3
CH3
CH3
CH3

O
CH3
CH3
°
O
°
CH3
CH3
°
°
CH3
CH3
°
°
CH3
CH3

O
CH3
CH3
O
CH3
CH3
°
O
°
CH3 CH3
°
O
°
CH3 CH3
2
3
2
3
Rotation about
C2-C3 Single Bond
Recombination

The recombination process is important even when hydrogen transfer
reactions occur, but loss of carbon monoxide in solution phase
photochemistry is a major reaction pathway only when the alkyl
radical centers are stabilized by inductive effect, by β,γ- unsaturation
or by cyclopropyl conjugation.
This reflects an increase in the rate of loss of CO from the acyl alkyl
biradical in these systems.
2,6-dimethyl-cyclohexanone gives carbon monoxide on photolysis in
solution at room temperature.
2,2,6,6-tetramethyl-cyclohexanone gives carbon monoxide in a yield
greater than 70%, 7,7,9,9 tetra methylbicyclo[4,3,0] non 1,6-en-8-one
gives 100% CO and hydrocarbon products

hν,Solution -CO
O
°
O
°
°
°
Highly
stable

hν Methanol
Carvone Camphor
H
hν

Norrish Type 1 process given by Cyclopentanone
• Cyclopentanone decarbonylates on irradiation in the gas phase at
147nm.
• CH2=CH2 + CH2=CH2
O
hν, Gas Phase
O
°
°
-CO ° °

• In solution the loss of CO from cyclopentanone is a major path
only where the radical centres formed are stabilized by alkyl
substitution, double bond or cyclopropyl ring
O
hν, Solution
O
°
°
°
°
Highly Stable
-CO

°
°
45%
O
hν, Solution
°
°
O
-CO
°
°

+
°
°
Major Product
80%- trans
Minor Product
13%- cis

• α cleavage is not only given by cyclic ketones. Other cyclic
compounds such as lactones, lactams and cyclic anhydrides
undergo α cleavage to give a biradical species on photolysis in gas
or solution phase
O
O
O
O
°
°
O
O
H
Intramolecular H
Abstraction
°
Unsaturated Formate
O
O
°
C H
H
O
O
C
H
H
dialdehyde

α cleavage Given by Cyclobutanones
The efficiency of α cleavage reaction of cyclobutanone is 10 times
more than the cyclopentanone due to angle strain. Angle strain and
steric strain increases the efficiency of α cleavage.
The photochemistry of α cleavage of cyclobutanone differs
significantly from the photochemistry of α cleavage of other cyclic
ketones.
Unlike other ketones, cleavage occurs from S1 (n-π*) and leads to the
formation of acyl alkyl radical. There are three following different
pathways for stabilization of the diradical:

(i) Loss of CO and formation of 1,3-diradical that undergoes either
recombination to cyclopropane or an hydrogen abstraction to
form propene.
hν S1 -CO
O
O
°
°
1,4 acyl alkyl diradical
°
°

(ii) By a subsequent β Cleavage and formation of ethylene and
ketene
CH2=C=O + CH2= CH2
(iii) 1,4 acyl alkyl biradical can undergo ring expansion by rebonding
to oxygen to give oxacarbene. This carbene can be trapped by polar
protic nucleophilic solvents. The overall reaction is a ring
expansion.
O
°
°
[2+2] -cycloaddition

O
°
°
1,4 acyl alkyl diradical
Ring expansion
O
°
°
ROH
O
OH
R
Acetal

Formation of 1,4 acyl alkyl radical can be proved by trapping
experiment. The diradical formed can be trapped by 1,3-butadieneat
low temperature (-78° C) because low temperature suppress the
decarbonylation and [2+2]-cycloadditionreaction.
Photolysis of cyclobutanone at -78° C in the presence of trapping
agent , 1,3 butadiene gives 3-vinylcyclohexanone
CH2
CH- CH=CH2
O
O
°
°
°
°

• Ring expansion of unsymmetrical cyclobutanones is highly
regioselective which indicates the need for a more stable alkyl
radical to attack the oxygen atom of the acyl radical. Indeed alkyl
substitution does increase the yield of ring expanded products.
Both 2,2-dimethyl and 2,2,4,4-tetramethyl cyclobutanone form ring
expansion product predominantly.
O
°
°
O O
°
°
Unstable alkyl diradical
stable alkyl diradical

O
°
°
O
°
°
Oxacarbene
ROH
O
OR
H
O
OR
H
+

Oxacarbene
ROH
+
O
°
°
O O
°
°
O
OR
H
O
OR
H

• Photolysis of cyclobutanone is also stereospecific reaction.
Stereochemistry at C-2 is retained during the rearrangement to the
oxacarbene. Ring expansion reaction always exhibit retention of
configuration in methanol.
O
CH2
Ph
H3C
CH3
Ph
hν /methanol
O
Ph
H3C CH3
Ph
CH2
OMe
H

Ring expansion reaction is also possible in those cases where
hydrogen transfer process are inhibited by steric factor. Certain
tricyclic ketones such as Fenchone gives ring expansion due to the
steric strain in the molecule.
°
°

β-Cleavage Reaction
• Some class of compounds have relatively weak Cα-Cβ bonds which
can undergo cleavage as a result of electronic excitation of the
carbonyl group. Cyclopropyl ketones are one such class , and
evidence for interaction between the carbonyl and cyclopropyl
group , which provides a mechanism by which energy may be
transferred from the carbonyl group to the bond which is broken,
is found in the UV spectrum.
• The mechanism involve the formation of 1,3 biradical
intermediate. Photolysis of acetyl cyclopropane leads to the
cleavage of the cyclopropane ring and this is followed by a 1,2-
hydrogen shift or alkyl shift.

C
O
CH3
O
α°
H
1,2 Hydrogen Shift
CH3-CH=CH-C-CH3
O
CH2=CH-CH2-C-CH3
O
Major Minor

In a similar way , bicyclo[4,1,0]heptane-2-ones undergo cleavage of
one of the cyclopropyl C-C bonds
O
1α
O
H
1,2 Hydrogen Shift
O
O
1α
O
O
O
O

In some cases the α-cleavage and β-cleavage are often in the
competition as shown below:
O O
O
H
β-cleavage product
α -cleavage product

α-β epoxy ketones have also a relatively weak Cα-Cβ bonds which can
be cleaved in the excited state. Epoxy ketone reacts by the way of β
cleavage and alkyl migration on photolysis.
This reaction arise from a singlet n-π* state and result in the fission
of the C-O bond.
Ph
Me
C
O
CH3
O
°
Ph
CH3
Ph-C-CH-C-CH3
O
O
CH3
1,2 methyl shift

Intramolecular Hydrogen Abstraction (γ-Hydrogen
Abstraction): Norrish Type II
• 1,3 (n-π*) excited carbonyl compounds having an accessible
hydrogen atom in the γ position undergo a characteristic 1,5-
hydrogen atom transfer by an intramolecular cyclic process
with the formation of ketyl like 1,4 diradical
O
R
β
α
H
γ
S1 or T1
°
°
O
R
β
α
H
γ
1,4 diradical
°
°
OH
R
3
1
4
2

Depending on the conformation of the initially formed 1,4 diradical ,
two different pathways to stabilisation are possible
(i) If only the sp-orbitals of the radical centres can overlap, a
cyclobutanol is the product
(ii) If the sp orbitals of the radical centres are parallel to the β bond ,
they participate in the formation of two double bonds(one in the
enol and one in the alkene), a result of the cleavage of double
bonds.

°
°
OH
R
3
1
4
2
°
°
R
3
1
4
2
Cyclisation
R
OH
°
R
3
1
4
2
Cleavage of β bond
CH2=CH2 +
O

• The second process of this reaction is known as Norrish type II
process which leads to the formation of alkene and alkenol.
• Although the reaction occurs from both the singlet and the triplet
states of n, π* transitions, the quantum yield from the singlet state
is generally lower than that from the triplet state.
• In case of aryl-alkyl ketones, the reaction occurs only with the triplet
state because aromatic ketones can undergo rapid intersystem
crossing.
• Solvents also affect the efficiency of the reaction. The singlet state
reactions are unaltered in the presence of polar solvents. Polar
solvents such as alcohol on the other hand, enhance the reaction
from the triplet state.

• The quantum yield of the reaction is poor since radiation-less
deactivation from the S1 and T1 state and reversal of the hydrogen
transfer can compete with reactions proceeding to products. The
reversal process is confirmed by using the optical active ketone
having a chiral γ carbon. Ketone undergoes racemization.
• Racemisation reaction confirmed that the reaction intermediate is
1,4 diradical. This also confirmed the back transfer of hydrogen
atom

Back transfer of hydrogen atom i.e. photoracemisation can be quenched by the
addition of 1,3-cyclopentadiene. This quenching experiment confirmed the
formation of triplet state.
Participation of a 1,4 diradical intermediate in the Norrish Type-II reaction has
been confirmed by trapping experiments and spectroscopic techniques.
Formation of 1,4-diradical has also been proven chemically. Photoracemisation
of a ketone with a γ chiral carbon atom and loss of the chirality in the product
was observed.
O
R
β
α
H
γ
C2H5
CH3 OH
R
°
°
C2H5
CH3 OH
R
°
°
C2H5
CH3

O
R1
β
α
H
γ
R2
R3
T1 (n -π*)
OH
R1
°
°
R2
R3 OH
R1
°
°
R3
R2
O
R1
β
α
H
γ
R3
R2
Reversal of
Hydrogen Transfer
R1
OH R2
R3
R1
OH R2
R3
+
+
OH
R3
+

The γ hydrogen transfer to the oxygen atom has been shown to be
intramolecular. The transfer involves a six membered cyclic transition
state . 5,5 dideuterohexan-2-one on irradiation gives 2-
deuteropropene and 1-deuteroacetone. Formation of these products
confirm the transfer of hydrogen from γ carbon and is intramolecular.
O
Ph
β
α
H
γ
CH3
H
O
β
α
D
γ
CH3
D +
CH3-C=CH2
OD
CH3-C-CH2D
O
+
CH3-C=CH2
D
+
Ph-C=CH2
OH
+
CH3-CH=CH2

• When a molecule has two γ carbons both having hydrogens,
transfer of hydrogen in the Norrish Type II process is marked by
the preference of cleavage for the weaker carbon-hydrogen bond
as in case of ketone.
CH-CH2-C-CH3
O
γ1CH3
CH3
CH3 γ2CH
CH2=CH-CH
CH3
CH3
C=CH-CH3
CH3
CH3
+
CH3-C-CH3
O
+

Intramolecular hydrogen abstraction is not possible if γ carbon has no
hydrogen.
For alkyl aryl ketones the electrons donating groups such as p-methyl
and p-methoxy substituents decreases the rate and quantum yield
for Norrish Type II cleavage. Following this trend p-hydroxy , p-amino,
and p- phenyl substituents inhibit the reaction completely. This
because in such cases energy for π-π* excitation is less than that for
n-π*excitation.
The rate of radical recombination to give cyclobutanols compared
with α, β- bond cleavage is often dependent on α-substitution.

Ar-C-C-CH-CH3
OR1
R2
Ar-C-C-R1
OH
R2
+ CH2=CH2
R2
R1
Ar
OH
Substituents Cyclisation
R1=R2=H 10%
R1=H, R2= CH3 29%
R1=CH3, R2=CH3 89%
Substituents at α position favors Cyclisation reaction

On the other hand substituents at β position favors Norrish type II
reaction. Thus ketone mainly gives Norrish Type II reaction i.e.
elimination reaction . The rate of radical combination to give
cyclobutanol compared to β cleavage also depends upon the stability
of the radicals. If 1,4 diradical is stable then this favors cyclisation
reaction and if 1,4 diradical is unstable then favors Norrish Type II
reaction.
Ar-C-CH2C-CH3
O CH3
CH3
Ar-C C-CH3
OH °CH2
CH3
°
Ar-C=CH2
OH
+ C=CH2
CH3
CH3

• The singlet state photoelimination reaction occurs with high degree
of stereospecificity in threo and erythro form of ketone. Threo form
gives trans product and erythron form gives cis product . This
elimination is syn elimination.
O
H
CH3
COOCH3
H
CH3
C=O
CH3
CH3
+
H CH3
COOCH3
CH3
Trans Product

OH
H
CH3
COOCH3
°
CH3
C=OH
CH3
CH2
+
H CH3
COOCH3
CH3
Trans Product
°

O
H
CH3
H3COOC
H
CH3
C=O
CH3
CH3
+
H CH3
CH3
H3COOC
Cis Product
• Generally the yield of cyclisation product are only 10-20% of those
of elimination in normal ketones. But for certain compounds
cyclisation is very efficient. Some of these systems where there are
steric factors promoting cyclisation as with cyclodocecanone.

Norborane also gives only cyclisation because alkene product
is highly strained
hν, Pyrex
77% yield

Hydrogen Abstraction from Other Sites
• The normal path of Norrish Type II involves a six membered
cyclic transition state, alternate pathways are also common. In
these cases either larger or small transition states provide
diradical which can not fragment. Alternative routes are also
possible when γ carbon has no hydrogen or biradical is
stabilized by hetero atom.

β-hydrogen Abstraction
• β-hydrogen Abstraction can only be possible if substrate has no γ-
hydrogen and the biradical is stabilized by the hetero atom.
C°
OH CH3
°CH
CH3
CH3
NH
C
C
C
O CH3
CH2
CH3
CH3
NH
OH
CH3
CH3
CH3
NH

CH2
C
O CH2
CH2
CH2
C N
O
C6H5
C6H5
CH2
C°
OH CH2
°CH
CH2
C N
O
C6H5
C6H5
C6H5
OH
N
C6H5
O
C
CH2 C6H5
CH2
C6H5

δ – and ε- Hydrogen Abastraction
• Intramolecular γ hydrogen abstraction by an excited carbonyl
oxygen is approximately 20 times faster than δ – and ε-
hydrogen abstraction.
• Nevertheless, the tendency of the half occupied n-orbital of
the carbonyl oxygen to abstract a hydrogen atom from other
positions in considerable.
• A necessary condition is the planarity of the transition state.
An important example of this class is cyclodecanone. In this
hydrogen abstraction takes place from C-6

There are other systems where hydrogen abstraction through a six-
membered of cyclic transition state is not possible because there is
no suitably placed hydrogen atom, 1-(8-benzylnaphthyl)- phenyl
ketone is such a compound
O
1
6
hν
Quartz, Hydrogen
abstraction from ε Position
OH
Ph
Ph-C=O
hν 366nm
Hydrogen abstraction from
δ Position
Ph
Ph
OH

Hydrogen abstraction from distant site
• In the photolysis of alkyl esters of benzophenone-4- carboxylic
acid, the planar transition state is possible only for > 1,8
biradical whose recombination produces a paracyclophane
O
R-CH2-(CH2)n
Ph-C
O
C
O
R-°CH-(CH2)n
Ph-°C
O
C
O
OH

R-°CH-(CH2)n
Ph-°C
O
C
O
OH
R-CH-(CH2)n
Ph- C
O
C
O
OH

Complete the following reaction:
(i)
(ii)
O
Ph
hν
O
hν
Ph
OH Ph
OH
O
Ph

Formation of Photoenols or Photoenolisation
The formation of photoenols arises by Norrish type II process in ortho
substituted aryl ketones and are produced by the following
mechanism:
C-Ph
CH3
O
ISC
S1
hν
T1
C-Ph
CH2
OH
Spin
Inversion
C°-Ph
CH2°
OH

C°-Ph
CH2°
OH
C-Ph
CH2
OH
+
C-OH
CH2
Ph
E-isomer Z-isomer

Photoenolisation can be quenched by molecular oxygen. This
confirmed the formation of triplet biradical. The product of the
reaction behaves as diene for Diels-Alder addition. Thus photoenols
can be trapped by addition of dienophile.
C-Ph
CH3
O
C-OH
CH2
Ph
O
O
O

Intermolecular Hydrogen Transfer: Intermolecular
Photo Reduction
• Photo reduction is one of the best known photo reaction of
carbonyl compounds.
• Initial step is the transfer of hydrogen atom to the oxygen atom
of the carbonyl excited state from a donor molecule which may
be a solvent, an added reagent or ground state molecule of
reactant.
• Ketone undergoes photoreduction in the presence of a variety
of hydrogen atom donors.
• Hydrogen atom donors are secondary alcohol, toluene and
cumene. This reaction is a bimolecular reaction.

(C6H5)2 C O S1 (n-π*)
ISC
T1 (n-π*)
(C6H5)2 °C O°
(C6H5)2 °C O° +
(CH3)2
C° OH
(C6H5)2 °C OH +
(CH3)2
CH OH

(CH3)2
C°
OH
(C6H5)2 C O
+ (C6H5)2 °C
OH
+ (CH3)2 C O
(C6H5)2 °C
OH
+ (CH3)2
C°
OH
(CH3)2
C
OH
(C6H5)2
C
OH

C6H5 C
O
C6H5 C6H5 °C
O°
C6H5
CH3OH
C6H5 °C
OH
C6H5 +
CH2OH
C6H5 C
OH
C6H5
This reaction takes place
when the concentration of
benzophenone is very low i.e
10-4 m
°CH2OH

O
(CH3)2 C O
OH OH
+
O
C H
COOH
(CH3)2 CH OH
O°
C° H
COOH

O°
C° H
COOH
(CH3)2 CH OH
OH
C° H
COOH
(CH3)2 C° OH
+
OH
CH
COOH
OH
CH
COOH
O
CH
C
O
CH
C
O O

N
hν
CH3OH
N
°
°
CH3 OH
H
N
°
+ °CH2 OH
H
N
H
N

Photocycloaddition Reaction (Paterno-Buchi Reaction)
• Photocycloaddition of ketones and aldehydes with alkenes
can result in formation of four membered cyclic ethers
(oxetanes), a process often referred to as the Paterno-Buchi
reaction.
• Benzophenone with isobutene gives a high yield of oxetane
C6H5 C O
C6H5
+
CH2 C CH3
CH3
O
C6H5
C6H5 CH3
CH3

• Paterno Buchi can be studied under two categories depending upon the
nature of alkene.
Addition to Electron Rich Alkene
Mechanistic pathway changes according to the type of carbonyl compound and
alkenes involved. Addition of simple aliphatic or aromatic ketones to electron
rich alkenes involves attack on ground state alkene by the n-π* triplet state of
the carbonyl compound in a non- concerted manner, giving rise to all the
possible isomers of oxetane.
The reaction is non-concerted because the reactive excited state is a triplet
state. The initial adduct of this reaction is a triplet 1,4 diradical which must
undergo spin inversion before product formation is complete. Stereospecificity
is lost, if the 1,4 diradical undergoes bond rotation faster than ring closure.

C6H5 C O
C6H5
S1 T1
ISC C6H5 C O
C6H5
Excited Triplet State
CH2 C CH3
CH3
CH2
C
CH3
CH3
C6H5 C O
C6H5
CH3
C
CH2
CH3
C6H5 C O
C6H5
+

Spin conversion
CH2
C
CH3
CH3
C6H5 C O
C6H5
CH3
C
CH2
CH3
C6H5 C O
C6H5
+
Spin conversion
CH2
C
CH3
CH3
C6H5 C O
C6H5 CH3
C
CH2
CH3
C6H5 C O
C6H5
O
C6H5
C6H5 CH3
CH3 O
C6H5
C6H5
CH3
CH3

• The reaction is stereospecific for at least some aliphatic
ketones but not for aromatic carbonyls. This result
suggests that the reactive excited state is a singlet for
aliphatics and a triplet for aromatics. With aromatic
ketones, the regioselectivity of addition can usually be
predicted on the basis of formation of the more stable of
the two possible diradical intermediates obtained by bond
formation between oxygen and the alkene.

• Stereochemistry can be interpreted in terms of
conformation effects in the 1,4-biradical intermediates.
Vinyl enol ethers and enamides add to aromatic ketones
to give 3-substituted oxetanes, usually with the cis isomer
preferred.

Although the reaction is not stereospecific, there is a preference for one
orientation of addition, which can be rationalized in terms of initial
attacks on the alkene by the oxygen atom of the excited carbonyl groups
to give a biradical intermediate.
The existence of biradical has been confirmed by picosecond
spectroscopy.
The more energetically stable of the two possible biradicals is formed
more readily. Thus reaction is regioselective. The biradical stability is
applicable to the major product of the reaction.
Two important rules for the successful synthesis of oxetanes:
(i) Only carbonyl with low lying n-π* state will form oxetanes.
(ii) The energy of the carbonyl excited state must be less than that of the
alkene to prevent energy transfer from the carbonyl excited state to
the alkene.

• In case of the addition of aromatic carbonyl compound, only the
triplet state is effective (due to highly efficient ISC) and
consequently a triplet biradical intermediate is produced.
• The reaction of alkyl ketones can be complicated by the less
efficient ISC thus permitting reaction from both singlet and triplet
state.
• The singlet state reaction is obtained at high conc. Of alkene, and
the triplet state at low conc. Of alkene.

Addition of Electron Deficient Alkenes:
Photocycloaddition of aliphatic ketones to electron deficient
alkenes, particularly dicyano-ethene, involves addition of singlet
state(n-π*) excited ketone to ground state alkene. The reaction is
stereospecific and the stereochemistry of the alkene is retained in
the product,
CH3 C O
CH3
+
CN
CN
O
CN
CH3
CH3
CN
hν
313nm

The course of the photocycloaddition of electron deficient alkenes
to ketones follows certain rules. While oxetanes are formed only
from S1 (n-π*) state, the T1 (n-π*) state stereo specifically sensitizes
the cis-trans isomerization of electron deficient alkenes and does
not lead to oxetanes.
CH3 C O
CH3
+
CN
CN
hν
313nm
O
CN
CH3
CH3
CN

CH3 C
O
CH3
T1 (n-π*)
CN
CN
CN
CN
CH3 C
O
CH3
+
CN
CN
CH3 C
O
CH3
CN
CN
+

• Stereo specificity of the oxetane formation with electron deficient
alkenes can be explained as follows:
• In case of electron deficient alkenes, oxetane formation takes place
via formation of exciplex. Exciplex formation takes place between
the singlet excited state of ketone and ground state of alkene.
Exciplex is stabilized by charge transfer as well as energy transfer
between the constituent molecules.
C° O°
C
C
C° O-
C+
C°
Exciplex

Charge transfer interaction in the formation of the carbonyl olefin exciplex
The exciplex ( excited state cyclic complex) has considerable charge transfer and the
stereospecific formation of the product is accounted for if both new bonds are formed
simultaneously in the complex or if the second is formed after the first at a rate faster than
the rate of bond rotation. There is again a preference for one orientation of addition, but this
is opposite to that expected on the basis of the most stable biradical intermediate. The
preference reflects the preferred orientation in the exciplex which is governed by charge
densities as shown:

Cδ- Oδ+ +
CN
δ-
δ+
O
CN
O
CN
Exciplex
n-π*

Oxetane formation with dienes and alkynes
• Addition of carbonyl compounds to conjugated dienes is also
feasible. The ET of dienes is usually less than that of carbonyl
compounds. However, the formation of oxetanes competes
successfully with excitation energy transfer because dienes quench
the T1 (n-π*) state. Thus formation of oxetanes occurs from the S1
(n-π*) state of the carbonyl compounds. Therefore, dienes give
stereospecific reactions with dienes. Examples are as follows:

C O +
C
O
C O +
O
C
H
H
O
C
H
H
+

(B) With Alkynes
Carbonyl compounds undergo photochemical cycloaddition reaction
to alkynes to give oxetenes which are not usually isolated but
isomerises to α,β unsaturated carbonyl compounds in a subsequent
thermal reaction.
C6H5 C O
H
+ CH3 C C CH3
hν, -78°C
C6H5 CH
O C CH3
C CH3
α,β unsaturated carbonyl compounds
Warm
C6H5 CH
O
C CH3
C
CH3
Oxetene

C6H5 C O
C6H5
CH3 C C OCH3
+ hν, C6H6
C6H5 C
O C OCH3
C CH3
C6H5
C6H5 C
O
C OCH3
C
CH3
C6H5
Warm
α,β unsaturated carbonyl compounds

Intramolecular Paterno Buchi Reaction
• It is mainly given by γ-δ enones. The efficiency of these reactions can
be attributed to the rapid rate of interaction between the excited CO
group and the ground C=C group. This combination of substrates
allow the formation of one regioisomer. Thus yield are high and there
is usually no byproduct. This reaction is highly efficient & versatile
method for the synthesis of a variety of compounds that are difficult
or impossible to be prepared by other methods.

CH2 CH
O
C CH3
CH2 CH2
CH2
CH3
O°
CH CH2
C°
CH2
CH2
CH3
O
°CH CH2
C°
CH2
CH2
CH3
O
CH CH2
C
CH2

O
Ph
°O
Ph
° O
Ph
O O°
°
O°
°
O
°
°
O
Pd-Al2O3
Azulene

[2+2] Cycloaddition reaction of Enones with Alkenes
The most useful reaction of α-β unsaturated enones is the [2+2]
photocycloaddition reaction with alkenes which affords cyclobutene
derivatives. A simplified mechanism is given below:
Alkene
+ Enone
[Enone]1
[Enone]3
Alkene
[Exciplex]3
Biradical
Product

The biradical can be formed by bonding at the α carbon or the β
carbon of the enone to the alkene. The reaction is stereoselective at
the fused junction. Cis fused 4/5, 4/6 and 5/5 systems are common
and are much more stable than their trans isomers. 5/6 can be cis or
trans. 6/6 can be cis or trans but prefers trans.
O
+
α bond
β bond
O
°
°
O
°
°

O H
H
+
O
O
°
°
The reaction is regioselective with respect to unsymmetrical alkenes. Electron rich
alkenes give head to tail adduct whereas electron deficient alkenes form head to head
adduct
O
+
OMe
O
OMe
H
H

Photorearrangements of β,γ- unsaturated ketones:
β,γ- unsaturation in carbonyl compound promote α cleavage because of
allylic stabilization of the radical produced. β,γ- unsaturated ketones, in
addition to undergoing normal photochemical reactions of saturated
ketones, undergo rearrangement reaction.
Their irradiation induces non-concerted sigmatropic reactions that are
directed by the electronic configuration and multiplicity of excited state.
The rearrangement are 1,2 acyl shifts (oxa-di-π methane rearrangements)
which occur from the lower triplet state of n-π* transition and 1,3 acyl
shift which occurs from the S1 or T2 (n- π* ) state with the formation of
an acyl alkyl radical pair. It has been suggested that while at higher
temperature the radical pair is formed predominantly from the S1 state,
at lower temperature the T2 state is involved. Although both
rearrangements involve biradical or biradical pair intermediates, they are
generally stereospecific.

1,2-Acyl Shift (Oxa-di- π Methane Rearrangements)
Mechanistic studies of this rearrangement have shown that the
excited state is low-lying π-π* triplet state. Because intersystem
crossing probability is low in β,γ- unsaturation ketones., the triplet
state reaction in general be initiated only by sensitizer.
Rearrangement also takes place in the absence of sensitises in those
cases where ISC is efficient.
This rearrangement affords cyclopropane derivatives and referred to
as oxa-di-π methane rearrangement.
The mechanism is similar to di-π methane rearrangement.

• 1,2 acyl migration does not involve α fission, but a stepwise process
is involved where the acyl group migrates to yield a biradical
intermediate. In this rearrangement migration takes place by
formation of 1,4 biradical which than converts into 1,3 biradical due
to 1,2-shift.
3
4
1
2
5
O
Ph
Ph
Ph
R R
hν
Sens
π-π*
S1
ISC
T1 3 °
4 °
1
2
5
O
Ph
Ph
Ph
R R

2 °
1 °
4
3
O
Ph
Ph
Ph
R R
2
1 °
4
3
° 5
O
Ph
Ph
Ph
R R
1,4 biradical
2
1 °
4
° 3
5
O
Ph
Ph
Ph
R R
2
1
4
3
5
O
Ph
Ph
Ph
R
R

The product of the rearrangement is cyclopropyl alkyl (or aryl)
ketone. Acyl group migrates from position 1 to position 2 on the
alkenyl chain in this rearrangement. Thus, rearrangement is 1,2-acyl
shift.
C6H5
C6H5 3C
O
C Ph
2CH 1C
R
R
hν
2
3
1
O
Ph
Ph
Ph
R
R

Product formation of the rearrangement in short can be represented
as follows (not based on the mechanism)
R
R C
O
C
CH3
CH CH2
hν
R
R °C
CH
COCH3
°CH2
R C
CH
COCH3
CH2

1,3 Acyl Shift
β,γ- unsaturated ketones on direct irradiation gives 1,3-acyl shift.
This shift takes place by formation of acyl radical and allyl radical due
to α cleavage of the substrate.
1,3 acyl shift occurs from S1 or T2 state of n-π* transition. It has
been suggested that, while at higher temperature the radical pair is
formed predominantly from the S1 state, at lower temperature the
T2 state is involved. Although intermediate is radical pair but the
rearrangement is generally stereospecific.

CH3 1C
O
CH 3CH2
1C
R
R
hν
S1 or T2
CH3
°C
O
CH CH2
°C
R
R
+
CH3 °C
O
CH °CH2
C
R
R
+
Recombination
CH CH2
C
R
R
COCH3

1,3 acyl shift in short can be represented as follows:
In this rearrangement acyl group migrates from position 1 to position
3 on the alkenyl chain.
Due to this reason, the rearrangement is known as 1,3-acyl shift, 1,3
acyl shift leads to migration.
R CH CH2
C
R
COCH3
CH CH2
C
R
R
COCH3

2-cyclopentyl methyl ketone were the first examples of the
rearrangement of β,γ-enones. Photo CIDNP and radical trapping
experiments indicate that the 1,3 acyl shift is a radical process that
proceeds predominantly via cage radical pair which also leads to α-
cleavage products.
O
R1
R1
C CH3
R1
R1
°
CH3 C°
O
+

R1
R1
°
Recombination
R1
R1
COCH3
CD3
C=O
hν
CD3
COCH3
2-Cyclopentylmethyl
ketones give 1,2-acyl
shift in the presence
of triplet sensitiser

R2
C=O
R1
hν, Sens
R2
° C=O
R1
°
CH3
R2
COCH3
R1
°
°
R2
COCH3
R1

Similarly bicyclo[2,2,2] octenone gives 1,3 acyl shift by direct
irradiation. In the presence of triplet sensitizer it gives 1,2 acyl shift.
O
hν
°
° O
°6
1
2
3
4
5
Recombination
2
1
3 °
O
°

Cyclic β,γ- unsaturated ketones undergo both 1,3 and 1,2 acyl shifts.
1,2 acyl shift leads to cyclopropane derivative while a 1,3 acyl shift
leads to isomerization.
Ring contraction and/or isomerization is main character of 1,3 acyl
shift.
Bicyclic ketones give isomerization by 1,3 acyl shift
O
°
O
°
°
O
°
1
2
3
4
°
O
3
2
1
4
°
O
Bicyclo[3,2,0]-hept-6-en-2-one

• On the other hand, Cycloocta-3-enone gives isomerization as well
as ring contraction reaction
O °
O
°
°
O
°
O
O

• 1,3 shift mainly results in isomerisation and/or ring contraction.
Sometimes it can also result in ring expansion.
O
R
O
R
°
°
O
R
°
°
O
R
O
R

Photochemistry of Carbonyl Compounds Explained

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Photochemistry of Carbonyl Compounds Explained

Similar to Photochemistry of Carbonyl Compounds Explained (20)

Recently uploaded

Recently uploaded (20)

Photochemistry of Carbonyl Compounds Explained