Photochemical Dimerization Reactions
Dimerization brought about by the action of light; formation of, or
conversion into, a photodimer known as photodimerization.
When thymine is exposed to ultraviolet light, it undergoes [2+2]-
dimerization with a suitably oriented nearby thymine, as shown above.
1 2 3
1
When this happens toxic products are generated that are directly
responsible for cell death via mutagenic action, suppression of DNA
transformation and/or activation of carcinogenic pathways.
A natural repair mechanism employing a photolyase enzyme system
reverses the dimerization.
(A) [4 + 4] Photodimerization of anthracene derivatives.
4 5
2
Photochemical Cyclization Reactions
Irradiation of o-alkynylaryl isocyanides and organic dichalcogenides
such as diselenides or ditellurides with light of appropriate wavelength,
the intramolecular cyclization of the isocyanides takes place to afford the
corresponding 2,4-bischalcogenated quinolines selectively.
The photochemical cyclization of 2-(phenylethynyl)phenyl isocyanide
could also proceed in the presence of hydrogen transfer reagents such as
tris(trimethylsilyl)silane, tributylgermyl hydride, alkanethiols, and
benzeneselenol, providing the corresponding 3-phenylquinoline as the
result of 2,4-dihydrogenation.
3
8
6
7
4
A sensitizer is needed in addition to direct irradiation.
The reason is that a π→π∗ singlet excited state (S1) produced by direct irradiation of an
alkene or arene crosses over to the triplet state (T1) inefficiently (compared to n→π∗excitation of ketones).
Also, the S1 state leads to other reactions beside isomerization which, in the case of 1,2-
diphenylethene and other conjugated hydrocarbons, produce cyclic products.
For example, cis-1,2-diphenylethene irradiated in the presence of oxygen gives
phenanthrene by the sequence of Equation below.
The primary photoreaction is cyclization to a dihydrophenanthrene intermediate 11, which,
in the presence of oxygen, is converted to phenanthrene:
5
910
1112
6
Photo-oxidation reactions
Photo-oxidation reactions is either classified as type I or type I1 reactions.
In the former, the action of light produces radicals which subsequently react
with oxygen:
Type I
The type I1 reaction involves singlet oxygen (usually 1ΔgO2) as the intermediate.
This species is readily produced by energy transfer from an excited state (usually
a triplet state) to oxygen:
Type II
7
where S0 and S1 are the singlet ground and first excited states respectively and
TI is the lowest triplet state; ISC denotes intersystem crossing.
Recently, accumulation of evidence suggests that the superoxide radical anion is
an intermediate in some photo-oxidation reactions.
The known chemical and biological activity of this materials is an added
motivation to discover how widespread such reactions might be.
i Fission of carbon-halogen bonds
Phenols have also been detected as products when chlorinated
biphenyls were irradiated in the presence of oxygen. 8
ii. Photolysis of azo-compounds
In this case, direct photolysis produces a singlet biradical which ring-closes
despite oxygen being present at a high concentration.
Production of a triplet biradical by use of a triplet sensitiser enables the
biradical to be intercepted.9
iii. Photolysis of diazo-compounds
Diazo-compounds, as derivatives of diazomethane, are readily photolysed to
give carbenes. The carbenes react with oxygen to give a species which may
react either as a zwitterion or as a biradical.
10
iv. Electron ejection
Irradiation of many carboxylates leads to decarboxylation, as in the case of 1-
naphthylacetates and (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T).
(Np= 1-naphthyl)
It should be noted that such decarboxylation reactions can be sensitised by a
wide range of compounds including heterocyclic and carbonyl compounds.
11
v. Intramolecular atom abstraction reactions
Biradicals, generated by intramolecular hydrogen atom abstraction reactions
can be intercepted by oxygen.
The efficiency of interception depends upon the availability and efficiency of
the various reaction pathways of the biradical and the oxygen concentration
12
13
Photochemical Reduction Reactions
Photoreduction of Diaryl Ketones
Diaryl ketones do not undergo photodissociation in the same way as alkyl
ketones, probably because cleavage to phenyl and other aryl radicals is
unfavorable.
Nevertheless, aromatic ketones are photochemically reactive in the
presence of compounds that can donate a hydrogen atom, with the result
that the carbonyl group is reduced.
Indeed, one of the classic photochemical reactions of organic chemistry is
the formation of 1,1,2,2-tetraphenyl-1,2-ethanediol (3, benzopinacol) by the
action of light on a solution of diphenylmethanone (2, benzophenone) in
isopropyl alcohol. The yield is quantitative.14
The light is absorbed by 2 and the resulting activated ketone, 2∗, removes a
hydrogen from isopropyl alcohol:
15
Benzopinacol results from dimerization of the radicals, 4:
Since the quantum yields of 2-propanone and benzopinacol both are nearly
unity when the light intensity is not high, it is clear that two of the radicals, 4,
must be formed for each molecule of 2 that becomes activated by light.
This is possible if the 2-hydroxy-2-propyl radical formed reacts with 2 to
give a second diphenylhydroxymethyl radical:
16
The reaction is energetically favorable because of the greater possibility for delocalization of
the odd electron in 4 than in the 2-hydroxy-2-propyl radical.
Photochemical formation of 3 also can be achieved from diphenylmethanone, 2, and
diphenylmethanol, 5:
The mechanism is similar to that for isopropyl alcohol as the reducing agent:
17
This reduction is believed to involve the triplet state of 2 by the following
argument:
Formation of 3 is reasonably efficient even when the concentration of the
alcohol, 5 is low; therefore, whatever the excited state of the ketone, 2∗, that
accepts a hydrogen atom from 5, it must be a fairly long-lived one.
Because solutions of 2 show no visible fluorescence, they must be converted
rapidly to another state of longer life than the singlet (S1). The long-lived
state is then most reasonably a triplet state.
In fact, if naphthalene is added to the reaction mixture, formation of
benzopinacol, 3 is drastically inhibited because the benzophenone triplet
transfers energy to naphthalene more rapidly than it reacts with the alcohol, 5.18
References
28.3 Organic Photochemistry - Chemistry LibreTexts.htm
Mechanisms of Photo-oxidation Reactions" Robert S. Davidson
Neuronal mapping: a photooxidation reaction makes Lucifer yellow useful for electron
microscopy, A. R. Maranto Science. 1982 Sep 3;217(4563):953-5. doi:
10.1126/science.7112109.
19
