CO2_2016 - page 47

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Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
from environmentally,
costly, and efficiently
point of views.
REFERENCES
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For reviews see: (a)
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Paih J., Zani, L., Chem.
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(a) Tamura M., Kochi J.,
Synthesis, 303–305
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radical formed in the
presence of [Fe(bpy)
3
]Br
2
using
N
-tertbutyl nitrone
(PNB) as radical trap. In
fact, from the values
obtained by EPR, the
typical adduct between
PNB and alkyl radicals
carrying carbonyl groups
in
α
-position was revealed.
Other experiments carried
out with femtosecond
laser absorption
spectroscopy were
performed on the iron
complex, but we were
unable to clearly indicate
the mechanism of a
photoinduced electron
transfer. Although quite
difficult to observe,
[Fe(bpy)
3
]Br
2
is able to
initiate a very efficient
radical chain reaction
process, comparable to other photocatalytic processes.
Differently to the mechanism illustrated in Scheme 3,a, the
formation of radical species is probably induced directly by the
better reducing ability of the photoexcited *[Fe(bpy)
3
]Br
2
.
However, the ET event that is closing the catalytic cycle could
be different from the case in which [Ru(bpy)
3
]
2+
is employed.
Probably, the direct reduction of the bromoderivative by the
amidoalkyl radical is occurring. In other words, the reaction is
proceeding via a radical chain mechanism, where the quite
inefficient and rare initiating event, determined by iron, is serving
only to keep up to a good level the presence of radical species.
This radical chain behaviour can be clearly appreciated when
the light is turned off and on. Contrary to other photocatalyzed
reactions, in which when the light is turned off the reaction is
completely shuttled down, in our case we can only reduce the
reaction rate, and the reaction is still continuing, even if with
minor efficiency, in the absence of light. The radical chain
reactions promoted by iron complexes are also well studied in
ATRA and ATRC polymerization reactions (21).
CONCLUSION
To conclude, the development of radical processes
initiated by iron salts or its complexes can be
advantageously used in accessing interesting products and
transformations. We have briefly introduced iron mediated
processes that can lead to C-C bond formation. Iron
photoinitiated radical processes are also particularly
intriguing, as iron was not considered a metal able to be
used as photosensitizer. In these new radical
transformations, the advantage to use cheap and relative
low toxic initiators in small amount, can be quite easily
translated in practical synthetic procedures, that have a
number of advantages over established ones. These new
C-C bond forming radical procedures can be performed
with quite straightforwardly synthesized iron complexes. The
scale-up of these and related procedure, and the
development of other synthetic transformations based on
iron, will be crucial for solving difficult synthetic problems,
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Scheme 3
.
a) Mechanism of a photocatalyzed reaction promoted by
[Ru(bpy)
3
]
2+
; b) Mechanism in the generation of radical with [Fe(bpy)
3
]
2+
; c)
Scope and reactivity photoinduced [Fe(bpy)
3
]Br
2
reaction with different bromo
derivatives; d) Application of the reaction in the synthesis of a natural products.
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