CO2_2016 - page 44

Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
KEYWORDS: Catalysis, iron complexes, radicals, alkylation, reduction, methylation.
The development of iron complexes as catalysts in organic transformations is attractive for a number of
reasons. Iron is one of the most abundant metals in the earth’s crust, it is cheaper than the rare metals,
and its toxicity is low. As the cost of rare and precious metals will increase in the Future, the development of new synthetic strategies
based on iron are critical. In this brief summary, we discuss two innovative iron-catalysed C-C bond forming reactions through radicals,
generated by redox chemistry, or by photocatalysis. These works shine light on the possibility to promote radical transformations by
iron complexes that can be advantageously used as starting point to develop new methodologies for the formation of C-C bonds
with a cheap and environmental friendly approach.
Teach new tricks for an old dog:
cheap and abundant iron complexes for new catalytic
Due to the low cost, availability and low toxicity of iron, its
chemistry is now under active re-investigation as demonstrated
by the increasing number of publications in this field.(1) Since
the price of coinage metals is increasing, and their availability
will probably become problematic, the development of new
iron promoted reactions deserves particular attention and
could be a useful tool for setting up new transformations,
particularly on industrial scale. Although the iron promoted
cross-coupling reactions were developed before the discovery
of palladium catalysts,(2) the difficulties to understand,
optimize, and develop this chemistry, retarded their practical
uses for almost 20 years. More recently, thanks to the high level
of sophistication that organometallic chemistry, ligands design,
and computational tools have now reached, the quite
capricious iron promoted reactions started to be reinvestigated
and iron salts and complexes have been successfully used as
catalysts in many different transformations (1). In these
transformations, cycles in which radicals are formed seem to
play a decisive role. On the other hand, iron is a redox-active
metal and iron complexes can promote innate catalytic
radical cycle in which the iron behaves as a standard initiator,
“smart initiator”,(3) or as a redox catalyst. In other reactions,
suitable iron hydride precursors are suggested, and their radical
addition to unsaturated substrates can give rise to a radical
cycle. The ability of iron to initiate radical processes can be
advantageously used for new transformations. As the role
played by radicals in this chemistry, we want to highlight the
formation of radical species promoted by iron complexes in
two topics. First, we will illustrate the straightforward
functionalization methodologies for the synthesis of complex
molecules through putative radical species, generated by iron
complexes in the presence of hydride donors, that can be
intercepted by Michael acceptors or nitroso derivatives. In the
second topic, cheap and available iron complex in the
presence of light was used to generate radicals from electron
poor bromine derivatives. In this case, the electrophilic radical
species generated can be intercepted by electron rich olefin
derivatives such as enamines. Although the two processes
have in common radical intermediates, the reactions illustrated
in the two topics shows different mechanistic pictures; in the first
case, a hydrogen transfer radical addition is happening, while
a radical formation by electron transfer process is occurring in
the latter. These iron promoted processes should be considered
in developing and exploring new radical processes for
effective synthetic transformations.
During the total synthesis of Vinblastine and Vincristine, two
important bis-indole alkaloids widely used as antitumor drugs,
Boger reported an intriguing transformation based on the use
of FeCl
, O
and NaBH
,(4) that was able to introduce the OH
functionality in the complex precursor. The reaction, was
ultimately a “Fe-promoted hydrogen atom radical generation
*Corresponding author
ALMA MATER STUDIORUM, University of Bologna,
Dipartimento di Chimica “G. Ciamician”,
Via Selmi 2, 40126 Bologna, Italy
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