CO2_2016 - page 46

44
Chimica Oggi - Chemistry Today
- vol. 34(2) March/April 2016
Also in these fields rare, expensive, and toxic, ruthenium
polypyridine complexes were widely used. But just going up in
the periodic table we can encounter iron, that forms Fe(II)–
polypyridine complexes structurally quite similar to those of
Ru(II). However, there is a crucial difference concerning the
excited state lifetimes. The [Ru(bpy)
3
]
2+
complexes commonly
used in organic transformation, and quite employed in solar
cell devices, tend to have excited state lifetimes in the
microsecond range whereas, Fe(II)–polypyridines complexes
exhibit very short-lived excited-states (<100 fs), that make them
not good candidates for dynamic electron-transfer processes.
The origin of this unfavourable behaviour of Fe(II)–polypyridines
complexes is well understood,(17) and is due by the
deactivation of
1
MLCT and
3
MLCT(18) photoactive excited
states by intersystem crossing into low-lying, metal-centred
ligand-field states. As a pioneering study by Ferrere and Gregg
demonstrated the possibility of Fe(II)–polypyridines complexes
to perform an ultrafast electron injection into TiO
2
,(19) we
wondered if [Fe(bpy)
3
]
2+
could replace the costly
effective ruthenium in
stereoselective
photocatalytic reactions.
Now, we have recently
demonstrated, that the
combination of 2.5 mol%
of [Fe(bpy)
3
]Br
2
and
visible light can effectively
replace [Ru(bpy)
3
]
2+
or
other photosensitizers in
practical and effective
organocatalytic
stereoselective
transformations (20).
In the reaction illustrated
in the Scheme 3,c [Fe(bpy)
3
]Br
2
, used in catalytic amount (2.5
mol%), was a compelling and effective photosensitizer for
promoting the reaction between aldehydes (1) and bromo
derivatives (2), in the presence 20 mol% of the organocatalyst 3
and upon irradiation with visible light. Isolated yields and
enantiomeric excesses obtained were comparable to the
reaction promoted by [Ru(bpy)
3
]
2+
. As in the case of ruthenium
the reaction tolerates different functional groups on the
aldehyde partner, and different bromo derivatives, as for
example
α
-bromo ketones, could be employed. Similarly to
ruthenium, the limitation of this chemistry is due to the reduction
potential of the bromoderivatives. Indeed bromoesters, that
are harder to reduce compared to bromoketones or
malonates, gave lower yields. However, by the introduction of
electron-withdrawing groups in the ester moiety, such as
trifuoroethyl group (10), the corresponding bromo derivatives
afforded satisfactory yields. This methodology was directly
applied to the preparation of interesting lactones, key
intermediates for the synthesis of biologically active
compounds (Scheme 3, d).
As [Fe(bpy)
3
]Br
2
is not emitting, and the lifetime of the excited
states is quite short, the mechanism that allows the radical cycle
to start is quite intriguing. Understanding this matter is crucial to
design new effective “smart initiators” for an innate radical
cycle. Some peculiar experiments shows that the reaction does
not proceed in the absence of the iron photosensitizer by
irradiation with visible light (
λ
> 420 nm), also using a powerful
light sources (500 W). By EPR we were able to trap the malonyl
derivative, and to generate a
a
-radical species from an olefin,
a new reaction for the C-N bond formation can be established.
The radical specie recombines with the nitroso derivative, to
give the stable hydroxylamine that are
in situ
reduced by Fe(II)
specie or with the admission of zinc metal, to give the desired
amine. Using readily available and quite inexpensive precursors
as olefins and nitro(hetero)arenes, the protocol described by
Baran gave secondary amines by a formal hydroamination of
alkenes through an iron mediated radical reaction. Employing
this protocol a large variety of amines was successfully
obtained and, thanks to the nature of the process, the synthesis
of hindered amines is also possible (Scheme 2, b) (10). The
radical nature of these processes was quite established, but still
limitations were found, in particular, the use of alcohols as
reaction media. Quite recently Shenvi and co-workers
discovered that phenylsilane is not stable in the reaction media
and it is transformed in a more reactive and efficient mono
alkoxysilane, that is the kinetically preferred reductant in the
metal-catalysed radical
hydrofunctionalization
reactions reported (11). By
studying carefully the iron-
catalysed radical
reactions developed by
Baran, they discovered
that isopropoxy(phenyl)
silane is a more efficient
stoichiometric reductant
compared to
phenylsilane. Using this
silane derivative it is now
possible to significantly
decrease catalyst
loadings, reaction
temperatures, and use less
polar solvents (such as hexane) improving yields compared to
the original conditions.
IRON PHOTOCATALYSIS FOR STEREOSELECTIVE ALKYLATION OF
ALDEHYDES
Asymmetric catalysis promoted by visible light, a sustainable
and economical source of energy, is emerging as an active
new field of investigation (12). In 2008, MacMillan’s group
reported the first example of merging visible light induced
photoredox catalysis with asymmetric organocatalysis by using
[Ru(bpy)
3
]
2+
as photosensitizer (Scheme 3, a) (13). In the key
step an electron poor radical was generated by a photoredox
event involving ruthenium complex, and it was intercept by the
chiral enamine formed
in situ
from secondary amine and
aldehyde. The [Ru(bpy)
3
]
2+
could be replaced by other
photosensitizers as organic dyes (14) or semiconductors (15).
Clearly, a limitation of this methodology lies in the employment
of rare and expensive ruthenium and iridium complexes. The
discovery and employment of alternative metal complexes,
derived from earth-abundant, first-row transition metals, for
photoinduced organic transformations would be a major
advance in the area of photocatalysis. Particularly, due to the
large availability and abundance, the use of iron(II) complexes
would be in fact quite attractive for photocatalytic
stereoselective reactions. The use of iron complexes in photo
induced transformations will be also quite crucial for the quest
of high-energy chemicals and energy, using sunlight (16).
Scheme 2.
a) Mechanistic proposal for the reaction of iron induced radicals with
nitroderivatives; b) Examples of molecules accessible with the synthetic procedure.
1...,36,37,38,39,40,41,42,43,44,45 47,48,49,50,51,52,53,54,55,56,...68
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