reaction in the presence of TEMPO and BHT, respectively).
Addition of the carbon-radical trap BrCCl322 slowed down the
reaction (quantitative conversion to methyl phenyl sulfide was
obtained in 6 h compared to the 3 h needed in the absence of
spin trap). These findings indicate the presence of free radicals in
the catalytic reaction, and suggest a radical-based mechanism.23
To the best of our knowledge, the only example of an iron
species catalysing the reduction of sulfoxides was recently
reported by Enthaler.24
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For a further mechanistic insight, the stoichiometric reac-
tion of 4 with AgBF4 was monitored by NMR spectroscopy in
C6D6 using a J Young-valve tube. However, we could not
observe any intermediate species. Probably, the insolubility of
the formed species in benzene hampers their observation.25
The reaction of 4 with 1 equivalent of AgBF4 was also
monitored by IR spectroscopy. The IR spectrum showed that
the nCO band corresponding to the CO ligand in complex 4
(at 1932 cmꢀ1) completely disappeared after 30 min of reaction.
The IR spectrum of the new species formed in the reaction did
not show any bands in the region 1700–2200 cmꢀ1, indicating
that under the reaction conditions, 4 loses the carbonyl ligand.26
In conclusion, we have set up a simpler protocol for the
synthesis of Fe(II) complexes bearing a cyclopentadienyl-
functionalised NHC ligand by direct reaction of the imidazolium
proligands and Fe3(CO)12. We have proved the catalytic
efficiency of these well-defined iron species in the deoxygena-
tion of sulfoxides. Excellent performance of the catalytic
system (Cp-NHC)Fe(CO)I/AgBF4 using PhSiH3 as a reducing
agent is demonstrated for both aliphatic and aromatic sub-
strates. The presence of radicals in the catalytic reactions has
been proved by radical trap experiments. Further investigation
into the mechanism of the hydrosilylation reaction and reac-
tivity of iron complexes containing the Cp-NHC fragment is
currently underway in our laboratories.
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methyl phenyl sulfide under similar reaction conditions (2 equiv.
PhSiH3, toluene, 100 1C).
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catalytic system composed of [Fe2(CO)9] and silane has been
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However, a radical based mechanism was excluded for this system
based on spin trap experiments performed with TEMPO.
25 Similar NMR experiments carried out in deuterated THF did not
allow us to identify any iron species.
This work was supported by FCT of Portugal, POCI 2010,
FEDER through project PTDC/QUI-QUI/110349/2009.
J.M.S.C. thanks FCT for grant SFRH/BD/66386/2009. We
thank FCT for REDE/1517/RMN/2005.
Notes and references
z Typical experimental procedure for the reduction of sulfoxides: all
operations were carried out under nitrogen. In a small flask, a mixture
of sulfoxide (1 mmol), silane (2 mmol), catalyst/AgBF4 (ratio 1 : 1,
1 mol%), toluene (2 mL) was stirred at 100 1C and the reaction was
monitored by gas chromatography.
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26 The cationic iron [CpFe(CO)(IMes)(NCMe)][PF6] complex has been
fully characterised by Guerchais and co-workers (see ref. 18a). Our
attempts to characterise the new species formed in the reaction of 4
with AgBF4 in THF or toluene failed. Based on the IR data, we
exclude the formation of a related cationic [(Cp-NHC)Fe(CO)][BF4]
species.
c
4946 Chem. Commun., 2012, 48, 4944–4946
This journal is The Royal Society of Chemistry 2012