Angewandte
Chemie
DOI: 10.1002/anie.200905025
Iron-Catalyzed Reduction
Hydrosilane Reduction of Tertiary Carboxamides by Iron Carbonyl
Catalysts**
Yusuke Sunada, Hiroko Kawakami, Tsuyoshi Imaoka, Yukihiro Motoyama, and
Hideo Nagashima*
Iron and silicon are two of the most popular elements used for
effecting the catalytic transformations of organic molecules,
owing to their high natural abundance, low cost, and low
toxicity.[1,2] We have previously reported a series of catalytic
systems for the reduction of carboxamides to amines using
hydrosilanes. The most active catalyst we have reported thus
far is [(m3,h2,h3,h5-acenaphthylene)Ru3(CO)12],[3] which, in
À
Scheme 1. The iron-catalyzed reduction of carboxamides under either
thermal or photoassisted conditions (n=1, m=5, or n=3, m=12).
tandem with hydrosilanes that have two proximal Si H
groups (typically 1,1,3,3-tetramethyldisiloxane (TMDS) or
1,2-bis(dimethylsilyl)ethane), is able to successfully reduce
secondary and tertiary carboxamides to their corresponding
amines in high yields and under mild conditions. Commer-
cially available platinum compounds were thought to be
inactive for the silane reduction of carbonyl compounds prior
PMHS is accompanied by concomitant absorption of the iron
species into the insoluble silicon resin, also formed during the
reaction. Furthermore, in the presence of a nitro substituent
on the substrate, selective reduction of the nitro moiety was
observed, with the amide group remaining intact; this has not
been achieved by either the ruthenium catalysts or the
platinum catalysts described earlier.
When N,N-dimethyldihydrocinnamamide (1a) was
treated with TMDS in the presence of either [Fe(CO)5] or
[Fe3(CO)12] (10 mol%) in toluene at 1008C, the color of the
initial solution (yellow for [Fe(CO)5] and dark green for
[Fe3(CO)12]) gradually became purple within the first 30 min
and turned dark brown after 1 h. After 24 h, the iron residues
were removed, and 1H NMR spectroscopy of the crude
material revealed that 1a had been completely consumed
and the desired N,N-dimethyl-3-phenylpropylamine (2a) was
formed as a single product. Removal of the silicone waste
from the crude product afforded 2a in good yields (86% for
[Fe(CO)5] and 85% for [Fe3(CO)12]). In sharp contrast, less
than 5% of 2a was formed when other iron catalysts, such as
FeCl2, FeCl3, [FeCl2(PPh3)2], and [Fe(acac)2] were used, or
À
to our previous work, which used the two Si H groups of
TMDS or 1,2-bis(dimethylsilyl)benzene to successfully effect
this transformation.[4] Astonishingly, the reduction of carbox-
amides using poly(methylhydrosiloxane) (PMHS) in the
presence of the ruthenium or platinum catalysts described
above is accompanied by formation of an insoluble poly-
(siloxane) gel, into which all of the metallic species is
absorbed; the expensive ruthenium and platinum can be
recovered from the silicone resin once the reaction has
finished.[3c,d,4] The absorption and removal of metallic resi-
dues makes these processes particularly environmentally
friendly; nevertheless, the final goal for green processes
remains the replacement of noble metal catalysts, such as
ruthenium and platinum complexes, with iron compounds.[5,6]
Herein, we wish to report that two iron complexes, [Fe(CO)5]
and [Fe3(CO)12], can both act as the catalyst for the reduction
of tertiary carboxamides to their corresponding amines, using
TMDS as a reducing reagent (Scheme 1).
Although the thermal iron-catalyzed process requires a
higher reaction temperatures than those catalyzed by the
ruthenium or platinum catalysts, the reaction also proceeds
photolytically at ambient temperature. In both the thermal
and photoassisted reactions, the reduction reaction involving
À
with other hydrosilanes containing only one Si H group, such
as PhMe2SiH, (EtO)3SiH, and SiMe3OSiMe2H, were used as
the reducing agent under the same conditions (see the
Supporting Information). Although alternative solvents,
such as benzene, tetrahydropyran, and cyclohexane, were
successfully used in the [Fe(CO)5]- and [Fe3(CO)12]-catalyzed
reactions at 1008C,[7] lowering the reaction temperature to
808C resulted in the yield of 2a falling to below 20%. It is
noteworthy that high temperatures (> 1008C) are not
required when the reaction is carried out under irradiation.
Photoassisted reduction of 1a with TMDS in the presence of
[Fe(CO)5] or [Fe3(CO)12] under irradiation with a 400 W high-
pressure mercury lamp for 9 h afforded 2a in 94% yield and
73% yield, respectively.
[*] Dr. Y. Sunada, H. Kawakami, T. Imaoka, Dr. Y. Motoyama,
Prof. Dr. H. Nagashima
Institute for Materials Chemistry and Engineering
Graduate School of Engineering Sciences, Kyushu University
Kasuga, Fukuoka 816-8580 (Japan)
Fax: (+81)92-583-7819
E-mail: nagasima@cm.kyushu-u.ac.jp
[**] This work was supported by Grant-in-Aid for Science Research on
Priority Areas (No. 18064014, Synergy of Elements) from Ministry of
Education, Culture, Sports, Science and Technology (Japan).
We then examined the scope of the thermal (Table 1) and
photoassisted (Table 2) reductions using a variety of tertiary
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 9511 –9514
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9511