DOI: 10.1002/cssc.201100585
Amine Synthesis through Mild Catalytic Hydrosilylation of Imines using
Polymethylhydroxysiloxane and [RuCl2(arene)]2 Catalysts
Bin Li, Jean-Baptiste Sortais, Christophe Darcel,* and Pierre H. Dixneuf[a]
Dedicated to Prof. Hubert Le Bozec, a pioneer in the field of metal complexes for nonlinear optics, on the occasion of his 60th birthday
The development of synthetic strategies toward amines under
mild and green conditions is of interest in many areas, such as
the production of natural products, agrochemicals, pharma-
ceuticals, and medicinal compounds. Reductive amination of
carbonyl derivatives in the presence of primary amines,
through sequential condensation/catalytic reduction,[1] consti-
tutes a basic method for producing substituted amines, but it
often requires drastic reaction conditions. Tremendous prog-
ress in amine synthesis has recently been achieved using tran-
sition-metal catalyzed CÀN bond formation, in particular in the
field of Pd-catalyzed Buchwald–Hartwig[2] and Cu-catalyzed
Ullman reactions.[3]
sis point of view. Notably, PMHS has recently been used in the
catalytic reduction of imides into amines.[20]
Herein, we report that: i) the easily available [RuCl2(p-
cymene)]2 complex[21] is an efficient catalyst for the hydrosilyla-
tion of imines using the stable and safe hydrosilane reagent
PMHS, and ii) the reaction takes place simply in ethanol, under
air, at room temperature, and without further basic desilylation
step, leading to a variety of functional amines arising from aldi-
mines and ketimines.
The search for a catalytic system allowing the hydrosilylation
of imine 1a directly into amine 2a with Ph2SiH2 and PMHS
under mild conditions was undertaken. Our successful results
with ruthenium(II)-based catalysts are displayed in Table 1.
Firstly, it was shown that [Ru(OAc)2(p-cymene)] in the pres-
ence of Ph2SiH2 was not effective for the hydrosilylation of
imine 1a, whereas [RuCl2(p-cymene)]2 appeared to be an effi-
cient catalyst in diethyl ether (Table 1, entries 1 and 2); howev-
er, desilylation with NaOH/MeOH was still required. When the
reaction was performed in methanol, ethanol, or more impor-
tantly in ethanol under air using [RuCl2(p-cymene)]2 (2.5 mol%),
the reaction was complete after 0.5 h at room temperature,
and directly gave the amine (Table 1, entries 3–5). As PMHS is a
less expensive and greener hydrosilylation reagent than
Ph2SiH2,[8] it was then evaluated under closely related condi-
tions. The reaction required 2 h at room temperature to reach
completion (Table 1, entries 6 and 7). The amount of catalyst
could be decreased to 1 mol% in ethanol under air to reach
complete conversion and amine 2a was obtained in 90% iso-
lated yield (Table 1, entry 8). However, a shorter reaction time
and/or decreasing the amount of catalyst (0.5 mol%) led to
lower conversions (Table 1, entries 9–11). Other catalysts, such
as [Ru(OAc)2(p-cymene)] or [RuCl2(PMe3)(p-cymene)], were inef-
fective for this reaction (Table 1, entries 12 and 13) in the pres-
ence of PMHS.
An alternative solution arises from the transition-metal cata-
lyzed reduction of imines, easily produced from a variety of al-
dehydes and ketones, through hydrogenation[4,5] or hydrogen
transfer[6] promoted by catalysts such as ruthenium, iridium,
and rhodium; this is one of the most ubiquitous protocols in
organic synthesis to produce amines. Whereas hydrogenation
is the most desired reduction process from an economical and
environmental point of view, catalytic hydrosilylation,[7] which
does not require autoclave pressure, can be an interesting al-
ternative, mainly when the reaction is performed with polyme-
thylhydroxysiloxane (PMHS) under mild conditions and under
air. PMHS is not only an inexpensive and abundant hydrosilane
source arising from the silicone industry, but it is stable to air
and water, soluble in most organic solvents, and a non-toxic
reducing reagent.[8] In addition to the reported hydrosilylation
of amides,[9] hydrosilylation of imines has been performed
under smooth conditions in the presence of various metal cat-
alysts;[10–16] however, a further desilylation step was required to
obtain the amines. There have been only a few reports on
imine hydrosilylation using ruthenium-based complexes.[17,18]
We have recently shown that [RuCl2(p-cymene)]2 is able to
efficiently catalyze the hydrosilylation of bulky imines in diethyl
ether at room temperature in the presence of Ph2SiH2, leading
to the corresponding amines following a desilylation step
(NaOH/MeOH).[19] The use of an inexpensive, abundantly avail-
able, safe, and stable reducing silylating agent, such as PMHS,
should make this approach highly attractive for a green cataly-
The reaction of [RuCl2(p-cymene)]2 with PMHS under air in
ethanol (Table 1, entry 8) offered the best catalytic activity to
produce amine 2a at room temperature. These conditions
were applied to a variety of aldimines 1 to directly produce
amines 2. The results are shown in Table 2.
Aldimines 1a and 1h were reduced at room temperature for
2 h to give good isolated yields of 90 and 88% for 2a and 2h,
respectively (Table 2, entries 1 and 8). This hydrosilylation reac-
tion tolerated sp2CÀX (halide) bonds to produce amines 2b,
2c, and 2i in good yields (Table 2, entries 2, 3, and 9). In the
presence of cyano and nitro electron-withdrawing groups, the
hydrosilylation was more difficult to perform, and 2 mol% of
ruthenium(II) catalyst was used with a longer reaction time to
reach satisfactory yields for 2d and 2e (87 and 69%, respec-
[a] B. Li, Dr. J.-B. Sortais, Prof. Dr. C. Darcel, Prof. Dr. P. H. Dixneuf
Catalyse et Organomꢀtalliques, Institut Sciences Chimiques de Rennes
UMR 6226 CNRS-Universitꢀ de Rennes 1, Ave Gꢀnꢀral Leclerc
35042 Rennes Cedex (France)
Fax: (+)33223236939
Supporting Information for this article is available on the WWW under
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ChemSusChem 2012, 5, 396 – 399