Direct hydrosilylation of tertiary amides to amines by an in situ formed iron/N-heterocyclic carbene catalyst

Article abstract of DOI:10.1002/ejoc.201300010

Tertiary amides were efficiently reduced to their corresponding tertiary amines in high isolated yields by using the commercially available and inexpensive polymeric silane polymethylhydrosiloxane (PMHS) as the reducing agent. The reaction is efficiently catalyzed by an in situ generated iron/N-heterocyclic carbene complex (1 mol-%) obtained from iron(II) acetate and 1-(2-hydroxy-2-phenylethyl)-3-methylimidazolium triflate ([PhHEMIM][OTF]). A catalytic amount of lithium chloride (1 mol-%) present in the reaction mixture significantly reduced the reaction time and increased the chemoselectivity of the reduction process. Tertiary amides are reduced to their corresponding tertiary amines in high isolated yields by using the polymeric silane polymethylhydrosiloxane (PMHS) in the presence of an in situ generated iron/N-heterocyclic carbene complex (1 mol-%), generated from iron(II) acetate and 1-(2-hydroxy-2-phenylethyl)-3-methylimidazolium triflate ([PhHEMIM][OTF]), and a catalytic amount of lithium chloride (1 mol-%). Copyright

Full text of DOI:10.1002/ejoc.201300010

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300010
Direct Hydrosilylation of Tertiary Amides to Amines by an In Situ Formed
Iron/N-Heterocyclic Carbene Catalyst
Alexey Volkov,[a] Elina Buitrago,*[a] and Hans Adolfsson*[a]
Keywords: Hydrosilylation / Iron / Carbenes / Amides / Amines
Tertiary amides were efficiently reduced to their correspond- iron(II) acetate and 1-(2-hydroxy-2-phenylethyl)-3-methyl-
ing tertiary amines in high isolated yields by using the com- imidazolium triflate ([PhHEMIM][OTF]). A catalytic amount
mercially available and inexpensive polymeric silane poly-
of lithium chloride (1 mol-%) present in the reaction mixture
methylhydrosiloxane (PMHS) as the reducing agent. The re- significantly reduced the reaction time and increased the
action is efficiently catalyzed by an in situ generated iron/N-
heterocyclic carbene complex (1 mol-%) obtained from
chemoselectivity of the reduction process.
recent examples on the use of iron catalysts for the re-
duction of amides under hydrosilylation conditions.[10]
Beller showed that the iron–carbonyl complex [Fe3-
(CO)12] could be used together with polymethylhydrosilox-
ane (PMHS) for the direct reduction of tertiary amides to
amines at 100 °C.[11] Simultaneously, the group of Nagash-
ima presented that the iron–carbonyl complexes [Fe(CO)5]
and [Fe3(CO)12] efficiently catalyzed the same transforma-
tion. The reaction could be carried out under both thermal
and photoassisted conditions by using either PMHS or
1,1,3,3-tetramethyldisiloxane (TMDS) as the reducing
agent. They also observed that in the presence of a nitro
group, the amide functionality remained completely un-
touched in the reaction and that the nitro functionality was
selectively reduced.[12] Furthermore, the Nagashima group
showed that the use of 1,2-bis(dimethylsilyl)benzene
(BDSB) as the hydride source led to decreased reaction
times and a lower catalyst loading.[13] Sortais and Darcel
reported a series of well-defined piano stool iron/N-hetero-
cyclic carbene (NHC) complexes for the hydrosilylation of
aldehydes, ketones, and imines,[14] and recently, they showed
that their cationic catalyst also could be used for the re-
duction of secondary and tertiary amides at 100 °C.[15] An
efficient protocol for the reduction of primary amides to
their corresponding amines was recently presented by Beller
and co-workers. The combination of two different iron
complexes, [Et3NH][HFe3(CO)11] and iron(II) acetate, to-
gether with a phenanthroline ligand, catalyzed the forma-
tion of primary amines in a two-step reaction, in which the
iron–carbonyl complex reduced the amides to nitriles, fol-
lowed by reduction of the formed nitriles to the amines by
the iron(II) acetate/phenanthroline system.[16]
Introduction
Amines are common functionalities found in essentially
all pharmaceuticals, and a common route for their pro-
duction is the reduction of amides with aluminum hydride
and boron reagents. The direct reduction of amides to
amines by using stoichiometric reagents results in signifi-
cant amounts of waste formed from the reducing agents,
and alternative catalytic methods are highly desirable.[1] In
recent years, large effort has been put into the development
of transition-metal-catalyzed reductions of amides. There
are a few examples of the direct hydrogenation of amides;[2]
however, silanes are more commonly employed as reducing
agents in these methods. Several transition-metal-catalyzed
hydrosilylation protocols for the reduction of amides have
been reported. In 1998, Ito reported the use of an analogue
of Wilkinson’s rhodium complex for the catalytic hydro-
silylation of tertiary amides.[3] The simplicity and mildness
of the catalytic hydrosilylation process led to further devel-
opments, and currently a variety of different transition-
metal-catalyzed protocols are available. The most com-
monly employed protocols involve the use of catalysts that
are based on rhodium,[3,4] ruthenium,[5] platinum,[6] and ti-
tanium.[7]
Recently, Beller reported the use of copper-[8] and zinc-
based[9] systems for the catalytic hydrosilylation of second-
ary and tertiary amides. Iron, being the most abundant
transition metal and displaying low toxicity, is an interest-
ing alternative for this transformation, and there are some
[a] Department of Organic Chemistry, Stockholm University,
The Arrhenius Laboratory,
10691 Stockholm, Sweden
We recently reported that a highly active, in situ formed
iron/NHC complex formed from iron(II) acetate and
1-(2-hydroxyethyl)-3-methylimidazolium triflate [HEMIM]-
[OTF] efficiently catalyzed the reduction of aldehydes and
E-mail: elina@organ.su.se
hansa@organ.su.se
Homepage: http://www.organ.su.se
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300010
2066
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Eur. J. Org. Chem. 2013, 2066–2070

Direct Hydrosilylation of Tertiary Amides to Amines
ketones under hydrosilylation conditions (Scheme 1).[17] served the same activity and a slightly higher selectivity by
Herein, we evaluated this catalytic system for the reduction using the phenyl-substituted ligand (Table 1, entry 3), and
of amides to amines.
this compound was therefore chosen for further optimiza-
tions.
A series of different silanes were compared to the origi-
nally used PMHS in an attempt to improve the selectivity
of the reaction. The use of TMDS gave excellent conver-
sion, but unfortunately, extensive byproduct formation was
observed (Table 1, entry 4). When the catalytic hydro-
silylation was performed with diethoxymethylsilane, the
Scheme 1. Hydrosilylation of aldehydes and ketones catalyzed by selectivity was similar to that of PHMS (Table 1, entry 5),
an in situ generated iron complex with [HEMIM][OTf].
and the use of triethoxysilane gave slightly better results
(Table 1, entry 6). Owing to its higher price and the known
hazards involved with handling triethoxysilane,[18] we chose
to continue our optimizations by using PMHS as the hy-
Results and Discussion
dride source. Changing the solvent from THF to 1,4-diox-
ane resulted in no conversion of the starting material
(Table 1, entry 7); however, performing the reaction in tolu-
ene gave full conversion and moderate selectivity (Table 1,
entry 8). The lack of reactivity in dioxane is possibly ex-
plained by the presence of water in the solvent, which inter-
feres with the reduction process. Further optimizations in-
cluding changing the number of equivalents of the silane
and the concentration of the reaction mixture did not lead
to any major improvements. Blank experiments performed
separately with only the NHC precursor in the presence of
nBuLi or with only iron acetate under otherwise unchanged
reaction conditions resulted in no conversion of the starting
material (Table 1, entries 9 and 10). As expected, no reac-
tion occurred when the reduction was performed with only
the silane and no catalyst components added. The purity of
the iron acetate used in all experiments was 99.995%, which
strongly indicates that trace amounts of other metals are
not the active catalysts in the amide reductions.
Initially, we chose N,N-dimethylbenzamide (1) as the
model substrate and subjected it to the same conditions
used for the reduction of ketones and aldehydes (Scheme 1).
For the amide to react, a prolonged reaction time was re-
quired, and full conversion of the starting material was ob-
served after 16 h (Table 1, entry 1). Along with the forma-
tion of desired N,N-dimethylbenzylamine (2), we also ob-
served the formation of benzaldehyde (3) and benzyl
alcohol (4). The promising result obtained in the initial ex-
periment encouraged us to investigate the reaction condi-
tions further to improve the formation of the amine.
Increasing the catalytic loading to 2.5 mol-% resulted in
full conversion and slightly higher selectivity in favor
of the amine product (Table 1, entry 2). The ionic liquid
[HEMIM][OTf] is rather difficult to handle in the small
amounts used for the catalytic reactions, and we therefore
investigated if the solid phenyl-substituted analogue 1-(2-
hydroxy-2-phenylethyl)-3-methylimidazolium triflate ([Ph-
HEMIM][OTf]) would show similar behavior as a ligand
To summarize the optimization study of the hydro-
for iron in the hydrosilylation process. Gratifyingly, we ob- silylation of N,N-dimethylbenzamide, full conversion of the
Table 1. Reduction of N,N-dimethylbenzamide under hydrosilylation conditions.[a]
Entry
Fe(OAc)2
[mol-%]
Ligand
Silane
Solvent
Conversion
[%][b]
Product ratio
2
3
4
1
2
3
4
5
6
7
8
9
10
1
2.5
1
1
1
1
1
1
0
HEMIM
HEMIM
Ph-HEMIM
Ph-HEMIM
PMHS
PMHS
PMHS
TMDS
THF
THF
THF
THF
THF
Ͼ95
Ͼ95
Ͼ95
Ͼ95
Ͼ95
Ͼ95
–
Ͼ95
–
–
53
65
74
48
75
82
–
54
–
–
20
11
11
14
15
10
–
19
–
–
27
24
15
38
10
8
–
27
–
Ph-HEMIM (EtO)2MeSiH
Ph-HEMIM
Ph-HEMIM
Ph-HEMIM
Ph-HEMIM
–
(EtO)3SiH
PMHS
PMHS
PMHS
PMHS
THF
dioxane
toluene
THF
5
THF
–
[a] General conditions: Fe(OAc)2 (x mol-%), solvent (3 mL), N,N-dimethylbenzamide (1 mmol), PMHS (3 equiv.), NHC precursor
(1.1x mol-%), nBuLi (2.2x mol-%), 65 °C, 16–18 h. For workup details, see the Exp. Section. [b] Conversion and product ratio determined
1
by H NMR spectroscopy.
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A. Volkov, E. Buitrago, H. Adolfsson
SHORT COMMUNICATION
Scheme 2. Proposed schematic mechanism for the reduction of tertiary amides to tertiary amines.
starting material was obtained by using different silanes in slow decomposition of intermediate A to generate B
the presence of the in situ formed iron catalyst (1 mol-%). (Scheme 2), which opens the alternative path to form inter-
However, in all of the experiments we observed the forma- mediate C. We anticipated that the addition of an oxophilic
tion of a significant amount of benzyl alcohol and a smaller Lewis acid could assist in the formation of iminium ion B,
amount of benzaldehyde. The proposed schematic mecha- which would thereby lead to a higher yield of the amine.
nism for the amide reduction under hydrosilylation condi- Lithium chloride, which was previously successfully used as
tions is depicted in Scheme 2. Addition of a hydride to the an achiral Lewis acid promoter in asymmetric catalytic re-
starting amide leads to the formation of tetrahedral inter- duction reactions,[19] was therefore chosen, and a small
mediate A, which upon elimination of the silylated alcohol amount of LiCl was added to the reaction mixture. Most
gives iminium ion B. Subsequent addition of a second hy- gratifyingly, with only 1 mol-% of LiCl present as co-cata-
dride to B produces desired amine 2. Benzyl alcohol (4) lyst, the hydrosilylation reaction of piperidylbenzamide was
can, in principle, be formed by two different pathways: not only free from byproducts, but the reaction reached
(1) Hydrolysis of imine intermediate B to benzaldehyde (3). completion in a substantially shorter period of time. In only
(2) Collapse of intermediate A to silylated oxo-carbenium 5 h all of the starting material was consumed, and the prod-
ion C with elimination of dimethylamine. In both cases, re- uct amine could be isolated in a yield of 89%. A control
duction of either intermediate C or benzaldehyde will lead experiment using 1 mol-% LiCl as the sole catalyst resulted
to the formation of benzyl alcohol. In addition to benzyl in no conversion of the starting material, which excluded
alcohol, we observed various amounts of benzaldehyde the possibility that the additive, or trace amounts of other
when analyzing the reaction mixtures after workup metals present in the alkali salt, catalyzed the hydro-
(Table 1). We previously used the same reaction conditions silylation reaction. Hence, the optimized conditions found
for the hydrosilylation of aldehydes and ketones, and in for the iron-catalyzed hydrosilylation of tertiary amides are
those experiments, we observed very rapid reduction of, in presented in Scheme 3.
particular, the aldehydes (ca. 15 min).[17b] The latter sug-
gests that the stability of tetrahedral intermediate A is sig-
nificant, as both iminium ion B and benzaldehyde formed
by hydrolysis of B would be reduced to either the product
or benzyl alcohol by the catalyst. Hence, the transformation
of A into B could be the rate-limiting step of the process.
Furthermore, the fact that benzaldehyde can be observed in
Scheme 3. Optimized conditions for the iron-catalyzed reduction of
tertiary amides.
a reaction performed under anhydrous conditions suggests
that the aldehyde is formed in the aqueous workup of the
reaction mixture.
To investigate the scope of the reaction with the Fe-
Attempts to isolate rather volatile N,N-dimethylbenzyla- (OAc)2/[Ph-HEMIM][OTf] catalyst system, a set of dif-
mine led to significantly lower yields than indicated by ferent tertiary amides was subjected to the optimized reac-
1
analysis of the worked-up crude reaction mixtures by H tion conditions presented in Scheme 3. The results of the
NMR spectroscopy. We therefore decided to change the hydrosilylation reactions are presented in Table 2. Both aro-
model substrate to piperidylbenzamide to explore methods matic and synthetically useful heteroaromatic amides re-
to further suppress the formation of benzyl alcohol and to acted under these conditions to generate the corresponding
be able to isolate the formed amine in a higher chemical amines in good isolated yields. Amides bearing halogens
yield. To our delight, piperidylbenzamide was fully reduced that are susceptible to reduction or that can be replaced
under the reaction conditions used in Table 1, entry 3, and under reductive conditions remained intact under these re-
we were pleased to see that the amount of formed benzyl action conditions. Sterically hindered N,N-dibenzyl benz-
alcohol was significantly lower with the use of this sub- amide was reduced with good conversion within 14 h
strate. As suggested above, a possible explanation for the (Table 2, entry 12). Overall, the catalytic system performed
formation of benzaldehyde and benzyl alcohol can be the well for the examined substrates, and the amines were ob-
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Direct Hydrosilylation of Tertiary Amides to Amines
Table 2. Substrate scope for the iron/NHC-catalyzed hydro- tiary amines by using an iron catalyst formed in situ from
silylation of tertiary benzamides.[a]
iron(II) acetate, the imidazolium salt [Ph-HEMIM][OTf],
and the inexpensive and mild silane PMHS. The catalytic
system showed improved selectivity upon the addition of
LiCl, which is an effect believed to arise from Lewis acid
mediated decomposition of the quaternary intermediate
formed in the first step of the reduction reaction. A variety
of tertiary benzamides were reduced in good yields in rela-
tively short reaction times compared to previously reported
similar systems. Unfortunately, primary and secondary
amides remained unreactive under these reaction condi-
tions.
Experimental Section
General Procedure for the Iron-Catalyzed Hydrosilylation Reaction:
The iron source, Fe(OAc)2 (1.7 mg, 0.01 mmol), [Ph-HEMIM]-
[OTf] (3.9 mg, 0.011 mmol), and LiCl (0.4 mg, 0.01 mmol) were
treated under vacuum for 10 min. THF (2 mL) was added under
a nitrogen atmosphere followed by nBuLi (0.022 mmol) and the
substrate (1 mmol) in THF (1 mL). The temperature was increased
to 65 °C, and PMHS (0.18 mL, 3 mmol) was added. The mixture
was stirred at 65 °C for 5–14 h. The reaction mixture was cooled
to room temperature, and NaOH (2 m in H2O, 10 mL) was added.
After stirring for 3–6 h, the product was extracted with CH2Cl2
1
(3ϫ 20 mL). The crude products were analyzed by H NMR and
13C NMR spectroscopy.[20]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, and copies of
1
the H NMR and 13C NMR spectra.
Acknowledgments
This work was financially supported by the Swedish Research
Council, the K & A Wallenberg Foundation, and the Carl Trygger
Foundation.
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SHORT COMMUNICATION
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Received: January 4, 2013
Published Online: March 1, 2013
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