Ruthenium-catalyzed transfer hydrogenation of nitriles: Reduction and subsequent N-monoalkylation to secondary amines

Article abstract of DOI:10.1002/ejoc.201300151

The selective synthesis of amines continues to be of importance because of their application in the bulk and fine chemical industries. Herein, domino ruthenium-catalyzed transfer hydrogenation of nitriles with subsequent N-monoalkylation by using alcohols is described. With this novel approach, various nitriles were reductively N-monoalkylated in excellent yields. A simple method for the synthesis of secondary amines starting directly from nitriles by using a ruthenium catalyst is described. With this novel domino system, various nitriles were reduced and subsequently N-monoalkylated in excellent yields (up to 99 %). In addition to isopropanol, other alcohols were also used as a reductant and N-monoalkylation reagent. Copyright

Full text of DOI:10.1002/ejoc.201300151

Job/Unit: O30151
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Date: 26-04-13 12:32:46
Pages: 5
SHORT COMMUNICATION
Reductive N-Monoalkylation
A simple method for the synthesis of sec-
ondary amines starting directly from ni-
triles by using a ruthenium catalyst is de-
scribed. With this novel domino system,
various nitriles were reduced and sub-
sequently N-monoalkylated in excellent
yields (up to 99%). In addition to isopro-
panol, other alcohols were also used as a
reductant and N-monoalkylation reagent.
S. Werkmeister, C. Bornschein, K. Junge,
M. Beller* ......................................... 1–5
Ruthenium-Catalyzed Transfer Hydrogen-
ation of Nitriles: Reduction and Sub-
sequent N-Monoalkylation to Secondary
Amines
Keywords: Amines / Nitriles / Alkylation /
Ruthenium / Transfer hydrogenation
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S. Werkmeister, C. Bornschein, K. Junge, M. Beller
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300151
Ruthenium-Catalyzed Transfer Hydrogenation of Nitriles: Reduction and
Subsequent N-Monoalkylation to Secondary Amines
Svenja Werkmeister,[a] Christoph Bornschein,[a] Kathrin Junge,[a] and Matthias Beller*[a]
Keywords: Amines / Nitriles / Alkylation / Ruthenium / Transfer hydrogenation
The selective synthesis of amines continues to be of impor-
tance because of their application in the bulk and fine chemi-
cal industries. Herein, domino ruthenium-catalyzed transfer
hydrogenation of nitriles with subsequent N-monoalkylation
by using alcohols is described. With this novel approach,
various nitriles were reductively N-monoalkylated in excel-
lent yields.
Notably, in recent years the alkylation of amines with
alcohols has emerged to be an important tool for the selec-
tive synthesis of secondary amines with the main focus on
controlled monoalkylation.[9] Owing to their availability
and economic constraints, the application of alcohols rather
than alkyl halides is favored.
Introduction
Numerous pharmaceutical products, agrochemicals, dyes,
polymers, and naturally occurring compounds constitute
amine derivatives.[1] They are known to be important
for the chemical industry and are interesting for academic
research. To date, various methods are available for the
synthesis of secondary amines. Typically, reductive
amination,[2–4] reductive hydroamination,[5] selective imine
hydrogenation,[6] and alkylation of primary amines by using
alkyl halides[7] or alcohols[8] are performed. In Scheme 1, a
general overview of the most important reaction pathways
is shown.
Instead of applying amines, the direct use of nitriles to
form secondary amines represents a feasible alternative to
common syntheses. Nitriles are available in nature; for ex-
ample cyanogenic glycosides, cyan lipids, and cyanohydrins
(α-hydroxynitriles) are found in bacteria, algae, sponges,
and fungi, but they are also found in higher organisms.[10]
In addition, nitriles are an interesting class of substrates
with significant importance for industry and economy, and
they are produced on major industrial scale. Consequently,
they constitute suitable starting materials for various trans-
formations. To the best of our knowledge, no catalytic
methodology is known for the domino N-monoalkylation
of nitriles under transfer hydrogenation conditions. Here,
we describe for the first time a one-pot ruthenium-catalyzed
nitrile reduction followed by subsequent N-monoalkylation
of the in situ formed primary amine by using alcohols/
NaOH as the reducing system.
Results and Discussion
Recently, we developed the ruthenium-catalyzed transfer
hydrogenation of nitriles to amines, in which N-monoalkyl-
ation was observed as a minor side reaction.[11] Given the
importance of secondary amines and with respect to our
experience in amination chemistry,[4f,4g,8i,8j,12] we became in-
terested in investigating this unusual one-pot procedure.
Our initial investigations were carried out by using benzo-
nitrile as a model substrate in the presence of [Ru(p-
cymene)2Cl2]2 in combination with various ligands. As
shown in Table 1, diphosphane ligands including 1,2-bis(di-
phenylphosphanyl)ethane (dppe), 1,3-bis(diphenylphos-
Scheme 1. Selected synthetic methods for the preparation of sec-
ondary amines.
[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
Albert-Einstein-Straße 29a, 18059 Rostock
Fax: +49-381-1281-5000
E-mail: matthias.beller@catalysis.de
Homepage: www.catalysis.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300151.
2
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Ruthenium-Catalyzed Transfer Hydrogenation of Nitriles
phanyl)propane (dppp), 1,4-bis(diphenylphosphanyl)butane tolerated. Both electron-donating and electron-withdrawing
(dppb), 1,5-bis(diphenylphosphanyl)pentane (dpppe), and groups on the aryl rings were accepted, and this led to high
1,1Ј-bis(diphenylphosphanyl)ferrocene (dppf) (Table 1, en- isolated yields of the products up to 93%. A direct influence
tries 1–5) revealed only low product yields (6–28%). Simi- of the catalyst system on the electronic situation was not
larly, standard monophosphane ligands did not form the recognizable (Table 2, entries 2–4, 6–8, 10). Noteworthy, un-
secondary amine with satisfying results (Table 1, entries 6–
8). In the absence of phosphorus ligands, no conversion was
observed with [Ru(p-cymene)2Cl2]2, [Ru(cod)Cl2]n (cod =
Table 2. Ru-catalyzed N-monoalkylation of nitriles: substrate
scope.[a]
1,5-cyclooctadiene), or [Ru(benzene)2Cl2]2 as catalyst
(Table 1, entries 9–11). However, applying the defined com-
plex RuCl2(PPh3)3 provided an excellent yield of 99% even
after reproducing the reaction several times (Table 1, en-
try 12). In contrast to the publication by Sasson et al.[13] in
which they noticed the formation of ruthenium(0) nanopar-
ticles during the reduction of ketones with alcohols, we did
not observe the formation of ruthenium(0) nanoparticles.
Owing to the fact that sodium hydroxide is very hygro-
scopic, we studied the influence of various bases (see the
Supporting Information). Indeed, no other base revealed a
better yield than sodium hydroxide.
Next, to demonstrate the applicability of this novel cata-
lytic transformation we tested 15 different aromatic, hetero-
aromatic, disubstituted, and aliphatic nitriles for the re-
ductive N-monoalkylation process. As shown in Table 2,
ortho-, meta-, and para-substituted aromatic nitriles were
Table 1. Reduction and N-monoalkylation of benzonitrile: varia-
tion of catalyst precursors and ligands.[a]
[a] Reaction conditions: benzonitrile (0.5 mmol), Ru precursor
[a] Reaction conditions: benzonitrile (0.5 mmol), Ru precursor
(1 mol-%), ligand (2 mol-%), NaOH (10 mol-%), isopropanol (2 mol-%), NaOH (10 mol-%), isopropanol (5 mL), 120 °C. [b] Iso-
(5 mL), 6 h, 120 °C. [b] Yield determined by GC analysis by using
hexadecane as an internal standard. [c] 2 mol-%.
lated yield. [c] Yield determined by GC analysis by using hexa-
decane as an internal standard. [d] Yield of isolated HCl salt.
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S. Werkmeister, C. Bornschein, K. Junge, M. Beller
SHORT COMMUNICATION
der the reaction conditions the ester substituent in the para N-monoalkylation. With this domino method, several aro-
position to the nitrile group underwent transesterification. matic, heteroaromatic, aliphatic, and disubstituted nitriles
Nevertheless, the product was obtained in 90% yield were converted with isopropanol into the secondary amines
(Table 2, entry 8). Furthermore, various aliphatic substrates with excellent yields up to 99%. Furthermore, other sec-
were reduced and N-alkylated in 6 or 20 h. In addition to ondary alcohols form the corresponding N-monoalkylated
linear aliphatic nitriles, branched compounds were also amines in high yields.
smoothly reduced (Table 2, entries 11–15).
Finally, we were interested in the more challenging trans-
Experimental Section
fer hydrogenation with different alcohols. Apart from iso-
propanol, we tested ethanol, 2-butanol, and 2-pentanol in
the N-monoalkylation of benzonitrile to form the corre-
sponding secondary amines. Extending the reaction time to
48 h, we were able to obtain low to very good yields of the
products (13–97%). Not surprisingly, the primary alcohol
ethanol showed the lowest activity. In contrast, 2-propanol,
2-butanol, and 2-pentanol gave the desired products in high
yields (Scheme 2).
General Procedure for the N-Monoalkylation of Nitriles: A 12 mL
vial capped with a septum was charged under an argon atmosphere
with RuCl2(PPh3)3 (0.01 mmol). Afterwards, a magnetic stirring
bar and dry isopropanol (5 mL) were added. The solution was
stirred for 5 min at room temperature before NaOH (0.05 mmol)
was added, and the mixture was stirred for another 5 min. Then,
the nitrile (0.5 mmol) was added, and the reaction mixture was
heated to 120 °C in an alloy plate with wells for 6 or 20 h. After
the reaction was finished, hexadecane as a standard was added,
and the yield was determined by GC for N-benzylpropan-2-amine,
N-benzylethanamine, and N-benzylbutanamine. For all the other
substrates, the yield was determined after purification (P1, P2, or
P3; see below) by mass.
P1: The solvent was removed under vacuum. The residue was dis-
solved in a small amount of CH2Cl2 and filtered through filter
agent Celite® 545. The secondary amine was then analyzed by
NMR spectroscopy, GC–MS, and HRMS.
P2: After removing the solvent under vacuum, the residue was dis-
solved in methanol. The methanol solution was applied to an Iso-
lute SCX-2 column (2 g/15 mL, Biotage), and the alcohol was
passed through. The column was washed with methanol before the
product was eluted with an ammonia solution in methanol (7 n).
Methanol was removed under vacuum to give the corresponding
N-monoalkylated secondary amine, which was then analyzed by
NMR spectroscopy, GC–MS, and HRMS.
Scheme 2. Variation of different alcohols as transfer hydrogenation
and N-monoalkylation reagent.
Regarding the mechanism of this novel N-monoalkyl-
ation reaction, we propose the following sequence
(Scheme 3): First, nitrile 1 is hydrogenated by using
RuCl2(PPh3)3 with isopropanol and sodium hydroxide as
the base to give primary amine 3. Whilst isopropanol is
reduced to acetone, the ketone reacts with 3 through re-
ductive amination to give desired secondary imine 4 by re-
leasing water as a side product, which does not have any
influence on the reaction procedure. Finally, imine 4 is sub-
sequently hydrogenated to desired secondary amine 2.
P3: A 1 m HCl solution (1 mL) was added to the solution. After-
wards, the solvent was removed under vacuum to afford the HCl
salt. After washing the salt with ethyl acetate, the product was ana-
lyzed by NMR spectroscopy, GC–MS, and HRMS.
Supporting Information (see footnote on the first page of this arti-
cle): General information, general procedure for the N-monoalkyl-
ation of nitriles, variation of bases, characterization of amines,
NMR spectra.
Acknowledgments
This research was funded by the Federate State of Mecklenburg-
Western Pomerania, the Bundesministeriums für Bildung und For-
schung (BMBF), and the Deutsche Forschungsgemeinschaft
(DFG) (Leibniz Prize). The authors thank Dr. W. Baumann, Dr.
C. Fischer, S. Buchholz, S. Schareina, and A. Koch (all at the
LIKAT) for their excellent technical and analytical support.
Scheme 3. Proposed reaction mechanism.
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In conclusion, we have developed the first catalytic
transfer hydrogenation of nitriles including subsequent
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Received: January 29, 2013
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