COMMUNICATION
DOI: 10.1002/chem.201100007
Synthesis of Primary Amines from Secondary and Tertiary Amines:
Ruthenium-Catalyzed Amination Using Ammonia
Sebastian Bꢀhn, Sebastian Imm, Lorenz Neubert, Min Zhang, Helfried Neumann, and
Matthias Beller*[a]
Amines are of significant importance for the bulk and
fine chemical industry; for example, for the synthesis of
pharmaceuticals and agrochemicals, but also as building
blocks for polymers and dyes.[1] Primary amines in particular
are useful intermediates for further derivatization reactions.
Owing to their numerous applications, the development of
novel methods for the synthesis of primary amines continues
to be an active area of research.[2]
methodology,[15] also known as “hydrogen auto-transfer”
(Scheme 1).[16] Thereby, the secondary amine is in situ dehy-
drogenated to the corresponding imine. Subsequent conden-
Interesting approaches for their synthesis include reduc-
tive amination[3] and reduction of amides[4] or carboxylic
acids[4d] in the presence of ammonia. Furthermore, the heter-
ogeneously catalyzed amination of alcohols using ammonia
is performed on an industrial scale. Unfortunately, this reac-
tion usually results in a mixture of primary, secondary, and
tertiary amines.[5]
Homogeneously catalyzed aminations of alcohols using
primary and secondary amines have been known since the
pioneering work of Grigg and co-workers in 1981.[6] To date
a number of alcohol aminations producing secondary and
tertiary amines in the presence of mainly ruthenium- or iri-
dium-based complexes have been described. Recent elegant
examples came from the groups of Williams,[7] Fujita,[8]
Kempe,[9] us,[10] and others.[11] Despite the importance of pri-
mary amines, so far only two catalytic systems have been re-
ported from the groups of Milstein, Vogt, and us for the se-
lective amination of alcohols to give primary amines.[12] Un-
fortunately, these catalytic systems still cannot fully avoid
the formation of undesired secondary amines for a number
of alcohols. In this respect, the conversion of secondary
amines into primary ones is an attractive transformation. Al-
though Williams and co-workers[13] and our group[14] have
shown that secondary and even tertiary amines can be acti-
vated and used in the alkylation of aliphatic and aromatic
amines, to the best of our knowledge no similar reactions
with ammonia have been reported, yet.
Scheme 1. Proposed mechanism for the splitting of secondary amines
using ammonia.
sation with ammonia results in the corresponding primary
amine and primary imine. The latter is finally re-hydrogen-
ated. Notably, the hydrogen required for the final hydroge-
nation of the primary imine is generated completely by de-
hydrogenation of the secondary amine in the first reaction
step. Hence, no additional hydrogen is required. It has to be
pointed out that all the reaction steps are reversible. Thus,
typically such a reaction should lead to an equilibrium of
primary and secondary amines.
Based on the experience in amine–amine coupling reac-
tions,[14] we chose the splitting of dicyclohexylamine as a
model reaction for our investigations (Scheme 2). First, we
examined different catalytic systems using 1 mmol dicyclo-
hexylamine, 2 mol% of catalyst, 1 mL tert-amyl alcohol, and
1 g ammonia at 1308C for 16 h.[17]
In analogy to the amination of alcohols, such a transfor-
mation is based on the so-called “borrowing hydrogen”
Unfortunately, Milsteinꢀs catalyst (1)[12c] and our
[Ru3(CO)12]/cataCXiumPCy system (2),[18] both known to
convert alcohols selectively into primary amines, gave no
[a] S. Bꢁhn, S. Imm, L. Neubert, Prof. Dr. M. Zhang, Dr. H. Neumann,
Prof. Dr. M. Beller
Leibniz-Institut fꢂr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-51113
Supporting information for this article is available on the WWW
Scheme 2. Splitting of dicyclohexylamine using ammonia.
Chem. Eur. J. 2011, 17, 4705 – 4708
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4705