Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnIand FeIIPNP Pincer Complexes

Article abstract of DOI:10.1002/chem.201603148

Herein, we describe an efficient coupling of alcohols and amines catalyzed by well-defined isoelectronic hydride Mn^Iand Fe^IIcomplexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alcohol dehydrogenation concept. A range of alcohols and amines including both aromatic and aliphatic substrates were efficiently converted in good to excellent isolated yields. Although in the case of Mn selectively imines were obtained, with Fe—exclusively monoalkylated amines were formed. These reactions proceed under base-free conditions and required the addition of molecular sieves.

Full text of DOI:10.1002/chem.201603148

DOI: 10.1002/chem.201603148
Communication
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Pincer Complexes
Divergent Coupling of Alcohols and Amines Catalyzed by
Isoelectronic Hydride MnI and FeII PNP Pincer Complexes
Matthias Mastalir,[a] Mathias Glatz,[a] Nikolaus Gorgas,[a] Berthold Stçger,[b] Ernst Pittenauer,[b]
Gꢀnter Allmaier,[b] Luis F. Veiros,[c] and Karl Kirchner*[a]
1) acceptorless dehydrogenation (AD) of alcohols;[5] 2) imine
Abstract: Herein, we describe an efficient coupling of al-
cohols and amines catalyzed by well-defined isoelectronic
formation; and 3) in situ hydrogenation of imines, provided
that the hydrogen generated from the first step can be tempo-
hydride MnI and FeII complexes, which are stabilized by
rarily stored on the metal complex (borrowing hydrogen meth-
a PNP ligand based on the 2,6-diaminopyridine scaffold.
odology). Key features are that the process is hydrogen neutral
This reaction is an environmentally benign process imple-
and that the only stoichiometric by-product is water and H2 (in
menting inexpensive, earth-abundant non-precious metal
the case of imines).
catalysts, and is based on the acceptorless alcohol dehy-
Despite the significance of such coupling reactions, homo-
drogenation concept. A range of alcohols and amines in-
geneous catalysts mostly employ precious metals, such as
cluding both aromatic and aliphatic substrates were effi-
Ru,[6] Rh,[7] Ir,[8] and Os.[9] In comparison, the same reaction with
ciently converted in good to excellent isolated yields. Al-
catalysts that utilize non-precious, earth-abundant metals[10] is
though in the case of Mn selectively imines were ob-
much less developed although base metals demonstrated the
tained, with Fe—exclusively monoalkylated amines were
ability to oxidize alcohols via AD.[11,12] Recently, Hanson and co-
formed. These reactions proceed under base-free condi-
workers[13a] and Zhang and co-workers[13b] reported a Co cata-
tions and required the addition of molecular sieves.
lyst, stabilized by
a bis(phosphino)-amine (PNP) ligand
The catalytic coupling of amines with alcohols represents an
environmentally benign and atom-economic pathway for the
synthesis of substituted imines or amines that have important
synthetic applications in the synthesis of dyes, fragrances, fun-
gicides, pharmaceuticals, and agricultural chemicals.[1–3] In
terms of sustainability, the choice of alcohols as substrates is
highly desirable, because they are readily available by a variety
of industrial processes and can be obtained renewably by fer-
mentation or catalytic conversion of lignocellulosic biomass.[4]
The catalytic cycle involves two or three successive steps:
[a] M. Mastalir, M. Glatz, N. Gorgas, Prof. Dr. K. Kirchner
Institute of Applied Synthetic Chemistry
Vienna University of Technology, Getreidemarkt 9, 1060 Wien (Austria)
Fax: (+43)1-58801-16299
E-mail: kkirch@mail.tuwien.ac.at
Homepage: www.ias.tuwien.ac.at
Scheme 1. Efficient base metal catalysts for the coupling of alcohols and
amines involving acceptorless dehydrogenation of alcohols.
[b] Dr. B. Stçger, Dr. E. Pittenauer, Prof. Dr. G. Allmaier
Institute of Chemical Technologies and Analytics
Vienna University of Technology, Getreidemarkt 9, 1060 Wien (Austria)
[c] Prof. Dr. L. F. Veiros
Centro de Quꢀmica Estrutural, Instituto Superior Tꢁcnico
Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa (Portugal)
(Scheme 1), which is able to give imines and/or amines de-
pending on the reaction conditions. Kempe and co-workers
described a new Co PNP pincer catalyst based on a triazine
backbone, which was highly active for the alkylation of aro-
matic amines.[14] The groups of Feringa and Barta,[15a] Wills,[15b]
and Zhao[15c] reported the alkylation of amines with alcohols to
give amines by utilizing Fe catalysts featuring functionalized
cyclopentadienone or hydroxycyclopentadienyl ligands based
upon Knçlker’s complex or derivatives thereof.[16] We described
Supporting information for this article and ORCIDs for some of the authors
are available on the WWW under http://dx.doi.org/10.1002/
chem.201603148. It contains complete crystallographic data, experimental
1
details of the synthesis of all complexes including H, 13C{1H}, and
31P{1H} NMR spectra, computational details, atomic coordinates of opti-
mized species, and technical details in CIF format for 1 (CCDC 1479482 con-
tains the supplementary crystallographic data for this paper. These data
are provided free of charge by The Cambridge Crystallographic Data
Centre).
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the application of well-defined CoII catalysts, which feature
a PCP ligand based on the 1,3-diaminobenzene scaffold.[17]
Very recently, Milstein and co-workers[18] discovered the first
Mn catalyst, which is active for the dehydrogenative coupling
of alcohols and amines to form selectively imines. This catalyst
features a deprotonated 2,6-bis-(di-tert-butylphosphino-meth-
yl)pyridine pincer ligand. Noteworthy, this reaction does not re-
quire any additives, such as base or Lewis acids.
Table 1. Screening of Mn and Fe complexes as catalysts for the coupling
of p-toluidine and 4-fluorobenzyl alcohol.[a]
Intrigued by these recent discoveries, we describe herein
the efficient coupling of alcohols and amines catalyzed by iso-
electronic hydride MnI and FeII complexes, which are stabilized
by a PNP ligand based on the 2,6-diaminopyridine scaffold.
The aromatic pyridine ring and the phosphine PiPr2 moieties
are connected by NH or NMe linkers. The broad applicability of
this ligand class has been demonstrated recently in the case of
Fe, which are highly efficient catalysts for the hydrogenation of
ketones, aldehydes and CO2.[19] New Mn complexes 1 and 2
were synthesized, characterized, and applied in addition to the
known Fe complexes 3 and 4. The molecular structure of
Entry
Catalyst
Yield [%][b]
I
II
1
2
3
4
5[c]
6[c]
7[d]
8[d]
9[e]
10[f]
11[f]
1
2
3
4
1
3
3
1
3
1
3
84
81
14
39
45
22
19
41
70
1
was determined by X-ray crystal-structure analysis
73
(Scheme 2).
[a] Reaction conditions: 1.0 mmol 4-fluorobenzyl alcohol, 1.4 mmol p-tol-
uidine, 0.3 g 3 ꢂ MS, 3.0 mol% catalyst, 4 mL toluene, 1408C, 16 h. [b] Iso-
lated yields. [c] Without MS. [d] Without MS, 60 h. [e] Open system in o-
xylene. [f] 1.0 mmol LiOTf.
the absence of MS, even after 60 hours, the yields were consid-
erably lower (entries 5–8). When the reaction with 3 was per-
formed in an open system, a mixture of imine and amine were
obtained (entry 9). When the reaction was performed in the
presence of LiOTf instead of MS, good yields were also ach-
ieved (entries 10 and 11) suggesting that Lewis acid property
of these additives may also play a role.
Having established 1 and 3 as active catalysts, this method-
ology was applied to other substrates including benzyl alco-
hols and nBuOH, as well as aromatic and aliphatic amines.
These results are shown in Tables 2 and 3. The resulting imines
and N-alkylated amines were isolated in good to excellent
yields. An exception is the Mn-catalyzed reaction of nBuOH
and p-toluidine were only 17% of the respective imine was ob-
tained (Table 2, entry 6). The low yield was due to polymeri-
zation of the product under these reaction conditions. More-
over, in the case of Fe, exclusively monoalkylated amines were
formed.
Scheme 2. Mn and Fe complexes 1–4 tested as catalysts, and structural view
of 1 showing 30% thermal ellipsoids (most hydrogen atoms omitted for
clarity). Selected bond lengths [ꢂ] and angles [8]: Mn1ꢀP1 2.2074(7), Mn1ꢀ
P2 2.2059(9), Mn1ꢀN1 2.060(2), Mn1ꢀC18 1.747(2), Mn1ꢀC19 1.775(2), Mn1ꢀ
H1 1.46(2); P1-Mn1-P2 161.00(3), N1-Mn1-C18 171.3(1), N1-Mn1-C19 96.2(1).
First, the Mn and Fe catalysts 1–4 were screened for the
coupling of 4-fluorbenzyl alcohol with p-toluidine (1.4 equiv) in
toluene (4 mL) at 1408C in a closed vial with added 3 ꢂ molec-
ular sieves (MS), and the results are summarized in Table 1. The
Simplified catalytic cycles with 1 and 3 as catalyst and pre-
catalyst, respectively, are shown in Schemes 3 and 4. Key spe-
cies are AMn and AFe with the latter being formed initially from
3 in the presence of base.[19c] Both compounds feature a depro-
tonated PNP ligand and are coordinatively unsaturated with
the important difference that the first contains two, the latter
only one inert CO co-ligand, but additionally a hydride ligand,
which can participate in the catalytic reaction. Accordingly, the
Fe system is capable of performing both alcohol dehydrogena-
tion (oxidation cycle) and imine hydrogenation (reduction
cycle) via an insertion mechanism. In the reduction cycle, coor-
dination of dihydrogen (EFe) and subsequent protonation of
1
products were analyzed by H and 13C{1H} NMR spectroscopy
and identified by comparison with authentic samples. In gener-
al, isolated yields after purification by column chromatography
are reported. When 1 (3.0 mol% based on alcohol) was used
as catalyst, selectively 1-(4-fluorophenyl)-N-(p-tolyl)methane
imine (I) was obtained in 84% yield (entry 1), whereas with 3,
the corresponding amine, N-(4-fluorobenzyl)-4-methylbenzene
amine (II), was isolated in 81% yield (entry 3). Catalysts 2 and
4 bearing NMe linkers were catalytically inactive, and no reac-
tion took place (entries 2 and 4). This emphasizes the impor-
tance of the acidic NH moieties for the catalytic reaction. In
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Table 2. Dehydrogenative coupling of alcohols and amines to form
imines utilizing catalyst 1.[a]
Table 3. Coupling of alcohols and amines catalyzed by 3.[a]
Entry Product
1
Entry Product
Entry Product
1
Entry Product
91% 2
82%
90% 2
84%
3
5
88% 4
80% 6
67%
61%
3
5
92% 4
68% 6
71%
17%
7
93% 8
61%
89%
79%
7
90% 8
60%
92%
73%
9
82% 10
82% 12
9
86% 10
87% 12
11
11
[a] Reaction conditions: 1.0 mmol alcohol, 1.4 mmol amine, 0.3 g 3 ꢂ MS,
3.0 mol% catalyst, 4 mL toluene, 1408C, 16 h. [b] Isolated yields.
[a] Reaction conditions: 1.0 mmol alcohol, 1.4 mmol amine, 0.3 g 3 ꢂ MS,
3.0 mol% catalyst, 4 mL toluene, 1408C, 16 h. [b] Isolated yields.
The overall reaction has a high barrier of 33 kcalmolꢀ1 in ac-
cordance with the experimental conditions, that is, reaction
temperature of 1408C, and a thermodynamically unfavorable
free energy balance of 9 kcalmolꢀ1. Thus, the driving force
must be provided by the consecutive reaction, that is, conden-
sation between aldehyde and amine and probably by water re-
the imine N atom with formation of the amine and regenera-
tion of the hydride (AFe) is essential. The same mechanism was
recently proposed for the hydrogenation of ketones and alde-
hydes.[19c] The Mn system, on the other hand, is only able to
oxidize the alcohol with concomitant irreversible H2
release.
For the Mn-catalyzed reaction, we carried out pre-
liminary DFT calculations[20] to establish a reasonable
mechanism for the dehydrogenation of EtOH as
model substrate. A summary of these results with the
most relevant points along the catalytic cycle is pre-
sented in Scheme 3 (for details, see the Supporting
Information). The first step is a proton transfer from
the N atom of the PNP ligand to the hydride with for-
mation of dihydrogen complex BMn. Hydrogen trans-
fer is assisted by a nearby EtOH molecule acting as
proton shuttle. This process is endergonic (DG=
¼
10 kcalmolꢀ1) with the highest barrier DG along the
path being 33 kcalmolꢀ1. In the next step, H2 release
from BMn gives the five-coordinate intermediate AMn
.
This occurs easily with a maximum barrier of 12 kcal
molꢀ1 in an almost thermoneutral step. In the final
step, an EtOH molecule approaches the metal center
in outer-sphere fashion and protonates the PNP N
atom regenerating 1 and releasing acetaldehyde.
This process occurs in a single concerted step with
a barrier of 32 kcalmolꢀ1. In a very recent work, a simi-
lar mechanism was proposed for the hydrogenation
of nitriles with a related Mn PNP complex.[21]
Scheme 3. Simplified catalytic cycle with 1 as catalyst. Free-energy values ([kcalmolꢀ1],
barriers in italic) are referred to 1.
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Scheme 4. Simplified catalytic cycle with 3 as pre-catalyst.
nology is acknowledged for financial support and for providing
access to the single-crystal diffractometer.
moval by molecular sieves, as was indicated by the experimen-
tal observations.
Alternatively, dehydrogenation of EtOH may proceed via the
ethoxide complex CMn, which undergoes b-elimination with re-
generation of 1 and formation of aldehyde. However, this alter-
native pathway is less favorable than the one calculated for
the concerted step.
Keywords: alcohols · amination · homogeneous catalysis ·
iron · manganese
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Received: July 1, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Pincer Complexes
M. Mastalir, M. Glatz, N. Gorgas,
B. Stçger, E. Pittenauer, G. Allmaier,
L. F. Veiros, K. Kirchner*
&& – &&
Divergent Coupling of Alcohols and
Amines Catalyzed by Isoelectronic
Hydride MnI and FeII PNP Pincer
Complexes
Efficient coupling of alcohols and
phatic substrates, was efficiently con-
verted giving in the case of Mn selec-
tively imines, whereas with Fe, exclu-
sively amines were formed (see
scheme).
amines catalyzed by well-defined isoe-
lectronic hydride MnI and FeII complexes
is described. A range of alcohols and
amines, including both aromatic and ali-
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!