Communication
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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Chem. Eur. J. 2016, 22, 1 – 6
2
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!