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based on simple and easily accessible bidentate aminophos-
phine ligands. They show good performance at an unprece-
dented loading of only 0.2 mol%, bringing Mn-catalyzed
hydrogenation a step closer to practical implementation.
The use of P,N ligands for Ru-catalyzed ester hydro-
genation was first reported by Saudan et al.[15] We prepared
complexes 1 to 3 by reaction of Mn(CO)5Br with 1 or
2 equivalents of the corresponding P,N ligand in toluene at
1008C for 24 h. The isolated complexes were fully charac-
decreased substantially at higher temperatures owing to
formation of methyl benzyl ether (Table 1, entries 2,4,5).
Increasing the amount of KOtBu led to improved yields
(Table 1, entries 6–8). Ultimately, quantitative BnOH yield
was obtained with 0.75 equivalents of KOtBu relative to the
substrate (Table 1, entry 8).
After full conversion was achieved, we sought to optimize
crucial process parameters such as solvents, bases, reaction
temperature and H2 pressure to enable use of 2 at reduced
catalyst loading. With 0.5 mol% of 2 in THF a BnOH yield of
87% could be achieved in just 3 h. Importantly, 2 could also
be formed in situ without significant loss of activity, thus
eliminating the need for catalyst isolation (Table S1 in the
Supporting Information). Mercury poisoning did not evi-
dence inhibition, suggesting the homogeneous nature of
catalysis with 2 (Table S1).[18] Replacement of THF for 1,4-
dioxane resulted in a higher product yield, while the use of 2-
methyl-THF and MTBE led to inferior performance
(Table S2). KOtBu was found to be the superior base for the
current catalytic system (Table S3). An increase in temper-
ature and reduction in H2 pressure resulted in lower BnOH
yields (Table S4).
1
terized by H/31P-NMR, ESI-MS, FTIR, elemental analysis,
and single-crystal X-ray structure analysis (see Supporting
Information). Single-crystal X-ray structure determination
revealed the cis-coordination of the N-donor groups of the
P,N ligands and CO ligands in 1, with the two phosphine
moieties bound trans to each other (Figure 1). Their chemical
Next, we expanded the scope of the substrates and further
decreased the catalyst loading to 0.2 mol%. Under the
optimized conditions, 2 was able to convert aromatic and
aliphatic esters into their corresponding alcohols in good to
excellent yields (Scheme 2). Reduction of hexanoate esters
A1–A3 led to good yields of 1-hexanol with hexyl hexanoate
as the only by-product. Interestingly, more sterically hindered
esters (A4–A6) were almost quantitatively hydrogenated,
whereas these are typically more difficult to reduce than their
methyl and ethyl analogues.[1] Aromatic benzoate esters with
varied steric bulk or electronic properties were all hydro-
genated to benzyl alcohol in high yield (B1–B4). Similar to
aliphatic esters, the reduction of bulky tert-butyl benzoate was
more efficient than the less-sterically hindered substrates.
Figure 1. ORTEP diagrams of 1 (left) and 2 (right). Thermal ellipsoids
are set at 30% probability. Hydrogen atoms have been omitted for
clarity.
equivalence was also detected in solution by 31P NMR,
revealing a single resonance for 1 at d = 79.3 ppm. Complex
2 contains a single P,N ligand. The amine and Brꢀ in 2 are
bound in a cis fashion, providing a favorable environment for
heterolytic H2 activation across the Mn–N moiety.[16]
Complexes 1–3 are active catalysts for ester hydrogena-
tion. Table 1 summarizes the results of the initial catalytic
tests using methyl benzoate as a model substrate. Mono-
ligated complex 2 was found to be considerably more active
than 1 and 3 (Table 1, entries 1–3). This is remarkable as the
related Ru-PN catalyst is biligated.[15] Reaction at 80–1008C
gave similar benzyl alcohol (BnOH) yields, while the yield
Table 1: Hydrogenation of methyl benzoate with 1–3.[a]
Entry Catalyst KOtBu [mol%] T [8C] Conv. [%] YBnOH [%][17]
1
2
3
4
5
6
7
8
1
2
3
2
2
2
2
2
10
10
10
10
10
25
50
75
100
100
100
80
120
100
100
100
43
75
13
74
57
86
96
99
24
66
3
65
43
80
91
98
Scheme 2. Hydrogenation of various esters with 2. Conditions: 1 mmol
substrate, 75 mol% KOtBu, 0.2 mol% 2, 2 mL 1,4-dioxane, 1008C,
50 bar H2, 16 h. [a] 0.5 mol% 2, 6 h.
[a] Conditions: 1 mmol methyl benzoate, 10–75 mol% KOtBu, 1.0 mol%
Mn, 2 mL THF, 80–1208C, 50 bar H2, 20 h. Yield determined by GC.
2
ꢀ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
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