Hydrogen autotransfer and related dehydrogenative coupling reactions using a rhenium(i) pincer catalyst

Article abstract of DOI:10.1039/c6cc09977g

A novel rhenium complex bearing a non-innocent PNP pincer ligand was prepared. This novel catalyst is active in hydrogen autotransfer reactions to form new C-C and C-N bonds. More specifically, valuable alkylations of ketones and sulfonamides with primary alcohols are herein presented. In addition, the first examples of rhenium-catalysed synthesis of pyrroles are described by dehydrogenative coupling of diols, amines and ketones.

Full text of DOI:10.1039/c6cc09977g

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DOI: 10.1039/C6CC09977G
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Hydrogen Autotransfer and Related Dehydrogenative Coupling
Reactions using a Rhenium(I) Pincer Catalyst
Patrick Piehl, Miguel Peña-López, Anna Frey, Helfried Neumann and Matthias Beller*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A novel rhenium complex1 bearing a non-innocent PNP pincer can be synthesised using such methodologies or related
ligand was prepared. This novel catalyst is active in hydrogen dehydrogenative coupling processes.11 So far, such protocols
autotransfer reactions to form new C-C and C-N bonds. More are mainly performed using ruthenium and iridium complexes,
specifically, valuable alkylations of ketones and sulfonamides with although recently significant attention has been paid to use
primary alcohols are herein presented. In addition, the first alternative metals. For example, a variety of complexes
examples of rhenium-catalysed synthesis of pyrroles are described bearing PNP pincer ligands have been developed using several
by dehydrogenative coupling of diols, amines and ketones.
metals such as ruthenium,12 iron,13 cobalt14 and lately
manganese (Scheme 1).15
Compared to other transition metals, molecularly defined
rhenium complexes remained largely unexplored in catalysis.2
Selected examples for reductive processes and C-C bond
formation include the dehydration of alcohols,3 hydrosilylation
reactions,4 as well as coupling of propargylic alcohols.5 Most
recently, some rhenium-pincer complexes have been also
described for the stoichiometric activation of nitrogen.6 In
addition, several rhenium catalysts were applied for the
dehydrogenative coupling of carbonyl compounds with
alcohols,7 as well as the alkylation of primary amines with the
same substrates, in the latter case to give the corresponding
imines8 or secondary amines.9
Transition metal-catalyzed hydrogen autotransfer (also called
borrowing hydrogen) of alcohols are atom economical,
operational simple and eco-friendly methodologies where
water is formed as the only stoichiometric residue.10
Mechanistically, readily available alcohols are initially
dehydrogenated to afford more reactive carbonyl compounds.
Further condensation with an appropriate nucleophile
provides the corresponding unsaturated intermediates, which
are finally reduced with the hydrogen extracted in the first
step, regenerating the active species. This strategy has been
widely used to form new C-C and C-N bonds. More specifically,
the alkylation of amines or ketones with non-activated, cheap
and abundant alcohols continues to be challenging.
Furthermore, a plethora of relevant heterocyclic compounds
Scheme 1. Ruthenium, iron and manganese PNP pincer complexes. Synthesis of
cationic rhenium(I) pincer complex 5.
All these complexes showed reactivity in hydrogenation and
dehydrogenation reactions, as well as hydrogen autotransfer
processes. Notably, the reactivity for individual catalytic steps
can be tuned by varying the metal centre and not the ligand
scaffold. Hence, we are interested in the preparation of novel
related complexes. Following our previous experience on this
topic, we herein describe the synthesis of a rhenium-based
pincer complex and its application in hydrogen autotransfer
reactions, which are barely known in the presence of rhenium.
Initially, the reaction of Re(CO)5Br with the PNP pincer ligand
in toluene at 100 °C overnight, provided the cationic complex
quantitatively (Scheme 1). Crystallisation from
chloroform/heptane provided the X-ray molecular structure
5
shown in Figure 1. To our delight,
5
showed a good reactivity
in the α-alkylation of ketones with alcohols,15d, a greener
alternative for the carbon-carbon formation compared to
classical enolate procedures.
16
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
E-Mail: Matthias.Beller@catalysis.de
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
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Chem. Commun., 2016, 00, 1-4 | 1
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such transformations. Advantageously, the reaction takes
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places with only one equivalent of basDeOwI:i1t0h.10re39s/pCe6cCtCt0o99t7h7eG
catalyst although the activation of the catalyst as well as the
product formation require basic conditions. Finally, we chose
the reaction of 6a (1 mmol) with 7a (1.2 mmol), rhenium
complex
5 (2 mol%) and Cs2CO3 (2 mol%) as base in tert-amyl
alcohol at 140°C as model system, giving the product 8a in a
good 86% yield.
Table 2. Rhenium-catalysed reaction of ketones (6) with primary alcohols (7).[a]
Figure 1. X-ray molecular structure of rhenium complex 5 (ellipsoids correspond to 30%
probability. Hydrogen atoms except the NH-proton and solvent molecules are omitted
for clarity. There are two molecules in the asymmetric unit of which one is shown).
Entry
1
Ketone
Alcohol
Product
Yield (%)[b]
86
More specifically, we optimised the reaction of acetophenone
6a with benzyl alcohol 7a to form 3-phenylpropiophenone (8a
)
(Table 1). By testing different bases (Table 1, entries 1-4), we
found Cs2CO3 to be the most effective one for this
transformation. Lowering the amount of catalyst led to a slight
drop in yield while a higher amount does not improve the
reaction (Table 1, entries 6-7).
2
91
3
4
5
60
53
88
Table 1. Rhenium-catalysed α-alkylation of acetophenone (6a) with benzyl alcohol (7a):
Variation of reaction conditions.[a]
Entry Re catalyst
Base
Solvent
T (°C) Yield (%)[b]
1
2
3
4
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re(CO)5Br
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re complex 5
Re complex 5
—
KOH
KOtBu
K2CO3
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 1,4-dioxane
Cs2CO3 toluene
t-amyl alcohol
t-amyl alcohol
t-amyl alcohol
140
140
140
140
140
140
140
140
140
74
81
65
84
—
81
88
86
85
86
86
72
75
—
4
6
7
40
38
5
6[c]
7[d]
8
8
9
80
72
9
10[e]
11[f]
12[e]
13[e]
14
Cs2CO3 t-amyl alcohol 140
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
140
130
150
140
140
140
140
140
—
t-amyl alcohol
10
83
15[e]
16[e]
17[e]
18[e]
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Cs2CO3 t-amyl alcohol
Ru complex 2
Fe complex 3
Mn complex 4
41
13
64
11
12
68
37
[a] Unless otherwise specified, all reactions were carried out with 6a (1 mmol), 7a
(1.2 mmol), Re catalyst (0.02 mmol), base (0.05 mmol) in a solvent (1 mL) at
indicated temperature for 22 h. [b] Isolated yields. [c] Catalyst loading: 1 mol%.
[d] Catalyst loading: 5 mol%. [e] Base: 2 mol%. [f] Base: 10 mol%.
[a] Unless otherwise specified, all reactions were carried out with 6 (1 mmol), 7
(1.2 mmol), Re catalyst 5 (0.02 mmol), Cs2CO3 (0.02 mmol) in t-amyl alcohol (1
mL) at 140°C for 22 h. [b] Isolated yields.
Different solvents do not have a strong influence on the
reaction (Table 1, entry 8-9). To our delight, it was possible to
reduce the base loading to only 2 mol% without a significant
decrease of the yield (Table 1, entry 10). This is an important
improvement compared to most other catalysts known for
With optimised conditions in hand, we compared the activity
of our novel rhenium complex with previously described
2 | Chem.Commun., 2016, 00, 1-4
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ruthenium-, iron- and manganese-pincer complexes
2
-
4
bond is formed using simple and easily available alcohols as
View Article Online
(Scheme 1).11g, 11h, 15d, 17 Reaction of acetophenone (6a) with electrophiles. In this line, we found thaDt OsuI:l1f0o.n10a3m9/iCd6eCsCc0a9n97b7Ge
benzyl alcohol (7a) catalyzed by
2
and
3
(2 mol%) under the efficiently N-functionalized using the rhenium complex
5
as
19
same conditions provided 8a in lower yield (41 and 13%, catalyst (Scheme 2).18d,
Applying the aforementioned
respectively; Table 1, entries 16 and 17). Similarly, the developed conditions although increasing the reaction
manganese complex
4, recently reported to catalyse this temperature to 150°C, p-toluenesulfonamide (10a) was N-
alkylation of ketones,15d gave the desired product in 64% yield alkylated with benzyl alcohol affording N-benzyl-p-
(Table 1, entry 18). Comparing all the product yields in the toluenesulfonamide (11a) in 84% yield. Likewise, N-benzyl-
presence of the different pincer complexes showed clearly the tert-butylsulfonamide
superiority of the rhenium system (86% yield). These results whereas 2-thiophenesulfonamide
suggest that this catalyst is more reactive than analogous functionalized, albeit in lower yield. In addition, higher base
complexes bearing other metals at low concentration of base. concentration was required in the latter case to get a modest
(
11b
)
was obtained in 79% yield,
(
10c was also
)
Next, the versatility and generality of the method was studied 30% yield. Gratifyingly, different alcohols can be used as
(Table 2). Initially, differently 4-substituted acetophenones alkylating agents in this transformation. For example, 4-
were tested in the rhenium-catalyzed reaction with benzyl methoxybenzyl alcohol and 2-thiophenemethanol were
alcohol. The synthesis of methoxy-, bromo- and applied for the alkylation of p-toluenesulfonamide (10a) giving
trifluoromethyl-substituted 3-phenylpropiophenones
(
8b-d
)
the desired products 11d and 11e in 94 and 69% yield,
was possible in moderate to very good yields (53-91%, Table 2, respectively (Scheme 2). Finally, the reaction of 10a with
entries 2-4). Moreover, heteroaryl derivatives such as 2- cyclohexanemethanol gave only traces of product 11f, even in
acetylthiophene (6e) and 2-acetyl-N-methylpyrrol (6f) were presence of higher amounts of caesium carbonate.
applied affording the desired products in 88 and 40% yield,
respectively (8e-f, Table 1, entries 5 and 6). Tetralone was also

-alkylated in secondary carbon to give compound 8g in a
moderate 38% yield (Table 1, entry 7). On the other hand,
structurally diverse alcohols were assayed in the reaction
7
with acetophenone (6a). Electron-rich 4-methoxybenzyl and
electron-deficient 4-bromobenzyl alcohol were effectively
used affording products 9b and 9c in good yields (80 and 72%,
respectively, Table 1, entries 8 and 9). In addition, 1-
naphthalenemethanol and 2-thiophenemethanol were applied
as alkylating agents giving the corresponding coupled products
in 83 and 68% yields (Table 1, entries 10 and 11). Finally, we
found that aliphatic alcohols such as n-butanol can also be
used, obtaining the desired ketone in modest yield (Table 1,
entry 12).
Scheme 3. Rhenium-catalysed reaction of ketones 12 with amines 13 and alcohol 14.
Unless otherwise specified, all reactions were carried out with 6 (0.5 mmol), 13 (1.0
mmol), 14 (5.0 mmol), Re catalyst 5 (0.01 mmol), Cs2CO3 (0.01 mmol) in t-amyl alcohol
(1 mL) at 150°C for 22 h. Isolated yields.
Besides the formation of new C-C and C-N bonds, one of the
most relevant applications of the hydrogen autotransfer
methodology is the synthesis of heterocycles. In this case,
related dehydrogenative coupling processes have been also
described to obtain unsaturated heterocyclic compounds.
Here, in addition to water, hydrogen gas is obtained as by-
product. As an example, the multistep ruthenium-catalysed
synthesis of pyrroles from ketones, amines and diols was
recently developed by our research group.11c, 11d With the aim
of extending the scope of the novel cationic rhenium catalyst,
we also assayed such a three-component reaction (Scheme 3).
Indeed, the reaction of 1,2-diphenylethanone, ethylene glycol
and
phenethylamine
provided
N-phenethyl-2,3-
diphenylpyrrole (15a) in 66% yield. It is worth mentioning that
two C-C and one C-N bonds are sequentially formed in this
domino reaction. Using hexylamine allowed to obtain the
corresponding N-hexyl-2,3-diphenylpyrrole (15b, 49% yield),
while the reaction with cyclohexanone as ketone gave rise to
1-phenethyl-4,5,6,7-tetrahydroindole (15c) in modest yield.
In conclusion, we have synthesised a new cationic rhenium(I)
pincer complex and demonstrated for the first time that such
complexes can be conveniently used in several hydrogen
Scheme 2. Rhenium-catalysed reaction of sulfonamides (10) with primary alcohols (7).
Unless otherwise specified, all reactions were carried out with 10 (1 mmol), 7 (1.2
mmol), Re catalyst 5 (0.02 mmol), Cs2CO3 (0.02 mmol) in t-amyl alcohol (1 mL) at 150°C
for 22 h. Isolated yields. [a] 5 mol% Cs2CO3.
After studying the rhenium-catalyzed C-alkylation of ketones,
we focused our interest on N-alkylation processes using
hydrogen autotransfer methodology, which had been
described earlier.18 In this reaction, a new carbon-nitrogen
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autotransfer reactions. More specifically, the
-alkylation of
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We thank the state of Mecklenburg-Vorpommern and the
Bundesministerium für Bildung und Forschung (BMBF) for
financial support. We also thank analytical department of
LIKAT for technical support.
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