Phosphine- and hydrogen-free: Highly regioselective ruthenium-catalyzed hydroaminomethylation of olefins

Article abstract of DOI:10.1002/anie.201402368

A highly regioselective ruthenium-catalyzed hydroaminomethylation of olefins is reported. Using easily available trirutheniumdodecacarbonyl an efficient sequence consisting of a water-gas shift reaction, hydroformylation of olefins, with subsequent imine or enamine formation and final reduction is realized. This novel procedure is highly practical (ligand-free, one pot) and economic (low catalyst loading and inexpensive metal). Bulk industrial as well as functionalized olefins react with various amines to give the corresponding tertiary amines generally in high yields (up to 92), excellent regioselectivities (n/iso>99:1), and full chemoselectivity in favor of terminal olefins.

Full text of DOI:10.1002/anie.201402368

Angewandte
Chemie
DOI: 10.1002/anie.201402368
Amine Synthesis
Phosphine- and Hydrogen-Free: Highly Regioselective Ruthenium-
Catalyzed Hydroaminomethylation of Olefins**
Samet Gꢀlak, Lipeng Wu, Qiang Liu, Robert Franke, Ralf Jackstell, and Matthias Beller*
Abstract: A highly regioselective ruthenium-catalyzed hydro-
aminomethylation of olefins is reported. Using easily available
trirutheniumdodecacarbonyl an efficient sequence consisting
of a water-gas shift reaction, hydroformylation of olefins, with
subsequent imine or enamine formation and final reduction is
realized. This novel procedure is highly practical (ligand-free,
one pot) and economic (low catalyst loading and inexpensive
metal). Bulk industrial as well as functionalized olefins react
with various amines to give the corresponding tertiary amines
generally in high yields (up to 92%), excellent regioselectivities
(n/iso > 99:1), and full chemoselectivity in favor of terminal
olefins.
to aldehydes,[6] condensation with amines, and subsequent
reduction step. Since its seminal discovery by Reppe in 1949,[7]
the past decades have witnessed several developments
especially by the groups of Eilbracht[5a,8] and Zhang,[9] as
well as ourselves.[10]
Despite all achievements, most of the known catalytic
procedures require expensive rhodium catalysts in which the
n-selectivity is ensured by using a large excess of bidentate
phosphine ligands like Naphos and Xantphos derivatives.
Hence, it is of interest that recent examples of ruthenium-
[11]
À
catalyzed C C bond-formation reactions
were reported
and showed their potential in hydroformylation reactions.[12]
Nevertheless, powerful alternatives to rhodium-catalyzed
HAMs have been rarely reported.[13] Last year our group
presented a general protocol for a highly n-selective ruthe-
nium-catalyzed HAM using syngas.[14] Here, the use of special
imidazole-substituted phosphine ligands was essential for
achieving high yields and regioselectivites. Based on our
continuing interest in the development of practical and
economic procedures for the synthesis of amines, we herein
present a general hydrogen-free ruthenium-catalyzed HAM
system (Scheme 1). Notably, excellent regioselectivities are
obtained without any phosphine ligands present.[15]
Amines and their derivatives are of substantial importance
in the chemical industry and are produced on a multimillion
ton scale per year.[1] In particular, aliphatic amines constitute
valuable compounds which are used as solvent-free coatings,
civil engineering compounds (flooring, patch repair), chem-
ical-resistant tank linings, adhesives, composites, castings,
moldings as well as dyes and agrochemicals.[2] Because of their
importance many methodologies have been developed for the
synthesis of all kinds of amines. Traditionally, nucleophilic
substitution reactions of alkyl halides, hydrocyanation/reduc-
tion reactions,[3] and reductive amination of carbonyl com-
pounds are the most common routes to aliphatic amines.[4]
Despite all the known processes, there is continuing interest
in the development of more efficient catalytic routes to this
class of products.[5] One of the most promising catalytic
syntheses of amines in terms of atom-efficiency, selectivity,
and operational practicability is the so-called hydroamino-
methylation (HAM) of olefins. This effective domino
sequence consists of an initial hydroformylation of alkenes
[*] Dr. S. Gꢀlak,[+] L. Wu,[+] Dr. Q. Liu, Dr. R. Jackstell, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Scheme 1. Selective ruthenium-catalyzed HAM with water as the hydro-
gen source.
Prof. Dr. R. Franke
Evonik Industries AG
Paul-Baumann-Strasse 1, 45772 Marl (Germany)
and Lehrstuhl fꢀr Theoretische Chemie, 44780 Bochum (Germany)
Our initial studies were carried out with 1-octene and
piperidine as a model reaction and the results are summarized
in Table 1.[16] Notably, HAM reactions proceeded similarly
well when using either syngas or CO in water. To our surprise
the regioselectivity for 1-nonylpiperidine under water-gas
shift conditions were outstanding. The use of various P li-
gands did not additionally improve the n-selectivity. A
moderate product yield was obtained with NMP (NMP = N-
methyl-2-pyrrolidone) as the solvent and 0.5 mol%
[Ru3(CO)12] in the presence of 20 mol% K2CO3 as the base
(entry 1). Other solvents and solvent mixtures led to lower
yields (entries 2 and 3). The amount of base can be reduced to
[+] These authors contributed equally to this work.
[**] We are particularly grateful to the Bundesministerium fꢀr Bildung
und Forschung for financial support under the PROFORMING
project (No. 03X3559) and the Chinese Scholarship council (grants
for L.W.). Q.L. thanks the Alexander von Humboldt Foundation for
financial support. We thank Dr. C. Fischer, S. Buchholz, S.
Schareina, A. Koch, Dr. W. Baumann, Dr. N. Rockstroh, and P.
Bartels for their technical and analytical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402368.
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Table 1: Variation of reaction parameters.[a]
Table 2: Hydroaminomethylations of 1-octene with various amines.[a]
Entry Amines
1
Major product
Yield [%][b] n/iso[b]
Entry
[Ru3(CO)12]
[mol%]
Variation from standard
conditions
Yield
[%][b]
n/iso[b]
82
76
97:3
96:4
1
2
3
4
5
6
7
8
9
10
0.5
0.5
0.5
0.5
0.5
0.5
–
0.5
0.5
0.5
–
61
34
55
10
49
60
0
63
75
81
97:3
97:3
98:2
96:4
97:3
97:3
–
97:3
97:3
97:3
Tol/MeOH/H2O (8:8:2)
MeCN/H2O (15:2)
without water
without K2CO3
5 mol% K2CO3
2
3
72[c]
75
97:3
97:3
5 mol% K2CO3
5 mol% K2CO3, 1:1.3[c]
5 mol% K2CO3, 1.3:1[c]
5 mol% K2CO3, 1.3:1,[c]
NMP/H2O (15:1)
5 mol% K2CO3, 1.3:1,[c]
NMP/H2O (15:1)
4
80[c]
72
97:3
98:2
11
0.25
81, 53[d]
97:3
5
6
71,[e] 45[f]
[a] Reaction conditions: [Ru3(CO)12] (0.5 mol%), 1-octene (1 mmol),
piperidine (1.2 mmol) in THF/H2O (15:2 v/v, 1.7 mL), K2CO3
(20 mol%), CO (40 bar), 1308C, 20 h. [b] Determined by GC. [c] 1-
octene/piperidine. [d] 20 bar CO. [e] 1408C. [f] 1008C.
7
8
75
97:3
97:3
73[c]
5 mol% (entry 6). Control experiments showed that the
ruthenium catalyst and water are essential for the reaction
(entries 4 and 7). Notably, the reaction without base showed
lower, but still significant activity (entry 5).[17] This activity is
explained by the presence of the basic amines which also
serve as organic bases in the reaction.[18] Considering the fact
that internal olefins barely react under these reaction
conditions and 1-octene slowly underwent isomerization, it
was beneficial to use a slight excess of the olefin (entry 9),
whereas an excess of piperidine led to a lower yield
(entry 8).[19] The water concentration in the reaction is of
relevance, too. By lowering the amount of water, the yield was
increased to 81% (entry 10). It is worth noting that higher
catalyst loadings (1 mol%) did not increase the yields, and
lowering the catalyst loading (0.25 mol%) led to the same
result (entry 11). Neither the variation of the CO pressure nor
the temperature could improve upon the reaction (entry 11).
With the optimized reaction conditions in hand, we
explored the substrate scope and potential applications of
our protocol (Table 2). First, reactions of 1-octene with
various secondary and primary amines were studied. With
cyclic secondary amines like piperidine, morpholine, 1-
phenylpiperazine, and 2,3-dihydroindole as starting materials,
high yields (72–82%) and excellent regioselectivities (n-
selectivity up to 97%) were achieved (entries 1–4). Acyclic
amines like diethylamine, more bulky amines like dibenzyl-
amine, and the alcoholic substrate (2-methylamino)ethanol
led to similar yields (72–80%) and regioselectivities
(entries 5–9). However, primary amines such as aniline, 1-
hexaneamine, or cyclohexylamine gave somewhat lower
yields (22–45%; entries 11–13). GC analysis indicated a selec-
tivity problem resulting from consecutive reactions of the
secondary amine products. In all cases mixtures of the
secondary and tertiary amines were observed. Our efforts to
9
77[c]
98:2
10
11
0
–
44
95:5
12
13
45
24
95:5
95:5
[a] Reaction conditions: [Ru3(CO)12] (0.25 mol%), 1-octene (1.3 mmol),
amine (1.0 mmol) in THF/H2O (15:1 v/v, 1.6 mL), K2CO3 (5 mol%), CO
(40 bar), 1308C, 20 h. [b] Determined by GC. [c] Yield of isolated product.
increase the selectivity towards the tertiary or the secondary
product, by variation of the olefin to amine ratio (olefin to
amine 1:1 or 2.6:1), did not give superior results. Nevertheless
the n-selectivity is still high.
Then, the reactions of bulk industrial as well as function-
alized olefins with piperidine were studied (Table 3). Sub-
stituted linear terminal olefins like 1-hexene and higher
analogues gave generally high yields and n-selectivities
(entries 1 and 2). A double HAM of 1,7-octadiene gave an
interesting diamine in a similar manner (entry 3). Various
functionalized linear olefins were tested, too. Olefins con-
taining ester, alcohol, and nitrile groups were efficiently
converted into the amines (73–86%; entries 4, 5, and 8).
Interestingly, in these cases excellent chemoselectivity is
obtained and no amide formation or alcohol amination is
observed. Styrene provided a 70:30 mixture of amine isomers
but a very high overall yield (entry 6). It is expected that
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Table 3: Hydroaminomethylations of various alkenes with piperidine.[a]
Table 4: Selective hydroaminomethylations of terminal and internal
olefins.[a]
Entry Alkene
Major product
Yield n/iso[b]
[%][b]
Entry Alkene
Major product
Yield n/iso[b]
[%][b]
1
2
81
80
97:3
97:3
1
2
97:3
82
10
85:15
86:14
11[e]
3
4
5
70[d]
73
93:7
98:2
97:3
3
<2
–
>99:1[d]
>99:1[d]
90[c]
77[c]
4
5
83
[a] Reaction conditions: [Ru3(CO)12] (0.25 mol%), 1-octene (1.3 mmol),
amine (1.0 mmol) in THF/H2O (15:1 v/v, 1.6 mL), K2CO3 (5 mol%), CO
(40 bar), 1308C, 20 h. [b] Determined by GC. [c] Yield of isolated product.
[d] Greater than 99% chemoselectivity in favor of the terminal alkene.
[e] 0.5 mol% [Ru3(CO)12] and 1 mol% 2-(dicyclohexylphosphino)-1-
phenyl-1H-pyrrole.
6
92[e]
30:70
terminal olefin by isomerization, we decided to use 2-
(dicyclohexylphosphino)-1-phenyl-1H-pyrrole as a ligand,
which is known to promote isomerization reactions.[20]
Unfortunately, this attempt did not result in a higher yield
of the amine; instead larger amounts of internal olefins were
detected. In accordance with these results, cyclic olefins like
cyclohexene gave a low yield of the amine product (entry 3).
Gratifyingly, the poor reactivity of internal olefins allows the
chemoselective transformation of olefins containing both
internal and terminal double bonds. Thus, highly chemo-
selective HAMs in favor of the terminal double bond were
possible with 4-vinylcyclohexene and b-citronellene (entries 4
and 5). In both cases the linear amine products were formed
exclusively in high yields and excellent n-selectivities.
Finally, some mechanistic experiments were performed.
To get more insight into the domino sequence, we analyzed
the composition of the reaction solution and the gas phase.
The ruthenium-catalyzed water-gas shift reaction produces
carbon dioxide and hydrogen. The latter is used initially in the
ruthenium-catalyzed hydroformylation reaction. After base-
promoted condensation reaction with the amine the corre-
sponding imine/enamine intermediate is formed. Subsequent
hydrogenation led to the final product. At no period of the
reaction could we detect the aldehyde or the imine/enamine
intermediate, thus indicating that the hydroformylation step is
rate determining. This assumption is supported by the
detection of excess amounts of hydrogen and carbon dioxide
in the gas phase after 3 hours.
7
8
92
90:10
95:5
86[c]
11
12
90[c]
82[c]
90:10
>99:1
[a] Reaction conditions: [Ru3(CO)12] (0.25 mol%), alkene (1.3 mmol),
piperidine (1.0 mmol), THF:H2O (15:1 v/v, 1.6 mL), K2CO3 (5 mol%),
CO (40 bar), 1308C, 20 h. [b] Determined by GC. [c] Yield of isolated
product. [d] 1,7-octadiene (1.3 mmol), piperidine (2 mmol). [e] Com-
bined yield.
styrene forms a p-allyl intermediate stabilized by ruthenium,
and results in lower n-selectivity. However, allylbenzene and
the fluoro-substituted analogue gave comparable yields to
that of styrene, but with much higher n-selectivies (entries 7
and 11). As an example of a sterically hindered substrate,
neohexene led to the corresponding amine in excellent yield
with greater than 99% n-selectivity (entry 12).
After achieving good activity with various terminal
alkenes, we turned our attention to the HAM of internal
olefins and dienes. As shown before, internal olefins barely
react under the optimized reaction conditions. Hence, 2-
octene gave significantly lower yields than 1-octene (Table 4,
entries 1 and 2). Surprisingly, the n-selectivity is still com-
parably high. To promote the formation of the more reactive
In conclusion, we have developed the first general
protocol for a hydrogen-free ruthenium-catalyzed HAM
reaction. Contrary to established hydroformylation method-
ologies this reaction provides high yields (up to 92%) and
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excellent regioselecitvities (n/iso > 99:1) without the use of
any P ligands. The broad applicability is demonstrated by the
conversion of various alkenes into structurally interesting
linear amines. The reactions display tolerance of alcohol,
ester, nitrile, and halide substituents. Moreover, chemoselec-
tive HAMs of terminal olefins can be achieved in the
presence of internal double bonds, thus allowing the selective
conversion of multiple unsaturated substrates, even alkene
mixtures. Additionally, this novel protocol is an operationally
simple, economical, and practical one-pot procedure.
Experimental Section
General procedure: Experiments were carried out in 4 mL reaction
vials which were placed in a Parr stainless steel autoclave. The
reaction vial was charged with [Ru3(CO)12] (0.25 mol%), alkene
(1.3 mmol), amine (1.0 mmol), degased water (0.1 mL), and NMP
(1.5 mL) under argon atmosphere. The autoclave was pressurized
with 40 bar CO and heated to 1308C for 20 h. After the reaction time,
the autoclave was cooled with ice water and the pressure was
released. The crude reaction mixture was analyzed by gas chroma-
tography with isooctane as an internal standard. For isolation the
product was extracted with diethyl ether from the mixture and NMP
was removed by washing with water. After drying and removing the
solvent the crude raction mixture was purified by column chroma-
tography on silica gel (gradient elution using hexanes/ethyl acetate).
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Supporting Information.
Received: February 13, 2014
Published online: && &&, &&&&
Keywords: amines · chemoselectivity · homogeneous catalysis ·
.
ruthenium · synthetic methods
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Amine Synthesis
S. Gꢁlak, L. Wu, Q. Liu, R. Franke,
R. Jackstell, M. Beller*
&&&& — &&&&
Phosphine- and Hydrogen-Free: Highly
Regioselective Ruthenium-Catalyzed
Hydroaminomethylation of Olefins
HAM it up: A highly regioselective ruthe-
nium-catalyzed synthesis of amines,
through hydroaminomethylation (HAM),
is reported. This novel procedure is
practical (P-ligand-free, one pot) and
economic (low catalyst loading and inex-
pensive metal). Bulk industrial as well as
functionalized olefins give the corre-
sponding tertiary amines in high yields
(up to 92%) with excellent regioselectiv-
ities (n/iso>99:1) and full chemoselec-
tivity.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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www.angewandte.org
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