Highly Selective Synthesis of Enamines from Olefins

Article abstract of DOI:10.1002/anie.200352320

Astonishing chemoselectivity is observed in the rhodium-catalyzed formation of enamines from olefins, CO, H₂, and amines (see scheme). This new reaction can be used with a broad range of substrates and is high-yielding and highly selective (n/iso = 99:1 in most cases).

Full text of DOI:10.1002/anie.200352320

Angewandte
Chemie
Synthesis of Enamines
Highly Selective Synthesis of Enamines from
Olefins**
Moballigh Ahmed, Abdul Majeed Seayad, Ralf Jackstell,
and Matthias Beller*
Dedicated to Professor Manfred T.Reetz
on the occasion of his 60th birthday
A major challenge for synthetic chemists constitutes the
development of cleaner (green) and practical catalytic
technologies for organic synthesis. In this respect, we have
been interested for some years in the development of novel
methods for the synthesis of nitrogen-containing compounds,
for example, amines,[1] enamines,[2] and imines.[3] Members of
this class of compounds are useful as dyes, fine chemicals, and
biologically active substances for pharmaceuticals. An ideal
chemical synthesis of amines and their derivatives should
make use of easily available raw materials and produce a
minimum of waste.[4] On the one hand, atom economy[5] or
atom efficiency[6] of the reaction should be as high as possible,
on the other hand, a minimum amount of reaction steps
should be performed to avoid tedious workup procedures and
to save solvents and energy. These latter points are especially
important for large-scale synthesis. Hence, the use of multi-
component coupling reactions[7] or domino reactions[8] is
highly desirable compared to classic multistep procedures.
An environmentally benign synthesis of amines consti-
tutes the so-called hydroaminomethylation[9] of olefins. This
catalytic domino reaction starting from readily available
olefins and consists of an initial hydroformylation step, to give
aldehydes, and a subsequent reductive amination (Scheme 1).
Since the discovery of this reaction by Reppe and Vetter
at BASF,[10] the hydroaminomethylation reaction has been
considered mainly in industry.[11] Notable advances in the last
decade have been especially reported by Eilbracht and co-
workers, who developed elegant domino variants of hydro-
aminomethylation reactions.[12] More recently, we presented a
rhodium catalyst based on the xantphos ligand (3;
Scheme 2),[13] which allowed for the first general, efficient,
and highly regioselective (typically n/iso > 98:2) hydro-
aminomethylation of simple as well as of functionalized
[*] Prof. Dr. M. Beller, M. Ahmed, A. M. Seayad, R. Jackstell
Leibniz-Institut für Organische Katalyse
Universität Rostock e.V. (IfOK)
Buchbinderstraße 5–6, 18055 Rostock (Germany)
Fax: (+49)381-46693-23
E-mail: matthias.beller@ifok.uni-rostock.de
[**] The authors thank Dr. Holger Klein (IfOK) for help with the
preparation of the ligand iphos and for his work on the high-
pressure infrastructure, Susann Buchholzfor GC analyses, and
Hannelore Baudisch for GC–MS experiments. Financial support
from the BMBF (Bundesministerium für Bildung und Forschung),
the Ministry of Education, Science, and Cultural Affairs of
Mecklenburg-Vorpommern and the Fonds der Chemischen
Industrie (FCI) is gratefully acknowledged.
Angew. Chem. Int. Ed. 2003, 42, 5615 –5619
DOI: 10.1002/anie.200352320
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Scheme 1. Hydroaminomethylation of alkenes.
catalytic syntheses of enamines.[18] Furthermore, this trans-
formation is interesting because it allows the synthesis of
unsaturated products in the presence of hydrogen. Therefore,
we investigated this new hydroaminomethylenation reaction
in more detail. In general the reaction was carried out in the
presence of [Rh(CO)2(acac)] (0.1mol%), ligand (0.2 mol%),
and a 1:1 mixture of synthesis gas. Ligands such as triphenyl-
phosphane and bidentate phosphanes such as naphos, xant-
phos, or their derivatives were used.
Examination of different solvents, temperatures, time,
pressure, and concentration of starting materials in the
presence of naphos led to highly selective reaction under
remarkably mild conditions (Table 1, entries 1–9). At 658C
and under 10 bar pressure of synthesis gas (CO/H2 = 1:1),
good conversion (90%) and excellent chemoselectivity
(> 97%) as well as regioselectivity (> 99:1) are observed.
The high chemoselectivity is noteworthy because of the
relatively easy hydrogenation of the produced enamines. We
thought that the ligand controls this unusual selectivity to a
Table 1: Hydroaminomethylenation of 1-pentene with piperidine in the presence of different Rh
complexes and ligands.[a]
Scheme 2. Ligands used in the hydroaminome-
thylenation of 1-pentene with piperidine
(Table 1).
a-olefins.[14] During our studies on the
improvement of tailor-made catalyst
systems for hydroaminomethyla-
tions,[15] we noted that the reaction
steps of the sequence (1. hydroformy-
lation, 2. imine/enamine formation,
3. hydrogenation) are influenced by
the ligand in different ways. Among
the ligands tested in the model reac-
tion of 1-pentene with piperidine, 2,2’-
bis(diphenylphosphanylmethyl)-1,1’-
binaphthyl (naphos (1); Scheme 2)[16]
and its derivatives provided relatively
large amounts of the corresponding
Entry Ligand
T
CO/H2 Conv. Yield [%]
Selectivity [%][b]
iso
n-
n-
n
aldol n/iso
[8C] [bar]
[%] enamine enamine enamine amine product
1
2
1
1
65 20
85 20
100 20
120 20
75 10
65 40
65 10
65 20
65 20
125 40
65 10
65 10
65 10
65 10
125 40
69 50
85 76
83 75
80 70
95 85
42 41
90 87
92 44
99 15
72
89
90
87
89[c]
98
97
48
15
–
8
1
1
5
–
–
–
5
1
2
–
–
–
–
11
3
90:10
99:1
3
1
3
99:1
4
1
4
95:5
5
1
1
–
–
–
>99:1
>99:1
>99:1
62:38
94:6
6
1
7[d]
8[e]
9[f]
10[i,j]
11
12
13
14
15[i,j]
1
1
1
1
–
PPh3
2
3
3
29 23
1
–
–
76[g]
2
100
57
–
–
89
–
94:6
–
35[h]
18
17
23
–
–
76 37
79 61
39 39
49
77
39
–
20 13
71:29
99:1
99:1
enamine
(N-1-hexenylpiperidine).
–
–
–
–
–
97
After studying the literature, we
were surprised to find that there was
no general protocol for the direct and
highly selective synthesis of enamines
from olefins.[17] Clearly, such a domino
reaction has advantages over the step-
wise procedure of hydroformylation
and amination, but also over other
100
–
–
98:2
[a] Reaction conditions: 1-pentene (15 mmol), piperidine (15 mmol), [Rh(CO)2(acac)] (0.1 mol%),
ligand (0.2 mol%), toluene (30 mL), 12 h. [b] Selectivities were determined by GC analysis with
bis(methoxyethyl) ether as an internal standard. [c] Linear aldehyde (5%), iso-aldehyde (4%). [d] 16 h.
[e] THF (30 mL). [f] MeOH (30 mL). [g] Iso-amine (6%). [h] Iso-amine (48%). [i] Alkene (10 mmol),
amine (10 mmol), methanol (15 mL), (toluene) 15 mL, CO (7 bar), H2 (33 bar), 1258C, 5 h.
[j] [Rh(cod)2]BF4. cod=cycloocta-1,5-diene.
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Chemie
Table 2: General synthesis of enamines.[a]
large extent. Indeed, as shown in
Table 1 (entries 7, 11–14), both the
chemoselectivity and the regiose-
lectivity (n/iso)[19] of the reaction
are controlled by the added ligand.
Whereas in the absence of any
phosphane ligand essentially no
enamine and a nearly 1:1 mixture
of linear and iso-amine is formed,
the use of the “standard ligand”
triphenylphosphane leads to lower
conversion (76%), chemoselectiv-
ity (69%), and regioselectivity (n/
iso = 71:29). Furthermore, it is
shown that rhodium catalysts
based on xantphos or iphos (3)[20]
also gave significantly lower con-
version and selectivities. Apart
from the ligand, the solvent
appeared to be important for pre-
venting hydrogenation of the
enamine. In this case, aromatic
solvents, which are known to coor-
dinate to the rhodium center and
thus slow down hydrogenation
reactions,[21] seem to be especially
suitable. Interestingly, at higher
temperature (1258C) and slightly
higher pressure (40 bar), high
selectivity to the corresponding
amine is observed (Table 1,
entries 10 and 15).[14]
Entry
Alkene
Amine
Major product
Conv.
[%]
Selec.
[%][b]
n/iso
1
2
3
100
95
97
95
99:1
99:1
99:1
90
>99
4[c]
5[d]
80
>99
99:1
100
90
73:27
6[d]
75
>99
99:1
7
8
40
98
>99
99:1
85[d]
89:11
9
10
11
100
98
98
>98
>98
99:1
99:1
99:1
100
12[e]
95
86
99:1
Evidently, the new procedure
[a] Reaction conditions: substrate (1:1, 15 mmol), [Rh(CO)2(acac)] (0.1 mol%), naphos (0.2 mol%),
for enamine synthesis is only of
significant importance to synthetic
organic chemists if different ali-
phatic and aromatic olefins with
toluene (30 mL), PCO/H (1:1, 10 bar), 658C, 16 h. [b] Selectivities were determined by GC analysis with
bis(methoxyethyl) ether as an internal standard. [c] Thiomorpholine (30 mmol). [d] 20 h. [e] THF
(30 mL).
2
various functional groups as well as various amines can be
applied with success. Therefore, we were interested in the
scope and limitations of our new reaction (Table 2).
the different reactions studied, the hydroaminomethylenation
of a-methylstyrene with pyrrolidine and of 1-pentene with N-
methylaniline needed longer reaction time and/or higher
reaction temperature for complete conversion.
We were pleased to find that not only lower but also
higher aliphatic as well as aromatic olefins react well with
piperidine to give the linear products with good to excellent
selectivity (Table 2, entries 1–8). N-Methylaniline, N-ethyl-
benzylamine, and other aliphatic secondary amines (morpho-
line, thiomorpholine, pyrrolidine, N-methyl-N-butylamine)
react well to give the corresponding enamine in high yield and
selectivity. Notably, an unprotected allylic alcohol is effi-
ciently converted into the corresponding N,O acetal, which is
the cyclization product of the 4-hydroxyenamine, with high n/
iso ratio (99:1) (Table 2, entry 9). In most cases, the reaction
proceeds with an extremely high degree of chemo- (typically
> 95%) and regioselectivity (> 99:1) towards the linear
enamines. In case of aliphatic olefins, only the E enamine is
detected. This is of special importance because the separation
of mixtures of the branched and the linear enamine products
is often very tedious owing to similar physical properties of
both compounds. Hence, regioselectivities > 98% are an
important factor for the application of the method. Among
Notably, chiral secondary amines such as 2-methoxyme-
thylpyrrolidine also produce the chiral aliphatic enamines in
excellent yields and selectivities (chemoselectivity > 98%; n/
iso > 98:2; Table 3, entry 1). In the case of styrene, the
regioselectivity is somewhat lower. It is evident that these
chiral enamines are useful building blocks for asymmetric
synthesis.[22]
In conclusion, we have shown the first direct synthesis of
enamines from olefins. Remarkably, excellent selectivities are
observed under mild reaction conditions. Aliphatic olefins
give the corresponding linear products in general with
regioselectivities of 99:1, which allows easy isolation of the
product. Key to the success of the reaction is the use of a
catalyst system consisting of a neutral rhodium precursor
together with naphos as ligand. The reported catalyst system
is tolerant to a variety of potentially reactive functional
groups, making the procedure valuable for the synthesis of
interesting organic building blocks, including chiral enamines.
Angew. Chem. Int. Ed. 2003, 42, 5615 –5619
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Communications
Table 3: Synthesis of various enamines using chiral amines.[a]
(m, 1H of the two isomers), 2.71–2.40
(br m, 4H), 1.98–1.78 (m, 4H), 1.53–
1.36 (m, 6H), 1.16 and 1.11 ppm (d, J =
Entry
1
Alkene
Amine
Major product
Conv. [%]
100
Selec. [%][b]
98
n/iso
6.0 Hz and d, J = 6.0 Hz, 3H of the two
isomers); 13C NMR (100 MHz, CDCl3):
98:2
d = 95.9, 95.7, 74.3, 73.4, 48.3, 47.8, 32.9,
32.1, 28.2, 25.5, 24.0, 20.7, 19.8 ppm; MS
(EI, 70 eV): m/z (%): 169 [M+], 154,
2
100
90
98
99:1
140, 125, 114, 98, 84, 69, 55, 41, 29;
HRMS: calcd for C10H19NO [M+]:
169.14443; found: 169.1466.
N-(1-Hexenyl)-2-methoxymethyl-
3[c]
82[d]
63:37
pyrrolidine: Yield: 98% (GC);
1H NMR (400 MHz, CDCl3): d = 6.21
(d, J = 14 Hz, 1H), 4.16 (quint, J =
[a] Reaction conditions: substrate (1:1, 10 mmol), [Rh(CO)2(acac)] (0.1 mol%), naphos (0.2 mol%),
14 Hz, J = 7 Hz, 1H), 3.36 (d, J = 3 Hz,
toluene (30 mL), PCO/H (1:1, 10 bar), 658C, 16 h. [b] Selectivities were determined by GC analysis with
2
2H), 3.34 (s, 3H), 3.22–3.17 (m, 1H),
bis(methoxyethyl) ether as an internal standard. [c] 20 h. [d] Major side product is amine (15%, 80:20).
2.82 (t, J = 8.12 Hz, 2H), 1.98–1.93 (m,
2H), 1.89–1.79 (m, 2H), 1.73–1.69 (m,
2H), 1.31–1.27 (m, 4H), 0.87 ppm (t,
Clearly, this new synthesis of enamines is atom-economic and
environmentally friendly (i.e., water is the only by-product),
and the starting materials are both inexpensive and readily
available.
J = 7 Hz, 3H); 13C NMR (100 MHz, CDCl3): d = 135.4, 99.0, 75.7,
60.2, 59.0, 49.0, 33.9, 30.3, 28.4, 23.5, 22.1, 14.0 ppm; MS (EI, 70 eV):
m/z (%):197 [M+], 182, 166, 152, 122, 108, 94, 81, 70, 54, 41, 27;
HRMS: calcd for C12H23NO [M+]: 197.15984. found: 197.16231.
Received: July 7, 2003 [Z52320]
Experimental Section
Keywords: amines · enamines · homogeneous catalysis ·
rhodium · synthetic methods
1H and 13C NMR spectra were recorded on a Bruker ARX 400
spectrometer (1H: 400.1, 13C: 100.6 MHz). Chemical shifts (d) are
given in ppm relative to residual solvent as internal standard. Gas
chromatographic analyses were performed on a Hewlett Packard
HP 5890 chromatograph with flame-ionization detector and an HP5
column (cross-linked 5% PH ME siloxane). Mass spectra (GC–MS)
experiments were conducted on an Agilent-6890. The products were
isolated from the reaction mixture by evaporation of the solvent and/
or further purified by vacuum distillation wherever necessary. All
yields reported in Tables 1–3 refer to GC yields using bis(methoxy-
ethyl) ether as an internal standard. All yields of isolated compounds
(which vary by 5–10% from those determined by GC) were estimated
to be > 95% pure as determined by GC and NMR. All new
compounds were further characterized by HRMS. Linear/branched
ratios were determined by GC analysis of the crude reaction mixtures.
Compounds known in the literature were characterized by comparing
their 1H NMR, 13C NMR, and GC–MS data with the previously
reported data. The purity of known compounds was confirmed by
GC.
.
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General procedure: All hydroaminomethylenation experiments
were carried out in a Parr stainless-steel autoclave (100 mL). In a
typical experiment, the autoclave was charged with [Rh(CO)2(acac)]
(0.1mol%), naphos (0.2 mol%), olefin (51.0 mmol), amine
(15.0 mmol), and toluene (30 mL) under argon. The autoclave was
pressurized with CO (5 bar) and hydrogen (5 bar), and the reaction
was carried out at 658C for 16 h. The autoclave was then cooled to
room temperature and depressurized. The reaction mixture was
transferred to a Schlenk flask under an argon atmosphere, dried over
MgSO4, and analyzed by GC with bis(methoxyethyl) ether as internal
standard.
1
N-1-Hexenylpiperidine: Yield: 99% (GC); H NMR (400 MHz,
CDCl3): d = 5.79 (d, J = 14 Hz, 1H), 4.36 (quint, J = 14 Hz, J = 7 Hz,
1H), 2.78–2.69 (m, 4H), 1.95–1.90 (m, 2H), 1.58-1.46 (br m, 4H),
1.29–1.26 (m, 6H), 0.86 ppm (t, J = 7 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d = 140.1, 101.4, 50.1, 33.5, 30.2, 25.4, 24.3, 22.0, 13.9 ppm;
MS (EI, 70 eV): m/z (%): 167 [M+], 152, 138, 124, 110, 96, 80, 68, 55,
41, 27; HRMS: calcd for C11H21N [M+]: 167.16795; found: 167.16740.
2-Methyl-5-piperidinyltetrahydrofuran: Yield: 99% (GC);
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