Rhodium-catalyzed amination of aromatic olefins [1]

Article abstract of DOI:10.1007/s007060070012

The oxidative amination of styrene with secondary amines in the presence of cationic rhodium catalysts yields regiospecifically the corresponding anti-Markovnikov enamines. Styrene as the hydrogen acceptor gave concomitantly ethylbenzene. In the presence

Full text of DOI:10.1007/s007060070012

Monatshefte fuÈr Chemie 131, 1327±1334 (2000)
Rhodium-Catalyzed Amination
of Aromatic Ole®ns [1]
Annegret Tillack1, Harald Trauthwein2, Christian G. Hartung1,
Martin Eichberger2;a, Stephan Pitter3, Achim Jansen3, and Matthias Beller1;Ã
1
È
Institut fuÈr Organische Katalyseforschung an der Universitat Rostock e.V. (IfOK), D-18055
Rostock, Germany
2
3
È
È
Anorganisch-Chemisches Institut, Technische Universitat Munchen, D-85747 Garching, Germany
Forschungszentrum Karlsruhe, Institut fuÈr Technische Chemie, Bereich Chemisch-Physikalische
Verfahren, D-76344 Eggenstein-Leopoldshafen, Germany
Summary. The oxidative amination of styrene with secondary amines in the presence of cationic
rhodium catalysts yields regiospeci®cally the corresponding anti-Markovnikov enamines. Styrene as
the hydrogen acceptor gave concomitantly ethylbenzene. In the presence of 1,5-cyclooctadiene (cod)
preferential reduction to cyclooctene takes place. The addition of cod reduces the rate of the reaction,
but also the amount of ethylbenzene produced. Here, for the ®rst time the ratio of enamine:
ethylbenzene is > 1, which is favourable in case of more expensive styrene derivatives. A screening of
various ligands for oxidative amination reveals that hemilabile 2-(!-phosphino-n-alkyl)-pyridines are
superior ligands for this reaction compared to simple alkyl and aryl phosphines.
Keywords. Amination; Enamines; Homogeneous catalysis; Rhodium; Transition metal.
Introduction
Amines and their derivatives are of fundamental importance for organic chemistry
as natural products, pharmacological agents, ®ne chemicals, and dyes [2]. In
general, the synthesis of amine derivatives involves classical organic chemistry such
as nucleophilic substitution or nitration of aromatics and subsequent reduction.
Apart from reductive amination of carbonyl compounds, the atom-ef®cient
synthesis of amines is rare. Hence, there is considerable interest in the development
of new ef®cient catalytic routes for the construction of carbon-nitrogen bonds. In
this respect, the catalytic amination of ole®ns is a particularly convenient method
for the synthesis of amine derivatives [3]. Amination of ole®ns can take place either
as hydroamination to give alkylamines or as oxidative amination to yield enamines
or imines [4]. Recently, we discovered the ®rst intermolecular oxidative amination
of aromatic ole®ns providing enamines in anti-Markovnikov regiochemistry
(Scheme 1) [5].
a
Deceased 1997
Ã
Corresponding author

1328
A. Tillack et al.
Scheme 1
Results and Discussion
By means of cationic rhodium catalysts, the oxidative amination of styrene was
achieved using secondary amines such as piperidine or morpholine in re¯uxing
THF. Concurrently, a second molecule of styrene is reduced to give ethylbenzene. In
general, the reaction tolerates various substituents at the aromatic core of the ole®n
[6] as well as on the amine. In order to get satisfactory yields of enamines, an excess
of styrene (4±10 equivalents referred to the amine) has to be applied. Due to the
price and availability of substituted styrenes we were interested whether oxidants
other than styrene might be used for this reaction. Therefore, we tested several
oxidants for the reaction of styrene with piperidine, diethylamine, and morpholine.
The results are summarized in Table 1.
Applying previously optimized conditions (2.5 mol% [Rh(cod)2]BF4/2 PPh3,
THF, re¯ux, 20 h) [7], enamine yields of 40±74% were obtained. In the presence
of 0.5 equivalents of N-methylmorpholine-N-oxide (NMO) or benzoquinone no
reaction occured at all. We assume that the cationic rhodium(I) catalyst is oxidized
to an inactive rhodium(III) species. Furthermore, N-oxides can act as anionic
ligands [8] which destroy the catalytic activity of cationic catalysts. Next, we tested
ole®ns (in addition to styrene) to regenerate the active catalyst due to their hydrogen
acceptor capability. It is obvious that only ole®ns can be applied which are not
aminated by the rhodium catalyst under the given conditions. Here, we used
different amounts of cyclohexene, 1,3-cyclohexadiene, and 1,5-cyclooctadiene
Table 1. Amination of styrene in the presence of various oxidantsa
Enamine
Amine
Oxidant
(mol%)
Enamine
(%)
Ethylbenzene
(%)
Ethylbenzene
Piperidine
Piperidine
Piperidine
±
55
57
1
1
±
Cyclohexene (40)
17
18
1,3-Cyclohexadiene
< 0.1
< 0.1
(100)
Diethylamine
Diethylamine
Morpholine
Morpholine
±
40
23
74
27
54
24
84
6
0.8
1
Cyclohexene (40)
±
0.9
4.5
cod (25)
a
Molar ratio of styrene:amine ˆ 4:1, 2.5 mol% [Rh(cod)2]BF4/2 PPh3 relative to the amine, 20 h,
re¯ux in THF; the yield refers to the amine and was determined by GC with hexadecane as internal
standard

Amination of Ole®ns
1329
Scheme 2
(cod). In general, we observed a decreasing enamine yield with increasing ole®n
concentration. However, by adding 0.25 equivalents of 1,5-cyclooctadiene the ratio
of enamine:ethylbenzene is above unity for the ®rst time. This demonstrates that
reduction of styrene is suppressed at the expense of hydrogenation of cod as the
product, cyclooctene (coe), is detected in the reaction mixture (Scheme 2). The
complete reduction of cod to cyclooctane takes place only to a very small extent
(< 2%).
In order to study this effect more thoroughly, the reaction of styrene with
morpholine was performed in the presence of different concentrations of cod; the
results are shown in Fig. 1.
In the presence of 0.1 equivalent of cod (refers to the amine) a reduced yield of
ethylbenzene (51% vs. 84%) was observed, whereas the enamine yield (74%) was
nearly constant compared to the reaction without addition of cod. Addition of
more cod led to decreased enamine yields, but an increase of the enamine:ethyl-
benzene ratio was observed, e.g. the addition of 0.5 equivalents of cod gave an
enamine:ethylbenzene ratio of 7.5 (ethylbenzene < 2%).
The decrease of the yield of enamine with increasing concentration of cod is
explained by the favourable coordination of cod to the central metal, thus blocking
coordination sides needed for styrene to react with the amine [9]. Nevertheless,
synthetically useful yields of enamine can be achieved even in the presence of 0.25
equivalents cod if the reaction time is prolonged to 65 h. Here, the yield of enamine
again is 74%. Interestingly, the addition of cod seems to have a stabilizing effect on
Fig. 1. Oxidative amination of styrene with morpholine in the presence of cod

1330
A. Tillack et al.
Scheme 3
the active catalyst, since without cod catalyst deactivation is observed after
approximately 20 h.
It is noteworthy that in case of amination of styrene with morpholine we also
observed the formation of 2-phenylethyl-morpholine. The amount of this hydro-
amination product is not affected by the presence of cod. This gives additional
support for a parallel anti-Markovnikov hydroamination pathway [7], since
hydrogenation of 2-phenylethenyl-morpholine is unlikely in the presence of cod.
In addition to the search for alternative oxidation reagents, we were interested
in improving the catalyst system. So far, [Rh(cod)2]BF4/2 PPh3 was assumed to be
the most ef®cient catalyst system for the oxidative amination of ole®ns. Previously
we have shown that variation of the triphenylphosphine ligand (substitution with
electron withdrawing groups (e.g. F, CF3) or electron donating groups (e.g. OCH3,
CH3) lowers the enamine yield [7]. Phosphines with large Tolman angles (e.g.
P(o-tolyl)3) inhibit the reaction at all. Trialkylphosphines, e.g. tricyclohexylpho-
sphine, trimethylphosphine, or tri-n-butyl-phosphine give lower yields of N-2-
phenylethenyl-morpholine compared to PPh3. Chelating phosphines like 1,2-bis-
diphenylphosphino-ethane (dppe), but also 1,10-bisphenylferrocenylphosphine again
inhibit the activity of the catalyst. Hence, the tolerance of the catalyst for the added
ligand is very small. It seems that a subtle balance between stabilization of the
central metal and creation of free coordination sites on the rhodium center is a
prerequisite for successful catalysis. We assume that phosphines with an additional
hemilabile coordination site might ful®ll these requirements. Hence, we tested a
series of P,N-ligands of the general formula 2-pyridine-ꢀCH2†n-PPh2 (n ˆ 1, 5±7;
Scheme 3) [10].
Table 2 shows the obtained results for the amination of styrene with morpholine,
piperidine, and di-n-butylamine. Applying standard conditions (styrene:amine
ratio ˆ 4:1, 2.5 mol% [Rh(cod)2]BF4/2 PPh3, THF, 20 h, 100ꢁC, pressure tube),
enamine yields of 74, 55, and 48% were obtained. Next, the P,N-ligands were tested
with Rh:P ratios of 1:1 and 1:2. In the presence of 2-diphenylphosphinomethyl-
pyridine (PyPh-1) and 2-(5-diphenylphosphinopentyl)-pyridine (PyPh-5), very
small yields of enamines (0±10%) were observed. However, applying 5 mol% of
2-(6-diphenylphosphinohexyl)-pyridine (PyPh-6) or 2-(7-diphenylphosphinohep-
tyl)-pyridine (PyPh-7) as the ligand, improved results compared to PPh3 were

Amination of Ole®ns
1331
Table 2. In¯uence of hemilabile P,N-ligands [10] on the oxidative aminationa of aromatic ole®ns in
comparison of PPh3
Ligand
PPh3
PyPh-1
PyPh-5
PyPh-6
PyPh-7
Rh:L
Rh:L
Rh:L
Rh:L
Rh:L
Rh:L
Rh:L
Rh:L
Rh:L
1:2
1:1
1:2
1:1
1:2
1:1
1:2
1:1
1:2
Enamine
(%)b
(%)b
(%)b
(%)b
(%)b
2
3
4
5
74
55
48
18
6
10
1
1
< 1
< 1
±
4
5
±
±
4
5
±
±
46
31
14
±
74
72
42
±
64
51
21
±
76
72
52
30
±
a
Reaction conditions: molar ratio of styrene:amine ˆ 4:1, 2.5 mol% [Rh(cod)2]BF4/L or 2L relative
to the amine, THF, 20 h at 100ꢁC in a pressure tube, b the yield refers to the amine (Experimental) and
was determined by GC with hexadecane as internal standard
obtained. Similarly, the reaction of 4-¯uorostyrene with piperidine proceeds signi-
®cantly better in the presence of PyPh-7 (30 vs. 18%).
In conclusion, we report two improvements of the rhodium-catalyzed oxidative
amination of aromatic ole®ns. Our studies demonstrate that 1,5-cyclooctadiene acts
as an improved hydrogen acceptor compared to styrenes, thus reducing the amount
of ethylbenzene. This result is important for the oxidative amination of more
expensive substituted styrenes. In addition, for the ®rst time ligands are described
which give superior results compared to the standard system ([Rh(cod)2]BF4/2
PPh3). We expect this ®nding to be of value for oxidative aminations of other
substrates, too.
Experimental
All operations were carried out in an inert atmosphere (Ar). THF was freshly distilled from sodium
tetraethylaluminate under Ar prior to use. Amines, ole®ns, and oxidants were purchased from
Ê
Aldrich or Fluka. Amines were distilled from CaH2. Ole®ns were dried over 4 A molecular sieves
before use. [Rh(cod)2]BF4 [11] and !-2-pyridyl-n-alkylphosphines [10] were prepared according to
literature procedures.
Physical measurements
1H and C NMR spectra were recorded on a Bruker AM 360 spectrometer at 25ꢁC. Mass
13
spectroscopic analysis was performed on a Finnigan MAT 311A. GCs for analysis of catalytic
reactions were recorded with a HP 6890 gas chromatograph using a HP-1 capillary column. GC/MS-
studies were conducted on a HP 5890 with 70 eV electron impact ionization (detector: HP 5970 B).
General procedure for the amination of styrenes with amines in the presence of cod or oxidants
45 mg (0.11 mmol) [Rh(cod)2]BF4 and 58 mg (0.22 mmol) PPh3 were suspended in 10 cm3 THF.
Subsequently, 4.40 mmol of the amine, 17.6 mmol of styrene, and the corresponding amount of cod

1332
A. Tillack et al.
Scheme 4
(see Fig. 1) or oxidant (see Table 1) were added at room temperature. The mixture was heated to
re¯ux for 20 h. The yields were determined by gas chromatography using hexadecane as internal
standard.
General procedure for the amination of styrenes
[Rh(cod)2]BF4 (0.11 mmol) and the phosphine (0.11 or 0.22 mmol) were suspended in 10 cm3 THF.
Subsequently, the amine (4.40 mmol) and styrene (17.60 mmol) were added at room temperature.
The mixture was heated in a pressure tube for 20 h at 100ꢁC. The yields were determined by GC with
hexadecane as the internal standard. In the case of 4-¯uorostyrene, the reaction mixture was
hydrogenated to the corresponding alkylamine (24 h, 1 bar H2, 0.5 g 5% Pd/C). After hydrogenation
the catalyst was separated by ®ltration, the solvent was removed in vacuo and the residue was
taken up in CH2Cl2 (20 cm3). After extraction with 5% HCl (3 Â 20 cm3), the combined aqueous
phases were carefully brought to pH ˆ 9 (NaOH) and then extrated with CH2Cl2 (3  20 cm3). The
combined organic phases were dried over MgSO4. The solvent was removed in vacuo and the
product was puri®ed by column chromatography and dried in high vacuum. The yields were
determined by GC and refer to the amine.
(E)-N-(2-Phenylethenyl)-diethylamine (1; C12H17N)
According to the general procedure, 0.45 cm3 diethylamine (4.4 mmol), 2.0 cm3 styrene (17.6 mmol),
45 mg [Rh(cod)2]BF4 (0.11 mmol), and 58 mg PPh3 (0.22 mmol) were re¯uxed in 10 cm3 THF for
20 h. The title compound was isolated by distillation.
GC-yield: 40%; 1H NMR (360 MHz, ꢀ, CDCl3): 7.22±7.18 (m, 4H, H-2, H-3), 7.00±6.96 (m, 1H,
3
3
H-4), 6.76 (d, J(H,H) ˆ 14.1 Hz, 1H, CH±N), 5.28 (d, J(H,H) ˆ 14.1 Hz, 1H, Ph±CH), 3.19 (q,
3J(H,H) ˆ 7.2 Hz, 4H, N±CH2), 1.17 (t, 3J(H,H) ˆ 7.2 Hz, 6H, N±CH2±CH3) ppm; 13C NMR
(91 MHz, ꢀ, CDCl3): 140.2 (C-1), 138.3 (N±CH), 128.2 (C-3), 123.0 (C-2), 122.3 (C-4), 95.1 (Ph±
‡
‡
‡
‡
CH), 46.7 (N±CH2), 13.8 (CH3) ppm; MS (EI, 70 eV): m/z ˆ 175 [M ], 160 [M -CH3], 146 [M -
‡
‡
C2H5], 130 [C6H5±C2H2NCH ], 117 [M -CH2±N±(CH3)2], 103 [C6H5±CH=CH ].
(E)-N-(2-Phenylethenyl)-morpholine (2; C12H15NO)
According to the general procedure, 0.38 cm3 morpholine (4.4 mmol), 2.0 cm3 styrene (17.6 mmol),
45 mg [Rh(cod)2]BF4 (0.11 mmol), and 58 mg PPh3 (0.22 mmol) were reacted in 10 cm3 THF for
20 h at 100ꢁC. The title compound was isolated by distillation (148ꢁC, 0.1 mbar).
GC-yield: 74%; 1H NMR (360 MHz, ꢀ, CDCl3): 7.27 (m, 2H, H-2), 7.24 (m, 2H, H-3), 7.10 (m,
1H, H-4), 6.53 (d, 3J(H,H) ˆ 14.0 Hz, 1H, CH±N), 5.36 (d, 3J(H,H) ˆ 14.0 Hz, 1H, Ph±CH), 3.68 (t,
3J(H,H) ˆ 4.8 Hz, 4H, O±CH2), 2.95 (t, 3J(H,H) ˆ 4.8 Hz, 4H, N±CH2) ppm; 13C NMR (91 MHz, ꢀ,
CDCl3): 139.8 (N±CH), 138.6 (C-1), 128.5 (C-2), 124.4 (C-4), 124.2 (C-3), 101.5 (Ph±C), 66.5 (O±
‡
‡
‡
C), 49.1 (N±CH2) ppm; MS (EI, 70 eV): m/z ˆ 189 [M ], 158 [M -CH3O], 130 [C6H5±C2H2NCH ],
‡
‡
‡
104 [C6H5±C2H3 ], 91 [C6H5±CH2 ], 77 [C6H5 ].

Amination of Ole®ns
1333
(E)-N-(2-Phenylethenyl)-piperidine (3; C13H17N)
According to the general procedure, 0.44 cm3 piperidine (4.4 mmol), 2.0 cm3 styrene (17.6 mmol),
45 mg [Rh(cod)2]BF4 (0.11 mmol), and 58 mg PPh3 (0.22 mmol) were reacted in 10 cm3 THF for
20 h at 100ꢁC. The title compound was isolated by distillation.
GC-yield: 55%; 1H NMR (360 MHz, ꢀ, CDCl3): 7.22±7.19 (m, 4H, H-2, H-3), 7.02±6.99 (m, 1H,
3
3
H-4), 6.67 (d, J(H,H) ˆ 14.2 Hz, 1H, CH±N), 5.37 (d, J(H,H) ˆ 14.2 Hz, 1H, Ph±CH), 3.03 (t,
3J(H,H) ˆ 4.8 Hz, 4H, N±CH2), 1.66±1.57 (m, 6H, N±CH2±CH2±CH2) ppm; 13C NMR (91 MHz, ꢀ,
CDCl3): 140.3 (N±CH), 139.5 (C-1), 128.4 (C-3), 123.8 (C-2), 123.6 (C-4), 99.4 (Ph±CH), 49.7 (N±
‡
CH2), 25.3 (N±CH2±CH2), 24.3 (N±CH2±CH2±CH2) ppm; MS (EI, 70 eV): m/z ˆ 187 [M ], 130
‡
‡
[C6H5±C2H2NCH ], 104 [C6H5±C2H3 ].
(E)-N-(2-Phenylethenyl)-di-n-butylamine (4; C16H25N)
According to the general procedure, 0.74 cm3 di-n-butylamine (4.4 mmol), 2.0 cm3 styrene
(17.6 mmol), 45 mg [Rh(cod)2]BF4 (0.11 mmol), and 58 mg PPh3 (0.22 mmol) were reacted in
10 cm3 THF for 20 h at 100ꢁC. The title compound was isolated by distillation.
GC-yield: 48%; 1H NMR (360 MHz, ꢀ, CDCl3): 7.22±7.19 (m, 4H, H-2, H-3), 7.02±6.99 (m, 1H,
3
3
H-4), 6.81 (d, J(H,H) ˆ 14.1 Hz, 1H, CH±N), 5.16 (d, J(H,H) ˆ 14.1 Hz, 1H, Ph±CH), 3.12 (t,
3J(H,H) ˆ 7.2 Hz, 4H, N±CH2), 1.59 (quin, 3J(H,H) ˆ 7.2 Hz, 4H, N±CH2±CH2), 1.39 (sext,
3J(H,H) ˆ 7.3 Hz, 4H, N±CH2±CH2±CH2), 1.00 (t, 3J(H,H) ˆ 7.3 Hz, 6H, CH3) ppm; 13C NMR
(91 MHz, ꢀ, CDCl3): 140.4 (C-1), 138.5 (N±CH), 128.5 (C-3), 123.1 (C-2), 122.6 (C-4), 95.4 (Ph±
CH), 51.5 (N±CH2), 30.1 (N±CH2±CH2), 20.3 (N±CH2±CH2±CH2), 14.0 (CH3) ppm; MS (EI,
‡
‡
‡
‡
70 eV): m/z ˆ 231 [M ], 188 [M -C3H7], 146 [M -C6H14], 130 [C6H5±C2H2NCH ], 103 [C6H5±
‡
‡
CH=CH ], 84 [CH=N±(CH2)3±CH3 ].
(E)-N-(2-(4-Fluorophenyl)-ethenyl)-piperidine (5; C13H16FN)
According to the general procedure, 0.44 cm3 piperidine (4.4 mmol), 2.1 cm3 4-¯uorostyrene
(17.6 mmol), 45 mg [Rh(cod)2]BF4 (0.11 mmol), and 58 mg PPh3 (0.22 mmol) were reacted in
‡
‡
‡
10 cm3 THF for 20 h at 100ꢁC. MS (EI, 70 eV): m/z ˆ 205 [M ], 162 [M -C3H7], 148 [M -C3H7N],
‡
‡
135 [M -C4H8N], 122 [F±C6H4±C2H3 ]. The isolation was performed by hydrogenation according
to the general procedure, and the product 6 was puri®ed by column chromatography (n-hexane:ethyl
acetate:NEt3 ˆ 1:1:0.01). GC-yield: 18%.
N-(2-(4-Fluorophenyl)-ethyl)-piperidine (6; C13H18FN)
3
1H NMR (360 MHz, ꢀ, CDCl3): 7.22±7.11 (m, 2H, H-3), 6.94 (t, J(H,H) ˆ 8.6 Hz, 2H, H-2), 2.83±
3
2.75 (m, 2H, Ph±CH2), 2.55±2.44 (m, 6H, N±CH2±CH2±Ph, N±CH2), 1.61 (quin, J(H,H) ˆ 5.4 Hz,
4H, N±CH2±CH2), 1.47 (m, 2H, N±CH2±CH2±CH2) ppm; 13C NMR (91 MHz, ꢀ, CDCl3): 161.3 (d,
1J(C,F) ˆ 243.2 Hz, C-4), 136.1 (C-1), 129.9 (d, 3J(C,F) ˆ 7.8 Hz, C-2), 114.9 (d, 2J(C,F) ˆ 65.4 Hz,
C-3), 61.3 (N±CH2), 54.5 (N±CH2±CH2±CH2), 32.7 (Ph±CH2), 25.9 (N±CH2±CH2±CH2), 24.3 (N±
‡
‡
‡
CH2±CH2±CH2) ppm; MS (EI, 70 eV): m/z ˆ 207 [M ], 109 [F±C6H4±CH2 ], 98 [C4H8N±CH2 ].
Acknowledgements
We thank the State of Mecklenburg-Vorpommern for funding and Mrs. Christine Mewes for technical
assistance. In addition, we are grateful to Degussa-HuÈls AG, Aventis R&T, and Heraeus AG for gifts
of rhodium catalysts. We also thanks the Deutsche Forschungsgemeinschaft (BE 1931/3-1) for
®nancial support.

1334
A. Tillack et al.: Amination of Olefins
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Received May 30, 2000. Accepted July 11, 2000