Zwitterionic rhodium complex catalyzed hydroaminomethylation of arylethylenes

Article abstract of DOI:10.1016/S0040-4039(01)00202-7

The hydroaminomethylation of arylethylenes can be achieved in high regioselectivity using the zwitterionic rhodium complex [Rh⁺(cod)(η⁶-PhBPh₃)^-] (1) as the catalyst under relatively mild conditions.

Full text of DOI:10.1016/S0040-4039(01)00202-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2423–2425
Zwitterionic rhodium complex catalyzed hydroaminomethylation
of arylethylenes
Yong-Shou Lin, Bassam El Ali† and Howard Alper*
Center for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa,
Ont., Canada K1N 6N5
Received 5 January 2001; revised 31 January 2001; accepted 1 February 2001
Abstract—The hydroaminomethylation of arylethylenes can be achieved in high regioselectivity using the zwitterionic rhodium
complex [Rh+(cod)(h6-PhBPh3)−] (1) as the catalyst under relatively mild conditions. © 2001 Elsevier Science Ltd. All rights
reserved.
The hydroaminomethylation of olefins has attracted
attention as an atom-economical efficient, one-pot syn-
thesis of amines.1,2 Originally discovered by Reppe and
co-workers,3 hydroaminomethylation consists of a reac-
tion sequence of hydroformylation of an olefin followed
by condensation with an amine and hydrogenation.
Catalysts used initially were iron and cobalt carbonyl
complexes, with more efficient rhodium and ruthenium
complexes developed in the last two decades.4
[Rh(CO)2Cl]2 and [Rh(cod)Cl]2 are the most common
rhodium catalysts employed in hydroaminomethylation
reactions.1a
mild conditions, affords the corresponding branched
methylated amines in high regioselectivity.
Hydroaminomethylation of styrene (2a, CO/H2=1:1,
200 psi) in the presence of isopropylamine and a cata-
lytic amount of complex 1 in THF afforded N-isopro-
pyl-2-phenylpropylamine (3a) in 85% isolated yield,
with the linear isomer as a by-product (4a, entry 1 of
Table 1 and Eq. (1)). The branched/linear ratio
(11.5:1.0) is higher than that previously reported.6c,7 In
general, hydroaminomethylation has been carried out
at relatively higher pressures of syngas (]100 bar),
except when one uses the dinuclear rhodium complex
[Rh2(m-SBut)2(CO)2(PPh3)2] as the catalyst for the
aminomethylation of oct-1-ene.8 However, the latter
gave linear amine as the major product. If the CO/H2
pressure was increased to 1000 psi in the present reac-
tion, a much higher branched/linear ratio was obtained
(15.3:1, entry 2). A decrease or increase of the reaction
temperature resulted in a lower conversion of styrene
(entry 3), or a lower ratio of 3a to 4a together with the
formation of other by-products (entry 4).
The zwitterionic rhodium complex [Rh+(cod)(h6-
PhBPh3)−] (1) is an excellent catalyst for the hydro-
formylation of olefins with high regioselectivity.5
Complex 1-catalyzed hydroformylation of aryl olefins
gives aldehydes in a high branched/linear ratio.5a Rela-
tively few studies on the hydroaminomethylation of
styrenes have been reported in the literature.6 We have
found that the hydroaminomethylation of various aryl
olefins using complex 1 as the catalyst, under relatively
NH
1
(1)
+
NH2
Ar
NH
+
Ar
CO/H2
Ar
2
4
3
Various para-substituted styrenes (2) have been
aminomethylated to the corresponding branched amine
(3) as the major product, together with the linear
isomer (4, Table 1, entries 5–10, and Eq. (1)). Both
electron-withdrawing and electron-donating sub-
stituents in the para position gave the branched amine
as the major product. Somewhat lower yields of
Keywords: amines; hydroaminomethylation; hydroformylation;
rhodium complex.
* Corresponding author. Tel.: +1-613-562-5189; fax: +1-613-562-5871;
e-mail: halper@uottawa.ca
†
Present address: KFUPM, Chemistry Department, Dhahran 31261,
Saudi Arabia.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00202-7

2424
Y.-S. Lin et al. / Tetrahedron Letters 42 (2001) 2423–2425
Table 1. Hydroaminomethylation of arylethylenes in the
branched amines resulted using para-tert-butylstyrene
and para-chlorostyrene as substrates (entries 4 and 5,
respectively).
presence of isopropylaminea
Entry Ar
Yield (mol%)b
Ratio
The hydroaminomethylation reaction of styrene can be
applied to both primary and secondary amines. The
results using different amines (5), to form branched and
linear arylamines (6 and 7, respectively), are summa-
rized in Table 2 (Eq. (2)). Cyclohexylamine (5a) gave
higher conversion with the corresponding branched
amine (6a) as the predominant product (entry 1). A
substantially higher ratio of branched/linear amines
was again obtained by increasing the CO/H2 pressure
(entry 2). n-Butylamine gave a higher branched/linear
ratio than tert-butylamine (entries 4 and 5). Benzy-
lamine as well as secondary amines including diethy-
lamine and piperidine also afforded higher yields of the
corresponding branched amines, with linear amines as
minor products (entries 6, 9, and 10). Higher CO/H2
pressures led to higher regioselectivity for branched
amines in the case of benzylamine (entry 7). However,
aniline formed the corresponding branched and linear
amines in a ratio of only 1.9:1 (entry 8). It is also
noteworthy that hydroaminomethylation of an internal
olefin, 1-phenyl-1-propene (8), in the presence of iso-
propylamine, gave isopropyl-(2-phenylbutyl)amine (9)
as the principal product (61%), together with isopropyl-
(2-methyl-3-phenylpropyl)amine (10, 7%, Eq. (3)).
3
4
3/4
1
C6H5- (2a)
C6H5- (2a)
C6H5- (2a)
C6H5- (2a)
p-CH3C6H4- (2b)
p-CH3OC6H4- (2c)
92 (85, 3a)
92 (3a)
25 (3a)
34 (3a)
85 (65, 3b) 15 (8, 4b)
90 (74, 3c) 10 (7, 4c)
8 (4a)
6 (4a)
0 (4a)
29 (4a)
11.5
15.3
–
1.2
5.7
9.0
3.7
5.5
6.1
6.7
2c
3d
4e
5
6
7f,g
8f,h
9
p-(CH3)3CC6H4- (2d) 70 (62, 3d) 19 (14, 4d)
p-ClC6H4- (2e)
p-FC6H4- (2f)
p-PhC6H4- (2g)
77 (57, 3e) 14 (4e)
86 (67, 3f) 14 (10, 4f)
87 (76, 3g) 13 (4g)
10
a Reaction conditions: arylethylene (2 mmol), isopropylamine (2.2
mmol), catalyst (0.01 mmol), CO/H2 (1:1, 200 psi), THF (5 mL), 80°C,
24 h, unless otherwise described.
b The yields were determined by 1H NMR and GC (the isolated yield
is in parenthesis).
The reaction was carried out under CO/H2 (1:1, 1000 psi).
d The reaction was run at 50°C, and 75% of styrene was recovered.
The reaction was run at 110°C, and N,N-bis(2-phenylpropyl)-
isopropylamine and its isomers were detected as by-products.
c
e
f
A trace amount of ArCOCH3 was detected.
g 8 mol% of olefin was recovered.
h 9 mol% of by-products were unidentified.
NRR'
1
+
Ph
NRR'
+
RR'NH
Ph
CO/H2
(2)
(3)
Ph
7
5
6
1 (0.01 mmol), 80 °C
NH
Ph
+
+
NH2
Ph
THF, 24 h
CO/H2 (1:1, 600 psi)
NH
10 (7%)
8
Ph
1.2 mmol
1 mmol
9 (61%)
Table 2. Hydroaminomethylation of styrene with various aminesa
Entry
RR%NH
CO/H2
Yield (mol%)b
Ratio
5
(1:1, psi)
6
7
6/7
1
2
Cyclohexylamine (5a)
Cyclohexylamine (5a)
n-Butylamine (5b)
n-Butylamine (5b)
tert-Butylamine (5c)
Benzylamine (5d)
Benzylamine (5d)
Aniline (5e)
200
1000
200
600
600
600
1000
600
200
200
78 (68, 6a)
91 (6a)
45 (6b)
89 (68, 6b)
58 (40, 6c)
84 (6d)
84 (78, 6d)
58 (41, 6e)
81 (60, 6f)
88 (74, 6g)
22 (7a)
7 (7a)
10 (7b)
6 (7b)
14 (7, 7c)
14 (7d)
6 (7d)
30 (7e)
18 (11, 7f)
12 (10, 7g)
3.5
13.0
4.5
14.8
4.1
6.0
14.0
1.9
3c
4d
5e
6
7f
8g
9
Diethylamine (5f)
Piperidine (5g)
4.5
7.3
10
a Reaction conditions: styrene (2 mmol), amine (2.2 mmol), catalyst (0.01 mmol), CO/H2, THF (5 mL), 80°C, 24 h.
b The yields were determined by 1H NMR and GC GC (the isolated yield is in parenthesis).
c 23 mol% of the imine, PhCH(CH3)CHꢀNRR%, was found, and 22% of styrene was recovered.
d 5 mol% of styrene was recovered.
e 19 mol% of imine was formed together with a small amount of unidentified products.
f 9% of ethylbenzene was formed.
g Small amounts of acetophenone (6%) and unidentified products (<6%) were detected.

Y.-S. Lin et al. / Tetrahedron Letters 42 (2001) 2423–2425
2425
In conclusion, the zwitterionic rhodium complex 1 is an
efficient catalyst for the hydroaminomethylation of
arylethylenes under relatively mild conditions, to give
branched chain amines in high regioselectivity.
Roggenbuck, R.; Schmidt, A. Chem. Rev. 1999, 99, 3329;
(b) Beller, M.; Cornils, B.; Frohning, C. D.; Kohlpaintner,
C. W. J. Mol. Catal. A 1995, 104, 17.
2. For recent examples, see: (a) Kranemann, C. L.; Eilbracht,
P. Eur. J. Org. Chem. 2000, 2367; (b) Rische, T.; Ba¨r-
facker, L.; Eilbracht, P. Eur. J. Org. Chem. 1999, 653; (c)
Eilbracht, P.; Kranemann, C. L.; Ba¨rfacker, L. Eur. J.
Org. Chem. 1999, 1907; (d) Rische, T.; Eilbracht, P. Tetra-
hedron 1999, 55, 7841; (e) Rische, T.; Eilbracht, P. Tetra-
hedron 1999, 55, 1915; (f) Rische, T.; Mu¨ller, K.-S.;
Eilbracht, P. Tetrahedron 1999, 55, 9801; (g) Ba¨rfacker, L.;
Rische, T.; Eilbracht, P. Tetrahedron 1999, 55, 7177; (h)
Zimmermann, B.; Herwig, J.; Beller, M. Angew. Chem.,
Int. Ed. 1999, 38, 2372; (i) Breit, B. Tetrahedron Lett. 1998,
39, 5163; (j) Kranemann, C. L.; Eilbracht, P. Synthesis
1998, 71; (k) Schaffrath, H.; Keim, W. J. Mol. Catal. A
1999, 140, 107.
3. Reppe, W.; Vetter, H. Liebigs Ann. Chem. 1953, 582, 133.
4. Iqbal, A. F. M. Helv. Chim. Acta 1971, 45, 1440.
5. (a) Amer, I.; Alper, H. J. Am. Chem. Soc. 1990, 112, 3674;
(b) Alper, H.; Zhou, J.-Q. J. Org. Chem. 1992, 57, 3729;
(c) Alper, H.; Zhou, J.-Q. J. Chem. Soc., Chem. Commun.
1993, 316; (d) Totland, K.; Alper, H. J. Org. Chem. 1993,
58, 3326; (e) Lee, C. W.; Alper, H. J. Org. Chem. 1995, 60,
499; (f) Park, H. S.; Alberico, E.; Alper, H. J. Am. Chem.
Soc. 1999, 121, 11697.
General experimental procedure: A mixture of olefin (2
mmol), amine (2.2 mmol), zwitterionic rhodium com-
plex 1 (0.01 mmol), and THF (5 mL) was placed in a 45
mL stainless steel autoclave equipped with a glass liner
and magnetic stirrer. The autoclave was purged three
times with carbon monoxide and then pressurized to
the desired level by CO and then H2. The reaction was
carried out in an oil bath, and once complete, the
autoclave was cooled to room temperature and the gas
was released. After evaporation of volatile components,
CDCl3 (1 mL) and Cl2CHCHCl2 (0.5 mmol, as an
internal reference) were added, and the products were
1
analyzed by H NMR and GC–MS. Purification of the
branched amines was attained by column chromatogra-
phy (alumina), preparative TLC or by HPLC. The
branched amines 3, 6, and 9 obtained were fully charac-
1
terized by H and 13C NMR, MS, and HRMS.
Acknowledgements
6. (a) Kostas, I. D. J. Chem. Res. (S) 1999, 630; (b) Rische,
T.; Kitsos-Rzychon, B.; Eilbracht, P. Tetrahedron 1998,
54, 2723; (c) Rische, T.; Eilbracht, P. Synthesis 1997, 1331.
7. By using [Rh(CO)2Cl]2 or [Rh(cod)Cl]2 as the catalyst
instead of complex 1, the hydroformylation of styrene in
the presence of isopropylamine gave a lower branched to
linear ratio (4:1) under the same reaction conditions.
8. (a) Baig, T.; Molinier, J.; Kalck, P. J. Organomet. Chem.
1993, 455, 219; (b) Baig, T.; Kalck, P. J. Chem. Soc.,
Chem. Commun. 1992, 1373.
We are grateful to the Natural Sciences and Engineer-
ing Research Council of Canada for support of this
research.
References
1. (a) Eilbracht, P.; Ba¨rfacker, L.; Buss, C.; Hollmann, C.;
Kitsos-Rzychon, B. E.; Kranemann, C. L.; Rische, T.;
.
.