Rhodium-catalyzed regioselective hydroaminomethylation of terminal olefins with pyrrole-based tetraphosphorus ligands

Article abstract of DOI:10.1039/c3cy01069d

A concise and elegant protocol was developed to prepare linear amines by regioselective hydroaminomethylation of terminal olefins with pyrrole-based tetraphosphorus ligands. It has been documented that the reactivity of the ligand is modulated by the substituent of the biphenylphosphane moiety. Ligand L5 containing electron-donating groups exhibited the highest reactivity, with up to 70.9 n/i ratio and 99.5% amine selectivity for 1-pentene and a 31.3 n/i ratio and 97.9% amine selectivity for 1-hexene. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01069d

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Rhodium-catalyzed regioselective
hydroaminomethylation of terminal olefins
with pyrrole-based tetraphosphorus ligands†
Cite this: Catal. Sci. Technol., 2014,
4, 917
a
abd
b
Guodu Liu,abc Zhao Li, Huiling Geng
Received 17th December 2013,
Accepted 10th January 2014
*
*
and Xumu Zhang
DOI: 10.1039/c3cy01069d
www.rsc.org/catalysis
A concise and elegant protocol was developed to prepare linear
amines by regioselective hydroaminomethylation of terminal
olefins with pyrrole-based tetraphosphorus ligands. It has been
documented that the reactivity of the ligand is modulated by the
substituent of the biphenylphosphane moiety. Ligand L5 containing
electron-donating groups exhibited the highest reactivity, with up
to 70.9 n/i ratio and 99.5% amine selectivity for 1-pentene and a
31.3 n/i ratio and 97.9% amine selectivity for 1-hexene.
reaction were documented by Eilbracht's group, in which
they prepared a series of highly functionalized diamines
and triamines with potential biological activities utilizing
Rh-catalyzed hydroaminomethylation of heterofunctionalized
olefins.9 A series of rhodium catalysts based on modified
Naphos- and Xantphos-ligands were developed for n-selective
hydroaminomethylation of terminal and internal olefins by
Beller and coworkers. Furthermore, they also ingeniously
designed and synthesized a series of other rhodium complexes
to realize the preparation of various bioactive compounds.10
Moreover, many other excellent catalysts based on rhodium
and novel synthetic applications were reported by the groups of
Vogt,11 Alper,12 Kalck,13 Ding14 and others.15 Meanwhile, meth-
odology improvements on biphasic systems16 and microwave-
assisted hydroaminomethylation were presented as well.17
Recently, we have reported the synthesis and application
of the pyrrole-based tetraphosphorus ligand TPPB (2,2′,6,6′-
tetrakis(dipyrrolyl phosphoramidite)-1,1′-diphenyl) and a series
of its derivatives, which showed high regioselectivity for the
isomerization–hydroformylation of internal olefins, styrene deriv-
atives and other functionalized olefins.18 To explore the reactivity
of this catalytic system, we designed and synthesized two new
pyrrole-based ligands: a 3,3′,5,5′-tetra-t-butylphenyl-substituted
diphenylpyrrole-based tetraphosphorus ligand L1 and a 4,4′-
dimethyl-substituted diphenylpyrrole-based tetraphosphorus
ligand L2 (Fig. 1). These two new ligands and seven other known
pyrrole-based tetraphosphorus ligands were then chelated with
rhodium to catalyze the hydroaminomethylation of terminal ole-
fins. To our delight, most complexes could efficiently transform
terminal olefins into the corresponding linear amines with up to
>99% conversion and high linear amine selectivity. Herein, we
present our recent results of the hydroaminomethylation of
terminal olefins with these ligands.
Hydroaminomethylation represents one of the most
important catalytic protocols for the synthesis of amines as it
is an environmentally benign and atom-efficient method
using ubiquitous and readily available olefins as starting
materials.1,2 These amine products are versatile intermedi-
ates and building blocks for many valuable pharmaceuticals,
bioactive natural products, dyes, agrochemicals and fine
chemicals.3 Most commonly, in hydroaminomethylation,
bisphosphorus ligands along with rhodium metal precursors
are used.4 The representative ligands include Iphos with a 9
n/i ratio and 97% amine selectivity for 2-butene, Naphos with
a 99 n/i ratio and 6% amine selectivity for 2-butene,5
Xantphos with a 48 n/i ratio and 97% amine selectivity for
1-pentene,6 and Xantphenoxaphos with a 24 n/i ratio and
98% amine selectivity for 2-hexene.7
Pioneering work on the hydroaminomethylation reaction
was first reported by Reppe in 1949.8 More than sixty years
later, the efficiency and comprehensive applications of this
a College of Science, Northwest A&F University, Shaanxi 712100, Yangling, China.
E-mail: genghuiling5@163.com
b Department of Chemistry and Chemical Biology and Department of Medicinal Chemistry,
Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA.
E-mail: xumu@rci.rutgers.edu; Fax: +1 732 445 6312
c Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
d Key Laboratory of Production and Utilization of Biological Resources in Tarim Basin,
Xinjiang Production & Corps, Xinjiang 843300, Alar, China
† Electronic supplementary information (ESI) available: Synthetic procedures
and the NMR characterization and enantioselectivity analysis of the products.
See DOI: 10.1039/c3cy01069d
Firstly, two new pyrrole-based ligands (L1 and L2) were
synthesized according to the following procedures. Diphenyl
with 3,3′,5,5′-tetra-t-butylphenyl could be prepared in good
yield from an iodo-substituted derivative and its boronic
acid via Suzuki coupling. Treatment of the substituted
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successfully. These two new ligands were air-stable solids. The
unoptimized yields for them were around 25–30%.
TPPB (L3) was chosen as the standard ligand for the
hydroaminomethylation of 1-hexene and piperidine. Reaction
conditions were optimized based on the previous study of this
reaction.20,21 Rh(acac)(CO)2 gave the best n/i ratio of 19.2 in
the 1 : 1 mixed solvent of methanol and toluene at 125 °C
(Table 1, entries 1–3), thus, it was used as the standard precur-
sor for further optimization. Various solvents and pressures
were tested for better amine selectivity and higher n/i ratios.
Although the n/i ratio was 27.8 in the 2: 1 mixture of
2-propanol and ethanol, the amine selectivity was not ideal
Fig. 1 The structures of new pyrrole-based tetraphosphorus ligands.
2,2′,6,6′-tetramethoxybiphenyl with boron tribromide gave
the parent tetraol, which was allowed to react with chloro-
dipyrrolylphosphine to furnish ligand L1 in the presence of
NEt3 at room temperature (Scheme 1).18b Ligand L2 was
synthesized from 5-methylbenzene-1,3-diol in four steps
(Scheme 2). This diol was firstly protected with iodomethane
to form 1,3-dimethoxy-5-methylbenzene. Then, following the
known procedures,18a the compound tetraol could be afforded
in two simple steps.19 In the presence of NEt3, tetraol
smoothly reacted with freshly made chlorodipyrrolylphosphine
to yield the desired pyrrole-based tetraphosphorus ligand L2
Table
1 Hydroaminomethylation of 1-hexene and piperidine with
TPPB under different reaction conditionsa
Linear sel.b
n-Aminec
CO/H2 Temp. Con. Amine
Entry Cata. Solvent (bar) (°C)
(%) sel.b
n/i (%)
1
2
3
4
5
6
7
8
A
B
C
A
A
A
A
A
Me/To = 7/35
1 : 1
Me/To = 7/35
1 : 1
Me/To = 7/35
1 : 1
Et/To = 7/35
2 : 1
Pr/Et = 7/35
2 : 1
Pr/Me = 7/35
2 : 1
Pr/Me = 7/35
1 : 1
Pr/Me = 7/35
1 : 2
125
125
125
125
125
125
125
125
125
99
99
99
80
99
99
99
99
81.7
84.7
91.1
65.8
83.2
91.6
94.0
96.1
19.2 95.0
11.8 92.2
7.6 88.4
25.3 96.2
27.8 96.4
13.1 92.9
9.6 90.6
6.6 86.8
Scheme 1 Synthesis of ligand L1.
9
A
A
A
A
A
A
A
A
A
2-PrOH 7/35
2-PrOH 10/50 125
99
99
99
99
99
99
99
99
99
91.6
83.5
83.6
75.8
87.5
90.6
92.4
92.1
92.0
29.3 96.7
14.8 93.2
18.3 94.8
45.2 97.8
27.1 96.4
32.0 96.9
30.1 96.8
17.2 94.5
16.9 94.4
10
11
12
13
14
15
16
17
2-PrOH 7/7
2-PrOH 5/5
2-PrOH 5/10
2-PrOH 5/30
2-PrOH 5/35
2-PrOH 5/40
2-PrOH 5/35
125
125
125
125
125
125
130
a
Reaction conditions: S/Rh = 1000, 1 μmol Rh precursor, 4 μmol
TPPB (L3), 1 mmol 1-hexene, 1 mmol piperidine, 125 °C, 8 h, and
3 mL solvent. A = Rh(acac)(CO)2, B = [Rh(cod)2]BF4, C = [Rh(cod)Cl]2,
Me = methanol, Et = ethanol, Pr = 2-propanol, To = toluene.
b
Selectivity and n/i ratios were determined by GC analysis using
c
2-methoxyethyl ether (0.1 mL) as an internal standard. Percentage
Scheme 2 Synthesis of ligand L2.
of linear amines in all amines.
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Table
2
Hydroaminomethylation of 1-hexene and piperidine with
(entry 5). We could conclude that polar solvents are beneficial
for full conversion of 1-hexene and higher amine selectivity;
however, under these conditions the regioselectivity was a little
lower than other circumstances (entries 6–8). Using 2-propanol
as the solvent, higher regioselectivity (n/i = 29.3), higher amine
selectivity (91.6%) and full conversion were observed for
1-hexene (entry 9). Consequently, with 2-propanol as the
solvent, different pressures were evaluated to search for the
optimal CO/H2 pressure. When the pressure of CO/H2 was 5/5
bar, the highest n/i ratio of amine was achieved (45.2), but
amine selectivity was somewhat lower as the intermediates,
enamines, couldn't be fully hydrogenated in 8 hours (entry 12).
When the pressure of H2 was gradually increased, to our sur-
prise, a 30.1 n/i ratio and 92.4% amine selectivity were
achieved with 5/35 bar CO/H2 in 8 hours (entry 15). However,
further increasing the pressure of H2 to 40 bar resulted in the
decrease of amine selectivity and the n/i ratio (entry 16). We
also increased the temperature to 130 °C; unfortunately, the n/i
ratio and amine selectivity were decreased simultaneously
(entry 17). The preliminary optimal conditions were as follows:
Rh(acac)(CO)2, S/L/Rh = 1000/4/1, 2-propanol, 125 °C, and 5/35
bar CO/H2, which were applied in the next ligand screening.
Under optimized reaction conditions, a series of pyrrole-
based tetraphosphorus ligands (L1–L9, Fig. 2) were subjected
to regioselective hydroaminomethylation of 1-hexene (Table 2).
In all cases, no N-formylpiperidine was detected and the
conversion of 1-hexene was more than 99%. However, the
fluctuations of the n/i ratio and amine selectivity were totally
different. The introduction of substituents to the diphenyl-
phosphane moiety of TPPB dramatically affected both the
regioselectivity of amines and the activity of the Rh catalytic
system. For ligand L5, the methyl groups at the 3,3′,5,5′-
positions of the diphenylphosphane moiety exerted
electronic effects on the regioselectivity of the amine prod-
ucts. The electron-donating alkyl-substituted ligands (L2, L5
and L6) showed higher reactivities than the chloric-
substituted ligand L4 and aromatic-substituted ones (L1 and
L7–L9) (entries 3–4, 9 vs. 2, 5–8). Those ligands with
electron-withdrawing groups, especially L4, gave the lowest
amine selectivity with moderate regioselectivity (entry 2). The
other electron-donating aromatic-substituted ligands showed
good amine selectivity while the regioselectivity was relatively
pyrrole-based ligandsa
Linear sel.b
Entry Ligand Con. (%) Amine sel.b n/i n-Aminec (%) TONd
1
2
3
4
5
6
7
8
9
L3
L4
L5
L6
L7
L8
L9
L1
L2
99
99
99
99
99
99
99
99
99
93.2
92.6
97.9
99.2
99.6
97.1
98.2
98.6
99.8
30.1 96.8
938
853
940
919
902
886
893
903
830
13.5 93.1
31.3 96.9
14.5 93.5
10.8 91.5
11.9 92.2
11.3 91.9
12.4 92.5
5.22 83.9
a
Reaction conditions: S/Rh = 1000, 1 μmol Rh(acac)(CO)2, 4 μmol
ligand, 1 mmol 1-hexene, 1 mmol piperidine, 125 °C, 8 h, and 3 mL
b
2-propanol. Selectivity and n/i ratio were determined by GC analysis
using 2-methoxyethyl ether (0.1 mL) as an internal standard. c Percentage
of linear amines in all amines. d Turnover numbers of linear amines.
low (entries 6 and 8). The 4,4′-dimethyl-substituted ligand L2
gave the highest amine selectivity (up to 99.8%), but the
linear regioselectivity was decreased (entry 9). Therefore, as
far as the total catalytic reactivity is concerned, L5 is the best
ligand with 97.9% amine selectivity, a 31.3 n/i ratio and a
turnover number of 940 for the regioselective hydroamino-
methylation of 1-hexene (entry 3).
These pyrrole-based ligands were also applied in the regio-
selective hydroaminomethylation of 1-pentene and piperidine
under the same reaction conditions. Given the amine selectivity
and regioselectivity, all ligands revealed better reactivity for
1-pentene than for 1-hexene, and the influence of the substitu-
ents was consistent with that for 1-hexene. Most of the amine
Table 3 Hydroaminomethylation of 1-pentene and piperidine with
pyrrole-based ligandsa
Linear sel.b
Entry Ligand Con. (%) Amine selb n/i
n-Aminec (%) TONd
1
2
3
4
5
6
7
8
9
L3
L4
L5
L6
L7
L8
L9
L1
L2
99
99
99
99
99
99
99
99
99
95.1
93.8
99.5
98.7
99.4
99.6
99.2
99.2
99.9
14.1 93.4
878
869
971
947
929
931
941
941
954
14.6 93.6
70.9 98.6
32.2 97.0
17.3 94.5
16.9 94.4
23.5 95.9
22.7 95.8
27.7 96.5
a
Reaction conditions: S/Rh = 1000, 1 μmol Rh(acac)(CO)2, 4 μmol
ligand, 1 mmol 1-pentene, 1 mmol piperidine, 125 °C, 8 h, and 3 mL
b
2-propanol. Selectivity and n/i ratio were determined by GC analysis
using 2-methoxyethyl ether (0.1 mL) as an internal standard. c Percentage
of linear amines in all amines. d Turnover numbers of linear amines.
Fig. 2 The structures of pyrrole-based tetraphosphorus ligands for
the hydroaminomethylation of terminal olefins.
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selectivity was up to 99%. The highest n/i ratio was 70.9 with
99.5% amine selectivity, which was offered by ligand L5, the
best ligand for 1-hexene (Table 3, entry 3). A surprising result
was achieved with L2, the 4,4′-dimethyl-substituted ligand. Its
amine selectivity was up to 99.9% (almost 100%), the n/i ratio
was 27.7, and the total amine product was 96.5% (entry 9).
Thus, two ligands, L5 (TON of 971) and L2 (TON of 954),
worked excellently for the hydroaminomethylation of 1-pentene.
Notes and references
1 W. Reppe, Experientia, 1949, 5, 93–110.
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Conclusions
In conclusion, a concise and green method was developed to
synthesize linear amines by rhodium-catalyzed regioselective
hydroaminomethylation of 1-hexene and 1-pentene with
pyrrole-based tetraphosphorus ligands. The substituents of the
diphenylphosphane moiety of the ligands greatly affected the
amine selectivity and regioselectivity. The 3,3′,5,5′-tetramethyl-
substituted pyrrole-based tetraphosphorus ligand L5 was found
to be the best ligand at hand, with up to 70.9 n/i ratio and
99.5% amine selectivity for 1-pentene and a 31.3 n/i ratio and
97.9% amine selectivity for 1-hexene. Moreover, the 4,4′-
dimethyl-substituted ligand L2 also showed excellent reactivity,
99.9% amine selectivity and a 27.7 n/i ratio, for the regio-
selective hydroaminomethylation of 1-pentene. The mechanism
of the effect of the substituent on the ligands' stereoselectivity
is not very clear; this will be disclosed in our further study.
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Experimental section
General procedure for hydroaminomethylation of 1-hexene
with piperidine20
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All hydroaminomethylation experiments were performed in a
nitrogen-filled glove box. In a typical experiment, a 10 mL
long neck vial with a magnetic stirring bar was charged with
TPPB (4 μmol, 3.5 mg) and Rh(acac)(CO)2 (1 μmol, 0.1 mL of
10 mmol solution in toluene). After the mixture was stirred
for 10 min, 1-hexene (1 mmol, 0.125 mL) and piperidine
(1 mmol, 0.098 mL) were added, followed by the addition of
2-propanol (3 mL) and 2-methoxyethyl ether (0.1 mL, internal
standard). The reaction mixture was transferred to an auto-
clave; all vials were covered with a simple lid. The autoclave
was purged with H2 three times and subsequently charged
with CO (5 bar) and H2 (35 bar). After the reaction was car-
ried out at 125 °C for 8 h, the autoclave was then cooled to
room temperature and depressurized carefully in a well-
ventilated hood. The reaction mixture was immediately ana-
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC 31101469), the New Century
Excellent Talents by Ministry of Education of China (NCET-
12-0475), the Fund of Youth Science and Technology Stars by
Shaanxi Province (2012KJXX-16), Xinjiang Production & Corps
funding (BRYB1102), and the Fundamental Research Funds
for the Central Universities (QN2011035).
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