Cobalt-Catalyzed Synthesis of Aromatic, Aliphatic, and Cyclic Secondary Amines via a "hydrogen-Borrowing" Strategy

Article abstract of DOI:10.1021/acscatal.6b02218

The replacement of precious metals with inexpensive, less toxic, and earth-abundant elements in typical noble-metal-mediated organic transformations is a major goal in current synthetic chemistry and industries. The metal-catalyzed N-alkylation of amines with other amines through a "hydrogen-borrowing" principle represents a green and atom-economical reaction for the synthesis of secondary amines. However, catalysts developed thus far that are effective for this process remain quite scarce and are only limited to a few ruthenium and iridium complexes. In this work, we present a cobalt-catalyzed selective alkylation of amines with amines to synthesize a large variety of secondary amines. A range of amine substrates have been converted to the corresponding products through hetero- or homocoupling between amines. Cyclic sec-amines are also achieved from diamine precursors as rare examples.

Full text of DOI:10.1021/acscatal.6b02218

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Letter
Cobalt-Catalyzed Synthesis of Aromatic, Aliphatic and Cyclic
Secondary Amines via a “Hydrogen-Borrowing” Strategy
Zhiwei Yin, Haisu Zeng, Jing Wu, Shengping Zheng, and Guoqi Zhang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02218 • Publication Date (Web): 30 Aug 2016
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Cobalt-Catalyzed Synthesis of Aromatic, Aliphatic and Cyclic
Secondary Amines via a “Hydrogen-Borrowing” Strategy
Zhiwei Yin,†,‡ Haisu Zeng,†,‡ Jing Wu, †,‡ Shengping Zheng*,‡ and Guoqi Zhang,*,
†
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† Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The Graduate Center of the City Uni-
versity of New York, New York, 10019 NY, USA.
‡ Department of Chemistry, Hunter College, the City University of New York, New York, 10065 NY, USA.
ABSTRACT: The replacement of precious metals with inexpensive, less
toxic and earth-abundant elements in typical noble metal-mediated
organic transformations is a major goal in current synthetic chemistry
and industries. The metal-catalyzed N-alkylation of amines with other
amines through a “hydrogen-borrowing” principle represents a green
and atom-economical reaction for the synthesis of secondary amines.
However, catalysts developed thus far that are effective for this process
remain quite scarce and are only limited to a few ruthenium and iridi-
um complexes. In this work, we present a cobalt-catalyzed selective
alkylation of amines with amines to synthesize a large variety of sec-
ondary amines. A range of amine substrates have been converted to the
corresponding products through hetero- or homo-coupling between
amines. Cyclic sec-amines are also achieved from diamine precursors as
rare examples.
KEYWORDS: cobalt, pincer complex, catalysis, secondary amine, H-borrowing
Amines are fundamental organic chemicals that have
widespread applications in pharmaceuticals, agrochemi-
cals, polymer materials, and synthetic industries.1 There-
fore, considerable efforts have been made into efficient
syntheses of amines.2 The alkylation of primary amines to
form secondary amines proved to be an affordable route,
whereas the challenge on the control of monoalkylation
remains.3 Traditionally, alkyl halides have been utilized as
the alkylating agents for amines, but they are mostly toxic
and mutagenic. Recently, alcohols have been employed
for transition-metal-catalyzed mono-alkylation of amines,
and highly efficient monoalkylation of amines have been
achieved predominantly by using ruthenium or iridium
catalysts through a “hydrogen-borrowing” process.4-6 Re-
markably, non-precious metals including iron and cobalt
were also found to reach precious metal activity in alkyla-
tion of amines with alcohols.7
In 2007, Beller and coworkers reported an effective syn-
thesis of aromatic sec-amines by using the well-defined
ruthenium-based Shvo catalyst via
a
“hydrogen-
borrowing” mechanism [Scheme 1, eq. 1].9 Later on, the
Williams group employed an iridium complex, [Cp*IrI2]2
to carry out the coupling between aryl- or alkyl-amines
and bis(isopropyl)amine to synthesize isopropyl deriva-
tives [Scheme 1, eq.2].10 More recently, a cyclometallated
Ir complex was developed to achieve alkylation of anilines
with bis(isopropyl)amine with moderate yields in the
presence of a base additive.11 Besides the limited examples
of precious metal-catalyzed amine synthesis, however, the
selective, earth-abundant metal catalyzed coupling be-
tween amines through the “hydrogen-borrowing” strategy
has not been explored (Scheme 2).
In sustainable chemistry, there is current urgency
for the replacement of precious metals with earth abun-
dant ones in catalytic organic transformations, in particu-
lar concerning large-scale applications where the cost and
environmental toxicity of precious metals can be a severe
concern.12 Major progress has been made in the past dec-
ade in replacing precious metal catalysts with iron, cobalt
and nickel, while involving hydrogenation and dehydro-
Alternatively, transition-metal-catalyzed alkylation
of amines by amines provides a new pathway to second-
ary amines. However, earlier efforts on this reaction fre-
quently suffered from poor product selectivity.8 Selective
monoalkylation of amines with another amines has been
only realized by using noble metal catalysts very recently.
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genation processes.13, 14 Recently, a CoII pincer complex
[(PNHPCy)Co(CH2SiMe3)][BArF4] (2, Scheme 3) has been
established by Hanson and Zhang15a-e as a highly active
catalyst for the hydrogenation of alkenes, aldehydes, ke-
tone and imines under mild conditions, as well as the
o
of aniline at 120 C in toluene in a sealed tube (Scheme 3
and Table 1). Although the neutral Co(PNP)(CH2SiMe3)
complex (1) was inactive, we delightfully found that ionic
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Scheme 3. Cobalt(II) pincer complexes 1-6 used as catalysts.
complex 2 (2 mol% based on 1-hexylamine, generated in
situ by the reaction of 1 and H[BArF ]·(Et2O)2)) was highly
4
efficient, producing the amine alkylation product 7a in
almost quantitative yield after 24 h according to GC anal-
ysis [entries 1 and 2, Table 1]. The modified ionic com-
plexes 3-5 were also tested under the same conditions. It
was found that 3 incorporating a diphenylphosphine lig-
and gave 86% GC yield of 7a, whereas 4 and 5 were essen-
tially inactive [entries 3-5]. In contrast, the reaction did
not proceed in the presence of the air-stable complex 6 or
without a cobalt catalyst [entries 6 and 7]. This is con-
sistent with the previous findings that 2 was the most
active catalyst in “acceptorless” alcohol and amine dehy-
drogenation reactions.7a,15 We next conducted the reac-
tion using 2 in various solvents. It was found that the re-
action in other solvents proceeded more sluggish than
that in toluene [entries 8-11]. In addition, both the reac-
tion temperature and the volume of reaction vessels were
important for the highest yield [entries 12 and 13]. It is
worth mentioning that the temperature applied for the
current cobalt catalysis is lower than that for Ru or Ir sys-
tems as reported.
Scheme 1. Alkylation of amines by amines catalyzed by tran-
sition-metal complexes
.
Table 1. Cobalt-catalyzed amination of aniline with 1-
hexylamine: Screening of catalysts and reaction conditions.a
Scheme 2. “Hydrogen-borrowing” strategy for the cobalt-
catalyzed monoalkylation of an amine with another amine.
acceptorless dehydrogenation of alcohols, and the N-
alkylation of amines with alcohols through the “hydro-
gen-borrowing” process.7a Considering the ability of the
same catalyst in the dehydrogenation of cyclic amines to
afford imines as established by the Jones group,15f we envi-
sioned that this CoII complex might be feasible for the
alkylation of amines with amines through an initial amine
dehydrogenation step. Herein, we report for the first time
an earth-abundant cobalt-catalyzed selective monoalkyla-
tion of amines with other amines for the efficient synthe-
sis of aromatic, aliphatic and cyclic sec-amines via the
“hydrogen-borrowing” pathway (Scheme 2).
Entry
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12c
13d
Catalyst
1
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
Hexane
benzene
1,4-dioxane
toluene
toluene
Yield (%)b
0
98
86
<5
0
2
3
4
5
6
-
2
2
2
2
2
2
0
0
43
25
90
47
42
85
We began our initial reactivity tests with the cou-
pling between aniline and 1-hexylamine. A variety of co-
balt(II) complexes developed previously were examined
for the reaction using 2 mol% catalyst and 1.2 equivalents
a Conditions: 1-hexylamine (0.5 mmol), aniline (0.6 mmol),
cobalt 2 (2 mol %, formed in situ by mixing 1 and
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H[BArF ]·(Et2O)2)), and solvent (4 mL) are heated to 120 oC in
are heated to 120 oC in a 100 mL Schlenk tube, 24 h. Yields are
of isolated products.
4
a 100 mL Schlenk tube, 24 h. b Determined by GC analysis. c
1
2
d
Reaction was run at 100 °C. Reaction was run in a 50 mL
3
4
5
6
7
8
9
Schlenk tube.
The cobalt(II)-catalyzed alkylation of anilines has
been then applied to different alkyl amines (Scheme 5). n-
Pentyl and n-octylamines can be converted to the corre-
sponding anilines 8a and 8b in excellent yields. Benzyla-
mine and 4-fluorobenzylamine are also suitable sub-
strates for the reactions, affording the desired products 8c
and 8d in reasonable yields, even though in the latter case
26% byproduct from the homocoupling reaction of 4-
fluorobenzylamine has been detected by GC analysis. In
addition, 2-furfurylamine and phenethylamine were test-
ed under the standard conditions, and products 8e and 8f
were isolated in moderate yields, along with the observa-
tion of 28% and 19% homocoupling products, respective-
ly. Branched amines such as sec-butamine and cyclohex-
ylamine were also found to react with aniline to provide
the corresponding products in 56% and 92% yields, re-
spectively, whereas in the former reaction an imine by-
product was detected (37%).
Having established the optimized conditions for the
cobalt-catalyzed N-alkylation of aniline with 1-hexylamine,
we next investigated other substituted anilines in order to
extend the substrate scope. Thus, a variety of anilines
have been employed for the reaction with 1-hexylamine,
and the results are summarized in Scheme 4. 4-methyl,
3,5-dimethyl and 4-isopropyl substituted anilines react
efficiently with 1-hexylamine, producing the correspond-
ing sec-amine products 7b, 7c and 7e in high isolated
yields, respectively, while the reaction involving 2,4-
dimethylaniline resulted in product 7d in only moderate
yield, due to the steric effect of an o-methyl group. In
addition, 4-phenylaniline also furnished the reaction with
reasonable yield. Substrates incorporating electron-
donating methoxy group in the ortho- or para-position
were tested and again it proves that the ortho-substituted
substrate was relatively less effective. However, fluoro-
substituent on the ortho-position of aniline was found to
have little influence on the resulting yields (7i and 7j), in
contrast to those for 7g and 7h. Aniline bearing both 3-
fluoro and 4-methoxy substituents was also suitable sub-
strate, affording 7k in good yield. It was noted that aniline
with a strong electron-withdrawing trifluoromethyl group
was a challenging substrate for this reaction, and the
product 7l was isolated in 32% yield. Interestingly, the
bis-alkylation in one molecule also proceeded well. Thus,
the reaction of 1,4- and 1,3-diaminobeneze with two
equivalents of 1-hexamine resulted in the isolation of bis-
alkylated products 7m and 7n in 70% and 74% yields,
respectively.
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H
2 (2 mol%), 120oC
NH2
N
toluene
Alkyl
Alkyl-NH2
+
- NH3
8, yield
Scheme 5. The substrate scope of aliphatic amines. Condi-
tions: aliphatic amines (0.5 mmol), aniline (0.6 mmol), cobalt
o
2 (2 mol %), and toluene (4 mL) are heated to 120 C in a 100
mL Schlenk tube, 24 h. Yields are of isolated products. a 26%
b
homocoupling product observed; 28% homocoupling prod-
c
d
uct observed; 19% homocoupling product observed; 37%
imine product observed.
As observed above in some cases the homo-coupling
of alkyl amines may occur, we were interested to explore
the cobalt-catalyzed reactions in the presence of only one
type of amines (Scheme 6). To our delight, aliphatic
amines including 1-pentylamine, 1-hexylamine and 1-
octylamine proceeded well under the same conditions,
furnishing the reactions with the isolation of homo-
coupling products 9a-c in excellent yields. The use of
benzylamines or furfurylamine also led to symmetrical
sec-amines in good or moderate yields, while an imine
byproduct (14%) was observed during the synthesis of 9d.
It was also noted that phenethylamine is an excellent sub-
strate for the homocoupling, giving 9g in 96% yield. A
branched amine, cyclooctylamine was also examined in
Scheme 4. The substrate scope of aniline derivatives for sub-
stituted sec-arylamines. Conditions: 1-hexylamine (0.5 mmol),
anilines (0.6 mmol), cobalt 2 (2 mol %), and toluene (4 mL)
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the presence of catalyst 2 and the resulting dicyclooctyl-
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1
2
3
4
amine product was isolated as the only product in 48%
yield. Previously, such homocoupling of primary amines
to synthesize secondary amines has been only achieved by
using Pt, Ru, or Ir catalysts.16
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Scheme 7. Examples of cobalt-catalyzed synthesis of cyclic
sec-amines. Yields are of isolated products.
In conclusion, we have revealed the first non-
precious metal cobalt-catalyzed alkylation of amines with
other amines through a “hydrogen-borrowing” methodol-
ogy for the synthesis of a broad range of aromatic, ali-
phatic and cyclic sec-amines. The well-defined cobalt pin-
cer complex (2 mol% catalyst loading) was observed to
show comparable catalytic activity and efficiency to the
known Ru and Ir catalysts for similar transformations,
even at relatively lower reaction temperature. Significant-
ly, the cobalt catalyst also represents the first example of
base metal-catalyzed intramolecular alkylation of linear
diamines, opening new opportunities for the synthesis of
pharmaceutically important N-heterocycles.
Scheme 6. The homo-coupling of various amines for sym-
metrical sec-amines. Conditions: primary amines (1.0 mmol),
cobalt 2 (2 mol % relative to one half amount of amine), and
toluene (4 mL) are heated to 120 oC in a 100 mL Schlenk tube,
a
24 h. Yields are of isolated products. 14% imine product
observed. b Isolated as a HCl salt.
Encouraged by the results above showing a broad range of
amines suitable for the cobalt-catalyzed alkylation, we
investigated the possibility of intramolecular alkylation of
amines, i.e. the cyclization of diamines. To the best of our
knowledge, metal-catalyzed cyclization of diamines to
form cyclic secondary amines remains extremely rare in
the literature.8d,17 The only homogeneous ruthenium cata-
lyst, RuCl2(PPh3)3 was reported to catalyze the cyclization
of several aliphatic α,ω-diamines at 180 oC in diphenyl
ether.17 Our experiments further confirmed the ability of
cobalt complex 2 in carrying out this challenging trans-
formation (Scheme 7). It was observed that when 1,4-
diaminobutane was reacted under the standard catalytic
conditions, pyrrolidine was isolated as a hydrochloride
salt (10) as the only product in 64% yield after workup
and recrystallization. 1,6-Diaminohexane was found to be
more effective substrate, leading to the desired cyclic sec-
amine, azepane isolated in a hydrochloride salt form (11)
in 91% yield. However, when 1,5-diaminopentane and 1,7-
diaminoheptane were subject to the same reactions, no
desired cyclic products were detected, but only the start-
AUTHOR INFORMATION
Corresponding Author
* Email for S. Zheng: szh0007@hunter.cuny.edu;
* Email for G. Zhang: guzhang@jjay.cuny.edu
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information for this article is available free of
charge via the Internet at http://pubs.acs.org.
Detailed experimental procedures, characterization data
(PDF).
ACKNOWLEDGMENT
We are grateful to donors of the American Chemical Society
Petroleum Research Fund for partial support of this work
(#54247-UNI3). We also acknowledge the support from
CUNY Collaborative Research Incentive Program, the PSC-
CUNY award (#69069-0047) and a “Seed Grant” support
from the Office for the Advancement of Research at John Jay
College, CUNY. Partial support from National Science Foun-
dation (CHE-1464543) is also gratefully acknowledged.
ing
materials
were
recycled.
Finally,
2-(2-
aminoethyl)aniline containing both aniline and aliphatic
amine units proved to be suitable substrate for the cy-
clization, affording indoline (12) in 52% yield after column
chromatography, while indole was also detected as a mi-
nor product (< 5%).
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