An Efficient Homogenized Ruthenium(II) Pincer Complex for N-Monoalkylation of Amines with Alcohols

Article abstract of DOI:10.1002/ejoc.201700486

An ionic 2,6-bis(imidazo[1,2-α]pyridin-2-yl)pyridine-based N^N^N pincer ruthenium(II) complex exhibited high efficiency in the C–N bond formation between amines and alcohols by the “borrowing hydrogen” (BH) or “hydrogen autotransfer” (HA) concept. The synthetic protocol selectively generated monoalkylated amines without formation of tertiary amines during the reaction. The unique selectivity enabled the formation of symmetrically and asymmetrically substituted diamines. This methodology features several advantages including a low catalyst loading (as low as 0.5 mol-%), a short reaction time (as short as 2 h), and excellent N-monoalkylation selectivity.

Full text of DOI:10.1002/ejoc.201700486

DOI: 10.1002/ejoc.201700486
Full Paper
Homogeneous Catalysis
An Efficient Homogenized Ruthenium(II) Pincer Complex for
N-Monoalkylation of Amines with Alcohols
Fa-Liu Yang,[a] Ying-Hui Wang,[a] Yong-Feng Ni,[a] Xiang Gao,[a] Bing Song,*[a] Xinju Zhu,*[a]
and Xin-Qi Hao[a]
Abstract: An ionic 2,6-bis(imidazo[1,2-α]pyridin-2-yl)pyridine- during the reaction. The unique selectivity enabled the forma-
based N^N^N pincer ruthenium(II) complex exhibited high effi-
ciency in the C–N bond formation between amines and alco-
hols by the “borrowing hydrogen” (BH) or “hydrogen autotrans-
fer” (HA) concept. The synthetic protocol selectively generated
monoalkylated amines without formation of tertiary amines
tion of symmetrically and asymmetrically substituted diamines.
This methodology features several advantages including a low
catalyst loading (as low as 0.5 mol-%), a short reaction time (as
short as 2 h), and excellent N-monoalkylation selectivity.
Introduction
Amines are an important class of building blocks with wide
application in pharmaceuticals, agrochemicals, dyes, food addi-
tives, and the chemical industry.[1] In the past decades, much
effort has been made to construct the C–N bond. Among the
various known methodologies, such as the Ullmann reaction,[2a]
Buchwald–Hartwig amination,[2b] to access amines, N-alkylation
of amines with alcohols represents an environmentally benign
and atom-economic strategy through a facile “borrowing
hydrogen” (BH) or “hydrogen autotransfer” (HA) process, which
produces water as the only by-product.[3]
In 1904, Nef first reported N-alkylation of amines or amides
with alcohols under strongly basic conditions (Scheme 1a).[4]
Following this original discovery, other alkali metal hydroxides
or alkoxides were also successfully applied to N-alkylations of
amines.[4–10] Recently, base-catalyzed direct alkylation of
amines was also achievable with either catalytic amounts
of aldehydes/alkyl halides[11] or heteroarylamines/sulfon-
amides,[12,13] which avoided the utilization of harsh conditions
and excess amounts of bases (Scheme 1b). In addition to base-
catalyzed synthesis of amines, acids, photocatalysts, and hetero-
geneous catalysts also exhibited good activities towards the for-
mation of substituted amines.[14–16]
Scheme 1. N-alkylation of amines with alcohols.
Grigg[17] and Watanabe,[18] significant achievements have been
made by Milstein,[19] Beller,[20] Williams,[21] Kempe,[22] and
Fujita[23] for Ru[24] and Ir[25] catalysts. Other catalysts based on
noble metals[26] as well as non-noble metals[27] were also
reported.
Ru pincer complexes have exhibited high efficiency in a
broad range of transformations.[28] In our previous work, we
have reported a series of Ru pincer complexes with catalytic
applications for transfer hydrogenation of ketones and transfor-
mation of aldoximes to primary amides.[29] In view of the grow-
ing interest in the Ru-catalyzed BH reaction and excellent per-
formance of Ru catalysts (Figure 1), we herein present an effi-
cient Ru-catalyzed system for highly selective N-monoalkylation
of anilines and heteroaromatic amines with a low catalyst load-
ing (0.5 mol-%) and a short reaction time (2 h) under thermal
heating.
Homogeneous transition-metal-catalyzed reaction is another
promising strategy to generate N-alkylated amines, which could
be applied to a broader range of substrates under milder
conditions (Scheme 1c). Inspired by the pioneering work of
[a] College of Chemistry and Molecular Engineering, School of Life Sciences,
Zhengzhou University,
No. 100 of Science Road, Zhengzhou, Henan 450001, P. R. China
E-mail: bingsong@zzu.edu.cn
zhuxinju@zzu.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.201700486.
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Figure 1. Ru complexes a–c.
Results and Discussion
of a broad range of amines 1a–1v with benzyl alcohol 2a were
investigated, which afforded selectively N-monoalkylated prod-
ucts 3aa–3va in each case (Scheme 2). Electron-donating and
electron-withdrawing groups at the para- and meta-positions
of the anilines were all tolerated to provide 3aa–3ma in excel-
lent yields (91–99 %). However, ortho-substituted anilines 1n–
1q were less reactive than para- or meta-substituted analogues,
probably due to steric effects. In the case of halide-substituted
anilines, the dehalogenation process was not observed within
a short reaction time.[30] In addition, the coupling reaction of
2-naphthalenamine 1r and disubstituted anilines 1s–1v with
benzyl alcohol also proceeded smoothly to furnish the desired
product 3ra–3va in moderate to good yields. Furthermore, a
gram-scale production was conducted, which generated the
desired product 3aa in 97 % yield. However, attempts to
achieve N-alkylation of aliphatic or secondary amines proved to
be unsuccessful, indicating the necessity of specific classes of
amine substrates.
Aniline 1a (1 mmol) and benzyl alcohol 2a (1.1 mmol) were
chosen as the model substrates to investigate the catalytic ac-
tivity of Ru complexes a–c (Table 1). Preliminary results indicate
that ionic Ru pincer complex a (0.3 mol-%) exhibits the highest
activity in the presence of KOH (20 mol-%) at 120 °C for a reac-
tion time of 12 h (Table 1, entry 1). The N-alkylated product
3aa was obtained in 87 % yield when the temperature was ele-
vated to 140 °C (Table 1, entry 7). Subsequently, the influence
of solvent and base was studied, which demonstrates that tolu-
ene and KOH was the optimal choice. (see the Supporting Infor-
mation). When the Ru catalyst loading was increased to 0.5 mol-
% with respect to the aniline substrate, N-benzylaniline 3aa was
isolated in 90 % yield (Table 1, entry 8). The ratio of 1a/2a was
also varied systematically; the amine/alcohol ratio of 1:1.5 is the
best choice (Table 1, entry 9). The reaction efficiency was fur-
ther improved by increasing toluene concentration (1 mL) to
generate the alkylated product in 99 % yield (Table 1, entry 10).
Finally, the desired product 3aa could still be obtained in 99 %
yield within 2 h without formation of the over-alkylated amine
and the corresponding imine (Table 1, entry 11). No product
was detected in the absence of KOH and Ru complex a, imply-
ing the necessity of both base and catalyst to fulfil the N-alkyl-
ation reaction.
Table 1. Optimization of reaction conditions.[a]
Entry
[Ru] cat.
Temp [°C]
1a/2a
Yield[b] [%]
1
2
3
4
5
6
a
b
c
120
120
120
120
120
120
140
140
140
140
140
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.5
1:1.5
1:1.5
27
12
6
RuCl2(PPh3)3 (d)
RuHCl(CO)(PPh3)3 (e)
RuH2(CO)(PPh3)3 (f)
a
a
a
a
a
N.R.
13
15
87
90
92
99
99
Scheme 2. Ru-catalyzed N-alkylation of various primary anilines with benzyl
alcohol. Reaction conditions: aniline 1 (1 mmol), alcohol 2a (1.5 mmol), Ru a
(0.5 mol-%), KOH (20 mol-%), toluene (1 mL), 140 °C, 2 h. (a) Aniline 1
(10 mmol), alcohol 2a (15 mmol). (b) Ru a (1.0 mol-%), 12 h.
7
8[c]
9[c]
10[c,d]
11[c,d,e]
To further evaluate the applicability of the reaction, aniline
1a was monoalkylated with various substituted alcohols 2b–2y
(Scheme 3). Similar to the cases in Scheme 1, para- and meta-
substituted benzyl alcohols 2b–2m were all compatible to gen-
[a] Reaction conditions: aniline 1a, alcohol 1b, Ru catalyst (0.3 mol-%), KOH
(20 mol-%), solvent, 12 h. [b] Isolated yield. [c] Ru catalyst a (0.5 mol-%).
[d] Toluene (1 mL). [e] 2 h.
With the optimized conditions in hand, the generality of the erate the secondary amines 3ab–3am in good to excellent
synthetic methodology was then examined. First, the reactions yields (85–97 %). In this case, ortho-substituted alcohols 2n–
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2r also proved to be effective substrates, albeit with slightly
decreased yields. No cleavage of halogen atom was observed
when halide-substituted alcohols were utilized. Also, hetero-
atom-containing and aliphatic alcohols 2s and 2t reacted with
aniline to provide the corresponding amines in 69 % and 79 %
yields, respectively. The substrates could be further extended to
aliphatic alcohols, which provided the corresponding products
3au–3ax in 32–72 % yields. Coupling reaction of 1,4-diols 2y
with 1a also provided 3ay in 28 % yield without formation of
N-heterocycle even in a longer reaction time. Finally, olefin-con-
taining alcohol 2z was compatible with the current methodol-
ogy. As discussed above, Ru catalyst a is highly selective for
N-monoalkylated products, and no dialkylation of amines was
observed, even when extremely excessive amounts of alcohols
were utilized.
Scheme 4. Ru-catalyzed N-alkylation of 2-aminopyridine with various alco-
hols. Reaction conditions: aniline 4 (1 mmol), alcohol 2 (1.5 mmol), Ru a
(0.5 mol-%), KOH (20 mol-%), toluene (1 mL), 140 °C, 2 h.
yields. Product 7i could also be obtained in 85 % yield when
5 equiv. of 2-thienyl methanol was used as the coupling part-
ner.
Scheme 5. Ru-catalyzed symmetrically N,N′-dialkylation of 2,6-diamino-
pyridine with various alcohols. Reaction conditions: 2,6-diaminopyridine 6
(0.4 mmol), alcohol 2 (1.0 mmol), Ru a (1.0 mol-%), KOH (40 mol-%), toluene
(1 mL), 140 °C, 12 h. (a) 2-Thienyl methanol 2t (2.0 mmol).
Moreover, asymmetrically substituted diamines were synthe-
sized by installing two different alcohols onto the 2,6-diamino-
pyridine scaffold in sequence, which has scarcely been explored
in Ru catalytic systems (Scheme 6). Firstly, unilaterally mono-
alkylated products 8a–8c were isolated as the main products in
67–71 % yields when a threefold equivalent of the diamine was
utilized. Next, the coupling reaction of 8 with another molecule
of alcohols 2 afforded asymmetrically N,N′-dialkylated products
Scheme 3. Ru-catalyzed N-alkylation of aniline with various alcohols. Reaction
conditions: aniline 1a (1 mmol), alcohol 2 (1.5 mmol), Ru a (0.5 mol-%), KOH
(20 mol-%), toluene (1 mL), 140 °C, 2 h.
Aminopyridines are among the less studied deactivated sub- 9a–9c in 83–90 % yields.
strates. Only a few examples of Ru-catalyzed N-alkylation of
aminopyridines have been reported.[31] Pleasingly, catalyzed by
Ru complex a, 2-aminopyrine 4 reacted with a range of
(hetero)aromatic and aliphatic alcohols 2 to give N-mono-
alkylated products in 74–95 % yields. Although the strong coor-
dinating potential of 4e might be detrimental for the Ru com-
plex, the desired product 5e could still be achieved in 96 %
yield. 1-Dodecanol was also used as an effective alkylating rea-
gent to deliver 5f in 81 % yield (Scheme 4). It is noteworthy
that only monoalkylated products were formed, and no N,N-
dialkylation of 2-aminopyrine was detected.
To explore the reaction pathways, a series of control experi-
ments were conducted (Scheme 7). Without Ru catalyst a, the
highest yield of product 3aa obtained was 24 % [Scheme 7,
Equation (1)], suggesting the dual roles of Ru catalyst and
base.[33] In the presence of Ru catalyst a and KOH, dehydrogen-
ation of primary alcohol 1a led to the formation of benzalde-
hyde 10, suggesting Ru complex a was necessary to tempora-
rily remove hydrogen from the alcohol [Scheme 7, Equation (2)].
The coupling reaction of 1a with 11 failed to yield the N-alkyl-
ated product 12 without alcohol [Scheme 7, Equation (3)]. In-
stead, the corresponding imine product was isolated. However,
Encouraged by the above results, it would be of great inter- when an external alcohol, 2b, was added to the reaction mix-
est to extend the current protocol to N,N′-dialkylation of di- ture of 1a and 11, both N-alkylated products 12 and 3ab were
amines. 2,6-Diaminopyridine is an important scaffold with isolated in 44 % and 26 % yield, respectively [Scheme 7,
hydrogen bond-forming sites (Scheme 5), which is widely util- Equation (4)]. These observations imply that the aldehyde is the
ized in supramolecular chemistry.[32] By increasing the reaction reaction intermediate, and the alcohol serves as the hydrogen
time, alkylated products 7a–7h were obtained in 33–89 % supplier during the BH or HA process.
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Scheme 6. Ru-catalyzed asymmetrically N,N′-dialkylation of 2,6-diaminopyr-
idine with different alcohols. Reaction conditions: (a) 2,6-diaminopyridine 6
(3.0 mmol), alcohol 2 (1.0 mmol), Ru a (1.0 mol-%), KOH (20 mol-%), toluene
(2 mL), 140 °C, 12 h; (b) compound 8 (0.4 mmol), alcohol 2 (0.6 mmol), Ru a
(1.0 mol-%), KOH (50 mol-%), toluene (1 mL), 140 °C, 12 h.
Scheme 8. Proposed catalytic cycle.
Conclusions
We have reported amine alkylation with alcohols catalyzed by
a N^N^N pincer-type Ru complex. The Ru complex enabled the
highly selective N-monoalkylation of primary aromatic amines
in two hours with a low catalyst loading (0.5 mol-%). Under
the optimized conditions, a wide range of primary alcohols and
aromatic amines were well tolerated to afford secondary amines
in good to excellent yields, irrespective of electronic effects.
Heteroaromatic amines, including 2-aminopyridines and 2,6-di-
aminopyridine, were also exploited, which provided access to
unilateral monoalkylated, symmetrically N,N′-dialkylated, and
asymmetrically N,N′-dialkylated diamines in a controlled man-
ner. Furthermore, a 10 mmol scale reaction was conducted to
provide the desired product in 97 % yield.
Experimental Section
Experimental Details: Solvents were dried with standard methods
and freshly distilled prior to use if needed. Ru catalysts a, b, and c
were synthesized according to previous reports.[29] Melting points
were measured with an XT4A melting point apparatus. Compounds
3ua–va, 3ar, 7e–g, 7i, 8b–c, and 9a–c are new compounds, and
were fully characterized. 1H NMR and 13C NMR spectra were re-
corded at 600 MHz with TMS as an internal standard. Data are re-
ported as follows: chemical shift (δ ppm), multiplicity (s: singlet, d:
doublet, t: triplet, q: quartet, m: multiplet), integration, and coupling
constants (J) in Hertz (Hz). HRMS were determined with a Q-Tof
Micro MS/MS System ESI spectrometer. Flash column chromatogra-
phy was performed on silica gel (200–300 mesh). Analytical and
preparative thin-layer chromatography (TLC) plates coated with
commercial silica gel were used to monitor the reactions and purify
amine products.
Scheme 7. Control experiments.
On the basis of the above mechanistic investigation and our
previous reports,[29,34] a plausible catalytic cycle was proposed
(Scheme 8). The reaction begins with the formation of Ru–alk-
oxide species I by the interaction between Ru complex a and
alcohol in the presence of KOH. Subsequent ꢀ-elimination of Ru
complex I yields a Ru–H species II and aldehyde intermediate A,
which undergoes condensation with the amine to form imine
intermediate B. Finally, coordination and insertion of intermedi-
ate B into the Ru–H bond results in the generation of Ru species
III, which reacts with the alcohol to liberate the amine product
and regenerate Ru–alkoxide species I.
General Procedure for the Syntheses of N-Alkylated Products
3 and 5: A 15 mL sealed tube was charged with amine 1 or 2-
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Acknowledgments
Financial support from the National Natural Science Foundation
of China (Grant Nos. 21528201 and 21672192), the Outstanding
Young Talent Research Fund of Zhengzhou University
(1421316036), and the Program for Science & Technology Inno-
vation Talents in Universities of Henan Province (17HASTIT004)
is gratefully appreciated.
[21]
[22]
Keywords: Homogeneous catalysis · Ruthenium · Pincer
complexes · Alkylation · Hydrogen autotransfer
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Received: April 6, 2017
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