Direct synthesis of amides and imines by dehydrogenative homo or cross-coupling of amines and alcohols catalyzed by Cu-MOF

Article abstract of DOI:10.1039/d1ra03142b

Oxidative dehydrogenative homo-coupling of amines to imines and cross-coupling of amines with alcohols to amides was achieved with high to moderate yields at room temperature in THF using Cu-MOF as an efficient and recyclable heterogeneous catalyst under mild conditions. Different primary benzyl amines and alcohols could be utilized for the synthesis of a wide variety of amides and imines. The Cu-MOF catalyst could be recycled and reused four times without loss of catalytic activity.

Full text of DOI:10.1039/d1ra03142b

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Direct synthesis of amides and imines by
dehydrogenative homo or cross-coupling of
Cite this: RSC Adv., 2021, 11, 20788
amines and alcohols catalyzed by Cu-MOF†
*
Soheil Zamani Anbardan, Javad Mokhtari,
and Abolfazl Hassani Bozcheloei
Ahmad Yari
Oxidative dehydrogenative homo-coupling of amines to imines and cross-coupling of amines with alcohols
to amides was achieved with high to moderate yields at room temperature in THF using Cu-MOF as an
efficient and recyclable heterogeneous catalyst under mild conditions. Different primary benzyl amines
and alcohols could be utilized for the synthesis of a wide variety of amides and imines. The Cu-MOF
catalyst could be recycled and reused four times without loss of catalytic activity.
Received 22nd April 2021
Accepted 5th June 2021
DOI: 10.1039/d1ra03142b
rsc.li/rsc-advances
a challenge. Recently, MOFs have attracted much more attention
due to their structural and chemical diversities and have become
very popular in the diverse research areas such as catalysis,43 drug
Introduction
Imines are of great importance for the synthesis of chemical
and biologically active compounds such as amines, chiral
amines, amides, pyrrolines, hydroxyamines, and oxazir-
idines.1–3 Typically for the synthesis of imines, a mixture of an
aldehyde or ketone with an amine in the presence of a catalyst is
required. On the other hand, many useful oxidation methods
for the synthesis of imines have been developed4 which include
dimerization of primary amines5–10 oxidation of secondary
amines11–14 and other methodologies.15–23
delivery44 gas adsorption and storage45 and etc. As part of our
ongoing work to develop catalytic activity of metal–organic
frameworks (MOFs) as efficient heterogeneous, green and recy-
clable catalyst in organic synthesis46 we report herein an improved
oxidative homo-coupling of amines to imines and oxidative cross-
coupling of amine with alcohols to amides, using inexpensive and
readily available Cu2(BDC)2(DABCO) as the recyclable catalysts and
TBHP as oxidant (Scheme 1).
The formation of amide bonds is one of the most commonly
used organic reactions due to the widespread presence of this
functional group in natural products, pharmaceutical compounds,
and synthetic polymers.24 Usually, amides are synthesized by the
reaction of an amine and carboxylic acid, which needs a coupling
reagent25 or conversion into reactive derivatives.26 These methods
have several drawbacks, such as the use of hazardous and expen-
sive reagents, generating stoichiometric amounts of waste which
lead to environmental problems. To solve these problems, there is
great interest in the development of atom economic and envi-
ronmentally friendly routes for the synthesis of amides such as
named reactions like the Beckmann rearrangement,27 the Schmidt
reaction28 and Ugi29 reaction. A more recent method has consid-
ered the use of direct oxidative amidation from alcohols and
aldehydes.30–41 However, these methods require the use of expen-
sive transition metals as catalyst and in some cases the reactions
require hazardous, expensive, dual catalyst and in most cases is
a problem and the catalyst is not recyclable.42 Therefore, the search
of a green, low cost and heterogeneous catalyst system remains
Experimental section
Materials and methods
All the chemicals were purchased from commercial sources and
used without further purication. Cu2(BDC)2(DABCO) was
synthesized according to the our previously reported proce-
dure.46a All reactions were monitored by thin layer chromatog-
raphy (TLC) using plates coated with Merck 60 HF254 silica
under UV light. Melting points were measured on an Electro-
thermal 9100 apparatus. 1H-NMR spectra were recorded with
BRUKER DRX 400 and 500-AVANCE FT-NMR instrument (CDCl3
solution) at 400 MHz and 500 MHz, respectively. Scanning elec-
tron microscope (SEM) images were captured with a ZEISS
scanning electron microscope at 30 kV with gold coating. X-ray
powder diffraction (XRD) measurements were performed using
an X'pert MPD. Philips diffractometer with Cu radiation source (l
˚
¼ 1.54050 A) at 40 kV voltage and 40 mA current.
Synthesis of amides via dehydrogenative cross-coupling of
amines and alcohols catalyzed by Cu2(BDC)2(DABCO)
Department of Chemistry, Science and Research Branch, Islamic Azad University, P. O.
Box 14515/775, Tehran, Iran. E-mail: j.mokhtari@srbiau.ac.ir
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra03142b
To a solution of amines (1 mmol) and alcohols (1 mmol) in THF
(5 ml) was added Cu2(BDC)2DABCO (10% mol) and TBHP 70%
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Scheme 1 Oxidative homo and cross-coupling of amine with alcohols catalyzed by Cu-MOF.
(2 mmol) and the reaction mixture was stirred at room catalyst in this reaction, 5, 10 and 20 mol% of the catalyst was
temperature for 24 h. The reaction progress was monitored by used and as shown in Table 1, 10 mol% of Cu-MOF was the best
TLC. Aer the reaction was completed, the catalyst was ltered for the synthesis of imines and amides (Table 1, entries 5, 7) and
and the ltrate was evaporated under reduced pressure, and the the higher amounts of the catalyst did not signicantly effect on
residue was puried using silicagel column chromatography the reaction yield. TBHP was used as oxidant because of our
(hexane/ethyl acetate (3 : 1)).
previous experience in oxidative coupling reactions.43 Control
experiments revealed that in the absence of Cu-MOF as catalyst
(Table 1, entry 10) and TBHP as oxidant (Table 1, entry 11) the
product 3a and 4a were not formed.
Aer optimization of the model reaction in hand (Table 1,
entry 5), to know the substrate scope a range of benzylic amine
Selected spectral data
1
N-Benzylbenzamide (3a), white solid; yield: 78%; H NMR (500
MHz, CDCl3): d 7.81 (d, J ¼ 7.1 Hz, 1H, CH of Ar), 7.43–7.45 (m,
3H, CH of Ar), 7.36–7.37 (m, 4H, CH of Ar), 7.29 (d, J ¼ 7.1 Hz,
2H), 6.45 (s, 1H, NH), 4.85 (s, 2H, benzylic CH2).
Table 1 Optimization of reaction conditions for synthesis of imine and
Synthesis of imines via dehydrogenative homo-coupling of
amines catalyzed by Cu2(BDC)2(DABCO)
amide from amines and alcoholsa
To a solution of amines (2 mmol) in THF (5 ml) was added
Cu2(BDC)2DABCO (10% mol) and TBHP 70% (4 mmol) and the
reaction temperature was stirred at room temperature for 24 h.
The reaction progress was monitored by TLC. Aer reaction
completion, catalyst ltered and ltrate was evaporated under
reduced pressure, and the residue was puried using silicagel
column chromatography (hexane/ethyl acetate (3 : 1)).
N-(Benzylidene)benzylamine (4a).47 1H NMR (400 MHz,
CDCl3): d 8.44 (s, 1H, CH), 7.97–7.76 (m, 2H, CH of Ar), 7.57–
7.42 (m, 3H, CH of Ar), 7.40 (d, J ¼ 4.4 Hz, 4H, CH of Ar), 7.34–
7.27 (m, 1H, CH of Ar), 4.88 (s, 2H, benzylic CH2).
Catalyst Cu-MOF
(mol%)
Yield 3a
(%)
Yield 4a
(%)
Result and discussion
Entry
Oxidant
Solvent
Cu2(BDC)2DABCO (Cu-MOF) catalysts were prepared and char-
acterized according to our previous work.46a The synthetic
Cu2(BDC)2DABCO was employed as a catalyst in the dehydro-
genative homo-coupling of amines to imines and cross-
coupling of amines with alcohols to amides. At the rst, for
getting to the optimum reaction conditions, various parameters
such as solvent, amount of the catalyst and oxidant were
examined for the model reactions (Table 1). Three solvent
including THF, DMF and CH3CN were investigated for this
homo-coupling reaction in presence of Cu-MOF as catalyst and
TBHP as oxidant. The results are summarized in the Table 1 and
can be seen that the maximum yield was obtained in the THF as
solvent (Table 1, entries 4–6). For the study of the amount of
1
2
3
4
5
6
7
8
5
10
20
5
10
20
5
10
20
—
10
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
—
CH3CN
CH3CN
CH3CN
THF
THF
THF
DMF
DMF
DMF
THF
52
60
65
75
86
81
10
25
25
—
—
35
40
50
60
79
74
25
35
35
—
—
9
10
11
THF
a
Reaction condition: benzylamine 1a (1.0 mmol), benzyl amine 1a or
benzyl alcohol 2a (1.0 mmol), TBHP (2.0 mmol), solvent (5 ml), time:
24 h, rt.
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Table 2 Cu-MOF catalyzed dehydrogenative cross-coupling of amines and alcohols to amidesa
Entry
1
Amine
Alcohols
Product 3
Yield (%)
78
2
3
4
5
72
70
74
65
6
7
62
76
8
72
74
72
9
10
11
69
a
Reaction conditions: 1 (1.0 equiv.), 2 (1.0 equiv.), TBHP (2.0 equiv.), Cu-MOF (10 mol%) in THF (5 ml) at 25 ꢀC for 24 h.
and benzylic alcohols for the synthesis of amides (Table 2) were
For the investigation of substrate scope of oxidative dehy-
used under the optimized reaction conditions. Benzylic alco- drogenative homo-coupling of benzyl amines to imines, benzyl
hols and amines with different substituent provided the corre- amines with electron-donating group at ortho and para position
sponding amides (3a–k) in excellent yields. Propyl amine was were used and imines were formed in good yields (Table 3,
favored substrate, provided good yields of amide products.
entries 1–7) but benzyl amines with electron-withdrawing group
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Table
3
Cu-MOF catalyzed dehydrogenative homo-coupling of Table 4 Recovery and reuse of Cu-MOF in the dehydrogenative
amines to iminesa
coupling of amines and alcohols
Entry Amine 2
Product 4
Yield (%)
1
2
75
69
Run
Yield (%)
1
75
2
75
3
74
4
72
3
4
5
68
65
72
at para position led to formation of a trace of product (Table 3,
entry 8).
A possible mechanism for these type of reactions is shown in
Scheme 2. Cu(II) oxidized TBHP in such a way that TBHP
transformed into tert-butylproxy radical. tert-Butylproxy radical
captured a hydrogen from benzylamine or benzyl alcohol,
thereby converting the benzylamine and benzyl alcohol into 5a
and 7a proxy intermediates. Elimination of TBHP from this two
intermediates led to imine 6a and benzaldehyde. Addition of
benzyl amine to imine 6a and elimination of ammonia resulted
imine 3a. Also, aminal 8a was obtained by addition of benzyl
amine to benzaldehyde and continued oxidative dehydrogena-
tion of aminal 8a by Cu(II) and TBHP led to amide 4a. The latter
mechanism was conrmed by reaction between benzaldehyde
and benzylamine under the same conditions and led to exclu-
sive formation of the corresponding amide. This indicates that
the reaction proceeds through an aldehyde. On the other hand,
as shown in Table 1 products 3 and 4 were not formed in the
6
7
71
76
8
Trace
a
Reaction conditions: 2 (1.0 equiv.), TBHP (2.0 equiv.), Cu-MOF
(10 mol%) in THF (5 ml) at 25 ꢀC for 24 h.
Scheme 2 Proposed mechanism for the synthesis of amides and imines catalyzed by Cu-MOF.
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Table 5 Comparison of activity for different catalytic systems in the coupling of benzyl alcohol and benzyl amine
Entry Catalyst
Reaction condition
Yield (compound 3a or 4a) Ref. no.
1
2
3
4
5
6
7
Au6Pd/resin
NaOH (1.1 eq.), O2 balloon, H2O, r.t., 2 h
51% (3a)
78% (3a)
48
49
50
51
52
53
Ru(COD)Cl2/PCyp3.HBF4
Cu2O/CQDs
KOtBu, toluene, 110 ꢀC, 24 h
CH3CN, O2, white cold LED l > 400 nm, 24 h 95% (4a)
Co2(CO)8/trioctylphosphine oxide (TOPO) Mesitylene, 164 ꢀC, 24 h
79% (4a)
81% (4a)
88% (3a)
86% (3a), 75% (4a)
Silicagel supported salicylic acid
Manganese pincer complex
Cu-MOF
O2 (0.1 MPa), toluene, 90 ꢀC, 24 h
KOtBu, toluene, 110 ꢀC, 48 h
TBHP, THF, r.t
Present work
absence of Cu-MOF and TBHP as catalyst and oxidant and all of
these evidence supports our proposed mechanism.
Molander, Academic Press, Oxford, 2nd edn, 2014, pp.
365ꢁ394.
The Cu-MOF catalyst also shown good recyclability and
stability. The catalyst was recovered by simple ltration and
washed with methanol and dried in oven and reused for 4 times
(Table 4). The catalyst could be stored for a long time under air
atmosphere without signicant loss of catalytic activity. Also,
the XRD pattern of Cu-MOF shows that the crystalline structure
of Cu2(BDC)2DABCO is maintained aer four run (Fig. S1†).46a
A comparison with other catalytic systems in the dehydro-
genative homo- or cross-coupling of amines and alcohols
demonstrated that our present Cu-MOF catalyst system exhibi-
ted a higher conversion and yield under milder conditions
(Table 5).
4 G. M. Robertson, Imines and their N-substituted derivatives:
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Conclusion
In conclusion, we have identied Cu-MOF as a green and
recyclable heterogeneous catalyst for the efficient dehydrogen-
ative coupling of alcohols with amines for the synthesis of 11 H. Yuan, W.-J. Yoo, H. Miyamura and S. Kobayashi, J. Am.
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synthesis of amides and imines under the mild conditions.
Further studies on the catalytic application MOFs are in prog-
ress and would be presented in the future.
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Conflicts of interest
There are no conicts to declare.
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© 2021 The Author(s). Published by the Royal Society of Chemistry
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