Tert-Butyl hydroperoxide (TBHP)-mediated oxidative self-coupling of amines to imines over a α-MnO2 catalyst

Article abstract of DOI:10.1039/c3gc42312c

We here demonstrate a simple, efficient and eco-friendly protocol for the direct synthesis of imines from amines via a facile α-MnO₂ catalyzed-procedure at rt. Up to 13 benzylic, heterocyclic, and normal aliphatic imines were synthesized with 95-99% selectivity at 82-99% conversion. the Partner Organisations 2014.

Full text of DOI:10.1039/c3gc42312c

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DOI: 10.1039/C3GC42312C
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ARTICLE TYPE
Tert-Butyl Hydroperoxide (TBHP)-Mediated Oxidative Self-Coupling of
Amines to Imines over α-MnO2 Catalyst
Zhe Zhang,a,b Feng Wang,a,* Min Wang,a Shutao Xu,a Haijun Chen,a,b Chaofeng Zhang,a,b JieXua,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
We here demonstrate a simple, efficient and eco-friendly
protocol for the direct synthesis of imines from amines via a
facile α-MnO2 catalyzed-procedure at rt. Up to 13 benzylic,
heterocyclic, and normal aliphatic imines were synthesized
Table 1. Oxidative transformation of 1a to 2a. [a]
N
NH2
N
O
1a
2a
2b
2c
10 with the 95-99% selectivity at the 82-99% conversion.
45
Imine is a significant class of compounds due to their versatile
applications as synthetic intermediates.[1] Among the existing
methods for imine syntheses, the oxidative self-coupling of
amines to imines has attracted more attentions. Relatively high
15 yields have been achieved using homogeneous catalysts, such as
Cu/TEMPO,[2] Cu/ABNO,[3] and TBHPQ[4], and heterogeneous
catalysts, such as precious metal catalysts (Pt[5], Ru[6], Pd,[7] Au,[8]
Ir[9]), vanadium oxides,[10] Cu-Ce-O,[11] Fe/MOF,[12] GO,[13] and
photocatalysts.[14] However, all of the above methodologies either
20 require complicated catalyst preparation and reaction setup,
involve cumbersome work-up or show limited compatibility for
the inactivated amines, such as the normal aliphatic amine and
benzyl amines substituted by electron-withdrawing groups.
Moreover, the use of harsh reaction conditions usually causes
25 side reactions such as hydrolysis and oxidation to aldehydes and
nitriles. Therefore it is desirable to develop an efficient catalytic
system working under mild conditions, preferably at room
temperature, easily separable and reusable, and suitable for
various amines.
Select. [%][b]
Entry
Catalyst
Solvent Conv. [%][b]
2a
74
-
2b
1
2c
25
-
1[c]
2[d]
3
4[e]
5[f]
6[g]
7
α-MnO2
α-MnO2
α-MnO2
-
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
>99
0
-
3
2
-
>99
28
0
95
90
-
2
8
α-MnO2
α-MnO2
α-MnO2
α-MnO2
α-MnO2
α-MnO2
α-MnO2
α-MnO2
OMS-2
β-MnO2
δ-MnO2
Mn2O3
Mn3O4
Fe2O3
-
98
90
70
52
50
59
>99
>99
36
73
47
27
25
30
41
20
31
98
98
94
94
89
90
70
71
98
97
95
93
90
90
95
90
89
1
1
3
2
4
2
11
9
1
2
2
3
3
3
2
3
4
1
1
8
CH2Cl2
PhCl
3
9
4
10
11[h]
12[i]
13
14
15
16
17
18
19
20
21
22
DMF
7
DMF
8
H2O
19
20
1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
1
3
4
7
30
Manganese dioxide (MnO2) is an oxidant in organic synthesis.
N. Mizuno and co-workers have reported the oxidation of amines
to amides with stoichiometric amount of MnO2 (MnO2:amine
=2.3:1) at 130 oC.[15] Imine (19%) as byproduct was detected over
β-MnO2.This provides a possibility of using MnO2 as catalyst.
V2O5
7
CuO
3
Co3O4
7
MoO3
7
35 We herein report this endeavor on a MnO2-TBHP catalytic
system in the oxidative self-coupling of amines to imines. Up to
13 amines covering substituted benzylic amines, normal aliphatic
amines, and O- and S-containingheterocyclic amines were high-
selectively converted into the corresponding imines with
40 excellent yields. To the best of our knowledge, this is the first
example with satisfied imine yield, operational simplicity, and
good tolerance with diverse amines.
[a] Reaction conditions: catalyst (0.05 mol), 1a (0.5 mmol), TBHP (1.0
mmol), solvent (2 mL), 17-25 oC, open air, 4 h. [b] As determined by GC-
MS using dodecane as an internal standard.[c] Under 0.3 MPa O2 and
50 100oC. [d] With 1 mmol H2O2. [e] Without catalyst. [f] In open air
without TBHP. [g] In Ar gas. [h] 12 h. [i] 1 h.
We initiated the investigations with the benzyl amine (1a)
conversion as a model reaction. A reaction under 0.3 MPa O2
over α-MnO2 (10% mol 1a) at 100 oC afforded >99% conversion
55 of 1a and 74% of N-benzylidenebenzylamine (2a) with 1%
benzaldehyde (2b) and 25% benzonitrile (2c) (Table 1, entry 1).
Further optimization tests failed to realize both the conversion of
1a and the selectivity of 2a larger than >90%. We found the
formation of 2b and 2c was thermally favourable and thus
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Page 2 of 6
the heterogeneous nature of the α-MnO2 catalyst.
decreasing reaction temperature could suppress the side reaction.
But no reaction occurred when conducting the reaction with O2 at
rt. We then changed to H2O2 as oxidant. However, H2O2 (30%
aq.) quickly decomposed over α-MnO2 even at 0 C and 1a was
45
The search for new synthetic technologies hasVidewevAertlioclpeeOdnline
DOI: 10.1039/C3GC42312C
o
continuous process approach. Its main advantages are facile
automation, reproducibility, safety, and process reliability,
because constant reaction parameters can be assured.[19] Up to
now, no study has reported continuous synthesis of imines. This
5
not converted at all (entry 2). Unexpectedly, when 2.0 TBHP, 0.1
α-MnO2 and 1.0 equivalent 1a was stirred in a 2 mL acetonitrile
at 20oC, the remarkably high conversion of 1a (>99%) was
obtained with 95% selectivity for 2a (entry 3). The reaction was
finished in 4 h in an open air without protection. The easy
10 handling and high efficiency mark it as the best methodology for
the self-coupling of amines to imines in open publications. The
background reaction without catalyst was sluggish in 4 h (entry
4). The reaction without TBHP gave no product (entry 5). A
reaction in Ar atmosphere proceeded comparably well (entry 6),
15 indicating TBHP was the main oxidant. Notably only 2 equvi. of
TBHP was used in this study, compared with 5-10 equivalent
amount.[10a, 16] A control test using the 4:1 mixture of 1a to TBHP
gave a 32% conversion of 1a, indicating the 64% of TBHP
utilization efficiency, much higher than literature value.[17]
50 is a challenging task partly because it requires reaction rate to be
fast enough to achieve high throughput. In this study, a column
reactor (20 cm in length, 4 mm in diameter) packed with α-MnO2
pellet (60-80 mesh) was employed for this purpose (Figure 2).
Two acetonitrile solutions of TBHP and 1a were fed into the
55 reactor by two respective pumps. The reactor temperature was
kept at 20 oC. The product mixture was collected at the other end
of the reactor. After several optimization tests, we obtained >99%
conversion of 1a and average 93% selectivity for 2a.The global
space-time yield was 0.15 mol (mol cat)–1 h–1. Although we
60 sacrificed ca. 1% selectivity compared with batch reactor, we
were able to continuously run the reaction for more than 100 h
without losing catalyst activity, showing a good promising
industrial application.
20
Solvent had great effect on the reaction. Acetonitrile was the
best among the investigated solvents, such as tetrahydrofuran,
dichloromethane, chlorobenzene, and dimethyl formamide
(entries 7-10). Even if extending reaction time to 12 h, the
reaction in dimethyl formamide only gave 59% of 2a (entry 11).
25 Particularly, when water was used as solvent, the conversion
>99% was obtained but with 70% selectivity (entry 12).
Various oxidative oxide catalysts were compared with α-
MnO2. An octahedral molecular sieve (OMS-2), reported by S. L.
Suib,[18] was less selective (entry 13, 71% of 2a) but much
30 oxidative giving 20% of 2c. Other manganese oxides, such as β-
MnO2, δ-MnO2, Mn2O3 and Mn3O4, exhibited much less
performance (Table 1, entries 14-17). Metal oxides such as
Fe2O3, V2O5, CuO, Co3O4 and MoO3 were inactive for the
reaction (Table 1, entries 18-22).
65 Fig. 2 A continuous synthesis setup and the catalytic result.
a)
Normal reaction
Hot filtration in 2 h
100
Table 2.Scope of the α-MnO2 catalyzed conversion of amine to imine.[a]
80
60
40
20
0
0
1
2
3
4
5
6
Time / h
35
Fig. 1 Filtration test. Reaction conditions: 0.5 mmol benzylamine,
1 mmol TBHP, 2 mL MeCN, α-MnO2 (10 mol%), r.t.. The
catalyst was filtrated at 2 h.
70 [a] Reaction conditions: 0.5 mmolamine, α-MnO2 (10 mol%), 1 mmol
TBHP, 2 mL MeCN, 17-25 oC, 4 h. The results were presented as conv. /
select. Byproducts were aldehydes and nitriles. The conversion and
selectivity were determined by GC-MS using dodecane as an internal
standard. [b] 8 h.
The α-MnO2 catalyst was separated by centrifugation and
40 reused for at least three times (Figure S1). ICP analysis did not
detect Mn species in the reaction mixture. The reaction stopped
after the removal of MnO2 from the reaction solution (Figure 1),
indicating no dissolved species contributed to the catalysis and
2
| Journal Name, [year], [vol], 00–00
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Page 3 of 6
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A
broad scope of imines were converted using the
organic reactant being oxidized by lattice oxide ions of MnO2 at
elevated temperature. In our reaction, three facts that (i) MnO2
alone could not oxidize amine under curreDnOt cI:o1n0d.1i0ti3o9n/C, (3iGi)CT42B3H12PC
methodology (Table 2). Regardless of the presence of electron-
donating or electron-withdrawing groups, benzylamines were
converted to the corresponding imines with >85% conversion and
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alone did oxidize amine but with low activity, and (iii) the
5
>90% selectivity (entries 1-7). 1-Phenylethylamine was relatively 50 complete oxidation of amine was achieved in the presence of both
inert to be oxidized (82% conversion) (entry 8), which could be
due to the steric hindrance effect. Heterocyclic amines and
normal aliphatic amines were usually difficult to be converted.[14a,
20] In this study, furan-2-ylmethanamine (entry 9) and thiophen-2-
MnO2 and TBHP, may infer that the formation of TBHP-MnO2
adduct is responsible for the catalysis. We postulate that some
surface defect sites or Mn-OH groups, instead of redox property
of MnO2, play a key role at rt.
10 ylmethanamine (entry 10), bearing proximal oxygen and sulphur
atom, were converted at 85% and 97% conversion with 96% and
>99% selectivity, respectively. Moreover, we achieved high
imine yields for n-butyl amine (95%) and2-phenylethan-1-amine
(83%), respectively (entries 11-12).
15
To gain insights into the mechanism details, several kinetic
experiments were conducted using in situ ATR-IR. Experimental
results indicated that the reaction was a pseudo-first-order with
respect to 1a (Figure S2). The apparent activation energy, Ea,
was determined from the Arrhenius plot in the temperature range
o
20 of 30-70 C (Figure 3a). The least-square fit analysis yielded an
Ea value of 62.8 kJ mol–1. A linear relationship between
log(kX/kH) and Brown-Okamoto constant σ+ (Figure 3b and
Figure S3) was established for the substituted benzylamines (X =
p-Cl, p-H, p-Me and p-OMe). The slope of the linear plot gave a
25 ρ value close to zero, suggesting no electronic perturbation in the
Cα-H or N-H activation step.[21]
55
Fig. 4 ESR spectra of (a) α-MnO2 and (b) a combination of α-MnO2 +
TBHP at room temperature.
This conclusion is supported by ESR characterization (Figure
4). The α-MnO2 is featured with wide line (a) with g value of
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
1.5
(a)
k=-7.557, R2=0.978
(b)
60 2.016, which is larger than 1.94 of the fully oxidized α-MnO2.[22]
Combining with the appearance of six visible and weak hyperfine
structure lines in line (a), we may postulate the existence of
exchange pair of Mn3+-Mn4+ in α-MnO2.[22-23] Upon adding
TBHP into MnO2, six clear symmetrical hyperfine structure lines
65 with g value of 2.022 are superimposed on the line (a). The
increase in g value and decrease in signal intensity suggest that
the addition of TBHP may increase the concentration of Mn3+
ions, stabilized by forming a TBHP-MnO2 adduct.
1.0
0.5
p-Cl
p-Hp-F
p-CH3O
0.0
-0.5
-1.0
-1.5
2.9
3.0
3.1
3.2
3.3
-0.2
0.0
0.2
+
-1
(1/T) / 10-3 K
Fig. 3 (a) Arrhenius plot for the oxidation of benzylamine. (b) Hammet
plot for the self-coupling of the substituted benzylamines over α-MnO2
30 using TBHP at rt. σ+ is the Brown-Okamoto constant.[21]
We added 2 equiv. of (2,2,6,6-tetramethylpiperidin-1-yl)oxy
(TEMPO) as a radical scavenger or butylated hydroxytoluene
(BHT) as a radical initiator to a reaction of 1a. As a result, the
35 reaction was not either inhibited or accelerated (Scheme S1),
indicating a non-radical process. This was reinforced by in-situ
ATR-IR test which did not observe induction period (Figure S4).
This is different to the usual role as radical initiator played by
TBHP.[10a, 17]
40
To further investigate the reaction route, several control
experiments were performed. First, reaction of 1a at rt over α-
MnO2 without TBHP gave no product in 4 h. The continuation of
the reaction by adding TBHP reached 96% conversion and 95%
selectivity of 2a (equ 1). This result can not be explained by the
70 Fig. 5 Oxidation of benzylamine without solvents. Reaction conditions:
2.5 mmol substrate, 2.5 mmol TBHP, 0.25 mmolα-MnO2, reaction
temperature 20 oC
45 general Mars-van-Krevelen mechanism over MnO2, with the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

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Page 4 of 6
Conclusions
In summary, we have demonstrated a simple, efficient and
View Article Online
45 eco-friendly protocol for the direct synthDesOisI: 1o0f.10im39i/nCe3sGCfr4o2m312C
amines via a facile α-MnO2 catalyzed-procedure at rt. The
catalysis was truly heterogeneous and α-MnO2 could be reused
without an appreciable loss of its high catalytic performance. A
continuous synthesis approach proved the catalyst long-life
50 stability in more than 100 h. The clean and mild synthetic
procedure described herein is expected to contribute to its
utilization for the development of new products and processes.
Acknowledgments
This work was supported by the National Natural Science
55 Foundation of China (Project 21303189, 21273231, 21233008),
and Hundred Person Project of the Chinese Academy of Sciences.
Notes and references
a Prof. Dr. F. Wang, Prof. Dr. J. Xu, Z. Zhang, Dr. M. Wang, Prof. Dr.
S.T. Xu, H.J. Chen, C.F. Zhang
60 State Key Laboratory of Catalysis, Dalian National Laboratory for Clean
Energy, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023 (China). E-mail: wangfeng@dicp.ac.cn;
xujie@dicp.ac.cn. Website: www.fwang.dicp.ac.cn
Previous studies proposed benzaldimine as an intermediate in
the dehydrogenation of benzyl amine.[20] Hermans and co-
workers did not detect it in GC-FID/MS and thought it was
instable in reaction or analysis.[24] Jones and co-workers recently
proposed the same intermediate but still did not have
experimental proof.[11] At first we wondered that benzaldimine
could not survive from high reaction and analyses temperature or
10 probably the sampling method. Because our reaction can be
conducted at rt, we tried to monitor the reaction by in situ NMR
technique, a strong tool to detect intermediate. Likely, we did not
detect benzaldimine from beginning to end (Figure 5). However,
dibenzylamine and N-methyl-1-phenylmethanamine majorly
15 generated the corresponding benzaldimines as detected by GC
(equ. 2 and 3), suggesting benzaldimine to be the intermediate.
This observation allowed us to conclude that benzaldimine was
not generated as free molecule during reaction. Because
benzaldimine has stronger basicity than benzyl amine and
20 benzaldehyde,[25] it may exist as the adsorbed species on catalyst
surface in an activated state after the oxidative dehydrogenation
reaction. The reaction was accelerated after adding a certain
amount of water (Figure S5), indicating very possibly one of the
steps involved the hydrolysis of the adsorbed benzaldimine.
25 Similar to Hermans and Jones’s discoveries, we also found
benzaldehyde was easily condensed with 1a to 2a without
5
65 b Z. Zhang, H.J. Chen, C.F. Zhang
Graduate University of Chinese Academy of Sciences, Beijing 100049
(China)
† Electronic Supplementary Information (ESI) available. See
70 DOI: 10.1039/b000000x/
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413-656). These data showed PA (benzalimine, 220 kcal mol-1) > PA
(benzyl amine, 218 kcal mol-1) > PA (benzaldehyde, 199 kcal mol-1).
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Green Chemistry
Page 6 of 6
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DOI: 10.1039/C3GC42312C
Graphical Abstract
A methodology for the direct synthesis of imines from amines via a facile α-MnO2
catalyzed-procedure at room temperature with TBHP as oxidant was described, which is
simple, efficient and eco-friendly. The catalysis is suitability for 13 aromatic and
aliphatic amines, and it is heterogeneous, recyclable and reusable.