A metallopeptoid as an efficient bioinspired cooperative catalyst for the aerobic oxidative synthesis of imines

Article abstract of DOI:10.1016/j.jcat.2017.09.018

Enzymatic catalysis is largely based on intramolecular cooperativity between a metal center and functional organic molecules located on one scaffold. Inspired by this concept we have designed the metallopeptoid trimer BT, which is a unique intramolecular cooperative oxidation catalyst incorporating two catalytic centers, phenanthroline-copper and TEMPO, as well as one non-catalytic benzyl group. Herein we explore the capability of BT to act as an efficient catalyst for the oxidative synthesis of imines, which are versatile intermediates in the fine chemicals and pharmaceutical industries. We demonstrate that BT, combined with CuI, can catalyze the production of benzyl, aryl, heteroaryl, allylic and aliphatic imines from various alcohols and amines with a turn-over-number up to 45 times higher than this achieved when phenanthroline, copper and TEMPO are mixed in solution. Moreover, in low catalyst(s) loading, BT enables transformations that are not possible when a mixture of the individual catalysts is employed.

Full text of DOI:10.1016/j.jcat.2017.09.018

Journal of Catalysis 355 (2017) 139–144
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A metallopeptoid as an efficient bioinspired cooperative catalyst for the
aerobic oxidative synthesis of imines
⇑
Darapanani Chandra Mohan, Arghya Sadhukha, Galia Maayan
Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Haifa 3200008, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Enzymatic catalysis is largely based on intramolecular cooperativity between a metal center and func-
tional organic molecules located on one scaffold. Inspired by this concept we have designed the metal-
lopeptoid trimer BT, which is a unique intramolecular cooperative oxidation catalyst incorporating two
catalytic centers, phenanthroline-copper and TEMPO, as well as one non-catalytic benzyl group. Herein
we explore the capability of BT to act as an efficient catalyst for the oxidative synthesis of imines, which
are versatile intermediates in the fine chemicals and pharmaceutical industries. We demonstrate that BT,
combined with CuI, can catalyze the production of benzyl, aryl, heteroaryl, allylic and aliphatic imines
from various alcohols and amines with a turn-over-number up to 45 times higher than this achieved
when phenanthroline, copper and TEMPO are mixed in solution. Moreover, in low catalyst(s) loading,
BT enables transformations that are not possible when a mixture of the individual catalysts is employed.
Ó 2017 Elsevier Inc. All rights reserved.
Received 25 July 2017
Revised 19 September 2017
Accepted 21 September 2017
Keywords:
Imines
Oxidation catalysis
Copper
Homogeneous
Peptide
Peptoid
1. Introduction
lactonization of diols [9] and oxidative coupling of alcohols and
amines to imines [10] or to nitriles and amides [11,12]. Among
The search for new catalysts is one of the highest priorities for
chemists, both in academia and industry, seeking to run processes
at ambient temperatures and pressures, with reasonable reaction
rates and a high turnover numbers, and with high selectivity. The
gains from improving catalysts are enormous both financially
and environmentally, leading to increased reaction efficiency,
selectivity and products availability, as well as to the reduction
of harmful waste. In nature, enzymes promote chemical and bio-
logical reactions with exquisite efficiency and selectivity under
very mild conditions. Enzymatic catalysis is largely based on the
cooperativity between a metal center and functional organic mole-
cules located at the surrounding folds of the protein (intramolecu-
lar cooperative catalysis). This cooperativity leads to the creation of
catalytic pockets, which enable enzymes’ high specificity and effi-
ciency [1]. The concept of cooperativity has inspired the design of
cooperative catalytic systems [2], specifically the combination
between a transition metal catalyst and an organocatalyst, which
are used as a mixture in solution (intermolecular cooperative catal-
ysis) [3,4]. One example is the combination between Cu(I) com-
plexes and nitroxyl catalysts, which has led to the development
of various homogeneouse catalytic systems for the aerobic oxida-
tion of alcohols to aldehydes and ketones (Scheme 1a, left) [5–8],
these transformations, the production of imines is especially
important because the highly reactive C@N bond can undergo a
variety of subsequent organic reactions such as reduction, addition,
cyclization, and aziridination [13], enabling the synthesis of fine
chemicals as well as of pharmaceutically and biologically active
compounds [14]. Several protocols for the synthesis of imines have
been developed [15], with the most attractive one being the oxida-
tive cross-coupling of alcohols with amines (Scheme 1a, right) [16],
which has a potential to utilizes widely available and inexpensive
starting substrates, while producing only hydrogen and water as a
byproducts [17]. Various homogeneous catalytic systems have
been reported for this reaction sequence, some exploit earth abun-
dant metal ions such as Cu [10,18] and Fe [19], mild conditions and
air or molecular oxygen as the oxidant. These systems, however,
employ 2–5 mol% catalysts, therefore the highest turn-over-
number (TON) possible, at the maximum of 100% conversion is
50. Although these systems are practical and applies to a large
scope of substrates, the low TON is a significant drawback. More-
over, such systems often require addition of base, ligands, and
the use of excess alcohols or amines, which can lead to separation
problems. Consequently, recyclable heterogeneous catalytic sys-
tems were also developed, and high TON could be achieved after
multiple recoveries of the catalyst [20]. But these systems also
have some drawbacks including requirements for high reaction
temperatures, stoichiometric oxidants and less available noble
⇑
Corresponding author.
E-mail address: gm92@tx.technion.ac.il (G. Maayan).
https://doi.org/10.1016/j.jcat.2017.09.018
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

140
D. Chandra Mohan et al. / Journal of Catalysis 355 (2017) 139–144
Scheme 1. Intermolecular (a) vs. intramolecular (b) catalytic system utilized for the oxidation of primary alcohols and for the oxidative synthesis of imines.
metal ions. In order to overcome the limitations of both homoge-
neous and heterogeneous procedures there is a need to design a
catalyst that will operate in mild conditions, preferably at room
temperature and under air or oxygen atmosphere, and will perform
with both high conversions and high TON towards the production
of imines from a broad scope of alcohol and amines.
Table 1
Optimization studies.a
Conversion (%)b
TON
One approach for increasing catalytic efficiencies (and selectiv-
ities) while maintaining mild reaction conditions is to design
intramolecular cooperative catalytic systems in which the catalytic
groups are tethered in close proximity to each other [21,22]. This
configuration will create a confined catalytic pocket similar to
the catalytic sites in enzymes [23]. Adopting such approach, we
have recently developed a peptidomimetic catalyst, namely a
metal-binding peptoid [24] trimer bearing 1,10-phenanthroline
(Phen), TEMPO and a non-catalytic benzyl group (BT, Scheme 1b)
[25]. We have discovered that the presence of at least one bulky
non-catalytic group induces steric constrains on the two catalytic
groups, leading to a highly active intramolecular cooperative cata-
lyst [25]. This catalyst, in the combination with Cu(I) and N-methyl
imidazole (NMI), enabled the oxidation of various benzylic, allylic
and less activated aliphatic primary alcohols at room temperature
with TON of up to 16 times higher than a mixture of the Phen,
TEMPO, CuI and NMI (the intermolecular cooperative catalytic sys-
tem, Scheme 1) [25]. Herein we were interested to exploit BT as an
intramolecular cooperative catalyst for the oxidative synthesis of
imines and investigate whether it can enable high TON in mild
conditions.
Entry
CuI (mol%)
BT (mol%)
Time (h)
1
2
3
4
5
6
7
1
1
12
12
12
12
12
24
24
12
12
12
24
24
24
>99
>99
>99
90
51
32
2
>99
54
77
98
100
200
400
450
510
640
–
500
540
770
980
–
0.5
0.25
0.2
0.1
0.05
0.01
0.2
0.1
0.1
0.1
–
0.5
0.25
0.2
0.1
0.05
0.01
0.2
0.1
0.1
0.1
0.2
–
8c
9c
10c,d
11c
12
13
NR
Trace
0.2
–
a
Reactions were performed in acetonitrile (0.5 mL) at room temperature and
under air, with 1.0 equiv. of benzyl alcohol, with 1.1 equiv, amine.
b
Conversions were determined by gas chromatography.
Under oxygen atmosphere.
30 °C.
c
d
CuI to 0.01 mol% (Table 1, entries 1–7). In the first three reactions,
>99% of (E)-N-benzylidene-1-phenylmethanamine 3 was obtained
as detected by GC, thus we have succeeded to increase the overall
TON of the reaction from 50, as was previously reported, to 400 by
using BT and Cu as an intramolecular cooperative catalytic system
(Table 1, entries 1–3). No other products such as benzyl aldehyde
or (E)-N-benzylidene-1-phenylmethanamide were detected, thus
the overall selectivity of the reaction was 100%. Further decrease
in the catalyst loading to 0.2 and 0.1 mol%, resulted in a decrease
in the overall conversion from about 100% to 90% or 51%, respec-
tively and 10% or 49% of unreacted alcohol were observed by the
GC in each case correspondingly (Table 1, entries 4–5). Using lower
catalyst loadings required a longer reaction time and the product
formation in these cases was not sufficient (Table 1, entries 6–7).
In order to maintain a low catalyst loading we decided to attach
a small oxygen balloon in atmospheric pressure to the reaction
vessel and conduct the reaction with 0.2 mol% of BT and CuI under
oxygen atmosphere. In these conditions, 99% of imine was
obtained, leading to an increase in the TON from 400 to about
2. Results and discussion
The peptoid catalyst BT was synthesized on solid support using
the submonomer method [26], as was previously published. The
peptoid was characterized by Reverse Phase High Performance Liq-
uid Chromatography (RP-HPLC) and mass spectrometry (MS),
cleaved from the resin, purified by HPLC (>99%) and lyophilized
to dryness. The ability of BT, in combination with a Cu salt, to cat-
alyze the oxidative coupling of alcohols and amines was initially
tested using benzyl alcohol 1 and benzyl amine 2 (Table 1), which
were chosen as the model substrates to perform and optimize the
reaction protocol described in Scheme 1b (right). We have started
our investigations by stirring these two substrates in acetonitrile
for 12 hours at room temperature under air, with 1 mol% of BT
and 1 mol% of CuI, and gradually decreased the loading of BT and

D. Chandra Mohan et al. / Journal of Catalysis 355 (2017) 139–144
141
Fig. 1. Kinetic description of BT catalyzed oxidative synthesis of imine 3 from alcohol 1 and amine 2. (a) Reaction profile for the formation of 3 and the conversion of 2 with
0.2 mol% of BT and CuI, at rt. under oxygen atmosphere. (b) Rates and order of the reaction with respect to the catalyst, in the first two hours of the reaction with 0.2, 0.3, 0.4
and 0.5 mol% of BT and CuI, at rt. under oxygen atmosphere.
Scheme 2. Proposed mechanism for the catalytic oxidative synthesis of imines by BT.
500. (Table 1, entry 8). Encouraged by these results we wished to
try and increase the TON even more by using only 0.1 mol% cata-
lyst. First, we conducted a reaction with 0.1 mol% catalyst under
oxygen atmosphere, but obtained only a slight increase in conver-
sion, 54% (Table 1, entry 9) rather than 51% that was obtained
under air. Aiming to improve the conversion further, we conducted
two more reactions under oxygen atmosphere, one at 30 °C instead
of at room temperature and one for 24 h, instead of 12 h. The first
reaction led to 77% imine formation (770 TON) while the second
reaction resulted in almost full conversion with 980 TON (Table 1,
entries 10–11).
to other copper salts tested in 0.5 mol% loading at room
temperature for 12 hours, including Cu(OTf)2 (98%), CuCl (72%)
and Cu(OAc)2 (6%). It is also notable from Table 1, that the
catalytic performance of (BT)Cu does not change in the loading
range of 1 mol% to 0.25 mol% and only at 0.2 mol%, the activity
starts to decrease with decrease in the catalysts concentration.
Moreover, control experiments in which 0.2 or 0.1 mol% of Phen,
CuI and TEMPO (the intermolecular catalytic system) were mixed
together in the presence of compounds 1 and 2, afforded only
21% and 16% conversion to imine
3
(105 and 160 TON),
support the
respectively. Overall, these observations
Notably, both BT and CuI are crucial for this reaction, and in lack
of either one there is no aldehyde or imine formation (Table 1,
entries 12–13). In fact, the catalyst is actually a (BT)Cu complex
[5a,b,25] generated in situ from BT and CuI under the reaction
conditions, as evident from both MS and UV–Vis spectra (Figs. S7
and S8). Additionally, in the presence of BT, CuI showed to
promote imine formation with the highest conversion compared
intramolecular cooperative action mode of the catalyst (BT)Cu,
demonstrating its ability to result in an increase of the TON in
this model reaction from 160 to about 1000 at almost full
conversion.
Before evaluating the substrate scope of the reaction, we have
performed a kinetic study aiming to reveal the reaction pathway
and to understand the effect of catalyst loading in the formation

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D. Chandra Mohan et al. / Journal of Catalysis 355 (2017) 139–144
Table 2
of imine 3 from alcohol 1 and amine 2. First, time-dependent
experiments were performed under optimized conditions (Table 1,
entry 8) and the product formation as well as the alcohol conver-
sion, both analyzed by GC, were detected by continues sampling
at different time intervals. The formation of 3 as the major product
was observed during the entire reaction along with a constant low
amount of aldehyde (<1%) indicating that this is the intermediate
in the reaction (Fig. 1a) [5a]. Second, to find out the rate of the
reaction (kobs) and the order of the reaction with respect to the
catalysts BT and Cu, we conducted time dependent experiments
in three more different loadings of both BT and CuI, namely 0.5,
0.4 and 0.3 mol%, which correspond to concentrations of 0.025,
0.02 and 0.015 mM, respectively. From the obtained data we
generated three more graphs similar to the one presented in
Fig. 1a, red line. Based on these results, we constructed four new
graphs by plotting the concentration of imine 3 vs. the time at
the first three time intervals. The slope of each linear curve
represents the kobs value of each reaction (Figs. S3–S6). The log of
each kobs value was than calculated and plotted against the log of
the concentration of BT (Fig. 1b). The obtained straight line with
a slope of a = 0 indicates that the reaction is zero order with
respect to the catalyst in the tested concentrations and in the
first two hours of the reaction, providing a clear proof for the
intramolecular reaction mode of our catalytic system. In contrast,
the rate of the oxidation of primary alcohols to the
corresponding aldehydes catalyzed by a mixture of the individual
catalysts Cu and TEMPO, was found to be one with respect to
these catalysts, as expected for the intermolecular system (see
Scheme 1a) [5b]. Subsequently, the mechanism of this oxidation
reaction was described in details by Stahl et al. and was
supported by their experimental results [5b]. Following this
proposed mechanism and the conclusion from our kinetic study,
we have monitored the oxidative synthesis of imine 3 from
amine 2 and alcohol 1 by MS, identified some intermediates that
are comparable to the ones envisioned by Stahl et al. [5b] and
proposed a full mechanism for this BT(Cu)-catalyzed oxidative
synthesis of imines (Figs. S9–S12 and Scheme 1S). A summary of
our proposed mechanism is shown in Scheme 2. This mechanism
includes the activation of BT by both CuI and the reacting amine
BT and Cu-catalyzed aerobic oxidative synthesis of imine from various alcohols and
amine 2.a
a
Reactions were performed in acetonitrile (0.5 mL) at room temperature with
2.5 mmol alcohol, 2.75 mmol amine, 0.2 mol% BT and 0.2 mol% CuI under oxygen
atmosphere for 12 h.
b0.1 mol% BT and CuI, 24 h.
c24 h.
dThe intermolecular catalytic system, namely a mixture of Phen (0.2 mol%) and
TEMPO (0.2 mol%) was used instead of BT. Conversions were determined by gas
chromatography.
These results demonstrate the strength of our intramolecular
catalytic system, which enables reactivity of alcohols that are unre-
active when the intermolecular cooperative catalytic system is
employed. Moreover, three selected imines, which were obtained
in >99% conversion, namely 3, 5 and 8, were isolated from the reac-
tion mixture by chromatography, purified and characterized by 1H
and 13C NMR spectroscopy (Figs. S13–S15). Isolated yields of all
three imines were about 90%, consistent with the high conversions
of the corresponding substrates, thus establishing the practicality
of our system.
to form the initial intermediate A. Thereafter,
a multi step
aerobic oxidation of A lead to intermediate B, in which Cu(I) is
already oxidized to Cu(II), and this further oxidizes the alcohol to
aldehyde, via dehydration. In the presence of the primary amine,
this aldehyde is converted to the corresponding imine by
condensation. As stated above, this proposed mechanism is
supported by previous literature and MS analysis.
Second, we have reacted different amines with benzyl alcohol 1
(Table 3) in order to evaluate the activity of the amines in this
reaction. As shown in Table 3, various imines (13–17) could be
obtained by the reaction of benzyl alcohol 1 with aromatic amines,
The conditions in which >99% conversion was obtained, namely,
a 12 h reaction with 0.2 mol% of BT/CuI at room temperature and
under oxygen atmosphere, were applied to a variety alcohols and
amines to extend the scope of this reaction. First, we have reacted
different alcohols with benzyl amine 2 (Table 2) in order to evalu-
ate the activity of the alcohols in this reaction. As shown in Table 2,
various imines (3À8) could be obtained by the reaction of benzyl
amine with electron-deficient and electron-rich substituent ben-
zylic alcohols at the para position including ones having reactive
functional group such as fluoro, chloro and nitro, in excellent con-
versions and high TON. Consequently, we have reduced the
amount of BT and CuI to 0.1 mol% and were able to maintain the
high conversion while significantly increasing the TON in the case
of imines 3–6 and 8 up to 980. The use of BT and CuI was also
efficient for the synthesis of imines from aliphatic alcohols (imines
9–12). Although both conversions and TON were lower when less
activated alcohols were used, these can still be considered excel-
lent as the intermolecular catalytic system failed to catalyze the
formation of these imines, producing only 3–8% conversion to imi-
nes 9–12, in the same catalysts loading, with TON lower than 40.
including the sterically more hindered a-substituted amines (chi-
ral substituents), which produce the corresponding chiral imines
in excellent to quantitative conversions with high TON. Conse-
quently, we have reduced the amount of BT and CuI to 0.1 mol%
and were able to maintain the high conversion while significantly
increasing the TON in all cases, up to 930 and 960, respectively, for
imines 14 and 16.
The use of BT and CuI was also efficient for the synthesis of imi-
nes from aliphatic amines (imines 18–24), especially in the case of
imine 18, where full conversion was achieved and high TON was
obtained even when the catalysts loading was reduced to 0.1 mol
%. The same trend of reduced conversions and TON when less
activated alkyl alcohols were used, is also seeing here with less
activated amines, but again, these can still be considered excellent
as the intermolecular catalytic system failed to catalyze the forma-
tion of the corresponding imines, producing only 4–10% conversion
of amines 19, 21–24, in the same catalysts loading, with TON lower
than 50. From this set, two selected imines, which were obtained in
>99% conversion, namely 16 and 17, were isolated from the reac-
tion mixture by chromatography, purified and characterized by

D. Chandra Mohan et al. / Journal of Catalysis 355 (2017) 139–144
143
Table 3
Table 4
BT and Cu-catalyzed aerobic oxidative synthesis of imine from various amines and
BT and Cu-catalyzed aerobic oxidative synthesis of azadienes and imine from various
alcohol 1.a
amines and alcohols including aryl- and heteroarylamines.a
a
Reactions were performed in acetonitrile (0.5 mL) at room temperature with
2.5 mmol alcohol, 2.75 mmol amine, 0.2 mol% BT and 0.2 mol% CuI under oxygen
atmosphere for 24 h.
a
b35-40 °C, 0.4 mol% NMI was added.
Reactions were performed in acetonitrile (0.5 mL) at room temperature with
cThe intermolecular catalytic system, namely a mixture of Phen (0.2 mol%) and
TEMPO (0.2 mol%) was used instead of BT. Conversions were determined by gas
chromatography.
2.5 mmol alcohol, 2.75 mmol amine, 0.2 mol% BT and 0.2 mol% CuI under oxygen
atmosphere for 12 h.
b0.1 mol% BT and CuI, 24 h.
c24 h.
dThe intermolecular catalytic system, namely a mixture of Phen (0.2 mol%) and
TEMPO (0.2 mol%) was used instead of BT. Conversions were determined by gas
chromatography.
both activated (benzyl) as well as less activated (aryl, heteroaryl,
allylic and aliphatic) primary alcohols and amines, with TON up
to 45 times higher than the ones obtained using the intermolecular
cooperative catalytic system, in which Phen and TEMPO are mixed
together in solution. Detailed kinetic studies showed that the high
activity does not depend on the catalyst concentration in the cata-
lyst loading range of 1–0.2 mol%, establishing the intramolecular
cooperative reaction mode of our catalytic system. Due to the high
activity of our catalytic system, we could demonstrated its high
potential in the preparation of products that are difficult to pro-
duce using the intermolecular cooperative catalytic system such
1H and 13C NMR spectroscopy (Figs. S16 and S17). Isolated yields in
both cases were about 90%, consistent with the high conversions of
the corresponding substrates.
Finally, we wished to demonstrate the more challenging synthe-
sis of a-b-unsaturated imines (azadienes), aryl- and heteroarylimi-
nes (Table 4). Azadienes play a crucial role in synthetic chemistry,
specifically in the construction of hetero Diels alder reaction prod-
ucts [27]. As shown in Table 4, cinnamyl alcohol reacted efficiently
with various amines in room temperature, when the reaction was
catalyzed by 0.2 mol% BT/CuI for 24 h, producing azadienes 27–30
in high yields and TON. Moreover, the intramolecular cooperative
catalytic system was also applicable for the synthesis of imines
31–36 from electron-deficient and much less reactive aryl- and
heteroarylamines, with good conversions and high TON of up to
450. As these amines are less basic and less nucleophilic than the
aliphatic ones, they were seldom successfully used as substrates in
previous reports because these reactions usually require large
amounts of bases and heating at high temperatures [19,28], while
in our cases, a slight heating to 35–40 °C and 2 equiv. of NMI
(0.4 mol%) are employed. Moreover, control experiments showed
that using the intermolecular catalytic system in the same reaction
conditions resulted in conversions lower than 11%, with the highest
TON being only 55. In the synthesis of imine 36, we have obtained
a TON 45 times higher when using our intramolecular cooperative
catalytic system (225 TON) than when using a mixture of the indi-
vidual catalysts (only 5 TON).
as
a-b-unsaturated and heteroaryl imines [10a]. The approach
described herein holds promise for wide spread implementation
in organic synthesis and enhancement of the practical utility of
methods that use solution mixtures of individual catalysts.
Moreover, This concept of intramolecular cooperative catalysis by
synthetic biomimetic sequences should be generally applicable to
different achiral and chiral peptidomimetic oligomers, with
numerous catalysts, for a variety of efficient chemical processes
including enantioselective transformations.
Acknowledgments
This research was supported by the Marie Curie Career Integra-
tion Grant (grant # 2017775). Darapanani Chandra Mohan thanks
the PBC for his post doctorate fellowship.
In summary, we have developed a general and highly efficient
method, based on an intramolecular cooperative catalytic system
that employs Phen and TEMPO, for the oxidative coupling of alco-
hols and amines to form imines under mild conditions and aerobic
atmosphere. This has enabled the synthesis of various imines from
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2017.09.018.

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D. Chandra Mohan et al. / Journal of Catalysis 355 (2017) 139–144
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