Tuning of cross-Glaser products mediated by substrate-catalyst polymeric backbone interactions

Article abstract of DOI:10.1039/c9cc08565c

Tuning of cross-Glaser products using different polymeric backbones supported by copper oxide nano-catalysts has been demonstrated by tweaking the substrate-catalyst interactions under greener conditions. Further, highly reactive magnetically separable and recyclable catalyst with scalability is demonstrated.

Full text of DOI:10.1039/c9cc08565c

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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Tuning of the Cross-Glaser Products mediated by Substrate-
Catalyst’s Polymeric Backbone Interactions
Sharanjeet Kaur,† Aritra Mukhopadhyaya,†,‡ Abdul Selim,†,‡ Vijayendran Gowri,† K. M. Neethu,†
Received 00th January 20xx,
Accepted 00th January 20xx
Arif Hassan Dar,† Shaifali Sartaliya,† Md. Ehesan Ali,†,* and Govindasamy Jayamurugan†,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Obtaining the hetero-selective product is the most challenging task reactivation before reuse, etc. In some cases, it is accompanied by
in cross-Glaser reactions. So far this has been achieved mainly the formation of undesired side products. An alternative approach
through the inherent reactivity of the substrates guided by metal based on solid-support was investigated,7 but this method requires
catalyst. Here we report a novel catalyst based on copper-oxide at least one alkyne to be used excessively along with additional
(CuI/IIO) nanoparticles with varying cellulose-polymeric backbone. detachment step from the resin. Transition metal based
The designed catalyst not only provides high hetero-selectivity but nanoparticles8,5a-c has received considerable attention in recent
also allows tuning the homo/hetero products by modifying the years over noble metals5d due to large abundance, high surface areas
catalyst cellulose-polymeric backbone either through chemical leading to high activity compared to bigger sized particles.
modification and/or by tweaking the catalyst-reactant interactions.
The atomistic details of the hetero-selectivity is explored based on
the ab initio Born-Oppenheimer molecular dynamics simulations.
The aerobic oxidation of terminal alkynes forming the C―C bond is a
powerful reaction that enable dialkyne formation in a single step.1
The dialkynes are quite important class of molecules not only
available in numerous natural products,2 but they also play major
roles in materials exhibiting exotic electronic properties comprising
of π-conjugated alkynes been utilised as advanced materials.3 Unlike
Glaser homo-coupling, the Glaser cross-coupling reaction is
challenging due to poor selectivity between homo- and cross-Glaser
hetero-products that limits its wide applications.4 To address this
issue various catalysts have been tried and achieved cross-hetero-
Fig. 1. (a) Structure of catalysts 1, 2 and 3 and (b) their synthetic pathway.
selectivity governed by inherent reactivity of substrates mediated by
metal-catalyst.5,6 For examples, mesoporous Cu/MnOx catalyst6a by
So far the hetero-selectivity was mostly achieved based on the
Rossi and Suib et al. as well as bimetallic catalysts6b by Wang et al.
inherent reactivity differences of differently substituted alkynes
with the catalyst.2e,6,7 However, in addition to the reactivity, the
polarity (H-bonding, hydrophobic, - or electrostatic
interactions) is also inherently different for the cross-Glaser-
products, which are always among two homo-products, one is
polar and the other is non-polar, whereas the hetero-product is
medium-polar. This trend is more pronounced especially when
one alkyne is highly polar (e.g. -OH, -NH2 containing groups) and
the other is non-polar (e.g. phenyl, alkyl, etc). Hence, we
hypothesise that tuning the catalyst’s backbone which can
interact differently among homo- and hetero-products might
influence the hetero-selectivity due to possible cooperative
effects (non-covalent interactions). To test this hypothesis, we
have designed the catalysts TC-S-CuI/IIO 1, EC-S-CuI/IIO 2, and C-
with 63–93% of hetero-selectivity’s have been achieved. Shi et al.
reported that good to excellent hetero-selectivity (57–93%) can be
achieved for gold catalysed cross-Glaser coupling in the presence of
PhI(OAc)2 as an oxidant and 1,10-phenanthroline as an additive.6c
Yao et al. reported Cu/C3N4-composite as a recyclable catalyst for
Glaser reactions.6d Interestingly, Zhou and Yin et al. have reported
that even simple Cu0 powder can catalyse the cross-Glaser reaction
under simpler condition.6e However, these catalysts suffer some
limitations like using organic solvents, expensive metals, excess
catalyst, oxidant, high temperature and longer reaction time,
‡ These authors contributed equally to this work
Institute of Nano Science and Technology, Mohali-160062, Punjab, India.
Email: jayamurugan@inst.ac.in.; ehesan.ali@inst.ac.in
†
Electronic Supplementary Information (ESI) available: Experimental details,
computational calculations and spectroscopic data. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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S-CuI/IIO 3 which are different mainly in the amine spacer such Section D1 in ESI). Preliminary studies revealed tVhieawtAertxiccleeOllnelinnet
DOI: 10.1039/C9CC08565C
as tris-(2-aminoethyl)amine (TREN)-functionalised cellulose yields were obtained when the homo-coupling was carried out
(TC), ethylenediamine-functionalised cellulose (EC), and with 1 mol% TC-S-CuI/IIO 1, 4 mol% TMEDA (N,N,N’,N’-
unfunctionalised cellulose (C), respectively (Fig. 1). The detailed tetramethyl ethylenediamine) as ligand under air at 25 °C. We
synthetic procedures and characterisation of the catalysts 1, 2 have compared the turn-over-frequency (TOF) of TC-S-CuI/IIO 1
and 3 have been described in Sections B and C, respectively in with the commercially available Cu-TMEDA (Di-μ-hydroxo-
the Electronic Supplementary Information (ESI). Amino bis[(N,N,N′,N′-tetramethyl
ethylene-diamine)copper(II)]
cellulose precursors for the synthesis of corresponding catalysts chloride) catalyst showing 2909 and 139 h–1, respectively (Table
1 and 2 have been prepared following the reported procedures S5). To the best of our knowledge, the observed TOF of 2909 h–1
starting from cellulose.9 Briefly, the synthesis of catalysts 1, 2 is ~20 times higher than reported TOF and the commercially
and 3 were achieved by two-steps processes (Fig. 1b, Scheme available Hay-catalyst (Cu-TMEDA) (Table S7). Moreover, we
S1, ESI), i.e. (i) treatment of functionalised-celluloses (TC, EC, examined the catalytic efficacy of TC-S-CuI/IIO 1 using various
and C) with thioglycolic acid (S) mediated by amounts of catalysts down to 0.0001 mol% of Cu and the
dicyclohexylcarbodiimide coupling agent to afford thiol- reaction was nearly quantitative (Table S6). It is noteworthy
functionalised celluloses (TC-SH, EC-SH, and C-SH),9,10 (ii) that the successful use of such an extremely low quantity of Cu
treatment of these celluloses with CuCl2 in 1:1 EtOH/H2O under eco-friendly conditions for oxidative homo-coupling of
afforded CuI/IIO nanoparticles (NP) anchored by thioglycolate ethynylbenzene is remarkable. Thus, it could be conferred that
part. It has been reported that thiol functionalised cellulose TC-S-CuI/IIO 1/TMEDA has very high catalytic activity with
paper could efficiently reduce the copper (Cu) ions into exceptionally high TOF. Furthermore, to demonstrate catalyst
copper(I) oxide NP.11 Whereas in our case with microcrystalline 1, its practicality in a scalable reaction, 10 mmol scale of 1a was
celluloses modified with thiol groups provided CuI/IIO (catalysts treated with 1 mol% of catalyst 1 and the corresponding diyne
1 to 3) nanoparticles which were characterised by various 2a (1.68 g) was obtained in 85% yield within 2 h.
characterisation techniques (Section C, ESI).
After optimising the conditions, we investigated whether
the catalyst 1 could be used for selective formation of hetero-
product in an aerobic oxidative cross-coupling of terminal
alkyne reactions to produce asymmetric 1,3-diynes (Table 1).
We have chosen relatively non-polar (1a-h) and a polar (1i)
terminal alkynes for cross-coupling reactions in order to see a
clear polarity discrimination between the products which can
be isolated by column chromatography. 2-Methylbut-3-yn-2-ol
1i was successfully cross-coupled with various terminal alkynes
to provide unsymmetric 1,3-diynes (3a-h) in high yields (68–
85%, Table 1, entries 1–8), in addition to the corresponding
homo-coupled 1,3-diynes in very moderate yields.
As we originally hypothesised the observed high selectivity
of the TC-S-CuI/IIO 1 towards the hetero-products (Table 1)
might be attributed due to catalyst backbone polarity/
interactions (H-bonding, hydrophobic, etc) facilitating moieties
which may selectively favour the similar polar nature of the
cross-coupled product (Fig. S47).13,4a To investigate the role
played by TREN moiety in 1 on hetero-selectivity, catalysts EC-
S-CuI/IIO 28b and C-S-CuI/IIO 310,9 in which TREN is replaced by
ethylenediamine and no TREN, respectively were also examined
under similar conditions for a model reaction between 1a and
1i (Fig. S47a, Table 2, entries 1, 4, and 7). Both catalysts 2 and 3
showed reduced selectivity of 59 and 25%, respectively,
towards hetero-product 3a as compared to 85% for catalyst 1.
It was observed that the selectivity towards the hetero-product
decreases as the amine part decreases in the catalysts similar to
the polarity/H-bonding interacting order of the catalysts 1>2>3.
To verify further, we changed the polarity/H-bonding
interacting part in 1, i.e. TREN moiety by adding additive
compounds with either polar (-COOH) or non-polar (dodecyl
chain) moieties for the same model reaction (Table 2, entries 2,
3, 5, 6, 8, and 9). In both cases, we chose carboxylic acid-
containing additives such as dodecanoic acid (DDA, non-polar)
and benzene-1,3,5-tricarboxylic acid (BTC, polar) because they
Fig. 2. (a) XPS survey spectrum of TC-S-CuI/IIO 1. (b) XPS copper profile describing the
coordination behaviour of Cu with TREN-Cellulose. (c) TEM images of TC-S-CuI/IIO 1. (d)
Magnified view of the selected area showing the particle size less than 10 nm. (e) Their
respective particle size distribution histogram.
Figs. 2a and 2b show the X-ray photoelectron spectroscopy
(XPS) survey of the as-prepared TC-S-CuI/IIO 1 and the presence
of Cu2O/CuO in 42:58 ratio, respectively.12 The transmission
electron microscopy (TEM) images of TC-S-CuI/IIO 1 is showed
that the spherical-shaped NP have been successfully deposited
over the porous cellulose and the particles size distribution of
the formed NP was found to be 1−7 nm (Figs. 2c–e). Further, the
loading of Cu on TC-SH was found to be 36.2 wt%, as
determined by inductively coupled plasma mass spectrometry
(ICP-MS). Thus, these results suggest that TC-SH acts as a
reducing agent as well as stabilizing agent for the synthesis of
CuI/IIO NP. After successful synthesis and characterisation of all
the three catalysts, we chose catalyst 1 to test and optimise the
reaction conditions to its catalytic activity in the Cu-catalysed
homo-coupling of ethynylbenzene 1a to form 2a (Table S4,
2 | J. Name., 2012, 00, 1-3
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cellulose (TC) functionalised with thioglycolic acidV(ieSw) AarnticclehOonrleinde
would bind with TREN moiety as it contains tertiary amine, thus
changing the polarity/H-bonding environment favoured atoms
in the catalyst’s backbone to non-polar and polar, respectively.
It was found that catalyst 1 alone showed 85% selectivity for
medium-polar hetero-product 3a (entry 1, Table 2). Whereas,
upon addition of DDA, the major compound observed was non-
polar 2a with 76% yield (entry 2). Interestingly, upon addition of
polar-additive BTC, the catalyst showed selectivity towards
polar homo-product 2i with 76% yield (entry 3). More evidently,
the similar significant effect was not observed in the case of
catalyst 2 and 3 which contain no-tertiary amine in the catalyst
backbone (entries 4–9, Table 2). In addition, catalyst 2
containing ethylenediamine moiety favoured moderate hetero-
product selectivity (50 – 61%). Whereas the catalyst 3
containing no-amine spacer favoured non-polar homo-product
selectivity (59 – 65%). These experimental results indicate that
TREN moiety in TC-S-CuI/IIO 1 is responsible for high selectivity
towards hetero-product and polarity does play a role which in
turn might be governed by the forces like non-covalent
interactions (Section F, ESI). Further, the fact that the effect of
additives is so prominent which implies that the catalyst
backbone largely controls the selectivity with very little effect of
chemo-selectivity caused by inherent reactivity of substrates.
on the CuIIO-111 surface, was used to model the catalyst NP
DOI: 10.1039/C9CC08565C
assembly (see Fig. 3a). To investigate the hetero-selectivity,
alkynes 1a and 1i were investigated by forming the copper–σ–
acetylide complexes with Cu(I/II)-atoms on the CuO surface as
proposed in the previous mechanistic study.13a Complete details
of the modelling of the catalyst and the copper (I/II)–σ–
acetylide complexes along with the computational methods are
given in Section F in ESI. The unit cell of CuO was optimised using
density functional theory14b with Perdew-Burke-Ernzerhof14
exchange correlation functional as implemented in the VASP
code.14d A time step of 1 fs was set for the AIMD simulations.
The dynamics were performed for all the three possibilities of
the coupling reaction; two homo-coupling reactions and the
cross-coupling reactions. The alkynes were anchored on the
surface by means of copper (I/II)–σ–acetylide complex
formation. For sake of simplicity, we will use σ–complex instead
of Cu (I/II)–σ–acetylide complex throughout our discussion.
Table 2. Effect of amine spacer and additives on aerobic oxidative cross-coupling of
terminal alkynes.a
En Catalyst
try
additi
ve
Time
(min)
Conversi Product
onb (%)
selectivityc
2a 3a 2i
Table 1. Aerobic oxidative cross-coupling of terminal alkynes.a
1
2
1
none
DDA
10
10
99
97
10 85
76 18
5
R1
H
1 + DDA
6
76
7
1a-h
1i
TC-S-CuI/II
1:1 EtOH/H2O, 25 oC, air
O 1, TMEDA
R1
OH
HO
OH
R1
R1
2a-h
3a-h
2i
3
4
1+ BTC
BTC
10
30
96
99
3
20
HO
H
en
Substrate R1
1a–h
Tim Conversi
e (h) onb (%)
Selectivityc (%)d
2
none
33 59
2a–h
3a–h
2i
5
6
2+ DDA
2+ BTC
DDA
BTC
30
30
96
96
43 50
30 61
6
8
try
0.08
0.75
0.75
0.33
4
99
99
99
95
99
96
10
85 (79)
5
1
1a
1b
1c
1d
1e
7
8
9
3
none
DDA
BTC
60
60
60
98
96
96
62 25
65 24
59 27
12
10
13
13
20
14
15
18
81 (75)
72 (70)
83 (78)
76 (73)
78 (72)
6
8
3
9
4
3+ DDA
3+ BTC
2
3
4
5
6
O
F
aReaction conditions: 1a-h (0.20 mmol), 1i (0.26 mmol), Cu-catalyst (1 mol%),
TMEDA (4 mol%), 1:1 EtOH/H2O (3 mL), air, 25 °C. bconversion and cselectivity were
determined by GC-MS analysis.
The snapshots at different stages during the progress of the
reaction are captured and few selected snapshots are shown in
Fig. 3b. The simulations revealed that the σ-bonded (nearby) C-
atoms from two different alkynes (1a and 1i) come within the
bonding proximity and re-hybridisation of their electron
densities facilitates the formation of the C—C bonds between
the alkynes in the cost of the σ–complexes. A detailed
description from electronic structure point of view is given in
Section F5, ESI. This is the key step of the coupling reactions,
however, the formation of the σ-bonded complexes in the
neighbourhoods is the prime conduction for such
heterogeneous chemical reactions. The detailed study about
the mechanism of formation of the activated σ-complex will be
reported elsewhere. The AIMD simulations also revealed the
relaxation process of the activated product states to the ground
state of the product. This occurs upon dissipating the electronic
and thermal energies through the configurations and
conformational changes along with the changes in the catalyst
backbone-product interactions which indeed remarkably
different between the catalyst backbone-reactant interactions.
The AIMD simulations within sub-picosecond timescale
convincingly describes the formation of C—C bond that results
in the coupled hetero-product 3a from the σ-bonded alkynes 1a
and 1i (see Fig. 3). This indeed qualitatively matches with the
experimental observations of the hetero-selectivity. We further
Cl
Br
4
1f
1g
1h
5
94
98
12
23
84 (73)
68 (65)
4
9
N
7
8
0.33
Si
aReaction conditions: 1a-h (0.20 mmol), 1i (0.26 mmol), TC-S-CuI/IIO 1 (1 mol%), TMEDA
(4 mol%), 1:1 EtOH/H2O (3 mL), air, 25 °C. bconversion and cselectivity were determined
by GC-MS analysis. disolated yields based on 1a-h are given in parentheses.
We infer that CuI/II
O NP in these catalysts bind preferentially
with the similar polar nature of alkynes possibly due to various
cooperative factors like H-bonding, electrostatic, and van der
Waals interactions, etc., which in turn lead to the formation of
different homo- and hetero-products. In order to obtain a
comprehensive understanding of hetero-selectivity of the
catalyst 1, the detailed investigations of the mechanism of the
chemical reaction with atomistic and electronic structures
details are indispensable. We have performed the ab initio
molecular dynamics (AIMD)14a simulations to explore the
possible interactions between the catalyst 1 and substrates and
also the formations of products. Tris2-aminoethylamine (TREN)-
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investigated the role of the catalysts in the hetero-selectivity Notes
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process, and realised that the H-bonding interaction (-NH···OH) The authors declare no competing financial interest.
DOI: 10.1039/C9CC08565C
between the amide N—H atom of the catalyst and the oxygen
atom in O—H of 3a play the central role in the selectivity ACKNOWLEDGEMENT
process. This H-bond formation influences the formation of the This work has been dedicated to Prof. Dr. François Diederich on
σ-bonded complex near to the catalyst anchored sites. The non- the occasion of his retirement from ETH-Zurich. This work was
polar alkyne 1a will have ample possibilities to form σ-bonded supported by DST-SERB, Grant Nos. SB/S2/RJN-047/2015,
complexes within the bonding proximity of the 1i and that lead ECR/2016/000441 and ECR/2016/000362. GJ and SK thank for
to hetero-product 3a. However, the same H-bond guided Ramanujan and NPD Fellowships, respectively. MEA thanks
grafting of 1i alkyne cannot promote the polar homo-product CDAC for computational facility.
2i. This is simply because of the bulkiness of the catalysts and
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distances along the MD trajectory, shown in Fig. 3b clearly
depicts that there is no H-bonding interaction present in this
case. Due to the absence of polar group no such interaction is
present in case of non-polar homo-product 2a.
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(I/II)–σ–acetylide complexes formed by alkynes 1a and 1i. The formation of the alkyne σ
-complexes on the neighbouring Cu-atoms is one of the crucial steps for the coupling
reactions occurred on the surface.13 (b) Progress of the coupling reactions are captured
in the snapshots at different time steps of the AIMD trajectory. The formation of the
C—C bond and the H-bond occurs simultaneously. (c) The plots describe the H-bonding
distances between the amide –NH and O atom of –OH group of polar homo- (inset) (2i)
and hetero-products (3a). In case of hetero-product (3a) formation, the H-bonding
interaction changes from moderate to strong as the reaction proceeds. The strength of
the H-bond is represented by the thickness of the broken H-bond in blue in (b). Different
chemical entities involved in the reaction are denoted as follows. The large spheres
represents CuO substrate (Cu in tan and O in red). The stick model represent the TREN-
backbone without H atoms. Colour Code: grey: C (TREN-backbone), cyan: C (for alkynes),
red: O (TREN-backbone and alkynes) blue: N, yellow: S and white: H.
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In conclusion, CuI/IIO NPs anchored by cellulose polymer
functionalised with TREN and thiol groups TC-S-CuI/IIO 1, served
as an effective water-soluble catalytic system for the homo- and
cross-Glaser-couplings of terminal alkynes under greener
conditions. The comparison studies of catalysts backbone and
carboxylic acid-based additives and the AIMD simulations
reveals the interactions between the catalyst backbone and
reactants, that play the key fundamental role in facilitating the
hetero-selectivity. Thus, any modifications in the catalytic
centres that tune such interactions will affect the hetero-
selectivity. Though the designed catalyst not providing 100%
hetero-product, but the approach is highly promising as the
desired goal could be achieved by playing with the catalyst-
backbone and substrates interactions. Further, we showed easy
recyclability by one-step conversion of soluble catalyst 1 to
magnetically separable heterogenous catalyst 4 by grafting on
to Fe3O4 NPs (for details see, section G, ESI).
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CC08565C
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TOC
Tuneable selectivity in cross-Glaser products is achieved by tweaking the catalyst-backbone. The interaction responsible for this
is unravelled.
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