Poly(: Meta -phenylene oxides) for the design of a tunable, efficient, and reusable catalytic platform

Article abstract of DOI:10.1039/c8cc00774h

We present poly(meta-phenylene oxides) as versatile and tunable scaffolds for immobilized catalyst design. Their modular synthesis enables the rational incorporation of different side chain groups with positional control along the polymer backbone, providing an easy means to modulate polymer properties, and their corresponding catalytic activity. The coordinative convolution of these polymers with (NH₄)₂PdCl₄ affords efficient, reusable catalysts for various organic transformations.

Full text of DOI:10.1039/c8cc00774h

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Zhang, A. LoTemplio, G. Benedetto, G. Hamasaka, Y. M. A. M. A. Yamada, J. L. Katz and Y. Uozumi, Chem.
Commun., 2018, DOI: 10.1039/C8CC00774H.
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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DOI: 10.1039/C8CC00774H
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Poly(meta-phenylene oxides) for the design of a tunable,
efficient, and reusable catalytic platform
Received 00th January 20xx,
Accepted 00th January 20xx
[
a,b,c]
[b]
[b]
[b]
[d]
R. Hudson,*
H. R. Zhang, A. LoTemp[al,dio] , G. Benedetto, G. Hamasaka, Y. M. A.
[
a]
[b]
Yamada,* J. L. Katz,* and Y. Uozumi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We present poly(meta-phenylene oxides) as versatile and tunable
scaffolds for immobilized catalyst design. Their modular synthesis
enables the rational incorporation of different side chain groups
with positional control along the polymer backbone, providing an
easy means to modulate polymer properties, and their
corresponding catalytic activity. The coordinative convolution of
these polymers with (NH
4 2 4
) PdCl affords efficient, reusable
catalysts for various transformations.
In the world of catalysis, researchers are often torn between
1
homogenous and heterogeneous systems. Homogeneous
catalysts often offer superior activity because of their equal
distribution within the reaction media, and the availability of
2
highly tunable ligand architectures. Heterogeneous catalysts,
on the other hand, typically suffer from decreased catalytic
3
activity but enable easier recovery and recycling.
4
Scheme 1. Tunable Synthesis of poly(mPOs)
Our pursuit of solid-supported nanocatalysts
fuels
investigations into the coordinative convolution of polymers
5
bearing imidazole, pyridine and similar N-heterocycles with
6
poly(mPOs) thus attracted our attention for their unexplored
5
a,6b
5b,6a
metal-binding ligands, as well as for their tunability.
Pd,
Cu,
and other metals. Despite its role as an
7
excellent Pd ligand, pyrimidine-bearing polymers have yet to
8
Reaction of dihalide electrophilic (A) and diphenol nucleophilic
5
,6,9
(B
) coupling partners affords these thermooxidatively robust
0a
be explored in this context.
The synthesis of poly(meta-phenylene oxides) (poly(mPOs))
provides convenient route to such pyrimidine-bearing
1
1
0
ABAB-type poly(mPOs).
1
electrophile (pyrimidine, pyrazine, pyridine, triazine, or
Typically, the diphenol and
1
11a
12
13
a
1
4
15
nitro-, nitrile, or acetylene- substituted benzene) coupling
partners will react to form small cyclic oligomers
amphiphilic polymers as main-chain polymer ligands, as well as
those containing bidentate 1,10-phenanthroline ligands.
Unlike many of the more conventional polymers thus far used
1
6
(oxacalixarenes).
However, with specific electrophiles,
1
0a
5
,6,9 poly(mPOs) can be generated under kinetic conditions. Prior
studies demonstrated the kinetic polymerization using two
electrophilic scaffolds, 4,6-dichloropyrimidines, and 2,6-
dihalobenzonitriles. Herein we expand the scope of
poly(mPOs) derived from 4,6-dichloropyrimidines (A1), and
for catalyst immobilization via coordinative convolution,
these poly(mPOs) offer straightforward and structured tuning
by the incorporation and variation of side chain groups, while
leaving the fundamental backbone unchanged. These
introduce 4,7-dichloro-1,10-phenanthroline
(
A2
)
as
a
multicyclic aromatic electrophile that has not previously been
used in either poly(mPO) or oxacalixarene syntheses. In this
work, we leverage the tunability and functional group
tolerance of this novel synthesis (scheme 1) toward industrially
relevant, recyclable catalysis for aqueous Suzuki, Heck, and
hydrogenation reactions effective in the PPM-PPB metal
loading range.
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1
hydrophobicity (R = H < CH
3
< C
5
H11), the yield for styrene
hydrogenation after 12 hours decreased in cyclohexane (94
ꢀ
95%). The
4
2%), but steadily increased in water (52%
ꢀ
increased reactivity in water likely stems from the more
hydrophobic polymer’s ability to better attract the nonpolar
styrene into the polymer matrix to interact with the active
Figure 1. SEM/EDX scan and elemental mapping (A).
HRTEM of nanoparticle lattice spacing (B). TEM images (C)
and XPS (D) of nanoparticles as-synthesized (before) and
after use in Suzuki coupling (after).
Poly(mPOs) were readily prepared by the reaction of a 1,3-
diphenol with an equimolar amount of dihalogenated
2 3
electrophile and excess K CO in DMSO at 100-150˚C (SI).
Without directly affecting the site of metal binding, the
hydrophobicity of the polymer systems could be modified by
1
varying the nature of R (H, methyl, n-pentyl) on the diphenol
Figure 2. Tuning Poly(mPO) Hydrophobicity for
Improved Aqueous Hydrogenation
unit. The coordination environment of the pyrimidine units
2
A1) could likewise be modulated by varying R from among H,
(
Ph, N-morpholino, or even by the incorporation of a second
pyrimidyl unit, enabling incorporation of a 2,2-bipyrimidine
scaffold integrated into the polymer backbone. Alternatively,
the bidentate 1,10-phenanthroline unit could be directly
incorporated by use of the electrophile 4,7-dichloro-1,10-
phenanthroline (A2).
palladium species.
We further explored Suzuki coupling under aerobic and
aqueous conditions, which tend to be problematic for
Pd/phosphine systems. In exploring our full range of as
prepared Pd@poly(mPOs), we found no advantage to
hydrophobicity tuning or the use of more complicated
bidentate phenanthroline or bipyrimidyl systems. Thus, we
proceeded with the simplest polymer (poly(mPO) 1a) derived
from the most abundant, widely available monomers. As
shown in Table 1, Pd@poly(mPO)1a efficiently catalyzed
Suzuki reactions of aryl boronic acids with aryl chlorides
bearing neutral substituents (Entries 1-2), electron
withdrawing groups (Entry 3), and strong electron donating
groups (Entries 4-6). Coupling of the less reactive trans-2-
phenylvinyl boronic acid proceeded smoothly with either
bromo- or iodobenzene, but negligible product was observed
for chlorobenzene (Entry 7). To investigate catalyst TON (Entry
Coordinative convolution of each polymer was achieved by
mixing a solution of (NH
4
)
2
PdCl
4
(0.15 mmol in 2 mL H
2
O) into a
solution of poly(mPO) (1 mmol in 2 mL CHCl
3
). After stirring
vigorously for 3 hours at 60˚C, the mixture was diluted with
acetone, and the globular, insoluble Pd-polymer complex was
washed with H
resultant (NH
mostly PdCl
2
O, acetone, and chloroform. XPS analysis of the
PdCl @poly(mPOs) indicated Pd present as
with some PdO (Figure 1a). TEM analysis
4
)
2
4
2
identified PdO nanoparticles (3.2 +/- 0.6 nm) distributed
evenly throughout the polymer matrix (Figure 1b). SEM/EDX
elemental mapping likewise demonstrated an even
distribution of both Pd and Cl (Figure 1c-d). Loading of each
Pd@poly(mPO) complex was carried out via ICP-MAS, ranging
from 0.32-0.89 mmol/gram. The coordination of Pd to multiple
pyrimidine or phenanthroline ligands serves the dual role of
cross-linking the polymer and stabilizing the reactive metal
center.
8
), coupling of 4-iodotoluene with phenylboronic acid was
carried out at Pd loadings of 400 mol PPM (99 %), 40 mol PPM
97%), 4 mol PPM (99%), and 400 mol PPB (75%, TON = 1.9
(
million). Reusability of the catalyst was demonstrated by
measuring the initial reaction rates for recycling runs (Entry 9).
Through 5 cycles of catalysis, the initial reaction rates did not
significantly decrease. In further support of a heterogeneous
mechanism, we found that (NH ) PdCl was incapable of
Styrene hydrogenation
catalyzed
by
(
NH PdCl @poly(mPO)1a-c was evaluated in order to probe
4
4
)
2
4
2
4
the relative effect of pendant side chain groups not directly
involved in metal binding (Figure 2). By increasing the
catalyzing the coupling of aryl chlorides to phenylboronic acid,
even with the addition of the closest available homogeneous
2
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analogue (4,6-dimethoxypyrimidine) to the polymeric complex, In order to further showcase the versatility of this catalyst
indicating the cooperative affect of the Pd/polymer complex platform, we demonstrated its utility in mediating Heck
(Entry 10). Furthermore, ICP analysis after catalysis likewise coupling under aqueous conditions (Scheme 2).
measured very little (0.17 PPB) Pd leached from the polymer
matrix during the course of a typical Suzuki coupling reaction
(
Entry 1). Pd@poly(mPO)1a outperformed the commercial
1
heterogeneous catalyst with our TON ranging from two to
9
1
five orders of magnitude greater than Pd/C
8
17-19
(Table 2). Our
21
20
system likewise outperformed Pd(PPh
3
)
4
2 3
and Pd (dba) ,
although our TON and TOF fell short of a homogenous (non-
recyclable) system catalyzed by trace impurities of Pd found in
Scheme 2. Aqueous Heck coupling
2
2
other reagents.
Table 2. Comparison of Pd@poly(mPO) vs. commercial
catalysts for Suzuki coupling.
Table 1. Aqueous Suzuki Coupling
TON
1,900,000
333
TOF
95,000/hr
28/hr
this work
Pd@poly(mPO)1a
ref 17
Pd/C
ref 18
Pd/C
28
28/hr
ref 19
Pd/C
20,000
33
16,600/hr
5.5/hr
Entry Ar
R
Yield
ref 20
Pd(PPh3)4
1
2
3
4
5
6
7
4-Me-C
6
H
4
Ph
Cl: 97 (isolated)
ref 21
Pd (dba) /PtBu
3
61
12/hr
2
3
Ph
4-Me-C
Ph
6
H
4
Cl: 98 (GC)
Pd (trace impurities in 61,875,000
ref 22
50,000,000/hr
Na CO )
2
3
4-CN-C
6
H
4
Cl: 97 (isolated)
Cl: 94 (isolated)
Cl: 95 (isolated)
Cl: 94 (isolated)
4-NH
2
-C
6
H
4
Ph
The modular synthesis of poly(mPOs) offers a tunable scaffold
from which to design reusable catalysts that are effective in
the PPM-PPB metal loading range. We introduce a novel 4,7-
dichloro-1,10-phenanthroline monomer previously unused in
the nascent poly(mPO) field, or the similar but more
developed oxacalixarene field. The active catalysts were
prepared by coordinative convolution of these poly(mPOs)
4-OH-C
6
H
4
Ph
4-OMe-C
Ph
6
H
4
Ph
a
I: 98 (isolated) Br: 95 (GC) Cl: 1
(GC)
b
c
d
e
with (NH ) PdCl in CHCl /H O. Reactivity toward styrene
4 2 4 3 2
8
9
4-Me-C
4-Me-C
6
H
H
4
4
Ph
Ph
I: 99, 97, 99, 75, (GC)
hydrogenation could be specifically regulated for aqueous or
non-polar environments by modulating the hydrophobicity of
the polymer scaffold. The catalysts also demonstrated aqueous
reactivity for Suzuki and Heck coupling reactions.
f
6
Cl (initial rates, µmol/min): 1.4,
.3, 1.5, 1.4, 1.3
g
h
i
j
1
k
1
0
4-Me-C
6
H
4
Ph
Cl: trace (homogeneous)
Reaction conditions: (NH ) PdCl @poly(mPO)1a (10 mg, 0.04
2
4
4
mol%), Ar-X (1.5 mmol), R-B(OH) (1.8 mmol), TBAB (1.5
2
Conflicts of interest
mmol), K PO (4.5 mmol), H O (0.6 mmol), 95˚C, 20 hr.
2
3
4
[
a]
There are no conflicts to declare
0
1
mmol TBAB
[
b]
[c]
0
mg (NH ) PdCl @poly(mPO)1a
2
,
400 PPM,
1
mg
mg
4
4
[
PPM, 0.1
d]
(
NH ) PdCl @poly(mPO)1a
4
,
40
2
4
Notes and references
(
NH ) PdCl @poly(mPO)1a, 4 PPM)
4
4
2
[e]
0
.1 mg (NH ) PdCl @poly(mPO)1a, 400 PPB, Ar-X (15 mmol),
4
1
a) J.M. Thomas, W.J. Thomas. Principles and Practice of
Heterogeneous Catalysis. John Wiley & Sons, 2014. b) R.
Hudson, Y. Feng, R.S. Varma, and A. Moores, Green Chem.
2
4
R-B(OH) (18 mmol), K PO (45 mmol), H O (6 mL), 95˚C, 20 hr.
4
2
3
2
Initial reaction rates were determined after
3
h.
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(NH ) PdCl @poly(mPO)1a was reused in five sequential
4
2
4
[f] st
[g] nd
2
[h] rd
[i] th
run, 3 run, 4 run, 5 run)
[j] th
reactions ( 1 run,
[
6,
k]
(
NH ) PdCl (1.2 mg), 4,6-dimethoxypyrimidine (4.7 mg), Ar-
2
4
4
2
3
a) M. Meuresch, S. Westhues, W. Leitner, J. Klankermayer,
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