Acid- and base-functionalized core-confined bottlebrush copolymer catalysts for one-pot cascade reactions

Article abstract of DOI:10.1039/c4cc06573e

We demonstrate a novel method that enables the formation of core-confined bottlebrush copolymers (CCBCs) as catalyst supports. Significantly, owing to the site-isolated effect, these CCBC catalysts with the incompatible acidic para-toluenesulfonic acid (P

Full text of DOI:10.1039/c4cc06573e

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Acid- and base-functionalized core-confined
bottlebrush copolymer catalysts for one-pot
cascade reactions†
Cite this: Chem. Commun., 2014,
50, 14778
Received 21st August 2014,
Accepted 6th October 2014
Linfeng Xiong, Hui Zhang, Aiqing Zhong, Zidong He and Kun Huang*
DOI: 10.1039/c4cc06573e
www.rsc.org/chemcomm
We demonstrate a novel method that enables the formation of core- site-isolating catalysts that can conduct one-pot sequential reac-
confined bottlebrush copolymers (CCBCs) as catalyst supports. tions still remains a significant challenge.
Significantly, owing to the site-isolated effect, these CCBC catalysts
Molecular bottlebrushes are single graft copolymer mole-
with the incompatible acidic para-toluenesulfonic acid (PTSA) and cules with a well-defined cylindrical shape which is a result of
basic 4-(dimethylamino)pyridine (DMAP) groups can conduct a simple the steric hindrance between the polymeric side chains forcing
two-step sequential reaction in one vessel.
the backbone to adopt a nearly extended conformation.18–24
Recently, considerable attention has been drawn towards these
In the past decade, many investigations have been focused on macromolecules due to their potential use in intramolecular
the development of green chemical processes and synthetic nanoengineering, for example as templates for inorganic nano-
methods due to environmental considerations.1,2 The capability particles or nanowires,25–31 and for development of materials
to carry out multiple reactions in one flask plays an important having novel properties, such as supersoft elastomers,32,33
role in the field of green chemistry because it decreases the photonic crystals,34–38 molecular tensile machines,39 drug-loading
number of work-ups and purifications, as well as the volume of vehicles,40,41 organic nanotubes,42–44 other carbon nanostructures45
the solvent used. These reactions are often called cascade or and water-soluble organo-silica hybrid nanotubes.46 Resembling
domino reactions, which represents the development trend dendritic, hyperbranched and star polymers, which contain multi-
of modern organic synthesis due to the simplified process with ple arms joined at a central core, core–shell bottlebrush copolymers
low cost, less waste and fewer product purification steps. A key are composed of highly branched polymeric side chains emanating
problem for the cascade reactions is how to site-isolate catalysts from a central backbone. Especially, the unique shape, easily
or reagents from each other such that they do not come into controlled dimensions and versatile synthesis routes of molecular
contact and poison one another to avoid undesired interactions. bottlebrushes render them useful in single-molecule manipulation
To date, several catalyst site-isolation techniques have been to create site-isolated nano-objects. To date, there have been no
developed by using solid supports or sol-gels to encapsulate reports for the precise introduction of catalytic functional groups
opposite catalysts.3–13 More recently, soluble polymers with into bottlebrush copolymers.
branched architectures, such as dendritic and hyperbranched
In this communication, we describe a novel method that
polymers, have emerged as attractive nanoscale reactors for the enables the formation of CCBCs as catalyst supports for one-pot
encapsulation and isolation of various catalytic groups within cascade reactions. Owing to the site-isolated effect, these CCBC
17
the interior of the polymers.14–16 In particular, Frechet et al.
catalysts with the incompatible acid and base groups can
´
reported the use of star polymers to combine the normally conduct a simple two-step sequential reaction in one vessel.
incompatible acid and base catalysts for one-pot cascade reactions. CCBCs containing acidic PTSA or basic DMAP catalysts con-
Although recent advancements in supramolecular chemistry, fined in their core were successfully synthesized by a ‘‘graft-from’’
nanomaterial synthesis, and catalyst design have significantly approach with the help of the reversible addition-fragmentation
improved our ability to construct incompatible multifunctional chain transfer (RAFT) polymerization and metathesis cross-
catalytic systems, developing new and easy methods for preparing linking reaction. The structure of the target bottlebrush copolymer
precursor is shown in Scheme 1. First, poly(glycidyl methacrylate)
(PGM) backbones with an average of 140 units were prepared by
RAFT polymerization mediated by 2-cyanoprop-2-yl-4-cyano-
Department of Chemistry, East China Normal University, Shanghai, 200241,
P. R. China. E-mail: khuang@chem.ecnu.edu.cn
dithiobenzoate (CPD). The polymer had an extremely narrow
† Electronic supplementary information (ESI) available: Experimental details and
Fig. S1–S6. See DOI: 10.1039/c4cc06573e
molecular weight distribution (Mw/Mn o 1.1). Pendent epoxide
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Scheme 1 Fabrication of core-confined bottlebrush copolymer supported
catalysts.
groups of PGM were then hydrolyzed to provide diols, which
served as initiators for S-1-dodecyl-S0-(a,a0-dimethyl-a00-acetic
acid)trithiocarbonate (TC) RAFT modification. The successful
outcome of the esterification was evident by the appearance of
a new signal at 3.28 ppm belonging to the methylene protons
adjacent to the trithiocarbonate group (Fig. S1, ESI†). Based on
the area integration analysis of the peak at 3.28 ppm (designated
‘‘e’’ in Fig. S1, ESI†) and the methyl peak (at around 0.90 ppm)
from the main chain and RAFT end groups (designated ‘‘a’’ and
‘‘g’’ in Fig. S1, ESI†), we can calculate the number of grafted
RAFT groups. Upon using an excess of RAFT agent acid chloride,
an average of 63% of hydroxy units were successfully converted
to RAFT groups. Then, phenyl 4-vinylbenzenesulfonate (PVBS) or
4-N-(4-vinylbenzyl)oxyethyl-N-methylaminopyridine (VEMAP) as
catalytic groups with cross-linkable 4-(3-butenyl)styrene (BS)
groups were randomly grafted onto the PGM backbone by RAFT
polymerization to form acid or base-containing bottlebrush
copolymers. In order to improve the solubility and steric
hindrance of the final CCBC catalysts, a poly(N-isopropylacryl-
amide) (PNIAAm) shell layer was introduced into the bottle-
brush architecture to form a core–shell structural precursor.
Finally, the acid-containing bottlebrush copolymers were composed
of the poly(PVBS/BS) core block with an average of 23 PVBS and
10 BS units and a PNIAAm shell block with an average of
220 units, confirmed by 1H NMR spectroscopy analyses.
Similarly, the base-containing bottlebrush copolymers contained
the poly(VEMAP\BS\St) core block with an average of 10 VEMAP
and 4 BS and 4 St units and a PNIAAm shell block with an average
of 260 units (Fig. S2 and S3, ESI†). Every molecular bottlebrush
contained approximately 4057 acidic groups (or 1764 basic
groups) as measured by 1H NMR analysis of the bottlebrush
copolymer precursors. Size exclusion chromatography (SEC)
traces of the acid or base-containing bottlebrush copolymer
precursor exhibited monomodal molecular weight distribution
Fig. 1 TEM characterization of acid-containing CCBCs.
Scheme 2 Illustration of core-confined bottlebrush copolymer supported
catalysts for one-pot cascade reactions.
Table 1 Catalytic results for a one-pot cascade reaction using CCBCs
acid and base catalysts
Base
Acid
Conversion
of Ab (%)
Yield
Entry
catalysta
catalysta
of Cb (%)
1
2
3
4
5
6
7
M2
DMAP
M2
DMAP
M3
M2
M1
M1
PTSA
PTSA
M1
M4
M4
100
5
7
0
9
10
6
87
4
5
0
8
9
5
M3
a
Reaction conditions: 10 mol% acid and base catalysts were used. The
reaction mixtures were run for 48 h at 70 1C in DMSO/H2O (40 : 1), [D] =
b
300 mM, n(A) : n(D) = 1 : 4. Yields are based on GC-MS measurements.
traces with PDI = 1.17 and 1.10, respectively (Fig. S4 and S5, As we expected, clearly distinguishable cylindrical nanorods with
ESI†), which indicated efficient reinitiation and the formation a length of 30 ꢀ 6 nm and a diameter of 8 ꢀ 2 nm were observed.
of well-defined copolymers.
Next, we explored the possibility of our materials as catalyst
According to a previously reported method,44 the pendent supports for the one-pot cascade reactions. An acid-catalyzed
olefinic groups of the BS units were linked together by the acetal hydrolysis and the subsequent base-catalyzed Knoevenagel
cross-metathesis reaction to produce core-confined architecture condensation were used as model reactions (Scheme 2).
in copolymer precursors. For the acid-containing bottlebrush Reactions with either free acid or free base, and with uncross-
copolymers, the phenylsulfonate esters can be deprotected with linked bottlebrush copolymer catalysts were also performed as
KOH solution and then acidified to produce acidic para- control experiments (Table 1).
toluenesulfonic acid groups. The shape and size of the synthe-
The results showed complete hydrolysis of the acetal to
sized acid- and base-containing CCBCs were also characterized by benzaldehyde and 87% yield of target product C was observed
transmission electron microscopy (TEM) (Fig. 1 and Fig. S6, ESI†). when the reaction was performed with M1 and M2. When the
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This work was supported by the National Natural Science
Foundation of China grant 51273066, the Shanghai Pujiang
Program grant 13PJ1402300 and the Large Instruments Open
Foundation of East China Normal University (No. 2014-15).
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