Bifunctional [c2]daisy-chains and their incorporation into mechanically interlocked polymers

Article abstract of DOI:10.1021/ja0725100

A strategy for the formation of mechanically interlocked polymers is presented. Ring-closing olefin metathesis has been shown to provide a very high yielding route to [c2]daisy-chains suitably functionalized to allow their one-step conversion to bisolefin

Full text of DOI:10.1021/ja0725100

Published on Web 06/27/2007
Bifunctional [c2]Daisy-Chains and Their Incorporation into Mechanically
Interlocked Polymers
Erin N. Guidry,† Jean Li,† J. Fraser Stoddart,*,‡ and Robert H. Grubbs*,†
The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, California 91125, and California NanoSystems Institute
& Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, 405 Hilgard AVenue,
Los Angeles, California 90095
Received April 10, 2007; E-mail: rhg@caltech.edu; stoddart@chem.ucla.edu
Scheme 1. Synthesis of [c2]Daisy-Chain Dimers Using RCM and
Their Subsequent ADMET Polymerization to Form An Interlocked
Polymer.
Much attention has been devoted to the design and synthesis of
linear daisy-chain polymers1, a class of rotaxane-based polymers
in which both the host and guest recognition units are coupled
covalently together in the same monomer (Figure 1a). Repetitive
self-assembly processes leading to daisy-chain polymers (Figure
1c) have to compete with the formation of cyclic analogues2,3 where
the thermodynamic driving force lies in the production of small
cycles, for example, the [2]daisy-chain dimer shown in Figure 1b.
In addition to their appeal as challenging targets in polymer synthe-
sis, such macromolecules would also contain mechanical links rather
than covalent bonds between the monomer units. It is tempting to
speculate that the incorporation of flexible mechanical links, that
could also be rendered switchable, would have repercussions for
the polymer-chain behavior which is expected to affect both the
solid-state and solution properties.4 Despite the fascination of chem-
ists with these macromolecules, few examples5 exist on account of
the considerable challenge posed by their synthesis. Although num-
erous attempts have been made at their synthesis, most rely on tem-
plate-directed self-assembly as the polymerization step. More often
than not, however, this strategy results3 in cyclic dimer forma-
tion. While higher oligomers have been observed,1a-c,e the number
of repeat units is typically low. Here, we describe a strategy that
relies on the polymerization of a bisfunctionalized [c2]daisy-chain
monomer.
include the formation of imines,7 disulfides,8 and cyclic acetals9 as
well as olefins.10 The use of functional group tolerant ruthenium
alkylidene catalysts has been applied11 successfully to the synthesis
of [2]catenanes12 and [2]rotaxanes.13 Previously, we have demon-
strated that the ring-closing of appropriate bisolefin-containing
polyether substrates around disubstituted dibenzylammonium ions
result in the reversible formation of [2]catenanes14 and [2]-
rotaxanes.15 We now describe how we have extended this ring-
closing metathesis methodology to the synthesis of [c2]daisy-chains.
The formation of [c2]daisy-chains necessitates that both recogni-
tion units reside within the same molecule such that it will not un-
dergo self-complexation but rather form a dimeric daisy-chain. The
precursor 1-H‚PF6 was prepared (Supporting Information) in seven
steps. Ring-closing of the terminal olefin functions present in 1-H+
using the ruthenium catalyst 2 afforded (Scheme 1) the [c2]daisy-
chain 3-H2‚2PF6 in near quantitative conversion and as a 95% yield,
isolated as a mixture of (E) and (Z) isomers, a situation which con-
fers considerable complexity upon the 1H NMR spectrum (SI). Hy-
Figure 1. Graphical representation of daisy-chain monomer (1a), [2]daisy-
chain dimer (1b), and daisy chain polymer (1c).
Although [c2]daisy-chains, based on hydrophobic, hydrogen
bond, and metal-coordination templating motifs have been produced,
all these approaches involve irreversible stoppering events as the
final mechanical bond-forming reactions. On account of the
accompanying formation of unwanted byproducts, this strategy does
not always result in high yields of mechanically interlocked
products. An alternative strategy exploits dynamic covalent chem-
istry,6 an approach that relies on reversible reactions in which the
product distribution depends on thermodynamic rather than kinetic
control. Examples of reversible reactions employed as the final
bond-forming steps in the synthesis of catenanes and rotaxanes
† California Institute of Technology.
‡ University of California, Los Angeles.
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J. AM. CHEM. SOC. 2007, 129, 8944-8945
10.1021/ja0725100 CCC: $37.00 © 2007 American Chemical Society

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Figure 2. Ball and stick representation of 4-H2‚2PF6 generated from a
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Functionalization of the stoppers in these [c2]daisy-chains could
provide an alternative means to synthesize mechanically interlocked
polymers. Acyclic diene metathesis (ADMET) chemistry looked
promising to us since it requires only functionalization of the [c2]-
daisy-chain as a bisolefin which can be polymerized under mild
conditions catalytically using 2 and is driven by the removal of
ethylene. The synthesis of the [c2]daisy-chain 6-H2‚2PF6, containing
olefinic terminal groups on both stoppers was accomplished
(Scheme 1), starting from 5-H‚PF6 in 93% yield using the ruthenium
catalyst 2. The diol 6-H2‚2PF6 was hydrogenated using Adams’
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Metathesis, in two different guises is making a hitherto unreach-
able goal in synthesis a reality. Ring-closing olefin metathesis has
been shown to provide a very high yielding route to [c2]daisy-
chains suitably functionalized to allow their one-step conversion
to bisolefins which can be used as monomers in ADMET
polymerizations to afford mechanically interlocked polymers. While
the properties of these polymers are being investigated, their
potential for applications is also being explored.
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Acknowledgment. We would like to thank Drs. Stuart Cantrill,
Al Nelson, and Anna Sanchez for useful discussions. The ruthenium
metathesis catalyst was received as a generous gift from Materia.
We thank Lawrence M. Henling and Dr. Michael W. Day for the
X-ray crystallographic data. The work was supported by the Office
of Naval Research through its MURI Program.
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Supporting Information Available: Complete experimental pro-
cedures as well as full characterization. This material is available free
of charge via the Internet at http://pubs.acs.org.
(17) Crystals of 4-H2‚2PF6 diffract poorly and appear to be twinned. Data used
for least-squares refinement was restricted to 2θ < 40° with data between
36 and 40° being extremely weak. Disorder is observed in the ring-closed
portion of the molecules. Restraints were placed on temperature factors,
bond distances, and bond angles in this portion of the molecules. The
structure has been deposited in the CCDC; number 648323. See Supporting
Information for complete details.
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