One-step syntheses of very large cage-type molecules from aromatic sub-units

Article abstract of DOI:10.1039/b108124c

Polycondensation of a trifunctional, ketone-activated fluoroarene with bis- or tris-phenoxides under pseudo-high dilution conditions affords a series of very large macropolycyclic aromatic ether ketones; isolation and characterisation of these materials b

Full text of DOI:10.1039/b108124c

One-step syntheses of very large cage-type molecules from aromatic
sub-units†
Howard M. Colquhoun,*a Fabio Aricoa and David J. Williams*b
a Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD.
E-mail: h.m.colquhoun@rdg.ac.uk
b Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY.
E-mail: djw@ic.ac.uk
Received (in Cambridge, UK) 7th September 2001, Accepted 23rd October 2001
First published as an Advance Article on the web 13th November 2001
Polycondensation of a trifunctional, ketone-activated fluor-
oarene with bis- or tris-phenoxides under pseudo-high
dilution conditions affords a series of very large macro-
polycyclic aromatic ether ketones; isolation and character-
isation of these materials by NMR, MALDI-TOF MS and,
for one example (after reduction of the carbonyl groups to
methylene linkages) by X-ray crystallography, confirms that
polycondensations which would normally lead to highly
branched or cross-linked polymers can also give rise to large,
closed-network molecules.
of high-value condensation polymers.2 Macrocyclic aromatic
oligo-amides are, in addition, of considerable value for the
generation of novel supramolecular architectures including
The synthetic and structural relationships between linear
aromatic polymers and their macrocyclic homologues have
been intensively investigated over the past decade,1 work
focusing mainly on the potential of macrocyclic ring-opening
polymerisation for reactive fabrication of linear polymers. The
reverse reaction—ring closing depolymerisation—has however
also been explored for application in the recovery and recycling
catenanes,3 rotaxanes,4 and knots.5 We now describe some
preliminary experiments which demonstrate that the ring–chain
relationship can be extended to a third dimension. Thus,
polycondensations involving trifunctional monomers, which
would normally afford highly branched or even fully cross-
linked polymers, are here shown also to give, under pseudo-
high dilution conditions, a series of very large aromatic cage-
type molecules.6
Reaction of 4,4A-hexafluoroisopropylidenediphenol (1) with
1,3,5-tris(4-fluorobenzoyl)benzene (2) was carried out by slow
addition of a solution of the two monomers (3+2 mole ratio) in
dimethylacetamide (DMAc) to potassium carbonate in reflux-
ing DMAc–toluene, with continuous azeotropic removal of
water (Scheme 1). A complex mixture of oligomeric and
polymeric materials was obtained but the macrobicyclic cage-
compound 3 (mp 450 °C) could be isolated straightforwardly
from this mixture, albeit in low yield ( ~ 5%) , by column
chromatography.
Spectroscopic analyses of 3 by MALDI-TOF MS and by 1H
and 13C NMR were entirely consistent with the structure shown
in Scheme 1, but efforts to obtain single crystals suitable for X-
ray analysis were unsuccessful. However, reduction of the
carbonyl groups in 3 to methylene linkages using triethylsilane
and trifluoroacetic acid (Scheme 1)7 afforded the fully-reduced
cage-compound 4 (42% yield), together with a compound (5) in
21% yield in which one ketone group remains unreduced.†
Compounds 4 and 5 were readily separated by column
chromatography, and 4 eventually yielded crystals suitable for
X-ray analysis.‡ Its structure is shown in Fig. 1, from which it
is evident that the composition and topology proposed for this
compound (and thus by inference for 3) are correct.
Scheme 1
The molecule adopts a semi-collapsed and flattened con-
formation with no obvious intramolecular interactions other
than a weak p-stacking arrangement between a pair of aromatic
ether-containing ring systems in the two adjacent arms of the
† Electronic supplementary data (ESI) available: analytical and spectro-
scopic data for compounds 3–5 and 8. See http://www.rsc.org/suppdata/cc/
b1/b108124c/
2574
Chem. Commun., 2001, 2574–2575
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b108124c

An analogous polycondensation between the trifluoro-com-
pound 2 and tris-phenol 6 (derived from 2 by hydrolysis with
potassium hydroxide in DMSO) afforded not the expected cage-
compound 7—a direct analogue of 3—but its macrotricyclic
dimer 8 (Scheme 2). This compound was isolated in pure form
by column chromatography (4% yield) and characterised in
detail.† It shows a sharp, clearly-defined melting point by DSC
at the astonishingly high temperature of 556 °C, reflecting both
extreme rigidity of the molecular structure and a quite
remarkable thermal stability. The MALDI-TOF spectrum of the
original product mixture showed a strong [M + Na]+ peak for
compound 8, but gave no evidence for the monomeric cage 7.
The MALDI-TOF analysis did however indicate the presence of
higher polycyclic oligomers of 7, including the macro-
polycyclic trimer and tetramer. Evidence to date, mainly from
the 1H NMR spectra of partially-resolved chromatographic
fractions, suggests that the higher-order polycyclic oligomers of
7 comprise increasing numbers of the structural repeat (a six-
ring macrocycle with a two-ring linking umit) found in
oligomer 8.
This approach to large, closed-network molecules is clearly
not restricted to the aromatic polyetherketone systems described
here, but should be generally applicable to any type of
branching polycondensation, including esterification, amida-
tion and imidation. Such possibilities are currently under
investigation, as are the potential applications of these mole-
cules in supramolecular assembly and in cage-opening poly-
merisation.
Fig. 1 Molecular structure of the reduced cage-compound 4 (hydrogen
atoms are omitted for clarity).
macrocycle. The hexafluoroisopropylidene-linked aromatic res-
idues here display a consistently skewed conformation, but the
diarylene-ether units have geometries that range from symmet-
rically-skewed to near-orthogonal.
We thank Professor F. H. Kohnke of the University of
Messina for stimulating discussions.
Notes and references
‡ Crystal data for 4: C99H66F18O6.0.75 CH2Cl2, Mr = 1757.21, triclinic,
¯
P1, a = 14.291(3), b = 17.673(4), c = 20.181(3) Å, a = 97.04(2), b =
109.61(1), g = 112.31(2)°.V = 4255(1) Å3, T = 293 K, Z = 2, Dc 1.371
g cm23, m (Cu-Ka) = 1.371 mm21, F (000) = 1803. Independent measured
reflections 11444. R1 = 0.077, wR2 = 0.166 for 5472 independent observed
reflections [2q 5 115°, I > 2s(I)]. CCDC 171849. See http://www.rsc.org/
suppdata/cc/b1/b108124c/ for crystallographic files in .cif format.
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6 A number of smaller, semi-aromatic cage-type molecules (4–8 aromatic
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Scheme 2
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