Organogel formation and molecular imprinting by functionalized gluconamides and their metal complexes

Article abstract of DOI:10.1039/a608266a

Functionalized gluconamides and their metal complexes are shown to give supramolecular assemblies, in some cases chiral, and to form organogels in a large variety of organic solvents, e.g. methacrylate mixtures which can be polymerized, as well as o-xylen

Full text of DOI:10.1039/a608266a

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Organogel formation and molecular imprinting by functionalized
gluconamides and their metal complexes
a
a
a
b
a
Rudi J. H. Hafkamp, Bas P. A. Kokke, Iris M. Danke, Hubertus P. M. Geurts, Alan E. Rowan,
Martinus C. Feiters and Roeland J. M. Nolte*a
a
a
Department of Organic Chemistry, NSR Centre, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands
Central Electron Microscopy Facility, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands
b
Functionalized gluconamides and their metal complexes are
at concentrations of 1, 2, and 5 mg ml21, respectively. For all
gels, TEM showed fibres with approximately the same
diameters as 7a, but without a helical twist. In toluene, these
non-helical fibres in turn self-assemble in a hierarchical process
to give mm-sized twisted superstructures [Fig. 1(e)].
shown to give supramolecular assemblies, in some cases
chiral, and to form organogels in a large variety of organic
solvents, e.g. methacrylate mixtures which can be poly-
merized, as well as o-xylene, chloroform, ethyl acetate,
ethanol and tetrahydrofuran.
We were interested in using the organogels to prepare
membranes with pores by the technique of ‘imprinting’.1 Using
this approach, 3a and 4a (1% m/v) were dissolved in a warm
mixture of methyl methacrylate and n-butyl methacrylate (1:4,
v/v) containing 0.5% (m/v) polymerization catalyst (benzoin
ethyl ether), as adapted from the literature.11 Well defined
aggregates with high aspect ratios were formed and gelated this
solvent mixture. The organogel was transferred into gelatin
cups and readily polymerized overnight with UV light (l = 365
nm) in a refrigerator (5 °C), yielding a rigid polymer matrix.
Using a glass knife and a diamond microtome, slabs of this
polymer matrix were sliced in thin coupes (90 nm) which were
allowed to float on a water surface before they were transferred
0
1
Gluconamides are known to form suprastructures in water. We
2
have recently shown that the self-assembly behaviour of the
imidazole-functionalized N-octylgluconamide 1a in water can
be tuned. Dispersion of the compound in water at pH 8.5 results
in fibres and hollow tubuli, whereas at low pH vesicles are
formed. We report here that further functionalization of
gluconamides with different head-groups and the addition of
metal ions result in diverse suprastructures which induce
gelation in a large variety of organic solvents.
In recent years, a relatively broad range of compounds has
3
been found to gelate organic solvents. Gluconamide 2a has
been reported to aggregate into bilayer scrolls and form an
unstable organogel in o-xylene.4 We find, however, that a
number of ester derivatives of 2a, viz. the benzoate 3a and the
cyclohexanoate 4a (cf. Scheme 1 for structures and outline of
preparations)† form stable organogels in a number of solvents,
e.g. o-xylene, chloroform, and ethyl acetate (Table 1). In
chloroform, 3a yields fibres or bundles of fibres which do not
exhibit chirality [Fig. 1(a), (b)]. In contrast, 4a in ethyl acetate
HO
O
HO
HO
O
OH
iii
i, ii
H
N
OH OH
O
O
O
O
MeO
R
OH
OH
OH
O
OH OH
O
O
O
O
O
[
Fig. 1(c)] is able to express its molecular chirality in the
2
iii
viii
suprastructure, resulting in helical fibres. From electron micro-
scopic investigations, we propose that these fibres consist of
double and multiple-stranded helices, and that the knobs at the
end of the fibres represent hairpin-like turns of these helices.
In contrast to the functionalized gluconamides with free
hydroxy groups, most of the bis-methylene protected N-n-
octylgluconamide derivatives investigated, including the imi-
dazolyl derivative 1a as well as the 6-hydroxy analogue 5a and
the benzoate and picolinate (6) esters thereof, did not form
ix
H
N
OH OH
O
H
N
R
R
O
O
OH OH
O
3
5
H
N
OH OH
O
O
vii
iv
R
R
O
O
organogels (Table 1). We decided to investigate the palla-
O
O
OH OH
dium(ii)8 and platinum(ii) complexes of the pyridine-
4
functionalized gluconamides 6 and 7. trans-[Pd(7b)
2
Cl
2
] was
H
N
O
O
O
H
N
O
O
soluble in chloroform but precipitated in water. In thf a rigid gel
was obtained, which showed helical ribbons (width 36 nm) with
a regular twist (pitch 110 nm) and a (maximum) aspect ratio of
R
OTs
O
O
O
O
O
O
N
6
vi
2
5 [cf. Fig. 1(d)].
trans-[Pd(6b)Cl
v
2
] dissolved in thf without giving a gel. It is
Cl ] forms an organogel in thf and
does not. A slight difference is expected between
the conformations of [Pd(7b) Cl ] and [Pd(6b) Cl ], because in
H
N
O
not clear why [Pd(7b)
Pd(6b) ]Cl
2
2
R
O
N
H
N
O
O
[
2
2
O
O
R
N
N
2
2
2
2
7
the latter compound the pyridyl ring is meta-substituted and the
connection to the carbohydrate is via an ester group. These
differences can cause a bend in the overall shape of the molecule
which is not favourable for gelating solvents according to the
rule of thumb: ‘gelators of organic fluids should have the
capability of adopting rodlike shapes in their extended con-
O
O
O
1
Scheme 1 R = n-octyl a or n-hexadecyl b. Outline of syntheses of
gluconamide derivatives from gluconolactone. Reagents and conditions: i,
O–H ; ii, H -MeOH; iii, excess alkylamine;1 iv, TsCl in
+ 6
+
7
trioxane, H
pyridine, 0 °C; v, imidazole, CHCl
triethylamine, 115 °C; vii, picolinoyl chloride, CHCl , 0 °C; viii, benzoyl
2
3
, 15 kbar, 50 °C; vi, 4-hydroxypyridine,
9
formations’. We have also prepared the Pt complex, trans-
3
[Pt(7b)
2
Cl
2
], and find that it gelates thf, toluene, and methanol
chloride, pyridine, 0 °C; ix, C
6
H11C(O)Cl, 0 °C.
Chem. Commun., 1997
545

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Table 1 Gelation behaviour of functionalized gluconamides in organic
solventsa
Compound/
solvent
o-Xylene
CHCl
3
EtOAc
EtOH
1
2
3
4
5
6
a
a
a
a
a
a
R
G
G
G
T
S
S
I
G
G
S
S
S
I
G
G
S
S
S
R
G
S
S
S
b
a
G, gelates forming a high-viscosity mixture upon cooling; I, insoluble; R,
recrystallizes within 1 h upon cooling; S, dissolves without gelation; T,
gives a turbid mixture with slightly increased viscosity upon cooling.
b
Compound 3a also forms gel in methanol, acetonitrile, acetone, dioxane,
dichloromethane, toluene and benzene. It recrystallizes from water,
dissolves without gelation in thf, and is insoluble in n-hexane and in diethyl
ether.
order of magnitude larger than that of the helical fibres. The
increase in size as well as the loss of chirality are probably due
to shrinking of the methacrylate gel during polymerization.
In conclusion, we have shown that functionalized glucon-
amides and their metal complexes form a variety of chiral and
non-chiral aggregates in a large variety of organic solvents and
polymerizable methacrylate mixtures, and that it is possible to
make imprints of the gluconamide assemblies.
We thank Professor R. G. Weiss (Georgetown, USA) for
discussing his results with us prior to publication, and agreeing
to concurrent publication of their work12 and ours.
Footnote
†
1
The preparations of 6-deoxy-6-(1-imidazolyl)-N-n-octyl-d-gluconamide
2
1a
a, N-n-octyl-d-gluconamide 2a, and 2,4:3,5-dimethylene-N-n-octyl-
2
d-gluconamide 5 are in the literature. The syntheses and characterizations
of N-n-octyl-d-gluconamide-6-benzoate 3a, N-n-octyl-d-gluconamide-
6
6
-cyclohexanoate 4a, 2,4;3,5-dimethylene-N-n-hexadecyl-d-gluconamide-
-pyridylcarboxylic acid ester 6b and 6-(4-pyridyl)-2,4;3,5-dimethylene-
II
II
N-n-hexadecyl-d-gluconamide 7b, and their Pd and Pt complexes are
described elsewhere. The purity and assigned structures of all compounds
5
are fully supported by spectroscopy and elemental analyses.
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1
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4
a form helical aggregates [Fig. 1(c)], the pores observed in the
polymeric matrix did not have helical charcter [Fig. 1(f)]. It
should be noted that the size of the pores is approximately one
Received, 9th December 1996; Com. 6/08266A
546
Chem. Commun., 1997