Cooperative assembly of a double-stranded hydrogen-bonded porphyrin zip

Article abstract of DOI:10.1016/S0040-4039(01)00117-4

A linear zinc porphyrin dimer has been used for the first time to stabilise two identical non-covalent aggregates by a combination of N-pyridyl zinc binding and hydrogen bonding. An improved multigram synthesis of 5,15-bis-(3,5-di-tert-butylphenyl)-10,20-

Full text of DOI:10.1016/S0040-4039(01)00117-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2225–2229
Cooperative assembly of a double-stranded hydrogen-bonded
porphyrin zip
a,
a
b
M. John Plater, * Stuart Aiken and Grant Bourhill
a
Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, UK
b
Defence Evaluation and Research Agency, St Andrews Road, Malvern, Worcester WR14 3PS, UK
Received 28 November 2000; revised 8 January 2001; accepted 17 January 2001
Abstract—A linear zinc porphyrin dimer has been used for the first time to stabilise two identical non-covalent aggregates by a
combination of N-pyridyl zinc binding and hydrogen bonding. An improved multigram synthesis of 5,15-bis-(3,5-di-tert-
butylphenyl)-10,20-bis(trimethylsilylethynyl)porphyrin is described. © 2001 Elsevier Science Ltd. All rights reserved.
Porphyrin arrays are attracting attention because of
their novel electronic, optical and photophysical prop-
erties. Conjugated porphyrins are being investigated as
using the trimethylsilylpropynal presumably as a conse-
quence of the ability of the silylacetylene group to assist
in stabilisation of a pyrrolic carbocation intermediate.
5,15-Dibromo-10,20-(3,5-di-tert-butylphenyl)zinc por-
phyrin 1 was prepared by standard methods via
1
light-harvesting antenna for artificial photosynthetic
2
3
systems and for potential applications in sensing,
4
5
17
opto-electronic and magnetic materials. Conjugation
in linear porphyrin oligomers is maintained by connect-
ing them with acetylene linkages and is increased by
the assembly of double strands with DABCO which
dipyrrylmethane. This was converted to compound 2
1,18
by palladium catalysed ethynylation.
Compound 1
6–8
did not readily undergo palladium catalysed coupling
with triisopropylsilylacetylene so the monoprotected
derivative 4 required for dimer formation was prepared
9
may enhance their non-linear optical behaviour. Non-
covalent multiple strand arrays also occur in biological
macromolecules such as nucleic acid helices and are
9
following the protocol developed by Anderson. This
involved bis-desilylation with fluoride followed by
mono-lithiation with LiHMDS and quenching with tri-
10–12
topical targets for various biomimetic studies.
In
this paper we report for the first time the use of a linear
porphyrin dimer to cooperatively stabilise two hydro-
gen-bonded 4(3H)pyrimidone dimers in CDCl3.
isopropylsilylchloride. Dimer
acetylenic coupling using Pd(Ph P) Cl .
5 was prepared by
3
2
2
The association of dimer 5 with 4(3H)pyrimidone was
The porphyrin dimer 5 required for these studies was
prepared by the route shown in Scheme 1. An
improved large scale synthesis of a key building block
1
studied by H NMR titration. It was expected that
13
4
(3H)pyrimidone would bind strongly to the zinc por-
phyrins owing to the known tight binding of DABCO
5
,15-bis-(3,5-di-tert-butylphenyl)-10,20-bis(trimethylsi-
to zinc porphyrins. K values have been reported of
14
lylethynyl)porphyrin 2 has been developed. In our
hands the previously reported method of preparation
involving condensation of trimethylsilylpropynal with a
meso-aryl dipyrromethane to give 2 directly was unsatis-
factory on a larger scale owing to the difficulty of
separating porphyrins formed as by-products during
6
4
1
.8×10 and 9.9×10 determined in toluene and chloro-
9
form, respectively, by UV titration. The binding is too
strong to be determined by H NMR. An equilibrium
1
between monomer 6 and dimer complex 7 (Scheme 2)
was therefore anticipated to be established. Titrating
the pyrimidone with aliquots of porphyrin dimer 5 and
plotting the chemical shift of the pyrimidone NH versus
concentration of the dimer gave a straight line plot
which illustrates tight binding of the pyrimidone to
1
5
the reaction.
The acid catalysed scrambling of
1
6
substituents is a facile process in porphyrin syntheses
1
dimer 5. The H NMR chemical shift of the pyrimidone
NH proton was then monitored as a solution of the
expected complex 7 was diluted down (Figs. 1 and 2).
The proton moved upfield indicating that the equi-
Keywords: supramolecular; hydrogen-bond; zinc porphyrin;
4
*
(3H)pyrimidone.
Corresponding author.
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00117-4

2
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M. J. Plater et al. / Tetrahedron Letters 42 (2001) 2225–2229
Scheme 1. Reagents and conditions: (i) Me SiC H, Pd(Ph P) Cl , CuI, Et N, 91%; (ii) MeOH, Bu NF, 90%; (iii) LiHMDS then
3
2
3
2
2
3
4
i
(
Pr) SiCl, 48%; (iv) Pd(Ph P) Cl (1.0 equiv.), CuI (1.0 equiv.), Et N, 95%.
3 3 2 2 3
Scheme 2. Complex formation from porphyrin dimer 5 and 4(3H)pyrimidone.
librium was shifting to a non-hydrogen-bonded com-
plex. The data were analysed by standard methods for
as dimer and plotting a graph of mole fraction versus
the chemical shift. A straight line graph is obtained
with the correct value of K. This gave a value of
19,20
an equilibrium of the type 2[C]=[C2].
This involves
−
1
assuming a value for the association constant K, calcu-
lating a value for the mole fraction of monomer present
K=2500±40 mol
(DG=−4.6 kcal/mol) for an
assumed equilibrium between monomer 6 and dimer 7.

M. J. Plater et al. / Tetrahedron Letters 42 (2001) 2225–2229
2227
Figure 1. Plot of the chemical shift against concentration as the [porphyrin 5–4(3H)pyrimidone] complex 7 in CDCl is diluted.
3
Figure 2. Plot of the chemical shift against mole fraction as the [porphyrin 5–4(3H)pyrimidone] complex 7 in CDCl is diluted.
3
Comparison of the integrations of the pyrimidone pro-
tons and the porphyrin b protons showed that the
complex stoichiometry was 1 porphyrin: 2 pyrimidones.
Further evidence that the complex 7 exists as the dimer
shown and not as a linear polymeric species is provided
by comparing the association constant K of 2500 with
the association constant for two hydrogen-bonded
molecules of 4(3H)pyrimidone. This was determined to
increase the acidity of the amide protons and cause
peak broadening. However, in the porphyrin 5–pyrimi-
done complex 7 stronger cooperative aggregation must
sharpen the pyrimidone NH signal.
In summary this study illustrates the use of a covalent
porphyrin dimer to stabilise two side by side hydrogen-
bonded aggregates by cooperative assembly. There are
further exciting possibilities to use porphyrin ladders
for the stabilisation of arrays of hydrogen-bonded
aggregates. These studies are in progress.
−
1
be 445±2 mol (DG=−3.6 kcal/mol) in CDCl (Figs. 3
3
2
1
and 4). The pyrimidone NH signal was a sharp peak.
The significantly stronger association constant for
dimer complex 7 is evidence for cooperative binding
which would occur in the cyclic structure but not in an
acyclic polymeric complex species. Furthermore it was
not possible to determine a value of K for the associa-
tion of the porphyrin monomer 2–pyrimidone complex
into a dimer owing to broadening of the NꢀH peak
which occurred gradually upon addition of porphyrin 2
to 4(3H)pyrimidone. Presumably in the porphyrin 2–
pyrimidone complex binding to the zinc atom would
Acknowledgements
We are grateful to DERA and the University
of Aberdeen for funding the project and to the
National Mass Spectrometry Service Centre for the
mass spectra.

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M. J. Plater et al. / Tetrahedron Letters 42 (2001) 2225–2229
Figure 3. Plot of the chemical shift against concentration as 4(3H)pyrimidone in CDCl is diluted.
3
Figure 4. Plot of the chemical shift against mole fraction as 4(3H)pyrimidone in CDCl is diluted.
3
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