Fused porphyrin-imidazole systems: New building blocks for synthesis of porphyrin arrays

Article abstract of DOI:10.1039/a905507j

Reaction of porphyrin-2,3-diones with aromatic aldehydes and NH₄OAc in AcOH-CHCl₃ affords 2-aryl-1H-imidazo-[4,5-b]porphyrins which, with appropriate substitution, are useful building blocks for the synthesis of multi-porphyrin arrays; porphyrin-tetraones are similarly converted into the corresponding bis-fused systems.

Full text of DOI:10.1039/a905507j

View Article Online / Journal Homepage / Table of Contents for this issue
Fused porphyrin-imidazole systems: new building blocks for
synthesis of porphyrin arrays
Maxwell J. Crossley* and James A. McDonald
School of Chemistry, The University of Sydney, NSW 2006, Australia
Received (in Cambridge, UK) 8th July 1999, Accepted 26th July 1999
Reaction of porphyrin-2,3-diones with aromatic aldehydes
and NH4OAc in AcOH–CHCl3 affords 2-aryl-1H-imidazo-
[4,5-b]porphyrins which, with appropriate substitution, are
useful building blocks for the synthesis of multi-porphyrin
arrays; porphyrin-tetraones are similarly converted into the
corresponding bis-fused systems.
NH4OAc in a refluxing 1:1 mixture of AcOH–CHCl3 for 1 to
24 h (Scheme 1). These reactions could be carried out readily
on a multi-gram scale. The products 3–7 are easily purified by
column chromatography over silica and recrystallisation. The
1H NMR spectra of imidazoporphyrins 3–7 show a broad sing-
let at about 8.4 ppm, indicating the presence of the imidazole
NH and the lack of symmetry in the spectra show that imid-
1
Multi-porphyrin systems that occur naturally have a variety of
geometries between interacting chromophores. Most model
compounds designed to mimic such porphyrin systems have
relied on linkage through meso-positions or have used flexible
chains linked through single positions. The only previous multi-
porphyrin models with β-pyrrolic rings on adjacent porphyrin
rings were constructed by reaction of porphyrin-2,3-diones
with aromatic 1,2-diamines.1,2 The formation of the imidazole
ring in the synthesis of 2,3,5-triphenylimidazole (lophine) by
reaction of an α-dione, benzil, with benzaldehyde and ammonia
has been known since last century.3 We now report that por-
phyrin-2,3-diones, despite the α-dione system being attached to
a 5-membered heterocyclic ring, show similar reactivity towards
aromatic aldehydes and ammonia. This provides a new method
of functionalising the porphyrin macrocycle by introduction of
a 2Ј-arylimidazole ring fused across a β,β-pyrrolic position of
the porphyrin and allows the synthesis of more elaborated
systems by bridging of porphyrin units through the new
functionality.
azole tautomerism is slow on the H NMR timescale. In each
of the reactions producing compounds 3–7, the corresponding
22,23-diarylpyrazino[2,3-b]porphyrin product 9† was formed in
10–15% yield. More-highly substituted porphyrins are avail-
able from similar reactions of aldehydes and NH4OAc with
porphyrin-2,3,12,13-tetraone4 10. Linear extended bis(2-aryl-
1H-imidazo)porphyrins 11 and 12† were obtained (Scheme 2),
again in good yields; the corresponding pyrazino compounds
were also observed as minor products. 5,10,15,20-Tetrakis-
(3,5-di-tert-butylphenyl)porphyrin-2,3,7,8-tetraone4 also reacts
similarly with arylaldehydes to give bis-fused L-shaped
extended systems.
Compounds 14 and 15 are useful building blocks for con-
struction of a range of linear bis- and tris-porphyrin systems
with butadiyne linkages5 generated by CuCl-mediated coupling
of the terminal acetylenes.6–10 Compound 14 was obtained in
98% yield by desilylation of the ethynyl unit of zinc() por-
phyrin 13 (Scheme 2). Compound 15 was obtained in two steps
[metallation with zinc() and desilylation] from porphyrin 6 in
71% overall yield.
2-Aryl-1H-imidazo[4,5-b]porphyrins 3–7† were prepared in
good yields by the condensation of porphyrin-2,3-dione1
with the corresponding arylaldehyde in the presence of excess
1
The dizinc() butadiyne-linked bis(imidazoporphyrin) 16
was prepared in 90% yield by stirring a solution of zinc() 2-[4-
Scheme 1 Reagents and conditions: i, NH4OAc, AcOH–CHCl3, ∆ [3: 2 h (40%), 4: 6 h (54%), 5: 24 h (62%), 6: 4 h (48%) and 7: 2 h (66%)];
ii, Zn(OAc)2ؒ2H2O, CH2Cl2–MeOH, ∆, 1 h (85%).
J. Chem. Soc., Perkin Trans. 1, 1999, 2429–2431
2429

View Article Online
Scheme 2 Reagents and conditions: i, NH4OAc, AcOH–CHCl3, ∆ [11: 48 h (31%) and 12: 6.5 h (31%)]; ii, Zn(OAc)2ؒ2H2O, CH2Cl2–MeOH, ∆,
2 h (91%); iii, K2CO3, CH2Cl2–MeOH, ∆, 15 min (98%).
(ethynyl)phenyl]-1H-imidazo[4,5-b]porphyrin 15 in CH2Cl2
with an excess of freshly-prepared CuCl and TMEDA in air at
room temperature for 30 min (Scheme 3). The UV–vis spectrum
of bisporphyrin 16† shows slight broadening of the Soret band
and unaltered Q bands compared with the spectrum of the
monomer 15.† This indicated that there is negligible ground
state electronic communication between the porphyrin rings of
16.
Dizinc() bisporphyrin 16 was treated with 7 M HCl in a
two-phase system to afford the free base butadiyne-linked
bis(imidazoporphyrin) 17† in 93% yield. Bis(imidazo-
porphyrin) 17 was treated with Zn(OAc)2ؒ2H2O (1 equiv.) to
afford a mixture of free base 17 (28%), dizinc() 16 (36%) and
the desired monozinc() bis(imidazoporphyrin) 18† (31%)
after purification (Scheme 3). Photo-induced energy-transfer
in butadiyne bis(imidazoporphyrin) system 18 (1 × 10Ϫ6 M in
CHCl3) from the zinc() porphyrin to the free base porphyrin
was found to be 80% efficient which accords with a Förster-type
(dipole–dipole) electronic energy-transfer mechanism.11
The synthesis of linear tris(imidazoporphyrin) 19 illustrates
the utility of the linear zinc() bis{2-[4-(ethynyl)phenyl]-1H-
imidazo}porphyrin building block 14 to prepare larger ordered
multi-porphyrin arrays. The linear trizinc() butadiyne-linked
tris(imidazoporphyrin) 19† was prepared in 29% yield by a
cross-coupling reaction between 14 (1 equiv.) and zinc() 2-
[4-(ethynyl)phenyl]-1H-imidazo[4,5-b]porphyrin 15 (2.5 equiv.)
using an excess of freshly-prepared CuCl and TMEDA in air
at room temperature for 3 h (Scheme 3). Dizinc() butadiyne-
linked bis(imidazoporphyrin) 16 was obtained as a by-product
in 45% yield.
The utility of the condensation is illustrated further by reac-
tion of the [zinc() (imidazo)porphyrin]-appended arylaldehyde
Scheme 3 Reagents and conditions: i, CuCl, TMEDA, CH2Cl2, stir for 30 min in air; ii, HCl (7 M), CH2Cl2, stir for 2 min; iii, Zn(OAc)2ؒ2H2O,
CH2Cl2–MeOH, ∆, 1 h; iv, CuCl, TMEDA, CH2Cl2, stir for 3 h in air.
Scheme 4 Reagents and conditions: i, NH4OAc, AcOH–CHCl3, ∆, 72 h.
2430
J. Chem. Soc., Perkin Trans. 1, 1999, 2429–2431

View Article Online
᎐
R = C᎐CSiMe3: 382 (3.86), 438 (5.33), 476 (3.41), 526 (3.36), 563 (3.70),
8 with zinc() porphyrin-2,3-dione 2 and NH4OAc in AcOH–
CHCl3 at reflux for 72 h. This reaction gave the novel dizinc()
bisporphyrin 20† in 42% yield (Scheme 4). The zinc-to-zinc
distance in arene-linked bisporphyrin 20 was calculated12 to be
18.5 Å which is significantly shorter relative to the zinc-to-zinc
distance in the dizinc() bis(imidazoporphyrin) 16 (ca. 28.3 Å)
and between the zinc ions in the terminal porphyrins of the
trizinc() tris(imidazoporphyrin) 19 (56.6 Å).
᎐
597 (3.99), 653 (3.40), 692 (3.20); 12: 326 (4.65), 420 (5.48), 517 (4.45),
552 (4.15), 587 (4.09), 640 (3.69); 15: 310 (4.41), 427 (5.43), 480 (3.39),
492 (4.31), 514 (3.68), 586 (4.03); 16: 348 (4.74), 426 (5.70), 513 (4.16),
550 (4.71), 587 (4.45); 17: 242 (4.66), 427 (5.85), 519 (4.75), 556 (4.38),
589 (4.29), 649 (3.91); 18: 241 (4.60), 429 (5.39), 519 (4.50), 552 (4.56),
588 (4.36), 649 (3.56); 19: 250 (4.82), 352 (5.01), 369 (5.00), 429 (5.87),
454 (5.83), 519 (4.50), 554 (4.87), 596 (4.80); 20: (toluene) 321 (4.60),
431 (5.68), 458 (5.43), 516 (4.15), 554 (4.69), 589 (4.42).
The use of these novel compounds and systems to probe
further the influence of porphyrin orientation and alignment
on interporphyrin electronic communication is under active
investigation in our laboratory.
1 M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1987,
39.
2 M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1991,
1569.
3 B. Radziszewski, Ber., 1882, 15, 1493.
4 M. J. Crossley, L. J. Govenlock and J. K. Prashar, J. Chem. Soc.,
Chem. Commun., 1995, 2379.
5 D. P. Arnold, A. W. Johnson and M. Mahendran, J. Chem. Soc.,
Perkin Trans. 1, 1978, 366.
6 D. P. Arnold and L. J. Nitschinsk, Tetrahedron, 1992, 48, 8781.
7 D. P. Arnold, D. A. James, C. H. L. Kennard and G. Smith, J. Chem.
Soc., Chem. Commun., 1994, 2131.
8 D. P. Arnold and D. A. James, J. Org. Chem., 1997, 62, 3460.
9 H. L. Anderson, Inorg. Chem., 1994, 33, 972.
10 R. W. Wagner, T. E. Johnson, F. Li and J. S. Lindsey, J. Org. Chem.,
1995, 60, 5266.
Acknowledgements
We thank the Australian Research Council for a research grant
to M. J. C.
Notes and references
† All imidazoporphyrins have been fully characterised by elemental
analysis, mass spectroscopy, 1H NMR spectrometry and spectro-
scopic techniques. Visible spectroscopic data of selected porphyrins:
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin: λmax(CHCl3)/nm
405sh (log ε 4.91), 422 (5.66), 487sh (3.66), 519 (4.28), 555 (4.04), 593
(3.78), 648 (3.85); 3: 317 (4.30), 424 (5.50), 484 (3.75), 519 (4.23), 554
(4.02), 587 (3.89), 646 (3.45); 4: 422 (5.51), 519 (4.24), 554 (4.00), 588
(3.94), 647 (3.85); 6: 423 (5.51), 519 (4.35), 554 (4.01), 588 (3.92), 649
(3.62); 8: 346 (4.42), 431 (5.42), 524 (3.90), 550 (4.29), 592 (4.05); 9
11 T. Förster, Faraday Discuss. Chem. Soc., 1959, 27, 7.
12 SPARTAN version 4.0, Wavefunction Inc., Irvine, CA, USA, 1995.
Communication 9/05507J
J. Chem. Soc., Perkin Trans. 1, 1999, 2429–2431
2431