Synthesis and Nuclear Magnetic Resonance Shielding Effect of three Triazine-Linked Porphyrin Compounds

Três porfirinas, monômero Por I, dímero Por II e trímero Por III, foram sintetizadas através da reação de cloreto cianúrico, 5-(4-hidroxofenil)-10,15,20-trifenil porfirina e 1-fenilpiperazina. As propriedades destas porfirinas inéditas, no que diz respeito à proteção na ressonância magnética, aos acoplamentos e desdobramentos dos hidrogênios β foram estudadas por espectroscopia de RMN de H. Os resultados mostram que os hidrogênios β no dímero e no trímero encontram-se fortemente protegidos pelo anel porfirínico e exibem sinais com desdobramentos complicados.


Introduction
Since porphyrins have been covalently linked to form porphyrin dimers in the 1970s, bridged bisporphyrins and other porphyrin oligomers became a focus in porphyrin chemistry because of their unique physical and chemical properties. 1 In recent years, good many porphyrin dimers and oligomers have been synthesized. 2Porphyrin derivatives are studied widely as chemical sensors in supramolecular chemistry. 3Obviously, excellent photochemical properties of porphyrins have huge merits that can be used as detection units conveniently. 4riazines have been used widely in analytical chemistry as complexation agents, in electrochemistry as multi-step redox systems and as pesticide or herbicide components in agriculture. 5In recent years, owing to its high reactivity and trifunctionality that allows easy and controlled sequential replacement of the halogens by nucleophiles, cyanuric chloride (2,4,6-trichloro-1,3,5triazine) has attracted increasing attention for potential use in the synthesis of fluorescent labeling 6 and dendrimer. 7his compound has also been used to prepare porphyrin dimers and oligomers. 8In our work, we report three porphyrin compounds, monomer Por I, dimer Por II and trimer Por III using this triazine as a bridge.These compounds and their metal complexes are of interest in molecular recognition and catalysis.Their molecular structures are shown in Figure 1.

Results and Discussion
Three porphyrin derivatives were synthesized by sequential chloride substitution in cyanuric chloride by monohydroxylporphyrin 1 and 1-phenylpiperazine, as outlined in Scheme 1.
This sequential process was controlled by temperature and ratios of reagents.The first chlorine was substituted quickly by 1 equiv. of monohydroxylporphyrin 1 in THF in the presence of excess diisopropylethylamine (DIPEA), after 15 min, at 0 °C.This monosubstituted intermediate was used directly without purification for the synthesis of Por I, which was obtained after addition of excess 1phenylpiperazine and DIPEA and heating at 70 °C for 20 h.The yield was 62% after purification and recrystallization.The porphyrin dimer was synthesized from the same monosubstituted intermediate, to which 1 equiv. of 1 was added with excess of DIPEA.After 24 h stirring at room temperature, addition of excess Synthesis and Nuclear Magnetic Resonance Shielding Effect J. Braz.Chem.Soc.

1-phenylpiperazine and heating at 70 °C for 5 h led to
Por II in 42% yield, after purification.Porphyrin trimer Por III was synthesized directly by the reaction of cyanuric chloride with excess 1 at 70 °C for 40 h.This compound was obtained in 25% yield after purification and recrystallization.This relatively poor yield, compared to the other products is attributed to the huge steric effect.Attempts to synthesis Por II directly, as described for Por III, i.e. by reaction of 2 equiv. of 1 with 1 equiv. of cyanuric chloride to prepare the disubstituted intermediate were not successful.The first triazine chlorine being very reactive at room temperature, the result of this reaction was complicated and incomplete for 1.

1
H NMR spectroscopy was used to identify the three porphyrin targets and the corresponding spectra are shown in Figure 2. Complicated shielding effect on hydrogen of β-position of porphyrin was observed.
The chemical shifts and coupling constants (see Experimental) of β-H in Por I were similar to those of common tailed porphyrins. 9The β hydrogens were split The spectrum of Por III was similar to that of the dimer, and exhibited six protons in a position, and six in b position, both appearing as doublets.In this structure, the shielding effect of the porphyrin ring became much stronger, the peaks at d position being shifted to δ 8.28, at even higher field than the benzene ortho-H (at δ 8.38).The chemical shifts of the β-H of the three porphyrins are summarized in Table 1.
In summary, we have described the synthesis of three novel triazine-linked porphyrins and observed the shielding effect of porphyrin ring current on the â protons.Further work is being carried out on the recognition properties of these porphyrin compounds and their metal complexes.
All UV-visible spectra were obtained on a Shimadzu 1601 spectrophotometer.Proton NMR spectra were measured using a Varian Mercury-VX 300 spectrometer, chemical shifts (δ) were referenced to internal tetramethyl silane (TMS).The element analyses were measured with a Finnigan FLASH 1112 SERIES EA.Mass spectra were obtained on a TSQ 7000 instrument. For

Synthesis of Por II
To a solution of monohydroxylporphyrin 1 (50 mg, 0.08 mmol) in THF (5 mL) at 0 °C was added cyanuric chloride (14.6 mg, 0.08 mmol) and diisopropylethylamine (DIPEA, 2 drops, about 30 mg, 0.23 mmol), and the mixture was stirred for 15 min.After withdrawing ice bath, the solution was stirred at room temperature for 10 min.Then, the solution was added to DIPEA (2 drops) and monohydroxylporphyrin 1 (50 mg, 0.08 mmol), and was stirred at room temperature for 24 h.To the resulting solution was added excess 1-phenylpiperazine (25.7 mg, 0.16 mmol), and the solution was reacted at 70 °C for 5 h.With similar purification method of Por I (chloroform and methanol (v/v = 100:0.5)as eluent), Por II afforded (50. 4

Scheme 1 .Figure 1 .
Scheme 1. Synthetic route to Por I, Por II and Por III.
1 H NMR data, Ph o , Ph m and Ph p indicate corresponding ortho-H, meta-H and para-H of 10, 15, 20-triphenyl in porphyrin ring.Ph o 1 and Ph m 1 indicate corresponding ortho-H and meta-H of 5-phenyl in porphyrin ring.Ph o ortho-H, meta-H and para-H of phenyl which link to piperazine.

Figure 2 .
Figure 2. Shifts and compling of β hydrogens of: Por I, Por II and Por III.