Synthesis of substituted meso -tetraphenylporphyrins in mixed solvent systems

An efficient synthetic method of substituted meso -tetraphenylporphyrins with better isolated yields was proposed by using propionic acid, valeric acid and m -nitrotoluene as mixed-solvent systems. The porphyrin yields in mixed solvent systems were obviously higher than those in the single propionic acid or valeric acid as solvents. The further investigation showed that the acidity, polarity, viscosity and the property of oxidant played an important role to the synthesis of porphyrin. Compared with other oxidants, m -nitrotoluene as an excellent oxidant could completely transform tetraphenylporphyrinogen to tetraphenylporphyrin.


Introduction
meso-Tetraphenylporphyrin (TPPH2) as one of the simple and stable substituted tetrapyrrolic macrocycle compounds has been widely investigated in terms of synthesis and application [1][2][3][4][5][6] for several decades.Many porphyrin derivatives including free base porphyrin compounds with different substituents, 7 mononuclear metalloporphyrins 8 and binuclear metalloporphyrins 9 have been prepared by virtue of the efficient synthesis of TPPH2 with high yields.They can be used to mimic natural enzyme peroxidase, catalase and heme-containing proteins, 10 which are responsible for molecular binding, 11 oxygen transport 12 and energy transfer. 13][16] Almost all of the natural and synthetic porphyrins can be synthesized by means of the cyclocondensation of substituted aldehydes with pyrrole and herein several famous synthetic methods have been widely established to synthesize TPPH2.Rothemund firstly synthesized TPPH2 in 10% yield by heating the mixture of pyrrole and benzaldehyde with pyridine as solvent in a sealed tube filled with nitrogen. 17,18Subsequently, Adler and Longo converted benzaldehyde and pyrrole to TPPH2 in a single refluxing carboxylic acid with air oxidation. 19The yield of TPPH2 was up to 20% and the separation of product was relatively simple by using Adler-Longo methodology. 20,21Furthermore, Lindsey developed another synthetic strategy to form tetraphenylporphyrinogen by reacting benzaldehyde and pyrrole with dichloromethane as a solvent and BF3 etherate as a catalyst under N2 at room temperature. 22Then the tetraphenylporphyrinogen was transformed to TPPH2 with the addition of 2,3-dichloro-5,6dicyanobenzoquinone (DDQ) as a oxidant 23 and the better spectroscopic yield of porphyrin was obtained with the addition of salts. 24Besides, TPPH2 might also be synthesized by the direct condensation of benzaldehyde and pyrrole with AlCl3 as the catalyst in refluxing DMF and the separation yield was ~30%. 25In addition, tetraarylporphyrins could be obtained from pyrrole and substituted benzaldehydes in the gas phase without solvents 26,27 or using high-valent transition metal salts as aromatizing agents to synthesize porphyrins. 28Guo and co-workers developed the industrial method and the device to synthesize tetraaryl porphyrins by using the condensation of pyrrole and aromatic aldehyde with air oxidation. 294][35] However, the present synthesis of TPPH2 from the direct condensation of benzaldehyde and pyrrole is still inconvenient and the purity of products directly separated from the filtrate remains to be improved.
In our previous reports, 36,37 the synthesis of para-substituted tetraphenylporphyrins with the mixed-solvent method was developed on the research basis of Gonsalves. 30In this paper, the mixed solvent systems of binary carboxylic acids and nitrobenzene derivatives were used to synthesize TPPH2 by the condensation of substituted benzaldehydes and pyrrole.By adjusting the physicochemical properties and oxidizing intensities of the reaction systems, the synthetic processes of TPPH2 became very effective and simple and that the better yields were obtained.

Synthesis of TPPH2 in mixed solvent systems
In an attempt to examine the effect of carboxylic acids on the synthesis of TPPH2, the condensation reaction in the single C1-C8 saturated fatty acids with different physicochemical parameters was performed.The results were listed in Table 1. a Each reaction was performed via aerobic oxidation at 0.1 M benzaldehyde and 0.1 M pyrrole in 100 mL carboxylic acids under reflux for 2 h.b Acid strength.c Dipole moment.
From Table 1, it could be seen that the isolated yields of TPPH2 were obtained in the range of 0-22.5% in the single carboxylic acids with air as the oxidant.It was found that TPPH2 was difficult to form in the strongest formic acid (pKa=3.77),however, the highest yield (22.5%) of TPPH2 was obtained in valeric acid with the highest dipole moment (μ=2.66) and the stronger acid strength (pKa=4.81)than other carboxylic acids.Meanwhile, the yields of TPPH2 in single C2-C5 carboxylic acids (high polarity, moderate refluxing temperature and viscosity) were higher than those in C6-C8 carboxylic acids.The results indicated that the physicochemical properties of carboxylic acids had influences on the synthetic yields of TPPH2.
To further evaluate the roles of physicochemical parameters of carboxylic acids, binary carboxylic acids including valeric acid via aerobic oxidation were mixed and applied to the synthesis of TPPH2 and the results were listed in Table 2.As shown in Table 2, the synthesis in propionic acid -valeric acid mixed solvents with medium dipole moment and moderate refluxing temperature gave higher porphyrin yields than the corresponding synthesis in the single valeric acid or other binary mixed carboxylic acid systems.Therefore, the yields became higher in the given solvent compositions, which exhibited a lower viscosity and higher temperature.In addition, the yields in C2-C4 mixed carboxylic acid systems were higher than those in C6-C8 carboxylic acid combinations.The acids (including Lewis acid and Brønsted acid), as reported, 31 played a catalytic role in the condensation of benzaldehyde and pyrrole. 38But then the stronger acidity was liable to make pyrrole form straight-chain pyrrole polymers, 39 thus, almost all black intractable mixtures but the TPPH2 product were obtained in formic acid and valeric acid as the mixed solvents.

Scheme 1. Oxidation of tetraphenylporphyrinogen to tetraphenylporphyrin.
It is widely accepted that tetraphenylporphyrinogen is the key intermediate in the synthesis of tetraphenylporphyrin.The oxidation of tetraphenylporphyrinogen by O2 in the atmosphere (Scheme 1) 15,23 was a competitive procedure which formed the desired porphyrin (TPPH2) and tetraphenylchlorin (TPC). 19Hence, the oxidants played a crucial role in the synthesis of porphyrins and an ideal oxidant would selectively oxidize only tetraphenylporphyrinogen to TPPH2 without the formation of TPC. 33,40The results of reactions oxidized by different oxidants under the same conditions were listed in Table 3.
The results in Table 3 showed that the tetraphenylporphyrinogen oxidized by nitrobenzene derivatives gave better yields than those by air and dimethylsulfoxide.According to the UV-vis absorption spectra (Figure 1), the ratios of absorbances with nitrobenzene and air as oxidants between ~480 nm and ~650 nm was 1 (0.064/0.064) and 1.26 (0.111/0.088), which suggested the obtained porphyrin filtered from the reaction solution containing nitrobenzene was very pure without the formation of TPC. 41But the tetraphenylporphyrinogen was incompletely oxidized to TPPH2 only by O2 in the atmosphere if no stoichiometric oxidants were added into the reaction system.Nitrobenzene derivatives as the weak organic oxidants effectively promoted the formation of TPPH2, which had the same effect as dihydroquinoline oxidized to quinoline by nitrobenzene under acidic conditions in the Skraup reaction. 42Meanwhile, the yields of TPPH2 with nitrotoluene derivatives as oxidants were higher than those with nitrobenzene and nitrobenzoic acid derivatives as oxidants.Additionally, nitrotoluene derivatives were of lower toxicities versus nitrobenzene and the oxidative effect of m-nitrotoluene surpassed o/pnitrotoluene in the synthesis of TPPH2.Thereby, m-nitrotoluene was chosen as a preferable oxidant in the synthesis of TPPH2.a Each reaction was performed at 0.1 M benzaldehyde and 0.1 M pyrrole in 100 mL solvents under reflux for 2 h.Now that the concentration of the oxidant usually had a profound effect on the synthetic reaction, the influences of m-nitrotoluene concentrations on the yields of TPPH2 were studied and the results were listed in Table 4.In contrast to the 28.2% yield of TPPH2 without organic oxidants, the yield of TPPH2 exceeded 45% with 0.51 M of m-nitrotoluene as the oxidant (Table 4).The results indicated the oxidation of tetraphenylporphyrinogen became gradually complete as the increase of m-nitrotoluene concentration.However, superfluous m-nitrotoluene (dipole moment, 4.21 D) affected the polarities of mixed solvent systems, so the yields of TPPH2 decreased gradually when the concentration of m-nitrotoluene exceeded 0.51 M. On the basis of the investigation of the optimal oxidants, the yields of TPPH2 in the mixed solvent systems with different proportions were examined.The highest yield (45.1%) of TPPH2 was obtained by using propionic acid, valeric acid and m-nitrotoluene in the proportion of 47:47:6 (V/V/V) as the mixed solvent systems.In addition, the reactant concentrations and reaction time also had obvious effects on the yields of TPPH2 in the mixed solvent systems of binary carboxylic acids and m-nitrotoluene (Figure 2 and Figure 3).Figure 2 showed that the yields of TPPH2 were closely related to the concentrations of reactants.The maximal yield was generally obtained at 0.1 M of reactants and the reactions of higher concentrations gave lower yields than those in dilute solutions.As shown in Figure 3, the condensation rates of benzaldehyde and pyrrole in binary carboxylic acids and m-nitrotoluene as the mixed solvents were very fast and the of TPPH2 exceeded 35% in 0.5 h.But the yields of TPPH2 gradually decreased when the reaction time was more than 2 h because of the polymerization of TPPH2 for long time at high temperature.

Conclusions
In summary, a synthesis of substituted tetraphenylporphyrins from aromatic aldehyde and pyrrole in binary carboxylic acids and nitrotoluene derivatives solvent systems was systematically studied.The highest yield of TPPH2 exceeded 45% with propionic acid, valeric acid and m-nitrotoluene as the mixed solvents.The improvement of the TPPH2 yields could be realized by adjusting the acidity, polarity, refluxing temperature and viscosity of mixed carboxylic acids.Nitrotoluene derivatives as oxidants played an important role in the synthesis of TPPH2 and m-nitrotoluene as an excellent oxidant exhibited remarkable effects in the oxidation of tetraphenylporphyrinogen.The experimental results showed that it was possible to apply binary carboxylic acid and nitrobenzene derivatives as the mixed solvents to synthesize various substituted tetraphenylporphyrins in excellent yields.

Experimental Section
General.All chemicals were obtained commercially and used as received unless otherwise noted.Pyrrole was redistilled before use.Dichloromethane was dehydrated.Neutral Al2O3 was baked at 100 C for 5 h.Chromatography was performed on neutral Al2O3.Ultraviolet-visible (UV-vis) spectra were recorded in dichloromethane with a HITACHI U-3010 spectrophotometer.Infrared (IR) spectra were recorded as KBr pellets via a Nicolet AVATAR-360 spectrophotometer.1H NMR spectra were recorded with an AV 400 MHz Bruker spectrometer.The data of elemental analysis were obtained with an EURO EA3000 elemental analyzer.

General synthetic procedures of TPPH2
Propionic acid (47 mL) and valeric acid (37 mL) were added into a 250 mL three-neck roundbottom flask equipped with stirrer, reflux exchanger and dropping funnel.The mixture was stirred at refluxing temperature for 30 min.Then, benzaldehyde (1 mL, 0.01mol) was dissolved in valeric acid (10 mL) and freshly distilled pyrrole (0.7 mL, 0.01mol) was dissolved in mnitrotoluene (6 mL).Subsequently, above two kinds of solutions were simultaneously dropped into the flask through two dropping funnels in 15 min.The reaction mixture was stirred at refluxing temperature for 2 h.When the temperature of the reaction mixture was cooled to 50 C-60 C, methanol (30 mL) was added into the flask.After that, the reaction solution was allowed to stir for 15 min and then stood for 30 min.The resulting solution was filtrated under reduced pressure and afforded the blue-purple power.The crude product was washed with methanol and dried in 60 C for 30 min.Purification by column chromatography (Al2O3, CH2Cl2 as the eluent) afforded pure TPPH2.The isolated yield of the product was found to be 0.69 g

a
Each reaction was performed at 0.1 M benzaldehyde and 0.1 M pyrrole in 100 mL solvents (Vpropionic acid :Vvaleric acid =1:1) under reflux for 2 h.b Liquid.c Solid.

Figure 1 .
Figure 1.UV-vis absorbance spectra (benzene) of TPPH2 filtrated from the reaction solution with air and nitrobenzene as oxidants.

Table 1 .
Yields of TPPH2 in the single carboxylic acids a

Table 2 .
Yields of TPPH2 in binary mixed carboxylic acids a a Each reaction was performed via aerobic oxidation at 0.1 M benzaldehyde and 0.1 M pyrrole in 100 mL solvents (V/V=1/1) under reflux for 2 h.b Acid strength.c Dipole moment.

Table 3 .
Effects of various oxidants on yields of TPPH2 a

Table 4 .
Effects of m-nitrotoluene concentrations and solvent proportions on yields of TPPH2 a