Synthetic Pathways of trans-Substituted Porphyrins Bearing Pentafluorophenyl Groups from Dipyrromethanes ^†

Abstract

Different approaches were evaluated to obtain 5,15-bis [4-(N,N-diphenyl)aminophenyl]-10,20-bis(pentafluorophenyl)porphyrin and 5,15-bis [4-(9-carbazolyl)phenyl]-10,20-bis(pentafluorophenyl)porphyrin. First, the reaction of 5-pentafluorophenyldypyrromethane with the corresponding benzaldehyde catalyzed by boron trifluoride diethyl etherate in dichloromethane led to a high level of scrambling that produced a mixture of porphyrins. These products involve ABAB (3%), A₃B (15%), and A₄ (4%) symmetries, where A represents a pentafluorophenyl group. These porphyrins have similar polarities and they are very difficult to separate by column chromatography. Therefore, the reagents were changed to pentafluorobenzaldehyde and dipyrromethane. When a 0.5:1 molar ratio was used, A₄ porphyrin was not obtained, and the main products were ABAB (19%) and A₃B (6%). Therefore, condensation of a dipyrromethane with pentafluorobenzaldehyde provides a general method for the rational synthesis of ABAB-porphyrins in good yield with lower scrambling.

1. Introduction

trans-substituted porphyrins (ABAB) are commonly synthesized through the reaction of an aldehyde with a dipyrromethane at room temperature and under acid catalysis with trifluoroacetic acid (TFA) or boron trifluoride diethyl etherate (BF₃.O(C₂H₅)₂) [1]. In a second step, the oxidation of the tetraphenylporphyrinogen cycle (reduced form of porphyrin) is produced with the addition of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) [2,3]. In particular, these specific substituent patterns are crucial building blocks in obtaining different materials.

Nevertheless, in this acid-catalyzed condensation process, there is a continuous risk of fragmentation followed by recombination of the reagents (Figure 1). As a result, this method can cause the rearrangement of meso-substituents, leading to the production of a mixture of porphyrins [4].

In this work, several synthesis routes were explored to synthesize 5,15-bis [4-(N,N-diphenyl)aminophenyl]-10,20-bis(pentafluorophenyl)porphyrin and 5,15-bis [4-(9-carbazolyl)phenyl]-10,20-bis(pentafluorophenyl)porphyrin. These particular substituent arrangements are essential components in the development of diverse materials [5,6,7,8].

2. Materials and Methods

2.1. Equipment and Chemical Reagents

Proton NMR spectra were recorded using a Bruker Avance DPX400 FT-NMR spectrometer (Bruker BioSpin, Rheinstetten, Germany). Mass spectra were obtained with a Bruker micrO-TOF-QII (Bruker Daltonics, Billerica, MA, USA) featuring an ESI source (ESI-MS). Compounds from Sigma-Aldrich (Milwaukee, WI, USA) were utilized as received. Silica gel TLC plates (250 microns) were sourced from Analtech (Newark, DE, USA), and silica gel 60 (0.040–0.063 mm, 230–400 mesh) was supplied by Merck (Darmstadt, Germany).

2.2. Synthesis

5-(Pentafluorophenyl)dipyrromethane and 4-(9-carbazolyl)benzaldehyde were synthesized following a previously described methodology [9,10,11]. Diphenylaminophenyl)dipyrromethane and di(4-(9-carbazolyl))dipyrromethane were obtained through the typical synthesis of dipyrromethanes [12,13,14].

3. Results and Discussion

In this study, various synthetic pathways were investigated to produce 5,15-bis [4-(N,N-diphenyl)aminophenyl]-10,20-bis(pentafluorophenyl)porphyrin and 5,15-bis [4-(9-carbazolyl)phenyl]-10,20-bis(pentafluorophenyl)porphyrin. In the first instance, both porphyrins were synthesized by acid-catalyzed condensation from the 4-(N,N-diphenylamino)benzaldehyde or 4-(9-carbazolyl)benzaldehyde and perfluoro-substituted dipyrromethane (5-(pentafluorophenyl)dipyrromethane) in a 1:1 molar ratio (Figure 2). The reaction was carried out in dichloromethane (DCM) and maintained under an argon atmosphere for 3 h. Then, DDQ was added to obtain the oxidized porphyrin. The product was purified by flash column chromatography (hexanes/DCM 8:2) to obtain a 3% yield of the ABAB porphyrin.

In this case, acid catalysis by BF₃.O(C₂H₅)₂ led to a high level of scrambling, which produced a mixture of porphyrins [15]. The desired porphyrin (ABAB) was obtained in a low yield (~3%), while porphyrins A₃B and A₄ had yields of 15% and 4%, respectively. In this structure, A represents the pentafluorophenyl substituent. Due to the similarity in polarity of these porphyrins, purification of the porphyrin mixture by flash chromatography was difficult.

Therefore, due to these disadvantages, the reagents were exchanged to 5-(diphenylaminophenyl)dipyrromethane or di(4-(9-carbazolyl))dipyrromethane and 5-pentafluorobenzaldehyde to obtain the desired porphyrins (Figure 3).

Firstly, in a 1:1 molar ratio, these reagents were kept under an argon atmosphere for 15 min. Then, (BF₃.O(C₂H₅)₂) was added, and the mixture was stirred for 3 h in the dark. After this, the addition of DDQ allowed us to obtain the oxidized form of the porphyrins. The products were purified by flash column chromatography (hexanes/DCM 80:20). Under these conditions, the obtained products also resulted in a mixture of the above-mentioned porphyrins (ABAB, A₃B, and A₄).

Therefore, it was decided to carry out the reaction under the same experimental conditions but with a different molar ratio for the reagents. When molar ratios of 1:0.7 and 1:0.5 (dipyrromethane:aldehyde) were used, the yields obtained for the mixture of porphyrins increased to 20% and 25%, respectively. Furthermore, in the latter case (1:0.5 molar ratio), there was no formation of porphyrin A₄. After purification by flash column chromatography, the yields of the products were determined: 19% for porphyrin ABAB and 6% for porphyrin A₃B. These yields may be due to the stability offered by the different dipyrromethane to acidolysis and/or scrambling processes [15]. Therefore, the best conditions to obtain an adequate yield of these porphyrins involve acid-catalyzed condensation between 5-(diphenylaminophenyl)dipyrromethane and 5-pentafluorobenzaldehyde in a molar ratio of 1:0.5.

4. Conclusions

Two possible synthetic routes were explored to obtain trans-substituted porphyrins (ABAB). The acid-catalyzed condensation of 5-(pentafluorophenyl)dipyrromethane with triphenylamine or carbazole aldehydes led to an undesired mixture of porphyrins due to the scrambling phenomenon produced. When the reagents were exchanged, the yields of the desired products were improved. The best results were obtained with a 1:0.5 molar ratio of dipyrromethane derivative to pentafluorobenzaldehyde. Consequently, the condensation of substituted dipyrromethane with pentafluorobenzaldehyde offers an effective approach for the controlled synthesis of ABAB-porphyrins, achieving good yields with reduced scrambling.