Studies of the Catalytic Activity of Iron (III) Porphyrins for the Protection of Carbonyl Groups in Homogeneous Media

Gabriel Ferreira,Shirley Nakagaki,Charles Carvalho

doi:10.3390/catal9040334

Abstract

The protection of carbonyl groups that produce acetal products is a key reaction in fine chemistry due to the high reactivity of aldehydes and ketones in certain media. This process can be catalyzed by protic or Lewis acids. Since metalloporphyrins often possess free axial positions in the central metal, they can be applied as Lewis acid catalysts, allowing the additional coordination of substrates. Therefore, three ferric complexes were selected to be evaluated as catalysts for the acetalization of benzaldehyde with ethanol and triethyl orthoformate (TEOF) in the homogeneous phase: (i) 5,10,15,20-tetrakis(phenylporphyrin) iron (III) chloride (Fe0F); (ii) 5,10,15,20-tetrakis(2,6-difluorphenylporphyrin) iron (III) chloride (Fe2F); and (iii) 5,10,15,20-tetrakis(pentafluorphenylporphyrin) iron (III) chloride (Fe5F). The complex Fe5F showed the highest catalytic activity, and kinetic parameters were evaluated for this reaction, exhibiting an increasing rate of reaction of about 550-fold in comparison with the non-catalyzed reaction. The reaction scope was also investigated, and Fe5F was found to be active for the acetalization of benzaldehyde and acetophenone, with different protective agents such as alcohols, glycols, glycerol, and epoxide being selective for the formation of cyclic acetals. The protection of benzaldehyde with ethylene glycol and propylene glycol were also studied at different temperatures, and turnover frequency (TOF) values of up to 360 h−1 were determined at 40 °C in homogenous media without the need for solvent or drying agents.

Highlights

Porphyrins are macrocyclic compounds that are naturally found in diverse forms of life, where they perform distinct roles, including oxygen transport in cellular respiration processes, and mainly in catalytic processes, such as the detoxification of xenobiotic compounds and the oxidation of fatty acids [1,2].Over the last 40 years, these compounds have been intensively studied due to their application as bio-inspired models of heme proteins that present catalytic activity, e.g., the cytochrome P450 enzyme family [3]
The electronic effects of fluorine atoms were attributed to causing the of the catalytic activity in comparison to the results presented by the [Fe(TPP)]Cl used by Groves
The investigation of the catalytic activity of metalloporphyrins for benzaldehyde acetalization started with studies about the influence of peripheral fluorine atoms in the structure of three different iron (III) porphyrins, since these electronegative atoms can withdraw electron density from the macrocycle structure, modulating the charge density over the complex

Summary

Introduction

Porphyrins are macrocyclic compounds that are naturally found in diverse forms of life, where they perform distinct roles, including oxygen transport in cellular respiration processes, and mainly in catalytic processes, such as the detoxification of xenobiotic compounds and the oxidation of fatty acids [1,2].Over the last 40 years, these compounds have been intensively studied due to their application as bio-inspired models of heme proteins that present catalytic activity, e.g., the cytochrome P450 enzyme family [3]. In 1979, Groves et al demonstrated for the first time the catalytic activity of a cytochrome P450 biomimetic model based on a synthetic iron (III) porphyrin [Fe(TPP)]Cl (Figure 1A), which was able to catalyze the hydroxylation and epoxidation of hydrocarbons with iodosylbenzene as an oxygen source under mild conditions [8]. This transformation was indicated as occurring through the formation of a high valent metal–oxygen intermediate complex [3,9].

Methods

Results

Conclusion