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Microwave-Assisted Synthesis and Reactivity of Porphyrins

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Microwave-Assisted Synthesis and Reactivity of Porphyrins Send Orders for Reprints to [email protected] Current Organic Synthesis, 2014, 11, 89-109 89 Microwave-Assisted Synthesis and Reactivity of Porphyrins Marta Pineiro* Departamento de Química, Universidade de Coimbra, Rua Larga 3004-535 Coimbra, Portugal Abstract: This paper describes past and current microwave-assisted methodologies for the synthesis of porphryins and porphyrin derivatives, including porphyrin precursors such as dipyrromethanes. The review is organized in two main topics, porphyrin synthesis and porphyrin reactivity under microwave irradiation and covers solventless and solution reactions. Keywords: Hydroporphyrin, metalloporphyrin, microwave irradiation, microwave synthesis, porphyrin, tetrapyrrolic macrocycles. 1. INTRODUCTION stituted beta-porphyrins were obtained via cyclisation of linear The acknowledgement that porphyrins and related compounds tetrapyrroles, b-bilenes and a,c-biladienes [15] could be referred. had essential functionalities in nature prompted the interest in mul- Microwave-assisted organic chemistry has grown in the last tiple scientific domains, from the deeper comprehension of those decades as a valuable and versatile tool for organic chemists [16]. functions to the investigation of new applications which comprise In general, compared to conventional heating methods, microwave several analytical uses [1], dye-sensitised solar cells [2], molecular heating has been shown to drastically reduce reaction times, in- electronics and non-linear optics [3], sensors of small molecules crease reaction yields and enhance product selectivity, mostly re- such as O2, NO, NH3 or phosphines [4], catalysts in oxidation and ducing undesirable side reaction products. Microwave irradiation is, photo-oxidation reactions [5], and several biological applications in our days, fully recognized as a useful tool for organic synthesis such as photodynamic therapy of cancer, imaging and boron neu- used in multi-step total synthesis [17], medicinal chemistry and tron-capture therapy [6]. drug discovery [18], polymer synthesis [19], material science [20], nanotechnology [21] and biochemical processes [22]. The use of The remarkable popularity and versatility of porphyrins and microwave irradiation for the synthesis and derivatization of por- their derivatives relies in a great length, on the development and phyrins is reviewed herein. improvement of synthetic strategies over the years that make possi- ble the huge availability of these compounds. Since Rothemund’s report on the one-pot synthesis of meso-substituted porphyrins [7] 2. SYNTHESIS OF PORPHYRINS AND RELATED COM- several simple one-step or two-step approaches have been devel- POUNDS oped for the preparation of these compounds. Adler and Longo used equimolar quantities of pyrrole and benzaldehyde in refluxing 2.1. Solventless Reaction Conditions propionic acid [8]. Rocha Gonsalves and co-workers obtained The preparation of porphyrins under microwave (MW) activa- meso-tetraalkylporphyrins in a two-step procedure starting from tion was firstly described by Loupy and co-workers in 1992 [23]. pyrrole and the methyl acetal of the corresponding aliphatic alde- Irradiation of a mixture of pyrrole (1) and benzaldehyde (2) pre- hyde in refluxing carbon tetrachloride to prepare the corresponding adsorbed on the surface of silicon dioxide for 10 minutes, using a porphyrinogen followed by photo-oxidation or chemical oxidation microwave digester and open-vessel conditions, afforded with quinones [9]. Lindsey and co-workers followed a similar two- 5,10,15,20-tetraphenylporphyrin (3) in 9.5% yield (Scheme 1). One step strategy for the synthesis of meso-tetraarylporphyrins, the por- decade after the report by Loupy this procedure was adapted for phyrinogen is formed, from pyrrole and aldehyde, in dichloro- undergraduate experimental teaching [24]. methane doped with acid at room temperature, followed by oxida- The research group of Raghavan [25] published a solvent-free tion with quinone [10]. Using acetic or propionic acid as solvent microwave-promoted synthesis of three porphyrins 3 and 4a,b in and nitrobenzene as solvent and oxidant, Rocha Gonsalves and co- 2004. The reactions were carried out in a domestic microwave ap- workers synthesized aryl and alkyl meso substituted porphyrins via paratus operating at 1200 W during 12 minutes, using HZSM-5 a simple one step procedure [11]. zeolites or Al-MCM-41 mesoporous molecular sieves as solid More complex multi-step protocols, which involve the prepara- acidic catalysts, the latter exhibiting a better performance (Scheme tion of multi-substituted pyrrolic precursors, were used to prepare 2). Porphyrin yields fell to a negligible level when the surface of the porphyrins substituted at the beta position. In order to typify the catalyst was doped with tetraethoxy silane and also when the reac- diverse strategies the MacDonald methodology [12], also known as tion was performed with solvents. Starting from pyrrole and alkyl [2+2] strategy indicating the condensation of two dipyrromethanes, ketones and under the same reaction conditions, calix(4)pyrroles was used to prepare beta-substituted porphyrins with or without 6a-6d were synthesized in good yields, (Scheme 3). additional substituents at the meso-position; the “head to tail” con- Following this study Raghavan and co-workers [24] reported densation of four molecules of pyrrole under acidic reaction condi- the one-step reaction and separation of porphyrins and tions [13]; the [3+1] condensation between diformyl pyrrole and calix(4)pyrroles, i.e. porphyrinogen on a single thin layer chroma- tripyrrane affords beta-substituted porphyrins and was also applied tography (TLC) plate. The reaction of pyrrole and aromatic alde- for the synthesis of expanded porphyrins [14]; unsymmetrical sub- hydes or ketones over zeolite based molecular sieve catalysts as sorbents in TLC under microwave heating allows in situ synthesis and evaluation of the reaction products. The best results were ob- *Address correspondence to this author at the Departamento de Química, Universidade de Coimbra, Rua Larga 3004-535 Coimbra, Portugal; Tel: +351239854479; tained using Al-MCM-41 as solid support, yielding 56.5% and Fax: +351239852080; E-mail: [email protected] 79.1% of porphyrin 3 and porphyrinogen 6a, respectively. The 1875-6271/14 $58.00+.00 © 2014 Bentham Science Publishers 90 Current Organic Synthesis, 2014, Vol. 11, No. 1 Marta Pineiro CHO NH N 1a SiO2 + MW (135 W, 10 min) N HN N H 2 3 9.5 % Scheme 1. Ar Ar CHO 1a Ar = C6H5 1b Ar = 4-CH OC H 3 6 4 NH N 1c Ar = 4-CH3C6H4 Al-MCM-41 (0.5 g) + Ar Ar MW (12 min) N HN N H 2 Ar 3 Ar = C6H5 23.5% 4a Ar = 4-CH3OC6H4 16.0% 4b Ar = 4-CH3C6H4 40.1% Scheme 2. R1 R2 1 2 NH 6a R = R = CH3 1 2 O 1 HN 1 5a R = R = CH3 Al-MCM-41 (0.5 g) R R 6b R1 = CH ; R2 = CH CH 1 2 3 2 3 5b R = CH3; R = CH2CH3 + 6c R1 = R2 = CH CH 1 2 1 2 2 2 2 3 5c R = R = CH2CH3 R R N MW ( 12 min) R R 1 - 2 H NH HN 6d R = -(CH2)4 R 5d R1 = -(CH ) - R2 2 4 2 R1 R2 Scheme 3. process could have a possible application in high throughput paral- methyl 4-formylbenzoate, 3-hydroxybenzaldehyde and pyrrole, pre- lel synthesis and screening on a single micro plate employing mi- adsorbed on the surface of silica gel, heated for 12 minutes at 450 crowave irradiation in combinatorial chemistry. W, in a domestic oven with temperature and power control, the 5,10,15,20-Tetrakis(4-tert-butylphenyl)porphyrin (8a) and desired unsymmetrically substituted porphyrin 12a and the symmet- 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (8b) were obtained rical meso-substituted porphyrin 12b were obtained with 13% and in good yields, heating the corresponding aldehyde and pyrrole 38% isolated yield, respectively. These unsymmetrical porphyrins under microwave irradiation in a domestic oven at 240 W during 5 were typically synthesized in reflux of propionic acid for 3 h with min without solvent or solid support. As described previously by relatively poor yields (~ 10%) [28]. Zn(II) and Cu(II) complexes 13 and 14 of unsymmetrical meso-substituted porphyrins were synthe- Hu and co-workers [26], using the porphyrins, MgCl2 and 1,8- diazabicyclo[5.4.0]undec-7-ene (DBU) as starting materials, the sized by heating the same reagent mixture, the corresponding di- magnesium porphyrinates 9a,b were synthesized under microwave chloride salt and 1 mL of 2,6-dimethylpyridine in silica gel under irradiation in 5 minutes in yields up to 72%. Tetraperylene substi- microwave irradiation [29]. A similar procedure was used to syn- tuted porphyrin 10 was obtained in 59% yield from the reaction of thesize Cu(II)-5-(2-hydroxyphenyl)-10,15,20-tris(4-carboxymethyl- hydroxyporphyrin 8b and a perylene derivative in DMF with phenyl)porphyrinate 15a and Zn(II)-5-(2-hydroxyphenyl)-10,15,20- tris(4-carboxymethylphenyl)porphyrinate 15b with 40% and 38% K2CO3 under microwave irradiation at 240 W during 30 min, (Scheme 4) [27]. yield, respectively. The symmetrically substituted porphyrinates of Cu(II) and Zn(II) were obtained in the same reaction with yields of Unsymmetrical meso-substituted porphyrins bearing two differ- about 55% [30], (Scheme 5). ent aryl groups at the methylene positions in a 3:1 proportion (A3B type) were prepared under microwave heating using solid-supported Ni (II) 5,10,15,20-tetraphenylporphyrinate 16 was prepared in a and open-vessel reaction conditions. Using 3:1:4 molar ratio of two-step procedure, with first the porphyrin 3 being synthesized Microwave-Assisted Synthesis and Reactivity of Porphyrins Current Organic Synthesis, 2014, Vol. 11, No. 1 91 Ar Ar N H NH N N N 2 MgCl2, DBU Ar Mg Ar + Ar Ar MW ( 240 W, 5 min) MW ( 240 W, 5 min) N N Ar CHO N HN 7a Ar = 4-tBuC6H4 7b Ar = 4-HOC6H4 Ar Ar 8a Ar = 4-tBuC6H4 44% 9a Ar = 4-tBuC H 72% 8b Ar = 4-HOC H 48% 6 4 6 4 9b Ar = 4-HOC H 68% RO OR 6 4 9b K2CO3 DMF RBr N MW (240 W, 30 min) N Mg N N O O N N R = RO 10 OR O O Scheme 4.
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