Porphyrins are found widely in biology, including in hemoglobin, myoglobin, chlorophyll, and cytochrome c oxidase. As such, they are very often utilized in biomimetic chemistry where often times the aim is to construct synthetic analogs of enzymes. Recently, they have gained much attention when coupled with fullerenes as potential solar cell materials.
Porphin, the simplest porphyrin
Porphyrins are macrocyclic aromatic compounds. The four nitrogen atoms make it tetradentate and can bind a large range of metals including magnesium (chlorophyll), iron (heme), mercury, tin, and zinc, with varying degrees of stability. Porphin is the simplest porphyrin and is synthesized from formaldehyde and pyrrole. In general, porphyrins can be synthesized from four equivalents of aldehydes and four equivalents of pyrrole (which can be substituted at any carbon position) with an acid catalyst (propionic acid or acetic acid) in a procedure known as the Adler method. The aldehyde gives rise to substitution at the meso or 5,10,15,and 20 positions on the porphyrin ring. The yields are very low and asymmetric porphyrins must be separated from ugly statistical mixtures. For example, if it was trans-diphenyldi-p-tolylporphyrin you desired, you could react benzaldehyde, p-tolyl aldehyde, and pyrolle together, but statistically you would get a 1:4:3:3:4:1 ratio of tetraphenylporphyrin, triphenyl-p-tolylporphyrin, trans-diphenyldi-p-tolylporphyrin, cis-diphenyldi-p-tolylporphyrin phenyl, tri-p-tolylporphyrin, and tetra-p-tolylprophyrin, respectively. In practice, the actual ratio would not be this perfect due to the different reactivities of the two aldehydes.
In order to overcome at least some of the messiness of Adler’s preparation, Lindsay developed an alternative way of constructing porphyrin rings. First, an aldehyde is reacted with a large excess of the pyrrole under acid catalysis to give a dipyrromethane. The dipyrromethane is then reacted with, sensibly, a different aldehyde, to give an asymmetric porphyrin cleanly. Now it is entirely possible to react two different dipyrromethanes with one aldehyde or one dipyrromethane with two different aldehydes or even two different dipyrromethanes with two different aldehydes. But again, you get the idea. It will become messy.
My goal is to synthesize trans-alpha-beta-diaminophenyldi(p-tolyl)porphyrin. The synthesis scheme starts from p-tolualdehyde (4-methylbenzaldehyde) and pyrrole to give the dipyrromethane, meso-(p-tolyl)dipyrromethane, or 2,2’-(p-tolylmethylene)bis(1H-pyrrole). The clear liquid pyrrole slowly spontaneously self-polymerizes to give black-colored polypyrrolles, and the rate of this process increases with the presence of light or acid. In order to increase the purity of pyrrole, it is frequently distilled immediately before use in synthesis. p-tolualdehyde is reacted with a large excess of freshly distilled pyrrole to avoid more than one aldehyde from coupling to the dipyrromethane (this would, in part, lead to porphyrin product which is undesired at this stage). The reaction is catalyzed by trifluoroacetic acid and performed at room temperature for no longer than an hour.
The excess pyrrole is removed via vacuum distillation at room temperature as to not heat the dipyrromethane and cause unnecessary polymerization. The crude product is then diluted in dichloromethane and washed with dilute sodium hydroxide and then water to neutralize the acid catalyst. The organic layer is then dried with anhydrous magnesium sulfate, rotavapped, and the dipyrromethane is subsequently purified with column chromatography. When running the column, ~1% triethylamine is used in the eluent to make sure that the entire column is basic. This, again, is to limit polymerization of the dipyrromethane.
Once the dipyrromethane is isolated, it is reacted with o-nitrobenzaldehyde in an attempt to get the desired trans-alpha-beta-diaminophenyldi(p-tolyl)porphyrin. The dinitroporphyrin is then reduced to the final diaminoporphyrin. Now, you may be wondering if the reduction can be done before the porphyrin ring is formed. The answer is yes, but with caution. o-nitrobenzaldehyde can easily be reduced to o-aminobenzaldehyde. However, the aminobenzaldehyde cannot be directly coupled with pyrrole to give the desired porphyrin because acidic conditions are necessary, and the acid will just protonate the amine to give the unreactive cation. Therefore, the amine must be protected, most commonly through acetylation to give, what is clumsily called, N-(2-formylphenyl)acetamide. This substrate could then be reacted with pyrrole to give, in addition to many other porphyrins, trans-alpha-beta-diacetamidophenyldi(p-tolyl)porphyrin. The acetyl groups would then be cleaved through deprotection to yield the final trans-alpha-beta-diaminophenyldi(p-tolyl)porphyrin. It is unclear to me at this time whether or not it is more advantageous to reduce before or after the porphyrin ring formation. I will attempt to reduce the nitro groups after the ring formation first since this method is more straightforward.
The dipyrromethane reaction with o-nitrobenzaldehyde is performed in chloroform under nitrogen at room temperature with boron trifluoride diethyl etherate acting as a strong Lewis Acid catalyst. It has been proven, oddly enough, that dichloromethane is less effective in porphyrin formation than chloroform. After lots of research, it was discovered that a trace amount of ethanol helps promote the reaction and that ethanol is a common impurity in reagent grade chloroform. The immediate result of the condensation reaction is a porphyrinogen, a reduced porphyrin. This porphyrinogen, as it turns out, can result in a lot of trouble. It is unstable and so many bonds of the ring can break and reform, creating many side-products in what is known as scrambling. So, although the dipyrromethane formation, in this case, sets up the coupling so that the trans product will form, scrambling can also result in the cis product. In addition, rotation of a pyrrole in a broken porphyrinogen ring can result in what has been termed an N-confused porphyrin in which the nitrogen on the pyrrole ring shifts from the interior to the perimeter of the porphyrin ring. Different rotational isomers, called atropisomers, in porphyrins are also possible. The nitro groups can either be sticking above or below the plane of the porphyrin. So, in the case of the dinitroporphyrin, two atropisomers are possible- one with the two nitro groups pointing up or down (called alpha-alpha) or one with the two nitro groups pointing in opposite directions (called alpha-beta). I am seeking to synthesize the alpha-beta variety. Mild heating of porphyrins causes the interconversion of atropisomers as enough kinetic energy is given to the molecule to cause free rotation. All of this, along with polypyrrole formation leads to a vast variety of side-products, an ugly mess, pathetic yields, and lots of fun column chromatography for days. But this is the sad reality of porphyrin synthesis. If someone figures out a better way, I’m sure they will become famous for it.
Once the porphyrinogen ring is formed, it is converted to a porphyrin using a mild oxidizing agent, typically DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) or p-chloroanil (2,3,5,6-tetrachloro-parabenzoquinone). This reaction occurs rapidly at room temperature.
Ok, well actually I lied a bit. From the dinitroporphyrin mixture, trans-alpha-beta-dinitrophenyldi(p-tolyl)porphyrin does not have to be isolated yet (although it most certainly could be for analytical purposes). In my case, I’m more than content to just use column chromatography to get rid of the polypyrrole and other super polar junk from the dinitroporphyrins. While I’m at it, I can also get rid of the less polar tetratolylprophyrin which will occur due to scrambling. The mixture of dinitroporphyrin isomers can then be reduced to diaminoprohyrins by stirring it with stannous chloride dihydrate in concentrated hydrochloric acid at room temperature overnight (I discovered part of this set of reaction conditions on my own, see below). The aqueous solution is then neutralized slowly with ammonium hydroxide until just a little bit of stannic oxide is formed. The neutral amine is then extracted from the tin(IV) oxide-water slurry repeatedly with dichloromethane. The organic layer is then dried with magnesium sulfate and rotavapped and then the compound is purified by exhaustive column chromatography.
The experiment I describe below was performed in a professional chemistry laboratory. However, to keep things in the spirit of this website and of amateur chemistry, I decided to challenge myself and come up with a possible route to synthesizing trans-alpha-beta-diaminophenyldi(p-tolyl)porphyrin using only household chemicals. At first, this seemed like a difficult task since this is a relatively complicated molecule. But after a little bit of research and some thinking about it, I was able to come up with the following much to my delight.
With commonly available chemicals, I actually plan to do the porphyrin coupling reaction using the Adler preparation instead of the Lindsay method since the trifluoroacetic acid or boron trifluoride catalyst required are difficult and probably dangerous to make from scratch. But with the Adler method, glacial acetic acid is the catalyst and solvent that is needed! Also, with this method, the porphyrin is immediately formed and the intermediate porphyrinogen does not have to be oxidized with something like DDQ, which would most likely be difficult to synthesize. As described in my future projects, I plan to make glacial acetic acid from sodium acetate and sulfuric acid drain cleaner with urea helping to convert concentrated acetic acid to pure glacial acetic acid. The sodium acetate comes from the reaction of vinegar and baking soda, and urea would probably come from my own urine.
Now for a lot of the steps in this synthesis plan, there will be considerable side product formation that can only be separated by chromatography. So I will need to make my own TLC plates and column. I will need silica for this. Silica is just basically super fine and pure silicon dioxide. I could try grinding up quartz sand for this with my ball mill or I could perhaps precipitate a high mesh powder by acidifying a sodium silicate solution. The sodium silicate would be made by fusing sodium hydroxide with silicon dioxide, of the coarser variety (of course!). I’ve actually read about how to make homemade TLC plates before, but I’ll have to look up and describe the details later.
With the glacial acetic acid in hand, 2-nitrobenzaldehyde, p-tolylaldehyde, and pyrrole are the next chemicals that would need to be synthesized. Let’s start with the aldehydes since they are less interesting to make than pyrrole. For the synthesis of 2-nitrobenzaldehyde, I would start with toluene which is sold in some hardware stores as a common purpose solvent. This would be nitrated with a mixture of concentrated sulfuric and nitric acids (made from sulfuric acid and potassium nitrate) to give a mixture of a 2-nitrotoluene and 4-nitrotoluene since the methyl group on the benzene ring is ortho-para directing. The 2-nitrotoluene would be isolated somehow, probably by chromatography. This would be reacted first with boiling concentrated nitric acid to give the nitromethane derivative which would further be oxidized to the corresponding aldehyde with potassium permanganate (see the manganese dioxide page for information on a synthesis scheme towards this potent oxidizer). This two-step conversion of 2-nitrotoluene to 2-nitrobenzaldehyde is well described in an old patent. I am hoping to use an analogous set of reactions to convert xylene (also found in hardware stores) to p-tolyl aldehyde.
The synthesis of pyrrole, on the other hand, is quite intriguing! I have planned to use milk out of all things as the starting material! A useful aromatic 5-membered heterocycle from milk- how cool is that? So the milk is first curdled with a dilute acid and filtered to give milk plasma (whey). Lactose is then purified from the whey through recrystallization of an aqueous solution with ethanol. The disaccharide lactose is then hydrolyzed by boiling it with dilute sulfuric acid to give galactose and glucose. The former is isolated by fractional crystallization. The galactose is then reacted with nitric acid to give mucic acid which is neutralized with ammonium hydroxide to give ammonium mucate. The ammonium mucate is finally pyrolyzed in glycerin to give pyrrole. The pyrrole can be easily purified by distillation under atmospheric pressure.
To make the desired trans-alpha-beta-dinitrophenyldi(p-tolyl)porphyrin, 1 equivalent of each aldehyde and 2 equivalents of pyrrole would be refluxed in glacial acetic acid for a few hours. The porphyrins would then be filtered off and reduced to aminoprophyrins using perhaps steel (iron) in hydrochloric acid. The final trans-alpha-beta-diaminophenyldi(p-tolyl)porphyrin would then have to be isolated via column chromatography.
If I don’t want to go through the whole ordeal of making TLC plates and synthesizing or locating suitable silica from column chromatography, I could content myself with making a simpler porphyrin like tetraphenylporphyrin. This would be made simply with pyrrole and benzaldehyde, and filtering the solution from the Adler mixture, would give relatively pure tetraphenylporphyrin. Benzaldehyde would perhaps be made by oxidizing toluene with manganese dioxide and sulfuric acid.
As part of the research in the chemistry lab I work for, I am synthesizing a porphyrin to model the active site of cytochrome c oxidase. The goal is to have a model in which the influx of both protons and electrons is controllable and slow as is the case in the actual enzyme. The details of the model, unfortunately, I cannot yet reveal, but the first synthetic steps (where I am at this point) are general enough that I can describe them here.
30 mL reagent grade pyrrole, a viscous brown-black liquid, was distilled under atmospheric pressure. The head temperature was allowed to reach 130C (the boiling point of pyrrole) for at least 3 minutes before the fraction collected was discarded. The rest of the fraction that came over at 130C was collected came over as a clear liquid.
A solution of 2.16 mL p-tolualdehyde (18.3 mmol, 1 eq) in 88.9 mL freshly distilled pyrrole (1.3 mol, 70 eq) was stirred and degassed with nitrogen for 1 hour. 136 uL trifluoroacetic acid (1.8 mmol, 1eq) was then added dropwise, resulting in a bright orange solution. The reaction mixture was stirred at room temperature for 30 minutes. By this time, a TLC indicated that all of the starting material was consumed. The solution was then dumped in 250 mL dichloromethane before it was washed twice with 300 mL 0.1 M sodium hydroxide and 300 mL water. The organic extract was then dried with sodium sulfate and rotavapped down to an oil. The pyrrole was then removed via vacuum distillation at 40C. The dipyrromethane was the purified from residual pyrrole by repeated column chromatography in a 70:30:1 hexanes:dichloromethane:triethylamine solvent system. This gave 2.13g of a tan solid (50% yield). NMR of the product looked clean with perhaps a tiny bit of starting aldehyde impurity.
103 mg of o-nitrobenzaldehyde (0.68 mmol, 2 eq) and 161 mg meso-(p-tolyl)dipyrromethane (0.68 mmol, 2 eq) was added to 70 mL chloroform. The solution was degassed for 30 minutes with nitrogen before 1.1mL boron trifluoride diethyl etherate (9.0 mmol, 26.5 eq) was added dropwise to the solution. The reaction mixture was stirred at room temperature for 1.5 hr, and then 117 mg DDQ (0.52 mmol, 1.5 eq) was added, resulting in a black solution. The solution was stirred for an additional hour before it was concentrated down to ~5mL via rotavap. 105 mg tetratolylprophyrin (10.5% yield) was isolated as a purple solid as the most nonpolar spot in 1:1 hexanes:dichloromethane using flash column chromatography. 403 mg of the next three spots were isolated (40.3% yield) as a purple solid and were assumed to be nitroporphyrins.
The impure nitroporphyrins were added to 30 mL concentrated hydrochloric acid. 2.3 g stannous chloride was dissolved in 10 mL of concentrated hydrochloric acid and the solution was added dropwise to the reaction mixture, giving a green solution. The solution was stirred overnight at room temperature (one hour at 65C gave no product in a previous attempt) under slight positive nitrogen pressure. Concentrated ammonium hydroxide was then added slowly to the solution while it was on ice until a pH of 8 was attained and a white precipitate of stannic oxide was observed. During addition of the base, the temperature of the reaction mixture was never allowed to go above 35C. The solution was then extracted ten times with 200 mL chloroform. The organic layer was then dried with anhydrous magnesium sulfate and rotavapped down to give 352mg of a purple solid that contained three spots via TLC. The presumed aminoporphyrin isomers have not yet been purified via column chromatography.