Green chemistry for preparation of oligopyrrole macrocycles precursors: Novel methodology for dipyrromethanes and tripyrromethanes synthesis in water

A novel methodology for preparation of linear oligopyrroles is presented. Synthetic protocol uses water as a solvent for acid catalysed condensation of an aldehyde or ketone with unsubstituted pyrrole. While the most procedures for the above-mentioned compounds are performed in organic solvents, or large excess of pyrrole ( used as solvent), we present here a novel, mild and efficient procedure for selective preparation of linear oligopyrroles in aqueous environment. Preparation of dipyrromethanes 3 and tripyrromethanes 4 was optimised by varying the molar ratio and concentrations of starting compounds ( aldehyde to pyrrole) and acid catalyst. The high initial concentration of aldehyde and pyrrole leads to the preferential formation of 3, where the driving force for the product formation is precipitation of 3 from water solution; the reaction proceeds at room temperature within a couple of minutes in excellent yields. On the other hand, low concentration of starting compounds led to preferential formation of tripyrromethanes 4. Formation of 3 and 4 can also be controlled by the initial ratio of aldehyde and pyrrole, where generally the ratios over 1: 6 gave mostly 3. Application of this synthetic protocol for ketones revealed that only 3 are formed, regardless of the ratio of starting compounds, which is interpreted as a result of lower reactivity of ketones in this particular condensation reaction.