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.
提出了一种用于制备线性寡噁吡咯烷的新方法。合成方案使用水作为溶剂,用于酸催化下醛或酮与未取代吡咯的缩合反应。虽然上述化合物的大多数程序是在有机溶剂中进行的,或者使用大量的吡咯(作为溶剂),但我们在这里提出了一种新颖、温和且高效的方法,用于在水性环境中选择性地制备线性寡噁吡咯。通过改变起始化合物(醛与吡咯)和酸催化剂的摩尔比和浓度来优化二吡咯甲烷和三吡咯甲烷的制备。高浓度的醛和吡咯导致首选形成二吡咯甲烷,产物形成的动力是从水溶液中沉淀出二吡咯甲烷;反应在室温下在几分钟内以极高的产率进行。另一方面,起始化合物的低浓度导致首选形成三吡咯甲烷。通过醛和吡咯的初始比例也可以控制三吡咯甲烷的形成,通常比例超过1:6的情况下主要产生二吡咯甲烷。将这种合成方案应用于酮时发现,无论起始化合物的比例如何,只会形成二吡咯甲烷,这被解释为酮在这种特定缩合反应中的较低反应性所致。