The synthesis of symmetric α-free meso-H-dipyrrin hydrobromides from 5-H-2-formyl pyrroles was investigated. The self-condensation produces regioisomeric dipyrrins through adoption of two mechanistic pathways. The key difference between the two pathways lies in which position of the pyrrole directs nucleophilic attack. Through a systematic study involving various substituted and (or) isotopically labelled 5-H-2-formyl pyrroles, we herein provide evidence to suggest that not only do two mechanistic pathways exist, but the steric bulk of the substituent adjacent to the 5-unsubstituted position influences which pathway dominates.
对5-H-2-甲醛吡咯烷合成对称α-自由meso-H-二吡咯溴化物进行了研究。自缩合通过采用两种机理途径产生位置异构的二吡咯。这两种途径之间的关键区别在于吡咯的哪个位置引导亲核攻击。通过对各种取代和(或)同位素标记的5-H-2-甲醛吡咯进行系统研究,我们提供了证据表明不仅存在两种机理途径,而且邻近5-未取代位置的取代基的立体体积影响哪种途径占主导地位。