A Re-Evaluation of the Electrophilic Substitution Reactions of the Ramirez Ylide

Cyclopentadienylidenetriphenylphosphorane (the Ramirez ylide), unexpectedly and contrary to a number
of earlier reports, has been shown to be like pyrrole in undergoing electrophilic substitution on the
cyclopentadienide ring at either the 3- or the 2-position, depending on the electrophile. Formylation
under Vilsmeier conditions and addition of tetracyanoethylene occurs at the 3-position, while activated
acetylenes and the nitrosyl electrophile substitute at the 2 position. The 3-formylated product was reduced
to the 3-methyl derivative and it also reacted under Knoevenagel conditions to give a number of novel
condensation products. The results of single-crystal X-ray crystallographic analyses are given for four of
the compounds studied, and a careful 2D NMR analysis of all of the compounds was performed in order
to develop a reliable method for the unambiguous assignment of the regiochemistry of adduct formation.

与此前多项报道相悖且出乎意料的是，环戊二烯亚基三苯基膦叶立德（Cyclopentadienylidenetriphenylphosphorane，又称Ramirez叶立德，Ramirez ylide）可与吡咯（pyrrole）类似，在环戊二烯负离子环（cyclopentadienide ring）的2位或3位发生亲电取代（electrophilic substitution）反应，具体取代位点取决于亲电试剂的种类。在维尔斯迈尔反应条件（Vilsmeier conditions）下进行甲酰化，以及与四氰基乙烯（tetracyanoethylene）发生加成反应时，反应位点为3位；而活化炔烃（activated acetylenes）与亚硝酰亲电试剂（nitrosyl electrophile）则会在2位发生取代。所得到的3-甲酰化产物（formylated product）可被还原为3-甲基衍生物（3-methyl derivative），同时该产物还可在克脑文盖尔反应条件（Knoevenagel conditions）下发生缩合反应，生成多种新型缩合产物。本研究对其中4种目标化合物开展了单晶X射线晶体学分析（single-crystal X-ray crystallographic analyses），并对所有化合物进行了细致的二维核磁共振波谱分析（2D NMR analysis），旨在建立一套可精准判定加合物形成区域化学选择性（regiochemistry of adduct formation）的可靠方法。