Structure and Tautomerism of Cyclopentadiene Derivatives: IX. Synthesis and Structure of Substituted N-Cyclopentadienyl Amidinium Ylides

A procedure has been developed for the synthesis of N-cyclopentadienyl amidinium ylides of the general formula C₅(CO₂Me)₄ArNC(Ar')NHAr]. According to the X-ray diffraction data, ¹H and ¹³C NMR spectroscopy, and MNDO quantum-chemical calculations, the title compounds have a zwitterionic structure with the positive charge localized over the amidine NÄCÄN triad, and the negative charge, over the cyclopentadiene fragment. The configuration of the amidine moiety is stabilized by additional interaction of the NH hydrogen atom with the negatively charged cyclopentadiene ring (-bonding). The ylides are chiral due to atropoisomerism arising from a high energy barrier (G 298 >25 kcal/mol) to rotation of the Ar' substituent about the ordinary CÄC bond in the amidinium fragment.