摘要:
A procedure has been developed for the synthesis of N-cyclopentadienyl amidinium ylides of the general formula C-5(CO2Me)(4)[ArNC(Ar)NHAr]. According to the X-ray diffraction data, H-1 and C-13 NMR spectroscopy, and MNDO quantum-chemical calculations, the title compounds have a zwitterionic structure with the positive charge localized over the amidine N-C-N 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 (pi-bonding). The ylides are chiral due to atropoisomerism arising from a high energy barrier (DeltaG(298)(not equal) > 25 kcal/mol) to rotation of the Ar' substituent about the ordinary C-C bond in the amidinium fragment.