Push−Pull vs Captodative Aromaticity

Vinylogs of fulvalenes with cyclopropenyl and cyclopentadienyl moieties attached either to different carbon atoms (c-C₃H₂CHCHC₅H₄-c, 7) or to the same carbon atom [XC(c-C₃H₂)(c-C₅H₄), 10] [X = CH₂; C(CN)₂; C(NH₂)₂; C(OCH₂)₂; O; c-C₃H₂; c-C₅H₄; SiH₂; CCl₂] of the double bond inserted between the two rings are examined theoretically at the B3LYP/6−311G(d,p) level. Both types of compounds are shown to possess aromaticity, which was called “push−pull” and “captodative” aromaticity, respectively. For the captodative mesoionic structures XC(c-C₃H₂)(c-C₅H₄), the presence of both the two aromatic moieties and the CC double bond is the necessary and sufficient condition for their existence as energetic minima on the potential energy surface. Aromatic stabilization energy (ASE) was assessed by the use of homodesmotic reactions and heats of hydrogenation. Spatial magnetic criteria (through space NMR shieldings, TSNMRS) of the two types of vinylogous fulvalenes 7 and 10 have been calculated by the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Ragué Schleyer, and visualized as iso-chemical-shielding surfaces (ICSS) of various sizes and directions. TSNMRS values can be successfully employed to visualize and quantify the partial push−pull and captodative aromaticity of both the three- and five-membered ring moieties. In addition, the push−pull effect in compounds 7 and 10 could be quantified by the occupation quotient π*_CC/π_CC of the double bond inserted between the two rings.