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Push-pull alkenes: Structure and -electron distribution

Kleinpeter Erich (Universität Potsdam, Chemisches Institut, Potsdam, Germany)

Push-pull alkenes are substituted alkenes with one or two electron-donating substituents on one end of C=C double bond and with one or two electron-accepting substituents at the other end. Allowance for -electron delocalization leads to the central C=C double bond becoming ever more polarized and with rising push-pull character, the -bond order of this double bond is reduced and, conversely, the corresponding -bond orders of the C-Don and C-Acc bonds are accordingly increased. This push-pull effect is of decisive influence on both the dynamic behavior and the chemical reactivity of this class of compounds and thus it is of considerable interest to both determine and to quantify the inherent push-pull effect. Previously, the barriers to rotation about the C=C, C-Don and/or C-Acc partial double bonds (G±, as determined by dynamic NMR spectroscopy) or the 13C chemical shift difference of the polarized C=C partial double bond (C=C) were employed for this purpose. However, these parameters can have serious limitations, viz. the barriers can be immeasurable on the NMR timescale (either by being too high or too low; heavily-biased conformers are present, etc.) or C=C behaves in a non-additive manner with respect to the combination of the four substituents. Hence, a general parameter to quantify the push-pull effect is not yet available. Ab initio MO calculations on a collection of compounds, together with NBO analysis, provided valuable information on the structure, bond energies, electron occupancies and bonding/antibonding interactions. In addition to G±C=C (either experimentally determined or theoretically calculated) and C=C, the bond length of the C=C partial double bond was also examined and it proved to be a reliable parameter to quantify the push-pull effect. Equally so, the quotient of the occupation numbers of the antibonding and bonding  orbitals of the central C=C partial double bond ( *C=C/ C=C) could also be employed for this purpose. .

Keywords: push-pull alkenes, barriers to rotation, 13C chemical shift differences, ab initio MO calculations, NBO analysis, quantification of the push-pull effect