Study of the distribution of the π‐electrons in push‐pull alkenes by ¹H and ¹³C NMR spectroscopy

Abstract

Pusb-pull alkenes witb variable structure were investigated witb respect to tbe influence of the different substituents at the central C,C double bond on the dynamic bebaviour of this type of compound. 'H and "C NMR spectroscopic investigations were carried out for ketene-S,S-, -S,N-and -N,N-acetak HMO calculations were employed to discuss the related n-electron density distributions.

Push-pull and pull-push effects in isatylidenes

“…Compounds 2, 6, 7 and 12 were not measured since 2 was obtained in very low yield, 6 and 7 decompose in DMSO solution and 12 contains two kinds of exchangeable protons. In all the compounds measured, negative isotope effects on the carbons linked to the N atom were observed, as well as for C-3, C-3a and C-7, three bonds away from the deuterium, whereas for the 4  isotope effects an inversion of the sign on C-8 was observed. The 2  isotope effects involved in a cross-conjugated system C(sp 2 )-NH(D)-C(sp 2 ) are known to be very sensitive to the chemical shift of the exchangeable hydrogen atom.…”

“…3 This is associated with an impressive charge polarization of the ethylenic carbons, with a concomitant decrease in the activation barrier of the E-Z isomerization process, allowing it to occur even at room temperature. 4 The common feature of these push-pull systems is the presence of electron-donating and -attracting substituents on the opposite sides of a double bond. This substitution pattern is responsible for a high delocalization of -electrons and allows the stabilization of the polar transition state.…”

Physicochemical studies of molecular hyperpolarizability of imidazole derivatives

“…To quantify the push-pull effect in D--A compounds, bond length alternation (BLA) and out-of-plane distortions of the polarized C C double bonds, available from X-ray studies, have been employed for a long time [8]. Alternatively, dipole moment measurements [9], bond lengths [10], barriers to rotation about the partial C C double bonds [11] (from dynamic NMR studies), and the occupation quotients ( / *) [12] of the bonding (to quantify the acceptor activity) and anti-bonding orbital (to quantify the donor activities) of these C C double bonds were adopted. Not only the push-pull effect in D--A compounds could be quantified, but also a linear dependence of the push-pull quotient ( */ ) on molar hyperpolarizability of these compounds were detected [13].…”

π-Delocalization in Oligoalkynes Induced by Push−Pull Substituents and 1,3-Conjugation: A Combined13C NMR and Computational Study

“…Because we have learned to handle 13 C chemical shift differences with care as a measure of the push-pull effect in push-pull olefins with proximal structural differences, 1,4,6 both the size of the push-pull effect and π-delocalization in oligoalkynes 1 and 2 (n ) 1-4) were restudied by employing the generally applicable push-pull parameters occupation quotient π* CtC / π CtC 2,8 and bond length d CtC . 2,5 Hereby, not only should the push-pull effect in the oligoalkynes 1 and 2 (n ) 1-4) be unequivocally quantified, it was also intended to clear up the controversial discussion concerning the conjugation of 1,3-positioned CtC triple bonds.…”