Push-pull ethylenes, rotation barriers

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

The transition state to C—N rotation is less polar than the ground state, and therefore barriers to this rotation are increased by increased solvent polarity (20,83). For similar reasons, the barriers to passage through the planar state in Case 2 push-pull ethylenes increase moderately with increasing solvent polarity (143). 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

Rotational Barriers in Push-Pull Ethylenes An Advanced Physical-Organic Project Including 2D EXSY and Computational Chemistry 94... 

Table XIII gives typical examples of 7t-barriers for planar heterocycles comparing the electron-attracting or electron-donating moiety to push-pull ethylenes. Steric effects destabilize the ground state and thus greatly reduce the 71-barriers to rotation. In these push-pull ethylenes the large entropy of activation and solvent effects hampered easy comparison of the barriers without high-quality determination of the activation parameters.

However, push-pull ethylenes and polyenes are also of interest from a stereochemical point of view due to the observation of hindered rotation of donor and acceptor groups with considerable barriers, and also of low barriers to rotation about the carbon-carbon double bond. It may be appropriate here to lay out briefly, with 3-dimethylaminoacrolein as an example, the principles on which a discussion of the C1—N, C2-acceptor and C1=C2 barriers may be based. The most primitive approach considers only the electron delocalization in the ground state. The electron distribution is described by superposition of two limiting structures, the nonpolar A and the dipolar B (Scheme 1). 

Among the difference types of olefins known with barriers to rotation amenable to study by dynamic H NMR technique, the reported rotational barriers of push-pull ethylenes containing potentially heteroaromatic systems are rather low, ca. 50 kJ-mol [85MI1 88AHC(43)173], Moreover, Elguero and co-workers have studied the rotational barriers around the C—C interannular bond of several 2-(4-pyridyl)benzazoles and their pyri-dinium salts (areno-analogues of amides), since they are too low to measure by H NMR (60 MHz) at 173 K (77H911). 

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