Torquoelectronic effects

Table 1 Torquoelectronic effects in the transition structures associated with the conrotatory ring closure of monosubstituted ketenes...

Sordo et al. [144] explained the stereoselectivity on the basis of torquoelectronic effects. Low-temperature infrared spectroscopy was also used to identify the reactive intermediates [145]. Two mechanisms were proposed to explain the product distribution in the (3-lactam formation reaction. The ketene mechanism was observed in a low temperature infrared spectroscopy study [145], while the acylation of imine mechanism was believed to be involved in some [122]. Both mechanisms were supported by evidences. It had been hypothesized that cycloaddition of the imine occurs from the least hindered side of the ketene, and this process generates zwitterionic intermediates conrotatory cyclization of these intermediates then produce cis- and //Y/ .v-[S-lactanis. Acylation of the imine by the acid chloride to form /V-acyliminium chloride also produced zwitterionic intermediates (Scheme 10). 

DFT investigation of the eight-electron electrocyclization reactions of 1,8-disubsti-tuted (3Z,5Z)-octa-l,3,5,7-tetraenes has found that these reactions proceed via a Mobius helical aromatic transition state. It was also found that outward substituents were preferred to inward ones, regardless of the electronic nature of the substituents, and that torquoelectronic effects are overridden by secondary orbital, electrostatic,... 

Alajarin, M., Vidal, A., Tovar, F., Arrieta, A., Lecea, B., Cossio, F. P. Surpassing torquoelectronic effects in conrotatory ring closures origins of stereocontrol in intramolecular ketenimine - imine [2+2] cycloadditions. Chem.— Eur. J. 1999, 5,1106-1117. 

According to the computational results, the stereochemical outcome of the reaction is determined by electronic effects related to the torquoelectronic model [34-37] rather than by steric effects. In contrast, the stereochemical outcome was not very sensitive to solvent polarity. 

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