Abstract
| - A notable and obvious ketene substituent-dependent effect of temperature on the stereoselectivity in theStaudinger reaction was observed. Most Staudinger reactions show concave Eyring plots characterizedby two lines with an inversion point, following the principle of isoinversion. Their cis-selectivities decreasewith increasing temperature. Reactions involving intramolecular p−π and π−π interactions between theketene substituents and imine C-substituents reveal protruding, S-shaped or straight-line Eyring plots.Their cis-selectivities increase with increasing temperature in a certain temperature region because suchinteractions enhance the cis-selectivity. Staudinger reactions involving cyclic imines with different ketenesclearly indicate that the temperature-dependent stereoselectivity is caused by the different rate increasesof the direct ring closure, which are affected by the p−π and π−π interactions between ketene substituentsand imine C-substituents if they exist, and the isomerization of the zwitterionic intermediates generatedfrom ketenes and imines during the change in the reaction temperature, not by the competition of theimine exo and endo attacks to the ketenes. Our results also indicate that nonlinear Eyring plots do notalways reveal a change of the stereoselectivity-determining step. Thus, one should use them carefully todetermine any changes in the stereoselectivity-determining step during the change in the reactiontemperature.
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