Réactions de cycloadditions : études de réactivité et mécanismes

Nucleophilicity and Electrophilicity Parameters for the Analysis of Cycloaddition Reactions

Extensive investigations carried out during the last two decades have shown that the rates of reactions of electrophiles with nucleophiles follow equation (1) when only one new bond is generated in the rate-determining step.

 lg k_20°C = s_N (E + N)               (1)

This equation, where electrophiles are characterized by one parameter (E) and nucleophiles are characterized by the solvent-dependent nucleophilicity parameter N and sensitivity parameter s_N allows one to predict absolute rate constants with an accuracy of factor 10-100 in a reactivity range of more than forty orders of magnitude. More than 1100 nucleophiles and almost 300 electrophiles have been parameterized in this way. Equation (1) fails when very bulky reagents are involved, and we have repeatedly mentioned that eq. (1) cannot be applied to multicenter processes. Recent work has shown, however, that eq. (1) does not only provide rate constants for stepwise cycloadditions via zwitterionic intermediates, but also for concerted cycloadditions with highly unsymmetrical transition states.

                                                                                              

Applications to Diels-Alder reactions and 1,3-dipolar cycloadditions will be discussed.

Contact : G. Berionni (Dpt Chimie) - 081724537 - guillaume.berionni@unamur.be
Plus d'info : http://www.cup.lmu.de/oc/mayr/
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