Dimerization of Substituted Arylacetylenes-Quantum Chemical Calculations and Kinetic Studies.

The dimerization of substituted arylacetylenes is a very interesting tool to generate 1,3-butadiene 1,4-diradicals. Recently, it was shown that electron-withdrawing groups attached to the triple bond reduce the activation barrier and increase the stability of the diradical intermediates. Here, we investigate the influence of the π donor character of substituents, which are bound to the aryl system, on the dimerization reaction of arylacetylenes. Both quantum chemical calculations and kinetic studies reveal that the higher the π donor character of substituents, the lower the activation barrier. The highest observed difference between the model systems amounts to 4.0 kcal/mol, which represents an acceleration by a factor of 700. However, according to the calculations the π donor character of the substituents increases the diradical character of the intermediates.