Catalysis with inorganic cations. 2. The effect of some perchlorates on Diels-Alder reaction rates

Article abstract of DOI:10.1021/ja00071a010

The effects of sodium, lithium, barium, and magnesium perchlorate in acetone solution on the rates of different Diels-Alder reactions were kinetically investigated, ¹³C-NMR spectroscopy being used as a tool to infer the mechanism of coordination of the solvated cations acting as catalysts. The Diels-Alder reactions, whose rates increase with increasing electrophilic character of the solvent, are strongly accelerated in the presence of the metal cation. The order of the cation activity, Mg²⁺ > Ba²⁺ ≥ Li⁺ > Na⁺, parallels the order of their respective charge:radius ratios. The Diels-Alder reactions characterized by a nucleophilic or nonspecific solvent effect are nearly insensitive to the presence of any metal cation, since it cannot act as an electrophile. The small increase in the rate of such reactions can be due to nonspecific interactions.