The nature of lithium perchlorate and gallium chloride salt effect in cycloaddition reactions

Article abstract of DOI:10.1016/S0040-4020(98)00025-8

Comparing the acceleration effects of (4 + 2)-, (3 + 2)- and (2 + 2)-cycloaddition reactions in the presence of the salts of two types: gallium chloride in inert solvents and lithium perchlorate (LP) in diethyl ether (DE) it was observed that in the presence of GaCl₃ the acceleration effect is approximately the same (10⁴ times) for the studied reactions, while in LPDE medium for the same reactions strong increase (up to 10⁴ times), weak increase or even decrease of the rate and equilibrium constants take place even with the common dienophile depending on the nature of the second reagent. it was suggested that the acceleration effect of cycloaddition reactions in the presence of such Lewis acids as aluminum, gallium or boron halides is due to the sharp increase of Π-acceptor properties of dienophiles and therefore increasing energy of orbital interaction, whereas LPDE medium demonstrates strong stabilization of static and/or dynamic polar forms and favors reactions with charge control.