Principle of activation of electrophiles by electrophiles through dimeric association - Two are better than one

Electron-deficient metal compounds can be significantly activated as electr ophiles through dimeric (or polymeric) association with either the same or different electrophiles. Although this phenomenon has played an important r ole in many reactions, such as the Friedel - Crafts reaction and the Ziegle r-Natta polymerization, it has apparently only recently been fully recogniz ed. Its widespread exploitation promises to lead to many new synthetically useful reactions as well as ready and accurate interpretations of mechanist ic and structural phenomena in acid-catalyzed and -promoted reactions.