Anion activation by the bulky perhydrotribenzo[2.2.2]cryptand in aqueous–organic two-phase systems and in low polarity media. A comparison with the anionic reactivity induced by [2.2.2,C14]cryptand

Abstract

In the displacement of the methanesulphonate group by a series of nucleophiles, anionic activation promoted by perhydrotribenzo[2.2.2]cryptand (2) is practically identical to that observed in the presence of the less hindered [2.2.2,C₁₄]cryptand (1), both under phase-transfer catalysis conditions and in homogeneous chlorobenzene solution. However, the stability constants of cryptates noticeably decrease on passing from (1) to (2), especially in anhydrous solutions. The latter behaviour is best explained on the basis of the greater rigidity of cryptand (2) compared with (1), whereas the reactivity data seem to indicate that an optimal ionic separation, and therefore a maximum anionic activation, is already reached with the [2.2.2]ligand.