Macrobicyclic polyethers with bridgehead carbon atoms

Abstract

Syntheses of the carbon-bridgehead [2]cryptands (5)-(9) have been achieved in a number of different ways from the ditosylate (12) of diethylene glycol and either pentaerythritol (10) or 1,1,1-tris(hydroxymethyl)ethane (11). The [2]cryptands (5)-(9) all exist as their ' out.out ' isomers and form complexes in solution with alkali metal cations. Variable temperature 1H n.m.r. spectroscopy indicates that the free energy of activation for the dissociation of the potassium ion complexes with the bisbenzyloxymethyl- (5) and dimethyl- (9) [2]cryptands in deuteriochloro-form is ca. 12 kcal mol-1. Stability constant measurements on the complexes of these [2] cryptands [i.e. (5) and (9)] and the bishydroxymethyl[2]cryptand (6) with sodium, potassium, rubidium, and caesium chlorides in methanolic solution indicate, surprisingly, that the complexes are extremely weak (K′ ca. 100). All three [2]-cryptands [i.e. (5), (6), and (9)] exhibit a slight selectivity for binding postassium ions.