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 deuteriochloroform 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.