endo-, exo-Cavity hydride isomerism for an aluminium complex of dimetallated 1,3-dimethyl ether p-tert-butylcalix[4]arene

Reaction of 1,3-dimethyl ether p-fert-butylcalix[4]arene 1 with H₃AlNMe₃ in toluene gives an isomeric mixture of monomeric five-coordinate metallocalixarene species derived from dimetallation with the metal centres attached to four oxygens and a hydride either exo (2a) or endo (2b) to the calixarene cavity; 2a is converted to thermodynamically favoured 2b in the presence of H₃AlNMe₃, with the chloro-analogue of 2a, 3 (formed exclusively from the reaction of 1 and AlCl₃) preferentially affording 2b when treated with NaH.