Kinetically Stable Complexes of Alkali Cations with Rigidified Calixarenes: Synthesis, X-ray Structures, and Complexation of Calixcrowns and Calixspherands

Article abstract of DOI:10.1021/ja00201a045

Selective 1,3-dialkylation of the phenolic groups of p-tert-butylcalix<4>arene (3) with methyl or benzyl tosylate yields 7a (75percent) and 8 (95percent), respectively.Subsequent bridging of the two remaining phenolic groups in 7a or 8, by reaction with polyethylene glycol ditosylates, gives p-tert-butylcalix<4>arene crown ethers (4b, 4c, and 4e).Reaction of 7a with 3,3''-bis(bromomethyl)-2,2',2''-trimethoxy-5,5',5''-trimethyl-1,1':3',1''-terphenyl (10) in the presence of NaH or KH produces the corresponding NaBr and KBr complexes of the calixspherand 6.Decomplexation of the6*NaBr and 6*KBr complexes in H2O/CH3OH (4:1) requires long reaction times and high temperatures, indicating a high kinetic stability.The X-ray structure of 6*NaPic confirmed the flattened partial cone conformation of the calixarene moiety in this complex.The free energies of complexation (ΔG⁰) of the calixcrown ether alkali picrate (MPic) complexes vary from -6 to -12 kcal*mol^-1 (CDCl3).The calixspherand 6 forms kinetically "stable" complexes with Na(1+) (k_d²⁹⁸ = 6.0*1E-9 s^-1), K(1+) (k_d²⁹⁸ =1.0*1E-8 s^-1), and Rb(1+) (k_d²⁹⁸ = 6.9*1E-5 s^-1).The rates of complexation vary between 1.3*1E4 and 2.5*1E5 M^-1 s^-1.The calculated free energies of complexation of 6 with MPic salts in CDCl3 at 298 K are -16.8 <6*Na(1+)>, -18.1 <6*K(1+)>, and -13.0 <6*Rb(1+)> kcal*mol^-1, respectively.The high thermodynamic and kinetic stabilities of these complexes are explained in terms of a high degree of preorganization of the calixspherand and the highly hydrophobic collar around the molecular cavity.This prevents solvent molecules to assist in the decomplexation process.