Preparation and properties of some crown ethers incorporating stable carbocations

Reaction of 1,3-xylyl-18-C5[12(n= 3)] and 1,3-xylyl-21-C6[12(n= 4)] with diazomethane yields mixtures of crown ethers incorporating cycloheptatrienes, and these have been converted to crown ethers incorporating tropylium ions 5(n= 3 and 4) by hydride abstraction with triphenylcarbenium tetrafluoroborate. These compounds have pKR+≈ 3.8. Crown ethers containing the 1,8-dioxyxanthone residue, 22(n= 3–5), have been prepared by alkylation of 1,8-dihydroxyxanthone with polyethylene glycol dibromides. These have been converted into the corresponding xanthydrols, 16(n= 3–5). The conjugate acids of the ketones have –2.14 < pKa < –1.94. The alcohols exist in equilibrium with the bridged 9-xanthylium cations and have 0.78 < pKR+ < 1.08. For 1,8-diethoxyxanthone and 1,8-diethoxyxanthydrol, the corresponding values are pKa=–1.93 and pKR+= 1.38. The possibility that crown ethers incorporating relatively stable carbocationic centres may catalyse amide hydrolyses is briefly discussed.