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 pK(R+) approximate to 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 < pK(a) < -1.94. The alcohols exist in equilibrium with the bridged 9-xanthylium cations and have 0.78 < pK(R+) < 1.08. For 1,8-diethoxyxanthone and 1,8-diethoxyxanthydrol, the corresponding values are pK(a) = -1.93 and pK(R+) = 1.38. The possibility that crown ethers incorporating relatively stable carbocationic centres may catalyse amide hydrolyses is briefly discussed.