Synthesis and stability of 1,1-dialkyl-1 H-azulenium cations

Abstract

Starting from trimethylsilyl enol ether of 1-acetyl-1,3,5-cycloheptatriene, the title 1,1-dimethyl-, 1,1-diethyl-, and 1,1-dipropyl-1 H-azulenium cations 6– 8 were synthesized in five steps. The order of p K R+ values of these cations was found to be 7> 8> 6. A comparison of the values between 1,1-dialkyl- and 1,1-spiroalkylated 1 H-azulenium cations with the same number of carbon atoms at the 1-position provided the results of 7> 1 and 8< 3. The cation 8 shows a relatively lower p K R+ value to those of 3 and 7 probably due to its slightly bulkier propyl groups from which solvation stabilization of 8 under the conditions suffers. An intermolecular charge-transfer interaction between the cations and dibenzo-24-crown-8 was also studied.