SYNTHETIC AND STRUCTURAL STUDIES OF ULTRASTABLE CARBOCATIONS

Synthesis of hydrocarbon carbocations having 1H-azulenium ion as its partial structure and and evaluation of their thermodynamic stability were studied. It is found that double annulation of spiro[4,5]diene on the tropylium ion increases its stability with pK_R+ value of up to 13.2. This can be suggested to be resulted from hybrid of inductive effect and π-π and σo-π conjugations. The stability of cations was found to be affected by their structure, particularly partial structure around the cationic center of the molecules ; steric crowdness reduces the stability because of uneffective solvation. A novel compound spiroalkylated at the, 1 position of 1H-azulenium ion by an adamanthyl group was synthesized and its solid-state strucure was confirmed by X-ray crystallographic analysis, which indicates that the azulenium ion part is deviated from a plane relatively greater than other cations. This cation showed moderate stability probably because its unplanarity offsets the inductive effect, with increased number of the carbon atoms in the substituent at the 1 position. The aryl-substituted cations at the 1 position were also synthesized and were less stable than the alkyl-substituted ions. It was also found that thees aryl-substituted cations show a charge-transfer band in their UV spectra which can be attributed to excitation from the aryl substituent to the azulenium ion part despite to their orthogonal relationship.