1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks. One-electron and two-electron oxidations and formation of a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene radical cation with remarkable stability owing to a strong transannular interaction

A stable derivative of 1,2-dithiin annelated with bicyclo2.2.2]octene frameworks 4 was synthesized as red crystals by the reaction of a dilithiated dimer of bicyclo2.2.2]octene with elemental sulfur in 59% yield. The cyclic voltammetry of 4 in CH(2)Cl(2) at -78 degrees C showed two reversible oxidation waves at E(1/2) +0.18 V and +0.72 V versus Fc/Fc(+), indicating that the radical cation and dication of 4 are stable under these conditions. Upon chemical one-electron oxidation of 4 in a rather low concentration (4.0 x 10(-4) M) with a 1.5 equiv of SbCl(5) in CH(2)Cl(2), a radical cation 4.+ was formed, whose spin distribution was determined by ESR spectroscopy and by the results of theoretical calculations (UB3LYP/6-31G). The electronic absorption spectrum of 4.+ in CH(2)Cl(2) exhibited a maximum absorption at 428 nm (epsilon = 2.3 x 10(3)), which was hypsochromically shifted from that of neutral 4 (469 nm). When the radical cation 4.+ was produced in higher concentration (0.06 M) in CH(2)Cl(2), a disproportionation was found to take place to give a SbCl(6)(-) salt of remarkably stable radical cation 5.+ having a novel 2,3,5,6-tetrathiabicyclo2.2.2]oct-7-ene structure. In the X-ray structure of 5.+SbCl(6)(-), the transannular distance (2.794(3) A) between the sulfur atoms was found to be less than the sum of the van der Waals radii of a sulfur atom (3.70 A), suggesting the existence of a bonding interaction between the two disulfide linkages. The theoretical calculations (UB3LYP/6-31G) suggested that this transannular interaction could be described as the resonance between the limiting structures, each of them having a two-center three-electron bond between two sulfur atoms belonging to two different disulfide linkages: thus, both the spin and positive charge are equally delocalized to the four sulfur atoms, causing a great stabilization of 5.+. On the other hand, the 1,2-dithiin radical cation 4.+ was found to readily react with triplet oxygen with subsequent rearrangement to give the 1,2-dithiolium derivative 6+ having a carboxyl group. Finally, the reaction of 4 with an excess amount of SbF(5) gave the corresponding dication 4(2+), which was found to be a 6pi aromatic system on the basis of the results of NMR measurement and theoretical calculations.