Wakamiya Atsushi, Nishinaga Tohru, Komatsu Koichi
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
J Am Chem Soc. 2002 Dec 18;124(50):15038-50. doi: 10.1021/ja028297j.
A stable derivative of 1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks 4 was synthesized as red crystals by the reaction of a dilithiated dimer of bicyclo[2.2.2]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-tetrathiabicyclo[2.2.2]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.
通过双环[2.2.2]辛烯的二锂化二聚体与元素硫反应，以59%的产率合成了一种与双环[2.2.2]辛烯骨架稠合的1,2 - 二硫杂环戊二烯的稳定衍生物4，呈红色晶体。4在-78℃的二氯甲烷中的循环伏安法显示，相对于Fc/Fc⁺，在E(1/2)为 +0.18 V和 +0.72 V处有两个可逆氧化波，表明4的自由基阳离子和二价阳离子在这些条件下是稳定的。在二氯甲烷中用1.5当量的SbCl₅对4进行相当低浓度（4.0×10⁻⁴ M）的化学单电子氧化时，形成了自由基阳离子4⁺，其自旋分布通过电子顺磁共振光谱和理论计算（UB3LYP/6 - 31G）结果确定。4⁺在二氯甲烷中的电子吸收光谱在428 nm处有最大吸收（ε = 2.3×10³），相对于中性4（469 nm）发生了蓝移。当在二氯甲烷中以较高浓度（0.06 M）产生自由基阳离子4⁺时，发现发生歧化反应生成具有新颖的2,3,5,6 - 四硫杂双环[2.2.2]辛 - 7 - 烯结构的显著稳定的自由基阳离子5⁺的SbCl₆⁻盐。在5⁺SbCl₆⁻的X射线结构中，发现硫原子之间的跨环距离（2.794(3) Å）小于硫原子的范德华半径之和（3.70 Å），表明两个二硫键之间存在键合相互作用。理论计算（UB3LYP/6 - 31G）表明，这种跨环相互作用可以描述为极限结构之间的共振，每个极限结构在属于两个不同二硫键的两个硫原子之间具有一个双中心三电子键：因此，自旋和正电荷均等地离域到四个硫原子上，导致5⁺高度稳定。另一方面，发现1,2 - 二硫杂环戊二烯自由基阳离子4⁺容易与三线态氧反应，随后重排生成具有羧基的1,2 - 二硫鎓衍生物6⁺。最后，4与过量的SbF₅反应得到相应的二价阳离子4²⁺，根据核磁共振测量和理论计算结果，发现它是一个6π芳香体系。
