Elucidation of Biradical Character in pi-Conjugated Hydrocarbons Caused by Annelation with Strained Bicycloalkene Frameworks

（1）合成了最基本的芳烃苯（1）和环辛四烯（COT，2），后者与高张力双环体系双环[2.1.1]己烯（BCH）稠合。它们的芳香性和反芳香性进行了研究，通过使用理论计算和光谱和电化学测量。(2)用SbCl_5单电子氧化苯1，可使1重排生成一种新的萘衍生物3，其1，2-和3，4-位上分别与BCH单元稠合。3的X-射线结构分析表明，由于键长均衡，BCH-稠合的六元环比其它环更芳香。它可以被氧化成一个稳定的自由基阳离子，因为大大提高HOMO能级。(3)苯1表现出多烯而不是芳香族化合物的典型反应性。因此，它被m-CPBA氧化，并进行Symmons-Smith反应，以高产率得到相应的全顺式三环氧化物和三环丙烷衍生物。(4)由于萘3与应变BCH体系的加成作用，在室温光照下，萘3容易被空气氧化，得到一种新的具有两个羰基和两个顺式环丁烷-1，4-二基单元的大环炔。这被证明是由反[2+2+2]裂解反应的内过氧化物首先形成在BCH-稠合萘的1，4-位。这是由于萘3的高双自由基特性。(5)合成了一系列完全被双环[2.2.2]辛烯单元包围的低聚噻吩（2-，3-，4-，6-和8-mer），并将其转化为稳定的自由基阳离子盐或双阳离子盐，其精确结构由X射线晶体学确定。这些被认为是相关的模型的极化子或双极化子的p-掺杂态的寡聚噻吩。

(1)The most essential aromatic hydrocarbon, benzene (1), and cyclic polyene, cyclooctatetraene (COT, 2), annelated with a highly strained bicyclic system, bicyclo[2.1.1]hexene (BCH), were synthesized. Their aromaticity and antiaromaticity were examined by the use of theoretical calculations and spectral and electrochemical measurements.(2) One-electron oxidation of benzene 1 with SbCl_5 was found to cause rearrangement of 1 to a novel naphthalene derivative 3 annelated with the BCH units at 1,2- and 3,4-positions. The X-ray structural analysis of 3 demonstrated that the BCH-annelated 6-membered ring is more aromatic than the other ring owing to bond-length equalization. It can be oxidized to a stable radical cation because of the considerably raised HOMO energy levels.(3) Benzene 1 exhibited the reactivity typical for the polyene rather than an aromatic compound. Thus, it was oxidized by m-CPBA and underwent Symmons-Smith reactions to give the corresponding all-cis tri-epoxide and tri-cyclopropane derivatives in high yields.(4) Because of the annelation with strained BCH systems, naphthalene 3 readily underwent oxidation with air under irradiation of room light to give a novel macrocyclic alkyne having a two carbonyl groups and two cis-cyclobutane-1,4-diyl units. This was shown to be formed by retro[2+2+2] cleavage reaction of the endoperoxide first formed at the 1,4-positions of the BCH-annelated naphthalene. This is due to the high biradical character of naphthalene 3.(5) A series of oligothiophenes (2-,3-,4-,6-, and 8-mers) completely surrounded by bicyclo[2.2.2]octene units were synthesized, and were converted to stable radical cation salts or dication salts, whose precise structures were determined by X-ray crystallography. These are considered as pertinent models for polaron or bipolaron of the p-doped states of oligothiophenes.