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

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(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.