Synthetic and Theoretical Studies of Cyclobuta[1,2:3,4]dicyclopentene. Organocobalt Intermediates in the Construction of the Unsaturated Carbon Skeleton and Their Transformation into Novel Cobaltacyclic Complexes by C−C Insertion

Theoretical and synthetic studies of the tricyclic 10π-electron hydrocarbon cyclobuta[1,2:3,4]dicyclopentene (<b>1</b>), a nominally aromatic structure that has never been synthesized, are described. Geometry optimization by density-functional-theory calculations (B3LYP/6-31G(d,p)) predict that <b>1</b> is a <i>D</i><sub>2</sub><i><sub>h</sub></i> symmetric structure with nonalternant C−C single and double bonds. The calculations also predict that <b>1</b> is 4.7 kcal/mol higher in energy than the isomeric hydrocarbon 1,6-didehydro[10]annulene (<b>2</b>), a molecule known to isomerize to 1,5-didehydronaphthalene (<b>4</b>) above −50 °C. Calculated enthalpic changes of homodesmotic reactions support the notion that <b>1</b> is an aromatic molecule with a resonance stabilization energy (RSE) about half to two-thirds that of benzene on a per-molecule basis. Investigations of potential synthetic pathways to <b>1</b> initially utilized as starting material the tricyclic carbonate <b>11</b>, the product of an intramolecular [2 + 2]-photocyclization reaction. In these studies, <b>11</b> was transformed in several steps to the distannane <b>12</b>, which upon treatment with boron fluoride ethyl etherate is believed to have formed the unstable hydrocarbon bicyclopentadienylidene (<b>13</b>). In an effort to avoid cleavage of the central, four-membered ring of unsaturated tricyclo[5.3.0.0<sup>2,6</sup>]decane intermediates (perhaps the result of 10-electron electrocyclic ring opening of the tetraene <b>8</b>), synthetic approaches to <b>1</b> employing cobalt−cyclobutadiene complexes <b>18</b> and <b>19</b> were pursued. Treatment of <b>18</b> with excess methyllithium led to the novel cobaltacyclic product <b>30</b>, and dehydration of <b>19</b> in the presence of pyridine produced the ring-opening cobaltacyclic product <b>35</b>. It is proposed that both processes may occur by a 10-electron electrocyclic ring-opening reaction of η<sup>2</sup>-organocobalt intermediates. These processes may be related to the hypothetical transformation of tetraene <b>8</b> to bicyclopentadienylidene (<b>13</b>).