Myers Andrew G, Sogi Miki, Lewis Michael A, Arvedson Stephen P
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
J Org Chem. 2004 Apr 2;69(7):2516-25. doi: 10.1021/jo030368p.
Theoretical and synthetic studies of the tricyclic 10pi-electron hydrocarbon cyclobuta[1,2:3,4]dicyclopentene (1), 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 1 is a D(2h) symmetric structure with nonalternant C-C single and double bonds. The calculations also predict that 1 is 4.7 kcal/mol higher in energy than the isomeric hydrocarbon 1,6-didehydro[10]annulene (2), a molecule known to isomerize to 1,5-didehydronaphthalene (4) above -50 degrees C. Calculated enthalpic changes of homodesmotic reactions support the notion that 1 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 1 initially utilized as starting material the tricyclic carbonate 11, the product of an intramolecular [2 + 2]-photocyclization reaction. In these studies, 11 was transformed in several steps to the distannane 12, which upon treatment with boron fluoride ethyl etherate is believed to have formed the unstable hydrocarbon bicyclopentadienylidene (13). In an effort to avoid cleavage of the central, four-membered ring of unsaturated tricyclo[5.3.0.0(2,6)]decane intermediates (perhaps the result of 10-electron electrocyclic ring opening of the tetraene 8), synthetic approaches to 1 employing cobalt-cyclobutadiene complexes 18 and 19 were pursued. Treatment of 18 with excess methyllithium led to the novel cobaltacyclic product 30, and dehydration of 19 in the presence of pyridine produced the ring-opening cobaltacyclic product 35. It is proposed that both processes may occur by a 10-electron electrocyclic ring-opening reaction of eta(2)-organocobalt intermediates. These processes may be related to the hypothetical transformation of tetraene 8 to bicyclopentadienylidene (13).
描述了三环10π电子烃环丁烷并[1,2:3,4]二环戊烯（1）的理论和合成研究，该结构名义上为芳香结构，但从未被合成过。通过密度泛函理论计算（B3LYP/6 - 31G(d,p)）进行的几何优化预测，1是具有非交替碳 - 碳单键和双键的D(2h)对称结构。计算还预测，1的能量比异构体烃1,6 - 二脱氢[10]轮烯（2）高4.7千卡/摩尔，已知2在高于 - 50℃时会异构化为1,5 - 二脱氢萘（4）。同系物反应的计算焓变支持了这样一种观点，即1是一个芳香分子，其共振稳定能（RSE）按每个分子计算约为苯的一半到三分之二。对1的潜在合成途径的研究最初使用三环碳酸酯11作为起始原料，它是分子内[2 + 2]光环化反应的产物。在这些研究中，11经过几步转化为二锡烷12，据信用氟化硼乙醚处理12会形成不稳定的烃双环戊二烯叉（13）。为了避免不饱和三环[5.3.0.0(2,6)]癸烷中间体的中心四元环断裂（可能是四烯8的10电子电环化开环的结果），采用钴 - 环丁二烯配合物18和19对1进行了合成方法的探索。用过量甲基锂处理18得到了新型钴环产物30，在吡啶存在下19脱水产生了开环钴环产物35。有人提出这两个过程都可能通过η(2) - 有机钴中间体的10电子电环化开环反应发生。这些过程可能与四烯8向双环戊二烯叉（13）的假设转化有关。
