Naphthologs of overcrowded bistricyclic aromatic enes: (E)-bisbenzo[a]fluorenylidene
作者:Naela Assadi、Sergey Pogodin、Shmuel Cohen、Israel Agranat
DOI:10.1007/s11224-012-0146-4
日期:2013.8
(E)-11H-Bisbenzo[a]fluorenylidene (E-6) was synthesized by Barton’s double extrusion diazo-thione coupling method from 11H-benzo[a]fluoren-11-thione (11) and 11-diazo-11H-benzo[a]fluorene (13). The reaction is probably thermodynamically controlled; in the event that the less stable Z -6 is also formed, it would rapidly undergo Z → E diastereomerization to give E -6. The B3LYP/6-311G(d,p) calculated diastereomerization barrier for Z -6 → E -6 is ΔG 298 = 57.0 kJ/mol (13.6 kcal/mol). The calculated equilibrium constant K eq(E -6 → Z -6) = 92:8 (at 298 K) is indicative of a marked diastereoselectivity of the reaction leading to E -6. The structure of E-6 was established by 1H-NMR and 13C-NMR spectroscopies and by X-ray analysis. PAE E-6 crystallizes in the monoclinic space group C2/c. The unit cell of the crystal structure E -6 contains eight molecules, arranged as four pairs of enantiomers. PAE E -6 adopts a twisted conformation with the pure twist of the central C11=C11′ bond ω = 39°. The dihedral angle ν in E -6 is 60.6°, which is significantly higher than the respective dihedral angle in PAEs Z -6, 2, E -7, Z -7, 14, and 15. The large syn-pyramidalization angles at C11 and C11′ (χ = 12.6° and 14.8°) of E-6 indicates the enhanced strain in the fjord regions of the molecule. The enhanced twist is primarily attributed to the double benzo[a]annelation of the bifluorenylidene moiety at the fjord regions. The B3LYP/6-311G(d,p) calculated structure of E -6 is in a very good agreement with the experimental X-ray structure. PAE E -6 adopts a twisted conformation in solution, with the downfield chemical shift of H1/H1′ (8.31 ppm); H10/H10′ (δ = 7.20 ppm) and H9/H9′ (δ = 6.86 ppm) in E -6 are positioned above the planes of the opposing naphthalene rings. PAEs E -6 and Z -6 are significantly higher in energy than their corresponding benzo[b]annelated isomers E -7 and Z -7.
(E)-11H-Bisbenzo[a]fluorenylidene (E-6) 是通过 Barton 的双挤压重氮-硫酮偶联法,从 11H-苯并[a]芴-11-硫酮 (11) 和 11-重氮-11H-苯并[a]芴 (13) 合成的。该反应可能是受热力学控制的;如果也形成了稳定性较差的 Z -6,则会迅速发生 Z → E 非对映异构反应,生成 E -6。B3LYP/6-311G(d,p) 计算得出的 Z -6 → E -6 非对映分解障碍为 ΔG 298 = 57.0 kJ/mol (13.6 kcal/mol) 。计算得出的平衡常数 K eq(E -6 → Z -6) = 92:8(298 K 时)表明,生成 E -6 的反应具有明显的非对映选择性。 通过 1H-NMR 和 13C-NMR 光谱以及 X 射线分析,确定了 E-6 的结构。PAE E-6 在单斜空间群 C2/c 中结晶。晶体结构 E -6 的单胞包含八个分子,排列成四对对映体。PAE E -6 采用扭曲构象,中心 C11=C11′ 键的纯扭曲度为 ω = 39°。E -6 中的二面角 ν 为 60.6°,明显高于 PAE Z -6, 2, E -7, Z -7, 14 和 15 中各自的二面角。E-6 的 C11 和 C11′(χ = 12.6° 和 14.8°)处的同位斜角较大,表明分子峡湾区的应变增强。扭转的增强主要归因于双芴分子在峡湾区的双苯并[a]炔化。E -6 的 B3LYP/6-311G(d,p) 计算结构与实验 X 射线结构非常吻合。PAE E -6 在溶液中呈扭曲构象,H1/H1′(8.31 ppm)、H10/H10′(δ = 7.20 ppm)和 E -6 中的 H9/H9′ (δ = 6.86 ppm)位于对立萘环的平面之上。PAEs E -6 和 Z -6 的能量明显高于其相应的苯并[b]琥珀酸异构体 E -7 和 Z -7。