Configurationally Stable Longitudinally Twisted Polycyclic Aromatic Compounds

Two strategies for the synthesis of configurationally stable twisted polycyclic aromatic compounds (PACs) were pursued. The first approach employed dissymmetrically positioned 1-naphthyl substituents to bias the direction of twist in highly substituted PACs. 2,3-Bis(1-naphthyl)-1,4-diphenyltriphenylene (7) was prepared, and its meso cis-dinaphthyl and enantiomeric trans-dinaphthyl isomers were resolved by preparative supercritical fluid chromatography (SFC) on chiral supports. Similarly, several naphthyl-substituted derivatives of the more highly twisted 9,10,11,12,13,14-hexaphenylbenzo[b]triphenylene (2) were prepared. Of these, 10-(1-naphthyl)-9,11,12,14-tetraphenylbenzo[b]triphenylene (13) was resolved by SFC on a chiral support. The pure enantiomers of trans-7 showed moderately large specific rotations ([α]D25 = −330 and +320°), but the specific rotations for the enantiomers of 13 were unexpectedly small ([α]D25 = −23 and +23°). Computational studies suggest that the latter result is due to presence of a minor conformation of 13 possessing a larger rotation of opposite sign than the major conformation. Both 7 and 13 showed strong circular dichroism and moderately strong circularly polarized luminescence. A byproduct of these syntheses was 9,10,19,21-tetraphenyldiphenanthro[9,10-b:9,10-h]carbazole (15), a very crowded carbazole that exhibits an 81° end-to-end twist but is not resolvable. In the second approach, the large, twisted, polycyclic aromatic ligand 9,10,11,12,13,14-hexaphenylbenzo[h]naphtho[2,3-f]quinoline (21, an aza-2) was used to prepare the chiral, cyclometallated iridium(III) complex 4. The ligand 21 was prepared via an unusually stable benzannulated norbornadienone, for which the free energy of activation for decarbonylation was a remarkable 33.5 kcal/mol. The iridium complex 4 proved to be configurationally stable and resolvable by analytical HPLC on chiral supports, but the low solubility of 4 prevented its resolution on a preparative scale. A much more soluble dibutyl analogue of 4 (complex 28) was then prepared, but it was not resolvable on any of the available media.

本研究开发了两种策略，用于合成构型稳定的扭曲多环芳烃（polycyclic aromatic compounds, PACs）。第一种策略采用非对称排布的1-萘基取代基，对高取代PACs的扭曲方向进行调控。我们成功合成了2,3-二(1-萘基)-1,4-二苯基三苯基烯（7），并通过手性载体上的制备型超临界流体色谱（supercritical fluid chromatography, SFC）对其内部消旋的顺式二萘基异构体与对映异构的反式二萘基异构体进行了拆分。类似地，我们还合成了扭曲程度更高的9,10,11,12,13,14-六苯基苯并[b]三苯基烯（2）的多款萘基取代衍生物。其中，10-(1-萘基)-9,11,12,14-四苯基苯并[b]三苯基烯（13）可通过手性载体上的SFC实现拆分。反式-7的纯对映异构体展现出中等强度的较大比旋光度（[α]D25 = −330° 和 +320°），但13的对映异构体比旋光度却意外微弱（[α]D25 = −23° 和 +23°）。计算研究表明，该异常现象源于13存在一种次要构象：该构象的旋光度数值更大且符号与主要构象相反。7和13均展现出强烈的圆二色性（circular dichroism）与中等强度的圆偏振发光（circularly polarized luminescence）。该合成路线的副产物为9,10,19,21-四苯基二菲并[9,10-b:9,10-h]咔唑（15），这是一种空间位阻极大的咔唑衍生物，其端到端扭转角达81°，但无法实现拆分。第二种策略则以大位阻扭曲型多环芳烃配体9,10,11,12,13,14-六苯基苯并[h]萘并[2,3-f]喹啉（21，即aza-2）为原料，制备手性环金属化铱(III)配合物4。配体21通过一种异常稳定的苯并降冰片二烯酮中间体合成得到，该中间体的脱羰自由能活化能高达33.5 kcal/mol，性能出众。实验表明，铱配合物4构型稳定，可通过手性载体上的分析型高效液相色谱（high performance liquid chromatography, HPLC）实现拆分，但由于4的溶解度较低，无法进行制备级拆分。随后我们合成了溶解度更佳的4的二丁基类似物（配合物28），但该类似物无法通过现有任何介质实现拆分。