Regiospecifically Fluorinated Polycyclic Aromatic Hydrocarbons via Julia–Kocienski Olefination and Oxidative Photocyclization. Effect of Fluorine Atom Substitution on Molecular Shape

A modular synthesis of regiospecifically fluorinated polycyclic aromatic hydrocarbons (PAHs) is described. 1,2-Diaryl­fluoroalkenes, synthesized via Julia–Kocienski olefination (70–99% yields), were converted to isomeric 5- and 6-fluorobenzo­[c]­phenanthrene, 5-and 6-fluorochrysene, and 9- and 10-benzo­[g]­chrysene (66–83% yields) by oxidative photocyclization. Photocyclization to 6-fluorochrysene proceeded more slowly than conversion of 1-styrylnaphthalene to chrysene. Higher fluoroalkene dilution led to a more rapid cyclization. Therefore, photocyclizations were performed at higher dilutions. To evaluate the effect of fluorine atom on molecular shapes, X-ray data for 5- and 6-fluorobenzo­[c]­phenanthrene, 6-fluorochrysene, 9- and 10-fluorobenzo­[g]­chrysene, and unfluorinated chrysene as well as benzo­[g]­chrysene were obtained and compared. The fluorine atom caused a small deviation from planarity in the chrysene series and decreased nonplanarity in the benzo­[c]­phenanthrene derivatives, but its influence was most pronounced in the benzo­[g]­chrysene series. A remarkable flattening of the molecule was observed in 9-fluorobenzo­[g]­chrysene, where the short 2.055 Å interatomic distance between bay-region F-9 and H-8, downfield shift of H-8, and a 26.1 Hz coupling between F-9 and C-8 indicate a possible F-9···H-8 hydrogen bond. In addition, in 9-fluorobenzo­[g]­chrysene, the stacking distance is short at 3.365 Å and there is an additional interaction between the C-11–H and C-10a of a nearby molecule that is almost perpendicular.

本研究报道了区域特异性氟化多环芳烃（polycyclic aromatic hydrocarbons, PAHs）的模块化合成方法。通过尤利雅-科琴斯基烯化反应（Julia–Kocienski olefination）合成的1,2-二芳基氟代烯烃（收率70%~99%），经氧化光环化反应可转化为异构体5-氟苯并[c]菲、6-氟苯并[c]菲、5-氟屈、6-氟屈以及9-氟苯并[g]屈、10-氟苯并[g]屈（收率66%~83%）。相较于1-苯乙烯基萘转化为屈的反应，6-氟屈的光环化进程更为缓慢。提高氟代烯烃的稀释度可加快环化反应速率，因此本研究采用高稀释条件进行光环化反应。为评估氟原子对分子构型的影响，我们获取了5-氟苯并[c]菲、6-氟苯并[c]菲、6-氟屈、9-氟苯并[g]屈、10-氟苯并[g]屈，以及未氟化的屈和苯并[g]屈的X射线衍射数据并进行对比分析。氟原子对屈类衍生物的平面性造成小幅偏离，而在苯并[c]菲衍生物中则降低了其非平面性，其影响在苯并[g]屈类衍生物中最为显著。在9-氟苯并[g]屈中观察到显著的分子扁平化现象：其湾区的F-9与H-8之间存在2.055 Å的短原子间距，H-8出现低场位移，且F-9与C-8之间存在26.1 Hz的耦合常数，上述结果表明二者之间可能存在F-9···H-8氢键。此外，9-氟苯并[g]屈的分子堆积距离较短，仅为3.365 Å，且邻近分子的C-11–H与C-10a之间存在近乎垂直的额外相互作用。