Polyphenylenes, i.e., compounds made from several phenyl rings, can have a variety of linear, branched, or even dendritic structures. Much less is known about related saturated structures made from cyclohexyl rings. There are a few studies on linear polycyclohexylene and branched “oligocyclohexyls”. However, larger oligocyclohexyls are rare, and 1,2,3,4-tetracyclohexylcyclohexane, i.e., a cyclohexane with four cyclohexyl substituents, had been the largest example reported so far.
Zijie Qiu, Akimitsu Narita, Klaus Müllen, Max Planck Institute for Polymer Research, Mainz, Germany, and colleagues have investigated the hydrogenation of hexaphenylbenzene (HPB) and obtained the partially hydrogenated hexacyclohexylbenzene (HCB), as well as fully hydrogenated 1,2,3,4,5,6-hexacyclohexylcyclohexane (HCC, pictured). The hydrogenation of HPB was carried out with a commercially available Pd/C catalyst in an autoclave at 150 °C and 250 bar. Mass spectrometry and high-performance liquid chromatography (HPLC) were used to monitor the reaction.
The crude product contained several compounds with different degrees of hydrogenation. Due to their similar structures, separation using silica-gel chromatography or large-scale recrystallization proved impossible. However, the team was able to grow two differently shaped types of crystals via a slow vapor diffusion of methanol into a chloroform solution of the crude product. The crystals were sorted and separated by hand. X-ray crystallography showed that these two types of crystals were composed of HCB and HCC, respectively.
According to the researchers, the work could serve as a model study for preparations of other fully saturated polyphenylene counterparts. However, this would require catalyst systems with higher hydrogenation efficiency.
- From Hexaphenylbenzene to 1,2,3,4,5,6-Hexacyclohexylcyclohexane,
Marcel Dillenburger, Zijie Qiu, Cheng-Wei Ju, Beate Müller, Svenja Morsbach, Dieter Schollmeyer, Akimitsu Narita, Klaus Müllen,
J. Am. Chem. Soc. 2020.