Hydrogenation and Dehydrogenation of Pentaphenylcyclopentadienes and Pentaphenylcyclopentenes

Abstract

Pentaaryl‐substituted cyclopentadienes and cyclopentenes have been employed in catalytic hydrogenation and photochemical cyclodehydrogenation reactions, targeting strained bowl‐shaped structures. Both types of reactions generally stop at the monohydrogenation and monocyclization stage, respectively.

Mechanochemical Scholl Reaction on Phenylated Cyclopentadiene Core: One‐Step Synthesis of Fluoreno[5]helicenes

“…Naturally, our first choice was the previously published CuCl 2 /AlCl 3 system. [23] Repeating the literature procedure did not produce the product (Table 2, entry 1). Only when both the oxidant loading and the time were increased, the product was isolated in a moderate yield (Table 2, entry 2-3).…”

“…Next, we explored whether different oxidant systems would be compatible with our milling conditions. Naturally, our first choice was the previously published CuCl 2 /AlCl 3 system [23] . Repeating the literature procedure did not produce the product (Table 2, entry 1).…”

“…However, with a considerably lower performance as compared to the mechanochemical FeCl 3 conditions. Lastly, we used the previously published CuCl 2 /AlCl 3 oxidant system, [23] which provided only the unreacted starting material after 5 min of stirring. An increase in the reaction time and the oxidant loading was necessary to obtain the product in moderate yield.…”

“…In contrast, Dyker and co‐workers demonstrated an efficient route to fluoreno[ 5 ]helicenes in one step by the solution‐phase cyclodehydrogenation of pentaarylcyclopentadiene precursor (Scheme 1). However, the isolated yield was modest [23] …”

“…However, the isolated yield was modest. [23] In the realm of polycyclic aromatic hydrocarbons, the mechanochemical approach has recently emerged as an alternative to the conventional solution-phase chemistries. [24] The work of Borchardt and coworkers on FeCl 3 -mediated Scholl reaction is particularly encouraging in this regard.…”