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

Abstract

In this study, we explore feasibility of the mechanochemical approach in the synthesis of tetrabenzofluorenes (fluoreno[5]helicenes). For this, commercially available phenylated cyclopentadiene precursors are subjected to the Scholl reaction in the solid state using FeCl3 as an oxidant and sodium chloride as the solid reaction medium. This ball milling process gave access to the 5‐membered ring containing‐helicenes in one synthetic step in high (95‐96%) isolated yields. The solution‐phase reactions, however, were found to be moderate to low yielding in this regard (10‐40%).

Copper-catalysed perarylation of cyclopentadiene: synthesis of hexaarylcyclopentadienes

“…In compound 6c, the torsion angle C31–C3–C4–C34 is worth 28°, and this angle is reduced to 23° in fluoreno[5]helicene 6b incorporating tert -butyl instead of methyl para -substituents. These values are in line with those reported in literature for 17-monoaryl- 34 and 17,17-dialkyltetrabenzofluorenes, 38 and the variation of amplitude may be ascribed to the packing force in the crystal. In any case, the racemisation barrier for such helicenic species is low and both enantiomers cannot be separated at room temperature.…”

“…In addition, on top of their intrinsic properties, propeller-shaped compounds 1 can also be viewed as direct precursors of highly attractive π-extended scaffolds for organic electronics, such as 17,17-diarylcyclopenta[ l , l ′]diphenanthrenes (also referred to as 17,17-diaryltetrabenzo[ a , c , g , i ]fluorenes, Scheme 6 ) and the corresponding spirobifluorenes. 30 b ,33 In this context, Stuparu et al very recently disclosed the successful conversion of pentaphenylcyclopentadiene 2a into 17-phenyl-tetrabenzo[ a , c , g , i ]fluorene upon mechanochemical Scholl reaction, 34 thus improving initial attempts by Dyker et al 35 …”

“…In addition, on top of their intrinsic properties, propellershaped compounds 1 can also be viewed as direct precursors of highly attractive p-extended scaffolds for organic electronics, such as 17,17-diarylcyclopenta[l,l 0 ]diphenanthrenes (also referred to as 17,17-diaryltetrabenzo[a,c,g,i]uorenes, Scheme 6) and the corresponding spirobiuorenes. 30b,33 In this context, Stuparu et al very recently disclosed the successful conversion of pentaphenylcyclopentadiene 2a into 17-phenyl-tetrabenzo [a,c,g,i]uorene upon mechanochemical Scholl reaction, 34 thus improving initial attempts by Dyker et al 35 Hexaphenylcyclopentadiene 1a and its tert-butyl and methyl para-substituted counterparts 1b and 1c, respectively, were next submitted to a Scholl reaction, 36 with the aim to trigger the formation of p-extended scaffolds. In spite of the wide set of conditions tested, spirobiuorenes were never detected, which can be accounted for by the high strain associated with the formation of the second uorene pattern upon 5-membered ring closure.…”

Completely solvent-free synthesis of double heterohelicenes and their further ring fusion using mechanochemical reaction

“…17 There have been several recent reports on the application of mechanochemical reactions to oxidative coupling reactions of aromatic compounds. 18–27 In 2018, Borchardt et al reported the mechanochemical Scholl reaction, an oxidative cyclodehydrogenation forming aryl–aryl C–C bonds. 18 They established the mechanochemical synthesis of fully planarized PAHs, such as hexa- peri -hexabenzocoronene (HBC) and its larger analogs, from non-planar dendritic oligophenylene precursors using ball mills as reactors.…”