9-Ethynylfluoroenyl Radicals: Regioselective Dimerization and Post Ring-Cyclization Reactions

Abstract

9-Ethynylfluorenyl radical derivatives were readily prepared in situ and underwent simultaneous intermolecular coupling reactions. Interestingly, the dimerization process took place in either a head-to-tail or a head-to-head mode between the acetylenic or the allenic resonance forms dependent on the terminal substituents, which could be well explained by their different spin distribution and steric hindrance effects. The structures of the products were confirmed by X-ray crystallographic and other spectroscopic analyses. It was also found that the newly generated dipropinyl dimers underwent a rearrangement and ring-cyclization reaction at room temperature, eventually giving unique difluorenylidene cyclobutene derivatives.

Synthesis, properties and chemical modification of a persistent triisopropylsilylethynyl substituted tri(9-anthryl)methyl radical

“…S11, ESI†). For comparison purposes, the TMS-ethynyl substituted fluorenyl radical reacts to only form the asymmetric head-to-tail σ-dimer (allene–ethynyl) (Scheme 2b) 14 probably because the fluorenyl moiety is sterically less hindered than the TAntM unit. The combined results indicate that the TIPS-ethynyl unit in 1 stabilizes the radical both through a steric effect and electronically by delocalization of the unpaired electron.…”

Propargyl Radicals in Organic Synthesis

“…In‐situ propargyl radical generation has been observed in alkynyl‐fluorenol derivatives 118 upon ionization/reduction with tin (II) chloride [57] . The nature of the subsequent dimerization is dependent upon the type of alkynyl substitution; substrates ( 119 ) with alkynyl TMS groups predominately undergo propargyl‐allenyl coupling ( 120 ), while propargyl‐propargyl dimerization reactions are favored with the bulkier alkynyl triisopropyl and mesityl substituted analogues ( 121 ) (Scheme 29).…”

Platforms for Stable Carbon‐Centered Radicals

“…Fluorenyls with substituents, other than an anthryl group, at the 9‐position were also synthesized by Wu et al. The 9‐ethynylfluorenyls 11R 1 R 2 readily underwent intermolecular coupling to give either head‐to‐tail ( 12 ) or head‐to‐head ( 13R 1 R 2 ) dimers depending on the terminal substituents . Substitution with oxasmaragdyrin at a meso‐position provided the stable fluorenyl radical 14 , which was separable by a silica gel column …”