Intramolecular Reaction of Transient Phosphenium and Arsenium Ions Giving Rise to Isolable 9‐Phospha‐ and 9‐Arsena‐Fluorenium Ions

Abstract

Transient phosphenium and arsenium ions, generated by fluoride abstraction from bis(m‐terphenyl)fluoropnictogens, underwent intramolecular electrophilic attack prior to methyl group migration and gave rise to isolable 9‐phospha‐ and 9‐arsena‐fluorenium ions.

Isolation of an Antiaromatic 9‐Hydroxy Fluorenyl Cation

“…This rearrangement is strongly reminiscent of the reaction between 2,6‐(4‐ t ‐BuC 6 H 4 )C 6 H 3 Li with Cl 2 BH⋅SMe 2 , which gave 9‐bora‐fluorene rather than the expected bis( m ‐terphenyl)borane [20] . It also resembles our previous attempts to prepare stable bis( m ‐terphenyl)phosphenium ions by fluoride abstraction from (2,6‐Mes 2 C 6 H 3 ) 2 PF and [2,6‐(Me 5 C 6 ) 2 C 6 H 3 ] 2 PF, which gave a (protonated) 9‐phospha‐fluorene [21] and isomeric 9‐phospha‐fluorenium ions instead [22] . The latter two reactions also involved methyl group migrations.…”

“…[20] It also resembles our previous attempts to prepare stable bis(m-terphenyl)phosphenium ions by fluoride abstraction from (2,6-Mes 2 C 6 H 3 ) 2 PF and [2,6-(Me 5 C 6 ) 2 C 6 H 3 ] 2 PF, which gave a (protonated) 9-phospha-fluorene [21] and isomeric 9-phospha-fluorenium ions instead. [22] The latter two reactions also involved methyl group migrations. The 9-hydroxy fluorenyl cation 2 comprises a central five-membered ring with formally 4π electron, thus fulfilling the Hückel rule for antiaromaticity.…”

Cationic Carbene Analogues: Donor‐Free Phosphenium and Arsenium Ions

“… [10] Since then, our systematic efforts to apply a similar strategy for the isolation of the lighter phosphenium and arsenium congeners have failed to yield the targeted salts. In this endeavor we have noticed that the halide abstraction reagent must be carefully considered, [11] and that when generated, the phosphenium and arsenium ions are shortly lived and react intramolecularly with one of the flanking aryl groups of the m ‐terphenyl substituent [12, 13] . From these observations it became clear that the free rotation of these flanking aryl groups, which should confer steric protection around the pnictogen is of serious concern.…”

“…In this endeavor we have noticed that the halide abstraction reagent must be carefully considered, [11] and that when generated, the phosphenium and arsenium ions are shortly lived and react intramolecularly with one of the flanking aryl groups of the m ‐terphenyl substituent. [ 12 , 13 ] From these observations it became clear that the free rotation of these flanking aryl groups, which should confer steric protection around the pnictogen is of serious concern. We have thus turned our attention to the tetramethyl‐s‐hydrindacen‐4′‐yl based substituents, [14] more specifically to the 9‐fluorenyl substituted one, [15] which is both rigid and able to confer sufficient steric protection around the pnictogen.…”

Kationische Carben‐Analoga: Donorfreie Phosphenium‐ und Arsenium‐Ionen

“… [10] Seitdem haben unsere systematischen Bemühungen, eine ähnliche Strategie auf die Isolierung der leichteren Phosphenium‐ und Arsenium‐Ionen anzuwenden, nicht zum Ziel geführt. Wir haben festgestellt, dass die Abstraktionsreagenzien für die Halogenide sorgfältig ausgewählt werden müssen, [11] und dass die gebildeten Phosphenium‐ und Arsenium‐Ionen sehr kurzlebig sind und spontan intramolekular mit einer der flankierenden Arylgruppen der m ‐Terphenyl‐Substituenten reagieren [12, 13] . Von diesen Beobachtungen wurde klar, dass die ungehinderte Rotation der flankierenden Arylgruppen, die die sterische Abschirmung um das Pniktogen gewährleisten, ein Problem darstellt.…”