Studies of 9-Fluorenyl Carbocations. Intramolecular Hydride Migration in a Substituted 9-Fluorenyl Carbocation

Abstract

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.

Photochemical Generation of 9H‐Fluorenyl Radicals

“…9,9′-Bifluorenyls also appear to generate 9-fluorenyl radicals as evident from the presence of the reduced fluorenes present in the photolysate. The coupling of fluorenyl substituted by an electron-withdrawing group such as CF 3 gives rise to an unsymmetrical bifluorenyl 2. This transformation is enhanced in polar and hydrogen-bonding solvents.…”

“…Our interests in carbocations with destabilizing electronic configurations such as anti‐Huckel (4N) rings ( e.g . 9‐fluorenyl carbocations ) prompted the study of the analogous 9H‐fluorenyl radicals and their 9‐substituted derivatives. Carbon‐centered radicals can be considered as electron‐deficient species and hence would be expected to exhibit similar effects of substituents on their stabilities as their carbocation counterparts.…”

The 9-Phenyl-9-fluorenyl Group for Nitrogen Protection in Enantiospecific Synthesis

“…In solution the cations are deep red and with an absorption maximum at λ max = 494 nm. Quenching the cation with methanol forms Pf methyl ether as the main product [ 13 ].…”

Optical switching behavior of poly(9-alkoxyfluorene-phenylenevinylene) upon exposure to Lewis acid and base

“…If this scenario proved to be feasible, the photoelectronic property of the polymer should be changed, which means this polymer may act as a sensor responding to a Lewis acid. According to the literatures, it is known that 9,9 0 -disubstituted fluorene readily forms fluorenyl cation under acidic conditions or by photoexcitation [14,15]. Solvolysis of the delocalized carbocations can be successfully carried out by adding methanol as shown in Scheme 2.…”