Detection and Characterization of a Transient Zwitterion, the 9-Carboxylate-9-fluorenyl Cation, and Its Conjugate Acid¹

Abstract

Laser flash photolysis of 9-hydroxy-9-fluorenecarboxylic acid in hexafluoro-2-propanol (HFIP) generates a transient with λmax at 495 nm that reacts with nucleophiles such as methanol and bromide and is insensitive to oxygen. In the presence of 0.15 M trifluoroacetic acid, a different transient with λmax at 560 nm that also reacts with nucleophiles is formed. The 495 and 560 nm species are identified as a zwitterion (the 9-carboxylate-9-fluorenyl cation) and its conjugate acid (the 9-carboxy-9-fluorenyl cation), respectively. The assignment of the zwitterion is supported by the observation of infrared absorptions in the carboxylate region that are quenched by nucleophiles with rate constants similar to those obtained by UV−visible detection. Both the zwitterion and its protonated form are observed at intermediate acid concentrations, leading to an estimate of 1 × 10-2 M-1 for the equilibrium constant for the ionization of the 9-carboxy-9-fluorenyl cation in HFIP. Absolute rate constants for reaction of the zwitterion with alcohols, anionic nucleophiles, and substituted aromatics are reported and compared to data for other 9-fluorenyl cations.

Time‐resolved IR studies of α‐lactones

“…In addition, B3LYP/6-31G(d) calculations indicate that the corresponding triplet-biradical 18 is 16.1 kcal mol À1 higher in energy than zwitterion 17. Lew et al 13 photolyzed 9-hydroxy-9-fluorenecarboxylic acid in hexfluoro-2-propanol (HFIP) and detected a transient ( % 20 ms) with max ¼ 495 nm and strong IR bands at 1575, 1600, and 1620 cm À1 , which was assigned to zwitterion 20, in excellent agreement with our measured IR bands for ring-opened form 17 (Scheme 2). They examined the region 1988-1855 cm À1 and reported that no signals were observed for ring-closed form 21.…”

“…21 ) The trend of these estimated secondorder order rate constants agrees with that observed in low-temperature matrices by Sander and co-workers. Although this series of carbenes contains both groundstate singlets (9 and 12) and ground-state triplets (13)(14)(15), unlike in Sander and co-workers' low-temperature experiments, 14,15 we cannot rule out the thermal population of and reaction from the higher spin state in our room temperature studies.…”

“…Hence structures 1 and 2 both play important roles in -lactone chemistry; however, in essentially all cases where ringopened form 2 dominates the observed reactivity, it has not been detected spectroscopically. (We are aware of only one instance in which zwitterion 2 has been directly observed; 13 this study will be discussed in more detail below.) For example, the reactivity of -lactones 6 and 7, even at temperatures as low as À100 C, is consistent with ring-opened form 2.…”

Photochemical Pinacol Rearrangement

“…Destabilized carbocations in which one or more of the α-substituents are electron-withdrawing and cations incorporated in a 4n cyclic π-framework have been of interest as intermediates in solvolysis, − as species directly observable under stable ion conditions, , or as transients obervable under laser flash photolysis in neutral solutions. − The 9-fluorenyl cation can be readily produced by irradiation of 9-fluorenols unlike other diarylmethyl cations which require more photolabile leaving groups. This is associated with the ease of photosolvolysis processes of cyclic π-chromophores possessing 4n electrons in the internal cyclic array (ICA) .…”

Effect of oxime substituents on 9-fluorenyl carbocations

“…7 In a related studies, the carbonyl substituted cations 3 have been studied by Lee-Ruff and Johnston under laser flash photolytic conditions. 8,9 Recent computational studies by Schleyer and Tidwell suggest that the parent 9-fluorenyl cation is non-aromatic (as opposed to antiaromatic). 10 Our recent experimental and computational studies on fluorene system 4 suggests that the carbonyl group in this cation is rotated 90°out of conjugation with the cationic center.…”