Direct observation of a doubly destabilized cation

Hanan Afifi,Alexandre Mikhailine,Andrei Chtchemelinine,Evgeny Danilov,Tatiana Dyblenko,Gabriela Mladenova,Edward Lee-Ruff,Israt Sultana,Rene Fournier

doi:10.3998/ark.5550190.0013.631

Hanan Afifi, Alexandre Mikhailine + Show 7 more

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https://doi.org/10.3998/ark.5550190.0013.631

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Abstract

The 9-fluorenyl cation is a member of the 4N Huckel antiaromatic series of intermediates, first observed by time-resolved spectroscopy on UV photo-excitation of 9-fluorenol. [1] 9Trifluoromethyl-9-fluorenol incorporating an electron-withdrawing substituent was subjected to preparative and laser flash photolysis. Photoproduct studies in methanol indicated products derived from the corresponding fluorenyl cation and radical intermediates. Time-resolved spectroscopy in hexafluoroisopropanol (HFIP) showed a transient which was assigned to the corresponding cation as evident from methanol quenching. The lifetimes and methanol quenching rates of this transient was compared with that of 9-methylfluorenyl cation. The kinetic stabilities of these ions were compared to thermodynamic parameters obtained from theoretical calculations.

Highlights

Carbocations incorporated in destabilizing configurations have been well investigated in terms of angular distortions,[1] incorporation into antiaromatic electronic configurations[2] and substituent effects.[3]
1,3-cyclopentadienylium, indenylium, fluorenylium cations to which an electron-withdrawing substituent such as trifluoromethyl is bonded have been referred to as “doubly destabilized” carbocations, and have been the subject of solvolytic and theoretical studies.4a,b,d The 9-fluorenylium cation can be conveniently generated by photoexcitation of the corresponding 9-fluorenol derivatives.2b,c These cations have not been directly observed in previous studies, and their kinetic stabilities not measured
9-methylfluorenol (1a) and 9-trifluoromethyl fluorenol (1b) which were obtained from 9

Summary

Introduction

Carbocations incorporated in destabilizing configurations have been well investigated in terms of angular distortions,[1] incorporation into antiaromatic electronic configurations[2] and substituent effects.[3] The placement of a carbocation center involving two of these factors gives rise to enhanced destabilized carbocations.[4] 1,3-cyclopentadienylium, indenylium, fluorenylium cations to which an electron-withdrawing substituent such as trifluoromethyl is bonded have been referred to as “doubly destabilized” carbocations, and have been the subject of solvolytic and theoretical studies.4a,b,d The 9-fluorenylium cation can be conveniently generated by photoexcitation of the corresponding 9-fluorenol derivatives.2b,c These cations have not been directly observed in previous studies, and their kinetic stabilities (nucleophile quenching rates) not measured. Computational data were obtained to assess their thermodynamic stabilities

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